Almost any major maintenance problem in an irrigation system will cause a unusual pressure level or flow level in your irrigation system. Therefore pressure and/or flow monitoring is a good way to detect problems. Most of the time the response to a abnormal pressure or flow level would be to shut down the system, or possibly to shut down the current valve zone and try another one. Irrigation systems are typically shut down using what is called a master valve. A master valve is a single valve located at the water source that can shut off all the flow of water into the irrigation system. For more details see my article on master valves. On systems with a pump you will probably want to shut off the pump. Sometimes, as with booster pumps, you will need to both shut down the pump and close a master valve.
So what problems might an abnormal pressure or flow indicate? A very low pressure may indicate that perhaps the pump is broken (if you have a pump), an intake screen is clogged, a filter is dirty, a valve failed to open, or a pipe has broken. Abnormally high pressure could be the result of a valve not opening when it should, a dirty filter (if the pressure is measured upstream of the filter rather than downstream) or some obstruction in the pipes. Low flow could indicate a valve failed to open, a filter is dirty, or that a pump isn’t working as it should. High flow could indicate a broken pipe, a broken sprinkler, or a valve that is stuck open. In most cases monitoring either flow or pressure is sufficient as opposed to monitoring both.
How to Monitor Your Irrigation System
There are a number of different ways to detect and respond to abnormal pressure or flows. Following are a few or these. If you would like to suggest other methods, please contact me. I realize this is not an exhaustive list.
Use a Smart Irrigation Controller that has a Sensor Input and Response Feature:
This is probably the easiest way to add pressure detection and response. It is also what I consider to be the preferred method, as it is reliable and gives you the most control. Some high-end irrigation controllers can use an electronic sensor hooked up to the mainline pipe to monitor the water in the irrigation system. Some of these controllers use flow sensors, some use pressure sensors, some can use both types. These controllers with advanced features are typically sold as Smart Controllers and are expensive compared to ones typically found on a residential irrigation system. Prices for these controllers typically start around $300.00 and go up into the thousands for ones that handle dozens of stations. Some controllers can increase the number of zones with an irrigation controller expansion module. But then you get a lot more with them too. They are sold through professional irrigation supply stores, both online and locally.
WARNING: Be sure the controller will do exactly what you want BEFORE you purchase it! Not all controllers marketed as “Smart Controllers” have these sensor input features, many only work with specific types or even models of sensors, and some controllers may not provide the response options you want or need. You need to research the controller carefully. Don’t rely on a simple check list of features! “Sensor input” can mean almost anything, you need details! I have seen controller feature lists where the unit sounded fantastic and ultra flexible, only to discover after closer examination that the actual response features don’t do what I need or want. Read the actual owner’s manual (most controller manufacturer’s have them available on their websites) to see what the true capability of the controller is. Read the sections of the manual on how to hook up the sensor, then there will also be a separate section on how to program the sensor you should look through. Some controllers allow for time delayed responses, some don’t. If you have a pump you will almost always need a time delay feature to bypass the sensor when the pump is starting up. Even those controllers that do allow you to add delay times may not allow as much or little time as you need. It is critical that you do as much research as possible before you go to the expense and effort of purchasing, installing and programming the controller.
For example, I have a Rainmaster Eagle Smart Controller on my own irrigation system, as well as using it on the majority of the commercial systems I design. This particular Smart Controller has flow sensing capabilities, but it does not have built-in pressure sensing capability. It does have a delayed response allowing delays of 1-6 minutes, but only in one minute intervals. It will also allow the use of one additional simple on/off type sensor (most controllers have a circuit for this type of very simple sensors. A simple rain switch is an example of this type of sensor.) It has an audible “chirp” alarm that alerts you that a sensor response has been activated. While this particular controller meets my needs, it certainly will not meet everyone’s. Almost every major irrigation company makes a Smart Controller, and each has different features and capabilities. Be sure you are using up-to-date resources when checking out models. Smart Controller models are introduced each year, and often the capabilities of existing models change from year to year, so it is hard to keep up with them.
When using a controller with a pressure and/or flow sensor you start by installing the actual sensor on the mainline pipe. The method varies with the brand and model of sensor, most are pretty easily installed. The sensor is wired to a special terminal on the irrigation controller. Typically the wire used must be a special shielded communications cable, rather than standard irrigation valve wire. Consider installing communications cable in PVC conduit to protect it, as it is very sensitive to even the smallest nicks from shovels, animals digging it up, or rodents chewing on it. Most pressure sensors work by sending a reading of the current pressure to the controller every few seconds. A typical flow sensor has a small paddle that turns as the water flows through the pipe. Flow sensors normally send a signal based on the amount of flow, for example they might send a signal each time 5 gallons of water has flowed past the sensor. The controller then interprets that data from the sensor and responds. In most cases you will pre-decide what the response will be when you set up the controller. For example; if you have a system with a pump, you could program the controller to shut down the irrigation system if the pressure was below 10 PSI for more then 2 minutes during the set irrigation period. The 2 minute qualifier (delay) for shut down would allow the pump time to pressurize the system during start up and also avoid “false alarms” caused by brief dips in pressure.
Using a Simple Pressure Switch with a Pump Operated System:
This method is for those with pumps. What I am describing here is for emergency shut off only. I’m assuming you already have something set up to turn on or off the pump during normal irrigation operation. That might be a standard pressure tank with a pressure switch to control it. Or you may be using the pump start feature on the irrigation controller to actually start and stop the pump using a 120v relay. The new pressure switch we are talking installing in this case is used only to detect pressures that indicate a problem and turn off the pump. So if all is hooked up properly, in the event of blockage or no water going into the irrigation system the pressure will drop and the new pressure switch will shut the pump off.
This method requires that your irrigation system is leak free and can hold pressure for days between irrigations. If the system is not leak free see #4 below.
1. Make sure you have a really good quality spring-loaded check valve on the irrigation mainline pipe. The check valve goes someplace after the pump, but before the pressure switch. A good quality check valve is needed to keep the water from leaking backwards out of the system through the pump. Typically the self-priming feature of the pump is not good enough by itself to do this, you need a separate check valve.
2. You will need to use a pressure switch that works backwards from normal ones used for household water systems, since you want the switch to shut off the pump at low pressure (standard switches used on household water systems turn on the pump at low pressure.) Some switches can be wired to work either way, others can’t. Keep in mind that the low end on many common pressure switches in around 25-30 PSI. That might be a bit higher than you want for a low end shut off, especially if your system will be operating at less than 45 PSI. You don’t want accidental “false” shut offs since the only way to get the system back on will be to manually start the pump and hold it on until the pressure is back above the shut-off level.
3. There a problem to be dealt with. The problem is that valves close slowly, taking as much as a minute or two to close after the controller tells them to. At the end of the last irrigation cycle a typical controller closes the last valve and immediately shuts off the pump. But it takes the valve several seconds up to a minute or two to actually close. During this closing period the system will depressurize. With no pressure in the system the pump will not restart for the next irrigation cycle, because the low pressure shut-off switch is detecting low pressure and shutting off the power to the pump. There are two ways to deal with this.
A. You can fool the controller into keeping the pump running after the last valve circuit has finished watering. Your controller needs to have the capacity for one extra valve on it to do this, so if you have 10 valves you will need a controller with 11 stations. The last station on your controller needs to not have a valve attached to it. Program 1 minute of time on that last station. Now the controller thinks it is operating one last valve, so it keeps the pump running. That will keep the system pressurized while the final valve closes. If one minute is not enough time for the final valve to close then add another minute of run time to that last empty station.
B. Some controllers have a built in delay feature that keeps the pump running after the last valve closes. This feature keeps the pump start circuit energized, which keeps the pump running for a minute or two after the last valve is signaled to close. This gives the valve time to close before the pump is shut off. Some less expensive controllers have this feature. But typically only high-end controllers have this feature, so this method isn’t very practical. If you are going to buy an expensive controller you might as well forget about using a pressure switch and use a Smart Controller and a sensor to shut the system down, as described in the first section of this article.
4. Often a small leak will cause the system to depressurize between irrigation runs. This can be a major problem. The pump will not start if the pressure is low, the low pressure switch is going to shut off the power to it.
If the leak is very small you can install a pressure tank, just like on a typical house water system. Assuming a small leak, the tank keeps the system pressurized. But that only works with a very small leak and it can take a huge pressure tank to supply enough water to keep the system pressurized. If your system has a larger leak you will need to find and repair the leak. If you can’t get the system leak free, you will need to take a different approach, as described below.
You can use a timer to over-ride the low pressure switch, and allow the system to start even with no pressure. You will need a “Time Delay Relay”. The time delay relay needs to be the type that allows the power to flow when energized, then shuts it off after a minute or two of delay. It needs to have an automatic reset. You then install the relay on a bypass wire around the low pressure switch. That way the pump can start even when the pressure switch is “off” due to low pressure. You will need to work with someone knowledgeable when ordering the time delay relay to be sure you get the correct relay, as they make many different kinds.
Using a Pump Controller with a Sensor:
This is essentially the same method as the Smart Controller method I described earlier. Only the “smarts” are in the pump controller rather than in the irrigation controller. Some of the newer digital pump controllers (don’t get confused here, we’re talking about a separate pump controller, not the sprinkler controller) are programmable, they are simply a small computer that operates a relay that starts and stops the pump. You hook them up to a pressure sensor, also to the irrigation controller, and to any other sensor you want (wind, rain, temperature, light, flow, you name it.) Then you can program them to do just about anything using that information input. They can turn off the pump if a low pressure occurs for more than x number of seconds, turn off the pump if a high pressure occurs for x number of seconds, turn on the pump at a given time of day, etc. Pretty much any input you want can cause the pump to turn on or off. The capability depends on the brand and model of the pump controller. The downside is it takes electronics know-how to set the thing up and someone tech savvy to program it. Typically you hook up a laptop to the pump controller to program in the logic, then once it is programmed it runs by itself. The laptop just gives you an interface that is easier to work with. I really can’t give you much more details beyond that, this type of pump control is beyond my expertise, I just have seen pump system experts use them to do amazing things.
A compression coupling or tee is a special fitting designed for joining existing metal and PVC pipes or tubes. They are sometimes sold as “pipe repair couplings”. They are primarily used for underground connections. Other types of compression fittings, such as threaded adapters, are also available for situations where something other than a coupling or tee is needed. Compression couplings are primarily used for pipe repairs, compression tees are used to tap a new pipe/tube into an existing one. A compression fitting may be constructed of PVC or metal. PVC should only be used for connections that will be hidden from sunlight, like underground or in a box.
Where and How to Connect Your Irrigation System to Your Water Supply:
This page provides some specific rules, tips, and techniques for tapping into a house water supply pipe for a new irrigation system. Where and how you tap into the water supply can be critically important, not just for the proper operation of the irrigation system, but also for the preservation of your sanity!
Grab a piece of string about 6″(152mm) long. Strip away any insulation so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once. This is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe. But school’s out so let’s forget about doing geometry calculations! Based on the string length use the table below to find your pipe size.
For Copper or PEX Pipe
2.75″ (70mm) = 3/4″ pipe
3.53″ (90mm) = 1″ pipe
4.32″ (110mm) = 1 1/4″ pipe
5.10″ (130mm) = 1 1/2″ pipe
For Steel Pipe or PVC Plastic Pipe
3.25″ (83mm) = 3/4″ pipe
4.00″(102mm) = 1″ pipe
5.00″(127mm) = 1 1/4″ pipe
6.00″(152mm) = 1 1/2″ pipe
For Flexible Polyethylene Pipe
2.96-3.33″ (75-85mm) = 3/4″ pipe
3.74-4.24″ (95-108mm) = 1″ pipe
4.90-5.57″ (124-141mm) = 1 1/4″ pipe
5.70-6.28″ (145-160mm) = 1 1/2″ pipe
Your string length will probably not be exactly the same as the lengths in the chart. Measurements vary a little, depending on how much the string stretches, dirt on the pipe, manufacturing tolerance of the pipe, how accurate you are at measuring, etc.
If you wish to print out the entire Drip Guidelines Package for reading off-line, print this page and each of the ones listed in the links above.
Drip irrigation is the most efficient method of irrigating. While sprinkler systems are around 75-85% efficient, drip systems typically are 90% or higher. What that means is much less wasted water! For this reason drip is the preferred method of irrigation in the desert regions of the United States. But drip irrigation has other benefits which make it useful almost anywhere. It is easy to install, easy to design, can be very inexpensive, and can reduce disease problems associated with high levels of moisture on some plants. If you want to grow a rain forest however, drip irrigation will work but might not be the best choice!
Drip irrigation (sometimes called trickle irrigation) works by applying water slowly, directly to the soil, bloop, bleep, bloop, bleep. The high efficiency of drip irrigation results from two primary factors. The first is that the water soaks into the soil before it can evaporate or run off. The second is that the water is only applied where it is needed, (at the plant’s roots) rather than sprayed everywhere. While drip systems are simple and pretty forgiving of errors in design and installation, there are some guidelines that if followed, will make for a much better drip system. The purpose of this tutorial is to guide you toward materials and methods that will increase the benefits of your new drip system, while steering you away from some common misconceptions and practices that can cause you trouble.
“What’s with the Metric measurements? !!” Come on, quit whining, the rest of the world uses metric without problems!!! OK, don’t flame me, I give up, I’ll compromise… While a lot of drip irrigation research has occurred in the USA, most of the credit for making drip irrigation what it is today really should go to Israel and South Africa. So I’m going to honor that contribution by using the metric system as the primary measurement units for these guidelines. After all, metric is really the “native” measurements of drip irrigation. When I started using drip irrigation (back in the dark ages of irrigation) all drip data and products were in metric! But because I’m such a nice guy (inflated ego alert!! Dump some ice water on this guy!), I will provide English measurements also. So don’t panic.
This tutorial is setup in a multilevel format. Each of the guidelines below describes a basic rule for drip irrigation design, the guidelines follow in the logical order for creating a design. You can think of the guidelines as design steps if it helps. This page is the top level, here you will find a brief description of each design guideline. For many of the guideline topics there is a link to another page with expanded information on the guideline topic. There may be additional links from there to allow you to dig even deeper into the drip irrigation knowledge base. So you choose how much you want (or need) to learn. My recommendation is that if you want to print out something, print this page. Then refer to the other levels (and print them if necessary) as needed. That will save you a lot of unnecessary wear and tear on your printer. It might also save a tree from going to the paper mill!
Parts of a Drip system:
If you don’t know a lateral from a pressure regulator start by learning about the basic parts of a typical drip irrigation system. I strongly suggest that even if you are familiar with drip irrigation you start be reading through The Basic Parts of a Drip System page now. It contains a lot of tips and recommendations.
Suggestion: Click on the image above for a pdf version of the drawing that prints better.
Prescriptive Drip Design Guidelines:
These guidelines will provide you with all the information necessary to design a residential drip system for a typical yard. These guidelines are what is termed a “prescriptive standard” in the building industry. A prescriptive standard is a set of rules and/or methods that, when followed, allow you to skip the engineering calculations for a design. Obviously this saves a lot of time and effort in preparing a design. The downside to a prescriptive standard design is that it tends to “over-design” in order to make the design “one size fits all”. Unlike sprinkler irrigation, drip irrigation systems are much more forgiving of design error, the cost of over sizing the materials is minimal, and so a prescriptive design method works very well for almost everyone. To prepare a fully engineered drip irrigation design requires a massive number of difficult mathematical calculations. If there was ever a great place to use prescriptive standards for the design, it is drip irrigation!
Emitter Type and Flow:
Use pressure compensating emitters if you have an elevation difference of over 1,5 meters (5 feet) in the area you are irrigating. For more level areas turbulent flow emitters will work great and are often less expensive. For gravity flow systems use short-path emitters, they typically work better than the others at very low water pressures.
For most soil types 2,0 l/hr (0.6 gph) emitters work well and are more economical. For sandy soil use 4,0 l/hr (1 gph) emitters.
1 or 2 emitters per plant, depending on the size of the plant. Trees and large shrubs may need more. Obviously, using two allows for a backup if one clogs up (which happens now and then, even on the best designed and maintained drip systems.) But just as important, more emitters also wet more soil area. This results in more roots, and a healthier, happier plant. Exception: if the plants are very close together you may need to use less than 2 per plant in order to maintain the minimum spacing between emitters. Minimum spacing for emitters: In most situations install emitters at least 450mm (18″) apart. A good default spacing for quick and dirty design is to space the emitters 600mm (24″) apart. For supplemental watering of low-water-use plants, use one emitter per plant. Supplemental watering is used for establishment of drought tolerant plants that are not likely to need irrigation once they have developed a good root system, or might be used to apply a little extra water now and then to make them a bit more lush. Use of low-water plants with supplemental drip irrigation is considered very “green” and is the current trend in landscape design.
Rule of thumb- install emitters 600mm (24″) apart under 80% of the leaf canopy of the plant. That’s where the roots are, and the roots need water. If the soil is very permeable install emitters 300mm to 450mm (12-18 inches) apart. For more information and a better method of determining spacing see Drip Emitter Spacing.
Drip emitters rest directly on the soil so it is especially important to have a backflow preventer to prevent water contamination by soil-borne disease. There are several types that will work depending on your situation and local codes. For more information see Irrigation Backflow Preventers.
What valve type and size to use:
Use a 20mm (3/4″) valve for most systems. Any type of valve may be used. For more information see Drip Irrigation Valves.
How many emitters per valve?
Use the charts below to determine how many emitters to install on each valve circuit. If you don’t know what size your water supply pipe is, see How to Find the Size of a Pipe.
Emitter volume used
Any water supply that comes out of a building, such as a hose bib. Any system with a pump*.
20mm (3/4″) water supply. Use a 20mm (3/4″) valve.
25mm (1″) water supply. OK to use a 20mm (3/4″) valve.
2,0 l/hr (0.6 gph)
4,0 l/hr (1 gph)
*Pumps can be tricky. This is a conservative figure in order to make it work with the majority of pump fed systems. You may be able to use a larger number of emitters by calculating the actual output of your pump. See the Irrigation Pumping Systems tutorial for more information about using pumps.
Water supplies coming out of a building are also a problem. The piping in buildings is almost never designed to carry large amounts of water such as is used by irrigation systems. To be safe I assume you have significant restrictions. 95% of buildings have these restrictions so don’t increase the flow unless you really know what you’re doing. Increasing the flow could cause extreme damage to the plumbing in the building!
Mainlines & Laterals.
Use 25mm (1 inch) PVC, PEX or polyethylene irrigation pipe for mainlines (“mains”) and laterals. The total length of the mainline and the lateral together should not be more than 120 meters (400 feet). So you could have 100 meters of mainline and 20 meters of lateral, for a total of 120 meters of both. But you should not have 80 meters of mainline and 60 meters of lateral because the total of both would be more than 120 meters. Remember mainline is the pipe before the control valve, lateral is pipe after the control valve. Many drip systems won’t need mainlines or laterals. Or they may need just a mainline, or just a lateral. For more information see the sections on mainlines and laterals in the The Basic Parts of a Drip System.
Maximum drip tube length.
The length of drip tube (or drip hose) may not exceed 60 meters (200′) from the point the water enters the tube to the end of the tube. Thus you could have 120 meters (400′) of tube if the water entered the tube in the middle (that would be 60 meters from the point the water enters the tube to the end of the tube in each direction, which would be OK). You can extend one tube off of another as long as the total length of the tubes that are connected is not more than 60 meters (200′). For more information see the drip tube section of The Basic Parts of a Drip System.
Never bury emitters underground unless they are made to be buried. If you bury the emitter roots will grow into it and clog it. If you do want to bury the emitters do a search for “subsurface drip irrigation” to find specialty drip products designed to be buried. Follow the manufacturer’s recommendations for those products as they must be designed and installed to very exacting standards to avoid problems.
Don’t bury the drip tube. If you do bury drip tube don’t complain to me if gophers, moles or other rodents chew it up. I’ve seen them gnaw to pieces a buried drip system over night. One day it works, the next, it’s garbage. It only takes one gopher (or mole, squirrel, etc.), and one evening! You’ve been warned! Other wildlife (and most dogs), will also chew the tubes. It helps if you provide a water source for them to drink from if possible. A water bowl with an emitter over it to keep it full sometimes will distract wildlife from the tubes. You may need to train your dog not to chew the tubes, dogs seem to chew on the tubes for no real reason other than to annoy you. If you want to hide the tube, dig a shallow trench for it, so that it is just below the level of the surrounding soil. Don’t put dirt over the tube. Throw some mulch or bark over the top to hide the tube, or plant a low spreading plant that will grow over it and hide it.
Feeder, Spaghetti, and Distribution Tubing
Avoid using feeder, spaghetti, or distribution tubing if possible. For more on this topic see the section on spaghetti tubing on The Basic Parts of a Drip System page.
Hard-Piped Drip Systems
A type of drip system used in commercial and high quality landscapes called “hard-piped” uses buried PVC pipe rather than poly drip tubing. The PVC pipe is installed underground and a pipe goes to each plant location, so it takes a lot of pipe. At each plant the emitters are installed above ground on short poly tubes called “risers”. Hard pipe systems can be pretty expensive due. For a detail drawing of this click here. The design of a hard-piped drip system is essentially the same as shown here, except you would use PVC or larger size poly irrigation pipe in place of the inexpensive drip tubing.
Fittings- Use the correct size!
This is really important! There are many different sizes of drip tubing sold, and the fittings have to be made for the exact size tube you are using! If they aren’t, they will either be very hard to install, or the tube will blow off the fitting. Sometimes it takes a week or so for the tube to come loose, but if the fitting is even 1mm too large, the tubing will come off eventually. Never heat the drip tube or use oil on it to make it easier to insert into or onto the fittings. See the section on drip tube in The Basic Parts of a Drip System for more information on fittings and tips and tricks for installing fittings.
Stake down the Drip Tubes!
Stake the drip tubes to the ground once every meter (about 3 feet). This keeps the tubes from wandering. No kidding, they tend to move around by themselves! Staking them also helps protect them from damage. I prefer to use metal stakes as the plastic ones I’ve tried pull loose too easily. Wire that rusts holds even better, as the rust binds the wire to the soil. After a few days they can be almost impossible to remove. They will rust away in a few years, but by then the tubing has adapted to its position and stays in place pretty well. Standard 12 gauge wire works well, as does pieces of wire coat-hangers. Buy some coat-hangers at a yard sale or thrift store and help recycle! Bend a 300mm (12 inch) length of wire into a”U” shape to make a tubing “staple”. Or you can buy metal staples that are made for holding down erosion control blankets, they work great.
Check Valves, Slopes, Hillsides:
Install check valves if the drip system is on a hillside of slope to prevent the water in the tubes from draining out through the lowest emitter each time the system stops running. For more information see the drip tube section of The Basic Parts of a Drip System.
Install an air vent at the highest point on each drip valve circuit. If there are multiple high points you an air vent installed at each one. Air vents should always be used for drip systems on sloped areas. Air vents are often not installed on small homeowner drip systems without any slopes. If air vents are not used be sure the emitters at the highest points are not installed where dirt could be sucked into them. For more information see Drip Systems for Slopes and Hillsides.
Flush Valves and End Caps
Install a flush valve or end cap at the end of each drip tube. Automatic flush valves are available, however my personal preference is for manual flush valves. See the section on flush valves in The Basic Parts of a Drip System for more information.
Patios with Potted Plants and Trellises:
You will probably want 6mm (1/4″) feeder/spaghetti/distribution tube running to the plants if they are in pots just to make it less obtrusive visually. Try to use as little 6mm (1/4″) distribution tube as possible, keep the tube lengths short as much as possible, and only put 2 emitters on a single 6mm (1/4″) tube. If a 6mm (1/4″) tube is longer than 5 feet, use only one emitter on it. I like to staple the tubes to something to keep them in place if possible (like stapling the tube to a trellis for hanging plants.) Use a wire stake to hold the emitter in place in a pot. Don’t pull any of your tubes tight, snake them a little, leaving some slack in them to allow movement. The tubes will expand and contract with temperature changes, you don’t want them to tear or pop the fittings off.
So for example, I run standard 15-16-17mm (1/2″) tube along the patio perimeters, trying to put it in places it will be out of the way or I can hide it. I also run it up onto the trellis if there are lots of hanging plants, putting it on the back side out of view and clamping it to the trellis using standard conduit or pipe clamps. (I’ve found conduit clamps are cheapest, look in the electrical dept at any hardware store.) From the 15-16-17mm (1/2″) tube I run short lengths of 6mm (1/4″) tube to the potted plants. Remember: more 6mm (1/4″) tube = more problems.
Backflow preventers are always an issue if you have hanging plants and trellises. Vacuum breaker or anti-siphon type of backflow preventers must be installed above the trellis or they won’t work. Both those types of backflow devices must be installed at least 150mm (6″) higher in elevation than any of your emitters. This is generally not very practical to do. I have seen people run copper pipe up a trellis and put an anti-siphon valve 150mm (6″) above the trellis. But in most cases you need to use a double check, or preferably a reduced pressure type of backflow preventer. Those can be installed at any elevation (a reduced pressure type should be above ground.) I recommend a reduced pressure type. See the backflow preventer page for more detailed information.
Beyond these issues, the other basic drip guidelines in this tutorial all apply to patio and trellis drip systems.
This is just for those who want to know all the little details. Everyone else can ignore this information. Here are the assumed pressure losses for the prescriptive drip system design used in these guidelines:
Valve 0,4 bars
Backflow Preventer 0,8 bars
Pressure Regulator 0,0 bars
Filter 0,2 bars
Mainline & lateral 0,4 bars
Drip Tube 0,2 bars
Emitters 1,0 bars
Total Pressure required 3,0 bars (44 PSI)
Based on 0,2 l/s flow for 20 mm valve with smaller supply, 0,4 l/s flow for 20 mm valve, and 0,9 l/s for 25 mm valve.
The short section of tube that attaches a sprinkler to the underground lateral pipe is called a “riser”. But the riser does much more than attach the sprinkler. It must hold the sprinkler in the correct position, it must allow you to adjust the sprinkler location, and we can also use it to protect the sprinkler from damage. The riser type you use is an important choice and deserves some attention. A good riser choice can save you time and money over the years. This article will take you through the many choices and the pros and cons of each. (“Lateral pipe” is the name given to the pipes that go from the zone valve to the sprinkler heads.)
The path ahead viewed dimly through the fog??? What we’re about to embark on here is known as “doing it the right way”. We are going to start by figuring out what the maximum water supply would be if you had perfect conditions, such as a very short pipe from the water meter to your house, lots of water pressure, a small yard, a happy family, a low interest rate mortgage, and good neighbors! Then we are going to modify that number later in the tutorial to reflect your actual conditions (long pipe, lousy water pressure, bad neighbors, whatever.)
The end result is that we will determine what the exact, optimum water supply is for designing your sprinkler system. What that means for you is that your sprinkler system will use less water, last longer, and there won’t be dry spots! Now it’s going to be a little more work than “guesstimating” would be, but it will be worth it. Don’t get discouraged, be patient, and it will all come together. The worst thing you could do right now is to try deciding what sprinkler you want to use. That would be “putting the cart before the horse”. Trust me, I know what I’m doing. Now let’s get on with it…
A. Find your water supply pipe.
Hopefully you already know where the water service pipe comes onto your property, or at least where it enters your house.
Mild Winter Climates: In milder climate areas there is typically a shut-off valve and or a water meter at the location where the pipe enters the property. From there the pipe generally goes to the house, then surfaces above ground where a house shut-off valve is located, then the pipe turns and runs into the side of the house. Often this location where the pipe enters the house is where the tap for the irrigation system will be made.
Note the “W” etched in the curb in front of the concrete water meter box in the photo above. Often there will be some type of mark on a curb at the location that the water supply pipe to the house runs under it.
The photo above is of a typical mild-climate water supply line where it enters the house. This one has a rather unusual model of pressure regulator (the gizmo with the white adjustment knob on top) to reduce the water pressure. Many houses do not have a pressure regulator. A ball valve (with a blue handle, the handle is in the “off” position) is on the incoming water supply pipe. The pipe going into the wall is the house supply. The pipe exiting the photo at the lower left goes to a hose bib.
Cold Winter Climates: In colder climates the water line often enters directly into the basement or crawl space under the house from underground. This water pipe to the house is often buried very deep to keep it below the frost line. The shut-off valve, and possibly a water meter, are often located in the basement or crawl space to help protect them from freezing.
The photo above shows a typical water supply line in a cold-winter climate. A copper water pipe enters through floor, goes up into a ball valve (yellow handle), then through a pressure regulator, then a remote-reading water meter. You would tap in for the sprinklers after the water meter. The mainline supply size would be measured on the copper pipe coming out of the floor. The water pressure in this case could be measured at any water faucet after the regulator (probably any faucet in the whole house would work). Photo credit and thanks to Ed Pletsch.
What type of pipe is it?
Once you find your supply pipe you need to know what type of pipe or tubing it is. Keep in mind that there may be more than one type of pipe or tubing used at different locations! Often copper is used under concrete slabs and then it converts to PEX for other locations.
Steel Pipe. Steel pipe comes in two types, black steel (used mostly for gas lines) and galvanized steel (galv. steel) which is used for water pipes. Galvanized steel pipe will be a silver gray color, and a magnet will stick to it. It will have threaded joints. Steel pipe is made in conformance with IPS (iron pipe size) standards. Galvanized pipe is often found on homes in inland areas, especially on less expensive tract homes.
Brass Pipe. Brass pipe is sometimes used for homes. Like copper it can take on a greenish tint with age. A magnet will not stick to it. It will have threaded joints. Brass pipe is made in made in conformance with IPS (iron pipe size) standards. Brass is not very common except for short sections of pipe, due to cost.
PVC Pipe. PVC plastic pipe is almost always white or gray, and is more rigid than the other commonly used types of plastic water pipe. Standard PVC pipe is made in conformance with IPS (iron pipe size) standards. It should have the letters “PVC” printed on the pipe. PVC is fairly rigid, and it is not easily scratched with your fingernail. PVC tends to be more commonly used in mild climate areas. Another type of PVC called CPVC is sometimes used inside homes and often is found in older mobile homes. It is similar to regular PVC, but will be labeled “CPVC”. Most often it is a yellowish, gold, or tan color. CPVC in homes is usually made to copper tube sizes (look for “SDR-11” printed on the pipe), but is also sometimes iron pipe size (labeled IPS). PVC is often used for house supply pipes in mild winter areas.
Copper Tube. Copper tube is very common in homes. It takes on a dirty green color as it ages. A magnet will not stick to it. Most joints will be soldered, look for silver color solder at the joints to identify it. Copper tube has a different diameter than iron pipes, and is made in sizes known as CTS (copper tube size). Copper has been the standard “high quality” tube used on better homes for decades. Often used in areas near the coast where salt air causes rapid corrosion of steel.
PEX and PE Tube. Both are both polyethylene (poly) products. Both tend to be used in areas with severe winters and/or rocky soil. There is a lot of confusion over these two poly-based products. Be careful, both are sometimes called “poly”, especially by the sales people in the big home improvement stores. True PEX is a stronger form of cross-linked polyethylene that has become popular in recent years. Both PEX and PE are flexible, and both have a glossy appearance and slick surface. So how do you tell which one you have? Older PE is almost always black, and in most cases PEX is not black. PE is almost never used inside a house if the house was built to code. The surest thing to look for are the letters “PEX” printed on the tubing. Making things even worse, white PEX looks a lot like PVC, especially if it is old or dirty! PEX is easily scratched with a fingernail, PVC scratches, but not easily. PEX was not invented until the ’70s, and it is seldom found in homes built before 1975. (It wasn’t officially sold in the USA until 1985. Of course, if your house has been remodeled, you could still have it in a older house.) PEX is almost always made to conform with CTS sizes. The heavy duty PE tube used for plumbing is most often made to a uniform size standard (labeled “SDR-7”), but many different PE products used for irrigation do not conform with this size standard. Be careful when working with PE tube, if possible take a sample with you when you go shopping for parts so you can test fit them at the store. PEX is quickly becoming the default tube for piping new homes due to low cost and ease of installation.
Warning: PEX pipe has a very thick wall, thus it has a smaller inside area for the water to flow through. This means it has much higher pressure losses when the water passes through it. For this reason you need to be careful when replacing a copper or PE tube with a PEX tube. Often when replacing a copper or PE tube with a PEX tube it is necessary to use PEX that is one size larger than the tube it is replacing. So if you are replacing a 3/4″ copper tube with PEX, you should consider using 1″ size PEX tube for the replacement. Otherwise you may notice a drop in water pressure after the replacement is made.
One good hint to the type of pipe is the way the pipes are connected to each other. PEX and PE are never glued at the joints. Sometimes PEX & PE are heat welded together, but most of the time they are connected together with fittings using clamps, teeth, or compression-nuts that hold the tube onto or into the fitting. (“Fittings” is the term we use for the various connectors that are used to join two or more pipes together.) If the pipe has glued joints it is almost always going to be PVC or ABS. (ABS plastic is typically black rigid pipe, almost always 3″ or larger in diameter, and is mostly used for sewer and drainage pipes. ABS can be other colors so don’t assume a pipe is PVC just because it is white or gray!) Another hint is that poly pipe tends to be used in colder climates, and PVC tends to be used in warmer climates. If you have to regularly shovel snow from the driveway, chances are the pipe is PE or PEX. Copper pipe is often soldered to the fittings. Look for the silver color solder at joints. Steel and brass pipe have threaded connections, a few threads almost always are visible at the joints. Confused yet? Your best bet is to find lettering on the pipe that says what type it is.
B. Find your Water Meter:
Now we need to know if you have a water meter. Most, but not all, water companies use a water meter to measure the amount of water you use. If you don’t have a meter, there will almost always be a shut-off valve at the point your house water line connects to the water provider’s pipes. Often the valves are buried, sometimes several feet down, and a sleeve comes up to the surface with a small lid or box over it. The water company uses a special tool that can reach down and open or close the valve. Often grass has completely grown over the lid and you can’t find it. Try probing the ground with a pitchfork, metal rake, or screwdriver to find the hard cover of the box.
The water meters are normally installed in an underground box as close as possible to the property line. This is usually at the street or alley. Most of the time the box will have “water meter” or the water company name stamped on the lid. In areas with severely cold winters the water meter is often installed in the house basement or a utility room of the house. If you still can’t find it, call your water company and request their assistance.
Try to find a size stamped on the meter. If you can’t find a size, ask your water company or just assume the meter is the next size SMALLER than the pipe running to the house. It is common for the meter to be one size smaller than the pipe. Standard water meter sizes are: 5/8″, 3/4″, 1″, 1 1/2″.
Spiders and snakes: If the meter is in a box, watch out for spiders and ants in the meter box! Most of the “pro” irrigation repair guys I know carry a can of spider spray with them! Sometimes we find snakes, rats, gophers, and other beasts in the boxes too! I found a turtle shell in a box once. No tunnels or holes into the box that I could find. I have no idea how it got in there.
Enter the meter size on your Design Data Form. If you don’t have a meter, enter 0 (zero).
C. Measure Your Water Pressure
Water pressure is the energy that powers your sprinkler system, so it is very important. If you work with it, it will make your sprinklers do the “rain dance”. If you ignore it, it can bite you hard in the wallet! For this tutorial I use the pressure units “PSI” which means “pounds per square inch”. When pros talk about pressure readings we almost never say the words “pounds per square inch”, we just say the letter names “P. S. I”. Outside of the United States pressure is most often measured in “bars”.
First off, grab the phone and call up your water supplier. Ask them for the “static water pressure” for your neighborhood. Don’t be shy, people call them all the time to ask! They may give you a pressure range, like 40-60 PSI. If so, write down the LOW number of this range. You can also measure your own water pressure using a pressure gauge that attaches to a hose bib on your house (you can purchase a 0-120 PSI gauge with a hose adapter on it at pretty much any hardware store).
Pressure regulators (also called pressure reducing valves)
Pressure regulators are devices used to reduce the water pressure and are commonly found on home water supplies in towns with hills. It takes lots of water pressure to lift water uphill. So in order to get the water to the houses on top of the hill the water pressure in the water system has to be very high. But this causes the pressure at the homes at the bottom of the hill to be too high. So pressure regulators are installed on the water supply pipes to homes in the lower areas of town, where the pressures are very high. The pressure regulators are generally set to someplace between 50 and 65 PSI.
If the water company tells you your neighborhood pressure is over 65 PSI, you probably have a pressure regulator installed someplace on the water supply line to the house. The pressure regulator reduces the water pressure in your house, so that it doesn’t damage your plumbing fixtures. Look around and see if you can find it (see the pressure regulators in the pictures above). The regulator may be installed near the water meter or at the point where the water supply pipe enters the house. This is important, because if you have a regulator and you tap into the water supply for your sprinklers after the regulator, the pressure will be a lot lower.
If you have a pressure regulator on your house you must use a gauge to test the water pressure yourself. Most pressure regulators are adjustable, so the water company has no idea what pressure the regulator is set at. When in doubt, test the water pressure with a gauge.
At this point you should make at least a preliminary decision as to where you want to tap into the house water supply pipe for the irrigation system water. Typically, the closer you can tap to the point the water enters your property, the better. Of course, you must tap into the pipe after the water meter. In areas where it gets very cold some people like to tap into a pipe in the basement or someplace else inside a heated building. That way they don’t have to worry about the shut-off valve for the irrigation freezing. (Be sure to install a drain valve after the shut-off valve to drain the water out of the irrigation pipe during freezing weather!) If you have a pressure regulator, consider if it would be better to tap before or after it.
A static water pressure higher than 70 PSI can damage the fixtures and appliances in a household. If you measure a static water pressure higher than 70 PSI when you do your water pressure check as described below, then you should consider installing a pressure regulator on your house water supply if there is not one already. It will help your faucets, pipes, washing machine, dish washer, etc. to all last a lot longer. Make sure it is a good quality brass-body pressure regulator.
For a pressure regulator to work accurately the pressure setting on it must be at least 15 PSI lower than the inlet pressure. So if your static pressure is 70 PSI, the highest pressure you should set on the pressure regulator would be 55 PSI. 55 PSI is a good pressure for both the needs of a house and a sprinkler system.
Hose Bibs as a Water Supply Source = BAD!
Using a hose bib or even a “sprinkler system stub-out pipe” provided on the side of the house for sprinklers is not a good idea. There are often unknown restrictions in the house piping that cause the water supply from these hose bibs to be severely limited. The water running through the house pipes can also be very noisy at night and disturbs some people’s sleep. Do this only as a last resort, when there is no other reasonable way to get water for your sprinkler system. I would suggest you assume the pipe is 1/2″ size, even if it appears larger. If you have concrete that prevents running a new pipe around the house, call a boring contractor and find out how much it would cost to bore a 1″ pipe under the concrete. It may be worth the price. Directional boring technology now allows them to bore and install curved pipes around obstacles.
How to Measure the Water Pressure with a Gauge
Important: If you want to test the pressure yourself, everything that uses water in your home: faucets, ice makers, toilets, etc., MUST be turned off when you take the measurement (that’s why its called “static” water pressure, the water isn’t moving.) Everything! This is critical or you will get a false low reading! You can test the pressure at any faucet that is at about the same height as the proposed irrigation tap. If all the water is turned off, the pressure will be exactly the same regardless of where you test it. (Try it and see!) The easiest place to test the pressure is usually a hose bib or garden valve on the outside wall of the house.
To test the water pressure using a gauge, attach the gauge to a water outlet, like a hose bib or washing machine connection. The place where you attach the gauge can be anywhere in the house, as long as it is about the same height (elevation) as the place where you will tap in the sprinkler system supply. Ie; don’t check it on the 3rd floor if you plan to attach the sprinklers at the first floor! (It is one of those weird, hard to understand hydraulic laws that as long as the water is not flowing the pressure is the same at any point on a pipe that has the same elevation above sea level.) Double check that all the water so water is turned off and not flowing in the house pipes. Then turn on the valve the gauge is connected to and allow the water to enter the gauge. Read the pressure on the gauge. That’s all there is to it, it’s very easy to do! Turn off the water and disconnect the gauge, you’re done!
OK, I realize I may have confused you, because earlier I told you not to use a hose bib to tap the sprinklers into, and now I just told you that you can use a hose bib to measure the static pressure. This is because you can get an accurate pressure measurement from a hose bib– if the water is not flowing, as described. The small pipe can’t restrict the flow if the water isn’t flowing! Confused? Hydraulics is hard to understand. I may sound crazy but I know what I’m doing! Often users of the tutorial have an “ah ha!” moment when they get about 95% done with their first design and suddenly it all makes sense.
The static water pressure that you were given (or you measured with a gauge) is your Design Pressure. Write down the “Design Pressure” on your Design Data Form!
D. Measure the Maximum Available Flow (GPM)
Flow is the traveling companion of water pressure. Pressure is the “energy” that moves the water through the pipes. Flow is the measure of how much water is moved in a given amount of time. Flow is measured in this tutorial using Gallons per Minute (GPM). Other common units used to measure flow include cubic feet per second (commonly used to measure river flows here in the USA), liters per minute, cubic meters per hour, and many others. Now that you know your Design Pressure you need to determine how much water you can use at a time, or your available flow.
Measure Your Supply Pipe Size
You need to find the water supply pipe and measure it’s size. Grab a piece of string about 6″(152mm) long, then find the location where your water supply pipe enters the house. Strip away any insulation, so you can get at the pipe and wrap the string around it. Measure how many inches of string it takes to go around the pipe once.
The string length is the circumference of the pipe (yikes, bad memories of high school geometry!). Using the circumference we can calculate the diameter of the pipe, which allows us to look up the pipe size, from which we can calculate the flow of water using the formula… zzzzzzzzzz….. Let’s forget all those calculations! Based on the string length use the table below to find your pipe size.
For Copper Pipe & PEX Tube
2.75″ (70mm) = 3/4″ pipe
3.53″ (90mm) = 1″ pipe
4.32″ (110mm) = 1¼” pipe
5.10″ (130mm) = 1½” pipe
For Steel, Brass or PVC Plastic Pipe
3.25″ (83mm) = 3/4″ pipe
4.00″(102mm) = 1″ pipe
5.00″(127mm) = 1¼” pipe
6.00″(152mm) = 1½” pipe
For most PE Tube
2.96-3.33″ (75-85mm) = 3/4″ pipe
3.74-4.24″ (95-108mm) = 1″ pipe
4.90-5.57″ (124-141mm) = 1¼” pipe
5.70-6.28″ (145-160mm) = 1½” pipe
Your string length will vary a little, depending on such unavoidable variables as string stretch, dirt on pipe, manufacturing tolerance, what kind of mood you’re in, etc.
Enter the supply pipe size on your Design Data Form! Also make a note of the type– copper, brass, steel, PVC, PEX, or PE.
Find Your Maximum Available GPM:
Your maximum available GPM is the maximum flow of water you have available for your sprinkler system. Actually, it would be more accurate to call this the Maximum Safe GPM. In most cases it is possible to push a higher flow (GPM) through the pipe. However, at high flows the water actually damages the inside of the pipe.
Use the smallest pipe to determine the Maximum Available Flow. Often the water supply coming into your property will not be a single type and size of pipe. You may have a plastic pipe running underground from the water company to your house. When the pipe enters the house it might switch from plastic to copper pipe, or possibly it might be galvanized steel. Then as the water supply pipe runs through the house it likely branches off in several directions with the pipe becoming smaller and smaller in size as it goes. When determining your Maximum Available GPM you will need to check the Maximum Available Flow for each of the types of pipe that the water will pass through, then use the lowest value as the Maximum Available GPM for your sprinkler design. You only need to be concerned about the pipes the water will pass through before it reaches the point where you are going to tap into it for the irrigation system.
There is an exception to the statement above. Often a short section of a smaller pipe size will be present on the water supply for one reason or another. Maybe the plumber didn’t want to drill a larger hole in the wall for the pipe. As long as this smaller pipe section is less than 5 feet long, you can ignore it and use the larger pipe size to determine maximum flow. The higher flow will be able to squeeze through the smaller pipe. The smaller pipe may wear out faster over time, but typically these short pipes are in places where they are easy to replace. Plus, the smaller pipe is often brass or steel, which has a higher resistance to wear than copper or plastic. You have to make a judgement call on this. In most cases I choose to ignore the small section of pipe.
Small Valves. It is not uncommon to find that a shut-off valve installed on the water supply pipe is a smaller size than the pipe. Don’t worry about it. It will not impact the available flow and valves are constructed to handle higher flows than the pipe.
Example 1: You find the water supply pipe entering the house, examine and measure it, and find that it is 1″ copper pipe. But you’re an ambitious type, so you also have done some digging around in the yard and discovered that the pipe going to the house through the yard is 1 1/4″ PE plastic. It just changes to copper about 6 feet away from the house (this is actually a fairly common situation.) After the copper pipe enters the house it quickly branches off in multiple directions and becomes smaller, but this doesn’t matter to you, because you have already decided that you are going to tap your irrigation system into the 1″ copper pipe right where it enters the house. So the irrigation water will not pass through any of those smaller pipes inside the house and you can ignore them.
Looking at the table you find that 1 1/4″ PE gives a flow of 23 GPM. But looking at 1″ copper pipe in the table shows a flow of only 18 GPM. Since the copper pipe is over 5 feet long you can’t just ignore it. This means you must use the lower 18 GPM value. But wait a minute! What if instead of tapping into the copper pipe, you decide to tap into the PE pipe out in the yard before it switches to copper? Now you can use the higher 23 GPM value because the water will no longer go through the 1″ copper pipe!
Example 2: You found you have a 3/4″ copper pipe that comes into the basement but you have no idea where or what type of pipe is used in the yard. It’s 0 degrees outside, and you couldn’t get a shovel into the frozen ground even if you wanted to, which you don’t! In this case it’s reasonably safe to assume the pipe in the yard is 3/4″ copper also. So you would use 11 GPM from the table.
Example 3: You have no idea where the water pipe enters the house, you have no idea where it is in the yard, and you have no desire to try to find out. In this case you must face reality, it’s time to hire a sprinkler contractor!
Maximum Available GPM Table (Maximum Safe GPM)
Maximum Available GPM (Maximum Safe GPM)
PE (poly) Tube
PEX (CTS) Tube
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
6 GPM(7 ft/sec)
3 GPM(7 ft/sec*)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
11 GPM(7 ft/sec)
7 GPM(7 ft/sec*)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
18 GPM(7 ft/sec)
12 GPM(7 ft/sec*)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
23 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
32 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
52 GPM(5 ft/sec)
CTS = Copper tubing size.
Caution: The values in the table above are the maximum safe flows for the given size and type of pipe.
These values are NOT the amount of flow you actually will use for your sprinkler system! Step #2 will show you how to modify these values to reflect your actual flow.
Velocities (ft/sec) are shown for reference only.
* PEX tube has an extremely small inside diameter when compared with the other pipe/tube types, this limits flow. Some manufacturers suggest that PEX will not be damaged by higher flows, up to 10 ft/sec. I don’t feel there is sufficient evidence yet to warrant damaging your plumbing by using flows that are too high, so I am sticking with the old industry standard for plastic pipe of 7 ft/sec maximum velocity. If you wish to take the chance, values at 10 ft/sec are
1/2″=6 GPM, 3/4″=11 GPM and 1″=18 GPM. Use these higher values at your own risk. They could cause serious damage to your both your house plumbing & irrigation piping. Read More on Water Hammer.
A flow test is optional, but suggested if you are not positive about the size or type of water supply pipe. The flow test should be run at a faucet as close as possible to the point you will tap into the water pipe for your irrigation system.
Get a 5-gallon bucket. Old paint buckets work great. Since most 5-gallon buckets actually hold more than 5 gallons of water, you will need to calibrate the bucket as follows: Find an accurate measuring container, and measure out 5 gallons of water into your bucket. Then mark the water level on the side of the bucket with a marking pen so you can easily see it. ?The test is simple. Put the bucket under your water outlet pipe and time how long it takes to fill the bucket to 5 gallons. The formula for calculating the flow in GPM is: 300 divided by the seconds it takes to fill a 5 gallon bucket = GPM.
If the result of the bucket test is lower than the Maximum Available GPM from the table above, something is restricting the flow. It may be the faucet you are using for the test, or there may be a restriction someplace in the house water supply pipe. You can try to find the restriction and get rid of it, or you can simply use the lower flow test GPM for your Initial Design Flow below.
If the result of the bucket test is higher than the Maximum Available GPM you determined in the table above, use the lower value from the table. The Maximum Available GPM Table gives you the maximum SAFE flow. The bucket test is only used to determine if there is an unseen restriction in the water supply pipe that reduces the flow below the level given in the table. Yes, many sprinkler tutorials and sprinkler salespersons may tell you a bucket test should be used for the design flow, they are wrong! In most cases a bucket test like this one gives you an unsafe flow. See the answers to common questions at the bottom of this page for details on why this happens.
Enter your Maximum Available GPM on your Design Data Form.
E. Initial Design Flow
Your Design Flow is the maximum amount of water you will design your sprinkler system to use. For now, use the same number as the Maximum Available GPM, or use the actual Flow Test GPM, whichever is lower.
You will probably need to reduce your Design Flow later, so additional lines are provided for Adjusted Design Flows on the Design Data Form. The initial flows here are very optimistic, 20 to 30% too high for most situations. You will make the adjustment, if needed, later in step #2. Don’t worry about it now. This is just an advanced warning so you won’t be surprised when you need to change the flow later.
Enter your Design Flow on your Design Data Form. Use a pencil so you can change it later!!!!
F. Do you have enough water available?
You are going to need about 20 GPM of water to irrigate 1 acre of grass with sprinklers. One acre is equal to 43,560 square feet (or 4047 square meters). So if you have a 2 acre grass yard you will need to have 40 GPM of water available in order to water it. If you have shrubs, they typically only use 1/2 as much water as grass, so 20 GPM would water 2 acres of shrubs.
There are only so many hours in the day to water. The amount of water needed varies with the climate, these values are typical for hot summer areas where most sprinkler systems are installed (daily high temperatures over 90 degrees F., 32 degrees C.) These values assume you would be willing to water as many as 10 hours per day. If you are willing to water more hours per day you can increase the area irrigated by a similar percentage.
If you don’t have enough water I can suggest a few ideas for you to look into.
Another option is to use drip irrigation for shrub areas. With drip irrigation you only water the area the plant foliage actually covers. Therefore, if the plants only cover half the actual ground area, you only need half the water.
Consider reducing the amount of lawn and replacing it with shrubs. Shrubs use about half the amount of water as lawn.
Another option for getting a higher flow is to install a larger water supply pipe. A description of how to do that is at the bottom of this page.
Why is the flow you measured with a bucket often too high? The GPM rates in the Maximum Available GPM Table above are based on a SAFE water velocity. When you do the bucket test, there are few restrictions on the flow, so the water velocity may easily exceed that safe limit. If you design your sprinkler system to exceed these flows some really bad things can happen. The first of these is called “water hammer”. Water hammer is a pressure surge which declares its presence by destroying the weakest point in your plumbing. The weakest point is usually that little water tube that runs between the shut off valve and the toilet in your bathroom, or possibly the ones that go to the sink faucets. The result is a flooded house, and that’s something you don’t need. Water hammer is exponentially related to water pressure. The higher the water pressure, the greater the water hammer danger. If your water pressure is over 80 PSI, I suggest that you reduce your maximum flow found in the table above by 20% and read carefully the High Pressure Alert below! The other bad thing that happens at high flows is called “scrubbing”. Scrubbing is what happens when the high water velocity actually scrubs molecules loose from the inside of the pipe. Eventually it wears away enough that the pipe develops a leak. The higher the velocity, the more scrubbing you get. A little scrubbing may take 20-30 years to create a leak. With a higher velocity the problem becomes much worse. I have seen 7 year old homes need a total replacement of all the copper pipes due to scrubbing damage. This is extremely expensive to repair! In my 30-year-old neighborhood, most of the homes have now had to replace the water supply pipes to the house due to scrubbing damage caused by sprinkler systems installed back in the bad old days before any of us realized the dangers of high flows. There are still a lot of old tutorials and literature being published that were written before the dangers of high flows were discovered, so be careful when comparing advice on this topic. A lot of industry professionals still haven’t gotten the word on this either!
But, but, but… you didn’t hear any water hammer when you did the bucket test, and nothing broke, so what’s the deal? After all, that higher flow could save some serious money on sprinkler parts! The deal is that you are only human. You can’t close the valve fast enough by hand to create water hammer, but don’t worry, an automatic sprinkler valve can! It can snap that valve closed almost instantly. The higher the water pressure, the faster the valve closes. When that valve snaps closed, it sends a shock wave through the pipe (water hammer). It may take weeks or even years for it to wear down the weak point in your plumbing and break, but it will! Then the cost savings on sprinkler parts will seem trivial. Do it right the first time! Water hammer and scrubbing are insidious and relentless. They just keep working away, little by little, day after day. Clunk, clunk, clunk, chew, chew… until the day you come home to a flooded house.
Clunk, clunk, clunk? Pipe noise!!! I hear those loud noises every time the washing machine or dishwasher runs! Is that water hammer??? You bet it is, and you better do something about it! First, if the water pressure in your house is over 65 PSI, install a pressure regulator to lower the pressure. If that doesn’t get rid of it, go down to your local hardware store and buy a water hammer arrestor. You can get one that screws onto the washing machine or dishwasher fill pipe. They cost about $10-15 and they work pretty well for water hammer caused by appliances. They don’t work nearly as well for water hammer caused by sprinkler systems. This is because many sprinkler systems exceed the maximum water velocities by so much that the arrestor is over-whelmed by it. I’ve written a whole tutorial on this topic: Water Hammer and Air in Pipes.
Q. I have 3 zones for my sprinkler system. I need to remove the valve/pipe/heads from one of the 3 zones in my backyard.
A. You may not even need to turn off the irrigation system water for this project. But it is a good idea to know how to turn it off. You never know when you may need to.
Definition: “Zone valve” when used in irrigation, is the valve that turns on and off a group of sprinkler heads. In most cases the zone valve is an electric activated valve and has a solenoid with wires leading into it on top of the valve. The wires connect the zone valve to the irrigation controller (sometimes called the “timer” or “control box.”) The power to the valve is typically 24 volts AC. It usually will not harm most people if they touch a live wire, but it will give you enough of a shock that you will never want to do it again! Obviously if you have a pacemaker or sensitivity to electrical current you will want to be extra careful around the wires. If you touch your cell phone to a bare wire it may become an expensive paperweight.
Shut off the water. (Optional, if you are not going to remove the zone valve you don’t need to do this.) Turn off the water to the entire sprinkler system. Many sprinkler systems have a main shut off valve that turns off all the water to the sprinkler system. Look around for the shut off valve. It may be in a box underground. Often it it near the location where the pipes enter the house. Often it it in a basement if other water pipes are located in the basement. Once you found a possible shut off valve, turn on one of your sprinkler zone valves so you can see that the system is running. Now try turning off the possible shut-off valve. It the sprinklers stop running you know the valve shuts off water to the sprinkler system. Now check and see if it also turned off the water to the house. If it did, you just found the house main water shut off valve. You may not find a valve that turns off only the sprinkler water. A lot of homeowner installed sprinkler systems don’t have them. You may just have to turn off all the water to the house in order to work on the sprinkler system.
The easiest way is to leave the zone valve installed and not remove it. Just plug it. I’ll tell you how to do that first.
Identify the valve. Now you need to figure out which of the sprinkler zone valves is the one you want to remove. Hopefully you know where the valves are. If not, see the article on how to find missing valves. To determine which valve you want to remove, you manually turn on the zone valves (without using the control box) and see which one turns on the sprinkler you want to remove. On top of your zone valves is a solenoid, written on it you will see ON/OFF arrows. Turn the solenoid in the “ON” direction about 1/4 turn or so. This should open the valve and the sprinklers should come on. Note: Some valves have a lever that turns them on and off, some have a bleed screw you partially turn to make them manually open. Each valve make and model is a little different, so you may have to use some deductive skills to figure out how to manually open your valve. By turning them on one at a time you should be able to determine which valve operates the sprinklers you want to remove. When finished, turn off the valve by by reversing the procedure you used to turn it on. If your valve uses a bleed screw to open it, DO NOT completely remove the bleed screw. Just unscrew it slowly until the valve turns on.
3. Now that you know which valve you want to remove, carefully dig the dirt away from the valve and expose the pipe on the downstream side of the zone valve. If you clear the dirt off the top of the zone valve it should have a flow direction arrow someplace on the valve body that points toward the outlet side. (It may be on the side of the valve, using a small mirror makes it easier to find it.)
Once you know which direction the water flows through the valve, cut out a short section of the pipe right after the valve. Water may squirt out when you make the first cut into the pipe, so be prepared to get some muddy water sprayed at you! A lot of water may drain out when you cut the pipe, depending on how much water was in the pipes and the slope of your yard. You may have to bail water out of the hole with a bucket to remove it. With the pipe section removed you can now use a wrench to unscrew the remaining pipe from the valve outlet. Take the pipe section you removed from the valve (with the threads on it) to a hardware store and buy a threaded plug of the same size and a roll of Teflon tape. Wrap several layers of the Teflon tape sealant onto the threads of the plug and then put the plug into the valve outlet opening. Hand tighten the plug, then use the wrench to tighten it another half turn. Do not overtighten it, if you overtighten the plug the valve body may split open. Now that valve zone is plugged off. You can remove the wires for that valve from the controller if you wish. Now remove any of the pipe or sprinklers you want from that valve zone.
You can remove the entire valve if you want to. I didn’t have you remove the valve because that does not require you to turn off the water to the entire sprinkler system, which is easier for most homeowners to do themselves.
To remove the entire valve: Turn off the water to the entire sprinkler system. Then manually turn ON the valve you want to remove, the sprinklers will come on for a few seconds then slowly shut off as the water discharges from the pipes and the pressure is released. If the sprinklers keep running the water is not shut off! Now follow the directions above. Once the outlet pipe section is cut and removed, cut the wires off the valve, then unscrew and remove the entire valve. Seal the ends of the wires with PVC glue or silicon caulk/sealer if you think you may ever want to use them again. Put a threaded cap on the pipe that formerly connected to the valve.
Removing sprinklers. To remove a sprinkler you can sometimes just grab the top of it and turn it counter-clockwise. It will unscrew from the pipe below it and then you can lift it out of the ground. Often you will need to dig away grass from it so you can twist it out. In most cases you don’t need to dig a big hole around the sprinkler head, just dig away enough dirt and grass to allow you to grip the sprinkler. Fill in the hole with dirt after you remove it. Assuming you are abandoning the pipes, there is no need to cap the pipe off below the sprinkler, just leave it there. If you don’t plan to ever use it, it doesn’t matter if it gets dirt in it.
Removing Pipes. Most of the time we just leave the pipes in the ground. They are a lot of work to remove and most of the time they don’t bother anyone if left buried. If the pipes are not very deep you can often pull them up using “brute force”. Dig down to expose the end of the pipe, grab the end and pull it up out of the ground. If there is thick lawn you may need to cut a slit in the lawn surface to allow the pipe to be pulled up easier. Use a edger to cut the turf directly above the pipe. A string trimmer with heavy string in it may be able to cut the turf. It may use up a lot of string!
I don’t recommend using a vehicle to pull the pipe out, but I know some will try it. If you do this and get yourself injured or killed, you will be featured in those “knuckleheads in the news” columns! If you try attaching a rope to the pipe and the other end to a garden tractor or truck to pull the pipe out of the ground – be very careful. Wear protective clothing, gloves, eye protection and a hard hat. Keep everyone else far away. Have someone there watching from a distance who can call 911 if you get hurt! Here’s why I say you shouldn’t do this: Plastic pipe breaks suddenly and violently when pulled hard. If the pipe or rope breaks while pulling on the pipe both the rope and the pipe can whip around violently and cause injury or damage, ie; break a window. The white hard PVC plastic pipe can shatter and release small, very sharp pieces of plastic that act like shrapnel and cut like dozens of little knives. If the pipe does not come out easily and you see the rope stretching, STOP, it’s going to break! Don’t be an idiot, use common sense and extreme care.
If you can’t pull the pipe up and you absolutely can’t just abandon it in place, the only way I know of to get it out is to dig it out. Ugghh. Lots of work.
Q. How do I calculate sprinkler risers losses in a sprinkler zone where the risers are extra long, 3 ft or more above ground? I have 10 risers in a zone for my proposed sprinkler irrigation system.
A. If you are using my Sprinkler System Design Tutorial and a standard riser of the recommended size, then you don’t need to worry about pressure lose in the riser, the tutorial has friction loss for the risers built-in to the formulas it uses. So you can ignore the riser pressure loss. Some standard risers are shown on the page on Sprinkler Risers in the Irrigation Installation Tutorial. The recommended size for a riser? In most cases it should be the same size as the threaded inlet on the sprinkler. But please actually read that page on risers, as there are some exceptions to that rule for certain types of standard risers!
OK, I realize that didn’t answer your question, you are asking about a non-standard riser that uses a long pipe to hold the sprinkler high above the ground. In that case you must calculate what the friction loss will be in the longer-than-normal riser pipe. (In this case that would be the 3 ft long pipe you described in your question above.) To do that you simply use the same friction loss spreadsheets that you use to calculate the friction loss in any other pipe. Just use this link to get the proper spreadsheet from my website for the type of pipe you are using. Then open the spreadsheet and on the first line enter the pipe size, GPM of the sprinkler you will install on the riser, and the length of the riser. Enter an error factor of 1.4 rather than the default 1.1. This is because even your “longer” riser is shorter than the typical pipe length that the default error factor is based on. Now read the friction loss. That’s it, you have the friction loss for your non-standard riser! Don’t worry about the fittings like ells and couplings that are part of the riser, that is part of what the error factor is compensating for.
When adding the riser friction loss into the total friction loss calculations for your whole sprinkler system, just add in the loss for a single riser. Use the friction loss value for the riser that has the highest friction loss. (This is most likely the one with the highest GPM sprinkler, or it may be the longest riser if you have different riser lengths. You may have to calculate the friction loss for several different risers to figure out which of them has the highest loss.) Why do you add in the friction loss for only one sprinkler, rather than the combined loss for all of them? Because as a single drop of water goes through the sprinkler system it only goes through one sprinkler, not all of the sprinklers. You have to think about the water as a collection of millions of drops, not as one solid body. So the pressure loss is what a single drop would experience as it travels through the system. As a drop of water enters the sprinkler system it travels through a water meter, lots of pipe, a valve or two, then it finally blows out through a single sprinkler onto the landscape. The pressure loss calculation for the whole sprinkler system is determined by what the worst case pressure loss values would be for a single drop of water traveling through the sprinkler system.
OK, so you calculated the friction loss, but what if it is a really high value, or maybe the calculator complained about the velocity being to high. In this case you need to use a larger size pipe for your riser. For the velocity in a riser you can go all the way up to the 7 ft/sec maximum without too much risk. Velocities in the marginal “use caution” zone are generally OK for risers. High velocity in a riser will seldom cause a water hammer problem, unless you are using a special type of sprinkler that has a solenoid valve built in to it. Those sprinklers are called “valve-in-head sprinklers”, they are very expensive, and are mostly used for golf course greens.
PVC pipe types labeled “schedule” (abbreviated “SCH“) are made based on the traditional dimensions used for steel pipe. Unfortunately steel has very different strength characteristics from plastic, so it is a system that isn’t very logical for use with PVC pipe. But when plastic first came along it was made to the same size standards that were already in use for steel. The common PVC pipe schedules you will see in stores are SCH 40 and SCH 80. As the pipe sizes rated SCH increase, the strength and pressure rating of SCH pipe decreases. So 1/2″ SCH 40 PVC pipe is very strong, while 2″ SCH 40 PVC has comparatively a low pressure rating, and is more easily damaged. In sizes 1/2″ to 1 1/2” SCH 40 is a thick wall pipe with a reasonably high pressure rating and good resistance to physical damage. It is often used for mainlines and other situations where a tough high pressure pipe is needed. Sch 80 is generally used for making threaded plastic nipples because the plastic walls are thick enough to have threads cut into them (although most now have molded threads rather than threads “cut” with a die.)
As you can see, the pressure ratings drop as the pipe size increases. Note that the industry standard rule is that your normal operating pressure should not exceed 1/2 of the rated pipe pressure. In other words, you shouldn’t use 1 1/2″ pipe for pressures higher than 165 PSI (330 x 0.5 = 165 PSI). This is because pressure surges created by closing valves can easily double the water pressure in the pipe. This rule applies to all PVC pipe, including that labeled SCH and CL.
Class rated pipe
PVC pipe types labeled “Class” (abbreviated “CL“) are based on the pipe’s pressure rating. So Cl 200 PVC pipe is rated for 200 PSI of water pressure. Cl 315 PVC pipe is rated for 315 PSI of water pressure. The strength of CL labeled pipe is directly related to the pressure rating. The standard “Cl” pipes are Cl 125, Cl 160, Cl 200 and Cl 315. Of these Cl 200 and Cl 315 are most common. Cl 125 is sold as a low cost pipe for use in sprinkler laterals for those for whom low price is everything. It has a very thin wall and breaks easily if not handled carefully or nicked with a digging tool.
1/2″ size pipe is generally only available in SCH 40. This is because of the thin wall of 1/2″ pipe makes it very easy to break. I don’t recommend using 1/2″ PVC pipe at all, however if you must, you should use SCH 40. Sometimes you will find 1/2″ Cl 125 PVC pipe at discount stores due to the very low price.
The Class system is obviously a more logical system for labeling pipe as you know immediately how strong the pipe is based on the label. Unfortunately the more confusing “SCH” system became entrenched in the industry and remains.
What Pipe Type to Use
All PVC pipe labeled for a given size in the USA has the same outside diameter. So any pipe labeled as 3/4″ will be the same diameter, whether it is SCH 40 or Cl 200 or any other type. That allows the same fittings to be used to join the various pipe types together. Most fittings are made to SCH 40 standards, although SCH 80 fittings are available, typically only at specialty plumbing and irrigation stores. Technically most codes require SCH 80 fittings for pipe sizes 2″ or over. In practice I’ve noticed that SCH 40 fittings are often used up to 3″ size. When dealing with sizes 4″ and above the use of non-glued “rubber ring-joint” fittings is recommended and usually required by code as well. Glueing joints on 3″ and larger PVC pipe is very, very difficult.
“Mainlines” are all of the pipes that are under constant pressure, that is, the pipes that are before the sprinkler zone valves. In most of the industry SCH 40 PVC pipe is used for irrigation mainlines up to 1 1/2″ size. For 2″ size and larger Cl 315 PVC is used. Most building codes prohibit the use of 2″ and larger SCH 40 PVC pipe for pressurized water lines. Depending on the jurisdiction, this rule may or may not be applied to irrigation systems. Those same codes generally require that all pressurized PVC pipes (mainlines) be buried at least 18″ deep to protect them from accidental damage, regardless of the type or size of pipe used.
“Lateral” pipes are the pipes after the sprinkler zone valve. These pipes are only pressurized when the sprinklers are operating. For lateral pipes the standard is to use Cl 200 PVC pipe. Where budget is a concern and you can find it, sometimes Cl 160 is used. As previously mentioned I recommend you avoid Cl 125 PVC pipe. Laterals can be buried any depth, but I generally recommend at least 10″ deep to avoid a lot of maintenance problems with broken pipes.
Well, if you have been reading my tutorials, I guess it’s pretty obvious that I’m not recommending you use 1/2″ pipe. I have a number of reasons for this.
1/2″ PVC pipe is generally not available in most areas except in SCH 40 type.
The water capacity of 1/2″ pipe or tube is very low.
1/2″ PVC pipe is hard to glue together without using too much glue. The glue piles up on the inside of the pipe when you insert it into the fitting and blocks the water flow. Too much glue also weakens the wall of the pipe and a leak develops after a few years.
Using 1/2″ pipe means you have to have another size of spare pipe and fittings on hand for repairs.
But the biggest reason is that 1/2″ pipe leaves no flexibility for future changes or additions to your sprinkler system. If you ever need to add another sprinkler to the pipe you’re screwed.
My conclusion: The small amount of money saved by using 1/2″ pipe just isn’t worth the hassle and risk.
As water moves through a pipe it loses pressure due to a phenomenon commonly called “friction loss”. Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. A complex formula (called the Williams/Hazen Formula) predicts the amount of pressure that will be lost due to friction loss. The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it’s way to the sprinkler head. Even a bend in the pipe causes pressure loss! Don’t panic over the formula, we’ll use a pipe sizing chart or a friction loss calculator!
You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. The manufacturer’s performance charts tell you how much pressure is required to achieve a specific radius for the water. As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to make the sprinklers operate as they should, we need to calculate the pressure losses between the water source and the sprinkler head. If the pressure loss is found to be too great, then we must reduce. The easiest way to do that is to use a larger size pipe.
DETERMINING THE SPRINKLER PIPE SIZE
There are several methods used to determine pipe sizes of sprinkler system lateral pipes. I’m going to explain two methods. One method is faster but less accurate, the other is very accurate but takes more time.
Pros: The fastest and easiest method. Requires a single, simple calculation and uses a chart to determine the sizes.
Cons: The learning curve to use it is a bit more difficult to understand. It uses an averaging system to arrive at pipe sizes.
See step-by-step tutorial for the Chart Method
Trial & Error:
Pros: Very accurate, calculates the pressure loss in each pipe section using a spreadsheet. Easier to understand.
Cons: Time consuming, need to enter data into the spreadsheet, uses trial and error to establish pipe sizes.
See step-by-step tutorial for the Trial & Error method
Which method should you use? For a beginner with a small irrigation system probably the Trial & Error system will be easier. Below are overviews of each method for experienced designers to use. Unless you are experienced you should probably read the full tutorial for the method you select.
Overview: CHART METHOD FOR LATERAL IRRIGATION PIPE SIZING
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
____ PSI. Insert the maximum PSI loss for the valve circuit laterals into the formula where it says “____PSI.”
____ Feet. Insert the distance from the valve to the farthest sprinkler on the valve circuit in the space labeled “____ Feet Total Length” in the formula.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
The Pipe Size Table or Chart:
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 = Desired PSI Loss in Lateral x 100 / Total length of Lateral
Flows shown red are over 5 feet/second. Use caution!
Find your PSI/100 value in the top blue row.
Read down the column to the value equal to, or higher than, the GPM in the pipe section.
Read across to the pipe size for that section in the right column.
Repeat steps 2 & 3 for next pipe section.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%. This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Notes about the Pipe Sizing Chart:
Warning: The sprinkler pipe sizing chart is based on using Cl 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Polyethylene, Polybutylene: After you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc.
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Change the pipe size if the velocity or total pressure loss is too high.
You must calculate the pressure loss for each of the possible water paths in the valve circuit.
Here is an example of the possible water paths for a valve circuit, shown in red, blue, and magenta.
Start with the water route that is the longest. In this case that would be the red route. There are 9 pipe sections in this route labeled 1-9. Enter the data from this route into the calculator. Use a larger pipe size if the velocity is not safe. Check that the friction loss “Total of All Sections” does not exceed your maximum allowable amount.
Write the pipe size for each section on your plan.
Now repeat the process for the blue water route and then the magenta color route.
It will not harm anything to use a larger pipe size. Period. If you are uncertain whether to use a 3/4″ or 1″ pipe, then you should use the 1″. Using a larger size pipe is ALWAYS the safest choice.
No, I don’t own stock in an irrigation pipe manufacturer and I’m not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be “too large”. Contrary to what might appear to be true, forcing water into a smaller pipe REDUCES the water pressure, and hurts sprinkler performance. This is because the smaller pipe creates more pressure loss due to friction and turbulence as the water flows through it. It’s another of those hard to grasp hydraulic principles! Just remember that when it comes to pipe, bigger is better! I’m always amazed at how many irrigation equipment sales people don’t know this most basic of irrigation rules. I’ve had clients tell me they were told to use a smaller pipe to keep the pressure up by tech support people at some of the major sprinkler manufacturer’s. That’s an industry disgrace!
So one more time to drill it into your head– You don’t decrease the pipe size to keep the pressure up- or down for that matter. That is totally, completely, wrong. The reason we use smaller pipe is to save money. Which of course, is a good reason! For those who want more specifics on this, there is a very boring scientific explanation at the bottom of this page.
Is it Pipe or Tube? For the most part I use the term “pipe” rather than “tube” on this page and elsewhere. Bad habit of mine (note that by reading carefully, you have found one of my faults!) The difference is the material they are made from. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually referred to as tube. I often screw up and call tube pipe! 🙂
TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE USING A SPREADSHEET
This method involves trying various pipe sizes until a good combination is found.
Definitions you need to know:
Lateral pipe: all the pipes between the control valve and the sprinkler heads.
Mainline: The pipe that goes from the water source to the control valves.
Control Valve: The valve that turns on and off a group of sprinklers. Most often it is an electric valve operated by a timer.
Valve circuit: a single valve, and all the pipe, fittings and sprinkler heads downstream from it. In other words, all the sprinkler heads that start working when you turn on the valve are part of the same valve circuit.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
You will need a spreadsheet Friction Loss Calculator.
Here’s a page with calculators for almost every type of pipe: Friction Loss Calculator Spreadsheets
Grab the appropriate spreadsheet for the type pipe you plan to use.
Now you just enter the appropriate data for each section of pipe into the calculator and then read the total pressure loss at the bottom of the spreadsheet. If the pressure loss is too high, then try making one of the lengths of pipe larger. The calculator will also give you the water velocity in each section of pipe, and warn you if the velocity is too high.
The best way to teach this is probably to walk you through a couple of examples.
If I may make a suggestion, download the spreadsheet for Cl200 PVC now, and open it up in a separate window. Then think about each step, enter the values I show into the spreadsheet, and actually try to duplicate what I do in the examples below. Something about actually doing this helps engage people’s brains. People tell me they read it twice and still don’t get it when they just read it, but as soon as they actually TRY it, then it suddenly makes sense. It’s called learning by doing, and it is considered the best teaching method. This process is simple, HOWEVER, it is not obvious and sounds illogical to those not trained in hydraulics.
A Simple Example:
The sketch above is an example of a very simple valve circuit with 5 sprinkler heads. In this example the sprinklers are 15′ apart and each sprinkler uses 3.7 GPM of water. The red numbers on the sketch are the total water flow for each pipe section in GPM.
Let’s assume we want to use Cl200 PVC pipe and we want a maximum total of 4 PSI of pressure loss in our lateral pipes.
If you are working through the Sprinkler Design Tutorial the maximum total pressure loss is entered on your Design Data Form in the Pressure Loss Table section. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems.
If you don’t understand how to calculate the water flow in each section (the red numbers) you should take a look at the Sprinkler Pipe Layout page.
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
If you don’t understand pressure losses in irrigation, see Pressure Loss & Selecting Your Sprinkler Equipment.
For advice on types of pipe (Cl200, poly, etc.) see Irrigation System Lateral Pipes.
To use the spreadsheet friction loss calculator to determine the pressure loss:
Download and open the Friction Loss Calculator.
There is a line on the spreadsheet for each section of pipe. So for this example you will enter data for 5 pipe sections.
Start with the pipe section closest to the valve as section #1, and work out to the farthest sprinkler head.
Start by selecting 3/4″ pipe for the pipe or tube size for all the sections. (See “why not 1/2″?”)
Enter the GPM for the section of pipe.
Enter the length of the section of pipe.
Use an error factor of 1.1
Go to the next line down and repeat steps 4-7 for the next pipe section.
The spreadsheet calculator will tell you the velocity and PSI Loss for each pipe section.
At the bottom of the calculator it will tell you the pressure loss total of all sections combined.
Here’s what the spreadsheet calculator looks like after we enter the data requested for each of the pipe sections using the example in the sketch above.
Note that the “Total of all Sections” shown at the bottom exceeds the 4 PSI maximum limit we set for pressure loss. Also notice that the velocity in two of the sections (highlighted in red) exceeds the safe level. The marginally high velocity highlighted in yellow is considered acceptable by most experts, since these are lateral pipes. (The marginal velocity level would not be as acceptable in mainlines.) Start by fixing the velocity problems. To decrease the velocity in those sections we will need to increase the pipe size. So let’s increase the pipe size for the two sections highlighted with red to 1″. Here’s what it looks like after the change:
Now the velocities are all within acceptable levels. Also note that increasing the pipe sizes reduced the pressure loss “Total of All Sections” shown at the bottom to 2.9 PSI, which is well below our maximum level of 4 PSI. That’s good, no more changes are needed. It is not possible for the pressure loss to be “too low.” As long as it is under the maximum it is fantastic. So what would happen if the pressure loss was still too high? If there was still too much pressure loss we would need to try increasing the size of some of the pipes to lower the friction loss.
So we now have pipe sizes that will work for each section of pipe in our lateral. I’m often asked at this point if it would be OK to make some of the pipes 1/2″ since the pressure loss is so low? The answer is yes, but you might not want to do it. See my explanation of the problems associated with the use of 1/2″ pipe.
A More Complex Example:
Now lets look at a more complex valve circuit. (Please note that this circuit is much larger than that found on a typical residential irrigation system. It would require much more water than most residences have available and is just used to show you an example of a much more complex layout.) As with the previous example we will assume that our maximum pressure loss value for the valve circuit is 4 PSI.
This valve circuit involves numerous paths the water may take. This makes the calculation a bit more complex, as a separate calculation is needed for each possible route that the water might take through the laterals on it’s way to the last sprinkler at the end of a pipe. If you look at the example above you will notice there are 3 sprinklers that are at the end of pipes, each sprinkler at the end of a pipe represents a different route the water can take. So this circuit has 3 and will therefore require 3 separate pressure loss calculations. The next drawing shows the possible water routes in magenta, blue, and red colors.
It may help to think of each path as the shortest route that a single drop of water could take to go from the valve to the last sprinkler on a pipe branch. For some people it helps to think of it as a road map and your looking for the shortest route to each of the dead ends at the end of the roads.
Start your calculations with the water route that is the longest. In this case that would be the route highlighted in red. There are 9 pipe sections in this route, I have labeled them 1-9 for clarity. Just as before, enter the data from this route into the calculator, and make all the pipe 3/4″ size. Here’s what the spreadsheet looks like:
As you can see there are a number of pipe sections highlighted red due to unsafe velocity. Change those pipe sections to larger pipe sizes until all the velocities are within safe levels.
Here’s the resulting spreadsheet calculator with the smallest possible pipe sizes. However, notice the Total of All Sections is 4.4 PSI, which is more than our 4 PSI maximum:
So we need to make some of the pipe sections larger in order to reduce the pressure loss (or friction loss.) Start by increasing the size of one of the the smaller pipe sections. Changing a 3/4″ pipe to a 1″ size is a lot less expensive than changing a 1″ pipe to 1 1/4″. So for the example lets change section 7 from 3/4″ to 1″. Doing that drops the Total of all Sections value to 3.77 in our example, below the 4 PSI maximum we set earlier. So now everything is good, these sizes will work for the “red” highlighted water route.
Now we add the pipe sizes from the spreadsheet to our circuit drawing (note that the pipe sizes for the red highlighted sections have been added on the next drawing below.)
Now we relabel our sections to follow the blue highlighted route.
Using the blue highlighted water route, repeat the same process used for the red one. Enter the GPM and pipe lengths for each section in the spreadsheet. This time we already know the sizes for sections 1-6, they were entered into the spreadsheet when we did the red section. So we just enter those for the new blue sections 7 and 8, again using 3/4″ size pipe. And it looks like this on the spreadsheet calculator:
Using 3/4″ pipe size for our two new sections works good. The velocity is safe and the Total of All Sections is 3.29 PSI, so the pressure loss for this route is also within the 4 PSI maximum we set. Write the size of the two new sections on the drawing and the blue water route is done.
Now all that remains is to do are the calculations for the magenta highlighted water route. That is done the same way, entering the data into the spreadsheet calculator for each pipe section. Start with 3/4″ then change to larger sizes until the velocity is safe. Then check that the total of all Sections is less than 4 PSI as before. Here’s the data entered into the spreadsheet calculator:
Now all that remains is to insert our lateral pipe sizes from the spreadsheet calculators into the drawing of the valve circuit.
All done! So the pressure loss for the entire circuit is the same as that for the highest water route. In this case the red route was highest at 3.77 PSI. So the pressure loss for the lateral circuit shown here is 3.77 PSI.
Often I get asked at this point why the “Total of all Sections” pressure losses for all 3 routes wasn’t added together? The pressure loss for the red route was 3.77 PSI, for the blue section it was 3.29 PSI, and for the magenta route it was 3.02 PSI. So the confusion is that it seems like there should be a total loss of 10.08 PSI! Nope, the pressure loss for the entire lateral is 3.77 PSI, the loss of the highest route. To understand this think of a single drop of water again. It can only travel on one route from the valve to the farthest sprinkler. It is not going to go backwards and try another route! So the pressure loss for the entire valve circuit is equal to the pressure loss from the valve to the farthest sprinkler.
This method is based on the assumption that you are using Cl 200 PVC pipe for the lateral pipes. With minor adjustments this method will also work reasonably well for SCH 40 PVC pipe or polyethylene irrigation tube. For other types of pipe or tube you will need to use the Trial & Error method to determine the pipe sizes.
While the Pipe Sizing Chart method described here seems rather complex when you read it the first time, it is actually extremely fast and easy once you figure it out. You will start with a simple calculation to obtain a “PSI/100” value. Then you will use that value in the Pipe Sizing Chart to figure out the maximum flow for various sizes of pipe. You will only do this once for each sprinkler system. Once you have that schedule you will fly through inserting pipe sizes into your plan. Most designers who “design in their heads” are using this method or a close variation of it. It is the method I use when designing my systems.
Lateral pipe: The pipes between the control valve and the sprinkler heads are called “laterals”.
Mainline: The pipes that go from the water source to the control valves are called “mainlines”.
Control Valve: The control valve is the valve used to turn on and off a group of sprinklers. Often it is an electric solenoid valve operated by a timer.
Valve circuit: A valve circuit consists of a single control valve, and all the fittings, pipes, and sprinkler heads that it turns on.
GPM: Gallons per minute, a measure of water flow rate. Use primarily in the United States.
PSI: Pounds per square inch, a measure of water pressure. Use primarily in the United States.
BASIC RULES TO KEEP IN MIND
When in doubt, always use a larger diameter pipe!
You may always use a larger size pipe. No, I don’t own stock in a irrigation pipe manufacturer. But using a larger size of pipe will not cause any harm to how well your sprinkler system works. Using a larger pipe will NOT noticeably reduce the water pressure. (Yes, I did condition that statement with a “noticeably”.) The only damage done by using a larger size of pipe is to your pocketbook. Larger pipe generally costs more. But from a irrigation system performance perspective you will NEVER hurt anything by using a larger size pipe. Now I realize that somewhere out there, somewhere will tell you this is not true. They are going to tell you that you need a smaller pipe to squeeze the water and create more pressure. They are totally wrong of course, but as you read this you are probably uncertain who is right, since they will claim I am wrong! Ask them to provide you with a scientific, documented explanation of why they are right. I will also provide both a basic and a very scientific explanation with references for you. Here’s mine: Using A Smaller Pipe to Increase Water Pressure. OK, sorry, I’ll climb down off my soapbox now.
Is it Pipe or Tube? I tend to call everything pipe. Habit, since here in La La Land (Los Angeles, California) we use mostly PVC pipe for irrigation. However some types of “pipe” are technically defined as “tube”. The difference is the material they are constructed of. Steel and PVC plastic are generally called pipe. Polyethylene, PEX, and copper are usually called tube or tubing. If I say pipe where I should say tube, please accept my apologies.
CALCULATING THE PSI/100 VALUE:
The PSI/100 value is a value used in the Pipe Sizing Chart (we’ll get to the chart in a moment.) The PSI/100 value determines which column of the chart you will use when finding the pipe sizes. A simple calculation will give you the PSI/100 value.
The PSI/100 formula:
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
For those who prefer variables, this is the same formula written using variables: (LPSI * 100) / FTL = PSI/100
Here are the values to insert in the blank spots (“____” ), or variables, in the formula:
____ PSI. (LPSI) Insert the maximum PSI loss for all laterals on the valve circuit into the formula where it says “____PSI.” .
If you are working through the Sprinkler Design Tutorial look on your Design Data Form for the Pressure Loss Table. There you will see a figure you entered called “_____ PSI – Laterals”. That is the maximum PSI loss for the laterals, use that number here. If in doubt, 3 PSI is a reasonably safe value for most sprinkler systems. If you don’t understand pressure losses in irrigation, see the Pressure Loss & Selecting Your Sprinkler Equipment and Lateral Pressure Loss pages. Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure. Example: 20 PSI sprinkler operating pressure. 20 x 0.20 = 4 PSI maximum pressure loss in circuit laterals.
____ Feet Total Length. (FTL) Insert the distance from the control valve to the farthest sprinkler in the space labeled “____ Feet Total Length” in the formula.
For this value you need to figure out the total length of pipe (in feet) that the water needs to travel through in order to get from the valve to the farthest sprinkler. Measure only the pipe sections that the water would pass through on the way from the control valve to that farthest sprinkler. Don’t add in the length of any side spurs going off to other heads that aren’t on the longest route. In the example below, the route from the control valve to the farthest sprinkler that you would measure the distance of is shown in red. Totaling each of the pipe sections along that route results in 118′. So 118 feet would be the ___ feet value you would use in the PSI/100 formula.
Example of A Typical Valve Circuit
Now use the PSI/100 formula above to calculate the PSI/100 value. ( ____ PSI x 100) / ____ Feet Total Length = PSI/100
Write down the PSI/100 value. ____________
Example: Let’s say the value “____ PSI – Laterals” is 4 PSI. Let’s also assume that the total length of the lateral as measured above is 118 feet. Those values inserted in the formula would look like this: (4 PSI x 100) / 118 feet Now do the math. 4 times 100 = 400. Then 400 divided by 118 = 3.389 Round that number to 3.4. Therefore when using this example your PSI/100 value to use in the Pipe Sizing Chart would be 3.4 PSI/100 .
You can repeat this procedure for each valve circuit. But the usual method is–
It is possible to use the same PSI/100 value for all the valve circuits. That’s how most professionals (myself included) do it. The only catch is that you must use the “worst case” PSI/100 value. In other words you need to figure out which of the valve circuits on your entire sprinkler system has the longest “Feet Total Length” between the valve and last sprinkler. Then use that valve circuit to calculate your worst case PSI/100 for the entire sprinkler system. The advantage of using the same PSI/100 value for everything is uniformity of design and, obviously, doing only one PSI/100 calculation for the entire sprinkler system saves time. For example, a pipe with five half circle spray heads downstream would always be the same size pipe. This is much less confusing for the installer, which is the main reason we do it this way.
Pipe Sections and GPM:
Each section of lateral pipe may be a different size. For example, the first section of pipe leading away from the valve might be 1 1/4″. The next two sections might be 1″, and the rest of the sections might be 3/4″. The pipe size to each section is based on the actual GPM flow passing through that section of pipe, so you will need to know what the GPM flow is for each section. If you have been working through the Sprinkler Design Tutorial you have already figured this out and written these GPM values down on your plan in an earlier step. If not, you will need to take a few minutes to do this now. See the page on Sprinkler Pipe Layout for instructions on figuring out the GPM for each pipe section.
THE PIPE SIZING TABLE or CHART:
before you use the chart…
Warning: The sprinkler pipe sizing table /chart is based on using Cl 200 PVC pipe. For other pipe types you will need to make an adjustment if you want to use the chart.
Schedule 40 PVC: If you plan to use Schedule 40 PVC pipe (“SCH 40”) for the laterals you need to make an adjustment before using the chart below, because SCH 40 PVC pipe has a much less water capacity than other PVC pipes. Reduce the PSI/100 value you just calculated for the valve circuits to 1/2 the original values.
Example for SCH 40 PVC pipe: In the example above you calculated a value of 3.4 PSI/100. But you have decided to use SCH 40 PVC pipe for the laterals, rather than Cl 200 PVC pipe. So you will need to reduce the PSI/100 value by half. 3.4 x 0.5 = 1.7 PSI/100. So your new value is 1.7 PSI/100. As you will see, this will result in much larger lateral pipes! This is why most people do not use SCH 40 PVC for laterals, and why I recommend you use Class 200 PVC. It makes a big difference in cost!
Class 125, Class 160, or Class 200 PVC pipe: The chart below is based on the use of Class 200 PVC pipe. It also works for Class 125 (not recommended) and Class 160 (hard to find).
Class 100 and 315 PVC pipe: As a general rule, these types of PVC pipe are not used for laterals.
Polyethylene, Polybutylene: Use the chart below. Then, after you obtain your pipe size from the chart you need to increase it by one size to get the proper size for poly pipe. In other words, if the chart says ¾” PVC pipe, then you should use 1″ poly pipe. 1″ would become 1¼”, 1¼” becomes 1½”, 1½” becomes 2″, etc. Note: PEX pipe is not the same thing as polyethylene irrigation pipe.
To use the chart you will use the PSI/100 value you calculated along with the GPM flow in the pipe section.
Sprinkler Pipe Sizing Chart for Laterals
PSI/100 (round down)
Flows shown in red are over 5 feet/second.
Sprinkler Pipe Sizing Chart, Copyright 1979, Jess Stryker, All rights reserved.
Permission is granted for reuse for any purpose and in any media, provided the copyright notice is maintained.
Sprinkler Pipe Sizing Table /Chart Instructions:
Start with the pipe section farthest from the valve (connecting to the last sprinkler head.)
Find the PSI/100 value in the top row (blue text, directly under the heading PSI/100.)
Read down that column and find a value equal to, or higher than, the GPM in the pipe section.
Now read across to the right column to find the pipe size to use for the pipe section.
Repeat steps 3-5 for the other pipe sections in the lateral valve circuit.
Flows over 5 ft/second are considered marginal (shown in red on chart.) Most experts believe that flows up to 7 ft/sec are acceptable for laterals. However flows over 7 ft/sec velocity are not considered safe, so they are not shown on the chart.
This table uses an averaging formula based on the assumption that all flows for any given size of pipe will not be at the maximum GPM for that size of pipe. In rare cases the PSI loss for the entire lateral may exceed the desired loss by up to 10%.
This table assumes the use of Cl 200 PVC pipe, adjustments to the pipe sizes are required for other pipe types, such as poly or SCH 40 PVC.
In the example above the flows for each pipe section are noted in gray text with an arrow pointing at the pipe section. The red pipe circuit has the longest distance between the control valve and the farthest sprinkler head. So for our example let’s use the red pipe circuit.
First we need to calculate the PSI/100 value.
We start with the maximum pressure loss we want in our lateral pipes. For this example we will use 4 PSI.
Now we measure the total pipe distance from the valve to the farthest head. I showed this route using a bold red line. It is 96 feet from the control valve to the farthest head when following this bold red route.
Now the PSI/100 formula with the values from this example inserted: ( _4_ PSI x 100) / _96_ Feet Total Length = _4.2_ PSI/100
Now we start using the chart to find the pipe sizes.
Our PSI/100 value is 4.2, so we look on the chart. Rounding down we see that 4.0 is the closest PSI/100 value on the chart, so we use the 4.0 column.
Now read down the 4.0 column. The numbers will tell us the maximum flow for each pipe size.
So the first number we see is 10. That would mean 10 GPM. Reading across to the right we see that 10 GPM is the maximum flow for 3/4″ size pipe.
Continuing in the 4.0 column, the next number is 20. Again we read across and see that 20 GPM will be the maximum flow for 1″ pipe.
Reading down one more line we see that 36 GPM is the maximum flow for 1 1/4″ pipe. And we can continue this on down the chart.
So now we can create a simple pipe size schedule to use for our plan, based on the values we took from the pipe sizing chart:
Up to 10 GPM = 3/4″ size pipe
Up to 20 GPM = 1″ size pipe
Up to 36 GPM = 1 1/4″ size pipe
Up to 48 GPM = 1 1/2″ size pipe
Up to 76 GPM = 2″ size pipe
Now go back and look at the flow for each section of pipe on your plan. Then based on the GPM flow, insert the pipe size from the schedule you made.
So the section with a flow of 2.5 GPM will be 3/4″ pipe.
The section with 1.3 GPM will also be 3/4″ pipe.
The section with 3.8 GPM will be 3/4″ pipe.
The section with 6.4 GPM will be 3/4″ pipe.
The section with 1.3 GPM will be 3/4″ pipe.
The section with 2.6 GPM will be 3/4″ pipe.
The section with 9.0 GPM will be 3/4″ pipe.
The section with 11.5 GPM will be 1″ pipe.
I’ve inserted these pipe sizes on the example sketch above.
See how fast and easy that is? Once you have the initial PSI/100 calculations done you can use the pipe sizing chart to create a custom pipe schedule for your plan. Then it is really fast to simply look at the flow in a pipe section, look it up on the schedule, and write in the pipe size! You can see why pros use this method, it allows them to fly through a large design with hundreds of sprinklers.
COMMON PROBLEMS AND QUESTIONS REGARDING USING THE PIPE SIZING CHART
Is your PSI/100 value off the chart? If your PSI/100 value is 6.0 or higher you should use the 6.0 column. At 6.0 you have reached the maximum safe capacity of the pipe sizes used on the chart.
Is the pipe size larger than the valve size? It is fairly normal for the first pipe after the valve to be one size larger than the valve. So you may have a 1″ mainline going into a 3/4″ valve and then have a 1″ lateral pipe coming out of the 3/4″ valve. This is very common, and is not a problem at all. So don’t worry if the pipe size you get from the chart is larger – or smaller – than the valve size.
Write the pipe size down next to the pipe on your plan. Repeat for each pipe section. Repeat for each valve circuit.
There is a very persistent misconception in the lawn sprinkler industry that using progressively smaller pipe sizes in a sprinkler system will help keep the water pressure high. The argument is that as the water moves through the pipes past the sprinklers, the pipe must get smaller in order to squeeze the water so that the pressure stays high enough to operate the sprinklers. Unfortunately, it’s not true. It would be nice if it was, because we could eliminate pumps. Plus think of all the money you would save on pipe. The smaller the pipe you used, the better your system would work! So why not use 1/4″ or even 1/8″ tube for the pipes? That would really pump up the pressure! Sounds a little silly when you look at it that way, right? OK, enough with the sarcasm. I’ll explain this whole mess.
Squeezing the water into a smaller pipe will not increase the water pressure!
Part of the reason this misconception persists is that it does seem logical. The example most often given to support this idea is what happens when holding your thumb over the end of a hose. As you press your thumb over the opening, making it smaller, you can feel the water pressure against your thumb increase. Pushing your thumb even tighter against the end of the hose, makes the opening even smaller, and you feel the pressure increase even more. That would seem to prove that decreasing the opening size is increasing the water pressure. So logically, using a smaller pipe would also increase the water pressure.
Unfortunately there is a lot more happening with this “thumb over the hose end” example than you realize. As water moves through a hose or pipe there is a lot of resistance caused by the hose or pipe surfaces. The water moves through the hose at the maximum speed it can while still overcoming this friction. When the water reaches the end of the hose it has close to zero pressure left as it exits. So if you have, say, 50 PSI of water pressure at the hose faucet, the water will move as fast as it can through the hose, such that it will use up almost all that 50 PSI of pressure by the time it reaches the end of the hose. If there were 60 PSI of pressure, the water would just move a little faster through the hose so that it used up almost all 60 PSI by the time it exits. So basically regardless of the pressure, almost all the water pressure is used up by the time the water flows through the hose. The nature of water is that it will reach the most efficient balance between flow rate and pressure loss that it can. (Note, I am oversimplifying this to make it digestible for the average person. If you have a degree in hydraulics you already know all the other related stuff about open vs. closed channels and nozzling effects.)
When you put your thumb over the end of the hose you change the flow dynamics in the hose. Your thumb restricts the flow of water through the hose. With your thumb over the end, the water is flowing much slower through the hose, and as a result, there is a lot less pressure loss due to friction. So with less pressure being lost in the hose, the pressure at the end of the hose where your thumb is increases. The tighter you squeeze your thumb, the more the flow is reduced, and the greater the pressure you feel will be. But you haven’t created any NEW pressure. You have simply traded reduced flow for increased pressure. You can easily test this yourself. Take a bucket and time how long it takes to fill it using an open end hose. Now time how long it takes to fill the same bucket with your thumb firmly pressed over the hose end. It will take longer to fill, because your thumb has reduced the flow! The same thing would happen in your sprinkler system if you used smaller pipe to increase the pressure. The smaller pipe would restrict the flow of water. The reduced flow would reduce the pressure loss in the pipes, resulting in more pressure. But of course the sprinklers would not work because they won’t be getting the flow they require! Sprinklers require both flow and pressure.
OK, that’s the layman’s explanation. But there are also some much more complex scientific theory that I have been asked about in relation to this topic. So here’s some very scientific explanations.
Grab your thinking caps for this. As you well know, Bernoulli’s Principle essentially says (paraphrased) that as the speed of a fluid increases, the pressure of that fluid decreases. If it didn’t, pigs wouldn’t fly.* Obviously as you force a given amount of water through a smaller size pipe, the velocity of the water must increase for it to get through the smaller pipe. According to Bernoulli’s Principle that will decrease the water pressure! This is called the Venturi effect. By suddenly forcing the water through a narrow passage you can actually create enough of a pressure decrease that it creates suction. This is how many fertilizer injectors work. It also is another reason why using a smaller pipe would not increase the pressure– it would actually decrease it!
Another less common argument is the pipe size must be decreased because the flow is decreasing at each sprinkler head location along the pipe route. Thus if the pipe were to remain the same size, the velocity in the pipe would decrease, resulting in an increase in pressure (according to Bernoulli’s Principle again.) This is actually a good, scientifically based point, and accurate too! So the argument is that the pipe sizes must become smaller in order to keep the velocity constant and avoid an increase in water pressure. (Are you bored yet?) Unfortunately when used as an argument for using smaller pipe, this one falls flat when you do the actual math. At a flow of 7 feet per second, which is the maximum recommended safe flow for PVC pipe, the maximum possible pressure increase due to velocity change would be a whopping 1/3 PSI. So in theory, using a smaller pipe would eliminate that 1/3 PSI pressure gain. But using a smaller pipe probably would also increase the pressure loss due to friction, as previously mentioned. The drop in pressure due to friction loss likely will offset most if not all of any gain that might have occurred due to decrease in velocity. Even if it didn’t the maximum possible pressure gain of 1/3 PSI is simply not significant and would not be noticed. So I stand by my statement that the only reason to decrease pipe size is to save money.
*Oh, by the way, Bernoulli’s Principle is why airplane wings create lift, which helps airplanes fly. Therefore, it is also the reason that people, and yes, even pigs, can fly!
Here are some spreadsheets I have created to help you calculate the capacity and water pressure loss through pipes and tubes of various types and sizes. These should be useful for both figuring pressure loss in mainlines and laterals. Each spreadsheet allows for multiple sections of pipe of various sizes and flows. All you do is select the proper spreadsheet for the type of pipe you are going to use, select the pipe size from a drop down list, enter the flow through the pipe in GPM, then enter the length of the pipe in feet. The spreadsheet calculator will then do the math to give you the water velocity in the pipe along with the pressure loss in PSI for that section of pipe. If there are multiple sections of pipe the spreadsheet will also total all of them for the total pressure loss.
Full instructions for using the spreadsheets are included on the spreadsheets.
Please read this paragraph before you try to use the spreadsheets.
These spreadsheets use Apache Open Office. That means you need to have the Open Office program installed on your computer for them to work. I use Open Office because it is a free, safe program that is available for just about every desktop and laptop computer make and model. That means just about everyone can use it, and nobody has to make a major software investment just to use the spreadsheets. If you don’t already have Open Office you will need to install it on your computer before you can use the spreadsheets. You can uninstall it when you are done using it if you want. Download it free from http://www.openoffice.org . It does take a while to download Open Office. It is a full office productivity suite. (Check it out, there are some cool apps in it!)
Your browser may try to “open” the spreadsheets if you left click on the links, even if you don’t have Open Office installed. This is because most computers have some type of minimal spreadsheet reader installed on them, so the reader will try to open these spreadsheets. If they were just simple spreadsheets they would probably work with the readers. But they aren’t. In most cases you will get a corrupted version of the spreadsheet that does not work. This is because these friction loss calculators use very complex formulas that the “stripped down” spreadsheet readers can’t handle. You need to install Open Office. I may have already mentioned this. 🙂
Remember, you can un-install Open Office when you are done if you don’t like it.
If you try to open the spreadsheets directly using your Internet browser they will probably open as Read Only and the spreadsheet won’t work. This is because the browser will not open an executable file directly. (It is trying to protect you from possible viruses.)
Solution: Right click on the link and “Save” the file to your hard drive. The actual wording varies, so depending on your browser you may select “Save link as..”, “Save Target as…”, etc. This should save the spreadsheet file to your hard drive. Then open it directly from your computer without using the browser plug-in. Tablet and phone users: you may need to get to a real computer to use the spreadsheets.
If the spreadsheets don’t work for you..
Do you have Open Office installed? If not, install it.
Are you using Open Office to read the spreadsheet? Sometimes another spreadsheet program will try to open it instead of Open Office.
Have you tried saving the spreadsheet file to your desktop, starting up Open Office, then opening the spreadsheet with Open Office?
If the spreadsheet says it is “Read Only” you probably are using a non-compatible plug in. Install & use Open Office.
Just dragging the spreadsheet link to your desktop may not actually save the file. You need to right click on the link and select Save as…
If you have an older version of Open Office you may need to upgrade it.
Try rebooting. I’ve experienced a problem where if I try to open one of these spreadsheets in a browser the computer gets messed up and won’t start Open Office. Rebooting fixed the issue.
SPREADSHEET CALCULATORS FOR PVC PIPE
Do not try to open these spreadsheets by left clicking on the links. Save the spreadsheets to your hard-drive first. See the explanation above.
PVC fittings only come in 90 degree and 45 degree angles. Sometimes you need a smaller bend. A website reader asked if it is safe to bend PVC pipe and if so, how much can PVC pipe be bend without damaging the pipe?
The answer is that, yes, it is OK to bend PVC pipe,but don’t bend it too sharp or too much. Each pipe manufacturer has rules on what degree curve you can bend the pipe to based on the type and size of pipe. You could look that up but it would take a lot of time and even then figuring out how much a 15% bend is out in the yard is not very practical for the average homeowner. So here is a simpler “rule of thumb” that I basically just made up. But it seems to work reasonably well, it’s easy to do, and it gives you a nice, visual answer!
To determine how much is the maximum bend you should allow grab one end of a length of the pipe you plan to bend and hold it so the other end is off the ground. The amount the pipe bends on it’s own is about the maximum amount of bend you should allow.
You can also make any angle you want simply by using two 45 degree ells. This is easier to demonstrate than to explain. Get two 45 degree PVC ells. Lightly push them together onto either end of a very short piece of pipe. (Don’t glue them for now, this is just a learning experience. If you do ever use them on a irrigation system then you can glue them!) Now start twisting them in different directions. You will see that you can make any angle curve from 0 degree up to 90 degree! Add another 45 degree ell and you can make even more angles. Have fun. It’s cool!
Looping your mainline often allows you to use a smaller pipe size for it, so using a loop system can be financially advantageous on a large irrigated area. A looped mainline also provides maintenance advantages on larger sprinkler systems, and almost all large landscape irrigation systems, like parks and golf courses, utilize a loop mainline layout. For smaller sprinkler irrigation systems they often provide little or no advantages. As a general rule if you have less than 10 valve zones you are not going to get much of an advantage from a looped mainline.
What is a Looped Mainline?
The irrigation mainline is the pipe that runs from your water source to the individual valves that turn on a group of sprinklers or a drip irrigation circuit. When the mainline is looped that simply means that all or part of it creates a continuous loop. Typically a looped mainline starts with a single pipe coming out from the water source (pump, water meter, etc.) then the single pipe splits into two pipes. the two pipes loop around the irrigated area and then rejoin each other to create the “loop”. Zone valves would be located at various points along the loop to supply groups of sprinklers. Normally you would put a isolation ball valve on each leg of the loop at the “split”, the location where the pipes separate into the loop. A third isolation ball valve is placed on the far side of the loop, allowing the loop to be divided into sections. (see the sketch of a looped mainline below) Additional isolation valves may be added anywhere along the loop if desired, to divide it into more segments. The isolation valves allow you to shut down sections of the mainline for repairs while the rest of it may still be operational. For a large irrigation system being able to shut down only a portion of the system for repairs can be very advantageous. Very large irrigation systems may have multiple loops, sometimes one loop will even be inside of another loop.
How to Design a Looped Mainline
When using a loop you should use the same size pipe for the entire looped portion of the mainline. (This is not a hard and fast rule, just a strong suggestion unless you really understand hydraulics!) The pipe leading from the water source to the loop may need to be a larger size than the loop pipe. It is also OK to have mainline “spurs” off of the loop leading to other valves or faucets. While unusual, the pipe size of the spurs may also be larger than the size of the loop pipe if they need to be. Valves for sprinkler zones, faucets, quick coupler valves or any other equipment may be placed anywhere along the loop, as well as on the mainline leading to the loop or even on spurs off of the loop. Normally drinking fountains would be on a separate pipe and not connected to the irrigation system due to the possibility of water contamination from the sprinklers. See the article on use of backflow preventers for more information on contamination.
You can have multiple loops, but I suggest that if you do, you size the pipe using the outside perimeter (largest) loop, as if there were not any cross pipes within the loop (using the method that follows below.) Then after you have determined what size the outside loop pipes need to be, use the same pipe size for any smaller loops or cross pipes inside the perimeter loop. (Technical note: The pipe sizes of inside loops and cross pipes do not necessarily need to be the same size as the outside loop, it is just that using the same pipe size will almost always work. Using a smaller size pipe may work, but it may not. So it is advisable for non-experts to stick with the same size!)
Loop Mainline Calculations
You must calculate both the friction loss AND the velocity for a looped mainline. We’ll go through the process step-by-step. You calculate the pressure loss for the non-looped mainline section from the water source to the beginning of the loop in the normal way (as described in the Irrigation Mainline Tutorial), using the pipe size, flow rate, and length of the pipe section. I suggest that you use one of the the Friction Loss Calculator Spreadsheets I’ve created, they are easier for non-mathematically oriented folks to use than the old manual calculations using charts. Choose the proper spreadsheet for the type of pipe or tube. Then enter the size, the GPM, and the length of the loop. Pressure loss for spurs off of the loop are also calculated using this same method.
Pressure Loss in the Loop:
To calculate pressure loss for the looped section we simply will assume that the water flow splits, and 1/2 of the water is going around 1/2 of the loop and the rest of the water is going the other way around. To do this start by determining the “highest flow GPM” that is found on the looped section. If you are planning to operate only one valve at a time that would be the flow for the largest zone valve to be installed on the loop section. If you will be running more than one valve at a time then the “highest flow GPM” will be the combined flow rate for the largest group of valves that will operate at the same time. Once you have the “highest flow GPM” you calculate pressure loss in the pipe using the calculator. But for the looped portion you will enter 1/2 of that “highest flow GPM” for the flow in the looped pipe and 1/2 of the total loop length (all the way around and back to the beginning) as the length of the pipe. The calculator result is the pressure loss in PSI for the entire looped section.
Total Pressure Loss for the Mainline:
Just total up the pressure loss for the mainline leading to the loop, the pressure loss for the loop, and if you have any spurs add to your total the pressure loss for the single spur having the largest loss value. The total of those is the pressure loss for your entire mainline network.
As mentioned, this method of calculating pressure loss uses an averaging system that assumes that half the water goes one direction to the valve, and the other half goes the other direction. While this is not a perfect method, it works good enough for figuring out the pressure loss. However, the flow doesn’t really split evenly in both directions. In reality the flow balances in each direction based on pressure loss, with most of the flow in the loop going the shortest distance to a valve. So if one of the zone valves is just a few feet from the split point on the loop, almost all the flow will go through that short distance rather than going the long way around and back to the valve. Also in the event you make a repair you may close those isolation valves mentioned earlier. This will force all of the flow in a singe direction through one side of the loop. So we come to the second rule of loops, the pipe size for the looped section must be large enough to handle ALL of the flow in one direction. If it is not, you may create excessive velocity of flow in a section of pipe which can cause major, and very expensive, problems. This means you must check the velocity of the flow while assuming all the flow may go in a single direction around the loop. If the velocity is over 7 feet per second you may create excessive pipe wear and water hammer. Pipe wear can seriously shorten the life of your system, water hammer is much worse, let’s just say you don’t want it. (Look it up if you really are curious.)
Calculate the Velocity:
Using the Friction Loss Calculator Spreadsheets mentioned above enter the pipe size, the “highest flow GPM”, and any random value for the length of the loop. Ignore the pressure loss it gives you, just check the ft/sec velocity result it gives you. The velocity MUST be less than 7 ft/sec. If it is not, you will need to use a larger size pipe for the loop.
That’s it! You should now know how to create a looped mainline. If it seems confusing try rereading it, it is admittedly a bit confusing after the first reading! It really is simpler than it first sounds, be patient, grab some coffee, take your time. Go back and actually work through it one sentence at a time, study the sample sketch of a looped mainline and try sketching your own on paper. Practice a bit with the Pressure Loss Calculator to see what happens when you change the input values. It should start to make more sense.
Q. 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.
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