Which condition should never be present when operating a positive displacement pump?

• A. Closed valve
• B. Open valve
• C. Bleed valve
• D. Pressure relief valve

Answer: (A)

Positive displacement pumps move a fixed quantity of fluid with each cycle, so there must be an open path for that fluid to follow. If the discharge path is blocked by a closed valve, the pump keeps trying to push fluid but nowhere for it to go. Pressure in the discharge line climbs rapidly, leading to deadheading, which can damage seals, gears, bearings, and even drive the motor harder or trigger safety relief. That is why having a closed valve on the discharge side is something to avoid during operation.

An open discharge valve is necessary for normal flow. A bleed valve is used to vent air or purge the line during startup or maintenance, so it serves a purposeful role when used correctly. A pressure relief valve is a safety device designed to limit the system pressure and prevent damage, so its presence is essential rather than problematic.