Output Torque on a Dynactair pneumatic actuators for 1/4 turn valves 2

[ICRH]

Has any one out there had experience with DYNACTAIR pneumatic actuator for 1/4 turn valves?

Note that this is a very unique actuator design.

The output torque appears to be lower than expected in the spring direction. No preventive maintenance since the preoperational tests, 26 years ago.

Thanks
Issi

 

[gerhardl]

As far as I can sea Dynactair is similar to other constructions in the way that the construction itself should not give 'weaker than expected' spring return torque over time.

Suggested checklist and questions:

Has the actuator been taken off the valve, but connected to all existing pipelines, solenoid valves, eventuel damping and throtteling devices etc, and torque been measured to be sure the valve and mechanical connections from actuator to valve is not causing the problems?

Are solenoid valves, damping and throtteling devices fully open, with no dirt or damages, no air leakages or dirt found in supply and air filter to ensure full air supply and air evacuation?

If OK for the two first: No internal leakages (this is what normally and most often do occur), especially past piston sealings. (Supply air pressure, and open return connections to discover air passing sealings)

No springs broken, weakened, damaged or unprecise because of wear?

General wear or dirt in actuator, especially inside piston borings and sliding part of walls?

 

[PeterIgg]

Further to what GerhardL has said, I looked at a brochure and I am not sure as to why you think that the actuator is of a unique design. I admit that I am not familiar with this specific brand, but it looks much like many other actuators of this type, unless they have changed their design in 26 yrs. And there, I think, we have the nub of the problem: If these valves have been cycling regularly over that period I would expect wear and deterioration generally with a resultant loss of performance. If they have been static for much of the time, one would assume with the springs compressed, I would still expect to see a drop off in performance.

How are you measuring the output?

 

[ICRH]

Both of you might be right saying that the construction is similar to other actuators in the industry. I am just familiar with a few designs.
The actuator has a piston and an accumulator, maybe like others. The difference that I find with respect to the ones that I know is its capability to exert a great torque as the fully closed position of the valve is approached. See attached curve.

The measurement has been done by using a strain gage on the shaft. With that gage the actuator is operated and stopped in different positions with a brake. The maximum value read by the gage every time the actuator is stopped is registered. This operation is done in both of the directions, the air supply and the air venting. The real test required additional devices, but essentially is like I explained.

Your questions and observations are being very helpful on the proccess to determine what to do next. By the time the issue is solved I will inform you.

 

[PeterIgg]

The torque characteristic curve that you show is indicative of the scotch yoke design, giving the minimum torque at around the 60° position rather than at the end, as a rack and pinnion design would. This mechanism is more common in the 'non aluminium' (more expensive) units such as Bettis CB series; Rotork CP series; to give two of the more common types - there are many, many others.

The way in which you are measuring the torque is as good as it gets, I am not aware of a better way and it rules out a lot of the other variables that can lead to the belief that the actuator is at fault. If the only issue is that the spring torque is down (and I assume significantly) from what it was I suggest that the only option will be to replace the springs, I would suggest that they are a nested set and one may well be broken. However it will probably be more economically realistic to replace the complete unit, it is 26 years old, after all!