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Sprinkler System Mainline Pipes

 

Quick & Dirty Summary

Definition of irrigation mainline:  The mainline is all the pipes between the water source (POC) and the irrigation zone control valves. Another definition is that mainline is any pipe that is always pressurized with water.

Worksheet for Choosing Your Mainline Pipe or Tube

 

Excessive Water Pressure:  In all cases if your static water pressure exceeds 100 PSI it is advisable to install a pressure regulating valve at the irrigation connection point to maintain a pressure lower than 100 PSI.  All of these pipes or tubes may burst at higher pressures.

Temperate Areas (ground doesn’t freeze in winter)

  • Rocky Soil: Is your ground very rocky, so that it would be impossible to keep rocks larger than 2″ diameter from contacting the pipe?  If “yes” consider using PEX tubing for you mainline.
  • Normal soil: If ground is not rocky consider using SCH 40 PVC pipe for your mainline.  If pipe larger than 2″ is needed use Cl 315 PVC pipe.

Frost Areas (ground freezes at least a couple inches deep in winter.)

  • If your static water pressure is less than 60 PSI the use of 125 PSI rated poly tube may be sufficient if cost is a major issue.  However it would be better to use 160 PSI tube if you can afford it.
  • For static water pressure between 60 and 80 PSI, use 160 PSI rated poly tube.
  • For static water pressure between 80 and 100 PSI, use 200 PSI rated poly tube.
  • Make sure you provide a method to blow out or drain the water from the mainline completely during winter.

Pipe or Tube Size:  There is no easy way to say what exact size you should use.  It you really don’t want to do calculations the following is a very rough educated guess that will work for most (but not all) situations.  Use a pipe or tube size that is the next size larger than the water supply pipe the irrigation system (POC) is tapped into.  Do not use smaller than 3/4″.  Ie; if the irrigation system mainline is going to tap into a 1″ house water supply pipe (POC), then use a 1 1/4″ size mainline for the irrigation.

Pipe Depth:  Bury the mainline pipe at least 18″ deep from the top of the pipe to the ground surface.  It is critical that this pipe be protected from accidental damage and light frosts.

No water running through the house!  To avoid nasty surprises, avoid using a water supply (POC) for your irrigation system that passes through a house inside the walls, under floors, or through the attic.

Keep reading for in-depth details and answers to “why?”

Why is this page so darn long?  It’s because a lot of the basics of irrigation system piping and hydraulics that you will need to know to create a good design are first introduced here.  So there are lots of explanations on this page.  Plus there are a lot of different mainline variables and I need to address all of them because I don’t know which ones will impact you and which won’t.  Finally, I give some rather detailed examples to help you understand all of this hydraulics stuff.

P.O.C. stands for Point of Connection, this is the standard name used in the irrigation industry to describe the point where the irrigation system taps into a water supply.  You might consider it the “start” of the irrigation system.  It is standard industry practice to label the location of the water source on irrigation plans with the letters “POC”.

Pipes or tubes?  I tend to use the term “pipe” because we use primarily PVC plastic pipe here where I live, but tube is an acceptable term as well.  Pipe tends to be those products with diameters similar to iron pipe, tube tends to be used for those products with diameters similar to copper tube.  I’ve have never found that either term is uniformly used, in the “real world” they tend to be interchanged a lot.

Mainline Background Information

Your water source may be a water company pipe in the street or alley, or it could be a pump, a well, lake, pond, stream, tank, or whatever.  For residential properties there is typically already a house water mainline that takes water to the house.  Most of the time you will be tapping into that house water mainline for your irrigation system water.  In that case you have two mainlines, a house mainline taking water to the house, and a irrigation mainline that takes water to the irrigation valves.

The irrigation mainline pipe is the primary or “main” pipe in your sprinkler system.  As mentioned above, it is the pipe that goes from the water source to the zone valves used to turn on the sprinklers or drip emitters.  It is typically always filled with water and pressurized.  For this reason it is typical to use a higher quality of pipe for the mainline. The mainline often has higher water pressures in it and also has a lot more pressure surges (water hammer) in it.  Typically it always has pressurized water in it, so if it breaks a lot of water may be wasted before the break gets noticed and repaired.

Most pipe has a maximum pressure rating. The pressure rating is not the recommended normal operating pressure, it is the maximum pressure the pipe should be exposed to.  Because of the higher demands on the mainline, normally the pressure rating of the pipe you use for mainline should be double the actual static water pressure. So if you have 60 PSI of static water pressure, the mainline pipe should be rated for 120 PSI or higher.  This may sound like over-kill at first, however huge pressure surges in an irrigation system are a normal occurrence, so you need to design the system to withstand them. Pressure spikes of twice the normal pressure may occur every time a sprinkler control valve is closed. That’s a lot of pressure spikes over the life of your irrigation system, and each one puts a huge stress on the pipe. Using a higher pressure rated pipe allows for a margin of safety and helps reduce the frequency of pipe breaks. Breaks are difficult and expensive to repair.

Building Codes:

Local building codes may dictate exactly what type of pipe or tube you may use. The Uniform Building Code,  National Electrical Code, and the International Plumbing Code are standard systems of code requirements that have been developed to try to create some uniform standards, and almost all areas of the USA have adopted these codes for uniformity.  However each local jurisdiction has the ability to add to, modify or waive the uniform codes.   Other countries also use similar, if not the same, codes.  So the basic rule is that you have to check with the local building officials to find out just what the codes are.

On the upstream side of the backflow preventer (the side the water is coming in from) the Uniform Plumbing Code applies, and it requires that all pipes and tubes be the same type as whatever the code requires locally for domestic potable water pipes.  Note: In some places the building officials require the ENTIRE irrigation mainline be installed to building code requirements.

In most places you need to get a building permit for your irrigation system before you install it.  As part of the permit process an inspector may need to check the water pipe or tube after you place it in the ground, but before you use or bury it.  Generally the authorities consider the irrigation mainline to be a potable water pipe (ie; it contains drinking quality water.)  They can be really, really anal about this.  I’ve never seen them fine anyone for using the wrong pipe, but I have seen a lot of pipe/tube have to be ripped out and replaced!   Before you decide to do your own thing with the mainline it is best to check the local requirements. While many don’t care, in some areas the building officials are very picky about what type of pipe you use for the entire irrigation mainline, and sometimes the entire irrigation system right out to the last sprinkler head. This is especially true for areas where water is in short supply, like the southern and western USA.  In these areas officials want to avoid possible leaks or breaks resulting from poor quality pipes and tubes that will waste water.  Please call them up or check the local building department website and check.  It is very expensive and a lot of hard work when they discover you used an unapproved pipe/tube and they make you remove and replace it.

Types of mainline pipe:

PVC Plastic. PVC pipe is the default pipe used for irrigation systems throughout the western and southern areas of the USA and is also used in most other temperate climates.  PVC pipe comes in different types, the common ones are “schedule 80″, “schedule 40″, “class 315″, “class 200″, and for really low-quality cheap systems, sometimes “class 125″ is used.  (There are other classes of PVC available that are not commonly used in irrigation.)  Schedule is often abbreviated “SCH” and class is abbreviated “Cl”.  For mainlines the normal recommended type of PVC is Schedule 40 PVC (SCH 40) with the letters UPC and NSHF stamped on the pipe.  Often the local authorities will require that 2″ size and larger mainline be class 315 PVC stamped UPC & NSHF, check with local building officials. For pipe sizes larger than 2 inch, Cl 315 PVC pipe is stronger than SCH 40 PVC and is a better choice.

Polyethylene (poly) Tube. In areas where the soil freezes heavy wall (rated 125 PSI) polyethylene tube has been used for many years.   Poly tubing rated at 160 PSI is now commonly available and is the most common poly tube to use for mainlines.  200 PSI poly tube is also available at some stores and is even better. “Poly tube”, as it is often called, is a flexible black plastic tubing which bends easily and has a slightly oily feel to it. You can easily scratch the soft surface with your fingernail. Polyethylene holds up better to freezing than PVC does. However, polyethylene as a general rule has a low pressure rating, and thus is subject to bursting and splitting open. The tubing sold as “Irrigation poly tube” is a lower quality, low pressure rated tube made for low pressure irrigation laterals. Do not use it for mainlines.

PEX tubing. PEX tube is now used in place of polyethylene mainlines in most places and is now used extensively for plumbing inside buildings.  PEX is a form of polyethylene which is reinforced to make it stronger while maintaining good flexibility and freeze resistance.  This does not mean you should allow the water in PEX to freeze, it is just less likely to be damaged by a light frost!  Be aware that PEX tube has less carrying capacity than polyethylene (as well as most other tubes and pipes), so you will need to use a larger size of PEX tube than you would have used for polyethylene.

caution  PEX is often sold as an “equivalent” or “equal” to copper tube, because it has the same outside diameter and can use compression type fittings similar to those used for copper tube.  However PEX has a much thicker tube wall so the inside of the tube has a smaller diameter than copper, and thus the area inside for the water to flow through is smaller. This smaller inside area results in considerably less water capacity than copper. So if you replace a 1″ copper tube with a 1″ PEX tube you may discover a large drop in water capacity and pressure through the tube.  As a general rule, when replacing a existing copper tube with new PEX tubing I feel it is best to increase the PEX tube size to be one size larger than the copper tube was.  If anyone argues this with you just ask them to get you both and hold them up side-by-side.  While the outside dimensions are identical, you will see that the PEX is much smaller inside.  Smaller means less capacity.  Yes the PEX is slicker inside than copper and allows the water to flow easier.  That helps a little but doesn’t solve the problem.  There are a series of spreadsheets on this website that calculate pressure loss and velocity in tubes.  Get the one for PEX and the one for copper and plug in the same size, flow and distance into both.  You will see a huge difference.  And yes, those spreadsheets do take the slickness into account.  Don’t misunderstand, PEX is a great product from my observations and I use it.  It hasn’t been around long enough to know for sure how it will hold up over time, but so far it’s good.  Just remember you do have to use a larger tube size than you use for copper.

Metal pipe or tube. 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.  I often use copper for short irrigation mainlines,.  The mainline on my house front yard irrigation is only 8 feet long so I used copper for durability and because my house plumbing is also copper.

Galvanized steel is not such a great choice for irrigation systems, it corrodes on the inside and will slowly choke off the water flow over time. Galvanized steel also flakes off bits of rust and corrosion inside the pipe which can cause massive, expensive problems with irrigation systems. Early irrigation systems used galvanized steel pipe, but the use was mostly stopped in the 1960’s.

Pipe Depth

The Uniform Plumbing Code says water pipes and tubes should be at least 18″ deep or below the frost line*, whichever is deeper.  The depth is measured from the average soil level to the top of the pipe (so burying it 12″ deep and putting a 6″ berm over the pipe location doesn’t meet the code requirement of 18″ deep.)  While digging a 24″ deep trench so that the mainline pipe/tube can be 18″ deep is a lot of work, there are really some good reasons to do it even if the authorities don’t make you.  The mainline pipe is your most important pipe in the irrigation system, when it breaks the whole system is out of service.  Having the mainline that deep protects it from most of the common sources of breakage (like gardening tools and car tires!)

Cold Climates & Winterizing

It is important to keep water from freezing inside your mainline pipe in cold climates.  The standard method for “winterizing” a mainline is bury it below the frost line, or more often, to drain or blow the water out of your mainline prior to each winter.  If you plan to winterize the irrigation system by removing the water you don’t need to bother with burying the pipe below the frost line, UNLESS of course, the local building officials say you do!   If you plan to drain or blow out the mainline you will need a winterization shut-off valve located someplace where the winterization shut-off valve and pipe upstream of it will not freeze (or you can use a special freeze-proof valve, and/or heating cable to prevent it from freezing.) Most people put the shut-off valve in their basement or a heated shed/pump house. I strongly recommend that you use a high-quality brass ball-valve for the shut-off valve (or a special frost-proof designed valve.)  A “ball valve” is a type of valve design, you will find them at any hardware store.  Do not use a “gate valve” as they tend to leak.  If you plan to blow the water out of the pipes you need a capped tee right after the winterization shut-off valve so you can connect the air compressor to it and blow air into the mainline.  See the Winterization Tutorial for more information on winterizing your irrigation system.

Pressure Loss Calculations

Common question: “Can I skip calculating the pressure loss in the house mainline? It is upstream of where I measured my static water pressure.”   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 in pipes 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 as well as the irrigation 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.  For purposes of irrigation design your “house mainline” starts where your house water supply pipe branches off of the water supplier’s big water mainline, which is typically in the street or alley.  The house mainline ends at the point where you tap into it for your irrigation water.  More on this later in this page.

whyWhy 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.

Pressure Loss in pipes and tubes is often called “friction loss“.  Often all pressure loss is called friction loss.  I sometimes slip into this bad habit as you may notice.  So if you find a reference to friction loss just think “pressure loss”.

In most cases you will have two (or more) different mainline 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. (A bigger irrigation mainline you ask?  Yes, that is perfectly fine.  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.  This is why the Pressure Loss Table in the Sprinkler Design tutorial has 3 entries for mainlines, “House Mainline 1″, “House Mainline 2″, and “Irrigation Mainline”.

Example: You have a 1″ polyethylene “house mainline” that starts at the water company’s main (your “water source”, a large water company pipe in the street that supplies water to all the houses in the neighborhood). Your polyethylene house mainline tube takes the water to the water meter and then continues to take it on to the house. At the house the mainline becomes copper, and extends into the basement then on to all the various faucets in the house. You decide to tap into this copper tube in the basement for the irrigation system water.  This location will be your Point of Connection (P.O.C..) for the irrigation system and you will want to install a good quality shut-off valve at the start of your irrigation mainline, for turning off the irrigation for repairs or winter.  You install a new 1″ copper irrigation mainline from the shut-off valve to a backflow preventer in the basement and the copper continues through the basement wall back outside again. Finally you use an adapter to attache a new 1 1/4″ PEX “irrigation mainline” to the copper mainline just outside the house. The PEX mainline runs a short distance across the yard to the location of the irrigation zone control valves.

OK, here’s what you do. You will need to calculate the pressure loss for each of those sections of pipe or tube 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 house mainline
+ 1″ copper irrigation mainline to backflow preventer and out through wall
+ 1 1/4″ PEX irrigation mainline to the valves
= Total mainline pressure loss

Now you need to know the length of each of these pipe sections.

Where are the existing pipes?  Existing pipes are sometimes not so easy to find or measure the length of, they may be already buried and you can’t see them, and they may even be under a street or driveway too!  So, if the water company’s big water supply pipe is somewhere out it the street, how do you know where it is so you can measure the length of your house supply mainline from it to the house? 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. If you can find two manholes, the pipe generally runs in a straight line between them. I know that’s not real helpful, and there is one other trick that sometimes works. If you call 811 (for those in the USA) they will send someone out to mark the underground utility locations for you using paint, including the location of the water company’s pipe, as well as the location of the pipe running onto your property.  You are going to need to do this anyway if you plan to install your irrigation anywhere near the street.  Often utilities are actually located in an easement on your property.  I’ve seen lot s of water pipes and especially high-voltage electrical wires that run right through peoples front yards, often in places I would never have expected.  The problem is if the water company’s pipe is off of your property out in the street you probably don’t plan to dig out there so they won’t mark it for you.  They will only mark locations where you plan to dig.  Generally you outline the area you are going to dig up with white paint (they will give you instructions on what to mark and how.)  If you can be there to meet them when they mark the pipes they will likely be happy to tell you where the water pipes are.  You could also just mark the street to indicate you are going to be digging up the street.  But be warned that they may notify the street maintenance people, who may have some questions for you as to why you are indicating you plan to be digging up the street!

Free Utility Location in USA

Dial 811 for Free Utility Location in USA & Canada

Irrigation mainline length.  So how are you supposed to know the length of your irrigation mainline before you even design the sprinkler system? At this point you probably don’t know how many valves you will need or where they will be located, so how can you know how long the mainline is going to be? 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?  Simple irrigation valves can be installed underground in a box almost anywhere you want, although in most cases it is best if they are near (but not within) the area that they will be watering. A special type of irrigation valves called anti-siphon valves can’t be buried and must be higher than the area to be watered, so that will limit where you can install them.  Also keep in mind that with an automatic irrigation system the valves don’t need to be in a single group. While I suggest you install them in small groups for convenience, it is perfectly fine to have several groups in various areas of the yard.

It is best not to locate a valve within the area that is watered by the sprinklers that the valve turns on and off. If you do, you are probably going to get sprayed by the sprinklers when you turn them on. This is not fun.

Example:
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, say, the front yard. The part of the irrigation mainline that is within the basement should be metal or PEX pipe (not all local building officials allow PEX pipe in a basement, so check first.) 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 inlet in the basement floor, 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 and water-proof 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 using an adapter. If the adapter uses threads make sure the female threads are metal, and the male threads are plastic. If you use plastic female threads with metal male threads the hard metal male threads will expand ad contract with temperature changes and cause the soft plastic female end to split open. If the pipe comes out of the wall above ground it will need to be metal until it is a few inches underground. Plastic pipe should never be used above ground where it would be exposed to sunlight. Sunlight will degrade it and cause premature failure.

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 reduced-pressure type backflow preventer in the basement and standard globe-type control valves out in the yard, 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 we will install underground in a box hidden by the shrubs. So far, so good. Hopefully you are now starting to get a mental image of what your irrigation system might look like.

Now let’s say you also want to water the backyard. You could put the valves for the backyard near 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 choose to run the mainline under the house in the crawl space, through an unfinished basement, even through an attic- this is OK but you should use metal or PEX pipe under or in the house. The type of pipe under or in the house must meet building code requirements.) You can even use metal pipe and strap it on the outside wall of the house, but it looks a little ugly! Now the question is where to put the valves for the backyard? As with the front yard, 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 well as a way to hide the valves.

Calculating the Mainline Size

What size should your new irrigation mainline be? I wish there was an easy answer, but, unfortunately, you will need to start with an educated guess. 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. Using a larger size pipe means there will be less pressure loss as the water squeezes through it. 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! Don’t panic, it is easier than it sounds and I will lead you through it step-by-step!

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.

Tip
Bigger is better!  Remember that with sprinkler system pipes a bigger pipe almost never hurts anything and is almost always better. Using a larger size pipe will not hurt anything except in a few very, very rare situations. The chance of you actually having one of those situations is close to zero.  In 35 years of doing this I have never run into one of those situations.  Never.  On the other hand, using a smaller pipe can hurt in many situations. This seems illogical, and you will run into a lot of professional sprinkler folks who will swear that a smaller pipe is needed to increase water pressure. They did not learn this in school!  It is not true. They aren’t bad people, they were probably taught this by whomever taught them irrigation, and the irrigation-myth gets passed along again and again, taking on a life of it’s own. Here’s the science-based truth: When you make the pipe smaller and try to force the same amount of water through it, the water must travel faster (higher velocity) to squeeze through that smaller pipe. Bernoulli’s Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases. Decreasing the size of the pipe will not create more water pressure, it does just the opposite. You can test it yourself- go to Animated Demonstration of Bernoulli’s Principle where there is a interactive graphic demo that allows you to change the size of a pipe and see what happens to the velocity and pressure. It’s fun, and a picture is worth a thousand words!

Hose Bibs and Yard Faucets

One more item to quickly remind you of. I strongly recommend that you do not use a hose bib or faucet on your house as your irrigation water source. It is best 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, 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 pipes inside the walls of houses are too small to handle the volume of water needed for an irrigation system. Those pipes are intended to supply the much lower flow requirements of household appliances, faucets, and showers. If you force more water through those pipes, the high flow velocity can create pin-holes in the house pipes. These pin-holes are extremely expensive to repair! Often the only fix is to completely replace the pies or install a plastic coating on all the pipe in the house. Plus the noise of the water flowing through the pipes when the sprinkler system is running may drive you nuts! It can be very loud.

Looped Mainlines

If you have a large yard it may be beneficial to use a looped irrigation mainline. Typically there is no benefit to this on a smaller residence, but if you have a big country estate with lots of irrigation it may be worth looking into. If you have more than an acre of property I would suggest you take a look at the tutorial How to Design a Looped Mainline for Irrigation Systems.  I almost always loop the mainline on large estates.  If nothing else it allows me to put lots of manual hose bibs around the yard for the owner to use.

Velocity

Water velocity, or speed, in the pipes is important.  it is possible to force the water to go  way faster through the pipe than is safe.  When the water goes too fast it causes two major problems.  The first is water hammer.  Water hammer is what happens when all that fast moving water is forced to stop moving almost instantly.  This happens when the automatic valves on your irrigation system close.  It also happens when the automatic valves on your dishwasher and washing machine close too!  The sudden stop causes a huge pressure surge, often doubling the water pressure in the pipes and tubes for a second or two.  This can cause the pipe or tube to burst.  It is also very loud!  The second problem cause by excessive velocity is called “scrubbing.”  It is pretty much what it sounds like, believe it or not, water is slightly abrasive.  At high speeds it scrubs away the inside layers of the pipe or tube.  This is really common in homes with copper pipe, since copper is a soft metal.  The result is tiny pin holes start to form in the tube.  Whole industries have sprung up re-plumb homes where the owners have installed appliances or sprinkler systems that use too much water and cause high velocity in the house pipes.  You better believe it is expensive to re-plumb a house!

The good news is that the pressure loss calculators listed below will also tell you the water velocity.  The spreadsheet calculators will even warn you if the velocity is too high.  For mainlines the velocity should be kept under 5 feet/second.  For other irrigation pipes that are downstream or after the zone valves (these are called “lateral” pipes) you can go up to 7 feet/second.  These lateral pipes have less pressure surges because they are after the valves, and scrubbing is not as big an issue as the water only flows through them when the irrigation valve is on.

 

How to calculate pressure loss

Water pressure loss in pipes is calculated by using the pipe size and the flow rate of the water through the pipe. This website has several calculators you can use, or you can do it the old-fashioned way and use tables.

Spreadsheets for Calculating Friction Loss

This is how I do it.  I’ve created a whole collection of spreadsheets that do all the calculations for pressure loss and velocity for various types of pipe and tubing. Hopefully you will find these spreadsheets pretty easy to use, I tried to make them do as much of the work for you as possible. They will calculate both pressure loss and velocity, and will warn you if the flow you have selected is too high. Spreadsheets are available for pretty much all the common types of pipe found in homes today. Spreadsheets for Calculating Friction Loss & Pressure Loss in Pipes and Tubes.


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.


 Pressure Loss Tables

There is a page on this website with a few old-fashioned pressure loss tables you can use to calculate the friction loss in your mainline. This is the old-school, low tech method.  See the Irrigation Mainline Pressure Loss Tables page.

Whew, this was a long page with a lot of information!  Glad you made it through.  You can do this!


This article is part of the Sprinkler Design Tutorial Series
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