Which characteristic is true of positive displacement pumps?

A characteristic of positive displacement pumps is that they inherently require a relief valve to prevent damage. Positive displacement pumps work by trapping a fixed amount of fluid and then forcing that volume into the discharge pipe. This means that as long as the pump is running, it will continue to push fluid, regardless of the pressure in the discharge line. If there is no route for the fluid to exit (such as a closed valve), the pressure will continually increase until a failure occurs, which can lead to mechanical failure of the pump or associated piping. Therefore, a relief valve is essential for safety, allowing excess pressure to be vented or directing flow back to the suction side of the pump.

While the other options address different aspects or types of pumps, they do not correctly characterize a standard feature of positive displacement pumps. For example, variable flow is typically associated with centrifugal pumps rather than positive displacement pumps, which have a constant flow rate under specific conditions. Positive displacement pumps can handle more viscous fluids effectively, which makes them suitable for pump applications outside of just water and thus not limited to low-viscosity fluids.