Which statement best describes a positive displacement pump's flow?

• A. It delivers a constant volume per cycle regardless of discharge pressure
• B. It delivers a flow that increases with energy input
• C. It delivers no flow at zero speed
• D. It vaporizes the liquid before pumping

Answer: (A)

In a positive displacement pump, the amount of liquid moved with each cycle is fixed. Because that volume is delivered per cycle, the flow rate at a given speed stays essentially constant even as discharge pressure changes. Pressure affects the head the pump must develop and the power it consumes, not the volume displaced per cycle. So as you raise back-pressure, the pump simply works harder to push against it, but the amount pumped per cycle remains the same. This is why PD pumps are valued for steady, predictable flow and dosing. By contrast, the flow in other pump types depends more on the system head, so increasing energy input or head can change the flow.