In our compression station we have frame 5 machines. After visual inspection of the gas valves actuator assembly (bolted to stationary base plate) of one machine, I noticed a SMALL oil leakage (see the picture in the link below) from the GCV actuator plug while the machine was running. (notice that the plug is situated on the top half of the actuator assembly and it is removable).
the machine was working normally and the valves were stable.
After discussing this subject with a GE engineer, he told me that it should not be there, and this oil leakage will cause the valve to be unstable. According to him there is a sealing problem in the actuator piston, and GE has made a TIL about this problem.
In the other hand, an other engineer told me, it is normal as long as it is not big, and we need that leakage during the actuation of the valve to evacuate the oil from the other side of the piston else the valve will not be able to reach higher stroke positions (50% or more).
Is anyone has the correct information about this subject or a copy of the TIL mentioned above, and what is the main purpose of this plug?
I'm not familiar with that particular actuator, but most of the actuators recently used seem to be capable of being either double- or single-acting depending on the type of hydraulic manifold used. The application for both the SRV and the GCV requires single-acting actuators that are pushed upwards by the flow of high-pressure hydraulic oil to push the valve stem up to open the valve(s) against closing spring tension.
Since the actuator is essentially a piston in a cylinder and the piston will have some type of seal around it and all seals will leak at least a small amount of oil if the area on the other side of the piston doesn't have a way for the leakage to drain away the area will eventually fill with oil which is a non-compressible fluid and the piston will not be able to rise to open sufficiently to allow the unit to operate properly. So, the "plug", which might be necessary if the actuator were double-acting needs to either not be present or it needs to allow any leakage to be able to get out of the unused side of the actuator. So, there should not be a blocking type plug in that hole, or if there is some kind of plug it should allow flow in only one direction.
It seems like the "plug" in the photo might actually be some kind of of check (non-return) valve that prevents contaminants from getting into the unused side of the actuator, or it has passages to allow a small amount of leakage to drain away.
If there is continual flow out of that hole (port) while the unit is running that would suggest a more serious leak of the piston seal(s) which if large enough could lead to instability if the servo can't flow enough oil to keep the valve open against spring tension. One way to tell if the leak is bad and/or getting worse is to monitor the servo currents being applied to the servo coils. If the servo currents are excessive and getting more excessive over time AND one sees evidence of continual leakage (such as is suggested by the flow in the photo) then the actuator should be examined and repaired or replaced to prevent what will most eventually become a stability problem.
Older and slightly different actuators had a small threaded hole (only about 1/4-inch or so in diameter) called a "weep hole" and the factory would sometimes forget to remove the plug used to prevent contaminants from getting through the hole during storage and shipping. This would eventually cause the actuator and valve to not be able to open fully. Sometimes when a new actuator was installed in the field the Mechanical Department would think the weep hole plug should not be removed and that would eventually lead to a problem.
But it appears the "plug" in the photo is allowing piston seal leakage to drain away, but it also appears that the leakage is more than the other units are exhibiting and may eventually become a serious problem.
It's difficult to see from the photo, but the "plug" does not appear to be blocking flow but the continual flow would suggest the piston seal(s) are leaking more than the other units, which may portend future problems. I wonder ... why you were checking the actuators in the first place.... Most sites wouldn't do that as a general practice.
Hope this helps! You should start, if you haven't already, monitoring the servo currents being applied to the GCV of that unit. You may find they already exceed the null bias current value upper limit (-1.2 mA, total) or that over time they are rising as the leakage worsens.
This check has been made because after start up of the unit we had a small instability in the gas valves. We were suspecting this leak, but after some tunning in the LP speed loop card (SSZB) (Mark II) the valves stabilized again. This was long ago, but after discussing this with two control specialist i wanted to share it and get the right information.
The servo current check has been made to the valve and it was normal (-0.8mA) in a steady state position, and the machine was stable after the tunning.
The plug in the photo has been loosen to let inside leaked oil to get out and prevent contaminants from getting into the unused side of the actuator,
I think we share the same direction towards this problem! as long as the leak is not too big and is not affecting the position of the valve (current instability or wrong PV reading from the LVDTs) it might be acceptable, but by time it may increase and cause a serious problem.
It's tuning, not tunning.
Thanks for the additional information. It would have been nice to have in the original post.
Glad you figured it out.
>It's tuning, not tunning.
Thank you for the correction and forgive my english.
> would have been nice to have in the original post.
Like i have mentionned before this problem was long ago and it is not the reason for this post, the reason was the main purpose of the plug after the discussion i made with the GE engineers, not in particular the plug in the picture, also i can remember seeing this leaks before on other actuators but was unware of its consequences.
Thanks again for your help.