Principle of operation (Fig. 2 and Fig. 3)
In control operation, the signal pressure pst controlled either by a temperature controller or positioner is applied to the actuator.
If the power fails or the safety limiting device interrupts the control current when the temperature or pressure limits are ex- ceeded, the solenoid valve switches back to the idle position. As a result, the supply pressure line is shut off, the actuator is vented and the force of the actuator springs closes the valve.