Airtightness is the most basic "health indicator" of a pneumatic actuator. Once it leaks, at best the action slows down and air consumption rises; at worst it won't open or close to position or even cannot move at all. The purpose of airtightness testing is very direct — to check whether any leakage exists at the various connection points. This article introduces the two most practical testing methods on site, as well as the sealing points that must be checked with priority.
1. Why Airtightness Testing Should Be Done Regularly
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Leakage is a gradual process: aging seals, worn sealing surfaces and loosened bolts do not happen suddenly — test early and deal with it early
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Leakage directly drags down performance: it shows as slow action, high air consumption and insufficient output torque
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Three must-test points: factory testing, reassembly after overhaul, and recommissioning after long-term shutdown
2. Method 1: Pressure-Drop Method (Pressure-Holding Observation)
This is the most standardized method, judging by "how much pressure was lost":
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Raise the pressure of the actuator (or the air chamber under test) to the test pressure
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Use a shut-off valve to cut off the air supply, so that the chamber under test forms a closed cavity
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After the pressure stabilizes, start timing and observe the change in air pressure on the pressure gauge or in the test diaphragm head
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Within the specified time, the pressure drop must not exceed the specified value, which means it is qualified
A few points that are easy to get wrong:
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Stabilize the pressure before timing: after pressurizing, the change in gas temperature causes pressure fluctuation, and timing too early leads to a misjudgment
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The pressure gauge must have sufficient accuracy, and the reading should be able to resolve a pressure drop of the order of the judgment value
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Do not exceed the actuator's rated air pressure for the test; overpressure turns a small leak into a large one
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The expansion and temperature drop of large cylinders and long piping bring a "false pressure drop", and the judgment value must follow the procedure
3. Method 2: Soapy-Water Method (Bubble Localization)
The pressure-drop method can only tell you "it leaked"; the soapy-water method can tell you "where it leaks":
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Raise the chamber pressure to the test pressure
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Apply soapy water (or a dedicated leak-detection liquid) to each sealing point
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Observe whether there are continuous bubbles — bubbles mean a leak, and the larger the bubbles, the larger the leak
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Advantage: intuitive localization, especially suitable for finding small leaks
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Note: wipe off oil and dust before applying; after testing, dry it and apply rust protection to avoid corrosion by residual liquid
The two methods are used together: first use the pressure-drop method to decide "is there a leak", then use the soapy-water method to decide "where the leak is".
4. Diaphragm-Type Actuator: 4 Must-Check Sealing Points
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The connection between the upper and lower diaphragm covers and the diaphragm
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The connection seal between the stop piece, the tray and the diaphragm
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The sealing point of the O-ring
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The wall surface of each air chamber, as well as each welding point
5. Rack-and-Pinion (AT/AW Type): Focus on These 5 Points
Most pneumatic actuators used in industrial sites are rack-and-pinion, and the corresponding leak-check focus is slightly different:
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The joint surface between the end cap and the cylinder barrel (including the end-cap seal)
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The piston seal between the piston and the cylinder barrel
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The shaft seal at the output shaft
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The inlet and outlet port threads, the air supply fittings, and the NAMUR mounting face (the mating face with the solenoid valve)
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The stroke limit bolt (prone to leakage after adjustment)
6. Preparation and Precautions Before Testing
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The air supply should be clean and dry: oil and water not only affect the judgment but also contaminate the actuator's interior
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Operate manually once before testing to confirm there is no jamming, then pressurize
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When testing together with a solenoid valve and positioner, pay attention to air-circuit isolation, to avoid the accessories' own exhaust interfering with the judgment
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After testing, release the pressure, restore the air circuit, and fill in the record
7. Leakage Judgment and Handling Quick-Reference Table
| Leak point | Common cause | Handling direction |
|---|---|---|
| Diaphragm cover / end-cap joint surface | Loosened bolts, insufficient seal compression | Re-tighten diagonally to the specified torque and replace the seal |
| Diaphragm / piston seal | Aging, scratches, medium corrosion | Replace the seals and improve the air supply cleanliness |
| O-ring | Assembly scratches, deformation, improper lubrication | Replace with an O-ring of the same specification and apply an appropriate amount of grease |
| Air port / fitting | Improper seal-tape wrapping, thread damage | Reseal, and replace the fitting if necessary |
| Cylinder body / weld seam | Blowholes, cracks | Replace the cylinder body; it must not be used under pressure |
8. Recommended Fields for Testing Records
| Field | Purpose |
|---|---|
| Date / unit tag number / model | Ledger and traceability |
| Test pressure, holding time | Testing conditions reproducible |
| Measured pressure-drop value | Compare with the specified value and watch the trend |
| Soapy-water check result (leak location) | Basis for localization and rectification |
| Conclusion / tester | Responsibility assigned to a person |
Conclusion
Airtightness testing of a pneumatic actuator comes down to two things: use the pressure-drop method to confirm whether there is a leak, and the soapy-water method to find where it leaks — then troubleshoot and handle each sealing point item by item. Putting this work at the three points of factory delivery, reassembly and after long-term shutdown will noticeably improve both the action reliability and the air-consumption cost of the actuator. For technical support on actuator airtightness testing fixtures, please contact our service team.
Zhejiang Huaershi Auto-control Instrument Valve Co., Ltd.
Since 1989 · 30+ years focused on pneumatic actuators and pneumatic valves
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