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Consider a safety valve with a hydraulic fail‐safe‐close actuator. The valve is held open by hydraulic pressure during normal operation. When a specific critical situation occurs, a closing signal is sent to the safety valve and the valve closes by the force of the fail‐safe actuator. The valve has two functioning states: open and closed. Transitions between these two states are facilitated by the actuator. The states and transitions are shown in Figure 3.1.

Figure 3.1 States and transitions for a safety valve.

The essential function in state “open” is to provide a conduct for the medium/fluid through the valve, and the essential function in state “closed” is to stop the flow through the valve. An auxiliary function for both states is to contain the fluid and thereby to prevent leakage to the environment.