Which item completes the list of three main factors that influence a centrifugal fire pump's discharge pressure?

• A. Speed at which the impeller is turning
• B. Temperature of the surroundings
• C. Pump mounting type
• D. Material of the impeller

Answer: (A)

Discharge pressure in a centrifugal fire pump is driven mainly by how fast the impeller is turning. The energy the pump imparts to the water—and thus the pressure it can develop at the discharge—rises as the impeller speed increases. For a given impeller size and design, increasing RPM increases the pump head (and thus the discharge pressure), roughly in proportion to the speed squared.

The other factors listed don’t directly set the discharge pressure. Surrounding temperature can slightly affect water properties, but not the hydraulic head produced at a given speed. Pump mounting type mainly influences installation and vibration, not the hydraulic discharge pressure. The material of the impeller affects wear and efficiency over time, but not the immediate pressure produced by a given operating speed. Therefore, the speed at which the impeller turns is the key factor that completes the list.