Note: This page contains a pressure loss calculator that uses a Java script. ("Active content" in geek speak.) Your browser may block it and alert you about it by beeping at you and showing a warning above. If so, and you wish to use the calculator, you will need to click on the warning line above and click on "Allow Blocked Content...". The calculator is harmless, all it does is calculate pressure loss in pipes.

Jess Stryker's Landscape Sprinkler Design Tutorial Step #2  Mainlines

Background Information:

The mainline consists of all the pipes from your water source to your irrigation valves. Your water source may be a water company pipe in the street or alley, a well, lake, stream, tank, or whatever. You will likely have two mainline sections, the existing house mainline which goes from your water source to the point where the irrigation system connects, and the irrigation mainline from the irrigation system connection point to the irrigation control valves. In most cases I recommend you use SCH. 40 PVC pipe with the letters UPC and NSHF stamped on the pipe for new mainlines. In most areas the requirement for the type of pipe are written into local laws. 2" size and larger must often be CL 315 PVC, but check with local building officials. If your house mainline is polyethylene (a flexible black plastic which usually has a slightly oily feel to it), then you will probably want to use heavy wall (rated 125 PSI) polyethylene pipe for the mainline also, however it is not as durable as PVC and you should check local codes to make sure it is still permissible. I do not recommend that you use poly pipe for mainlines if you have a static water pressure over 65 PSI. Normally the pressure rating of the pipe should be double the actual water pressure. You can also use copper or galvanized steel pipe for new mainlines if you wish. Copper is expensive, but a great choice as it is very durable. Galvanized steel is not such a great choice, it corrodes from the inside and will slowly choke off the flow over time. Galvanized steel also flakes off bits of rust which can cause problems with irrigation systems.

Common question: "Can I skip calculating the pressure loss in the house mainline? It is upstream of where I measured my pressure."   NO, NO, NO! Sorry to yell at you, but it's a huge, common mistake, so I want your attention. When you measured the water pressure you measured static pressure, that is, the pressure when the water is not moving. There is no pressure loss when the water is not moving. But when you run the sprinklers the water will be moving, so there WILL be pressure loss in the house mainline. Often the pressure loss in the house mainline is substantial. Therefore you must also calculate the pressure loss for the house mainline and add it in with your other pressure losses. Think of it this way- we call the loss "friction loss" and there isn't any friction when the water is not moving! Why not measure the pressure with the water flowing (dynamic pressure)? Because it is pretty difficult to get the correct flow rate and you would likely need to disassemble part of your house piping and build a special test pipe to get an accurate measurement. Not convinced? You're going to need to trust me on this one, or take a college level course in hydraulics.

Any new mainline pipe should be installed at least 18" deep, or below the frost level*, in order to meet code requirements.

*If you drain or blow the water out of your mainline each winter you do not need the mainline to be below the frost line. However it still should be at least 18 inches deep. If you do plan to drain or blow out the mainline you will need a shut-off valve located someplace where the valve will not freeze (or you can use a special freeze-proof valve, or use a heating cable to prevent it from freezing.) Most people put the shut-off valve in the basement. I strongly recommend that you use a brass body ball-valve for the shut-off valve (or a special frost-proof designed valve.) Gate valves are not a good choice, they tend to leak after a few uses.

In some cases you may have two (or more) different pipe sizes or types between the water source and irrigation valves. For example, you may have a 1" copper house mainline and a 1 1/4" PVC irrigation mainline. Often it is advantageous for the irrigation mainline to be larger than the house mainline. In any case, you will need to calculate the pressure loss separately for each different pipe size and type, then add them all together.

Example: You have a 1" polyethylene "house mainline" from the water source (a large water company pipe in the street that supplies water to all the houses in the neighborhood). This polyethylene pipe runs to the water meter and then continues on to the house. A 1" copper mainline then runs through the basement wall to a backflow preventer in the basement and then the copper goes back outside again (this copper pipe is also part of the "house mainline"). Finally you are planning to add a new 1 1/4" polyethylene "irrigation mainline" running from the copper pipe to your irrigation valves. You will need to calculate the friction loss for each of those sections of pipe separately, then add them all together for the total mainline pressure loss.

1" poly house mainline to meter and then continuing to house + 1" copper mainline + 1 1/4" poly irrigation mainline = Total mainline pressure loss

If the water company pipe is somewhere out it the street, how do you know where it is so you can measure the distance from it? You will probably need to make an educated guess. Look up and down the street for manhole covers labeled "water". Generally they are located directly over the pipe, and the pipes in most cases run parallel to the street curbs. I know that's not real helpful, but it is about the best you can do.

So how are you supposed to know the length of your mainline before you even design the sprinkler system? Good question! The answer is that you are going to need to guesstimate. As discussed above, the irrigation mainline essentially goes from the point of connection (that's the place where you tap into the existing water pipe for the new irrigation water supply) to the sprinkler control valves. So ask yourself where would you like for the control valves to be? Globe valves can be installed almost anywhere you want, although in most cases it is best if they are near (but not within) the area that they will control the water of. Anti-siphon valves need to be at the highest area of the yard. Perhaps an example will help!

So let's say you have a typical house where the water supply pipe comes from the street and runs to the house. You decide to tap into the water supply for the irrigation system in the basement, right after the pipe comes in from the yard. So your point of connection is in the basement. From there the irrigation mainline would run out through the basement wall to the front yard. The part of the irrigation mainline that is within the basement should be metal pipe. Many people install their backflow preventer in the basement also. That is what we will be doing for our example. So we will put a reduced pressure backflow preventer on the new mainline before it leaves the basement. (Since it is a reduced pressure backflow preventer we also assure that there is a drain in the basement as they sometimes spit out water onto the floor.) From the backflow preventer the metal irrigation mainline pipe continues through the wall and out to the yard. Be sure to seal the hole through the wall around the pipe! After the irrigation mainline is through the wall the metal pipe should extend another 12 inches beyond the edge of the wall, then you can change to plastic pipe. If the pipe comes out of the wall above ground it will need to be metal until it is underground. Plastic pipe should never be used above ground.

You plan to water the front yard with the new sprinkler system, so you will need some control valves for the front yard. If you are using anti-siphon valves you don't have a lot of choice as to where they are installed. They must be installed at the highest point in the front yard. In this case we are using a backflow preventer and globe valves,so we don't need the valves at the highest point. Where the new mainline comes through the wall there is already a big group of shrubs, perfect to hide the valves in. So the mainline will come out of the basement and extend to the front yard valves, which are located in a clump of shrubs. So far, so good. Now you also want to water the backyard. You could put the valves for the backyard in the same clump of shrubs with the front yard valves. But this would mean lots of pipes going to the backyard from the front. Plus it would be inconvenient- if you needed to work on the backyard sprinklers you would have to walk all the way around to the front yard each time you wanted to turn on the valve. So the best solution is usually to continue the mainline from the front yard, around the house, to the back yard. (Some people run the mainline under the house in the crawl space- this is OK but you should use metal pipe under the house.) Now the question is where to put the valves for the backyard? If you plan to use anti siphon valves you must install them at the highest point in the back yard. So your choice is already made for you. If you plan to use a backflow preventer and globe valves, then the choice of valve locations is up to you. Again, a good location is someplace out of the way, but also near the area they will control the water in. A good place for the valves is along the perimeter of the yard. I like to run my mainline about 24 inches away from the property line. Usually a shrub border is planted at the perimeter of a property, so this also works good to hide the valves.

One last note- it is best not to locate a valve within the area that is watered by the valves. If you need to manually turn on the valve you are going to get sprayed by the sprinklers when you turn them on. This is not fun.

What size should your new irrigation mainline be? I wish there was an easy answer, but, unfortunately, you will need to make an educated guess for starters. Then you need to calculate the total pressure loss in your mainlines and add it to all of your other pressure losses in the sprinkler system. If the pressure losses are too great, you may then need to increase the size of your new mainline. More on that later. Unfortunately, there are no "magic" ways to determine what size a pipe should be. Pipe size is based on many different variables, what works at one house may not work next door!

As previously stated, you need to start with an educated guess for the irrigation mainline size. Almost always the irrigation mainline will need to be the same size or larger as the house mainline, so that is one good place to start. Even better, start with one size larger than the house mainline as in most cases this will be needed, especially if your Design Pressure is less than 50 PSI. Another good starting point is one size larger than the water meter if you have a meter (use 2 sizes larger if the pipe is polyethylene).

Remember that with water pipes bigger is always better. You can never have a pipe that is too large, so when in doubt use a larger size! Anybody that tells you otherwise is wrong, and you should be careful when taking irrigation advice from them. You can not "squeeze" more pressure out by using a smaller pipe, in fact the opposite happens and the pressure is reduced. When you place your thumb over the end of the hose to make it spray, you are not increasing the water pressure. The water shoots farther because you are increasing the water velocity. The pressure is actually reduced slightly by your thumb. OK? The principle is referred to as a nozzle effect, and it is what makes a sprinkler spray water over long distances. It's not easy to understand, but please trust me on this, it is correct.

One more item to quickly remind you of. I strongly recommend that you do not use a hose bib on your house as the irrigation water source. It is better if you avoid running the irrigation water through the house. Connect to the house mainline between the water source and the house if possible. If you do need to install a backflow preventer in the basement to keep it from freezing, tap into the water supply as close as possible to the point where it enters the basement. You can then run a new pipe for the irrigation under or around the house if you want to get the water to the back yard. Most existing house pipes are too small to handle the volume of water needed for an irrigation system. Plus the noise of the water flowing through the pipes may drive you nuts!

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How to calculate pressure loss:

Water pressure loss in pipes is calculated using the pipe size and the flow rate of the water through the pipe. Below is a Java calculator that will figure the pressure loss for you if you have a Java enabled browser. It works with the latest versions of both Netscape and Internet Explorer. Because it is true Java you don't need to be online to use it! There is also a Windows pressure loss calculator you can download from the formulas page, it is a bit more complicated than this one.

FRICTION LOSS CALCULATOR FOR PVC PIPE

This calculator uses a Java script. Your browser will likely block you from using it unless you override the setting. This calculator only does calculations, it does not access your system files. It has been part of this website since 1999 and no one has complained of it doing anything evil. It is safe to allow it to operate.

Instructions:

1. Select pipe type.
2. Select pipe size in inches.
3. Enter GPM flow through this section of pipe (your "Initial Design Flow" from your Design Data Form).
4. Enter the pipe length.
5. Click on calculate.

Notes:

• The calculator is a blue box directly above. If you don't see it, then your computer is blocking it from displaying or does not have Java support installed. To install Java support on your computer click here: Link to Sun Microsystems Java page. Once Java support is installed you may need to reload this page and unblock the script.
• Friction loss calculations are for standard PVC pipe.
• Cl 160 PVC pipe is not commonly available in 1/2" size, therefore the calculator assumes that Cl 315 pipe will be substituted.
• Cl 200 PVC pipe is not commonly available in 1/2" size, therefore the calculator assumes that Cl 315 pipe will be substituted.
• Cl 160 PVC pipe is not commonly available in 3/4" size, therefore the calculator assumes that Cl 200 pipe will be substituted.
• This calculator uses a contracted form of the Hazen-Williams formula which is the industry standard for calculating friction loss.
• A conservative C coefficient of 140 is used for PVC pipe (many authorities use 150 which is risky as it leaves no margin for error).
• Velocities over 7 feet per second should not be allowed in PVC pipe.
• Velocities over 5 feet per second should be avoided, especially with pressures higher than 80 PSI, or in pipe larger than 2".

This program is Freeware and may not be sold.
Java programming by Dan Rasmussen, methodology by Jess Stryker, ©1999.
Special thanks to Dan for his contribution!

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Stand-alone Windows Pressure Loss Calculator

There is also a free stand-alone Windows friction loss calculator available, which was created by Steve Granger. Use the value of 1 for both the Specific Gravity and the Viscosity of water when using this calculator. Under the heading Material, "Plastic" is PVC plastic. The current version doesn't work for poly plastic. The windows calculator will calculate friction losses for galvanized steel several other materials in addition to plastic. Download the calculator from http://www.IrrigationTutorials.com/LineSize.exe. Many thanks to Steve for creating this calculator and providing it free for everyone to use.

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Pressure Loss Tables

For those who don't have a browser that supports Java (and don't "do Windows"), here are some old-fashioned pressure loss tables you can use to calculate the friction loss in your mainline.

How To Use a Pipe Pressure Loss Table:
(PSI = pounds per square inch = lbs./sq.in.)

• Select the proper table for the type of pipe, i.e.; SCH 40 PVC, SCH 40 Steel, Polyethylene, copper, etc.
• Locate the proper column on the table for the pipe size.
• Read down the column to the row for the flow rate (GPM) in the pipe section. You will find a PSI loss value (given as PSI/100).
• Multiply the PSI loss value shown by the total length of the pipe section, then divide the product by 100. (PSI loss on these tables is given in PSI per 100 feet of pipe.)
• Pressure loss values in red are over 5 feet per second. This is a high velocity, but considered acceptable for short distances (less than 50 feet of pipe length.)
• Pressure losses greater than those shown on the chart can cause permanent and expensive damage to your plumbing. You must use a lower flow (GPM) in the pipe.

PSI Loss Value x Length of Pipe / 100 = PSI loss in pipe

Example: 1" size, type SCH 40 PVC mainline. The length of the mainline pipe is 23 feet. The water flow rate through the mainline is 18 GPM. Using a pipe pressure loss table we find that the PSI loss for 1" SCH 40 PVC at a flow rate of 18 GPM is 8.12 PSI per 100'. Therefore:

      8.12 x 23 / 100 = 1.87 PSI - to simplify, you can round up the value to 2 PSI loss
 

(Note: PSI loss charts vary somewhat from each other. Other charts may result in an answer slightly different from the one in this example.)

Pressure losses shown in red reflect flow velocity between 5 and 7 feet/second. Use care utilizing flow velocities in this range. Water hammer damage can result from a combination of high pressure and high velocity. Pressure losses shown are in PSI per 100 feet of pipe length.

Pressure losses shown in red reflect flow velocity between 5 and 7 feet/second. Use care utilizing flow velocities in this range. Water hammer damage can result from a combination of high pressure and high velocity. Pressure losses shown are in PSI per 100 feet of pipe length. 

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Copper Pipe and Tube

Pressure Loss Table, Type K Copper Pipe or Tubing

Type K copper pipe has the thickest wall and highest pressure ratings of the common copper tubing types. In order of wall thickness, common copper tubing types are Type M (thinnest), Type L, and Type K (thickest). Type L is commonly used for household plumbing. If you don't know what Type the pipe is, assume it is Type K.

Pressure losses shown in red reflect flow velocity between 5 and 7 feet/second. Use care utilizing flow velocities in this range. Water hammer damage can result from a combination of high pressure and high velocity. Pressure losses shown are in PSI per 100 feet of pipe length.

 

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Pressure Loss Table, Type L Copper Pipe or Tubing

Type L copper tube is the type most commonly used for household plumbing. In order of wall thickness, common copper tubing types are Type M (thinnest), Type L, and Type K (thickest). If you don't know what Type the pipe is, assume it is Type K.

Pressure losses shown in red reflect flow velocity between 5 and 7 feet/second. Use care utilizing flow velocities in this range. Water hammer damage can result from a combination of high pressure and high velocity. Pressure losses shown are in PSI per 100 feet of pipe length.

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