Which of the following is a characteristic of a positive displacement pump?

A positive displacement pump is designed to move a fixed amount of fluid with each cycle or stroke of the pump. This means that the flow rate remains constant regardless of the pressure in the system. The fundamental operation of positive displacement pumps relies on trapping a volume of fluid and then displacing it into the discharge line, which is why their flow rates do not fluctuate even when the resistance against the flow changes. This reliability in maintaining a consistent flow makes positive displacement pumps particularly useful in various applications where a specific amount of fluid delivery is crucial.

The other characteristics listed can be misleading in the context of positive displacement pumps. Variable flow rates are more typical of centrifugal pumps, which adjust their flow based on line pressure. While regular maintenance is important for all types of pumps, it does not uniquely characterize positive displacement pumps, as maintenance needs depend more on the type and application of the pump rather than its operating principle. Lastly, low energy efficiency is often associated with certain types of positive displacement pumps, but many designs can achieve high efficiency levels depending on their application and operational conditions. Hence, the defining characteristic here is the ability to maintain constant flow rates across varying pressures.