Which statement describes the flow behavior of a positive displacement pump as system pressure changes?

Which statement describes the flow behavior of a positive displacement pump as system pressure changes?

• A. Flow is Highly Variable
• B. Flow Remains Constant as Pressure Changes
• C. Flow Decreases with Temperature
• D. Flow Is Proportional to Viscosity

Answer: (B)

Positive displacement pumps move a fixed volume of fluid with each cycle, so the flow rate is determined by the pump’s displacement and its speed, not by system pressure within normal operating ranges. When system pressure changes, the pump still delivers roughly the same amount of fluid per cycle; the pressure increase just means the pump does more work to push that same volume against a higher head. You’ll see the flow remain essentially constant until you hit limits like a relief valve opening or cavitation, which can then alter the flow. This is why the statement that flow remains constant as pressure changes is the best description. The other ideas don’t fit the typical behavior: flow isn’t inherently highly variable for a properly operating PD pump, it doesn’t inherently decrease with temperature, and it isn’t set by viscosity in the way those options imply.

Positive displacement pumps move a fixed volume of fluid with each cycle, so the flow rate is determined by the pump’s displacement and its speed, not by system pressure within normal operating ranges. When system pressure changes, the pump still delivers roughly the same amount of fluid per cycle; the pressure increase just means the pump does more work to push that same volume against a higher head. You’ll see the flow remain essentially constant until you hit limits like a relief valve opening or cavitation, which can then alter the flow.

This is why the statement that flow remains constant as pressure changes is the best description. The other ideas don’t fit the typical behavior: flow isn’t inherently highly variable for a properly operating PD pump, it doesn’t inherently decrease with temperature, and it isn’t set by viscosity in the way those options imply.