It is an important consideration in many systems is the position of the actuators when there is a loss of power, i.e., will chemicals or the fuel to the heaters continue to flow or will a total system shut down occur?.
Occasionally, service conditions require that the valve remains in the last operating position upon loss of air supply. For such operations, If air failure occurs, the system activates two pilot-operated lock-up valves that trip and lock the existing cylinder pressures on both sides of the piston, thus maintaining the last throttling position.
Two valve fail safes conditions:
For air close type valve, If the system fails, i.e., if there is a loss of pneumatic pressure, the spring acting on the piston will force the valve to revert back to its open position, which is shown in above figure, on the left side.
If the valve is air to open type, If the system fails and there is a loss of control pressure, the spring action will force the piston down and close the valve. Similar fail-safe electrically and hydraulically operated valves are available. Two-way and three-way fail-safe valves are also available which can be configured to be in a specific position when the operating system fails.
3 way switching:
The fail-safe system consists of a three-way switching valve and airset. The valve positioner operates as a three-way positioner and supplies air to one side of the piston only. A constant air pressure is maintained on the other side of the piston by the airset.
Supply pressure is sensed by the three-way switching valve. If supply pressure drops below a preset value, the threeway switching valve locks the air on the constant pressure side of the piston to drive the valve plug to its failure position.
When supply air pressure returns to normal, the three-way switching valve trips to permit normal throttling of the control valve. The setting of the three-way switching valve is factory set at a level close to the supply pressure