Pneumatic actuator leakage standards 1

[dmech13]

Is anyone aware of any standards/codes of practice relating to acceptable leakge rates for pnematic actuators?

A pressure-drop method is going to be used as a quality check post assembly but a leakage threshold must be known for this approach to work.

Any help with this matter would be greatly appreciated.

Kind regards,
S

 

[gerhardl]

I do not know of any official, common tests. Your question started me thinking about both purpose and limitations for such tests.

1. Leakage and construction?

In practical operational use any pneumatic actuator could leak across piston sealings (if piston), through diaphragm (if diaphragm), through unthight air connections, or leakage could be through attached components (solenoid valves, checkvalves, mechanical stroke adjustment screws etc).

Some actuars will have a thin, limited layer of gliding agent ('grease') that will prevent piston leakage for some (limited) time, some do instead have softer (gliding) sealings against harder (polished) metal.

Leakage across worn piston sealings over time is probably the most common failure. For actuators this would first be critical by leakage costing mony, or leakage large enough to give operation failure.

2. Leakage and purpose of lekage measurements.

a)For pneumatic actuators of good construction you would normally require functional test of valve/actuator together under load at site as startup procedure. Functional tests repated at regular maintenance and test intervals if critical. (See my point 4)

b)By fabrication you would obviously need a factory functional tests.

c)It would certainly also be sensible to include (as a fabrication quality test) a leakage measurement along with functional test for the actuator at fabrication. A high-qality producer should probably test all units. A torque or load force measurement control over some time, when closing off air supply, would indicate critical leakage if force weakens.

Suggestion: If lekage as such should be checked, and if not any other tests available, why not use same test criteria as one would use for the valvetype intended to be actuated, with adapted procedure?

3. If the actuator is operated by other gas than air, special tests suited to the gas characteristic must be done (flammable fluids, toxic fluids, sealing material and other material requirements).

4. On operational tests at site.

Actuators will as valves have most problems occuring under extreme operational conditions.

The purpose of any operational test is to give preventive maintenance at lowest possible total cost (operational stop loss, repair and part cost).

The test intervals must be intended to catch any irregularities 'at start' before they disturb process regular operation.

The operation regularity of the total unit (actuator with pneumatic supply and connections, inner sealing parts, torque or linear force mechanical application part, dirt and water air quality components, and valve itself) must be totally checked. Leakage of actuator alone would have limited purpose.

Depending on both actuator quality and construction, operational frequency, operational conditions, and same factors for attached valve, could give a lifetime (leakage or mechanical wear or both) shorter or longer for the actuator than for the valve.

(Sorry about the length of the answer, which probably should have been limited to 'no' from my side ;-))

 

[dmech13]

Hi gerhardl thank you for your response.

For this particular application 100% of actuators assembled will be leak tested. Currently a water bath technique is used where the actuator is pressurised with compressed air and immersed in water, if any air bubbles are present then the actuator fails.

I will look in to using the same criteria for the actuator as the valve.

Thank you for time and suggestions

 

[gerhardl]

Seems to be a sensible and simple methode!

In addition: all fabrication quality starts with quality control of ingoing materialss and accuracy quality control of produced single parts, also before assembly.

 

[rmw]

An adage repeated to me only yesterday by an old sage is "you get what you inspect."

rmw

 

[valvit]

ISO 12490, entitled "Petroleum and natural gas industries —
Mechanical integrity and sizing of actuators and mounting kits for pipeline valves" was published on 15 September 2011 and covers this topic.