What is Pump Discharge Pressure (PDP) and how is it used?

• A. Pressure at the intake side; used to set pump speed.
• B. Pressure at the discharge side; used with nozzle pressure and friction losses to set the pump operating pressure.
• C. Static water pressure in the supply line.
• D. Pressure at the hydrant outlet; used to compute nozzle distance.

Answer: (B)

Pump discharge pressure is the pressure on the discharge side of the pump—the pressure the pump must generate to push water through the hose lay to the nozzle. It’s used, together with the desired nozzle pressure and the friction losses along the hose, to determine the pump’s operating pressure. In practice you pick the nozzle pressure you want at the nozzle (for example, around 100 psi for many fog nozzles), estimate the friction loss in the hose length and size between pump and nozzle, and add these together. If there are elevation losses or appliance losses, include those as well. The sum is the pump discharge pressure you set on the pump to achieve the correct pressure at the nozzle.