Which type of valve should never be used to throttle a pump?

The correct answer is suction valve, as this type of valve is specifically designed to control flow in a system and should not be used for throttling purposes.

When a pump operates, it relies on a consistent flow of fluid into the suction side for optimal performance. Throttling a pump by partially closing a suction valve could result in a decrease in the flow rate entering the pump, potentially causing issues such as cavitation, which can damage pump components.

In contrast, other types of valves like relief valves, check valves, and drain valves are designed for specific purposes—such as pressure relief, preventing backflow, or draining fluid from a system—but are not meant to regulate or throttle the flow of fluids effectively. This limits the risk of damaging pump internals or affecting operational efficiency by using them in a throttling capacity.