The golden rule of irrigation
The fundamental rule that defines your maximum station rate is this point: in no case should you design a station such that the pressure drop from the public main to the last sprinkler exceeds 5 psi total. In many cases, you will have enough pressure to deliver so much water that the pressure drop will exceed 5 psi. This is very dangerous to do, and very unwise even if you still have enough working pressure at the sprinklers with this large pressure drop. If the pressure drop exceeds 5 psi, it indicates that there will be a large water hammer when the station valve shuts off. While the system may appear to work fine as installed, the excessive water hammer will fatigue the system, eventually causing joints or pipes in the main to rupture. The only way to solve the problem at that point is to replace the entire main (and perhaps additional pipes) and replace them with larger diameter pipe, or to subdivide stations into smaller areas (requiring new valve wires to be run and the controller upgraded) or all of the above!
If you learn one thing from this web site, make it this:
Use a station flow rate small enough, and pipe sizes large enough that the pressure drop from the public main to the last sprinkler will be less than 5 psi total.
In order to ensure that your design meets this requirement, you will have to review the pressure drop information below. As a starting point, use the pipe size indicated below. IMPORTANT NOTE: While this size may be adequate, on many designs, particularly large designs or designs with long mains, you will need to use a larger size than indicated below.
|Basis for Pressure Drop Calculations
(a larger size may be needed for your design)
|Pipe Size ||Maximum Flow|
|1/2 inch ||5 g.p.m.|
|3/4 inch ||10 g.p.m.|
|1 inch ||20 g.p.m.|
|1 1/4 inch ||35 g.p.m.|
|1 1/2 inch ||55 g.p.m.|
The basic steps
On the next couple of pages, you'll be guided through the basic steps of sizing pipe. Before you go further, you'll need to select as many sprinklers as you can supply at the flow rate determined in the flow check on the previous page, and place them where needed on your drawing. Draw in some pipe to connect them up, and measure the length of each section of pipe you will use. Determine the amount of water that will flow through every section of pipe to supply the sprinklers in your station. Using the information on the next page, add up the pressure drop through each section and see if it exceeds the 5 psi total pressure drop allowed. If it does, you'll need to upsize the pipe one size at a time, starting with the section having the largest single pressure drop, until the total pressure drop is below 5 psi.
Once you've done this a few times, you'll probably start to get the hang of it. It's important not to skip this step, or you could have a real problem on your hands just when you should be almost finished with the installation!
Links to continue to the next page are found at the bottom of this page.
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The staff at WaterTips.com