Friction Loss:
As water moves through a pipe it loses pressure due to a phenomenon commonly called “friction loss”. Much of this loss is caused by turbulence, but we call it friction loss for simplicity. The amount of friction loss is determined by the type of pipe, the diameter of the pipe, the amount of water flowing through the pipe, and the length of the pipe. A complex formula (called the Williams/Hazen Formula) predicts the amount of pressure that will be lost due to friction loss. The water also loses pressure each time it passes through a valve, a backflow preventer, or anything else it encounters on it’s way to the sprinkler head. Even a bend in the pipe causes pressure loss! Don’t panic over the formula, we’ll use a pipe sizing chart or a friction loss calculator!
You need lots of pressure at those sprinkler heads!
The sprinkler head needs a minimum amount of water pressure to work properly. The manufacturer’s performance charts tell you how much pressure is required to achieve a specific radius for the water. As the pressure increases so does the flow (GPM) and the radius of the throw. So in order to assure that there is enough pressure to make the sprinklers operate as they should, we need to calculate the pressure losses between the water source and the sprinkler head. If the pressure loss is found to be too great, then we must reduce. The easiest way to do that is to use a larger size pipe.
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DETERMINING THE SPRINKLER PIPE SIZE
There are several methods used to determine pipe sizes of sprinkler system lateral pipes. I’m going to explain two methods. One method is faster but less accurate, the other is very accurate but takes more time.
Chart Method:
Pros: The fastest and easiest method. Requires a single, simple calculation and uses a chart to determine the sizes.
Cons: The learning curve to use it is a bit more difficult to understand. It uses an averaging system to arrive at pipe sizes.
See step-by-step tutorial for the Chart Method
Trial & Error:
Pros: Very accurate, calculates the pressure loss in each pipe section using a spreadsheet. Easier to understand.
Cons: Time consuming, need to enter data into the spreadsheet, uses trial and error to establish pipe sizes.
See step-by-step tutorial for the Trial & Error method
Which method should you use? For a beginner with a small irrigation system probably the Trial & Error system will be easier. Below are overviews of each method for experienced designers to use. Unless you are experienced you should probably read the full tutorial for the method you select.
Overview: CHART METHOD FOR LATERAL IRRIGATION PIPE SIZING
See Determining Sprinkler Pipe Size Using a Pipe Sizing Chart for detailed step-by-step instructions.
Calculate the PSI/100 value:
( ____ PSI x 100) / ____ Feet Total Length = PSI/100
____ PSI. Insert the maximum PSI loss for the valve circuit laterals into the formula where it says “____PSI.”
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____ 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:
PSI/100 = Desired PSI Loss in Lateral x 100 / Total length of Lateral
PSI/100 |
||||||||||
0.2 |
0.5 |
0.8 |
1.0 |
1.5 |
2.0 |
3.0 |
4.0 |
5.0 |
6.0 |
SIZE |
2.2 |
3.3 |
4.4 |
5.0 |
6.2 |
7.1 |
8.5 |
10 |
11 |
13 |
¾” |
3.8 |
6.3 |
8.1 |
9.2 |
11 |
13 |
17 |
20 |
22 |
24 |
1″ |
7.1 |
12 |
15 |
18 |
22 |
25 |
31 |
36 |
37 |
37 |
1¼” |
11 |
16 |
22 |
24 |
31 |
35 |
44 |
48 |
49 |
49 |
1½ |
18 |
30 |
40 |
44 |
57 |
65 |
76 |
76 |
76 |
76 |
2″ |
28 |
46 |
60 |
67 |
83 |
96 |
114 |
114 |
114 |
114 |
2½” |
46 |
75 |
100 |
112 |
140 |
162 |
165 |
170 |
170 |
170 |
3″ |
87 |
140 |
185 |
208 |
250 |
280 |
280 |
280 |
280 |
280 |
4″ |
255 |
410 |
540 |
600 |
600 |
600 |
600 |
600 |
600 |
600 |
6″ |
Flows shown red are over 5 feet/second. Use caution!
Instructions:
- 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.
- Where’s the 1/2″ pipe? See “why not 1/2″?”
CONFUSING? DON’T PANIC: For detailed instructions see the page Irrigation Pipe Sizing Chart for Laterals.
Overview: TRIAL & ERROR METHOD TO DETERMINE LATERAL PIPE SIZE
See Calculating Sprinkler System Pipe Size Using a Spreadsheet for detailed step-by-step instructions.
This method involves trying various pipe sizes until a good combination is found. A spreadsheet does the calculations.
You will need a spreadsheet friction loss calculator: Friction Loss Calculator Spreadsheets
Remember that the maximum total pressure loss between the valve and the last sprinkler may NOT exceed 20% of the sprinkler head operating pressure.
Use the spreadsheet friction loss calculator to determine the pressure loss:
- Download and open the Friction Loss Calculator.
- There is a line on the spreadsheet for each section of pipe.
- Start with the pipe section after the control valve and work out to the farthest sprinkler.
- Select 3/4″ pipe for the pipe or tube size. (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.
- Change the pipe size if the velocity or total pressure loss is too high.
You must calculate the pressure loss for each of the possible water paths in the valve circuit.
Here is an example of the possible water paths for a valve circuit, shown in red, blue, and magenta.
Start with the water route that is the longest. In this case that would be the red route. There are 9 pipe sections in this route labeled 1-9. Enter the data from this route into the calculator. Use a larger pipe size if the velocity is not safe. Check that the friction loss “Total of All Sections” does not exceed your maximum allowable amount.
Write the pipe size for each section on your plan.
Now repeat the process for the blue water route and then the magenta color route.
Confused? For detailed instructions on using the spreadsheets see What Size Pipe for Sprinkler System Laterals?
This article is part of the Sprinkler Irrigation Design Tutorial
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