Sprinkler systems need flow and pressure to operate. Sometimes you might need to know the flow and pressure requirements of an existing sprinkler system. For example the flow and pressure are needed if adding on to the existing irrigation system or adding or replacing a pump. This article contains instructions on how to determine those values. This article uses the USA measurements GPM for flow and PSI for pressure, however conversion to metric is easy using standard conversion formulas.
How to Reverse Engineer the Flow (GPM)
This is going to take some time and a lot of crawling around on your hands and knees… sorry, that’s just how it is.
Look to see the sprinkler brand and which nozzles are installed in the existing sprinklers on a single valve circuit. Write them all down. You may have to look very close to find the nozzle number, it is usually imprinted using tiny text on the nozzle itself. It may be right next to where the water comes out, or it may be on the top of the nozzle. It is often hard to see. Each sprinkler may have a different nozzle, so you will need to look at every one of them and write them all down. Rotor type sprinklers (streams of water that rotate) tend to have numbered nozzles, #1, #3, #9, etc.. For rotors you often need to pull the pop-up riser up from the sprinkler body to see the nozzle outlet and the number. Spray type sprinklers (steady spray like a shower head) tend to have a number followed by a letter that indicates the arc, like 10F, 12H or 15Q.
Look up the flow requirement (GPM) for each sprinkler and nozzle using charts you should be able to find on the sprinkler manufacturer’s website. Add all the GPM values together to determine the total GPM for the valve circuit. For older discontinued sprinkler models you may have to contact the manufacturer’s consumer help department and ask them to email you performance charts.
The sprinkler manufacturer’s website will probably give you a chart that shows different flow values for the nozzles depending on the “PSI”. To determine the PSI to use measure the distance between adjacent sprinklers in feet. Measure several and determine the average distance. Now use that average distance between sprinklers as the “PSI” value– but do NOT use a value less than 30. Example: If the average distance between adjacent sprinklers is 45 feet, use 45 PSI to find the GPM in the chart. If the average distance between sprinklers is 25 feet, use 30 PSI. Do not use less than 30 PSI!
Repeat for each valve circuit.
Assuming you run one valve circuit at a time, your flow requirement for the pump will be the same as the single valve circuit with the highest flow value (GPM.) If you run more than one circuit at a time, add together the GPM values for the ones you run together as they essentially become one big circuit when run together.
To recap, calculate the total GPM of each group of sprinklers that run at the same time. The flow (GPM) value to use when determining the pump size that of the group of sprinklers that has the highest total flow (GPM).
Determining the pressure requirements can be difficult. First off, pump pressure in the USA is measured in either “Feet of Head” (Ft.Hd.) or “PSI”. (The rest of the world uses “bars” but this article is USA based. ) We’ll talk more about this later, but for now you can use either “Ft.Hd.” or “PSI.” You can switch back and forth between these two values using simple conversion formulas:
____ PSI x 2.31 = ____ Feet of Head (ft.hd.)
____ Feet of Head x 0.433 = ____ PSI
How to Reverse Engineer the Pressure
(3 different methods)
One quick rule of thumb- too much water pressure is much easier a problem to deal with than too little pressure. If you are uncertain at all, always go with the higher value. It is very easy to throttle down the pressure if you have too much, if you don’t have enough pressure it is very hard and expensive to increase the pressure. So if you are looking at a sprinkler head and asking yourself: “should this sprinkler head use 30 PSI or 35 PSI, I’m just not sure?”, the answer is easy, use the higher pressure, 35 PSI.
Method #1: Measure It with A Gauge.
If the irrigation system is still operational install a pressure gauge on the pipe as close as possible to where the water source, turn on the system, and the gauge will tell you the pressure the existing system uses. You may need to cut the water supply pipe and install a tee for the gauge to connect to, or possibly you can drill a hole in the pipe, thread the hole with a 1/4″ NPT thread tap, and then screw the gauge into the hole (most gauges have 1/4″ male NPT thread connections on them, but you might want to make sure before you drill and tap!)
If the irrigation system uses a pump to run it you typically would measure the pressure within a foot or two of the pump outlet. Remember that if you do have an existing older pump the pressure you measure may be lower than it should be due to the pump being tired, old, and worn out. You might want to add another 10 PSI to the pressure you measure at the pump to compensate for age.
Method #2: Calculate the Pressure losses.
If you can’t take a pressure reading on the existing system, the next best method is to completely redo all the calculations for the sprinkler system. Let’s be honest, this is too difficult and time consuming for most people, but it is the best way. If you want to try it you need to read through the Sprinkler System Design Tutorial to learn how pressure requirements for a sprinkler system are calculated. You should be able to then reverse engineer your existing sprinkler system by calculating the pressure loss in each section of pipe, each valve, each sprinkler head, etc. to figure out the PSI it requires to operate. You may have to dig up some pipes to determine what size they are. It is a lot of work, you will have to basically learn how to design irrigation systems to do it. Seriously, it is unlikely you are going to do this unless you are an engineer or just very anal-retentive… let’s move on.
Method #3: Guesstimate It.
If you really can’t figure out the pressure needed you can use a guesstimated pressure value. Most of you are likely to use this method. Obviously this method is not optimal and there are no guarantees it will work but it’s a lot less effort that the first two methods above. The idea here is to guesstimate high as previously mentioned in the introduction. Not sure? Add 5 PSI! Really feeling uncertain? Add 10 PSI. You get the idea. Instead of measuring with a pressure gauge, the guesstimate method is going to require you to grab a tape measure (which most people have) and you won’t need to cut into any pipes or dig anything up.
FYI: This guesstimate method is based on some hydraulic principles that define the water pressure needed at a sprinkler head in order to shoot water a specific distance, along with some very rough assumptions of how much pressure is typically needed to move water through all the various pipes, valves, etc.
Guesstimate formula: Sprinkler spacing in FEET + 30 = guesstimated PSI required at water source.
If the resulting total is less than 50, use 50. Do not use a guesstimated water pressure value less than 50 PSI !
To guesstimate the pressure needed take the largest distance in feet between adjacent sprinkler heads, and add 30 to it. If the answer is less than 50, use 50. Do not use a pressure lower than 50, very few sprinkler systems will function well at a pressure below 50 PSI. Which sprinkler heads to measure? Generally you should measure and write down the distance between all your sprinkler heads that are next to each other. It’s not uncommon to have two heads that are way further apart than all the others. If so you can choose to ignore that set of sprinklers. The examples below will help this make more sense.
Example: let’s say you have 6 sprinklers and you measure the distances between the adjacent ones and you get 50′, 32′, 29′, 33′, 35′, and 29′. In this case 50′ is way out of line with the other values so let’s ignore it and use the next greatest distance, which is 35′. So then 35′ + 30 = 65 PSI. So in this case 65 PSI is the guesstimated pressure required for the system. Understand there may be a dry spot between those two heads that are 50′ apart (there probably already is a dry spot there!) That 50′ distance is really a design error on the original sprinkler system and there should probably be another sprinkler head in the middle of that space.
More Guesstimate Method Examples:
Sprinklers 15 feet apart: 15 + 30 = 45 PSI. This is less the 50, so use 50 PSI. Sprinklers 35 feet apart: 35 + 30 = 65 PSI. Use 65 PSI. Sprinklers 45 feet apart: 45 + 30 = 75 PSI. Use 75 PSI.
Remember, 50 PSI is the minimum!
Remember: No guarantees, this method gives you a guesstimate! You understand that you are taking a risk using this. Buying a pump? It is strongly recommended that if you guesstimate the pressure you buy your pump from someplace with a generous return/exchange policy. You may need to return or exchange the pump.
STOP! If you’re using one of those design-it-yourself brochures or websites all bets are off! You need to either forget you ever read them or do not continue. You can NOT try to mix the “almost guessing” methods most of them use with the method in this tutorial. If you are going to use this tutorial (and you should!) you need to use ONLY this tutorial. If not you are going to be in big trouble. Gravity flow is tricky, this is not going to be an off-the-shelf irrigation system. Use this tutorial exclusively and save yourself a lot of grief and get a professional quality sprinkler system. Please, please, please! Thank you. You are helping save my sanity. Now on with the tutorial.
This is going to be interesting. The “Backwoods Water Method” is the catch-all category for everything that doesn’t fall into the other two categories (city water or systems that use pumps.) I’ve no idea what kind of water system you have, but it seems that most likely it will be some type of gravity flow system, so here’s some information about gravity flow systems. The principles here apply to just about any type of system, so you should be able to figure out at least a rough idea of your water supply using this information.
Do not use the methods below if you have a pump or if your water comes from a water company pipe!
When dealing with gravity flow systems your water supply is effected by at least three different factors. They are water availability, pipe size, and the elevation of the water supply above your irrigated area (known as “pressure head”). These are really the same factors that determine water supply for all irrigation systems, however you will measure them using different methods.
Measure the flow.
If your system ever runs dry, or the flow appears to vary from time to time, you will need to take that into consideration when measuring your flow. You really should measure flow at a “worst case” time, that is, at a time when you are experiencing low water availability. This is probably not practical, so you may need to do a bit of guessing and adjust your test results accordingly.
The “Bucket Method”. We’ll start by assuming your water is already being piped to the location of the proposed irrigation system (or someplace close to it.) A typical situation would be a small dam created using sandbags in a stream and a pipe is stuck in under the sandbags. Water collects in the area behind the sandbags and some of it is diverted into the pipe. Normally a piece of nylon or galvanized steel window screen or mesh hardware cloth is placed over the pipe inlet to keep out small fish and twigs. The pipe transports the water to the area you want to irrigate. The Bucket Method of measuring flow is pretty easy, but you may get wet! Simply measure the time in seconds it takes to fill a 1 gallon container from the pipe! Measure the flow at the downhill end of the pipe. If there is a hose on the pipe end, take it off as the hose will restrict the flow. To assure a more accurate measurement turn on the water and allow it to flow freely for a few minutes before you take the measurement. Avoid measuring the flow from a small valve such as a hose bib, as the valve may substantially reduce the flow. Remove the valve and measure the full flow from the open pipe end if possible. Get a one gallon container, and time how long it takes to fill it with water. For the best accuracy measure the flow 3 or 4 times and average the times together. The formula to find GPM is 60 divided by the seconds it takes to fill a one gallon container (60 / seconds = GPM).
Enter the Maximum GPM (inflow) on your Design Data Form.
Example: The one gallon container fills in 5 seconds. 60 / 5 = 12 GPM.
(60 divided by 5 equals 12 gallons per minute.)
If you have a high flow you may need to use a larger container to get an accurate reading. To determine GPM using a larger container take the container capacity in gallons, divided it by the number of seconds needed to fill container, then multiply times 60. The result is the GPM.
Container size in gallons / Seconds to fill container X 60 = GPM
Example: Using a 5 gallon container it takes 14 seconds to fill the container.
5 / 14 X 60 = 21.4 GPM.
(5 divided by 14, then multiplied times 60, equals 21.4 GPM.)
If you have a Storage Tank.
If you do not have a water storage tank skip down to the section titled “Pressure Head”.
Hopefully the tank is on a hill, or a tower or some other elevated location. If not, the tank won’t help, so skip down to the next section.
If you have a storage tank you MUST measure the water inflow to the tank. Do NOT design your system based only on system outflow, which often exceeds inflow. You must measure both inflow and outflow, and design based on whichever is LESS! To start, drain the storage tank. Now shut off the flow out of the tank completely. Time how long it takes to fill the tank. If you don’t know the tank capacity in gallons you will need to find it. (The formula is at the bottom of this page). Divide the capacity of the full tank in gallons by the time it takes in minutes to fill the tank. The result is your “Tank Inflow GPM”.
Example: A 200 gallon tank fills in 10 minutes. 200 / 10 = 20 GPM
If you have a storage tank with a capacity over 1000 gallons you may be able to increase your Tank Inflow GPM by “buffering” it. Multiply the number of hours you will be irrigating per day by 60 (you will probably need to guess the number of hours you will be irrigating, so guess low to be safe). Keep in mind that the irrigation hours per day plus the hours it takes to fill the tank may not be greater than 24 hours! Divide the tank capacity by this number to get the buffer GPM. Add this buffer GPM to the old Tank Inflow GPM to get the new higher Tank Inflow GPM. Buffering simply takes into account the fact that as the water is flowing out to the irrigation system, water is also flowing into the tank helping to refill it. It will not empty out as quickly as it would if there were no water flowing in! I know that was confusing, so look at the example below which will help clarify. Let me worry about why it works (it does), you just do the math!
Example: 2500 gallon tank capacity. You plan to run the irrigation system for 8 hours per day. When totally empty the tank takes less than 12 hours to refill. 8 hours + 12 hours is less than 24 so we can buffer the Tank Inflow GPM. The buffer formula is:
2500 / (8 * 60) = 5 GPM.
If the original Tank Inflow GPM was 4 GPM, the new buffered Tank Inflow GPM will now be 9 GPM (4+5=9).
Enter the “Tank Inflow GPM” on your Design Data Form.
Now we need to measure the amount of “pressure head”. There are two methods, you can use either method.
Method #1. Pressure head is based on elevation or, in this case, the height of the tank above the highest area to be irrigated. If you don’t have a tank it is the height from the point where the water enters the pipe. This height is the elevation difference, not the distance away. In other words, if you imagine a level line extending from the tank over your yard, it is the height that line would be above your yard. Take a look at the drawing above. The water pressure in PSI can be determined by multiplying 0.433 times the height (feet) of the tank above the yard. It’s that simple, don’t try to make it harder! It doesn’t matter if the tank is on the top of a cliff adjacent to the yard, or if the tank is a mile away on a hill. As long as the elevation is the same, the pressure will be the same! It’s one of those abstract hydraulic principles I told you about that are hard to understand. (Okay, no doubt some hot shot out there wants to argue with me. So here’s an exception. If the tank was far enough away, much farther than would ever apply here, the pressure COULD vary due to changes in the gravitational pull of the earth and moon. Wow, isn’t that “cosmic”!)
Example #1: Tank is 100′ away on a hill behind the yard. Tank elevation is 70′ higher than the yard.
70 * 0.433 = 30 PSI.
Example #2: Tank is 1000′ away on the side of a mountain. Tank elevation is 70′ higher than the yard.
70 * 0.433 = 30 PSI. Pressure is STILL 30 PSI!
Method #2. An alternate method of measuring pressure is to install a pressure gauge (you can buy them at most plumbing stores) on the water pipe at the pipe outlet or the point you plan to tap into it for the irrigation system. This is probably the easiest method for most people. The water must not be running (turn off all faucets) when you take the measurement! (That’s why it’s called “static” pressure.) Read the PSI from the gauge. Warning: your water system may already have a pressure gauge installed on it. Inexpensive gauges (an expensive top-quality gauge will say something similar to “liquid filled” on the dial) tend to loose their accuracy after a year or two of use, so you may not want to rely on an old gauge.
Enter the pressure you calculated or measured in the space labeled “Design Pressure”on the Design Data Form.
Possible bad news: If you have less than 25 PSI you just don’t have enough pressure for a standard automatic irrigation system to work well (the automatic valves need a higher pressure to work). You will need to use a manual control system, add a pump, or use a special type of valve used to heating systems. See my article about automation of a rain barrel irrigation system for more information on these options.
Initial Design Flow:
If you don’t have a storage tank the Initial Design Flow will be the same as the Maximum GPM you measured using the “Bucket Method”. Pretty simple! If you Do have a storage tank the Initial Design Flow will be the lower of your “Maximum GPM” or your “Tank Inflow GPM”. If your Maximum GPM was measured at 20 GPM, and your Tank Inflow GPM was 18 GPM, your Initial Design Flow will be 18 GPM because it is lower than 20 GPM. Use the lower number! Still pretty simple!
Enter the “Initial Design Flow” on your Design Data Form.
This is really important! Later in step #3 of the tutorial you will determine the friction loss in your mainline. With a gravity flow system your mainline includes the pipe that brings the water to your yard from the water source! If you have a pipe that goes from the water source to a tank, you do not need to include the pipe that goes to the tank. However, the pipe that goes from the tank to the yard is part of the mainline. Therefore, when you calculate the mainline friction loss you will need to calculate it for both the pipe going from the water source (or tank) to where your sprinkler system taps in to the pipe, and also the pipe from the sprinkler system tap to the valves. You then add the friction loss for both pipes together to get the “mainline friction loss”. You might want to write this down on the Design Data Form so you don’t forget!
Example: Joe Backwoods has a pipe (pipe #1) that runs from the creek way up the canyon to a storage tank on the hill above his house. From the storage tank, a second pipe (pipe #2) takes water down the hill to his house. Next to the house Joe plans to tap a new pipe (pipe #3) into the house supply pipe to take water to his new sprinkler system valves. Pipes #2 and pipe #3 are considered part of the irrigation system mainline. Joe will need to calculate the friction loss in both those pipes and add it together to find out his “mainline friction loss”. Joe isn’t worried about how to calculate the friction loss because Joe knows that he will learn how to do it in Step #3 of the tutorial!
Do you have enough water available?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters). Therefore, if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, in that case 20 GPM would water 2 acres of shrubs.
If you don’t have enough water you will either need to find a larger water supply, or reduce the amount of area watered. Another option is to plant shrubs and use drip irrigation on them. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these values are typical for hot summer areas where most sprinkler systems are installed (daily high temperatures over 90 degrees F., 32 degrees C.) These values assume you would water as much as 10 hours per day. Water more time per day and you can have more area irrigated.
You should probably consider installing a filter of some type on your system if the water is from a river, stream, pond, or lake. There’s a pretty good chance that all kinds of crud are in the water such as algae, sand, mineral deposits, fish, snails and clams. (No kidding!) All of these things can damage your irrigation system. For more information on filters see the irrigation filtration tutorial.
Storage Tank Capacity:
To find the capacity of an upright, round tank (measurements in inches):
radius * radius * depth * 0.01359 = gallons
example: 48 inch diameter X 48 inch high tank = 376 gallons (24 * 24 * 48 * 0.01359 = 376)
STOP! If you mess this up you can ruin your pump! Read this! Most do-it-yourself sprinkler design tutorials are not suitable for rural irrigation systems that use pumps!!! Be very careful if you are also looking at one of them for guidance. The methods they use to determine water supply are little better than taking a wild guess. You’ll understand after reading this page.
If you are going to use this tutorial (and you should!) you need to use ONLY this tutorial. If not, you are going to be in big trouble. A large number of questions I get from people who are confused are caused because they are trying to mix together this tutorial with some other guide or tutorial, or maybe it was advice they got from some well-meaning, but clueless, salesman. Even many professional irrigation contractors do not understand the hydraulics of systems using pumps! Use this tutorial exclusively and save yourself a lot of grief and get a professional quality sprinkler system. Please, please, please! Thank you. You are helping save my sanity. Now on with the tutorial.
Sorry this is such a long page, but pumps are tricky. There is a lot you need to know. You can really mess up big time with a pump and an irrigation system if you do it wrong. So please be patient, read carefully, then reread it all again. Try working the examples, it helps the old brain to kick in!
None of that domesticated city water for you! You have your own pump and water source! That may be a well, river, pond, lake, or the ocean. (Ocean water? Are you growing seaweed?) Unfortunately, pumps are tricky and so you’re going to have to do a bit more work than you would if you had a “City Slicker” water supply. If you have a well, you have probably been told at some time or another that you have a “XX GPM well”. Do NOT rely on that figure! That is most likely the capacity of your well, not the output of your pump, and there is a big difference between the two. There may also be a GPM & PSI noted someplace on the pump, pump panel, literature, or the box the pump came in. Same deal. Don’t rely on these numbers; they assume optimum conditions that exist pretty much only in the pump company’s test facility. You’re going to have to do a little research, do some tests, probably get wet, and do a few calculations. I don’t recommend calling your pump and well company and asking them for the GPM of your well or water system. While some have given me a correct figure, more have given me an incorrect one! It’s not that they want to lie to you, it’s just that there are several terms here that can be easily confused. If the figure they give you is too high, your irrigation system will not work, period. If it is too low you will burn up your pump. I have written a complete tutorial on pumps and related equipment. I strongly suggest the Pump Tutorial if you would like to know more about your pump system, or if you want to know more of the reasoning behind what follows on this page. Click here to go to the Pump Tutorial.
Planning to buy a pump or install a well, but don’t have it yet? First, if you have a well, you will need to know the GPM of the well itself. That’s not how much you will pump out of it, that’s the maximum the well will provide. Your well driller should have measured this when the well was drilled, if not you will need to have the well tested by a well drilling company. They will install a temporary pump in the well to test the output. Then you need to select a random Design Flow and Design Pressure. I suggest 20 GPM per acre and 50 PSI, as those are good starting values. Example: If you have a 2.5 acre mini-ranch you would want to use 50 GPM at 50 PSI. Now proceed with a trial irrigation design using those values. Once you have finished the trial design, you will have discovered whether those are good values for your sprinkler system. If not, what would be? Redesign it if you have to. I know, that’s a lot of work, I agree, but it is far easier to redesign it on paper than to try to fix it after it is installed! Once you have established a good Design Flow and Design Pressure it’s time to shop for a pump. Remember the Design Pressure is at the irrigation connection point, the pump will also need additional pressure to lift the water from the well or pond (see the pump tutorial.) After you have your pump installed and running, test it using the Wet Method below. Then create your final sprinkler design based on those results. I know this sounds like a lot of extra effort, but the result will be an almost perfectly matched pump and sprinkler system. It will be worth the effort! You should absolutely read the Pump Tutorial.
So-called “Sprinkler Pumps” and packaged pumps. Several retail hardware store chains sell what they call “sprinkler pumps”. They are also sold on a number of web sites and, of course, Ebay. At least one of them even calls their pump a “High Pressure Sprinkler Pump.” Sorry, but not by any stretch of the imagination! Typically these pumps do not provide enough water pressure for a standard sprinkler system, especially if you are on a hilly site or pumping from a water source that is more than 10 feet below the pump. (See the paragraph above for suggested pressures.) These pumps will operate small sprinkler heads on a level lot, using water from a shallow well. Few people outside of Florida have this situation. These pumps are really best suited for use as booster pumps. In general, they do not produce enough water pressure for automatic sprinkler systems. Also watch out for packaged pumps that say they are rated X GPM and X PSI on the box (insert any values for “X”.) Often these two performance figures are both the maximums possible for the pump, which may be accurate, but is also misleading. When a pump is operating at it’s maximum GPM, it will also be at the minimum PSI. There is an inverse relationship between the two values. With pumps (and this applies only to pumps) as the output pressure (PSI) goes up, the output flow (GPM) goes down. Often the figure they give you on the box is the maximum possible for each value. (Read the Pump Tutorial.) Just be very careful. Design your sprinkler system FIRST, then buy your pump! If your calculations say you will likely need a 1.5 HP pump, and you find a 1/2 HP pump at the store that says it will do the job, be very suspicious.
Measuring Pump Output:
By the way, this looks complicated but it’s really pretty simple if you take it step-by-step! There are two ways to do this; The Dry Method and The Wet Method. I suggest you read through both of the methods, and try both of them if possible.
The Wet Method is more accurate (but only when done correctly.) The advantage to the Dry Method is that it gives you a chance to check the condition of your pump. If the Dry Method results in a much higher GPM and flow than the Wet Method, then it probably means your pump is worn out. Consider replacing your pump.
If you think you may be getting a new pump in the next 5 years, wait until you have the new pump installed before designing your sprinkler system! A new pump will likely have much better performance than the old one. If you designed your sprinkler system for the old pump then it will not work well with the new one, and may even damage the new pump!
Submersible Pumps. Many people have submersible pumps. In those cases the pump is located down inside the well, rather than on top of the well as in the diagram above. All the calculations work the same regardless of where the pump is.
First you need the horsepower of your pump. This may be stamped on the well, on the pump, on the pump panel, or your pump company should have a record of it. You may notice a GPM and PSI stamped on the pump plate also. I’ll say it again because it’s important: don’t rely on these numbers!
(A) Enter your pump horsepower on the Design Data Form.
What if you don’t have a well? If you don’t pump out of a well, substitute river, lake, pond, spring, mud-puddle, or whatever for “well” in the following procedures. In this case “water level when pump is running” is the lowest expected water level in your pond, stream, etc. (i.e.; the level the water would be in a really dry year.) You obviously also don’t have a “top of well”, so for a submersible pump when the tutorial mentions the “top of well” you would use the highest possible water level of your water source. If the pump is mounted above the water level (non-submersible), then you will need to substitute the actual pump location for “top of well” in this tutorial.
When using a non-submersible pump (any pump not below the water level) it is very important that the pump be installed as close to the water surface level as possible. Pumps are made to push water, not to pull it. The farther and higher the pump has to pull the water, the less efficient the pump will be. Some pumps work better than others in this situation, but in general expect trouble if the pump is more than 10 feet higher than the water surface. The higher your elevation is above sea level, the closer the pump needs to be to the water level. Therefore, in Denver, Colorado, the “Mile High City”, you need to have your pump very close to the water surface (or better yet, use a submersible pump.) Also, avoid long intake pipes between the water and the pump. Long intake pipes/manifolds can also hurt the pump performance. Plus, a long intake pipe is more likely to have a leak in it, and a leak in the intake pipe can cause your pump to lose prime. Losing the pump prime is a real pain in the rear, and if you automate your system it can cause serious damage to the pump. The bottom line here is to keep the pump as close to the water as possible.
Now you need to find out the “Dynamic Water Depth” of the water in your well. Your pump company may have a record of this, however you really should have the well “sounded” to get a new reading, especially if the well is more than 5 years old. Water levels often drop over time. As a last resort you can use the pump depth or well depth, but if you do, you may experience expensive pump problems later. Better to sound the well now. This is something your pump company can do for you and in most cases, is relatively easy and inexpensive. If you don’t have a well (you have a pond, creek, etc.,) use the lowest “dry year” water level of your water supply.
As you can see from the diagram above, the Dynamic Water Depth is the distance in feet between the top of the well and the water level in the well when the pump is running (dynamic means moving, as in the water is moving when the depth is measured.) It is important that the pump be running when this is measured. This is because when the pump runs, the water level in the well drops. The distance it drops is known as “draw-down”. The further the pump must lift the water, the more energy it takes. So as the water level gets deeper, the pump will produce less water pressure (water energy) at the outlet. That energy (water pressure) is what runs the sprinklers, so you must have an accurate measurement of it. You need to measure the Dynamic Water Depth, not just the depth of the water table.
As you know, water flows downhill. When the pump runs it pulls water out of the well. This causes the water level to drop, and then water flows into the well from the surrounding soil. How far the water level drops depends on how hard it is for the water to move into the well from the soil. Some wells have very little draw-down, others may have 50 feet or more of draw-down. Don’t worry if you don’t understand, it will all come together later!
(B) Enter your “Dynamic Water Depth” on your Design Data Form.
Now you need to enter the elevation difference between the top of your well and the highest point in the area to be irrigated. That is, how much higher (or lower) is the highest point in the irrigated area than the top of the well. The best way to do this is to use a laser level and a tape measure. Place the laser level at the high point of the irrigation system and shoot a level beam toward the well. Then use the tape measure to measure the distance from the laser beam to the top of the well. That is the elevation difference. You may need to make several stepped measurements, or you may prefer to just make an educated guess at the elevation difference rather than try to measure it. Also, if the well is higher than the irrigated area the distance will be a negative number.
(C) Enter your elevation difference on your Design Data Form.
Add the elevation difference to the Dynamic Water Depth of the well (or subtract if it’s negative). This number (in feet) is called “elevation head” and is a measure of the height the pump must push the water to get it to your irrigation system.
(D) Enter your elevation head on your Design Data Form. B + C = D (Dynamic Water Depth + elevation difference)
One more thing needs to be factored in at this point, which is your Design Pressure. The Design Pressure is the amount of water pressure that is needed at the inlet of the irrigation system in order for the system to operate. Design Pressure is measured in PSI (pounds per square inch), but for this formula we need the pressure as measured in feet of head. To convert PSI to feet of head we simply multiply PSI times 2.31.
PSI x 2.31 = Feet Head (ft.hd.)
Well that’s all fine and dandy, but what IS our Design Pressure you ask? Good question! Guess what? “Guess” is the operative word here. You’re going to need to take an educated guess at this number. For most situations I recommend that you use a Design Pressure of 50 PSI. This is a good number that works with most small to mid-size irrigation systems. For sprinkler systems with large radius sprinklers (over 35′ between sprinkler heads) you will need a higher Design Pressure. A good rule of thumb for large systems is to take the distance you would like to have between sprinkler heads in feet, and add 15 to it to get a reasonable Design Pressure. For example, if you want to put the heads 50 feet apart, you will need a design pressure of 65 PSI (50 + 15 = 65). Don’t be surprised if your system won’t pump 65 PSI, most residential pump systems aren’t designed to supply more than 50 PSI. As a side note keep in mind that higher Design Pressures result in lower flows, so the higher the pressure, the more valves you will need. I do not recommend spacing sprinkler heads farther apart than 50 feet without having a professional design the system! It gets very tricky. Even most City parks now keep the spacing between sprinklers at 55 feet or less.
(E) Enter your desired “Design Pressure (PSI)” on your Design Data form. For most of you this will be 50 PSI as discussed above. Remember this number is not written in stone! You may want to try adjusting it up and down.
Now we need the “Design Head” so multiply Design Pressure (PSI) times 2.31 to convert it to feet head. This is just a conversion from one type of pressure measurement (PSI) to another (Feet Head). Pump calculations always measure pressure in Feet Head. Example: 50 PSI x 2.31 = 115 ft.hd. (rounded down from 115.5)
(F) Multiply your Design Pressure by 2.31 and enter it as “Design Head” on your Design Data Form.
Now we put all these numbers together to get our “total pressure head”. Total pressure head is the “elevation head” plus the “design pressure head”, all in feet of head.
elevation head (ft hd) + design pressure head (ft hd) = total pressure head (ft hd)
30 ft. Dynamic Water Depth is measured in well with the pump running. The high point of yard is 10 ft. higher than top of well. 50 PSI Design Pressure, which equals 115 feet of design pressure head.
Total Pressure Head = 30 + 10 + 115 = 155 ft. hd.
Pumping from a lake. The low water level is 20 ft. below the high water level. The lowest point of the irrigation system is 10 ft. higher than the high water level. 45 PSI Design Pressure (104 feet head).
Total Pressure Head = 20 + 10 + 104 = 134 ft. hd.
Yet another Example:
For a small park, pumping from a canal. We use the canal bank as our “top of well” level. The low water level in the canal is 8 ft below the top of bank. The irrigation system is downhill from the canal, 25′ below the top of bank. 65 PSI Design Pressure needed for large turf sprinklers, which equals 150 feet of design pressure head (65 * 2.31 = 150 ft hd).
Total Pressure Head = 8 – 25 + 150 = 133 ft. hd.
(G) Calculate your “Total Pressure Head” and enter it on your Design Data Form.
Now for the flow formula:
Multiply the pump Horsepower times 2178 (a constant value, see note at bottom of this page) and then divide by the Total Pressure Head in feet.
Horsepower x 2178 / Total Pressure Head (feet) = GPM (the “Design Flow”)
So for the first example above with a 2 h.p. electric pump:
2 h.p. x 2178 / 155 ft. hd. = 28 GPM Design Flow That’s it! The GPM resulting from the above formula is your “Initial Design Flow”. You will need the Initial Design Flow and Design Pressure values later in the tutorial.
(H) Calculate your “Initial Design Flow” and enter it on your Design Data Form.
Caution: When designing a sprinkler system with a pump you want to keep the actual flow of each valve zone as close to the “Design Flow” as possible without exceeding the Design Flow. This is to keep the pump from cycling on and off as it tries to match the demand of your irrigation system. Don’t worry about valve zones now, we’ll have more on that later. Just remember this: “Valve Zone GPM must be between 80% and 100% of Design Flow”. You may want to write that down someplace. Technical note: In order to simplify the pump formula I have factored a pump efficiency of 55% into the value of the formula constant (2178).
Pressure Up, Flow Down?!!! Wait a minute here! It seems like what this formula says is that if the pressure goes down the flow goes up! That just doesn’t make sense. What gives? O.K., by popular demand, here’s an answer to this little dilemma. It’s one of those obscure, hard to understand hydraulic principles I was yapping about earlier. Sit back. Grab a nice soothing cup of tea or whatever. Put on some soft, relaxing background music. Ready? Here we go… I know it doesn’t SOUND logical, but believe me it IS correct. You’re thinking if more pressure is added then more water will be moved, which is true, but you’re talking about ADDING pressure, not using the AVAILABLE pressure! And that is the key to the problem. Remember we are measuring your AVAILABLE pressure and flow, based on the current conditions at your house (or whatever).
If we were planning to add a big pumping system then we would use a different approach. The correct way of looking at it is “how much water can we move with the pressure that your existing pump can supply?” It takes energy to move water, and the water pressure is the energy that is used. As the water is moved, the energy is used up. Therefore, the water pressure goes down. The more water is moved, the more of the existing pressure is consumed moving it. As the flow (GPM) of the water increases, the pressure (PSI) of the water must decrease! (This is because the pressure is used up moving the water.) Does that make sense? Well, even if you don’t get it, that’s O.K. It takes most engineering students 2-3 months of college level class work before they fully understand hydraulics, so don’t feel bad if it still doesn’t make sense. You’ll just have to believe me that it’s correct (and don’t worry, it is!)
Wet Method (Also called the “Bucket Method”)
This is the most accurate way to test your pump. BUT… Before we start into this I need to warn you that you must follow the instructions below exactly.Do not skip any steps, do not shortcut. If you do not follow these steps exactly you will get a false reading, your sprinkler design will not work, and you probably will destroy your pump! Understand?
Sprinkler systems need pressurized water to operate. Without the pressure, the water doesn’t shoot out of the sprinklers! Think of water pressure as the “energy” that moves the water. Just measuring the water flow with a bucket is not good enough, we must also measure the water pressure at the same time, as we need that pressure to make the sprinklers spray water. If you had a city water supply we would measure the “Static Water Pressure”. This is the pressure when the water isn’t moving. The amount of water available on a city water system is determined by the size of the pipe supplying the water. For a pumped system the amount of water available is determined by the size of the pump. When using a pump we must measure the “Dynamic Water Pressure” also. Dynamic water pressure is the pressure of moving water. Dynamic pressure will always be lower than static pressure. This is because when the water is moving friction is created with the edges of the pipe. This friction consumes energy (remember, pressure is energy) so the pressure drops. The faster the water moves, the more friction, so the pressure is reduced even more. When pumping water there is an inverse relationship between flow and pressure, if you want to get more pressure, then you are going to get less water flow. If you just turn on a faucet and measure the flow into a bucket the pressure falls to zero (the water just falls into the bucket, therefore no water pressure.) That results in a higher than normal flow (lower pressure = higher flow.) To make matters worse, most people measure the flow from a standard faucet. The small size of the faucet restricts the flow, adding more error to the measurement. If you’re lucky the errors balance each other out, but most of the time that isn’t going to happen. Add in a little sloppy measuring, and the results can be off by 20% or more. That can be enough to cause your pump to cycle, which will result in pump failure. Replacing pumps is a major expense, so let’s do this thing right. It takes more time and effort, but it will be worth it. Need to know what size your pipe is? How to find the size of a pipe.
Pump and Pressure Tank Example #1:
The light green colors are the new pipes added for the test. Later you can use this new pipe for the supply pipe (mainline) for your sprinkler system supply tap, so installing this isn’t a waste of money. When the pipe from the pump goes into the tank and then another pipe comes out and goes to the house you should make your tap after the tank as shown. Note that the pressure tank may not be near the pump, especially in cold winter areas where the tank is often installed in the house basement. The new piping can be vertical if you wish, and you can add ells to the last section to get the pipe over the top of the bucket. The 8″ straight lengths of pipe before and after the pressure gauge are important, they must be 8″ long and there can’t be any ells in these sections. The purpose is to avoid creating turbulence in the water that could affect the pressure gauge accuracy.
Pump and Pressure Tank Example #2:
When the pressure tank only has a single pipe going into it you can tap the pipe anywhere. You can tap it between the pump and pressure tank as shown, or after the pressure tank. Most pressure tanks are set up this way. Note that the pressure tank may not be near the pump, especially in cold winter areas where the tank is often installed in the house basement. The new piping can be vertical if you wish, and you can add ells to the last section to get the pipe over the top of the bucket. The 8″ straight lengths of pipe before and after the pressure gauge are important, they must be 8″ long and there can’t be any ells in these sections. The purpose is to avoid creating turbulence in the water that could affect the pressure gauge accuracy.
Wet Method Test Step-by-Step:
Follow the instructions below exactly!
1. Select the location where you will connect the sprinkler system into your water system (your “tap location”). See the pump and pressure tank example diagrams above. In some (very few) pump systems you must tap the pipe after the pressure tank as in Example #1 (see example #1 above.) However, most systems are like Example #2, so you can tap into the water supply line anywhere you want. In this case it is often best to tap as close as possible to the pump. This will usually give you a higher pressure, which means a better sprinkler system (and often less expensive, too!) In some cases you may want to connect your new irrigation system into an existing pipe somewhere a considerable distance away from the pump, such as a faucet near a garden. This is fine, go ahead and try it. The problem is that often the existing pipe is not large enough. You may find that you don’t get a very high water pressure due to the small pipe. If this happens you can confirm the problem by making another tap near the pump and testing there also. If you get a much higher pressure and flow when you test near the pump, then the existing pipe is too small.
2. If there is already an outlet on the pipe at your desired tap location you may use it for the test, provided the outlet is the same size as the pipe coming from the pump. If there is not already an outlet you will need to cut the pipe and install one as follows. Measure the size of the pipe. Go to the hardware store and purchase a compression tee that will fit the pipe. Have the sales person show you how it works. I suggest using a metal compression tee rather than plastic. The side outlet of the compression tee should be the same size as the pipe you are tapping into. At your tap location, cut a short section out of the pipe out and install the compression tee.
Note, sometimes it is necessary to brace the compression tee in place to prevent it from moving and slipping off the pipe. In some situations compression tees will not work. In that case you may need to install a threaded or glue in place (pvc pipe only) tee. If you have the time and skill to install a threaded or glued tee, that is always a better option than a compression tee.
3. Install the parts shown in green as per the Wet Method Pump Output Test sample drawings above as follows:
Start with an 8″ long pipe section installed on the (compression) tee outlet, then another tee with a 0-100 PSI pressure gauge on it, then another 8″ long pipe section, then a ball valve, then add a temporary PVC pipe (maximum of 4 feet of pipe with 3 ells) so that you can fill a 5 gallon bucket to measure the water flow. All the pipes and the valve should be the same size as the pipe you cut to make the tap. Do not use a garden hose as it will restrict the flow and give you a false low flow reading! I strongly suggest using metal pipe and fittings between the tap and the ball valve. I also suggest using a brass (or bronze) ball valve. This ball valve will become the shut-off valve for your future sprinkler system. The last pipe section going to the bucket after the ball valve is temporary and can be plastic. See the example drawings above. The rest of this test process is going to dump a lot of water on the ground, so now is a good time to figure out where that water will go, before you’re up to your knees in mud!
4. Get a “5 gallon bucket”. Since most 5 gallon buckets actually hold more than 5 gallons of water you probably will need to “calibrate” it and mark the actual 5 gallon volume level on the side of the bucket. Fill it with 5 gallons of water using an accurate measuring container to measure the water, then mark the water level on the bucket with a marking pen so you can easily see it. Empty the bucket.
5. Open the ball valve and allow the water to flow freely from it for at least 5 minutes so the flow from the pump can stabilize. ( If your pump is manually controlled you will have to manually start it.) Assuming you have a pressure controlled pump like most are, the pump should start by itself and run continuously during this time.
If the pump shuts off by itself when running with your test outlet full open you have an unusual problem, probably too much restriction in the piping. First check to see if there is any kind of restriction in the pipe (I’ve found rocks, toy cars, rags, rats, fish, roots, etc. in pipes). If not, the pipe from your well may be too small. Best to call a pump company for help at this point as something is seriously wrong with the pumping system.
6. With the pump running, watch the PSI reading on the pressure gauge, and slowly start closing the ball valve until the pump shuts off. The water pressure shown on the gauge will increase as you close the valve. (As the flow from the pump is reduced the pump produces more pressure.) Make a mental note of the pressure when the pump shut off.
7. Reopen the ball valve and wait for the pump to start again. Now slowly close the ball valve again until you find the point at which you get the highest possible pressure reading on the gauge without the pump shutting off. When you find this “balance point” the pump should run continuously and the pressure should remain more or less constant. You will need to close the valve a little, then wait, then close it some more to do this.
What you are determining is the exact point where your pump produces the highest possible combination of pressure and water flow. Do not turn off the pump or adjust the ball valve from now until you finish the rest of the steps! In order to tell when the pump shuts off you may need someone to help out by standing close to the pump and signaling to you. The water coming out of the valve may be loud enough that you can’t hear the pump running. If you have a submersible pump it will be even harder to hear. Try removing one of the plugs in the top of the well so you can hear better. Another method is to see if you can feel the vibration of the water running in the pipe where it exits the well. Water will not be flowing through this pipe when the pump is off.
If you absolutely can’t tell if the pump is running you will need to just watch the flow and pressure. Adjust the valve until you find a flow where the pressure stays constant for at least 15 minutes.
If you don’t have a pressure switch on your pump you still use the same method for the test. Instead of listening for the pump to shut off, you will just have to play with the flow until you reach a good balance between flow and pressure. You will notice as you close the valve that there is a point where you have to severely reduce the flow to make the pressure go higher. Keep your pressure below this level, it is not good for the pump to restrict the flow too much.
The pressure reading is going to be your design pressure. If it is too low, you may have problems with your sprinkler design. For most situations I recommend that you use a Design Pressure of 50 PSI. This is a good number that works with most small to mid-size irrigation systems. For sprinkler systems with large radius sprinklers (over 35′ between sprinkler heads) you will need a higher Design Pressure. A good rule of thumb for large systems is to take the distance you would like to have between sprinkler heads in feet, and add 15 to it to get a reasonable Design Pressure.
For example, if you want to put the heads 50 feet apart, you will need a design pressure of 65 PSI (50 + 15 = 65.) Don’t be surprised if your system won’t pump 65 PSI, most residential pump systems aren’t designed to supply more than 50 PSI. Most people who use sprinklers with a radius over 45 feet need to buy a new high-pressure pump or add a booster pump in order to create enough pressure for the sprinkler system. Should you desire sprinkler heads that are farther apart than 50 feet apart you should have a professional design the system. Wide spacing between sprinklers gets very tricky as the water distribution just doesn’t remain uniform very well at long distances. Even most City parks now keep the spacing between sprinklers at 55 feet or less.
Most pumps have a pressure switch on them that turns the pump on and off at preset pressures. Typical settings are 35 PSI on and 45 PSI off. It works just like the heater and thermostat in your house do, only it measures water pressure rather than temperature. When you open the valve, the water flows out and this causes the water pressure to drop. When it reaches the preset “on” pressure the pump turns on. As you close the valve the pressure climbs. When it reaches the preset “off” pressure the pump shuts off. This creates a problem if you want to get 50 PSI and the pump turns off at 45 PSI! Fortunately, the pressure at which the pump turns on and off can be adjusted. If your pump is turning off at less than 50 PSI you may want to have the settings adjusted. You may be able to increase the off setting by 5 PSI, maybe a little more or less. Sometimes this requires the installation of a new pressure switch on the pump, most can only be adjusted within a limited range. (If you need to replace the pressure switch, have a pump professional do it. You can damage the pump if you use a switch with a pressure range that is too high. Your pump may not be designed to pump higher pressures! )
How do you adjust the pressure setting? Somewhere on the piping, usually near the pressure tank, there is a tee in the pipe and a small box is sitting on a short pipe above the tee. The box has electrical conduit running to it. This is the pressure sensor. It has an adjustment screw inside the box cover. I can’t tell you much more than that since the location of the screw will vary depending on the model. You may be able to find instructions for the pressure switch online by searching for the manufacturer’s website. Newer pumps often have solid state pressure sensing. Depending on the model you may be able to change the settings using the keyboard. Other models do not have a keyboard and must be hooked up to a programming device (often a laptop, tablet, or smartphone) to change the settings. You may need to call your pump company for help.
A. Write down the pressure gauge reading on the line “Design Pressure (PSI)” on your Design Data form.
8. Now measure the flow coming out of the pipe without adjusting the ball valve. The pressure gauge must stay at the “Design Pressure” and the pump must continue running while you measure the flow. Put your 5 gallon bucket under the flow and time how many seconds it takes to fill the bucket to the 5 gallon mark. Repeat this 3 times to make sure the the results are accurate (all three measurements should be about the same). Divide 300 by the number of seconds it takes to fill 5 gallons into the bucket to get the GPM. (300 / seconds to fill 5 gallon bucket = GPM) Example: 10 seconds to fill the 5 gallon bucket, therefore 300 divided by 10 seconds equals 30 GPM.
B. Write down the flow (GPM) you measure on the line “Initial Design Flow” on your Design Data Form.
You’re done measuring. You can close the valve now and shut off the pump if it is locked on. Leave the ball valve in place as you will connect your new sprinkler system to it.
A Few Other Important Items to Note
Caution: When designing a sprinkler system with a pump you want to keep the actual flow of each valve zone as close to the “Design Flow” as possible without exceeding the Design Flow. This is to keep the pump from cycling on and off as it tries to match the demand of your irrigation system. Don’t worry about valve zones now, we’ll have more on that later. Just remember this: “Valve Zone GPM must be between 80% and 100% of Design Flow”. You may want to write that down someplace.
Do you have enough water available from your pump?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters.) Therefore, if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs. If you don’t have enough water you will either need to find a larger water supply, or reduce the amount of area watered. Another option is to plant shrubs and use drip irrigation on them. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these estimates of the water quantity required per acre are typical for warm summer areas where most sprinkler systems are installed (daily high temperatures in summer over 90 degrees F., 32 degrees C.) These values assume you would water as much as 10 hours per day. If you are able to water more time per day then you will be able to irrigate more area.
Minimum pipe size:
If the pipe between the pump and the tap point for the irrigation system is longer than 10 feet (for submersible pumps measure from where the pipe exits the well) then the pipe must be as large or larger than the following:
Initial Design Flow
Minimum Pipe Size
0 – 5 GPM
5 – 15 GPM
15 – 30 GPM
30 – 40 GPM
40 – 70 GPM
70 – 100 GPM
100 – 160 GPM
If the pipe is smaller than the above minimum sizes you will need to replace it to avoid the possibility of water hammer which can damage your pump, pressure tank, and household plumbing, not to mention the sprinkler system. If your design pressure is over 50 PSI use one size larger pipe than what is shown.
Example: 55 PSI design pressure and 35 GPM Design Flow require a 2″ pipe (one size larger than chart says because design pressure is over 50 PSI).
Lots more on pumps…
I strongly suggest you take a look at my Irrigation Pumping Systems Tutorial. There’s a lot more information on pumps, pump controls, and pumping from wells, lakes, rivers, etc. in the Irrigation Pumping Systems Tutorial.
In this step you will make some preliminary selections of the equipment such as sprinkler heads, valves and more. When selecting your sprinkler equipment we need to also find out how much pressure loss each item creates. The amount of pressure loss may require that you reconsider one product over another. Keep reading and it will become clearer.
Like all other mechanical systems an irrigation system consumes energy when it operates. The irrigation system uses energy in the form of water pressure which, as we noted earlier, we will be measuring in PSI (pounds per square inch). Each component in the irrigation system that the water passes through consumes a little bit of that water pressure. It’s similar to how a car uses up fuel for each mile it goes. If we run out of water pressure before the water makes it through the system, then the irrigation system will not work. Therefore, we need to calculate how much pressure will be lost as the water passes through each component of the irrigation system. To start we will need to make some educated guesses, which are then confirmed and adjusted by using a trial and error process. Don’t worry, it’s easy to do…
Below is a Pressure Loss Table that lists items that you MAY need to factor into your pressure loss calculations. Some of the items may not be necessary in some situations. The tutorial has a page for each of the items that will tell you everything you need to know. I will explain to you all the pros and cons of the various product types available. For the more complex choices like backflow preventers, I will lead you through a series of simple questions that will guide you toward the best solution for your specific irrigation system. Then you will pick your actual equipment and enter the associated pressure loss value into the Pressure Loss Table on your Design Data Form.
Near the bottom half of your Design Data Form there is a copy of the Pressure Loss Table below.
Hopefully you picked up a copy of the Design Data Form earlier in the tutorial. If not, please go back to the appropriate page for your water supply source below so you can figure out the correct values for available GPM, PSI and get the form:
You’ll refer back to these values several times throughout the design process and you may need to change them a few times, so use a pencil so you can erase and rewrite values! If you have bad handwriting skills like me, you may wish to write a bit neater than normal so you can read it later! There’s nothing worse than having to go back and recalculate your data because you can’t read your own handwriting. Believe me, I’ve had to do it way too many times! If an item on the table doesn’t apply (for example, you don’t have a water meter) just enter n/a and a pressure loss value of 0 for that item.
OK, here’s a typical Pressure Loss Table.
Pressure Loss Table
Item (links jump to a page with details on each item)
Don’t panic! The next few pages of the tutorial goes through each item on this table and helps you to figure out the pressure losses to enter on each line of the table. If you’ve used the tutorial before and already know a lot of this, the links in the table above will allow you to jump ahead in the tutorial to the page where the details on that item are located. For most people you will want to just keep reading the pages in order.
Pressure Regulators & Pressure Reducers
Before we continue on we need to address pressure regulators/reducers. (You will see them also called a “pressure reducing valve,” “pressure regulating valve,” and various other names. I’m going to use the name “pressure regulator” to avoid confusion.) A pressure regulator is a special valve that reduces the water pressure to a set level and keeps it at that level. Some homes have these pressure regulators installed on the water supply, which can impact the values used in the Pressure Loss Table. If you have a municipal water supply, you already learned a little about pressure regulators on the City Slicker Water page of the tutorial. Back on that page you should have discovered if you have a pressure regulator and, if you do, you also decided if you would tap into your water supply before or after that pressure regulator.
If you have a pressure regulator on your house then you get to take a little shortcut. On your pressure loss table you get to ignore the pressure loss for everything upstream of the pressure regulator.
Installing a Pressure Regulator
If you are planning to install a new pressure regulator be aware that there are two types sold. The one you want to use on your house will be made of bronze or brass, it should have a pressure adjustment screw so you can set the downstream pressure you want, and generally it’s going to be pretty expensive. If you need one for a sprinkler system I suggest it also be this more expensive type. There are also cheaper pressure regulator models that use a different principle to work. These cheaper pressure regulators are often used on drip irrigation systems. They are not adjustable, are not nearly as accurate, and will often allow a damaging pressure surge to pass through them. They typically are barrel-shaped and constructed of plastic. They will not have a adjustment screw or knob on them.
Before you decide to take a shortcut and install a pressure regulator right before the valves so you can ignore pressure loss in the mainline, consider that the higher pressure may not be good for those upstream components. Generally I try to avoid pressures over 100 PSI in any portion of my sprinkler systems. I strongly recommend that you do likewise. Also remember that maximum water velocities also still apply to the mainline pipes, so you will still have to do the size calculations for the mainline.
When placing a pressure regulator on an irrigation system I normally install it right after my main irrigation system shut-off valve at the place where I tapped into the water supply. Thus I have:
connection to water supply –> emergency shut-off valve –> pressure regulator –> irrigation system.
Don’t forget the pressure setting of a pressure regulator must always be at least 15 PSI lower than the incoming pressure. If the incoming pressure is 80 PSI the pressure regulator must be set at 65 PSI or less. Otherwise the pressure regulator will not work accurately and may allow damaging pressure surges to pass through it. To restate this another way, the pressure regulator must reduce the pressure by 15 PSI or more for it to work accurately and reliably.
Elevation changes can add or subtract water pressure from your water system. That seriously changes how well the system works. Each foot of elevation change is equal to 0.433 PSI of water pressure. Think of a vertical column of water. At the bottom of the column the weight of all the water above is resting on the bottom of the column, this weight creates pressure. Have you ever swam down to the bottom of a deep swimming pool and felt your ears pop or hurt? That’s caused by the increased water pressure pressing against your eardrum. The deeper you go, the more pressure you feel.
Not Just for Irrigation
While this page is written for irrigation design, these same principles apply to any piping system that carries water in it. The elevation impacts described here would apply to a huge city water system, to the pipe bringing water from a well to a rural home, or a pipe taking water from a creek or pond to a remote tank. If you jumped here from an Internet search and are not working through the irrigation design tutorial this page is part of, remember that when designing a water piping system you must consider other sources of pressure loss in your design too, such as friction loss caused by the water moving through the pipe.
In the USA we measure water pressure most often in pounds per square inch (PSI). That’s the weight in pounds of the water on a one-square-inch surface area. Sometimes we measure pressure in “Feet of Head”, especially when dealing with pumps and wells. This is to confuse you. (Not really.) We also don’t use metric here in the good old USA. We do this to annoy the rest of the world. (No, we really do it because we are lazy and unwilling to adopt the metric system.) So if you are outside the USA water pressure is measured in bars… or kiloPascals (kPa). Or about a half dozen other measurements. Unfortunately the rest of the world is no more agreed on how to measure water pressure than we are! There are simply a lot of systems used to measure pressure. Fortunately a conversion calculator will allow you to switch back and forth between any of them. If you don’t like that calculator or it isn’t working there are many more, just search for “pressure conversion calculator.”
My tutorials mostly use PSI, although I use Feet Head in parts of the pump related tutorials and metric for drip systems. OK, now it’s class time!
You can skip down past this section if you wish. Look for the next horizontal line. This section is for those who need to know “why?” or want to understand hydraulics.
Since water is essentially a non-compressible liquid it exhibits the unique trait of transferring pressure horizontally when in a confined space. What this means is that water in a pipe (which is a confined space) exhibits the same pressure as it would if the pipe were perfectly vertical, even if the pipe isn’t. This isn’t an easy principle to understand, so be patient and re-read as needed. The best way to demonstrate this is with a picture.
In this picture the water pressure in the water tank at the top of the water surface level is 0 feet of head, or you could also say there is 0 PSI. This is because there is no water above it to create pressure. Head is another word that indicates pressure, it is mostly used when measuring pressure created by the depth of water. So 10 feet deep water will create 10 feet of head at the 10′ deep level. So 10 feet of depth = 10 feet of head. Ok? (Yes, I know there would be a small amount of additional water pressure due to the air pressure above the water, but let’s try not to confuse things. This is hard enough to understand! So we’re going to say that there is 0 feet of head at the water surface.)
Looking again at the picture above, we see that the ground level is 40 feet below the water level in the tank. Therefore the water pressure at ground level is 40 feet of head. Again 40 feet of depth = 40 feet of head. Now lets convert that to pressure measured in PSI. As noted earlier, 1 foot of elevation change creates 0.433 PSI of water pressure. So in this case 40 feet of head is going to be about 17 PSI. (40 ft head x 0.433 psi/ft = 17.3 PSI.) Again, the formula is “feet of head x 0.433 = PSI.” So far, pretty straight forward. Read again if you’re confused.
Static Water Pressure
Now the hard to understand part. In the drawing above, the water enters the house at a level 100 feet below the water level in the tank. So the static water pressure at the house is 100 feet of head, or about 43.3 PSI, using the formulas in the previous paragraph. Note that I said this is the “static pressure”. So now you’re likely wondering how this could be? The water level is not just 100 feet above the house there is also easily 180 feet of pipe between the tank and the house! The answer is that the length of a pipe does not matter when the water is static in the pipes. Static means the water is not flowing, it is not moving, it is standing still. This is very important! Because the water is a non-compressible liquid it transfers the pressure horizontally along the pipe route for pretty much any distance without any loss of pressure! Cool, right? You bet it is, it is a principle that is very handy and makes all sorts of neat gadgets used on machines work. This is why a small hose filled with hydraulic fluid can cause the brakes on every wheel of a mile long train to apply when the engineer hits the brakes!
Now on the other hand, if we measured the pressure with the water flowing, then the pressure would be termed “dynamic pressure”. With the water in a dynamic state (flowing in the pipe) the water would loose pressure due to friction on the sides of the pipe and we would get a lower pressure reading at the house shown in our previous diagram. (I’ll deal with dynamic pressure in the next paragraph.) So for now, just understand that static pressure means there is no flow in the system, so there is no friction, and no pressure loss! Read that last sentence again! Think about it for a second, go back look at the picture again if you need to. It makes sense if you think about it. Our professor spent a week drilling this concept into us back in college and a lot of people in the class never did understand it! So if you still don’t get it don’t feel bad and don’t get discouraged! Just accept it on faith (I wouldn’t lie to you) and continue on.
In most cases we use static water pressure values when designing irrigation systems (or any other water piping system for that matter.) Then we can use calculators, spreadsheets, or charts (if you really want to torture yourself you can even use a very complicated manual calculation) to estimate the “friction loss” that will occur in the pipes when the sprinkler system is operating. Then we will subtract the friction loss from the static pressure to arrive at the dynamic pressure. Why not just turn the water on and measure the dynamic pressure with the water flowing? It would seem simpler, then we would not have to prepare a separate calculation for friction loss, right? Well, that is correct, however dynamic pressure is extremely difficult to measure accurately! You have to get the flow just right, and then hold the flow at that level for a minute or two while the pressure stabilizes. This is a real pain in the rear to do and not nearly as easy as it sounds! Plus, it is a bit hard to do if the pipe isn’t installed yet! You can’t measure the dynamic pressure if the pipe isn’t installed! So, the result is that we almost always will work by using static water pressure and then use calculations to determine the dynamic pressure. Its just way easier to do, and who wants to do it the hard way?
Now go back and look at that picture at the top of this page of the tank and house again. As the water flows to the house the water level in the tank will go down (assuming water isn’t flowing into the tank to refill it.) So the elevation of the top of the water in the tank will drop as the tank empties. When the tank is almost empty the difference might be only 95 feet. So since the water depth is less, the water pressure would also be lower. This happens all the time and is normal! If the top of the water elevation varies, then the water pressure will also vary. So if the water level will vary at your water source, the pressure will also vary. I know I keep saying the same things over and over in different ways, but I’m trying to drive home some important, but hard to understand, principles! My apologies if you got it the first time through and are getting bored!
Still confused? Don’t worry about it, just follow through the procedures that follow and you’ll be all right even if you don’t fully understand why you’re doing some of these things! Just remember that whenever you measure water pressure with a gauge you need to turn off all the water outlets so the water is static, that is, not flowing.
Time to wake up!
Hills, Valleys, & Slopes Continued…
In a nutshell: Just remember every foot of elevation change causes a 0.433 PSI change in water pressure. If your pipe is going downhill add 0.433 PSI of pressure per vertical foot the pipe goes down. If the pipe is going uphill subtract 0.433 PSI for every vertical foot the pipe goes up. The word “vertical” is critical. If the pipe goes up a slope the vertical distance is how high the slope would be if the pipe were going straight up. Do not use the length of the pipe, use the change in elevation! If you don’t want to accept my word for it then you’re going to have to go back and read all that boring Hydraulics 101 stuff above!
Because elevation changes effect the water pressure we must take this into account when determining pressure loss in our water system. If the area to be irrigated is lower than the water source we will gain pressure, so we may be able to gain some beneficial added pressure to our system. Care must be taken though. We can only add pressure if ALL the irrigation system is lower. If portions of it are not lower, or are higher than the water source, then those portions aren’t going to be getting that extra pressure. It is safest when doing initial design work to just not add pressure for elevation changes unless you’re really sure.
On the other hand if portions of the irrigation system are higher than the water source you will always need to subtract out the pressure loss created by the elevation gain. Pressure gained can be easily disposed of, pressure lost however, is very difficult to replace. So, for every foot of elevation gain (higher) in the irrigation system, you should subtract 0.433 PSI from the design pressure.
The far corner of the irrigation system is 9 feet higher than the water source.
9 feet X 0.433 PSI = 4 PSI loss (loss because it is higher). The water pressure in the far corner will be 4 PSI lower than the pressure at the water source, simply because it is 9 feet higher.
One corner of the irrigation system is 20 feet lower than the water source. Another corner is 12 feet higher than the water source. 12 feet X 0.433 PSI = 5 PSI loss at the higher corner. However in the 20 feet lower corner the pressure will be higher. 20 feet x 0.433 PSI = 8 .7 PSI higher in the lowest corner. In some cases we might need to install a device to lower the pressure at the sprinklers in that low corner. But we’ll worry about that later. For now the high corner with the 5 PSI loss is more important. Remember, it is easy to lower the pressure if we need to, but it is hard to raise it.
A final example:
The water source is on a hill. The highest part of the irrigation system is 50 feet lower than the water source. The lowest part of the irrigation system is 60 feet lower than the water source. In this case you can add pressure because the ENTIRE irrigation system is lower. But the pressure added can only be the difference between the water source and the highest part of the irrigation system. 50 feet x 0.433 PSI = 22 PSI pressure GAIN. So you would subtract this amount from the total system pressure required. In other words you would enter a negative number in your Pressure Loss Table for Elevation Pressure Loss.
Too Much of a Good Thing:
What if one corner of the irrigation system is a lot lower than the other? While unusual, it is possible to have too much pressure! With too much pressure the sprinkler heads might not work as well, or they might even blow apart! For spray type sprinklers 40 PSI at the sprinkler head is the most pressure you want. For rotors it varies, but most small systems shouldn’t have more than 70 PSI at the rotor sprinkler head. If you have too much pressure you will need to reduce the pressure. Most sprinkler heads can be bought with a built in pressure reducing device. You can also buy an individual pressure reducing device that can be installed on the sprinkler head inlet pipe. These devices will reduce the water pressure to the optimum level for the sprinkler. Remember, the devices only reduce pressure, they can’t increase it! They will always reduce the pressure by at least a small amount, so they should not be used unless you have too much pressure. More on this topic will be covered when we get to discussing sprinkler heads so don’t worry about it now.
If you are working on a Sprinkler Design enter the pressure loss or gain caused by elevation changes on the “Elevation Change” line of the Pressure Loss Table. Enter the value in PSI. Remember, feet of elevation change x .433 = PSI.
The path ahead viewed dimly through the fog??? What we’re about to embark on here is known as “doing it the right way”. We are going to start by figuring out what the maximum water supply would be if you had perfect conditions, such as a very short pipe from the water meter to your house, lots of water pressure, a small yard, a happy family, a low interest rate mortgage, and good neighbors! Then we are going to modify that number later in the tutorial to reflect your actual conditions (long pipe, lousy water pressure, bad neighbors, whatever.)
The end result is that we will determine what the exact, optimum water supply is for designing your sprinkler system. What that means for you is that your sprinkler system will use less water, last longer, and there won’t be dry spots! Now it’s going to be a little more work than “guesstimating” would be, but it will be worth it. Don’t get discouraged, be patient, and it will all come together. The worst thing you could do right now is to try deciding what sprinkler you want to use. That would be “putting the cart before the horse”. Trust me, I know what I’m doing. Now let’s get on with it…
A. Find your water supply pipe.
Hopefully you already know where the water service pipe comes onto your property, or at least where it enters your house.
Mild Winter Climates: In milder climate areas there is typically a shut-off valve and or a water meter at the location where the pipe enters the property. From there the pipe generally goes to the house, then surfaces above ground where a house shut-off valve is located, then the pipe turns and runs into the side of the house. Often this location where the pipe enters the house is where the tap for the irrigation system will be made.
Note the “W” etched in the curb in front of the concrete water meter box in the photo above. Often there will be some type of mark on a curb at the location that the water supply pipe to the house runs under it.
The photo above is of a typical mild-climate water supply line where it enters the house. This one has a rather unusual model of pressure regulator (the gizmo with the white adjustment knob on top) to reduce the water pressure. Many houses do not have a pressure regulator. A ball valve (with a blue handle, the handle is in the “off” position) is on the incoming water supply pipe. The pipe going into the wall is the house supply. The pipe exiting the photo at the lower left goes to a hose bib.
Cold Winter Climates: In colder climates the water line often enters directly into the basement or crawl space under the house from underground. This water pipe to the house is often buried very deep to keep it below the frost line. The shut-off valve, and possibly a water meter, are often located in the basement or crawl space to help protect them from freezing.
The photo above shows a typical water supply line in a cold-winter climate. A copper water pipe enters through floor, goes up into a ball valve (yellow handle), then through a pressure regulator, then a remote-reading water meter. You would tap in for the sprinklers after the water meter. The mainline supply size would be measured on the copper pipe coming out of the floor. The water pressure in this case could be measured at any water faucet after the regulator (probably any faucet in the whole house would work). Photo credit and thanks to Ed Pletsch.
What type of pipe is it?
Once you find your supply pipe you need to know what type of pipe or tubing it is. Keep in mind that there may be more than one type of pipe or tubing used at different locations! Often copper is used under concrete slabs and then it converts to PEX for other locations.
Steel Pipe. Steel pipe comes in two types, black steel (used mostly for gas lines) and galvanized steel (galv. steel) which is used for water pipes. Galvanized steel pipe will be a silver gray color, and a magnet will stick to it. It will have threaded joints. Steel pipe is made in conformance with IPS (iron pipe size) standards. Galvanized pipe is often found on homes in inland areas, especially on less expensive tract homes.
Brass Pipe. Brass pipe is sometimes used for homes. Like copper it can take on a greenish tint with age. A magnet will not stick to it. It will have threaded joints. Brass pipe is made in made in conformance with IPS (iron pipe size) standards. Brass is not very common except for short sections of pipe, due to cost.
PVC Pipe. PVC plastic pipe is almost always white or gray, and is more rigid than the other commonly used types of plastic water pipe. Standard PVC pipe is made in conformance with IPS (iron pipe size) standards. It should have the letters “PVC” printed on the pipe. PVC is fairly rigid, and it is not easily scratched with your fingernail. PVC tends to be more commonly used in mild climate areas. Another type of PVC called CPVC is sometimes used inside homes and often is found in older mobile homes. It is similar to regular PVC, but will be labeled “CPVC”. Most often it is a yellowish, gold, or tan color. CPVC in homes is usually made to copper tube sizes (look for “SDR-11” printed on the pipe), but is also sometimes iron pipe size (labeled IPS). PVC is often used for house supply pipes in mild winter areas.
Copper Tube. Copper tube is very common in homes. It takes on a dirty green color as it ages. A magnet will not stick to it. Most joints will be soldered, look for silver color solder at the joints to identify it. Copper tube has a different diameter than iron pipes, and is made in sizes known as CTS (copper tube size). Copper has been the standard “high quality” tube used on better homes for decades. Often used in areas near the coast where salt air causes rapid corrosion of steel.
PEX and PE Tube. Both are both polyethylene (poly) products. Both tend to be used in areas with severe winters and/or rocky soil. There is a lot of confusion over these two poly-based products. Be careful, both are sometimes called “poly”, especially by the sales people in the big home improvement stores. True PEX is a stronger form of cross-linked polyethylene that has become popular in recent years. Both PEX and PE are flexible, and both have a glossy appearance and slick surface. So how do you tell which one you have? Older PE is almost always black, and in most cases PEX is not black. PE is almost never used inside a house if the house was built to code. The surest thing to look for are the letters “PEX” printed on the tubing. Making things even worse, white PEX looks a lot like PVC, especially if it is old or dirty! PEX is easily scratched with a fingernail, PVC scratches, but not easily. PEX was not invented until the ’70s, and it is seldom found in homes built before 1975. (It wasn’t officially sold in the USA until 1985. Of course, if your house has been remodeled, you could still have it in a older house.) PEX is almost always made to conform with CTS sizes. The heavy duty PE tube used for plumbing is most often made to a uniform size standard (labeled “SDR-7”), but many different PE products used for irrigation do not conform with this size standard. Be careful when working with PE tube, if possible take a sample with you when you go shopping for parts so you can test fit them at the store. PEX is quickly becoming the default tube for piping new homes due to low cost and ease of installation.
Warning: PEX pipe has a very thick wall, thus it has a smaller inside area for the water to flow through. This means it has much higher pressure losses when the water passes through it. For this reason you need to be careful when replacing a copper or PE tube with a PEX tube. Often when replacing a copper or PE tube with a PEX tube it is necessary to use PEX that is one size larger than the tube it is replacing. So if you are replacing a 3/4″ copper tube with PEX, you should consider using 1″ size PEX tube for the replacement. Otherwise you may notice a drop in water pressure after the replacement is made.
One good hint to the type of pipe is the way the pipes are connected to each other. PEX and PE are never glued at the joints. Sometimes PEX & PE are heat welded together, but most of the time they are connected together with fittings using clamps, teeth, or compression-nuts that hold the tube onto or into the fitting. (“Fittings” is the term we use for the various connectors that are used to join two or more pipes together.) If the pipe has glued joints it is almost always going to be PVC or ABS. (ABS plastic is typically black rigid pipe, almost always 3″ or larger in diameter, and is mostly used for sewer and drainage pipes. ABS can be other colors so don’t assume a pipe is PVC just because it is white or gray!) Another hint is that poly pipe tends to be used in colder climates, and PVC tends to be used in warmer climates. If you have to regularly shovel snow from the driveway, chances are the pipe is PE or PEX. Copper pipe is often soldered to the fittings. Look for the silver color solder at joints. Steel and brass pipe have threaded connections, a few threads almost always are visible at the joints. Confused yet? Your best bet is to find lettering on the pipe that says what type it is.
B. Find your Water Meter:
Now we need to know if you have a water meter. Most, but not all, water companies use a water meter to measure the amount of water you use. If you don’t have a meter, there will almost always be a shut-off valve at the point your house water line connects to the water provider’s pipes. Often the valves are buried, sometimes several feet down, and a sleeve comes up to the surface with a small lid or box over it. The water company uses a special tool that can reach down and open or close the valve. Often grass has completely grown over the lid and you can’t find it. Try probing the ground with a pitchfork, metal rake, or screwdriver to find the hard cover of the box.
The water meters are normally installed in an underground box as close as possible to the property line. This is usually at the street or alley. Most of the time the box will have “water meter” or the water company name stamped on the lid. In areas with severely cold winters the water meter is often installed in the house basement or a utility room of the house. If you still can’t find it, call your water company and request their assistance.
Try to find a size stamped on the meter. If you can’t find a size, ask your water company or just assume the meter is the next size SMALLER than the pipe running to the house. It is common for the meter to be one size smaller than the pipe. Standard water meter sizes are: 5/8″, 3/4″, 1″, 1 1/2″.
Spiders and snakes: If the meter is in a box, watch out for spiders and ants in the meter box! Most of the “pro” irrigation repair guys I know carry a can of spider spray with them! Sometimes we find snakes, rats, gophers, and other beasts in the boxes too! I found a turtle shell in a box once. No tunnels or holes into the box that I could find. I have no idea how it got in there.
Enter the meter size on your Design Data Form. If you don’t have a meter, enter 0 (zero).
C. Measure Your Water Pressure
Water pressure is the energy that powers your sprinkler system, so it is very important. If you work with it, it will make your sprinklers do the “rain dance”. If you ignore it, it can bite you hard in the wallet! For this tutorial I use the pressure units “PSI” which means “pounds per square inch”. When pros talk about pressure readings we almost never say the words “pounds per square inch”, we just say the letter names “P. S. I”. Outside of the United States pressure is most often measured in “bars”.
First off, grab the phone and call up your water supplier. Ask them for the “static water pressure” for your neighborhood. Don’t be shy, people call them all the time to ask! They may give you a pressure range, like 40-60 PSI. If so, write down the LOW number of this range. You can also measure your own water pressure using a pressure gauge that attaches to a hose bib on your house (you can purchase a 0-120 PSI gauge with a hose adapter on it at pretty much any hardware store).
Pressure regulators (also called pressure reducing valves)
Pressure regulators are devices used to reduce the water pressure and are commonly found on home water supplies in towns with hills. It takes lots of water pressure to lift water uphill. So in order to get the water to the houses on top of the hill the water pressure in the water system has to be very high. But this causes the pressure at the homes at the bottom of the hill to be too high. So pressure regulators are installed on the water supply pipes to homes in the lower areas of town, where the pressures are very high. The pressure regulators are generally set to someplace between 50 and 65 PSI.
If the water company tells you your neighborhood pressure is over 65 PSI, you probably have a pressure regulator installed someplace on the water supply line to the house. The pressure regulator reduces the water pressure in your house, so that it doesn’t damage your plumbing fixtures. Look around and see if you can find it (see the pressure regulators in the pictures above). The regulator may be installed near the water meter or at the point where the water supply pipe enters the house. This is important, because if you have a regulator and you tap into the water supply for your sprinklers after the regulator, the pressure will be a lot lower.
If you have a pressure regulator on your house you must use a gauge to test the water pressure yourself. Most pressure regulators are adjustable, so the water company has no idea what pressure the regulator is set at. When in doubt, test the water pressure with a gauge.
At this point you should make at least a preliminary decision as to where you want to tap into the house water supply pipe for the irrigation system water. Typically, the closer you can tap to the point the water enters your property, the better. Of course, you must tap into the pipe after the water meter. In areas where it gets very cold some people like to tap into a pipe in the basement or someplace else inside a heated building. That way they don’t have to worry about the shut-off valve for the irrigation freezing. (Be sure to install a drain valve after the shut-off valve to drain the water out of the irrigation pipe during freezing weather!) If you have a pressure regulator, consider if it would be better to tap before or after it.
A static water pressure higher than 70 PSI can damage the fixtures and appliances in a household. If you measure a static water pressure higher than 70 PSI when you do your water pressure check as described below, then you should consider installing a pressure regulator on your house water supply if there is not one already. It will help your faucets, pipes, washing machine, dish washer, etc. to all last a lot longer. Make sure it is a good quality brass-body pressure regulator.
For a pressure regulator to work accurately the pressure setting on it must be at least 15 PSI lower than the inlet pressure. So if your static pressure is 70 PSI, the highest pressure you should set on the pressure regulator would be 55 PSI. 55 PSI is a good pressure for both the needs of a house and a sprinkler system.
Hose Bibs as a Water Supply Source = BAD!
Using a hose bib or even a “sprinkler system stub-out pipe” provided on the side of the house for sprinklers is not a good idea. There are often unknown restrictions in the house piping that cause the water supply from these hose bibs to be severely limited. The water running through the house pipes can also be very noisy at night and disturbs some people’s sleep. Do this only as a last resort, when there is no other reasonable way to get water for your sprinkler system. I would suggest you assume the pipe is 1/2″ size, even if it appears larger. If you have concrete that prevents running a new pipe around the house, call a boring contractor and find out how much it would cost to bore a 1″ pipe under the concrete. It may be worth the price. Directional boring technology now allows them to bore and install curved pipes around obstacles.
How to Measure the Water Pressure with a Gauge
Important: If you want to test the pressure yourself, everything that uses water in your home: faucets, ice makers, toilets, etc., MUST be turned off when you take the measurement (that’s why its called “static” water pressure, the water isn’t moving.) Everything! This is critical or you will get a false low reading! You can test the pressure at any faucet that is at about the same height as the proposed irrigation tap. If all the water is turned off, the pressure will be exactly the same regardless of where you test it. (Try it and see!) The easiest place to test the pressure is usually a hose bib or garden valve on the outside wall of the house.
To test the water pressure using a gauge, attach the gauge to a water outlet, like a hose bib or washing machine connection. The place where you attach the gauge can be anywhere in the house, as long as it is about the same height (elevation) as the place where you will tap in the sprinkler system supply. Ie; don’t check it on the 3rd floor if you plan to attach the sprinklers at the first floor! (It is one of those weird, hard to understand hydraulic laws that as long as the water is not flowing the pressure is the same at any point on a pipe that has the same elevation above sea level.) Double check that all the water so water is turned off and not flowing in the house pipes. Then turn on the valve the gauge is connected to and allow the water to enter the gauge. Read the pressure on the gauge. That’s all there is to it, it’s very easy to do! Turn off the water and disconnect the gauge, you’re done!
OK, I realize I may have confused you, because earlier I told you not to use a hose bib to tap the sprinklers into, and now I just told you that you can use a hose bib to measure the static pressure. This is because you can get an accurate pressure measurement from a hose bib– if the water is not flowing, as described. The small pipe can’t restrict the flow if the water isn’t flowing! Confused? Hydraulics is hard to understand. I may sound crazy but I know what I’m doing! Often users of the tutorial have an “ah ha!” moment when they get about 95% done with their first design and suddenly it all makes sense.
The static water pressure that you were given (or you measured with a gauge) is your Design Pressure. Write down the “Design Pressure” on your Design Data Form!
D. Measure the Maximum Available Flow (GPM)
Flow is the traveling companion of water pressure. Pressure is the “energy” that moves the water through the pipes. Flow is the measure of how much water is moved in a given amount of time. Flow is measured in this tutorial using Gallons per Minute (GPM). Other common units used to measure flow include cubic feet per second (commonly used to measure river flows here in the USA), liters per minute, cubic meters per hour, and many others. Now that you know your Design Pressure you need to determine how much water you can use at a time, or your available flow.
Measure Your Supply Pipe Size
You need to find the water supply pipe and measure it’s size. Grab a piece of string about 6″(152mm) long, then find the location where your water supply pipe enters the house. Strip away any insulation, so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once.
The string length is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe, which allows us to look up the pipe size, from which we can calculate the flow of water using the formula… zzzzzzzzzz….. Let’s forget all those calculations! Based on the string length use the table below to find your pipe size.
For Copper Pipe & PEX Tube
2.75″ (70mm) = 3/4″ pipe
3.53″ (90mm) = 1″ pipe
4.32″ (110mm) = 1¼” pipe
5.10″ (130mm) = 1½” pipe
For Steel, Brass or PVC Plastic Pipe
3.25″ (83mm) = 3/4″ pipe
4.00″(102mm) = 1″ pipe
5.00″(127mm) = 1¼” pipe
6.00″(152mm) = 1½” pipe
For most PE Tube
2.96-3.33″ (75-85mm) = 3/4″ pipe
3.74-4.24″ (95-108mm) = 1″ pipe
4.90-5.57″ (124-141mm) = 1¼” pipe
5.70-6.28″ (145-160mm) = 1½” pipe
Your string length will vary a little, depending on such unavoidable variables as string stretch, dirt on pipe, manufacturing tolerance, what kind of mood you’re in, etc.
Enter the supply pipe size on your Design Data Form! Also make a note of the type– copper, brass, steel, PVC, PEX, or PE.
Find Your Maximum Available GPM:
Your maximum available GPM is the maximum flow of water you have available for your sprinkler system. Actually, it would be more accurate to call this the Maximum Safe GPM. In most cases it is possible to push a higher flow (GPM) through the pipe. However, at high flows the water actually damages the inside of the pipe.
Use the smallest pipe to determine the Maximum Available Flow. Often the water supply coming into your property will not be a single type and size of pipe. You may have a plastic pipe running underground from the water company to your house. When the pipe enters the house it might switch from plastic to copper pipe, or possibly it might be galvanized steel. Then as the water supply pipe runs through the house it likely branches off in several directions with the pipe becoming smaller and smaller in size as it goes. When determining your Maximum Available GPM you will need to check the Maximum Available Flow for each of the types of pipe that the water will pass through, then use the lowest value as the Maximum Available GPM for your sprinkler design. You only need to be concerned about the pipes the water will pass through before it reaches the point where you are going to tap into it for the irrigation system.
There is an exception to the statement above. Often a short section of a smaller pipe size will be present on the water supply for one reason or another. Maybe the plumber didn’t want to drill a larger hole in the wall for the pipe. As long as this smaller pipe section is less than 5 feet long, you can ignore it and use the larger pipe size to determine maximum flow. The higher flow will be able to squeeze through the smaller pipe. The smaller pipe may wear out faster over time, but typically these short pipes are in places where they are easy to replace. Plus, the smaller pipe is often brass or steel, which has a higher resistance to wear than copper or plastic. You have to make a judgement call on this. In most cases I choose to ignore the small section of pipe.
Small Valves. It is not uncommon to find that a shut-off valve installed on the water supply pipe is a smaller size than the pipe. Don’t worry about it. It will not impact the available flow and valves are constructed to handle higher flows than the pipe.
Example 1: You find the water supply pipe entering the house, examine and measure it, and find that it is 1″ copper pipe. But you’re an ambitious type, so you also have done some digging around in the yard and discovered that the pipe going to the house through the yard is 1 1/4″ PE plastic. It just changes to copper about 6 feet away from the house (this is actually a fairly common situation.) After the copper pipe enters the house it quickly branches off in multiple directions and becomes smaller, but this doesn’t matter to you, because you have already decided that you are going to tap your irrigation system into the 1″ copper pipe right where it enters the house. So the irrigation water will not pass through any of those smaller pipes inside the house and you can ignore them.
Looking at the table you find that 1 1/4″ PE gives a flow of 23 GPM. But looking at 1″ copper pipe in the table shows a flow of only 18 GPM. Since the copper pipe is over 5 feet long you can’t just ignore it. This means you must use the lower 18 GPM value. But wait a minute! What if instead of tapping into the copper pipe, you decide to tap into the PE pipe out in the yard before it switches to copper? Now you can use the higher 23 GPM value because the water will no longer go through the 1″ copper pipe!
Example 2: You found you have a 3/4″ copper pipe that comes into the basement but you have no idea where or what type of pipe is used in the yard. It’s 0 degrees outside, and you couldn’t get a shovel into the frozen ground even if you wanted to, which you don’t! In this case it’s reasonably safe to assume the pipe in the yard is 3/4″ copper also. So you would use 11 GPM from the table.
Example 3: You have no idea where the water pipe enters the house, you have no idea where it is in the yard, and you have no desire to try to find out. In this case you must face reality, it’s time to hire a sprinkler contractor!
Maximum Available GPM Table (Maximum Safe GPM)
Maximum Available GPM (Maximum Safe GPM)
PE (poly) Tube
PEX (CTS) Tube
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
3 GPM(7 ft/sec*)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
7 GPM(7 ft/sec*)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
12 GPM(7 ft/sec*)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
CTS = Copper tubing size.
Caution: The values in the table above are the maximum safe flows for the given size and type of pipe.
These values are NOT the amount of flow you actually will use for your sprinkler system! Step #2 will show you how to modify these values to reflect your actual flow.
Velocities (ft/sec) are shown for reference only.
* PEX tube has an extremely small inside diameter when compared with the other pipe/tube types, this limits flow. Some manufacturers suggest that PEX will not be damaged by higher flows, up to 10 ft/sec. I don’t feel there is sufficient evidence yet to warrant damaging your plumbing by using flows that are too high, so I am sticking with the old industry standard for plastic pipe of 7 ft/sec maximum velocity. If you wish to take the chance, values at 10 ft/sec are
1/2″=6 GPM, 3/4″=11 GPM and 1″=18 GPM. Use these higher values at your own risk. They could cause serious damage to your both your house plumbing & irrigation piping. Read More on Water Hammer.
A flow test is optional, but suggested if you are not positive about the size or type of water supply pipe. The flow test should be run at a faucet as close as possible to the point you will tap into the water pipe for your irrigation system.
Get a 5-gallon bucket. Old paint buckets work great. Since most 5-gallon buckets actually hold more than 5 gallons of water, you will need to calibrate the bucket as follows: Find an accurate measuring container, and measure out 5 gallons of water into your bucket. Then mark the water level on the side of the bucket with a marking pen so you can easily see it. ?The test is simple. Put the bucket under your water outlet pipe and time how long it takes to fill the bucket to 5 gallons. The formula for calculating the flow in GPM is: 300 divided by the seconds it takes to fill a 5 gallon bucket = GPM.
If the result of the bucket test is lower than the Maximum Available GPM from the table above, something is restricting the flow. It may be the faucet you are using for the test, or there may be a restriction someplace in the house water supply pipe. You can try to find the restriction and get rid of it, or you can simply use the lower flow test GPM for your Initial Design Flow below.
If the result of the bucket test is higher than the Maximum Available GPM you determined in the table above, use the lower value from the table. The Maximum Available GPM Table gives you the maximum SAFE flow. The bucket test is only used to determine if there is an unseen restriction in the water supply pipe that reduces the flow below the level given in the table. Yes, many sprinkler tutorials and sprinkler salespersons may tell you a bucket test should be used for the design flow, they are wrong! In most cases a bucket test like this one gives you an unsafe flow. See the answers to common questions at the bottom of this page for details on why this happens.
Enter your Maximum Available GPM on your Design Data Form.
E. Initial Design Flow
Your Design Flow is the maximum amount of water you will design your sprinkler system to use. For now, use the same number as the Maximum Available GPM, or use the actual Flow Test GPM, whichever is lower.
You will probably need to reduce your Design Flow later, so additional lines are provided for Adjusted Design Flows on the Design Data Form. The initial flows here are very optimistic, 20 to 30% too high for most situations. You will make the adjustment, if needed, later in step #2. Don’t worry about it now. This is just an advanced warning so you won’t be surprised when you need to change the flow later.
Enter your Design Flow on your Design Data Form. Use a pencil so you can change it later!!!!
F. Do you have enough water available?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters). So if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these values are typical for hot summer areas where most sprinkler systems are installed (daily high temperatures over 90 degrees F., 32 degrees C.) These values assume you would be willing to water as many as 10 hours per day. If you are willing to water more hours per day you can increase the area irrigated by a similar percentage.
If you don’t have enough water I can suggest a few ideas for you to look into.
Another option is to use drip irrigation for shrub areas. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
Consider reducing the amount of lawn and replacing it with shrubs. Shrubs use about half the amount of water as lawn.
Another option for getting a higher flow is to install a larger water supply pipe. A description of how to do that is at the bottom of this page.
Why is the flow you measured with a bucket often too high? The GPM rates in the Maximum Available GPM Table above are based on a SAFE water velocity. When you do the bucket test, there are few restrictions on the flow, so the water velocity may easily exceed that safe limit. If you design your sprinkler system to exceed these flows some really bad things can happen. The first of these is called “water hammer”. Water hammer is a pressure surge which declares its presence by destroying the weakest point in your plumbing. The weakest point is usually that little water tube that runs between the shut off valve and the toilet in your bathroom, or possibly the ones that go to the sink faucets. The result is a flooded house, and that’s something you don’t need. Water hammer is exponentially related to water pressure. The higher the water pressure, the greater the water hammer danger. If your water pressure is over 80 PSI, I suggest that you reduce your maximum flow found in the table above by 20% and read carefully the High Pressure Alert below! The other bad thing that happens at high flows is called “scrubbing”. Scrubbing is what happens when the high water velocity actually scrubs molecules loose from the inside of the pipe. Eventually it wears away enough that the pipe develops a leak. The higher the velocity, the more scrubbing you get. A little scrubbing may take 20-30 years to create a leak. With a higher velocity the problem becomes much worse. I have seen 7 year old homes need a total replacement of all the copper pipes due to scrubbing damage. This is extremely expensive to repair! In my 30-year-old neighborhood, most of the homes have now had to replace the water supply pipes to the house due to scrubbing damage caused by sprinkler systems installed back in the bad old days before any of us realized the dangers of high flows. There are still a lot of old tutorials and literature being published that were written before the dangers of high flows were discovered, so be careful when comparing advice on this topic. A lot of industry professionals still haven’t gotten the word on this either!
But, but, but… you didn’t hear any water hammer when you did the bucket test, and nothing broke, so what’s the deal? After all, that higher flow could save some serious money on sprinkler parts! The deal is that you are only human. You can’t close the valve fast enough by hand to create water hammer, but don’t worry, an automatic sprinkler valve can! It can snap that valve closed almost instantly. The higher the water pressure, the faster the valve closes. When that valve snaps closed, it sends a shock wave through the pipe (water hammer). It may take weeks or even years for it to wear down the weak point in your plumbing and break, but it will! Then the cost savings on sprinkler parts will seem trivial. Do it right the first time! Water hammer and scrubbing are insidious and relentless. They just keep working away, little by little, day after day. Clunk, clunk, clunk, chew, chew… until the day you come home to a flooded house.
Clunk, clunk, clunk? Pipe noise!!! I hear those loud noises every time the washing machine or dishwasher runs! Is that water hammer??? You bet it is, and you better do something about it! First, if the water pressure in your house is over 65 PSI, install a pressure regulator to lower the pressure. If that doesn’t get rid of it, go down to your local hardware store and buy a water hammer arrestor. You can get one that screws onto the washing machine or dishwasher fill pipe. They cost about $10-15 and they work pretty well for water hammer caused by appliances. They don’t work nearly as well for water hammer caused by sprinkler systems. This is because many sprinkler systems exceed the maximum water velocities by so much that the arrestor is over-whelmed by it. I’ve written a whole tutorial on this topic: Water Hammer and Air in Pipes.
Q. How do I calculate sprinkler risers losses in a sprinkler zone where the risers are extra long, 3 ft or more above ground? I have 10 risers in a zone for my proposed sprinkler irrigation system.
A. If you are using my Sprinkler System Design Tutorial and a standard riser of the recommended size, then you don’t need to worry about pressure lose in the riser, the tutorial has friction loss for the risers built-in to the formulas it uses. So you can ignore the riser pressure loss. Some standard risers are shown on the page on Sprinkler Risers in the Irrigation Installation Tutorial. The recommended size for a riser? In most cases it should be the same size as the threaded inlet on the sprinkler. But please actually read that page on risers, as there are some exceptions to that rule for certain types of standard risers!
OK, I realize that didn’t answer your question, you are asking about a non-standard riser that uses a long pipe to hold the sprinkler high above the ground. In that case you must calculate what the friction loss will be in the longer-than-normal riser pipe. (In this case that would be the 3 ft long pipe you described in your question above.) To do that you simply use the same friction loss spreadsheets that you use to calculate the friction loss in any other pipe. Just use this link to get the proper spreadsheet from my website for the type of pipe you are using. Then open the spreadsheet and on the first line enter the pipe size, GPM of the sprinkler you will install on the riser, and the length of the riser. Enter an error factor of 1.4 rather than the default 1.1. This is because even your “longer” riser is shorter than the typical pipe length that the default error factor is based on. Now read the friction loss. That’s it, you have the friction loss for your non-standard riser! Don’t worry about the fittings like ells and couplings that are part of the riser, that is part of what the error factor is compensating for.
When adding the riser friction loss into the total friction loss calculations for your whole sprinkler system, just add in the loss for a single riser. Use the friction loss value for the riser that has the highest friction loss. (This is most likely the one with the highest GPM sprinkler, or it may be the longest riser if you have different riser lengths. You may have to calculate the friction loss for several different risers to figure out which of them has the highest loss.) Why do you add in the friction loss for only one sprinkler, rather than the combined loss for all of them? Because as a single drop of water goes through the sprinkler system it only goes through one sprinkler, not all of the sprinklers. You have to think about the water as a collection of millions of drops, not as one solid body. So the pressure loss is what a single drop would experience as it travels through the system. As a drop of water enters the sprinkler system it travels through a water meter, lots of pipe, a valve or two, then it finally blows out through a single sprinkler onto the landscape. The pressure loss calculation for the whole sprinkler system is determined by what the worst case pressure loss values would be for a single drop of water traveling through the sprinkler system.
OK, so you calculated the friction loss, but what if it is a really high value, or maybe the calculator complained about the velocity being to high. In this case you need to use a larger size pipe for your riser. For the velocity in a riser you can go all the way up to the 7 ft/sec maximum without too much risk. Velocities in the marginal “use caution” zone are generally OK for risers. High velocity in a riser will seldom cause a water hammer problem, unless you are using a special type of sprinkler that has a solenoid valve built in to it. Those sprinklers are called “valve-in-head sprinklers”, they are very expensive, and are mostly used for golf course greens.
PVC pipe types labeled “schedule” (abbreviated “SCH“) are made based on the traditional dimensions used for steel pipe. Unfortunately steel has very different strength characteristics from plastic, so it is a system that isn’t very logical for use with PVC pipe. But when plastic first came along it was made to the same size standards that were already in use for steel. The common PVC pipe schedules you will see in stores are SCH 40 and SCH 80. As the pipe sizes rated SCH increase, the strength and pressure rating of SCH pipe decreases. So 1/2″ SCH 40 PVC pipe is very strong, while 2″ SCH 40 PVC has comparatively a low pressure rating, and is more easily damaged. In sizes 1/2″ to 1 1/2” SCH 40 is a thick wall pipe with a reasonably high pressure rating and good resistance to physical damage. It is often used for mainlines and other situations where a tough high pressure pipe is needed. Sch 80 is generally used for making threaded plastic nipples because the plastic walls are thick enough to have threads cut into them (although most now have molded threads rather than threads “cut” with a die.)
As you can see, the pressure ratings drop as the pipe size increases. Note that the industry standard rule is that your normal operating pressure should not exceed 1/2 of the rated pipe pressure. In other words, you shouldn’t use 1 1/2″ pipe for pressures higher than 165 PSI (330 x 0.5 = 165 PSI). This is because pressure surges created by closing valves can easily double the water pressure in the pipe. This rule applies to all PVC pipe, including that labeled SCH and CL.
Class rated pipe
PVC pipe types labeled “Class” (abbreviated “CL“) are based on the pipe’s pressure rating. So Cl 200 PVC pipe is rated for 200 PSI of water pressure. Cl 315 PVC pipe is rated for 315 PSI of water pressure. The strength of CL labeled pipe is directly related to the pressure rating. The standard “Cl” pipes are Cl 125, Cl 160, Cl 200 and Cl 315. Of these Cl 200 and Cl 315 are most common. Cl 125 is sold as a low cost pipe for use in sprinkler laterals for those for whom low price is everything. It has a very thin wall and breaks easily if not handled carefully or nicked with a digging tool.
1/2″ size pipe is generally only available in SCH 40. This is because of the thin wall of 1/2″ pipe makes it very easy to break. I don’t recommend using 1/2″ PVC pipe at all, however if you must, you should use SCH 40. Sometimes you will find 1/2″ Cl 125 PVC pipe at discount stores due to the very low price.
The Class system is obviously a more logical system for labeling pipe as you know immediately how strong the pipe is based on the label. Unfortunately the more confusing “SCH” system became entrenched in the industry and remains.
What Pipe Type to Use
All PVC pipe labeled for a given size in the USA has the same outside diameter. So any pipe labeled as 3/4″ will be the same diameter, whether it is SCH 40 or Cl 200 or any other type. That allows the same fittings to be used to join the various pipe types together. Most fittings are made to SCH 40 standards, although SCH 80 fittings are available, typically only at specialty plumbing and irrigation stores. Technically most codes require SCH 80 fittings for pipe sizes 2″ or over. In practice I’ve noticed that SCH 40 fittings are often used up to 3″ size. When dealing with sizes 4″ and above the use of non-glued “rubber ring-joint” fittings is recommended and usually required by code as well. Glueing joints on 3″ and larger PVC pipe is very, very difficult.
“Mainlines” are all of the pipes that are under constant pressure, that is, the pipes that are before the sprinkler zone valves. In most of the industry SCH 40 PVC pipe is used for irrigation mainlines up to 1 1/2″ size. For 2″ size and larger Cl 315 PVC is used. Most building codes prohibit the use of 2″ and larger SCH 40 PVC pipe for pressurized water lines. Depending on the jurisdiction, this rule may or may not be applied to irrigation systems. Those same codes generally require that all pressurized PVC pipes (mainlines) be buried at least 18″ deep to protect them from accidental damage, regardless of the type or size of pipe used.
“Lateral” pipes are the pipes after the sprinkler zone valve. These pipes are only pressurized when the sprinklers are operating. For lateral pipes the standard is to use Cl 200 PVC pipe. Where budget is a concern and you can find it, sometimes Cl 160 is used. As previously mentioned I recommend you avoid Cl 125 PVC pipe. Laterals can be buried any depth, but I generally recommend at least 10″ deep to avoid a lot of maintenance problems with broken pipes.
As water moves through a pipe it loses pressure due to a phenomenon commonly called “friction loss”. Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. A complex formula (called the Williams/Hazen Formula) predicts the amount of pressure that will be lost due to friction loss. The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it’s way to the sprinkler head. Even a bend in the pipe causes pressure loss! Don’t panic over the formula, we’ll use a pipe sizing chart or a friction loss calculator!
You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. The manufacturer’s performance charts tell you how much pressure is required to achieve a specific radius for the water. As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to make the sprinklers operate as they should, we need to calculate the pressure losses between the water source and the sprinkler head. If the pressure loss is found to be too great, then we must reduce. The easiest way to do that is to use a larger size pipe.
DETERMINING THE SPRINKLER PIPE SIZE
There are several methods used to determine pipe sizes of sprinkler system lateral pipes. I’m going to explain two methods. One method is faster but less accurate, the other is very accurate but takes more time.
Pros: The fastest and easiest method. Requires a single, simple calculation and uses a chart to determine the sizes.
Cons: The learning curve to use it is a bit more difficult to understand. It uses an averaging system to arrive at pipe sizes.
See step-by-step tutorial for the Chart Method
Trial & Error:
Pros: Very accurate, calculates the pressure loss in each pipe section using a spreadsheet. Easier to understand.
Cons: Time consuming, need to enter data into the spreadsheet, uses trial and error to establish pipe sizes.
See step-by-step tutorial for the Trial & Error method
Which method should you use? For a beginner with a small irrigation system probably the Trial & Error system will be easier. Below are overviews of each method for experienced designers to use. Unless you are experienced you should probably read the full tutorial for the method you select.
Overview: CHART METHOD FOR LATERAL IRRIGATION PIPE SIZING
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
____ PSI. Insert the maximum PSI loss for the valve circuit laterals into the formula where it says “____PSI.”
____ Feet. Insert the distance from the valve to the farthest sprinkler on the valve circuit in the space labeled “____ Feet Total Length” in the formula.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
The Pipe Size Table or Chart:
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 = Desired PSI Loss in Lateral x 100 / Total length of Lateral
Flows shown red are over 5 feet/second. Use caution!
Find your PSI/100 value in the top blue row.
Read down the column to the value equal to, or higher than, the GPM in the pipe section.
Read across to the pipe size for that section in the right column.
Repeat steps 2 & 3 for next pipe section.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%. This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Notes about the Pipe Sizing Chart:
Warning: The sprinkler pipe sizing chart is based on using Cl 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Polyethylene, Polybutylene: After you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc.
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Change the pipe size if the velocity or total pressure loss is too high.
You must calculate the pressure loss for each of the possible water paths in the valve circuit.
Here is an example of the possible water paths for a valve circuit, shown in red, blue, and magenta.
Start with the water route that is the longest. In this case that would be the red route. There are 9 pipe sections in this route labeled 1-9. Enter the data from this route into the calculator. Use a larger pipe size if the velocity is not safe. Check that the friction loss “Total of All Sections” does not exceed your maximum allowable amount.
Write the pipe size for each section on your plan.
Now repeat the process for the blue water route and then the magenta color route.
It will not harm anything to use a larger pipe size. Period. If you are uncertain whether to use a 3/4″ or 1″ pipe, then you should use the 1″. Using a larger size pipe is ALWAYS the safest choice.
No, I don’t own stock in an irrigation pipe manufacturer and I’m not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be “too large”. Contrary to what might appear to be true, forcing water into a smaller pipe REDUCES the water pressure, and hurts sprinkler performance. This is because the smaller pipe creates more pressure loss due to friction and turbulence as the water flows through it. It’s another of those hard to grasp hydraulic principles! Just remember that when it comes to pipe, bigger is better! I’m always amazed at how many irrigation equipment sales people don’t know this most basic of irrigation rules. I’ve had clients tell me they were told to use a smaller pipe to keep the pressure up by tech support people at some of the major sprinkler manufacturer’s. That’s an industry disgrace!
So one more time to drill it into your head– You don’t decrease the pipe size to keep the pressure up- or down for that matter. That is totally, completely, wrong. The reason we use smaller pipe is to save money. Which of course, is a good reason! For those who want more specifics on this, there is a very boring scientific explanation at the bottom of this page.
Is it Pipe or Tube? For the most part I use the term “pipe” rather than “tube” on this page and elsewhere. Bad habit of mine (note that by reading carefully, you have found one of my faults!) The difference is the material they are made from. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually referred to as tube. I often screw up and call tube pipe! 🙂
TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE USING A SPREADSHEET
This method involves trying various pipe sizes until a good combination is found.
Definitions you need to know:
Lateral pipe: all the pipes between the control valve and the sprinkler heads.
Mainline: The pipe that goes from the water source to the control valves.
Control Valve: The valve that turns on and off a group of sprinklers. Most often it is an electric valve operated by a timer.
Valve circuit: a single valve, and all the pipe, fittings and sprinkler heads downstream from it. In other words, all the sprinkler heads that start working when you turn on the valve are part of the same valve circuit.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
You will need a spreadsheet Friction Loss Calculator.
Here’s a page with calculators for almost every type of pipe: Friction Loss Calculator Spreadsheets
Grab the appropriate spreadsheet for the type pipe you plan to use.
Now you just enter the appropriate data for each section of pipe into the calculator and then read the total pressure loss at the bottom of the spreadsheet. If the pressure loss is too high, then try making one of the lengths of pipe larger. The calculator will also give you the water velocity in each section of pipe, and warn you if the velocity is too high.
The best way to teach this is probably to walk you through a couple of examples.
If I may make a suggestion, download the spreadsheet for Cl200 PVC now, and open it up in a separate window. Then think about each step, enter the values I show into the spreadsheet, and actually try to duplicate what I do in the examples below. Something about actually doing this helps engage people’s brains. People tell me they read it twice and still don’t get it when they just read it, but as soon as they actually TRY it, then it suddenly makes sense. It’s called learning by doing, and it is considered the best teaching method. This process is simple, HOWEVER, it is not obvious and sounds illogical to those not trained in hydraulics.
A Simple Example:
The sketch above is an example of a very simple valve circuit with 5 sprinkler heads. In this example the sprinklers are 15′ apart and each sprinkler uses 3.7 GPM of water. The red numbers on the sketch are the total water flow for each pipe section in GPM.
Let’s assume we want to use Cl200 PVC pipe and we want a maximum total of 4 PSI of pressure loss in our lateral pipes.
If you are working through the Sprinkler Design Tutorial the maximum total pressure loss is entered on your Design Data Form in the Pressure Loss Table section. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.
If you don’t understand how to calculate the water flow in each section (the red numbers) you should take a look at the Sprinkler Pipe Layout page.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
If you don’t understand pressure losses in irrigation, see Pressure Loss & Selecting Your Sprinkler Equipment.
For advice on types of pipe (Cl200, poly, etc.) see Irrigation System Lateral Pipes.
To use the spreadsheet friction loss calculator to determine the pressure loss:
Download and open the Friction Loss Calculator.
There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
Enter the GPM for the section of pipe.
Enter the length of the section of pipe.
Use an error factor of 1.1
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Here’s what the spreadsheet calculator looks like after we enter the data requested for each of the pipe sections using the example in the sketch above.
Note that the “Total of all Sections” shown at the bottom exceeds the 4 PSI maximum limit we set for pressure loss. Also notice that the velocity in two of the sections (highlighted in red) exceeds the safe level. The marginally high velocity highlighted in yellow is considered acceptable by most experts, since these are lateral pipes. (The marginal velocity level would not be as acceptable in mainlines.) Start by fixing the velocity problems. To decrease the velocity in those sections we will need to increase the pipe size. So let’s increase the pipe size for the two sections highlighted with red to 1″. Here’s what it looks like after the change:
Now the velocities are all within acceptable levels. Also note that increasing the pipe sizes reduced the pressure loss “Total of All Sections” shown at the bottom to 2.9 PSI, which is well below our maximum level of 4 PSI. That’s good, no more changes are needed. It is not possible for the pressure loss to be “too low.” As long as it is under the maximum it is fantastic. So what would happen if the pressure loss was still too high? If there was still too much pressure loss we would need to try increasing the size of some of the pipes to lower the friction loss.
So we now have pipe sizes that will work for each section of pipe in our lateral. I’m often asked at this point if it would be OK to make some of the pipes 1/2″ since the pressure loss is so low? The answer is yes, but you might not want to do it. See my explanation of the problems associated with the use of 1/2″ pipe.
A More Complex Example:
Now lets look at a more complex valve circuit. (Please note that this circuit is much larger than that found on a typical residential irrigation system. It would require much more water than most residences have available and is just used to show you an example of a much more complex layout.) As with the previous example we will assume that our maximum pressure loss value for the valve circuit is 4 PSI.
This valve circuit involves numerous paths the water may take. This makes the calculation a bit more complex, as a separate calculation is needed for each possible route that the water might take through the laterals on it’s way to the last sprinkler at the end of a pipe. If you look at the example above you will notice there are 3 sprinklers that are at the end of pipes, each sprinkler at the end of a pipe represents a different route the water can take. So this circuit has 3 and will therefore require 3 separate pressure loss calculations. The next drawing shows the possible water routes in magenta, blue, and red colors.
It may help to think of each path as the shortest route that a single drop of water could take to go from the valve to the last sprinkler on a pipe branch. For some people it helps to think of it as a road map and your looking for the shortest route to each of the dead ends at the end of the roads.
Start your calculations with the water route that is the longest. In this case that would be the route highlighted in red. There are 9 pipe sections in this route, I have labeled them 1-9 for clarity. Just as before, enter the data from this route into the calculator, and make all the pipe 3/4″ size. Here’s what the spreadsheet looks like:
As you can see there are a number of pipe sections highlighted red due to unsafe velocity. Change those pipe sections to larger pipe sizes until all the velocities are within safe levels.
Here’s the resulting spreadsheet calculator with the smallest possible pipe sizes. However, notice the Total of All Sections is 4.4 PSI, which is more than our 4 PSI maximum:
So we need to make some of the pipe sections larger in order to reduce the pressure loss (or friction loss.) Start by increasing the size of one of the the smaller pipe sections. Changing a 3/4″ pipe to a 1″ size is a lot less expensive than changing a 1″ pipe to 1 1/4″. So for the example lets change section 7 from 3/4″ to 1″. Doing that drops the Total of all Sections value to 3.77 in our example, below the 4 PSI maximum we set earlier. So now everything is good, these sizes will work for the “red” highlighted water route.
Now we add the pipe sizes from the spreadsheet to our circuit drawing (note that the pipe sizes for the red highlighted sections have been added on the next drawing below.)
Now we relabel our sections to follow the blue highlighted route.
Using the blue highlighted water route, repeat the same process used for the red one. Enter the GPM and pipe lengths for each section in the spreadsheet. This time we already know the sizes for sections 1-6, they were entered into the spreadsheet when we did the red section. So we just enter those for the new blue sections 7 and 8, again using 3/4″ size pipe. And it looks like this on the spreadsheet calculator:
Using 3/4″ pipe size for our two new sections works good. The velocity is safe and the Total of All Sections is 3.29 PSI, so the pressure loss for this route is also within the 4 PSI maximum we set. Write the size of the two new sections on the drawing and the blue water route is done.
Now all that remains is to do are the calculations for the magenta highlighted water route. That is done the same way, entering the data into the spreadsheet calculator for each pipe section. Start with 3/4″ then change to larger sizes until the velocity is safe. Then check that the total of all Sections is less than 4 PSI as before. Here’s the data entered into the spreadsheet calculator:
Now all that remains is to insert our lateral pipe sizes from the spreadsheet calculators into the drawing of the valve circuit.
All done! So the pressure loss for the entire circuit is the same as that for the highest water route. In this case the red route was highest at 3.77 PSI. So the pressure loss for the lateral circuit shown here is 3.77 PSI.
Often I get asked at this point why the “Total of all Sections” pressure losses for all 3 routes wasn’t added together? The pressure loss for the red route was 3.77 PSI, for the blue section it was 3.29 PSI, and for the magenta route it was 3.02 PSI. So the confusion is that it seems like there should be a total loss of 10.08 PSI! Nope, the pressure loss for the entire lateral is 3.77 PSI, the loss of the highest route. To understand this think of a single drop of water again. It can only travel on one route from the valve to the farthest sprinkler. It is not going to go backwards and try another route! So the pressure loss for the entire valve circuit is equal to the pressure loss from the valve to the farthest sprinkler.
This method is based on the assumption that you are using Cl 200 PVC pipe for the lateral pipes. With minor adjustments this method will also work reasonably well for SCH 40 PVC pipe or polyethylene irrigation tube. For other types of pipe or tube you will need to use the Trial & Error method to determine the pipe sizes.
While the Pipe Sizing Chart method described here seems rather complex when you read it the first time, it is actually extremely fast and easy once you figure it out. You will start with a simple calculation to obtain a “PSI/100” value. Then you will use that value in the Pipe Sizing Chart to figure out the maximum flow for various sizes of pipe. You will only do this once for each sprinkler system. Once you have that schedule you will fly through inserting pipe sizes into your plan. Most designers who “design in their heads” are using this method or a close variation of it. It is the method I use when designing my systems.
Lateral pipe: The pipes between the control valve and the sprinkler heads are called “laterals”.
Mainline: The pipes that go from the water source to the control valves are called “mainlines”.
Control Valve: The control valve is the valve used to turn on and off a group of sprinklers. Often it is an electric solenoid valve operated by a timer.
Valve circuit: A valve circuit consists of a single control valve, and all the fittings, pipes, and sprinkler heads that it turns on.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
BASIC RULES TO KEEP IN MIND
When in doubt, always use a larger diameter pipe!
You may always use a larger size pipe. No, I don’t own stock in a irrigation pipe manufacturer. But using a larger size of pipe will not cause any harm to how well your sprinkler system works. Using a larger pipe will NOT noticeably reduce the water pressure. (Yes, I did condition that statement with a “noticeably”.) The only damage done by using a larger size of pipe is to your pocketbook. Larger pipe generally costs more. But from a irrigation system performance perspective you will NEVER hurt anything by using a larger size pipe. Now I realize that somewhere out there, somewhere will tell you this is not true. They are going to tell you that you need a smaller pipe to squeeze the water and create more pressure. They are totally wrong of course, but as you read this you are probably uncertain who is right, since they will claim I am wrong! Ask them to provide you with a scientific, documented explanation of why they are right. I will also provide both a basic and a very scientific explanation with references for you. Here’s mine: Using A Smaller Pipe to Increase Water Pressure. OK, sorry, I’ll climb down off my soapbox now.
Is it Pipe or Tube? I tend to call everything pipe. Habit, since here in La La Land (Los Angeles, California) we use mostly PVC pipe for irrigation. However some types of “pipe” are technically defined as “tube”. The difference is the material they are constructed of. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually called tube or tubing. If I say pipe where I should say tube, please accept my apologies.
CALCULATING THE PSI/100 VALUE:
The PSI/100 value is a value used in the Pipe Sizing Chart (we’ll get to the chart in a moment.) The PSI/100 value determines which column of the chart you will use when finding the pipe sizes. A simple calculation will give you the PSI/100 value.
The PSI/100 formula:
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
For those who prefer variables, this is the same formula written using variables: (LPSI * 100) / FTL = PSI/100
Here are the values to insert in the blank spots (“____” ), or variables, in the formula:
____ PSI. (LPSI) Insert the maximum PSI loss for all laterals on the valve circuit into the formula where it says “____PSI.” .
If you are working through the Sprinkler Design Tutorial look on your Design Data Form for the Pressure Loss Table. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems. If you don’t understand pressure losses in irrigation, see the Pressure Loss & Selecting Your Sprinkler Equipment and Lateral Pressure Loss pages. Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
____ Feet Total Length. (FTL) Insert the distance from the control valve to the farthest sprinkler in the space labeled “____ Feet Total Length” in the formula.
For this value you need to figure out the total length of pipe (in feet) that the water needs to travel through in order to get from the valve to the farthest sprinkler. Measure only the pipe sections that the water would pass through on the way from the control valve to that farthest sprinkler. Don’t add in the length of any side spurs going off to other heads that aren’t on the longest route. In the example below, the route from the control valve to the farthest sprinkler that you would measure the distance of is shown in red. Totaling each of the pipe sections along that route results in 118′. So 118 feet would be the ___ feet value you would use in the PSI/100 formula.
Example of A Typical Valve Circuit
Now use the PSI/100 formula above to calculate the PSI/100 value. ( ____ PSI x 100) / ____ Feet Total Length = PSI/100
Write down the PSI/100 value. ____________
Example: Let’s say the value “____ PSI – Laterals” is 4 PSI. Let’s also assume that the total length of the lateral as measured above is 118 feet. Those values inserted in the formula would look like this: (4 PSI x 100) / 118 feet Now do the math. 4 times 100 = 400. Then 400 divided by 118 = 3.389 Round that number to 3.4. Therefore when using this example your PSI/100 value to use in the Pipe Sizing Chart would be 3.4 PSI/100 .
You can repeat this procedure for each valve circuit. But the usual method is–
It is possible to use the same PSI/100 value for all the valve circuits. That’s how most professionals (myself included) do it. The only catch is that you must use the “worst case” PSI/100 value. In other words you need to figure out which of the valve circuits on your entire sprinkler system has the longest “Feet Total Length” between the valve and last sprinkler. Then use that valve circuit to calculate your worst case PSI/100 for the entire sprinkler system. The advantage of using the same PSI/100 value for everything is uniformity of design and, obviously, doing only one PSI/100 calculation for the entire sprinkler system saves time. For example, a pipe with five half circle spray heads downstream would always be the same size pipe. This is much less confusing for the installer, which is the main reason we do it this way.
Pipe Sections and GPM:
Each section of lateral pipe may be a different size. For example, the first section of pipe leading away from the valve might be 1 1/4″. The next two sections might be 1″, and the rest of the sections might be 3/4″. The pipe size to each section is based on the actual GPM flow passing through that section of pipe, so you will need to know what the GPM flow is for each section. If you have been working through the Sprinkler Design Tutorial you have already figured this out and written these GPM values down on your plan in an earlier step. If not, you will need to take a few minutes to do this now. See the page on Sprinkler Pipe Layout for instructions on figuring out the GPM for each pipe section.
THE PIPE SIZING TABLE or CHART:
before you use the chart…
Warning: The sprinkler pipe sizing table /chart is based on using Cl 200 PVC pipe. For other pipe types you will need to make an adjustment if you want to use the chart.
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart below, because SCH 40 PVC pipe has a much less water capacity than other PVC pipes. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Example for SCH 40 PVC pipe: In the example above you calculated a value of 3.4 PSI/100. But you have decided to use SCH 40 PVC pipe for the laterals, rather than Cl 200 PVC pipe. So you will need to reduce the PSI/100 value by half. 3.4 x 0.5 = 1.7 PSI/100. So your new value is 1.7 PSI/100. As you will see, this will result in much larger lateral pipes! This is why most people do not use SCH 40 PVC for laterals, and why I recommend you use Class 200 PVC. It makes a big difference in cost!
Class 125, Class 160, or Class 200 PVC pipe: The chart below is based on the use of Class 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Class 100 and 315 PVC pipe: As a general rule, these types of PVC pipe are not used for laterals.
Polyethylene, Polybutylene: Use the chart below. Then, after you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc. Note: PEX pipe is not the same thing as polyethylene irrigation pipe.
To use the chart you will use the PSI/100 value you calculated along with the GPM flow in the pipe section.
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 (round down)
Flows shown in red are over 5 feet/second.
Sprinkler Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Permission is granted for reuse for any purpose and in any media, provided the copyright notice is maintained.
Sprinkler Pipe Sizing Table /Chart Instructions:
Start with the pipe section farthest from the valve (connecting to the last sprinkler head.)
Find the PSI/100 value in the top row (blue text, directly under the heading PSI/100.)
Read down that column and find a value equal to, or higher than, the GPM in the pipe section.
Now read across to the right column to find the pipe size to use for the pipe section.
Repeat steps 3-5 for the other pipe sections in the lateral valve circuit.
Flows over 5 ft/second are considered marginal (shown in red on chart.) Most experts believe that flows up to 7 ft/sec are acceptable for laterals. However flows over 7 ft/sec velocity are not considered safe, so they are not shown on the chart.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%.
This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
In the example above the flows for each pipe section are noted in gray text with an arrow pointing at the pipe section. The red pipe circuit has the longest distance between the control valve and the farthest sprinkler head. So for our example let’s use the red pipe circuit.
First we need to calculate the PSI/100 value.
We start with the maximum pressure loss we want in our lateral pipes. For this example we will use 4 PSI.
Now we measure the total pipe distance from the valve to the farthest head. I showed this route using a bold red line. It is 96 feet from the control valve to the farthest head when following this bold red route.
Now the PSI/100 formula with the values from this example inserted: ( _4_ PSI x 100) / _96_ Feet Total Length = _4.2_ PSI/100
Now we start using the chart to find the pipe sizes.
Our PSI/100 value is 4.2, so we look on the chart. Rounding down we see that 4.0 is the closest PSI/100 value on the chart, so we use the 4.0 column.
Now read down the 4.0 column. The numbers will tell us the maximum flow for each pipe size.
So the first number we see is 10. That would mean 10 GPM. Reading across to the right we see that 10 GPM is the maximum flow for 3/4″ size pipe.
Continuing in the 4.0 column, the next number is 20. Again we read across and see that 20 GPM will be the maximum flow for 1″ pipe.
Reading down one more line we see that 36 GPM is the maximum flow for 1 1/4″ pipe. And we can continue this on down the chart.
So now we can create a simple pipe size schedule to use for our plan, based on the values we took from the pipe sizing chart:
Up to 10 GPM = 3/4″ size pipe
Up to 20 GPM = 1″ size pipe
Up to 36 GPM = 1 1/4″ size pipe
Up to 48 GPM = 1 1/2″ size pipe
Up to 76 GPM = 2″ size pipe
Now go back and look at the flow for each section of pipe on your plan. Then based on the GPM flow, insert the pipe size from the schedule you made.
So the section with a flow of 2.5 GPM will be 3/4″ pipe.
The section with 1.3 GPM will also be 3/4″ pipe.
The section with 3.8 GPM will be 3/4″ pipe.
The section with 6.4 GPM will be 3/4″ pipe.
The section with 1.3 GPM will be 3/4″ pipe.
The section with 2.6 GPM will be 3/4″ pipe.
The section with 9.0 GPM will be 3/4″ pipe.
The section with 11.5 GPM will be 1″ pipe.
I’ve inserted these pipe sizes on the example sketch above.
See how fast and easy that is? Once you have the initial PSI/100 calculations done you can use the pipe sizing chart to create a custom pipe schedule for your plan. Then it is really fast to simply look at the flow in a pipe section, look it up on the schedule, and write in the pipe size! You can see why pros use this method, it allows them to fly through a large design with hundreds of sprinklers.
COMMON PROBLEMS AND QUESTIONS REGARDING USING THE PIPE SIZING CHART
Is your PSI/100 value off the chart? If your PSI/100 value is 6.0 or higher you should use the 6.0 column. At 6.0 you have reached the maximum safe capacity of the pipe sizes used on the chart.
Is the pipe size larger than the valve size? It is fairly normal for the first pipe after the valve to be one size larger than the valve. So you may have a 1″ mainline going into a 3/4″ valve and then have a 1″ lateral pipe coming out of the 3/4″ valve. This is very common, and is not a problem at all. So don’t worry if the pipe size you get from the chart is larger – or smaller – than the valve size.
Write the pipe size down next to the pipe on your plan. Repeat for each pipe section. Repeat for each valve circuit.
There is a very persistent misconception in the lawn sprinkler industry that using progressively smaller pipe sizes in a sprinkler system will help keep the water pressure high. The argument is that as the water moves through the pipes past the sprinklers, the pipe must get smaller in order to squeeze the water so that the pressure stays high enough to operate the sprinklers. Unfortunately, it’s not true. It would be nice if it was, because we could eliminate pumps. Plus think of all the money you would save on pipe. The smaller the pipe you used, the better your system would work! So why not use 1/4″ or even 1/8″ tube for the pipes? That would really pump up the pressure! Sounds a little silly when you look at it that way, right? OK, enough with the sarcasm. I’ll explain this whole mess.
Squeezing the water into a smaller pipe will not increase the water pressure!
Part of the reason this misconception persists is that it does seem logical. The example most often given to support this idea is what happens when holding your thumb over the end of a hose. As you press your thumb over the opening, making it smaller, you can feel the water pressure against your thumb increase. Pushing your thumb even tighter against the end of the hose, makes the opening even smaller, and you feel the pressure increase even more. That would seem to prove that decreasing the opening size is increasing the water pressure. So logically, using a smaller pipe would also increase the water pressure.
Unfortunately there is a lot more happening with this “thumb over the hose end” example than you realize. As water moves through a hose or pipe there is a lot of resistance caused by the hose or pipe surfaces. The water moves through the hose at the maximum speed it can while still overcoming this friction. When the water reaches the end of the hose it has close to zero pressure left as it exits. So if you have, say, 50 PSI of water pressure at the hose faucet, the water will move as fast as it can through the hose, such that it will use up almost all that 50 PSI of pressure by the time it reaches the end of the hose. If there were 60 PSI of pressure, the water would just move a little faster through the hose so that it used up almost all 60 PSI by the time it exits. So basically regardless of the pressure, almost all the water pressure is used up by the time the water flows through the hose. The nature of water is that it will reach the most efficient balance between flow rate and pressure loss that it can. (Note, I am oversimplifying this to make it digestible for the average person. If you have a degree in hydraulics you already know all the other related stuff about open vs. closed channels and nozzling effects.)
When you put your thumb over the end of the hose you change the flow dynamics in the hose. Your thumb restricts the flow of water through the hose. With your thumb over the end, the water is flowing much slower through the hose, and as a result, there is a lot less pressure loss due to friction. So with less pressure being lost in the hose, the pressure at the end of the hose where your thumb is increases. The tighter you squeeze your thumb, the more the flow is reduced, and the greater the pressure you feel will be. But you haven’t created any NEW pressure. You have simply traded reduced flow for increased pressure. You can easily test this yourself. Take a bucket and time how long it takes to fill it using an open end hose. Now time how long it takes to fill the same bucket with your thumb firmly pressed over the hose end. It will take longer to fill, because your thumb has reduced the flow! The same thing would happen in your sprinkler system if you used smaller pipe to increase the pressure. The smaller pipe would restrict the flow of water. The reduced flow would reduce the pressure loss in the pipes, resulting in more pressure. But of course the sprinklers would not work because they won’t be getting the flow they require! Sprinklers require both flow and pressure.
OK, that’s the layman’s explanation. But there are also some much more complex scientific theory that I have been asked about in relation to this topic. So here’s some very scientific explanations.
Grab your thinking caps for this. As you well know, Bernoulli’s Principle essentially says (paraphrased) that as the speed of a fluid increases, the pressure of that fluid decreases. If it didn’t, pigs wouldn’t fly.* Obviously as you force a given amount of water through a smaller size pipe, the velocity of the water must increase for it to get through the smaller pipe. According to Bernoulli’s Principle that will decrease the water pressure! This is called the Venturi effect. By suddenly forcing the water through a narrow passage you can actually create enough of a pressure decrease that it creates suction. This is how many fertilizer injectors work. It also is another reason why using a smaller pipe would not increase the pressure– it would actually decrease it!
Another less common argument is the pipe size must be decreased because the flow is decreasing at each sprinkler head location along the pipe route. Thus if the pipe were to remain the same size, the velocity in the pipe would decrease, resulting in an increase in pressure (according to Bernoulli’s Principle again.) This is actually a good, scientifically based point, and accurate too! So the argument is that the pipe sizes must become smaller in order to keep the velocity constant and avoid an increase in water pressure. (Are you bored yet?) Unfortunately when used as an argument for using smaller pipe, this one falls flat when you do the actual math. At a flow of 7 feet per second, which is the maximum recommended safe flow for PVC pipe, the maximum possible pressure increase due to velocity change would be a whopping 1/3 PSI. So in theory, using a smaller pipe would eliminate that 1/3 PSI pressure gain. But using a smaller pipe probably would also increase the pressure loss due to friction, as previously mentioned. The drop in pressure due to friction loss likely will offset most if not all of any gain that might have occurred due to decrease in velocity. Even if it didn’t the maximum possible pressure gain of 1/3 PSI is simply not significant and would not be noticed. So I stand by my statement that the only reason to decrease pipe size is to save money.
*Oh, by the way, Bernoulli’s Principle is why airplane wings create lift, which helps airplanes fly. Therefore, it is also the reason that people, and yes, even pigs, can fly!
Here are some spreadsheets I have created to help you calculate the capacity and water pressure loss through pipes and tubes of various types and sizes. These should be useful for both figuring pressure loss in mainlines and laterals. Each spreadsheet allows for multiple sections of pipe of various sizes and flows. All you do is select the proper spreadsheet for the type of pipe you are going to use, select the pipe size from a drop down list, enter the flow through the pipe in GPM, then enter the length of the pipe in feet. The spreadsheet calculator will then do the math to give you the water velocity in the pipe along with the pressure loss in PSI for that section of pipe. If there are multiple sections of pipe the spreadsheet will also total all of them for the total pressure loss.
Full instructions for using the spreadsheets are included on the spreadsheets.
Please read this paragraph before you try to use the spreadsheets.
These spreadsheets use Apache Open Office. That means you need to have the Open Office program installed on your computer for them to work. I use Open Office because it is a free, safe program that is available for just about every desktop and laptop computer make and model. That means just about everyone can use it, and nobody has to make a major software investment just to use the spreadsheets. If you don’t already have Open Office you will need to install it on your computer before you can use the spreadsheets. You can uninstall it when you are done using it if you want. Download it free from http://www.openoffice.org . It does take a while to download Open Office. It is a full office productivity suite. (Check it out, there are some cool apps in it!)
Your browser may try to “open” the spreadsheets if you left click on the links, even if you don’t have Open Office installed. This is because most computers have some type of minimal spreadsheet reader installed on them, so the reader will try to open these spreadsheets. If they were just simple spreadsheets they would probably work with the readers. But they aren’t. In most cases you will get a corrupted version of the spreadsheet that does not work. This is because these friction loss calculators use very complex formulas that the “stripped down” spreadsheet readers can’t handle. You need to install Open Office. I may have already mentioned this. 🙂
Remember, you can un-install Open Office when you are done if you don’t like it.
If you try to open the spreadsheets directly using your Internet browser they will probably open as Read Only and the spreadsheet won’t work. This is because the browser will not open an executable file directly. (It is trying to protect you from possible viruses.)
Solution: Right click on the link and “Save” the file to your hard drive. The actual wording varies, so depending on your browser you may select “Save link as..”, “Save Target as…”, etc. This should save the spreadsheet file to your hard drive. Then open it directly from your computer without using the browser plug-in. Tablet and phone users: you may need to get to a real computer to use the spreadsheets.
If the spreadsheets don’t work for you..
Do you have Open Office installed? If not, install it.
Are you using Open Office to read the spreadsheet? Sometimes another spreadsheet program will try to open it instead of Open Office.
Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in. Install & use Open Office.
Just dragging the spreadsheet link to your desktop may not actually save the file. You need to right click on the link and select Save as…
If you have an older version of Open Office you may need to upgrade it.
Try rebooting. I’ve experienced a problem where if I try to open one of these spreadsheets in a browser the computer gets messed up and won’t start Open Office. Rebooting fixed the issue.
SPREADSHEET CALCULATORS FOR PVC PIPE
Do not try to open these spreadsheets by left clicking on the links. Save the spreadsheets to your hard-drive first. See the explanation above.
Looping your mainline often allows you to use a smaller pipe size for it, so using a loop system can be financially advantageous on a large irrigated area. A looped mainline also provides maintenance advantages on larger sprinkler systems, and almost all large landscape irrigation systems, like parks and golf courses, utilize a loop mainline layout. For smaller sprinkler irrigation systems they often provide little or no advantages. As a general rule if you have less than 10 valve zones you are not going to get much of an advantage from a looped mainline.
What is a Looped Mainline?
The irrigation mainline is the pipe that runs from your water source to the individual valves that turn on a group of sprinklers or a drip irrigation circuit. When the mainline is looped that simply means that all or part of it creates a continuous loop. Typically a looped mainline starts with a single pipe coming out from the water source (pump, water meter, etc.) then the single pipe splits into two pipes. the two pipes loop around the irrigated area and then rejoin each other to create the “loop”. Zone valves would be located at various points along the loop to supply groups of sprinklers. Normally you would put a isolation ball valve on each leg of the loop at the “split”, the location where the pipes separate into the loop. A third isolation ball valve is placed on the far side of the loop, allowing the loop to be divided into sections. (see the sketch of a looped mainline below) Additional isolation valves may be added anywhere along the loop if desired, to divide it into more segments. The isolation valves allow you to shut down sections of the mainline for repairs while the rest of it may still be operational. For a large irrigation system being able to shut down only a portion of the system for repairs can be very advantageous. Very large irrigation systems may have multiple loops, sometimes one loop will even be inside of another loop.
How to Design a Looped Mainline
When using a loop you should use the same size pipe for the entire looped portion of the mainline. (This is not a hard and fast rule, just a strong suggestion unless you really understand hydraulics!) The pipe leading from the water source to the loop may need to be a larger size than the loop pipe. It is also OK to have mainline “spurs” off of the loop leading to other valves or faucets. While unusual, the pipe size of the spurs may also be larger than the size of the loop pipe if they need to be. Valves for sprinkler zones, faucets, quick coupler valves or any other equipment may be placed anywhere along the loop, as well as on the mainline leading to the loop or even on spurs off of the loop. Normally drinking fountains would be on a separate pipe and not connected to the irrigation system due to the possibility of water contamination from the sprinklers. See the article on use of backflow preventers for more information on contamination.
You can have multiple loops, but I suggest that if you do, you size the pipe using the outside perimeter (largest) loop, as if there were not any cross pipes within the loop (using the method that follows below.) Then after you have determined what size the outside loop pipes need to be, use the same pipe size for any smaller loops or cross pipes inside the perimeter loop. (Technical note: The pipe sizes of inside loops and cross pipes do not necessarily need to be the same size as the outside loop, it is just that using the same pipe size will almost always work. Using a smaller size pipe may work, but it may not. So it is advisable for non-experts to stick with the same size!)
Loop Mainline Calculations
You must calculate both the friction loss AND the velocity for a looped mainline. We’ll go through the process step-by-step. You calculate the pressure loss for the non-looped mainline section from the water source to the beginning of the loop in the normal way (as described in the Irrigation Mainline Tutorial), using the pipe size, flow rate, and length of the pipe section. I suggest that you use one of the the Friction Loss Calculator Spreadsheets I’ve created, they are easier for non-mathematically oriented folks to use than the old manual calculations using charts. Choose the proper spreadsheet for the type of pipe or tube. Then enter the size, the GPM, and the length of the loop. Pressure loss for spurs off of the loop are also calculated using this same method.
Pressure Loss in the Loop:
To calculate pressure loss for the looped section we simply will assume that the water flow splits, and 1/2 of the water is going around 1/2 of the loop and the rest of the water is going the other way around. To do this start by determining the “highest flow GPM” that is found on the looped section. If you are planning to operate only one valve at a time that would be the flow for the largest zone valve to be installed on the loop section. If you will be running more than one valve at a time then the “highest flow GPM” will be the combined flow rate for the largest group of valves that will operate at the same time. Once you have the “highest flow GPM” you calculate pressure loss in the pipe using the calculator. But for the looped portion you will enter 1/2 of that “highest flow GPM” for the flow in the looped pipe and 1/2 of the total loop length (all the way around and back to the beginning) as the length of the pipe. The calculator result is the pressure loss in PSI for the entire looped section.
Total Pressure Loss for the Mainline:
Just total up the pressure loss for the mainline leading to the loop, the pressure loss for the loop, and if you have any spurs add to your total the pressure loss for the single spur having the largest loss value. The total of those is the pressure loss for your entire mainline network.
As mentioned, this method of calculating pressure loss uses an averaging system that assumes that half the water goes one direction to the valve, and the other half goes the other direction. While this is not a perfect method, it works good enough for figuring out the pressure loss. However, the flow doesn’t really split evenly in both directions. In reality the flow balances in each direction based on pressure loss, with most of the flow in the loop going the shortest distance to a valve. So if one of the zone valves is just a few feet from the split point on the loop, almost all the flow will go through that short distance rather than going the long way around and back to the valve. Also in the event you make a repair you may close those isolation valves mentioned earlier. This will force all of the flow in a singe direction through one side of the loop. So we come to the second rule of loops, the pipe size for the looped section must be large enough to handle ALL of the flow in one direction. If it is not, you may create excessive velocity of flow in a section of pipe which can cause major, and very expensive, problems. This means you must check the velocity of the flow while assuming all the flow may go in a single direction around the loop. If the velocity is over 7 feet per second you may create excessive pipe wear and water hammer. Pipe wear can seriously shorten the life of your system, water hammer is much worse, let’s just say you don’t want it. (Look it up if you really are curious.)
Calculate the Velocity:
Using the Friction Loss Calculator Spreadsheets mentioned above enter the pipe size, the “highest flow GPM”, and any random value for the length of the loop. Ignore the pressure loss it gives you, just check the ft/sec velocity result it gives you. The velocity MUST be less than 7 ft/sec. If it is not, you will need to use a larger size pipe for the loop.
That’s it! You should now know how to create a looped mainline. If it seems confusing try rereading it, it is admittedly a bit confusing after the first reading! It really is simpler than it first sounds, be patient, grab some coffee, take your time. Go back and actually work through it one sentence at a time, study the sample sketch of a looped mainline and try sketching your own on paper. Practice a bit with the Pressure Loss Calculator to see what happens when you change the input values. It should start to make more sense.
Q. Is it possible to have two valves on at the same time or to run two irrigation valves at once?
A. Yes, it is often possible to run two valves at once. However there are several problems that can occur.
You must have a sufficient water supply for both valves to run at once. If the performance of the sprinklers suffers and you start seeing dry spots in the landscape, you obviously don’t have enough water. You may need to do some adjusting of the sprinklers as the water pressure operating them is likely to be less when two valves are on.
Both valves running at the same time may require more water than the pipe supplying them can reasonably handle. This can result in water hammer, or premature pipe wear/failure, due to high water velocity.
Water Hammer: Listen for a loud water hammer “thump” or “bang” noise when the valves close. A gentle thump is fine, but if the pipes reverberate from it that is not good. Run just one valve and listen to the sound when it closes. Assuming the irrigation is properly designed, that should be the “normal” closing sound. Now listen to the sound when both valves are closed together to see if it is significantly louder. If it is significantly louder, that is not good. You can possibly reduce or eliminate the water hammer problem by closing the valves separately, one at a time.
High Velocity: Premature wear due to velocity is harder to figure out. It generally isn’t a problem unless the water is really flowing fast through the pipe, like 8 feet per second or higher. The only way to determine if it is a problem is to do a couple of calculations. Start with the sprinklers. On top of each sprinkler is an identifying names and part numbers that tell you the brand, model, and hopefully the nozzle size. Write down that information for each sprinkler, then look up the water use (GPM value) for that sprinkler and nozzle at the sprinkler company’s website. (You may need to call the company’s help line to assist you, each brand and model is different so I can’t give exact instructions.) Now add together the GPM values for all the sprinklers that are running at the same time when two valves are turned on. This will tell you how much water the two valves require when running together. Next find the size and type of the water pipe that leads to the valves. (For example it might be a 3/4″ copper tube, or maybe a 1″ PVC pipe. It may be several different sizes and types of pipe, in which case you would use the smallest pipe size and type.) Using that information you can calculate the velocity of the flow in the pipe using the Friction Loss Calculator at https://www.irrigationtutorials.com/formulas.htm#sec8. Just enter the pipe type, size, and GPM into the calculator and it will give you the velocity.
If you decide to use a controller to operate the valves the controller must be a brand that provides sufficient amperage to run two valves at the same time (most do.) If you want the controller to run the valves at the same time, but start and stop them about one minute apart to reduce water hammer, you will need a controller that allows you to run two separate valve zones at the same time. Most controllers have a “stacking feature” that prevents them from doing this. You will need a controller that allows you to turn off the stacking feature. Most controllers can’t do this. You will probably need to enlist a knowledgeable controller salesman at a professional irrigation supply store to assist you in finding a controller that will work for this unique situation.
Q. We live on a river. I would love to plant some interesting things on the bank below our home but with the price of water these days I would love to be able to pump some river water up to do the job. Do you think that that is something we could do without spending a fortune? It would be great to have a soaker system.
A. First, you must have the right to take water from the creek, river,. pond, etc.. This almost always means you need to talk with the US Fish & Game Department, State regulators, and possibly the Environmental Protection Agency (or equivalent agencies for whatever country you are located in.) If you take water from a creek or pond or any other natural body of water in the USA without checking on the legal rights and requirements you can get into a lot of hot water, fast. The fines penalties and restitution costs can be enormous. So before you do anything, start doing some calling around. Be safe, not sorry. If you don’t know who to call, try calling the local County or Parrish Planning Department, they should be familiar with the agencies that regulate water and be able to point you to the right people.
Yes, from a physical standpoint it is not difficult to pump the water. The cost depends on how fancy you make it. My parents had a cabin on a river in Oregon. They simply had a small portable pump that sat on a concrete block and was chained to a tree. One end of a 15′ garden hose was attached to the pump intake, the other end of the hose had a piece of window screen tied around it to create a home-made filter and keep out small fish and junk. The end of the hose with the screen filter was tied to a concrete block and dropped into the river. The pump outlet was attached to a second garden hose, this one was 150 feet long. A long extension cord went from the pump to the power outlet at the cabin. They put a sprinkler on the end of the hose, placed the sprinkler where they wanted water, then plugged in the pump. Simple, cheap. You could easily semi-automate that by simply plugging the pump’s power cord into a timer to turn it on and off.
A fancier system is certainly possible. The pump still needs to be portable in most cases. The pump has to be mounted less than 8 feet above the water level (the closer the better.) You need a pad of some sort to put the pump on, but it is best if the pump can be easily moved, especially if the water level fluctuates in the creek or floods. There is also the possibility of using a submersible pump. A submersible should not sit on the bottom of the stream if there is a lot of mud and silt in the water that would get sucked into the pump. If you have a floating dock or a pier an alternative is to place the pump on it (or hang it below the dock in the case of a submersible pump.) Submersible pumps are often strapped to the side of pier pilings. Be sure to read installation instructions for the pump, many pumps have very specific positioning requirements, some submersibles must be installed inside a special sleeve.
You can get about as fancy as you want- using automatic controls to start and stop the pump and also to open and close multiple irrigation valves. Many irrigation controllers have built in circuitry that will start and stop the pump for you using a electrical relay. If you do it yourself, and you need only something similar to my parent’s small pump you could probably install a pump for around $200.00. The price can go up fast as you get bigger and fancier, $1000.00 is not an out of line figure for a pump system capable of watering an acre or so of yard. The wiring for the pump automated controls is a bit tricky, so most people would want to have that part done by a electrician. How much that costs depends on the length of wire needed to reach the pump. One option to look at when you get to larger irrigation systems is a pre-constructed pump unit. This consists of the pump and all of the needed controls for it pre-installed and pre-tested on a metal frame. You just hook up the pipes and wires to it and turn it on.
You may also need a storage tank for the water, especially if you have a small water supply (like a creek.) That way you could pump a small flow continuously from the creek to fill the tank. Once in the tank the irrigation water would either be pumped out of the tank to the irrigation system by a second pump, or if the tank can be located 30′ or so higher than the level of the irrigated area, you could use gravity flow from the tank. (If you want to use sprinklers the tank would need to be at least 60 feet higher to create enough pressure for a small sprinkler.) The tank will probably need to be a lot larger than you think. Typically they are 5,000 gallons or larger. To find out what size tank you will need you need to determine how much water it will take to irrigate your area. See How to Estimate Irrigation Water Quantity Needed for instructions on estimating your water requirements.
One last word of warning before you start: PLAN FIRST, BUY LATER! Don’t run out and buy an “irrigation pump” first! Most pumps sold with the description “irrigation pump” are designed to operate a single sprinkler on the end of a hose. You need to design the irrigation first, then you will now how much water volume AND water pressure the pump will need to produce. The Sprinkler System Design Tutorial takes you through the process of irrigation system design and finding the right pump size. It’s at https://www.irrigationtutorials.com/sprinkler00.htm
Q. I’m designing a pump system from a lake and have read and understand your calculation of FT HD needed for pump selection but it seems that the upstream (uphill) pipe diameter would be a factor in the calculation. I was going to use larger pipe to reduce pipe resistance and valve pressure drop but it seems to me the weight of the additional water (back pressure) would be higher for a larger diameter pipe than a smaller one. It must be easier to push water up a 3/4″ column than a 1 1/2 inch column. You mention nothing about this. Excluding pipe resistance, does the pipe diameter play a roll in taxing the FT HD required? Rephrased – Does a larger diameter column of water have any effect on the static pressure or force required to move it?
A. The short answer is that the larger pipe would be better because there would be less pressure loss in the pipe. This is due to less “friction loss” as the water flows through the larger size pipe. The larger amount of water in the bigger pipe has no impact on the water pressure. A smaller pipe may create more friction loss however, so it can be worse than a larger pipe. To find out, you need to calculate the friction loss in the different sizes of outlet pipe based on the flow and pipe size. See the Friction Loss Calculators to calculate the friction loss in pipes.
More detailed answer:
One of the really hard to grasp principles of hydraulics is the relation of volume of flow, pressure, and the weight of water. Odd as it seems a larger pipe will actually be easier for the pump. It’s not the volume of water, but the height it is lifted that matters. In a way this is a variation on the old saying “which weighs more, a pound of feathers, or a pound of lead?” Obviously both weigh a pound! This version could be phrased “which is easier for the pump, 5 GPM in a 1/2″ pipe or 5 GPM in a 2” pipe? Neither because 5 GPM is still 5 GPM regardless of the pipe size! Yes, you would need more power if you were actually lifting more water, also we would need more power to lift the water higher, but neither is not what is happening. The amount of water nor the height we are lifting hasn’t changed.
The other issue here is flow through a pipe. This is the issue that actually makes the smaller pipe potentially worse than the larger. Because the smaller pipe is smaller it is harder to force the water through it. The resistance of the walls of the smaller pipe causes pressure loss as water flows through. this is commonly called “friction loss”. How much friction loss occurs depends on the flow rate and pipe size. Both higher flows and smaller pipes sizes result in greater friction loss. This is the only reason a smaller pipe would be worse than the bigger pipe. How much worse is dependent upon the actual flow rate and pipe size.
As a general rule (ie: not always true, but is most of the time) the pipe size of the pump outlet is almost always smaller than the size of pipe that will provide optimal flow from the pump. In other words, if a pump has a 1″ threaded outlet, it is very likely that a 1 1/2″ pipe would be attached to the 1″ outlet for use as the outlet pipe. Pump manufacturer’s tend to use smaller size inlets and outlets to save money.
More technical answer:
Think about feet of head. As discussed in the Pump Tutorial, the number of feet of water depth determines the water pressure. So 80 feet of water depth equals a pressure of 80 ft. hd. This pressure will be the same regardless of the pipe size. The water pressure at the bottom of an 80′ high 1/2″ pipe is exactly the same as the water pressure at the bottom of an 80′ high 6″ pipe, even though the 6″ pipe holds a lot more water. A pump actually works by creating water pressure. So for the pump there is no difference between pumping into either size pipe, the water pressure required to move the water into the bottom of both pipes is the same. Now the pressure lost as water moves through the two pipes will be different. Assuming a high rate of flow, a lot more pressure will be lost due to friction in the smaller pipe. So for that reason using a larger pipe will be better. Depending on the flow, however, it may be only very marginally better. To find out you need to calculate the friction loss in the outlet pipe based on the flow and pipe size. See the Friction Loss Calculators to calculate the friction loss in pipes or tubes of various types.
The amount of water needed for irrigation depends on many different factors. A reasonably accurate estimate of the amount of irrigation water needed can be made using Eto data for your actual zip code. “Eto” is the amount of water needed for irrigation, based on scientific research. You can find the historic Eto for any zip code in the USA at the website http://www.rainmaster.com/historicET.aspx courtesy of the Rainmaster irrigation controller company, who makes very good “Smart” irrigation controllers. I use one of their Eagle model controllers on my own home. (Rainmaster get a plug from me as well as a big “thank you” for providing the ETo look up service online.) Unfortunately the Eto value only tells you how many inches per day are needed, which for most folks is a meaningless value. It makes more sense if you think about rainfall which is often also measured in inches. If you find you need 0.20 inches of irrigation, then 0.20 inches of rainfall would provide the required water. But most people in the USA want a value in gallons, which requires you to provide a little more information about your yard. Then you plug the values into a simple formula, and do a little multiplication and division on any calculator.
Formula to calculate the gallons of irrigation water needed per day: (Eto x PF x SF x 0.62 ) / IE = Gallons of Water per day
Values for the formula: Eto: Get this from http://www.rainmaster.com/historicET.aspx . Enter your zip code, or a nearby zipcode, and the website will give you the average daily ET value for each month of the year. Use the highest value or the “suggested reference value”. Usually they are the same thing.
PF: This is the plant factor. Different plants need different amounts of water. Use a value of 1.0 for lawn. For water loving shrubs use .80, for average water use shrubs use 0.5, for low water use shrubs use 0.3.
SF: This is the area to be irrigated in square feet. So for a 30 foot x 50 foot lawn you would use 1500.
0.62: A constant value used for conversion.
IE: Irrigation efficiency. Some irrigation water never gets used by the plant, this value compensates for that. I suggest using 0.75 as the value for this. Very well designed sprinkler systems with little run-off that using efficent sprinklers can have efficiencies of 80% (use 0.80). Drip irrigation systems typically have efficiencies of 90% (use 0.90).
A 1500 square foot grass lawn in zip code 85232 (Central Arizona)
Start by looking up the Eto for zip code 85232 at the Rainmaster website, which displays a suggested reference value of 0.3 inches per day using June, the driest month of the year in that area.
Now rewrite the formula inserting your values into it:
0.3 (Eto value) x 1.0 (grass value) x 1500 (sq ft) x 0.62 ÷ 0.75 (efficency factor) = gallons of water per day
Now do the math, just punch the values into a calculator and get your answer:
0.3 x 1.0 x 1500 x 0.62 ÷ 0.75 = 372 gallons per day
We could figure out the average daily water use for other months of the year also. Just use the same formula but insert the Eto value from the Rainmaster website for the month you want to get a valve for.
Remember this calculation just gives you an estimated value. There are many other factors that could make this value higher or lower. When planning for how much water a system that has not yet been designed or installed will use, it would be very wise to allow for error by adding 10% or more to the daily water use needed. It is generally better to have too much water, than to have too little! Play it safe!
A common related question is “how much water pressure will my irrigation system need?” The answer depends on a lot of factors, but as a rule of thumb, I would suggest 50 PSI of water pressure as a good starting point for sprinklers, 45 PSI for drip systems. If you have a large yard and want to put the sprinklers farther than 30 feet apart you will need more pressure. For example, if you want your sprinklers 45 feet apart you will probably need 65 PSI of water pressure. To get a real value you will need to create an actual sprinkler system design. See the Landscape Sprinkler Irrigation System Design Tutorial .
Never buy a pump, sprinklers, or any other materials before your sprinkler design is completed!
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