Why do positive displacement pumps have a relief valve on the discharge?

• A. If the downstream valve is closed, the pump will continue to deliver liquid until something breaks
• B. To improve energy efficiency
• C. To reduce vibration
• D. To reduce noise

Answer: (A)

Positive displacement pumps move a fixed amount of fluid with each cycle, so when the discharge path is blocked by a closed downstream valve, the pump keeps trying to push liquid and the pressure in the discharge line climbs rapidly. A relief valve on the discharge provides a controlled outlet that opens at a set pressure and bypasses liquid back to the source or tank. This prevents overpressure from damaging the pump seals, fittings, and piping, and stops components from failing when flow is blocked. It’s fundamentally a safety feature to protect the system from the consequences of continuing to pump against a closed outlet.