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Control Valve Air Lock Relay Working Principle

Control Valve Air Lock Relay

Air Lock Relay is designed such that when the header / supply air pressure falls below the desired set value (cut-off value) the Air Lock traps the air pressure in the diaphragm chamber of the actuator.

Air Lock

Air Lock is used when it is required to automatically hold the air pressure in control valve diaphragm chamber of the actuator, in an event of failure of air supply. When supply pressure drops below a preset value, the Air Lock Relay locks the air pressure in the diaphragm chamber of the actuator thereby holding the control valve in the same position, till the air supply is resumed.

Principle of operation

The pneumatic lock-up valve or air lock relay shuts off the signal pressure line either when the air supply falls below an adjusted value or upon complete air supply failure. This causes the pneumatic actuator to remain in its last position.

The supply air produces a force on the diaphragm (4) which is balanced by the spring (6). When the force produced on the diaphragm is greater than the spring force, input and output are connected, i.e. the signal pressure supplied by the positioner is transmitted unobstructed to the pneumatic actuator. When the supply air pressure falls below the adjusted value, the spring force dominates, and the spring (6) moves the plug (3) fully into the seat (9). As a result, the pressure in the pneumatic actuator is blocked.

control valve air lock relay Principle

Image Courtesy : Samson


1 Body 2 Cover 3 Plug 4 Diaphragm 5 Screw for set point adjustment 6 Spring 7 Lock nut  8 Cap 9 Seat

  • pz : Supply air
  • pe : Input
  • pa : Output

Air Lock Cut Off Setting

The cut-off is sat to a desired value by compressing the spring by spring adjusting screw. If the actuator pressure range is 0.2-1.0 Kg/cm², the setting of this relay could be much higher than 1.0 Kg/cm² as desired for cut off. Where many control loops are involved, the setting of this relay therefore needs impulse connection for cut off from the air supply header.

Cut off setting has been factory adjusted to the desired cut off value. Should it be required to re-adjust the cut off setting, this could be achieved either while the unit is already installed and connected to the system or separately on the test bench. Inlet supply air and the outlet output gauge should be available to re-adjust the setting.

To adjust the cut off setting, remove cap and adjust spring adjusting screw until desired outlet cut off pressure is achieved. Please note that air lock pressure differential is approx. 0.17 Kg/cm² higher than the set cut off pressure.

Turning the adjusting screw clock-wise will increase the cut off setting and turning anti-clockwise will reduce the cut off setting.

In this relay, a built-in manual relief valve is provided if the control valve has manual hand wheel. This exhausts the air from the diaphragm chamber when the hand wheel is operated to avoid damage to the actuator diaphragm.

Replace cap securely after having re-adjusting the cut off setting.

Applications :

Air Lock Relay Applications

Parts : 

  1. Pneumatic control valve
  2. Positioner
  3. Air Lock Relay / Lock-up valve
  4. Solenoid valve

Source : Samson

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tanmay kumar behera February 25, 2018 at 5:50 am

please sir help me to save the document

S Bharadwaj Reddy February 25, 2018 at 3:39 pm

Step by Step Procedure sent to your email. Please check.

venkataramana.k September 8, 2018 at 6:54 am

Dear sir
This is venkataramana from Kuwait KNPC we have a problem for flow control valve air fail to lock ,when air fail valve not lock on desire value it’s getting close. set the cut-off and relay screw adjusted but not stay air in actuator chamber, can you please solve my problem, functioning not good.

Natrajan September 11, 2018 at 1:50 pm

You can use airlock relay between positioner to actuator tubing so that at air fail condition if the air lock relay is working it will hold the pressure in the actuator and the valve position will remain undisturbed.


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