Which pump type requires a relief valve on its discharge side?

• A. A centrifugal pump
• B. A diaphragm pump
• C. A jet pump
• D. A positive displacement pump

Answer: (D)

Positive displacement pumps move a fixed amount of fluid per cycle, so if the discharge path is blocked or heavily throttled, the pump keeps pushing that same volume into a closed or restricted line. Since the flow cannot increase to match the blockage, pressure builds rapidly and can reach damaging levels. A relief valve on the discharge side is there to open and vent excess pressure, protecting seals, piping, and components.

The other pump types don’t inherently produce unbounded pressure when the discharge is blocked. Their pressure behavior is governed by the pump’s head curve and the system curve, so they don’t require a dedicated discharge relief in the same way PD pumps do.