Jess Stryker's Landscape Sprinkler Design Tutorial Step #5 Lateral Pipe Sizes

You're almost finished! As you remember, the term lateral pipe refers to all the pipes between the control valve and the sprinkler heads. A valve circuit is 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.

Why can't I just tell you what size pipe to use based on the number of sprinklers or something easy? Why all this hassle, you ask? Well, I wish I could! The problem is that as the water flows through the pipe, the water pressure drops. The farther the water goes, the more the pressure drops. Smaller pipe sizes result in even greater pressure drops. If the pressure drops too much, the sprinkler at the end of the pipe won't work right! So we not only need to know how much water is flowing through the pipe, we also need to know the length of the pipe. Then we can figure out the smallest pipe size that will work. Don't fight it! Just work through the process step by step. It looks much harder than it is!

Let's get going!

---

Calculating the PSI/100 value:

Select a valve circuit. Determine which pipes and sprinklers are connected to the valve. What you want to find out is 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 that the water would pass through on the way 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. What you're determining is the "worst case situation" for water coming out of this valve, that is, the farthest distance the water must travel to any single sprinkler head operated by this valve. See the example below.

Typical Valve Circuit

This distance will be inserted in the space labeled "____ Feet Total Length" in the PSI/100 formula below.  

Back when you estimated the pressure losses for your irrigation system you established a maximum amount of pressure that could be lost in the laterals. Look on your Design Data Form for the Pressure Loss Table. There you will see a figure you entered called "_____ PSI - Laterals". Insert that number in the blank space in the formula below (the space labeled "_____ PSI").

Here's the formula:

( ____ PSI x 100) / ____ Feet Total Length = PSI/100

Calculate the "PSI/100" value by multiplying the "PSI Loss" times 100, then divide the product by the "Total Length". The result is your PSI/100 for this valve only. You will use this "PSI/100" value later to determine the pipe sizes.

Example: Let's say the value from your Pressure Loss Chart for "____ 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 = 3.4 PSI/100

Write down your PSI/100 value next to the valve on your plan.

Repeat this procedure for each valve circuit. Or if you prefer--

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 valve circuit has the longest "Total Length" and use that valve circuit to calculate your PSI/100. The advantage of using the same PSI/100 value for everything is uniformity of design. For example, a pipe with five half circle spray heads downstream would always be connected to the same size pipe. This is much less confusing for the installer, which is the main reason we do it this way. The advantage of using separate PSI/100 values is that there is a possible savings in pipe cost.

The Pipe Size Table:

This table is used to determine what size each section of lateral pipe will need to be. As you remember, "lateral" pipe is the name used for the pipe after (downstream) of the sprinkler control valve. (The pipe upstream of the valve is called "mainline".) 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". Each section must be sized based on the actual GPM flow passing through that section of pipe. If you haven't yet determined the flow rate for each individual section of pipe, you need to return to the previous page of the tutorial and do it now.

Wait a minute...

If you haven't been using this tutorial for your sprinkler design please stop here and do yourself a big favor. Start your design all over from the beginning using this tutorial. I know, that's a lot of work, but give me a chance to explain. Many people pick up one of those "design it yourself" brochures at the hardware store, get stuck, search the Internet for help with the pipe sizes, and wind up here. The fact that you are searching for help should show you that there is something seriously wrong with the design guide you are using. I hear from dozens of people every year who wasted hundreds (even thousands) of dollars and countless hours of time on a sprinkler system that didn't work. They almost always used one of those short design guides. Please don't join them in their misery. Toss out the brochure and go back to the beginning of this free tutorial and use it. If you don't, don't say I didn't warn you. True story- I designed my first sprinkler system using one of those pamplets. Guess what? It didn't work!

Warning: What type of pipe are you planning to use for your laterals? It makes a big difference!

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 smaller 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: Do not use these types of PVC pipe for your laterals.

Polyethylene, Polybutylene: Use the chart below as if you were using Class 200 PVC. 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.

Previously you wrote the flow (GPM) for each pipe section next to the pipe on your drawing. Now you're going to use that information along with the PSI/100 value you determined at the top of this page.

1. Find your PSI/100 value in the top blue row.
2. Read down the column to the value equal to, or higher than, the GPM in the pipe.
3. Read across to the pipe size for that section in the right column.
4. If you don't understand #1 - 3, try reading this entire page starting at the top. (hint, hint)

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. This chart is derived from an earlier version copyrighted 1979 by Jess Stryker, all rights reserved.

Copyright 1997, Jess Stryker, All rights reserved.

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, so higher losses are not recommended. (Look at the highest values in the lower right side of the chart. These values represent the maximum safe flow for the given pipe size. Notice how they are all becoming the same as you move to the right?) The result of using the 6.0 column will be that the pressure losses will be lower, which is a good thing. It won't hurt, your system will simply work better!

Example: Assume the flow for your first pipe section out from the valve is 15 GPM. Assume that you calculated the PSI/100 constant value for this lateral as 3.4 PSI/100. The closest value on the chart that is less than or equal to 3.4 is 3.0. Using the column labeled 3.0, go down the column until you get to 17, which is the first number that is equal to, or greater than, your flow of 15 GPM. Read to the right across the row with 17 in it, and you find that the pipe size will be 1".

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 valve. On almost all of my designs the lateral pipe section immediately downstream of the valve is a size larger than the valve. So don't worry if yours works out that way too! But don't worry if it works out that the pipe is the same size as the valve either. In particular, on low-flow systems it is very common for everything to be 3/4"- valves, laterals, sometimes even the mainline!

Write the pipe size down next to the pipe on your plan. Repeat for each pipe section

.

Velocity: You want to avoid high water velocities in the pipes because the velocity can cause damage in your irrigation system. For mainlines the velocity should not be over 5 feet per second. For lateral pipes downstream of the valve the industry standard allows the use of velocities as high as 7 feet per second. In the chart above the dark red GPM figures are between 5 and 7 ft/second, and flows over 7 ft/second are not shown. You will note that in the example above the velocity of the water in the pipe is greater than 5 ft/second, but less than 7 ft/second. Never use more than the maximum water GPM shown on the chart.

Once all the pipe sections connected to the first valve are sized, go to the second valve and start over. Repeat this process for each valve.

This system may seem confusing at first but once you get started it will become easy and quick. Believe me it is much, much easier and faster than the old trial and error method!

When in doubt, always use a larger diameter pipe!

No, I don't own stock in a pipe manufacturer and I'm not getting kickbacks for pushing bigger pipe! Unlike clothing, pipe can never be a "size to 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. 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 this by tech support people at some of the major sprinkler manufacturer's. That's an industry disgrace! So one more time to drill it in to 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 enough reason by itself.

Where's the 1/2" size pipe you ask? Well, 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" CL 200 PVC pipe is generally not available in most areas. The water capacity of 1/2" SCH 40 and poly pipe is almost zero. 1/2" PVC pipe is hard to glue together without using too much glue. The glue piles up on the inside of the pipe 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 change it you're screwed. Conclusion: The small amount of money saved by using 1/2" pipe just isn't worth the hassle and risk.

---

*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, cows wouldn't fly. Some have argued that the pipe size MUST be decreased along a lateral because the flow is decreasing at each sprinkler head location. Thus the velocity in the pipe is decreasing, resulting in an increase in pressure (according to Bernoulli.) So the argument is that the pipe sizes must become smaller in order to keep the velocity constant and avoid a pressure change. (Are you bored yet?) This argument falls flat when you do the math. At a flow of 7 feet per second, which is the maximum recommended safe flow for PVC pipe, the pressure drop due to velocity would be a whopping 1/3 PSI. Of course, this 1/3 PSI gain in pressure is easily offset by the friction loss resulting from the water moving through the pipe. In other words, the pressure difference due to velocity is not significant in the first place, and even if it was, the friction loss in the pipe offsets it. 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 the reason airplanes fly, and therefore, it is also the reason that people, and yes, even cows, fly!

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

Glossary    Conversion Formulas    About the Author

Comments?   Questions?   Product Data & Reviews   More Irrigation Tutorials!

www.IrrigationTutorials.com

IrrigationTutorials.com is not affiliated with any irrigation product manufacturer, distributor, or retailer.

By using this tutorial you agree to be bound by the conditions and limitations listed on the disclaimer page.

Jess Stryker's Irrigation Tutorials
Copyright © Jess Stryker, 2003
All rights reserved.