Which plumbing term describes a valve that is usually closed and operates based on external conditions?

The term that describes a valve which is usually closed and operates based on external conditions is a relief valve. Relief valves are designed to automatically open when the pressure or temperature exceeds a predetermined limit, thereby allowing excess pressure to escape and preventing potential damage to the system, such as pipeline bursts or equipment failure.

Relief valves serve a critical safety function by continuously monitoring and responding to conditions. Their operation is typically linked to external factors, such as pressure buildup in a closed system, making them distinct from other types of valves that might operate based solely on user input or flow direction. This makes them vital in ensuring safe operation in various plumbing and industrial systems.

In contrast, flow valves tend to regulate the flow of fluid based on manual or automatic control under normal operational conditions, while a ballcock is primarily a mechanism used to fill tanks, operating based on float levels. A check valve, on the other hand, allows flow in one direction and prevents backflow, rather than reacting to external pressures.