Field Instrumentation

What is an airlock relay?

The airlock unit is very useful in a control valve. This could provide the stay-put condition of the control valve in case of the air supply failure. The air pressure is blocked inside the diaphragm chamber of the actuator when the supply air pressure falls below the desired set value. A double-acting air lock relay is used in pneumatically operated double acting actuators and power cylinders to hold its position in the event of air failure or in case if the airdrops below the preset pressure the airlock relay will be connected between the positioner and the actuator.

What are the features of an airlock relay?

  • It will be effective during the power failure
  • It can maintain the actuator position properly
  • Cut off pressure adjustable as desired
  • No external bleed
  • It is mostly used for air failure safe condition

How to install or connect an air lock relay?

Mostly an airlock relay is connected between an actuator and a solenoid valve, so if there is no pressure it would lock the actuator in position. So the actuator will maintain its position till the pressure comes to normal. So this relay can keep the actuator in position under the energized condition and also in case of air failure.

How does an air lock relay works?

The airlock relays are designed in a way that, if the headers supply air pressure falls below the cut-off value, then the airlocks trap the air pressure in the diaphragm chamber of the actuator. These relays set the cut-off value to the required value by compressing the spring by a spring adjusting screw. The cut-off value is varied and it is set higher than the value which is required for the control valve operation. The airlock would require an impulse connection from the air supply header for its operation. The cut-off value will be adjusted from the factory, we can re-adjust the set value while the unit is already installed and connected to the system or separately on the test bench. The inlet supply air and the outlet pressure gauge should be available to re-adjust. In order to vary the cut-off value, we can use the spring adjusting screw.

What are the required features for an air lock relay?

  • Stability and repeatability
  • Smooth action
  • It must be suitable for control valves and power cylinders
  • There shouldn’t be any leakage in the pressurized condition
  • There shouldn’t be any drift in the cut off value
  • Moving parts should be less
  • Good cut off range

How to select an air lock relay?

  • It must be selected according to the set pressure
  • Test pressure
  • We must also consider the inlet and differential pressure
  • Temperature and air connection
  • Weight


post-graduate in Electronics & communication.

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