We have our frame-6 GT tripped on low lube oil level. Found the lube oil IGV tubing burst. This tubing burst we think due to rubbing between the tubes which is due to high vibration between the tubing.

We studied the cause of this high vibration which leaded to tubing burst and think two reasons:

1- May be due to oil pressure to the IGV Actuator is not stable as we have fluctuation in it in range of 3 bar. This reading after the pump as there is no reading after the accumulator, and our operation team think this fluctuation is not too high and can not lead to the problem.

2- Another point of view that, the vibration due to a problem in the IGV Actuator itself that due to its travel adjustment nut is not locked properly. that leaded to the actuator fluctuating and vibrating strongly leading to the oil tubing rubbing with each other. This investigation as we found actually the traveling nut of the IGV actuator loose, but I told them this loose due to the non-stability of the oil pressure during oil rubbing as the pressure was fluctuating highly at that time in range of 10 bar so the nut became loose.

Now there is a confusion which come first and leaded to the other? The actuator problem leaded to the tube rubbing and oil bursting or the oil bursting leaded to the IGV actuator problem.

I appreciate to get the point of views of the experts. I want to know also what is the accepted limit of the oil pressure fluctuation to the IGV. Also we have YF 7645 series IGV actuator. I can see one big nut for the travel adjustment, but how to lock this nut after travel adjustment? there is no pin hole in it or no other Nut to lock it, So how to lock it?

Thanks and best regards

 

Dear hany5757;

I'm not familiar with Frame 6 GT but HDGT frame 5 <b>D model</b> has a derived axial compressor from the MS6001B so IGVs and its actuator may be the same!..

> there is no pin hole in it or no other Nut to lock it, So how to lock it?

This BIG nut <b>must</b> be locked otherwise you will have problems (looseness, wear, leakage, vibration). First thing to do is to check your P&ID and drawings if it's correctly installed and/or no parts are missing.

On the D model, we have here. there are both SMALL nuts (forward/After the big nut) to lock it. Another option could be done, if there is not enough space to add these SMALL nuts, is to drill two small holes on the BIG nut, thread them and install two screws to lock the BIG nut. But, again, don't make any modification until you checked all your machine drawings. How to lock this BIG nut should be specified there.

> This tubing burst we think due to rubbing between the tubes which
> is due to high vibration between the tubing.

<b>High vibration</b> on HDGT means casing, rotor vibrations or are you talking about located "High Vibration" on IGV actuator area?

On the D model (MS5002D);the two lube oil tubes (from/to IGV Actuator) are made to be one inside the other. This configuration will keep the tubes to remain away each other. I think the lube oil pipes should not get in contact and if it's the case, they should be tied properly to avoid any relative displacement. Because when tubes touch each other, even if you don't have "High vibration", time will do the rest and you will get the rubbing.

<b>ACCUMULATOR</b>: 3 to 5 bars fluctuation of hydraulic oil pressure, in my opinion (mechanical opinion), is not a big deal <b>if your accumulators are properly working.</b> So if you didn't yet check/calibrate them, I think you have to do it. Usually, on Frame 5, there are 3 accumulators and regarding my experience with two working correctly, the actuator woks fine!..

> Now there is a confusion which come first and leaded to the other?

My opinion, If the High Vibration is located only on the actuator area; the BIG nut looseness leads the vibration then tubing wear and tube bursting.

Regards
Karim

 

hany5757,

I agree with everything bkarim55 has said.

However, I believe that 3-5 barg oscillation is excessive for a machine with a properly charged and properly valved-in hydraulic accumulator.

It's very common for the bladder in the accumulator to have been damaged during attempts to check and/or charge the accumulator. While the fittings look very simple, and are, in fact, vehicle/bicycle tire fittings, they have a very important difference. There is usually a label attached to the top of the accumulator which has the brief instructions for connecting and using the charging hose supplied with the system.

Also, it's common for people to completely empty the internal bladder of the hydraulic accumulator while trying the check or charge it--again, because the fittings of the charging hose were not used properly.

The two valves at the bottom of the hydraulic accumulator are almost NEVER labeled, and are quite frequently not in the proper positions. One of the valves is the "block" valve--the one that can be used to isolate the accumulator from the hydraulic system (which uses L.O. as the high-pressure hydraulic medium), and the other valve is the "bleed" valve, which is used to drain any hydraulic oil/pressure in the accumulator during checking/charging activities.

The block valve must be open at all times during normal operation, and the bleed valve must be closed at all times during normal operation. Again, it usually takes some fiddling around with the valves to determine which one is the block valve and which one is the bleed valve--but site personnel SHOULD identify the valves and then LABEL them properly for future operation.

The pressure which the bladder must be charged to--when there is NO hydraulic pressure on the accumulator (because the bleed valve is closed and the block valve is open during checking/charging operations) is usually half of rated system pressure. If that's approximately 100 barg, then the charging pressure should be somewhere in the range of 50-60 barg (check the Device Summary to be sure). Most people think the accumulator has to be charged to the same pressure as the hydraulic system operates at--but that's wrong, and if the accumulator is charged to the same pressure as the hydraulic system it will be useless in helping to dampen pressure oscillations and high flow demands, especially during high flow demands.

A properly charged and valved-in hydraulic accumulator should dampen just about any pressure oscillation to much less than even 3-5 barg (on a 100 barg system).

As for locking the adjusting nut in place, some actuators use a "lock tab" which is a bar of metal bent at a 90 degree angle that is tack-welded to the nut and to a stationary part of the actuator rod. It can easily be removed during maintenance, if necessary. There are usually several examples of this kind of lock tab on most GE-design heavy duty gas turbines--it's also commonly used on foundation bolting.

But, as bkarim55 says, you should be able to look at the drawings provided with the turbine and see how the original nut was locked in place. (Some replacement actuator assemblies were not supplied with locking nuts or locking tabs.) But, it does have to be secured.

Finally, it's also very common for the hydraulic pressure relief valves to be used to set the hydraulic system pressure. And, they are RELIEF valves, and should NOT be used to set the pump output pressure--but to relieve pressure in the case the pump adjustment fails and causes hydraulic pressure to be much higher than normal. Again, consult the Device Summary for proper settings for the pump adjustment (the "compensator") and the relief valves and check to make sure they are all adjusted properly. Have a read of the 'Hydraulic System' System Description in the service manuals provided with the unit.

Hope this helps!!!

 

Are you sure that the accumulator is properly charged? I've never seen looseness on the clevis locknut cause this kind of problem. Why do you need to touch the locknut, have you been adjusting the IGV angle? Have you tried calibrating the IGVs when the unit is stopped, are they stable then? Have any checks been made on the IGV bushing wear and backlash?

 

Thanks for your reply and please find the following:

1- Our configuration of the hydraulic line supplying the IGV actuator is: Hydraulic Pump to Filter then orifice. then accumulator will give his discharge then the IGV servo. So, my question is the pressure before orifice is same as after orifice? Where is exactly system pressure point in this line? Also we do not have tapping point to measure pressure after this orifice and the location of accumulator discharge. Only we measure the pressure at discharge of the Pump. How to know the oil pressure direct at inlet of the IGV actuator if it is stable or fluctuating while there is no measuring point for pressure at this location?

2- Is there any way to confirm healthiness of accumulator while machine is running?

3- If the hydraulic system line pressure became low due to tubes leaking as in our incident, what is supposed to be the function of the accumulator? Is it supposed to supply oil to the line until which pressure? Does the Accumulator will supply until it becomes same as the Nitrogen charging pressure?

4- There is only location of test gauge pressure on the accumulator, what we should read on it? The charging nitrogen pressure (in our system it is 40 bar) or the system hydraulic line pressure (in our system it is 80 bar)?

5- For the Clevis lock nut, actually we installed a new IGV actuator from 2 years. I believe there was something done incorrectly during installation as we found during the incident that this nut is loose and the clevis became very loose. I found in the IGV actuator drawing, it is mentioned that the Clevis and the "L" bracket should be welded?! Is this correct? Also what is exactly the function of this Nut, Is to adjust the travel or to lock the Clevis? Also I found Locking plate bended at only one side by 90 degree but this bending not on the Nut side ,it is on the clevis side of the plate and not welded. So please shall we bend the other side of this locking plate by 90 degree or this plate is not installed correctly and this 90 degree bent place is not correct (i.e. the locking plate should be reversed)?

My best regards for your valuable advices.

 

hany5757,

1) Without being able to see the P&ID for <i>your</i> turbine it's impossible to say for sure what the function of the orifice is--but it's probably just to try to limit flow should there be a break and flow would try to increase very rapidly. As such, the pressure on both sides of the orifice will, under normal operating conditions, be the same (when it's not restricting flow). Again, under normal, so-called "steady-state" operating conditions there is very little flow in the hydraulic system--because all of the hydraulically operated devices are in their required positions. This is something that seems very difficult for people to understand--that when a control valve, or the IGVs, are in the called-for position (when the actual position feedback is equal to the position reference) there is NO flow to the device. Flow is only necessary when the device position needs to change.

The P&ID should show where the pressure gauge on the Accessory Gauge Panel is tapped into the system. It's usually between the pump and the manifold block with the air-bleed check valve(s) and the relief valves. The pressure should be constant all through the hydraulic system regardless of where it's measured--again, under normal operating conditions.

2) Yes. You can close BOTH valves at the bottom of the hydraulic accumulator, and then if you know which one is the bleed valve you can open it and then connect the charging hose/gauge assembly to the accumulator, and after following the instructions on the label on the top of the accumulator to "connect" the charging hose/gauge assembly to read the accumulator pressure.

If, when you close the accumulator bleed valve you see hydraulic pressure start to fluctuate more than 0.5 barg, then that may be an indication the accumulator was at least working a little bit before the isolation valve was closed.

3) The accumulator's only function is to smooth pressure fluctuations during normal operation, and one of the ways it does this is to supply a small volume of hydraulic oil (lube oil) when there is a high-flow condition which would tend to reduce hydraulic system pressure--such as if the IGVs were commanded to move a large amount in a very short period of time.

It's suggested you use your preferred Internet search engine to find a basic hydraulic accumulator function description. They only store a small amount of oil (approximately one-half the size of the accumulator assembly). If there is a large leak, the hydraulic accumulator won't be able to maintain pressure--but eventually, since hydraulic oil is lube oil, the lube oil tank level will drop--as you described. But, the accumulator can't maintain pressure when a large leak occurs.

4) "When the turbine is running, if the block valve is open, a test gauge on the top of the hydraulic accumulator should read the same as the hydraulic system pressure gauge on the Acc. Gauge Cabinet. If you close the block valve while the turbine is running and open the bleed valve, the test gauge pressure should drop to 40 barg.

5) Welding (or more correctly "tack welding" is many times how the L-shaped bracket is held stationary at both ends (of the bracket). It's quite commonly used on GE-design heavy duty gas turbines. Follow the drawings in the manual, and you should be fine.

I believe the function of the nut is to adjust the angle of the IGVs at one end of travel, and to allow for sufficient actuator movement to result in full opening of the IGVs.

I'm getting a little confused here about the intent of this thread/question. There should be very little, if any, fluctuation of hydraulic system pressure during normal operation. And, that's the function of the hydraulic accumulator--to smooth system pressure. Without an accumulator, sometimes the axial piston pump can cause as much as a 1-2 barg fluctuation in system pressure during normal operation. The other function of the accumulator is to <b>temporarily</b> supply hydraulic oil during a sudden increase is flow-rate, such as when the IGVs are commanded to open or close by a large amount very quickly (during a load disturbance, or similar situation). The accumulator does NOT continuously supply hydraulic oil to maintain pressure when there is a large leak; it can't, it's very small. And if the leak exceeds the ability of the pump to maintain flow then hydraulic pressure will drop.

If the IGV hydraulic actuator was replaced, the best guess is that, as with the lock nut not being properly "locked", the hydraulic tubing was also not properly reconnected or was bent such that it resulted in the rubbing that eventually wore through the tubing causing the leak.

Our suggestion that the hydraulic accumulator might not be working properly was made because, normally, hydraulic system pressure does not fluctuate by 3-5 barg when the turbine is running, and the hydraulic accumulator should be capable of maintaining pressure during normal operations to within 0.5-1 barg of rated system pressure.

If the relief valves were used to set system pressure instead of the pump compensator adjustment then that greatly increases the flow-rate of hydraulic (lube) oil through the hydraulic pump (because it's flowing out of the relief valve when it shouldn't be). Many times this increased flow-rate is sufficient to cause the Aux. Hyd. Pump motor overload relay to actuate, and it can even result in eventual damage to the Aux. Hyd. Pump motor windings if not corrected.

The cause of the tubing rubbing and leaking was most likely poor attention to detail when the IGV actuator was replaced--and that is borne out by the lack of a proper locking tab to secure the adjusting nut of the clevis. If one aspect of the replacement wasn't done correctly, then it's very likely another aspect might not have been done correctly, either.

But, we're NOT suggesting the problem was, or could have been prevented by, the hydraulic accumulator. We're simply suggesting the hydraulic accumulator may not be working correctly because, under normal operating conditions, the accumulator should be more than capable of maintaining a very stable hydraulic system pressure.

Really, the only way to properly check the operation of the accumulator is to know exactly which of the two valves on the accumulator is the block valve and which is the bleed valve. And, then use those valves to check the pressure in the accumulator bladder. During normal operation, hydraulic pressure compresses the bladder in the accumulator, raising it's pressure to be equal to hydraulic system pressure. It does this be allowing hydraulic oil to flow into the accumulator and storing that oil as it's compressing the bladder up to system pressure. If there are pressure fluctuations in the hydraulic system, say from pump discharge, the accumulator will allow some oil to flow out when the pump pressure tends to drop and then back in when the pump pressure increases--thereby maintaining system pressure. OR, if there a sudden large flow due to a large amount of hydraulic oil being required to move an actuator a long distance the accumulator will again release oil to help smooth out the pressure drop and thus not affect other hydraulically operated devices (like fuel control valves, or fuel stop valves).

If you're looking for a cause for the loss of oil from the rubbing of the tubing--it's probably a result of either poor reattachment during replacement of the IGV actuator, or the tubing was bent by a worker during a recent maintenance outage. But, the accumulator wasn't the cause of, nor could it have protected against, tubing leaks causing a major loss of oil (and a big clean-up, too). Normal vibrations on a turbine are quite high, and that's why tubing is generally securely clamped down and not usually run in parallel--unless the tubing is securely clamped together to prevent rubbing. And even then, over time, there will eventually be some rubbing--it's just going to happen.

AND, if the hydraulic system pressure isn't stable, this in only going to make the rubbing problem worse, resulting in a failure sooner rather than later. That's where the accumulator comes in, by helping to keep the system pressure relatively stable (which isn't 3-5 barg).

Hope this helps!

 

Dear hany5757

I think Sir CSA left nothing to add :-(

"Tack welding" is a common practice on <b>Heavy Duty</b> GT and we do it even if it's not on manuals and procedures. It's a "DOUBLE lock". And if it does exist in your OEM procedure, . <i>Just do it!</i>

Piston or Bladder Accumulators are used to improve <b>system efficiency</b>, absorb shocks, and compensate leakage (small leaks not tubing burst like you had)

I think the most important accumulator role is <b>shock absorber</b>. On vehicle (or car); Shock absorber which is an accumulator, its first job is to keep the wheel sticked to the road so when the shocks happened (bad road for example) shocks will not be transmitted to vehicle body but if the if the shock is too high, people inside the vehicle will may injury. Same thing with your IGV Bladder /Piston accumulator.. it maintains a steady pressure and absorbs any abnormal fluctuation but if it's too much, accumulator will lose its efficiency.

In some HDGT double shaft I have seen; It’s easy to check the nitrogen pressure because there is an additional Nitrogen reservoir (with pressure <b>gauge</b>) connected to the accumulators. But the main raison to implement this additional reservoir is because these accumulators work with both IGV and second stage variable NOZZLE actuators.

I think if you contact with your Accumulator Dealer/Manufacturer, they may be able to provide you with accessories (pipes, connections and gauge) to implement to your actual system.But, again, any modification should be approved first !!

The other way to check if your IGV system is working fine while GT is running is to watch to your actuator using a light torch and observe is there any leak, vibration, mechanical fluctuation, looseness.

One more thing; Your hydraulic line configuration looks quite simple! Are you sure that this line doesn't feed others Systems like fuel gas, DNL? These systems also require steady and smooth pressure. So when troubleshooting these systems should be considered.

Hope this is helpful
Regards
Karim