Which of the following is true about a positive displacement pump?

A positive displacement pump (PDP) operates by trapping a fixed amount of fluid and forcing it through the discharge pipe. This type of pump is characterized by its mechanism of displacing a volume of liquid with each stroke or rotation.

The statement that it must never be started with the discharge valve closed is accurate because starting the pump with the discharge valve closed can lead to excessive pressure build-up within the system. Unlike centrifugal pumps, which can handle a closed discharge condition by simply reducing flow, positive displacement pumps continue to generate pressure regardless of the discharge condition. This can lead to potential damage to the pump or even catastrophic failure of the system if the pressure exceeds safe limits.

In contrast, other options provide incorrect contexts. For instance, while a positive displacement pump can have specific applications that involve varying pressures, it typically does not maintain a constant flow rate under fluctuating pressures, negating the idea that it can operate with a closed valve without concern for pressure safety. Similarly, using a positive displacement pump only for water applications is misleading, as these pumps are versatile and can handle a range of viscosity and fluid types, including oils, slurries, and chemicals.