5.1 Sizing a Pump - The Specs

An irrigation pump does two things; it provides the water flow rate and the pressure head to make the irrigation system function properly. The "flow rate" and "pressure head" are the two factors used to size the pumping plant.

Flow rate is the total discharge of all sprinklers or emitters operating together.

Add the total water needed by all the sprinklers in the system. The total discharge cannot exceed the supply available. The irrigation system can be divided into several zones, if necessary, to avoid exceeding the supply. The power requirement will be proportional to flow.

    Head or Pressure Head of an operating pumping system is dynamic. The dynamic head is made up of several heads that are illustrated in the Total Pressure Head diagram, and are defined as follows:

  1. Static suction head. The vertical distance from the water surface to the pump center-line to which the pump must lift the water. For wells, estimate to the draw down point at which the water level will be when the pump is operating. If the pump intake pipe is lower than the water surface, subtract the submerged depth.
  1. Static elevation head. The vertical distance (elevation) from the pump to the highest discharge pipe or sprinkler. Consider this as a column of water the pump lifts when the pipes are full of water.
  1. Pressure head. Operating pressure, in psi, of the sprinkler or emitter. Convert psi to "feet of head" before adding to the other pressures to get total dynamic head. Remember 2.31 feet equals 1 psi.
  1. Friction head. Includes all pressure losses in the system due to friction. Since the pump must overcome all the friction losses and still deliver water at the desired pressure at the end of the pipe, add the friction head into the estimate of the total dynamic head. Convert psi to feet of head.

    The total pressure head of an operating system can be summarized as follows:

  • Total static head = static suction lift + static elevation head. This is the total pressure head to fill the pipes to the highest point.
  • Total dynamic head = total static head + pressure head + friction head. This is the total head to make the system operate properly.