The terms "fail open" and "fail closed" describe what part of a valve?

The correct terminology relating to "fail open" and "fail closed" pertains to the function of the spring in a valve. This terminology describes the position that a valve will assume in the event of a failure or loss of control signal.

In a valve mechanism, when the spring is responsible for closing the valve, it will be designated as "fail closed," meaning that if there is a loss of power or signal, the spring's inherent tension will pull the valve into a closed position. Conversely, if the spring design allows the valve to open in the event of a failure, it is referred to as "fail open." This characteristic of the spring plays a crucial role in the safe operation of process control systems, especially in preventing undesired flow conditions or maintaining safety protocols in automated systems.

Other components mentioned, such as the plug, seat, and handwheel, do not directly correlate with the terms "fail open" and "fail closed" in terms of their operational failure modes. These components may influence valve function, but it is the spring that primarily controls the default position of the valve under failure conditions, making it the core component to understand in this context.