A pneumatic control valve may be at an intermediate travel position when instrument-air pressure begins to fall. If the process cannot tolerate an immediate move towards fully open or fully closed, an air lock relay can isolate the actuator pressure path once supply falls below an adjusted switching point.
The resulting fail-in-place response is time-dependent. Trapped pressure can hold the valve near its last position, but leakage, actuator spring force and process load can eventually move the stem or shaft.
Correct application depends on the relay mechanism, its position relative to the positioner and solenoid valve, the actuator type, the cut-off setting and verified circuit leak-tightness. These details determine whether “hold last position” is a testable requirement or only an assumption on the control valve datasheet.
What is a control valve air lock relay?
A control valve air lock relay is a pneumatic accessory that monitors instrument-air supply pressure and isolates the actuator pressure line when supply falls below an adjustable switching point. It is also called a pneumatic lock-up valve, lock-up relay or actuator air lock.
Under normal air pressure, the relay allows the positioner’s output pressure to pass to the actuator. When the monitored pressure falls below the set point, the relay blocks the pneumatic path and traps the pressure already present in the actuator chamber. The valve can then remain near its last controlled travel rather than immediately moving under spring or process forces.
The air lock relay does not measure valve travel or correct positioning error; those are positioner functions. It also differs from a solenoid valve that switches, vents or redirects actuator air after an electrical command. The lock-up relay responds primarily to the pneumatic pressure condition applied to its sensing or pilot connection.
In this context, air lock has no connection with a rotary airlock used for powders or an air pocket in plumbing. It belongs to the actuator-accessory side of the control loop, alongside the positioner, solenoid valve, filter regulator and pneumatic actuator. These components are covered more broadly in MacoTango’s control valve basics.
How an air lock relay traps actuator pressure
An air lock relay switches by balancing the force created by supply pressure against an adjustable spring. A diaphragm senses the monitored air pressure, while a plug and seat open or isolate the pneumatic path between the positioner and actuator.
Normal instrument-air condition
With supply pressure above the switching point, pressure acting on the diaphragm overcomes the spring force. The plug remains away from its seat, connecting the relay input to its output. Air from the valve positioner can then enter or leave the actuator as the control signal changes.
In this state, the relay does not determine valve travel. It keeps the pneumatic passage available while the positioner regulates actuator pressure and corrects the difference between demanded and actual valve position.
Supply pressure below the set point
As the monitored pressure falls to the adjusted switching point, the diaphragm force becomes lower than the spring force. The spring moves the plug onto its seat, isolating the positioner side from the actuator side of the relay.
Air already present downstream of the closed seat remains in the actuator chamber and connecting tube. The relay does not generate replacement pressure or apply a mechanical brake to the stem. Valve travel is held only by the remaining pneumatic pressure acting against the actuator spring, friction and process forces.
What happens when supply pressure returns
When supply pressure rises above the relay’s reset threshold, diaphragm force again overcomes the spring and reopens the signal path. The precise relationship between cut-off pressure and reset pressure depends on the relay design, so the two values should not be assumed to be identical.
Once the path reopens, the current positioner output acts on the actuator. If the controller demand changed while the valve was locked, the actuator may move as soon as normal pneumatic control resumes. Port functions, switching direction and reset behaviour must therefore be checked against the specific manufacturer’s instructions.
Where the air lock relay sits in the pneumatic circuit
The air lock relay must isolate every pneumatic path that could release the pressure required to hold the actuator. Its position relative to the positioner, solenoid valve and actuator therefore affects what the control valve does during loss of air, electrical power or control signal.
Between the positioner and actuator
In a standard modulating circuit, the air lock relay is installed in the signal-pressure line between the positioner and pneumatic actuator. The relay remains open during normal control, allowing the positioner to load or vent the actuator chamber. When pilot supply falls below the set point, the relay closes and isolates the actuator side.
The relay should be mounted close to the actuator, using the port assignments specified by its manufacturer. Short, well-supported tubing reduces the number of downstream fittings that must remain leak-tight after lock-up. The sensing connection must monitor the intended instrument-air failure condition rather than a pressure point that remains trapped after the relay closes.
Lock-up priority versus solenoid-valve priority
When an SOV is included, component order determines which function can act directly on the actuator. In a common lock-up-priority circuit, the air lock relay is between the SOV and actuator. Once the relay closes, it isolates the actuator from an upstream SOV exhaust path and retains actuator pressure.
In a common SOV-priority circuit, the SOV is between the air lock relay and actuator. Changing the SOV state can then vent or redirect actuator air even if the upstream lock-up relay has closed. The actual result still depends on SOV porting, its energised or de-energised state and the actuator action. The solenoid-valve trip path should be checked for loss of air, loss of power and loss of control signal as separate cases.
Single-acting and double-acting actuator circuits
A single-acting actuator normally has one controlled loading line and a spring that drives movement in the opposite direction. Holding its last position requires the relay to trap pressure in the loaded chamber while preventing another accessory from venting that chamber through a parallel path.
A double-acting actuator uses separate air paths for its two chambers. A relay intended for one pneumatic line does not automatically lock both sides. The circuit may require a 4/2 lock-up relay, two coordinated isolation paths or another manufacturer-approved arrangement that traps both chamber pressures. Isolating only one side can still allow movement as air escapes from the other side or process torque acts on the valve.
The required number of pneumatic paths must therefore be matched to the single-acting or double-acting actuator circuit. The approved pneumatic schematic should also show boosters, quick-exhaust valves, manual vents and every branch that could bypass the air lock relay.
How to set the air lock cut-off pressure
There is no universal cut-off pressure for a control valve air lock relay. The setting must be high enough to isolate the actuator before falling supply pressure affects valve control, but low enough to prevent nuisance lock-up during acceptable variations in the instrument-air header.
Inputs required before adjustment
Start with the actuator pressure range, normal and minimum instrument-air pressures, positioner supply requirement and the relay’s adjustable range. The available setting window must also account for pressure loss through regulators, tubing, fittings and other accessories between the header and the relay sensing port.
The datasheet should identify which pressure the relay monitors and which failure condition must trigger lock-up. A setting based only on normal header pressure can be misleading if the actuator needs a higher residual pressure to resist spring force, process load or friction at the required valve position.
Switching accuracy and reset differential also affect the usable setting. A relay may close at one pressure and reopen only after the supply rises to a higher value. Both thresholds must remain inside the permitted operating range of the complete pneumatic circuit.
Follow the model-specific adjustment procedure
Adjustment normally changes spring compression through a screw or knob, but the direction, pressure margin and final locking method vary by design. Use a regulated air source and calibrated pressure gauges at the ports specified in the relay manual. Do not transfer the screw direction or pressure allowance from another manufacturer’s product.
For example, the SAMSON Type 3709 operating instructions describe a fine-tuning procedure for a fail-close actuator using a regulated supply set to the actuator’s upper pressure-range value plus 0.2 bar. That value and the stated screw movement belong to the Type 3709 procedure; they are not a general setting rule for every air lock relay.
Verify both lock and reset conditions
The final setting should be confirmed by changing pressure through the expected switching range while observing actuator travel. Perform the test under a controlled commissioning or bench procedure, with the process isolated where required and qualified personnel responsible for the valve movement.
- Place the control valve at an intermediate travel position that makes unintended movement easy to detect.
- Reduce the monitored supply pressure slowly and record the pressure at which the relay isolates the actuator.
- Change the positioner command across its test range and confirm that the actuator does not respond while the relay is locked.
- Raise the supply pressure gradually and record the pressure at which normal positioner control returns.
- Repeat the pressure cycle to check switching consistency, then secure the adjustment screw, lock nut or protective cap as required by the manufacturer.
A correct cut-off setting therefore includes two measured values: the pressure that initiates lock-up and the pressure that restores control. Recording only the adjustment-screw position does not prove how the assembled valve package will respond.
Why fail-in-place does not mean permanent position hold
An air lock relay can hold a valve only while the trapped pneumatic circuit remains sufficiently leak-tight. The ALR-3 instructions describe position holding on the assumption that the actuator and fittings have no leakage. That assumption defines the main limit of fail-in-place performance.
Pressure can escape through tube fittings, threaded connections, the lock-up relay seat, positioner passages, solenoid-valve seals, actuator diaphragms or piston seals. A small leak may not cause immediate travel, but it reduces the pneumatic force available to balance the actuator spring and process load.
In a single-acting actuator, falling trapped pressure eventually allows the spring to move the valve towards its spring-return position. In a double-acting actuator, unequal leakage from the two chambers changes the pressure balance across the piston. The valve may then move in the direction produced by the remaining chamber pressure and external load.
The direction and rate of drift also depend on valve mechanics. Differential pressure can create an unbalanced force on a globe-valve plug or torque on a rotary valve shaft. Packing friction may hold the stem temporarily, then release it after pressure decays further. Temperature changes can also alter trapped-air pressure, so drift may not be smooth or proportional to elapsed time.
A specification that states only “fail last” does not define an acceptance condition. It should state the required holding interval, maximum permitted travel change, initial valve position, test pressure and whether eventual drift open or drift closed is acceptable. The loss condition must also be identified because loss of instrument air, electrical power and control signal can produce different pneumatic responses.
The lock-up test should measure valve travel over the required interval rather than recording only the first position after pressure loss. If the process cannot tolerate the measured drift direction or duration, the pneumatic circuit may need tighter components, a different actuator arrangement, stored air or a defined final fail position. MacoTango’s guide to fail open, fail closed and fail last behaviour covers that wider process-safety decision.
How to specify an air lock relay for a control valve
Air lock relay selection starts with the actuator circuit. A relay that isolates one loading line may suit a single-acting actuator but cannot automatically hold both chambers of a double-acting actuator. The required fail response and pneumatic schematic must therefore be defined before choosing the relay body, port size or pressure range.
| Specification item | Why it matters | What to verify |
|---|---|---|
| Actuator type and action | Determines how many air paths must be isolated and which forces move the valve after pressure decays. | Single-acting or double-acting, spring direction, air-to-open or air-to-close, linear or rotary motion. |
| Pneumatic function and ports | The relay must block the correct input, output and chamber connections without leaving a bypass route. | Manufacturer port diagram, number of isolated paths, connection thread and compatibility with the approved circuit. |
| Supply and signal pressure | Exceeding pressure limits can damage the relay, while an unsuitable adjustment range prevents correct switching. | Maximum supply pressure, maximum signal pressure, adjustable set-point range and switching accuracy. |
| Reset behaviour | The relay may reopen at a different pressure from the cut-off point, affecting restart and pressure cycling. | Reset threshold, switching differential, automatic or manual reset and expected action after air returns. |
| Flow capacity and port size | An undersized passage can restrict normal actuator filling or venting and change valve response. | Cv or Kvs, actuator volume, tubing size, required stroke response and whether a booster version is needed. |
| Pilot-pressure source | The sensing line decides which pressure loss causes the relay to lock. | Header or regulated supply location, isolation arrangement and whether the sensed point represents the specified failure case. |
| Body, seals and temperature | Ambient conditions and contaminants affect corrosion resistance, sealing and switching performance. | Aluminium or stainless-steel body, seal material, ambient range and hazardous or corrosive location requirements. |
| Instrument-air quality | Dirt, moisture and thread-sealing debris can prevent the plug or spool from sealing correctly. | Filtration, dew point, oil restrictions, cleaned tubing and approved thread-sealing practice. |
| Holding requirement | The relay model alone does not define how long the assembled valve remains within an acceptable travel band. | Required interval, maximum travel change, initial position, drift direction and controlled acceptance test. |
Flow capacity should be checked for normal control as well as lock-up. A relay with small internal passages may trap pressure correctly but slow actuator response during ordinary modulation. Larger actuator volumes or rapid-stroke requirements may need a relay with an integrated booster or a separately engineered booster arrangement.
Pressure, material and reset data remain model-specific. For example, the Air-lock 101 device publishes defined operating-pressure, temperature, material and reset characteristics rather than a universal air-lock specification.
The final relay choice should be reviewed with the valve body, actuator, positioner, SOV, tubing and required failure response. An assembly-level review is necessary because compatibility between these components determines the actual lock-up behaviour.
Commissioning and troubleshooting checks
Air lock relay testing should be performed on a controlled bench or during an approved commissioning activity. The responsible personnel must know how the valve, actuator and connected accessories will move when supply pressure and control signals change. A live-process air-failure test is unsuitable unless the plant procedure, isolation and operating authority specifically permit it.
Controlled commissioning sequence
- Confirm the relay model, port assignments, adjustment range and actuator circuit against the approved pneumatic schematic.
- Inspect tubing and fittings, then apply clean, dry instrument air within the specified pressure range.
- Stroke the valve normally through the positioner and confirm that the relay does not restrict the required actuator response.
- Place the valve at an intermediate position, reduce pilot pressure gradually and record the pressure at which lock-up occurs.
- Change the control command while the relay is locked, monitor valve travel for the specified holding interval and check the trapped circuit for leakage.
- Restore supply pressure gradually, record the reset point and confirm that the valve resumes positioner control without an unexpected pneumatic restriction.
The test record should include the initial valve position, cut-off pressure, reset pressure, observed travel change, holding interval and final response after air restoration. Recording only “pass” does not show whether the result matches the process requirement.
Common symptoms and checks
| Observed symptom | Likely cause | Check | Corrective direction |
|---|---|---|---|
| Relay locks during normal pressure variation | Set point is too high, or pressure loss at the sensing connection is greater than expected. | Measure pressure at the pilot port during normal valve movement and peak air demand. | Correct the pressure loss or readjust within the manufacturer’s permitted range. |
| Relay does not lock as supply pressure falls | Set point is too low, pilot pressure is taken from the wrong location, ports are incorrect or the sensing mechanism is contaminated. | Compare port connections with the manual and measure pilot, input and output pressures separately. | Correct the circuit or setting; remove the device from service before manufacturer-approved cleaning or repair. |
| Relay locks but the valve drifts | Leakage exists downstream of the relay, or trapped pressure cannot balance spring and process forces. | Check fittings, tubing, relay seat, SOV, positioner passages and actuator seals using the approved leak-test method. | Repair the leaking path and repeat the timed holding test under the specified valve load. |
| Valve moves when the SOV changes state | The SOV has priority, or its exhaust path bypasses the locked relay. | Trace the air path from each actuator chamber through the SOV and lock-up relay in both electrical states. | Revise the arrangement only against the approved fail-action and pneumatic schematic. |
| Normal valve response becomes slow | Relay flow capacity, port size or tubing is too restrictive, or internal passages are contaminated. | Compare positioner output and actuator pressure during a controlled stroke. | Verify required Cv or Kvs, tubing size and whether a manufacturer-approved booster arrangement is needed. |
| Control does not resume after pressure returns | Supply has not reached the reset threshold, the setting is incorrect or the internal path remains blocked. | Measure supply pressure at the relay and compare it with the documented reset behaviour. | Restore the specified pressure; if the relay still does not reset, isolate it for qualified inspection. |
Do not dismantle or disconnect an air lock relay while its pneumatic circuit remains pressurised. Depressurisation, valve isolation and maintenance work must follow the relay manufacturer’s instructions and the site’s control-valve safety procedure.
Specify the hold requirement, not only “fail last”
A fail-last note should be written as a measurable response of the complete control valve package. Define the loss condition, initial valve position, required holding interval, maximum permitted travel change and acceptable eventual drift direction. Record the normal, minimum, cut-off and reset pressures used to verify that response.
The pneumatic schematic must show the actuator type, positioner, SOV, air lock relay, pilot connection, tubing and any booster, quick-exhaust or manual-vent path. This allows engineering, purchasing and commissioning teams to confirm which accessory has priority and whether every actuator chamber remains isolated during the specified failure.
For a new valve or replacement assembly, send these conditions with the actuator and process data when asking MacoTango to review the complete control valve package. The review should confirm a testable holding requirement rather than assuming that an air lock relay alone guarantees permanent fail-in-place behaviour.