Hydraulic supply problem in GT

C

Thread Starter

calvein

What will happen to a running gas turbine if the hydraulic oil pressure suddenly falls to zero? The GT is a GE make frame V machine. Hydraulic oil is used only for modulating the IGV and fuel bypass valve. I assume that it shall not trip the machine, but FSR/ fuel control will be out and IGV angle will be stuck at the last operating value. Please help.
 
B

Bob Johnston

Not quite sure why you think that a total loss of hydraulic pressure won't trip the GT, of course it will. Take a look at your P&IDs and see what the result would be on fuel control & IGV control. I've never seen it happen because usually the Aux. Hyd. Pump will cut in and save you (If you have one)
 
You didn't say what fuel the unit was running on when hydraulic pressure was lost; so, I'm presuming liquid fuel because you only mentioned the Liquid Fuel Stop Valve. (But I have a suspicion the unit has an electric gas fuel control valve, with a pneumatic gas fuel stop valve.)

You also didn't say what alarms were annunciated when the hydraulic pressure was lost, and if the unit tripped when hydraulic pressure was lost.

It would be very surprising to find a Liquid Fuel Stop Valve not actuated (opened) by hydraulic pressure. On loss of hydraulic pressure the closing spring on the Liquid Fuel Stop Valve would quickly close the Liquid Fuel Stop Valve shutting off fuel flow. (Hydraulic pressure is used to open the Liquid Fuel Stop Valve against a very strong closing spring.)

The LFBV (Liquid Fuel Bypass Valve) is closed by hydraulic pressure to increase the flow of fuel. So, a loss of hydraulic pressure would allow fuel pressure to open the LFBV, thereby dexreasing the fuel flow. That would only happen if the Liquid Fuel Stop Valve didn't close on loss of hydraulic pressure.

Air flow through the IGVs usually (for most non-F-class machines) tends to open the IGVs, so in the absence of hydraulic pressure the IGVs should not remain in their last position. Air flow would most likely open the IGVs since there is no spring to close the IGVs on loss of hydraulic pressure.

I'm curious. Do you believe the unit should continue to run on loss of hydraulc pressure?
 
You need to clear some things up - like how old is this machine and what type of control is on it? The older 5001N and 5001P machines had a separate hydraulic trip oil system (I don't remember exactly what the nominal pressure of this system was, but 120 psig sticks in my hazy mind). If you lose that pressure, the turbine will definitely trip because the fuel stop valve will close. However, I think it will also trip on loss of the 1200 psig hydraulic oil, on pressure switch 63HQ-2. There will be multiple switches on Mark IV and later control systems.
 
Sorry for not elaborating the exact problem. Actually the AHOP started automatically and is running with the hydraulic pressure fluctuating b/w 79 and 81 kg/cm2. Earlier it was 77 kg with MHOP running. Is it safe to stop the AHOP manually from the MCC panel, assuming that AHOP started due to a mild fluctuation in pressure; hence MHOP will take care now?

Also, the liquid fuel stop valve is held open by the trip oil pressure and not hydraulic oil pressure in the GT under consideration.
 
While it's not okay, we're becoming quite accustomed to people not providing sufficient information and asking completely unrelated questions when they mean to ask something entirely different.

On the unit under consideration, where does the Trip Oil system get its pressure/feed from? The L.O. system or the Hydraulic system?

Does the unit have discharge pressure gauges for both the Main- and Aux Hyd Oil Pumps? If so, can you see what the discharge pressure of the Main Pump is before you temporarily stop the Aux Pump motor?

Many older Frame 5 units did not have Hydraulic Accumulators. Assuming this unit does not have a Hydraulic Accumulator, then a fluctuating pressure is usually caused by a changing hydraulic flow-rate, such as a control valve or the IGV actuator oscillating or hunting.

Has anyone tried adjusting the compensator on either hydraulic pump?

Has anyone checked the settings of the hydraulic relief valves and the condition of the hydraulic air-bleed check valves to make sure they are operating properly?

What is the differential pressure across the hydraulic filters?

If there aren't separate gauges on the discharges of the two hydraulic pumps so that you can see what the discharge pressure of the Main Hyd. Pump is before you stop the Aux. Hyd. Pump, then you're going to have to have someone watching the hydraulic system pressure with a radio or some means of communicating to the person who will be switching the Aux. Hyd. Pump motor starter to OFF (temporarily). If, when the Aux. Hyd. Pump is stopped temporarily by switching the HAND/OFF/AUTO switch to HAND or MANUAL the hydraulic system pressure does not maintain sufficient pressure and drops below, say 15-20% of rated, then you will need to have the person at the Aux. Hyd. Pump starter put the starter back in AUTO and/or HAND (MANUAL) to re-start the Aux. Hyd. Pump.

You will probably have only a couple of seconds to keep the unit from tripping when you temporarily stop the Aux. Hyd. Pump, especially if there is no hydraulic accumulator. There typically isn't much flow in a hydraulic system, but if there is no hydraulic accumulator then the pressure may fall quickly and so it will be necessary to re-start the Aux. Hyd. Pump to keep the unit from tripping.

Please write back to let us know what you determine. And to tell us where the Trip Oil system obtains it's pressure/flow.
 
B

Bob Johnston

Your Main Hydraulic oil pressure looks a bit low, usually it should be around 80 Bar, check your spec. it could be either VPR-3 on the pump or VR-21 in the manifold. If you then get a system disturbance (change in load, etc)it can "bump" the Aux. hydraulic into service if the pressure drops below the setting of 63HQ pressure switch (check the setting for it as well). Do you have accumulators in the system, if yes check their fill pressure. If you are confident that the Main pump discharge is roughly OK, you can try and stop the aux. pump and let the Main pump run. Don't try and reset VPR-3 while running, wait until you can crank the machine and set it at crank speed as there is a very good chance you could trip it at full speed.
 
otised,

Thanks; that's a new one for me!

Did the Trip Oil pressure provide the force for opening the Liquid Fuel Stop Valve?

Or, was it like newer Frame 5s that used the Trip Oil pressure to operate a dump valve to allow high-pressure hydraulic oil to open the Liquid Fuel Stop Valve?

Curious minds want to know!
 
The trip oil line gets it's feed from the L.O line and is not related with the hydraulic oil as such. The pressure gauge is common for both the pumps. There is no accumulator in the hydraulic line and the filter DP is 0.65 bar.

We were thinking about stopping the AHOP manually and putting it on AUTO mode immediately as suggested, but in a completely unrelated development, the machine tripped yesterday on IGV control trouble trip due to a faulty feedback from one of the LVDTs. Upon rectification and startup, the AHOP disengaged itself at FSNL as per normal procedure and the hydraulic pressure is now steady at 82 bar. I am guessing the AHOP indeed got started due some minor disturbance in the system earlier.
 
CSA- In the frame 5 GE machine, the PG5371PA, the trip oil pressure itself holds the fuel stop valve open and is independent of the hydraulic oil pressure. I mean you can force the stop valve open when AOP is running without the AHOP, during standstill condition.
 
calvein,

Thank you very much for the clear response.

I have not seen, nor will I probably ever see, all of the various configurations that were used on GE-design heavy duty gas turbines over the decades they have been produced and sold. I have very recently (not on control.com) believed that what I was being told was complete and verified, and subsequently I (painfully) learned otherwise.

I learn a lot from these postings, and I've certainly learned from this one.
 
Hai,

Our GE Frame-6B too no hydraulic oil need to open liquid fuel stop valve, it is being operated by trip oil. Whereas in FRame-9E required hydraulic pressure to open LFSV.

Cheers
G.Rajesh
 
CSA- Can u please explain one more thing? The hydraulic supply to the IGV cylinder is routed via a 4 way infinite position servo valve. During standstill, we run the AOP and AHOP. Then we feed the CSRGV reference at, say 70 degrees. The IGV will open to 70. Now we fix a small iron piece at the IGV angle physical indicator and stop the AHOP. This entire procedure is done to clean the IGV blades during any shutdown. Now I am sure the small iron piece is just an afterthought and the IGV would have remained at position anyways once the AHOP is stopped due to the servo valve being at an closed port condition.

My question is, since the IGV stays at the last CSRGV reference during shutdown, why should it behave differently during running? Same should be the case with liquid fuel control valve, i.e. it should be stuck and fuel flow shall be held at the last healthy position. Hence, can the machine actually run on loss of hydraulic pressure, assuming that load and IGV reference does not change much, hence IGV position trouble trip or is not initiated. There is no tripping due to low hydraulic pressure as compared to, say, low trip oil pressure or low fuel oil pressure. Please explain.
 
B
Take a look at your P&ID and your software for the function of solenoid 20TV or logic L20TV?(maybe X)This is the IGV Trip Oil dump solenoid. This needs to be energized to operate the IGVs and when de-energized will dump the IGVs closed. Usually the logic should be L14HS + L4 = 98% Speed + Master Protective OK. So any Normal Stop or trip will close the IGVs
 
Bob: Though the L20 TV exists as a library function in the toolbox, there isn't any such solenoid in the P&I D or in the logic diagram. I think L20 TV is not meant to be used in Frame V machines. The library incorporates it as the same toolbox might be used for Frame VI or 9E. I have seen a trip oil operated dump valve in series with the IGV servo valve in the P&I D for a frame 9E machine but definitely not in my machine (Frame V).
 
I have got one strange behavior. GT frame 5, Mark II control system was running idle, then all of a sudden I could not increase the speed. I stopped the machine to check. unfortunately I could not start again. after start signal, firing is ok, burners 7&8 ok. then reaches 35% speed and stayed for about 2 minutes then trips. last time it did not trip. I stopped myself because it stayed longer time and the speed did not increase more than 1800 rpm and temp went more than 390c?

the hydraulic oil pressure found 0.8 bar hydraulic oil. I checked the IGV connections, VPR1, VPR2 and VR no leaks. pump discharge pressure also ok

what could be reason for this???
 
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