Which of the following is NOT one of the three main factors that influence a centrifugal fire pump's discharge pressure?

• A. Amount of water being discharged
• B. Speed at which the impeller is turning
• C. Pressure of water of an incoming source
• D. Temperature of the surroundings

Answer: (D)

Discharge pressure in a centrifugal fire pump comes from the energy the pump adds to the water, which depends on three main things: how fast the impeller is turning, how much water is being moved (the flow), and the pressure of the water at the pump’s inlet. Turning the impeller faster increases the energy given to the water, boosting discharge pressure. Pushing more water through the system raises the friction losses in hoses and fittings, so the pump must develop higher pressure to maintain that flow. The suction pressure, or incoming water pressure, provides the baseline head the pump starts from, so higher inlet pressure means the pump needs to generate less additional pressure to reach a given discharge value. The surroundings’ temperature doesn’t play a primary role in determining discharge pressure, as its effects on water properties are minimal for typical firefighting pump operation.