Which design of relief valve is more energy efficient?

The pilot-operated relief valve is considered more energy efficient in many applications compared to other designs because it utilizes a smaller control valve to manage the flow, which reduces the energy consumption associated with larger valves that need to open more fully. The pilot operates on a proportional basis, meaning it can maintain stable pressure with less fluctuation, ultimately resulting in lower system energy usage.

The compound relief valve, often utilizing both direct and pilot-operated features, can also enhance efficiency. This design can provide effective pressure regulation at varying flow rates by using the direct action for fast response and the pilot operation for energy-efficient flow management.

In contrast, while direct acting relief valves are simple and effective, they can be less efficient in systems where pressure varies often, as they do not adapt as well to changes, potentially leading to unnecessary energy use.

Thus, the combination of both the pilot-operated and compound relief valves being energy efficient supports the notion that option D is correct, as it includes designs that effectively manage energy consumption in pressure relief applications.