What type of pump is the only one that can be run against a closed head?

The correct choice, a centrifugal pump, is specifically designed to handle fluid flow in applications that may involve varying head conditions, including when the discharge side is closed. In centrifugal pumps, the rotational motion of an impeller generates centrifugal force, which allows them to effectively move liquids against a certain amount of head while maintaining operational efficiency.

This ability to run against a closed head is critical because it allows for flexibility in how the pump can be used, especially in systems where the discharge may occasionally be unintentionally obstructed. Centrifugal pumps can typically handle reductions in flow due to closed head conditions without suffering immediate damage, though long-term operation under such conditions can lead to overheating and wear.

Other types of pumps, such as piston pumps or diaphragm pumps, generally require specific conditions for optimal performance and may not be as capable of handling a closed head situation as effectively or safely as centrifugal pumps. This important distinction highlights the suitability of centrifugal pumps for a range of applications where changing or unexpected head conditions may occur.