Water Velocity and Pressure Loss

Premature wear due to velocity (aka pressure loss) is harder to figure out. It generally isn’t a problem unless the water is really flowing fast through the pipe, like 8 feet per second or higher. Hydro-Rain recommends keeping the water velocity/pressure loss below 5 feet per second.

The only way to determine if it is a problem is to do some calculations.

Sprinkler Nozzles #

On top of each sprinkler nozzle is the brand name, distance and pattern. Write down that information for each sprinkler, then look up the water use (GPM value) for that nozzle. Every irrigation company posts this information on their website. Now add together the GPM values for all the sprinklers that are running at the same time. This will tell you how much water you are using (GPM value).

Pipes #

Next find the size and type of the water pipe that leads to the valves. For example it might be a 3/4″ HDPE, or maybe a 1″ PVC pipe. You may find several different sizes and types of pipe, in which case you would use the smallest pipe size and type. Using that information you can calculate the velocity of the flow in the pipe using a Friction Loss Calculator.

Washington State University has a free calculator that is available through the link below.

Pipeline Pressure Loss Calculators (wsu.edu)

Fittings, Valves, and Sprinklers #

While not used in the calculations above every fitting, bend, valve, and sprinkler (rotor and spray) will affect the water velocity.

you can use the following calculator as a guideline to calculate those numbers.

Fitting Pressure Loss Calculator (wsu.edu)

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