Gas Turbine Gearbox Bearing Damage

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Thread Starter

Izhar

Hello there,

We have currently carried out combustion inspection of our Gas Turbine (Frame 9E gas turbine) with Flender Graffenstaden TYPE AA30CZ. In parallel we carried out overhauling of Gas Turbine gearbox, we had the following observation during the overhauling:

1) Bearing of Shaft 4 at the lube oil end was found badly damaged but the damage was at a specific area and was very deep. The bearing at the hydraulic pump was also damaged but it was suspected as the hydraulic pump was not function(it dropped after our Hot gas Path Inspection).

2) Coupling of hydraulic pump was also damaged (Teeth at the hydraulic pump side were worn out completely). However it was also suspected as the hydraulic pump was down already.

What could be the probable reason for such a specific damage on the lube oil bearing (is it electric frosting or something)?

3) Also recently (about 6 months before) we faced the same problem with our Gas Turbine drive end generator bearing. It was the same kind of damage that we currently have faced with our gearbox bearing.

I just wanted to know why specific area damage in journal bearing occurs? What is the reason for it?

Waiting for your kind response.

Thanks
Izhar
 
Hi Izhar,

It appears electric discharge. This is typical at accessory gearboxes and its affect is mainly seen on the No: 4 shaft which is at the bottom and closer to the ground. You must see similar frosting on the gear shaft journal as well. If you look through with a 50x magnifier you will see black dots all over the bearing surfaces. It damages coupling because bearing clearances becomes larger and gearwheel start to travel up and down. You are lucky that no catastrophic damage occurred on the gear wheels, which happened to us once. The easiest way to discharge this shaft voltages is to install a shaft grounding brush somewhere on the No: 1 gearwheel. You can consult Tom Sohre, Sohre Turbomachinery for suitable grounding brushes and shaft voltage measuring/monitoring devices. He helped us a lot.

Quick question; Is your turbine designed for dual fuel? If so, is liquid fuel pump driven by No: 3 gear shaft and equipped with a magnetic clutch? If so, verify if there is DC leakage from clutch.

In terms of generator; have a look at the NDE bearing insulation and measure the insulation resistance with 500 V DC. Also look at the grounding carbon brush at DE, check whether it is functioning or not.

Regards
 
Thanks alot

Let me make some correction. This bearing found damaged is of Shaft 02 which is the intermediate shaft and its purpose is to drive all the other shafts.

Yes our Turbine is designed for dual fuel and we do have a magnetic clutch installed at shaft 03.

Regarding the leakage and carbon bush, let me check and verify and then will come back to you.

Thanks again for the response.

Regards
Izhar
 
Hi Izhar,

I think you have to put the correct diagnosis to the problem. Otherwise your effort will be wasted. Bearing damage due to electric discharge is easy to distinguish from other type of bearing failures. If frosting appears on the bearing surface and on the journal as well and if you identify black spots with a magnifier on the surfaces, you can say it is electric discharge. Normally you can expect lube oil to act as an insulator, but in time it deteriorates and it becomes more conductive. I remember, at the time we had similar problem on our 6Bs, I included some tests that are normally carried out on transformer insulating oils, just to understand the dielectric strength of lube oil. In year 2001, this phenomenon occurred successively on our 3 units and affect was destructive as No: 2 gearwheel damaged on two units. But all these damages caused by excessive bearing and journal wear on No: 4 gear shaft together with flexible coupling damage (main hydraulic oil pump drive). For us it took sometime to diagnose the failure as electric discharge. Firstly, we installed a grounding brush (made by Sohre Turbomachinery) on shaft No: 1 and we measured the potential to ground while unit was running and recorded that the current was flowing to the ground. So we believed that the gearbox was protected and expected no more failures. It was true and we had no failures after that. Then we started to explore the cause of current flow. I recall it was the magnetic clutch.

For the generator, before it becomes severe, check the integrity of NDE (exciter end) bearing insulation and all insulation on bolts, instruments etc. and grounding brush on DE side.

Hope it helps.... Regards
 
Hi Toral

Thanks for the response.

1) Its interesting to see that the upper half and lower half of the bearing (the outer region that fixes into bearing housing) has black spots. in fact its almost completely black (shaft 02 bearings). And the inner babbit which is having the pitting (upper half) has black spot at the end of pitting. It seems to be an electric discharge. As far as I know discharge occurs between the shaft and bearing and in between the thin film acts as the insulation.

The black spots or the region at the outer side of the bearing can also be due to electric discharge. if yes than this means there is a discharge between the bearing and the casing and in between the oil film acts as the insulation.

I am quite convinced that this is an electric discharge problem. I have asked to electric guys to give me the readings of IR and grounding bushes health. I am just concerned about the leakage current of magnetic clutch, how do we find that leakage current?

2) Secondly the currently produced from the generator flows throughout the shaft (pls correct me if I am wrong). It flows through the complete shaft i-e Generator and Turbine. And for this reason shaft at Brg 4 is grounded to give the current a least resistance path. My question is why do we need another ground at shaft 01 of gearbox (as you mentioned earlier). Won't this be dangerous for Bearing 01, 02, 03 of our Gas Turbine?

3) Does the current produced also flows through the flexible coupling of Turbine to gearbox?

4) If you summarize the question it comes out to be like, Why the bearing gets grounded if there is a grounding bush already present?

5) If current doesn't flows through the flexible coupling, then the grounding bush at shaft 01 of gearbox makes sense (doesn't it)? But where from will this current be produced?

I hope to hear from you soon. And as soon as I get all the readings, I will share it with you.

Thanks again for your valuable inputs.

Izhar
 
Hi Izhar,

If you are certain about the problem, you have two ways to go. The remedial work to prevent re-occurrence and then to investigate the root cause. First thing first is to do the remedial work and put back the gearbox into servive safely. Installing a grounding brush will provide an easy path for shaft currents to flow, as it will choose the easiest less resistive way to flow. I will try to answer your questions.

1) At the time we had similar problem, we were keeping the units ready to switch to back up liquid fuel automatically, as there was short of gas in Northern Turkey in these years. I recall magnetic clutch was all the time energized. I am not sure, you can check through its manual, there must be a way to understand coil leakage by simply doing a resistance test or by just carrying out physical wiring check or just de-energize it.

2) This is an only advice as a remedial work. If you install a grounding brush on the gearwheel No: 1 the currents will flow to the ground through this easy path supposing that the current source is from magnetic clutch. I suppose current is conducted through gears and collect to the bottom gearwheel, that the potential becomes too high to break the oil film resistance and flow through the ground through the bearing.

3) No. Flexible coupling is made of polymer and it is a good insulator.

4) Installing a carbon brush on DE side of the generator aims to remove and direct the stray currents on the generator rotor safely to the ground and protect the driven equipment from detrimental affects of these current. It is all about to confine these currents on the generator and provide safe flow to the ground. On some generators both DE and NDE bearings are insulated, but installing a grounding brush before bearing allows to skip this installation, I believe.

Hope this helps...
 
Hi Izhar,

I have just realized your question No: 3 is about accessory coupling between the turbine and accessory gearbox. I thought it was for flexible coupling engaged to hydraulic oil pump.

Accessory drive coupling, I assume, comprises of metallic flexible laminated membranes bolted to the coupling hub, as I recall, please correct me if I am wrong, bolts are surrounded with flexible elements.

Regards
 
Thanks a lot Toral. We have overhauled the gearbox. It's working fine. I have also contacted sohro turbomachinery for some guidance.

If the accessory coupling is insulated then that means there are two possibilities for the flow of current: first is the current from starting motor and second is any kind of magnetization in the gears of gearbox(by any form) or third which cannot be due to insulation in coupling is that the evaporative cooler at the compressor suction may produce some static current which can flow to the gearbox(but as the coupling is insulated so it simply cannot be).

I am on holidays so couldn't get hold of the readings from the electrical guys however as soon as I am back I will share all the readings with you.

Thanks

--
Izhar
 
Hello Toral

I have taken some of the readings. I am surprised. Please see the below response. I yet have to take the readings of the gearbox.

1) Insulation resistance value between the NDE bearing pedestal and the ground is 0.00 ohms. However both the sandwiched insulation plates under bearing pedestal shown satisfactory results (550 Mega ohms at 500 VDC). This shows that insulation pads are good however the bearing is still grounded even though pipes flanges are insulated. This result is for Bearing # 5 i-e at the Non Drive end of the Gas Turbine Generator (this bearing should be insulated). For your information the shaft is already grounded at Bearing 4 i-e the drive end of generator.

2) Voltage across the DE generator shaft and ground is 12 V (the shaft is grounded though). Is it enough to break the oil film.

3) The voltage across both the ends of generator shaft is 2.1 V.

4) Voltage across bearing support and ground is found to 42 mV (NDE which has to be insulated). This confirms the ground doesn’t it?

5) The value of current from Bearing support to ground of NDE is 1.2 A.

Kindly give your expert opinion on the results. We will also check the values of the gearbox as well as suggested by you and then will let you know.
 
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