Tag Archives: hydraulics

How to Reverse Engineer a Sprinkler System

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.

  1. 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. 
  2. 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.
  3. 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!
  4. Repeat for each valve circuit.
  5. 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.
  6. 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.

How to Find Your GPM & PSI – Gravity Flow

caution
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.

Tip

There is a Design Data Form that you can print that will make things easier for you. If you would prefer a pdf version see PDF Design Data Form.

What in the World?

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.

cautionDo 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).

pencil

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).

pencil

Enter the “Tank Inflow GPM” on your Design Data Form.

 

 

How to measure flow in gravity flow systems.
Simple Gravity Flow Water System using a Tank.

 


Pressure Head:

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.

pencil

Enter the pressure you calculated or measured in the space labeled “Design Pressure”on the Design Data Form.

cautionPossible 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!

pencil

Enter the “Initial Design Flow” on your Design Data Form.

caution

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.

why

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.


Related things…

Filtration:

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)


This article is part of the Sprinkler Design Tutorial Series
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Elevation Pressure Loss in Irrigation Systems

Hills, Valleys, & Slopes:

“Consider the slope or you’ll look like a dope!”

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!


Hydraulics 101

why
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.

 sprinkler11aIn 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.

Example:

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.

Another example:

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.

pencil 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.


This article is part of the Sprinkler Design Tutorial Series
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By using this tutorial you agree to be bound by the conditions and limitations listed on the Terms of Use page.


 

Irrigation Lateral Sprinkler Pipe Size

Friction Loss:
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.

Chart Method:

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

See Determining Sprinkler Pipe Size Using a Pipe Sizing Chart for detailed step-by-step instructions.

Calculate the PSI/100 value:

( ____ 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

PSI/100

0.2

0.5

0.8

1.0

1.5

2.0

3.0

4.0

5.0

6.0

SIZE

2.2

3.3

4.4

5.0

6.2

7.1

8.5

10

11

13

¾”

3.8

6.3

8.1

9.2

11

13

17

20

22

24

1″

7.1

12

15

18

22

25

31

36

37

37

1¼”

11

16

22

24

31

35

44

48

49

49

18

30

40

44

57

65

76

76

76

76

2″

28

46

60

67

83

96

114

114

114

114

2½”

46

75

100

112

140

162

165

170

170

170

3″

87

140

185

208

250

280

280

280

280

280

4″

255

410

540

600

600

600

600

600

600

600

6″

 Flows shown red are over 5 feet/second. Use caution!

Instructions:

    1. Find your PSI/100 value in the top blue row.
    2. Read down the column to the value equal to, or higher than, the GPM in the pipe section.
    3. Read across to the pipe size for that section in the right column.
    4. 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.
  • Where’s the 1/2″ pipe?  See “why not 1/2″?”

CONFUSING?  DON’T PANIC:   For detailed instructions see the page Irrigation Pipe Sizing Chart for Laterals.



 

Overview: TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE

See Calculating Sprinkler System Pipe Size Using a Spreadsheet for detailed step-by-step instructions.

This method involves trying various pipe sizes until a good combination is found.  A spreadsheet does the calculations.

You will need a spreadsheet friction loss calculator:  Friction Loss Calculator Spreadsheets

Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.

Use the spreadsheet friction loss calculator to determine the pressure loss:

  1. Download and open the Friction Loss Calculator.
  2. There is a line on the spreadsheet for each section of pipe.
  3. Start with the pipe section after the control valve and work out to the farthest sprinkler.
  4. Select 3/4″ pipe for the pipe or tube size. (See “why not 1/2″?”)
  5. Enter the GPM for the section of pipe.
  6. Enter the length of the section of pipe.
  7. Use an error factor of 1.1
  8. Go to the next line down and repeat steps 4-7 for the next pipe section.
  9. The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
  10. At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
  11. 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.

Confused? For detailed instructions on using the spreadsheets see What Size Pipe for Sprinkler System Laterals?


This article is part of the Sprinkler Irrigation Design Tutorial
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||| Tutorial Index ||| Next Page >>>
By using this tutorial you agree to be bound by the conditions and limitations listed on the Terms of Use page.


Calculating Sprinkler System Pipe Size Using a Spreadsheet

THE GROUND RULES

First a tip that just may save your behind!

When in doubt, always use a larger diameter pipe!

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.

Detailed Instructions:

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:

Example Plan

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:

  1. Download and open the Friction Loss Calculator.
  2. There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
  3. Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
  4. Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
  5. Enter the GPM for the section of pipe.
  6. Enter the length of the section of pipe.
  7. Use an error factor of 1.1
  8. Go to the next line down and repeat steps 4-7 for the next pipe section.
  9. The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
  10. 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.

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Irrigation Pipe Sizing Chart for Laterals

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(Sometimes called a Pipe Sizing Table.)

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.

Definitions:

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.

PEX: Do not use the chart for PEX pipe.  PEX has extremely limited flow.  Use the Trial & Error Sizing Method for PEX!

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)

0.2

0.5

0.8

1.0

1.5

2.0

3.0

4.0

5.0

6.0

SIZE

2.2

3.3

4.4

5.0

6.2

7.1

8.5

10

11

13

¾”

3.8

6.3

8.1

9.2

11

13

17

20

22

24

1″

7.1

12

15

18

22

25

31

36

37

37

1¼”

11

16

22

24

31

35

44

48

49

49

18

30

40

44

57

65

76

76

76

76

2″

28

46

60

67

83

96

114

114

114

114

2½”

46

75

100

112

140

162

165

170

170

170

3″

87

140

185

208

250

280

280

280

280

280

4″

255

410

540

600

600

600

600

600

600

600

6″

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:

  1. Start with the pipe section farthest from the valve (connecting to the last sprinkler head.)
  2. Find the PSI/100 value in the top row (blue text, directly under the heading PSI/100.)
  3. Read down that column and find a value equal to, or higher than, the GPM in the pipe section.
  4. Now read across to the right column to find the pipe size to use for the pipe section.
  5. Repeat steps 3-5 for the other pipe sections in the lateral valve circuit.

Notes:

  • 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.
  • No 1/2″ pipe?  See my explanation of why I don’t use half-inch size pipe.

 

Example Using the Pipe Sizing Chart:

 Example Sketch of a Sprinkler System

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.

 

 Previous Page of Tutorial      Sprinkler Design Tutorial Index      Next Page of Tutorial

 

Using A Smaller Pipe to Increase Water Pressure

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.

Bernoulli’s Principle, Venturi Effect, & Flying Pigs

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!

Spreadsheets for Calculating Pipe Pressure Loss

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..

  1. Do you have Open Office installed?  If not, install it.
  2. Are you using Open Office to read the spreadsheet?  Sometimes another spreadsheet program will try to open it instead of Open Office.
  3. Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
  4. If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in.  Install & use Open Office.
  5. 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…
  6. If you have an older version of Open Office you may need to upgrade it.
  7. 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.

Cl 125 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 160 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 200 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Cl 315 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 40 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

SCH 80 PVC pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

 

SPREADSHEET CALCULATOR FOR POLYETHYLENE TUBE

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.

Polyethylene Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

 

SPREADSHEET CALCULATOR FOR PEX TUBE

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.

PEX Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

 

SPREADSHEET CALCULATORS FOR COPPER TUBE

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.

Type K Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type L Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

Type M Copper Tube Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.

 

SPREADSHEET CALCULATOR FOR SCH 40 STEEL 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.

SCH 40 Steel Pipe Calculator Spreadsheet for Velocity, Friction Loss or Pressure Loss.  (also use for galvanized steel)

 

 

 

Adding a Booster Pump with a Well Pump

Q.  I have a shallow well that was drilled this summer and a centrifugal pump pulling up about 15 gallons/min (HAPPY!).  The problem, it will only produce somewhere around 30psi (sad!).  Am I able to add a booster pump to this setup to produce more psi or should I just forget it and go for a submersible pump?  Obviously the booster pump would save me $…

A. You can add a booster pump but it is tricky.  The flow range of the booster pump needs to match that of the existing well pump.  Using two pumps will probably use considerably more electricity than a single new pump, especially if it is a submersible.  Submersibles are by nature more efficient than a centrifugal pump at the top of the well and now you are adding the friction drag of two pumps rather than one.  I can’t tell you how much the electricity cost difference would be, that’s beyond my knowledge level.  But ongoing electricity cost is certainly something to look at.

Essentially when you couple two pumps together they are going to have to play nice with each other.  You don’t want one to over-power the other and do most all the work while the other just causes drag.  Plus you need to deal with the wiring issues and how you will start the two pumps.  Hopefully they would both stay primed so, in most cases, you could start them both together using the irrigation controller connected to a relay connected to the pumps.  You might need two relays if the pumps exceed the capacity of the relay.

Finally you will need to deal with figuring out if and how you will handle problems such as the malfunction of one of the pumps.  If one burns out the drag created by the burned out pump could very quickly burn out the other.  Hopefully you would quickly notice the problem, since the irrigation system would not work well at all if only one pump was running.  But what if you were on vacation when it happened?

You probably should get a local pump professional who knows his/her stuff and has experience with two pump systems to help you if you use two pumps.

Basically if you want to keep this a simple do-it-yourself project I’m thinking buying a new submersible for your well would be the better way to go.

Valves Downstream from Anti-siphon Valve?

Q.  I have manual shut-off valves installed downstream from my electronic anti-siphon valves.  I installed them to turn off the water to parts of my yard where I grow annuals and only need to water for a few months out of the year.   I would really appreciate it if you would explain why valves downstream cause the anti-siphon valve backflow prevention to fail.

A.  If there are some sprinklers that are not shut off by the downstream valves (ie; there is always a sprinkler that will be on when the anti-siphon valve is on) then you should be fine.  The key to this is that when the anti-siphon valve is closed the water remaining in the pipe downstream of the anti-siphon valve MUST become depressurized.  Depressurizing normally occurs when you shut off the anti-siphon valve and the remaining water pressure in the downstream pipes is released through a sprinkler.   But if you have a valve downstream of the anti-siphon valve it will trap pressurized water in the pipe between the anti-siphon valve and the downstream valve and not allow it to “depressurize”.  Note that sprinkler heads with built-in check valves will also hold the water pressure in the pipe.  That is why when using anti-siphon valves you should remove the check valve from at least one of the sprinklers on each valve circuit (normally you would remove it from the sprinkler on the circuit with the highest elevation.)  the check valves are easy to remove from the sprinklers, normally you just unscrew the sprinkler cap and lift out the riser assembly.  You will see a rubber washer attached to the bottom of the riser assembly, pull it off.  That rubber washer is the check valve seal, with it removed the check valve won’t work.   Now reassemble the sprinkler.

How an anti-siphon valve works:
The  anti-siphon valve works by use of a little air vent that is located on the downstream side of the actual valve.  Look at the anti-siphon valve you will see there is a large cap directly above the water outlet of the valve, the air vent is under this cap.  If you look closely at the lower perimeter of the cap you will see holes or slits that allow the air to move in and out of the vent.  When the anti-siphon valve is turned off the pressure drops in the pipes downstream from it as the remaining water flows out of the sprinklers.  When the pressure drops the little air vent drops open and lets air into the pipe right behind the valve.   This air goes into the pipe and breaks any siphon effect (“anti-siphon”) so that sprinkler water can’t be drawn backward through the valve into the potable water supply.

(Water from the sprinkler pipes can be siphoned back into the water supply system when pressure is lost in the water supply system.  For example, the water company might depressurize their pipes to make repairs.  It doesn’t happen frequently, but it does happen.  When the pressure drops the flow reverses and water from the sprinkler pipes, along with dirt and other yucky stuff, can be sucked in through the sprinklers and then into the water supply system.  When the pressure returns that dirty sprinkler water may go back into the sprinkler system, but it may just as easily go to your kitchen or bathroom sink.  So why wouldn’t the closed anti-siphon valve stop this from happening?  After all the purpose of a valve is to stop water from flowing through it when it is closed, right?  Yes, of course, if the valve is a manual valve.  But electric solenoid valves are “directional” valves.  What that means is they are designed to stop the flow when the water is flowing in one direction only.  When the water flows backwards they don’t fully close!)

What the downstream valve does:
If you have another shut-off valve after the anti-siphon valve, then the water on the downstream side of the anti-siphon valve will stay pressurized even when the anti-siphon valve is closed.  This water pressure holds the little air vent in the closed position so it can’t let in air, and therefore the siphon effect is not broken.  This means the anti-siphon part of the valve will not work.  Even worse, when the little vent is held closed for days at a time due to the constant downstream pressure, it eventually just sticks in the closed position.  Then even if the pressure drops the anti-siphon won’t work.

My Friend or Irrigation Person Says This is All Just Something  YOU Made Up!
Unfortunately, this wrong practice of installing valves after an anti-siphon valve is pretty common in the irrigation industry.  I’ve been called some pretty ugly names over this issue.  Fortunately for me, you don’t have to take my word for it.  Tell your friend/buddy/pal to read the box the anti-siphon valve came in.  It says right on it “do not install valves downstream” or something similar.  If you don’t have the box or it didn’t come in one, then go to the manufacturer’s website and find the anti-siphon valve installation instructions.  You will find that same warning.  Here’s a sample from Rainbird if you want to check for yourself:  Rainbird Anti-siphon Valve Operation Manual. See the section that starts with the heading “CAUTION”.

Huge Grass Yard with Minimal Water Supply

Q.  My pump produces 10 GPM at 45 PSI or 7 GPM at 65 PSI.  Do you think the flow is decent for my yard?   When I had pro’s quote my job (which is why I’m doing it my self as the numbers were huge) they all said I’d need about 40 sprinkler heads.

A.  My gut feeling is that you don’t have enough water capacity from your pump and/or well to irrigate the size of area you have planted in lawn.  (Don’t panic yet, keep reading for some suggestions.)  40 sprinklers would be a lot to try to run off of 7-10 GPM of flow.  But it depends greatly on your climatic location and water needs.  If you only need irrigation for periodic supplemental watering you may be OK.  The rule of thumb is that 10 GPM will water about 1/2 an acre of lawn, assuming you need to water about 3 times a week to keep the grass lush.  So if you need to water only twice a week, then you could water more area with 10 GPM of water flow.  Also, the rule of thumb assumes you only wish to water during the night hours.  You could water more area if you are willing to water 24/7 during the peak hot season.  Keep in mind that if you share the water use with your house that 24/7 watering might not be a good idea as you won’t have any water left over for use in the house.  When your spouse gets in the shower and no water comes out because the sprinklers are using all of it, things are not going to be pretty!

The rule of thumb is that it takes 20 GPM to water an acre of lawn in a hot climate area.  So if you have a larger lawn you may need to think about adding a new pump and/or well.  I really don’t have enough info to say for sure since I don’t know the size of your yard or your climate location.  Take a look at this article which will show you how to calculate how much water you will need for your exact situation:
https://www.irrigationtutorials.com/how-to-estimate-water-useage-required-for-an-irrigation-system/

Options to consider if you don’t have enough water:
An option a lot of people in rural areas use is to create watering zones around the property.  They heavily water the area right around the house, possibly 25′ or so out from the foundation, sufficient to create a really lush green lawn.  Then for the next 25-50′ out they apply just supplemental water, watering maybe once (or twice) a week in hot weather.  This supplemental water area would tend to yellow a bit and show stress during the hottest part of the year.  Then they have a “no irrigation” area at the far reaches of the property that gets no irrigation water at all.

Tips on designing supplemental water areas: The sprinkler system design for a supplemental water area should be the same as for the lush area.  In particular, the distance between the sprinkler heads should be the same for both the lush and supplemental areas.  The only difference is that the supplemental area doesn’t get watered as often.  Don’t try to stretch out the sprinkler spacings to use less heads in the supplemental area, that will result in problematic dry spots that create a splotchy look to the grass.  A splotchy lawn really looks bad.  If you space the heads correctly then you will get a uniform looking lawn in the supplemental area, it will just have a yellower tint to it, and it will not be nearly as noticeably “ugly” as a splotchy lawn.   Also by designing the supplemental area for “full water coverage” you have the future option of turning that area lush by simply adding a new well or bigger pump.  If you skimp on the sprinkler spacing it is really difficult to correct the spacing problem if you ever wanted to make it lush.  You can add more heads, but I can tell you that over my 35 years in the business I have never had a customer who was happy with the results of adding more heads.   Your only real option to fix a system with heads installed too far apart is to rip it all out and start over.  Expensive!!!

Another option: If you don’t have the water supply or money to do all of it right, then install it in phases.  Start with the area around the house.  Then add more sprinkler zones to water the areas farther out from the house each year as you have funds and time.  Add another pump and/or well later when you need the water.   The critical thing is to “do it right” in regards to the sprinkler spacing and resist the temptation to stretch spacings between the heads to stretch the water supply or save money.  The results are always disappointing if you do that.

Can I Run Two Irrigation Valves at the Same Time?

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.

Can I Pump my Irrigation Water from a River, Creek, or Pond?

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

Why not use those huge sprinklers?

Q.   I don’t understand why I can’t apply the same guidelines from your tutorial and choose 2 or 3 heads with 70 foot spacing?  That would mean a lot less sprinkler heads on my large acreage lawn.  Other than not being able to aim them as selectively, I’m missing the reasons I shouldn’t go this route.  But you caution against it, so I’m sure I’m missing something.

A.  Someplace around 55 foot spacing things start to get all screwy.  They do make sprinklers that will shoot that large radius.  They are pricey, the cost works out about the same per square foot irrigated regardless of the spacing (funny how that happens!)   The problem is there’s just too much wind drift, evaporation, etc. at those wide spacings.  Plus to get water to fly those long distances you need big, heavy water drops with lots of momentum.  Those big drops just beat the crud out of the lawn, and cause compaction of the soil.  Think of what it would be like if a really hard rain storm occurred each time you watered.  Where the huge droplets don’t compact the soil they may erode it.  Golf courses and parks have fought this problem for years.  Most city parks have now settled for 55′ spacing rather than deal with the grief of citizen complaints about dead grass.

The bigger radius heads work better with pasture grasses, where long unmowed grass blades soften the droplet impact and a few dry spots and general “ugliness” aren’t as important.

It also takes lots of water pressure and volume to get that water out there.  70 feet radius means you need 70 PSI and 30 GPM at each sprinkler head.  That means probably 85 PSI or more coming out of the pump.  Most systems with big sprinklers like that run at over 100 PSI of pressure, which means lots more wear and tear on the system, and a shorter life-span.   With those high pressures, design becomes critical, mis-design a single thing and it is unforgiving; water hammer can rip the whole system apart in a big hurry.

Then there is the safety issue.  You ever been hit by a 30 GPM stream of water flying from a nozzle at 70 PSI?   I have, it knocks you on your butt and hurts like hell!  Keep in mind that the really big impact guns used on farms reverse with enough force to kill you if you are struck in the head by the sprinkler arm.   Liability is the biggest reason that parks and golf courses are ditching the big water guns for smaller sprinklers.

Bottom line is that using big radius sprinklers just gets really tricky and the results are ho-hum at best.  It’s not a good solution in the vast majority of situations.  If you do want to mess with it, get professional help with the design.  Most of the sprinklers over 70′ radius are only sold by agricultural irrigation dealers.

Large Radius Sprinklers
Big Irrigation Guns in Florida

Outlet Pipe Size for Pump- is a Bigger Pipe Better?

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.