Actuator not functioning after hydraulic oil replacement

[Hamza1988]

Hi, we have large actuators to control the operation of 20 inch butterfly valves. These valves are discharge valves of large vertical pumps. So recently we had to replace the oil (around 150 liters) of the control unit of the actuator. After replacement of the oil, the valve funcionality was checked and it was found that the valve was opening properly but was not closing fully. At first we thought that the 20inch valve was passing or the internal rubberlining was damaged but later we did the same preventive maintenance on another unit andthe same issued was faced again.

My question: Is the draining of oil from the oil circuit and renewing it causing the actuator to not function later during operation? Thanks

 

[IRstuff]

In hydraulically operated systems, such as car brakes, air is anathema. After every opening of the hydraulic circuit to air, you need to bleed off the entrained air. Did you bleed the systems to remove the residual air?

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[BEMPE16524]

common issue with air trap i guess. the system should have a bleed valve to relief the air.

R.Efendy

 

[MikeHalloran]

Yeah, it _should_ have a bleed valve.
... and the valve should be located where the air collects.

But many systems that are not expected to ever be serviced after installation,
as I suspect in this case,
there may not actually be a bleed valve,
so it may be necessary to open connections and bleed air,
starting from the high points and working down.

I assume Hamza's mechanic already figured that out.


Mike Halloran
Pembroke Pines, FL, USA

 

[Hamza1988]

Thanks guys, the problem is still there. You guys suggest that there might be air trapped in the system. We do have a bleed valve in the hydraulic circuit which we have used to bleed the air out. Plus if air is trapped in the system, what usually happens is that the pressure in the hydraulic circuit gradually drops which in our case is stable. The hydraulic circuit has an operating pressure of 140 bar and the pressure is stable during operation.

 

[3DDave]

It may be the viscosity of the oil is lower than it was, causing enough leakage that the actuator isn't developing enough force to close the valve. It's possible the seals have hardened and are difficult to make a seal and need to be replaced.

 

[MikeHalloran]

1. STOP doing this particular preventive maintenance until you have a procedure in place that does not degrade the system operation.

2. Consider using hydraulic oil sampling and analysis instead of just blindly replacing what may be perfectly good oil.

3. We can't see the system from here. A single bleed valve may eventually get all the air out, but it may take hundreds of full stroke cycles to carry all the air to the bleed valve, or it may never happen. Can you post photos of the troublesome installation?

4. You probably should seek guidance from the actuator system manufacturer to see if there are any special procedures necessary to get the system running from a completely drained state.

Examples:
- Using a vacuum pump to expand the trapped air and allow removal of most of it from the most remote cavities, before adding the new oil.
- Cracking open plugs on the high side of the actuator, or cracking open the cylinder ports to bleed out air after filling, perhaps with cycling, just like you bleed the brakes on a car. Again, it may still take multiple cycles.

5. You need to eliminate the possibility of a foreign object, e.g. a wrench, holding the butterfly open a little.





Mike Halloran
Pembroke Pines, FL, USA

 

[Oilyguy90]

Hi Hamza,

I am going to make two assumptions here.

[1]The hydraulic actuator in question is a linear double acting hydraulic cylinder and not a hydraulic motor.

[2]Closing of butterfly valve is during retraction stroke of cylinder. (My argument still applies even if closing is during extension)

I would consider the following possibilities to be the cause of the problem, some of which have been rightly pointed out by others:

[A]Make sure that nobody has tampered with the relief valve setting during maintenance and replacement of oil. Retraction force is more likely to be affected due to lesser area on which pressure acts.

Ensure you are using the right viscosity of oil! Very high viscosity will lead to higher pressure drops and therefore lesser available pressure (and hence force) for cylinder.

[C]Something is mechanically preventing the butterfly valves from closing. Ensure that the actuator is completely extending/retracting by detaching it from butterfly and keeping on the ground. Try eliminating at least the actuator by swapping it with a known working good one that has not had oil replaced. Ensure that old and new oils are compatible before swapping around actuators. Ensure pins,bushes and bearings are operational and lubricated.

I do not understand how entrapped air can be the cause of the problem you are seeing. It does not affect the force output of the actuator and shouldn't affect the stroke either. You might just experience jerky motion due to air in the system.