What is Air lock valve?

Air lock-up devices should be provided for all services requiring that the control valve remains in the position in which it was immediately before the air failure.

The lock-up valve should be mounted between the positioner output and the actuator on control valves with a positioner.

If air lock valves are specified, they should be installed as close to the valve actuator as possible. The air supply for the air lock should be the same as for the valve positioner.

Additionally, valves with the air lock function should have the following:

• For diaphragm actuators, a pressure gage showing real diaphragm pressure.
• A pressure gage for piston actuators indicating air volume reserve tank pressure.

Control Valve Air Lock Relay Principle

The pneumatic lock-up valve or air lock relay shuts off the signal pressure line either if the air supply falls below the adjusted value or if the air supply fails completely. This causes the last position of the pneumatic actuator to stay.

The supply air generates a spring-balanced force on the diaphragm(4). When the force generated on the diaphragm is higher than the force of the spring, the input and output are linked, i.e. the signal pressure provided by the positioner is transferred to the pneumatic actuator unhindered.

The spring force dominates when the supply air pressure drops below the adjusted value, and the spring (6) moves the plug (3) completely into the seat (9). As a consequence, the pneumatic actuator pressure is blocked.