Low Lube Oil Bearing Header Pressure

F

Thread Starter

flow

HI,

On our GE MS5001 gas turbine, we've observed that the lube oil bearing header pressure is 2.1bar when auxiliary pump(QA) is running but it drops to 1.5bar when the main lube oil pump which is shaft driven is running. the main lube oil pump discharge pressure is observed to be 6bar which is almost the same with the control oil pressure indicating that the oil discharge pipe is not clogged and the pressure relief valve is not malfunctioning.

we have tried to adjust the set point of the bearing header regulator but this still did not resolve the situation. Please what could be the possible reasons why we are having low bearing header pressure?

thanks.
 
Firstly Bearing Header Pressure should be 1.7 Bar, this is controlled by VPR-2. To get 2.1 Bar with Aux. Pump & 1.5 Bar with the Main Pump is kind of strange. Have you tried adjusting VPR-2 when the Aux. Pump is running? How did you make these two tests to check the pressures, at Full Speed? Is it possible to physically take a look at VPR-2 and see what is happening, when the pressure is < 1.7 VPR-2 should be trying to open to increase the Bearing Header Pressure, when > 1.7 it should be the opposite. What is the Main Lube Oil Filter Diff. Pressure when you are making these tests?

The other normal questions, when did this problem start? Was any maintenance done recently?
 
Thanks for your prompt response.

To start with, no maintenance work was carried out on the unit prior to when the problem started. Also, at start up, when the auxiliary lube oil pump is running, header pressure is seen to be okay but reduces to 1.5bar when the unit is on full speed and loaded.

the main lube oil filters were recently changed and the differential pressure across the filters is 0bar meaning the filters are not clogged.

As for the VPR-2, yes we can check when the unit is shut down and only QA running, but not possible to check when the unit is running. In fact, the last action carried out on the VPR-2 was to adjust the set point while QA was running but when the unit is on full speed, the condition still remains the same.
 
hello flow,

have you checked the check valve found on the output of the auxiliary lube oil pump functionality? it may be leaking the main pump output back to the oil tank. this leak will stop only when the lube oil auxiliary pump usually "88QA-1" is running.

first you may perform a DC backup lube oil pump usually "88QE-1" test and monitor the output pressure.

hope that helps.
 
It would be good to see a copy of your Lube Oil P&ID, can you post it on a file sharing site? Is VPR-2 installed as a dump from the Lube oil Header back to tank or after the filters in line with the Bearing Header.

If the Bearing Header is 6bar, VR-1 is completely closed (have you checked?) and VPR-2 is doing what it should do, the only other thing that can cause low pressure is excessive flow on the system. Flow on the system can be checked by measuring D.P. across the Test Orifice installed after the filters just before the Bearing Header. Somewhere in your manuals there may be a D.P. to flow rate graph but just measuring D.P. only to see if it changes when the pressure drops may give you a clue.

You say that no maintenance was done but was this problem sudden or progressive?

Asarhan's comment on the 88QA check valve is also a good idea.
Sorry for all the questions but that is troubleshooting.
 
Hello Asarhan,

Thanks for your suggestion, but i do think the check valve on the output of the auxiliary pump is okay. because the supply for control oil is after the 88QA check valve, and it reads almost the same value as the main lube oil pump discharge pressure meaning there is no dumping of oil back to the sump by the check valve.
 
Hello,

Thanks for your help so far. I have posted a copy of the lube oil P&ID on adobe document cloud, please follow the link below to visit.

https://files.acrobat.com/a/preview/81beb91b-faf8-452f-9f49-8621a9b082f0

Also for the questions you asked. from what i gathered, the lube oil header pressure has been that way for a long time since almost commissioning.

the VR-1 set point is 8bar from the factory and there is no indication that it is shattering since the control oil pressure is reading almost the same as the main lube oil pump discharge pressure.

the last action that was carried out was to open the bypass of the temperature control valve a little to further increase the header pressure. although it worked but we are a little bit concerned about the lube oil temperature which has increased a little bit since opening the by-pass valve from the the tank.
 
Dear flow and dear All;

> when the auxiliary lube oil pump is running, header pressure is
> seen to be okay but reduces to 1.5bar when the unit is on full
> speed and loaded.

The difference between auxiliary LO pump and Main LO pump is that the first one is usually used at low LO temperature (startup or cooldown), and the main pump is used GT loaded and LO temperature is HIGH. In this second case, your LO pass through the Lube Oil Air cooler. Is there any LO pressure drop through your Air Cooler? And Is there any increase of your LO header temperature even slightly?

I used to have a similar trouble but in my case the LO cooler was U tubes exchanger. The inlet and the outlet are located on the same side and a metallic plate with rubber gasket separate the inlet of the outlet flow. When the gasket is damaged an interne oil leak will happen consequently LO pressure will drop between the inlet and the outlet of the exchanger. This will conduct to both <b>LOW LO Header PRESSURE</b> and <b>HIGH LO Header TEMPERATURE</b>.

Hope this will help
Regards
Karim
 
Hello Karim,

Thanks for the suggestion. Just yesterday, with the using a pressure gauge, we checked the lube oil inlet pressure to the LO cooler and also the outlet pressure from the LO cooler. Both pressures were okay so there is no sign of blockage or leakage within the lube oil cooler system. The next step we intend to take is to check the temperature regulating valve to be sure it is opened enough to allow sufficient flow through it. I will keep you guys updated on the outcome.
Thanks all for your help so far.

Regards.
 
Dear flow;

Thank you very much for keeping us informed on progress

>Also for the questions you asked. from what i gathered, the lube >oil header pressure has been that way for <b>a long time since >almost commissioning.</b>

I'm wondering why do you want to change/increase your Lo header pressure if it has been set since long while and if it works fine? Because if with 1,5 bar as header and your GT is running properly why do you want to change it? Do you have other Frame5 <b>single</b> shaft? and what about their pressure readings?

On Frame 5 <b>double</b> shafts, huge LO flow is used by actuator of the second stage nozzle and this is why it's very critical to have 1,7 bars at your bearing header especially if this GT is driving centrifugal compressors. But for a single shaft GT, I think (my opinion) 1,5 will be great to lubricate the bearings.

Regarding your P&ID; The maximum LO flow is observed on the Gen Gearbox, you may need to check the calibrated orifice on that line if it' s the correct size?

Rega rds
Karim
 
Hello Karim,

Am quite sorry for this late response. To answer your question on why we want to change the LO pressure now since it has been that way since commissioning?

Actually,the logic for the QA had to be forced out since then because the QA tends to run with the main lube oil pump in operation. the logic had to be forced out in order to prevent it from running and this we know is not an ideal operation. Also, the unit always trip on gearbox bearing metal temperature high although the logic for this trip has been forced out but whenever it is unforced, the unit trips. We know that this could be due to faulty RTD or insufficient cooling for the bearing and since there is an history of low LO bearing header pressure, it is just natural to think the trip is being caused it. we are embarking on this maintenance action so as to put the machine in its proper operating state.

Also, we have other single shaft frame 5 turbines and their LO header pressure is 1.7bar as against the 1.5bar of the unit under discussion.

Again thanks for your suggestion, I will keep you guys updated on the progress of the job.

Kind regards
 
Same problem has been observed in one of our Frame 6 machine after MI shutdown. Lube oil header pressure of 2.3 bar observed with AOP running(88QA) but on GT load with MOP pressure dropped to 1.4 bar.

In our case, VPR-2 can be adjusted in running GT by opening the manhole window of lube oil tank. On FSNL we have adjusted the VPR-2 to 1.8 bar from 1.4 bar. We have low pressure 88QA start switch is set at 1.2 bar and generator side low pressure trip switch is set at 0.5 bar.

What is the pressure you are receiving at generator side tail end of lube oil line? By your P&ID it is showing that the GT trip on low lube oil pressure is set at 0.9 bar.
 
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