How to Figure Pump Horsepower for a Sprinkler System

Determining Water Horsepower

• 1

  Calculate the total static head of the irrigation system by measuring the difference in height between the point of intake at the water source and the highest point where the irrigation water comes out while discharging.

• 2

  Calculate the pressure head of the irrigation system in feet by using a pressure gauge to determine the pounds per square inch, referred to as psi, of water discharging from the irrigation system. Multiply this number by 2.31 to get the value of pressure in feet of head.

• 3

  Determine the amount of pressure lost due to friction as the water travels through the pipe system. This value is calculated by compiling the total length of the pipe network, diameter of the pipe, and number and degree of turns or angles in the system. These numbers are then used to calculate the total friction head, which is done by using a friction loss table, available online or in most plumbing reference manuals.

• 4

  Calculate the total dynamic head of the irrigation system by adding the three values from the first three steps.

• 5

  Check the flow rate of the system by placing a flow meter at the point where the water source enters the irrigation system. This value should be represented in gallons per minute.

• 6

  Multiply the value of the total dynamic head by the flow rate in gallons per minute. Divide this product by 3,960 to determine the water horsepower of the irrigation system.

Determining Brake Horsepower

• 7

  Determine the pump efficiency, which will be included with the pump and be a value between zero and one.

• 8

  Determine the drive efficiency of the pump. This value varies, depending on the type of pump. For pumps that pump directly from the water source with a straight shaft, this value is 1. For those that pump with an right-angled shaft, the value is 0.95. For belt-driven pumps, this value will be found within the manufacturer's literature and will be a value between 0.7 and 0.85.

• 9

  Multiply the drive efficiency of the pump by the pump efficiency. Divide the water horsepower value by the product of the efficiencies. This is the brake horsepower and is the required amount of horsepower to properly pump water through a given sprinkler system.