Dear CSA and other experts

Our issue is in one of 6 GE frame 7EA units, after recovery from generator failure back in 2013, GE had installed a new re-designed 7A6 air cooled generator without assistance vacuum blower (known as BQ). Things were running smooth until a few years ago when we started noticing oil accumulation under and beside the generator and generator bearings. I believed that we managed to resolve the issue temporarily while the root cause is still unresolved.

Lately when the same issue raised again we did the following:

• Replacing mist eliminator filters. Assembly and filters R K Case made.
• Fan assembly (fan +motor) was replaced, and rotation was checked.
• Check valve position and condition in the mist eliminator was checked.
• Bearing -2 outer pipe and breather pipe checked and reinstalled properly.
• Generator bearing oil and floating seals checked and some were replaced.
• All oil tank cover gaskets were replaced and secularly tightened.
• Oil level in the junction box was checked and confirmed that it felt below the vacuum pipe.
• The oil drainpipe from the demister was also checked.
• Generator filets were replaced.
• Bearing-5 vacuum gauge was calibrated.

Here is the situation now:

With the butterfly is fully opened, the readings of the generator end bearing (bearing-5) vacuum pressure are as the following:

At 0 speed (on ratchet) = -10 in.H2o

At FSNL = -4.5 in.H2o

At 30 MW = from -4.5 down to -1 in.H2o

At Base Load = Not yet tried.

As long as we keep the unit running on 30 MW, the vacuum pressure will keep dropping until it becomes almost ZERO and whence the unit goes down to FSNL the vacuum increases immediately to -4.5 in.H2o !

We believe that the oil leakage from the oil seals is stopped but we are unable to increase the load since the generator base and load compartment still splashing the remaining oil in the stator and we may need 4 to 5 starts until the oil goes away. Moreover, we are almost certain that the vacuum will drop to zero or +ve pressure when we raise the unit to Base Load and the oil leakage may show up again.

My questions are:

I am thinking of cooling and sealing air as one source of that problem but other than that

what are the possible causes that contribute in lowering the vacuum as load increases?

Is the vacuum at 0 speed is just fine or it should be higher than -10 in.H2o?

Is that drop in pressure from -10 to -4.5 in.H2o (0 to FSNL) normal?

Thank you ,,

Ibrahim

 

Hello Ibrahim,

A P&ID /schematic /drawing of the sealing/cooling system & lube oil /mist eliminator would be welcome here before to guide you on any path....

James

 

I will give you my experience on the 7EA .

The general idea is to provide sealing and cooling air for the bearing seals to prevent leaks. The bearing seal air valve has source air from either the 5th stage of the compressor, or the 11th stage. When the unit is started the sealing air valve is in a position to extract air from the 11 stage of the compressor, because the pressure from the 5th stage is very low. The bearing seal air valve actuator is fed air from the 5th stage, such that as the unit is loaded and 5th stage pressure rises to approximately 5 psi, the sealing air valve will shift and begin supplying air to the bearings from the 5th stage. This pressure will rise as the unit is loaded. The valve will stay in the position supplying bearing seal air from the 5th stage until the unit is unloaded and 5th stage pressure begins to fall below 5psi.

There is typically always a switch to indicate the position of the bearing seal air valve. It is there to generate an alarm if the valve is not in the correct or intended position. If the valve is stuck in the position of the 5th stage then bearing seal air pressure will be too low during startup or low load. If the valve is stuck in the 11th stage position then bearing seal air pressure and temperature with be too high at higher load conditions.

I have noted that the alarm that signals the valve out of position, comes in at times as a nuisance. The logic for valve out of position alarm is driven by the digital input of valve position, and logic that looks at CDP pressure. Since 5th stage pressure and CDP pressure can vary slightly based on ambient conditions and compressor efficiency the logic values sometimes indicate the valve out of position, even when it is correct.

So the question is did you monitored Bearing Seal air pressure?

Thx for clarifying,

I hope this is helpful, if you have a more direct question or problem you are experiencing please provide further informatio

 

ControlsGuy25

Thank you for writing back and i have to mention this :

• In our units and as per design , the cooling and sealing air is to cool and seal bearing -1,2,3 only and extracted directly from stage-5 and there is no such sealing air valve exist . Stage 11 is designed only for compressor air bleed purpose .
• Generator bearings are utilizing vacuum pressure -extracted from main mist eliminator system via vacuum pipe connected to surge oil tank in load compartment - for oil sealing purposes .

Thank you

Ibrahim

 

Ibrahim

Thanks for the inputs!

But we still do not have a diagram/schematic or P&ID to try to support as much as we can from here..

I know that one customer changed the Oil mist eliminator vessel for a non OEM one after have kind of case that you mentionned ...but each site problem can varying from on eto another

So without adding more gatherings datas like drawings I can not state on that case ...

 

Looks like sealing air is exctracted by Generator "rotor fan" did you check if that operating properly..
Is there any works have been done on the generator these days..

 

Again without be able to see the Unit P&ID it is difficult to state on..

 

Looks like sealing air is exctracted by Generator "rotor fan" did you check if that operating properly..
Is there any works have been done on the generator these days..

This is true in older frame 7E which the sealing air is extracted from Generator rotor fans ( very reliable design) BUT in this unit and other 5 frame 7EA units the vacuum is acting as a sealing air extracted as from main mist eliminator system via vacuum pipe connected to surge oil tank in load compartment.

Hope that was clear enough . I don’t have P&ID drawings right now and I will provide them at earliest.

Thank you ,,

 

Okay

sorry i did not notice that you already mentionned it on th eprevious post ...
As i am tryin to support various posters here..

Will be on the look out for these PIDS then we can have a better discussion/overview

 

Thank you

 

iabose,

I have seen this problem at a couple of older sites. The pipe between the generator and the L.O. reservoir/surge tank is not properly pitched (angled) to allow oil to drain to the reservoir/surge tank. This creates a build-up of oil residues in the pipe which restricts air flow. (Remember, most mechanics and maintenance technicians think all pipes are ladder rungs or hand-holds--so the pipes may have started service straight an possibly properly pitched (if someone paid attention to the Notes section on the Piping Arrangement Drawing or the P&ID), but eventually, they get bent and develop low points.

Both the sites where I saw this problem also had issues with the L.O. Mist Eliminator not working properly (the filters in one of them were completely caked (choked) with oil residue; a couple of the filter elements in the other had ruptured and all of the others were completely caked (choked)). Most sites don't inspect ot replace Mist Eliminator filter elements on a regular basis, and only when there's "a problem" and that's when other issues (such as caked (choked)) filters are discovered.

The interconnecting pipes at both sites were so badly clogged--and bent--they were just replaced. One site used an endoscope to look in one of the pipes; the endoscope was 1.5 meters long and only about 1/3 of the length could be inserted into the pipe because the blockage was so bad. We banged on the pipe with small hand-sledges and big wrenches and turned the pipe on end, and the amount of hard residue which came out was pretty difficult to believe.

This isn't the best system. There are companies that make small L.O. Mist Eliminators (they require 3-phase AC power for the motor) which can be mounted on the roof of the generator enclosure or the generator collector compartment enclosure. These are the best; they should be running any time the Aux. L.O. Pump is running or any time the unit is above zero speed (just like the large L.O. Mist Eliminator). They also have replaceable filter elements....

Hope this helps!