A fail closed valve is closed by which of the following mechanisms?

A fail closed valve is designed to close when a loss of power or control signal occurs, ensuring that the flow of fluid is stopped for safety or process reasons. The mechanism that facilitates this action is a spring.

In a fail closed setup, the spring holds the valve in a closed position by default. If the control input (such as electrical, pneumatic, or hydraulic pressure) fails or drops below a certain level, the spring force overcomes any opposing force, which allows the valve to transition to a closed state. This design is critical in applications where maintaining pressure or preventing leaks is essential during a failure event.

The other mechanisms mentioned, such as electric motors, pneumatic pressure, and hydraulic pressure, are typically associated with different types of valve operations. These may be used to open the valve under normal operating conditions, but in the event of a failure, they do not function independently to ensure that the valve remains closed. The spring mechanism is specifically suited for fail closed configurations, simplifying the safety measures during unexpected control failures.