When would you use an unloader (relief) valve on the pump, and what does it do?

• A. When you want to increase engine RPM; it increases fuel flow
• B. When you need to keep engine at lower RPM while maintaining a target PDP; the valve dumps water back to the pump or suction to regulate pressure.
• C. When you need to prime the pump; it seals suction
• D. When you want to close discharge; it blocks flow

Answer: (B)

Unloader valves are used to regulate the pump’s discharge pressure while letting the engine run at a lower speed. When the discharge pressure reaches the preset target, the unloader opens and diverts water back into the pump or suction line. That bypass reduces the hydraulic load on the pump, so the engine doesn’t have to work as hard and can stay at a lower RPM while still keeping the desired discharge pressure. When more flow is needed to raise pressure, the valve closes and the pump delivers water to the discharge. This isn’t for priming or sealing suction, and it doesn’t simply block discharge flow.