Dear all,
We have Frame 6 30 MW Gas turbine. It has only one mist eliminator which got tripped in overload. After 88QV was restored, we found that the hydraulic oil filter DP has increased by 1 kg/cm2 (0.9 to 1.9) and pinion side temperature in the reduction gear increased by 30 degree C. (76 to 106). For some time this temperature was in the range of 130 deg. C. But externally everything looked fine.
This incident happened on 12th Dec 2021 and on 31st night QV tripped again and again the hydraulic filter DP rose by 1 kg/cm2 but the pinion side RGB temperature was around 106 deg. C. However, after 8 hours of operation vibration in Reduction gear (all three probes) increased and the machine tripped on high vibration.
On inspection it was found that one teeth of the pinion gear was damaged.
So first, what could be the relation between hydraulic filter DP and Mist eliminator. We don't have means to check the main lube oil filter DP. Should we inspect the main lube oil filter too?
What could be the reason for the damage witnessed in the RGB? Poor alignment or something else with the Lube oil circuit?

 

A correction I need to make is that we do have Main lube oi filter DP gauge which read 0.46 kg/cm2. before and after 88QV tripping incidents.

 

Remember Hydraulic oil comes from Lube Oil, I would guess that no mist eliminator caused some kind of contamination which passed to hydraulic oil from L.O.
Load Gear damage could certainly be caused by increased temp. causing decreased lubrication and increasing Temp. (especially at 130 Deg.) Oil contamination from L.O. could cause lower flow through coolers than normal.
A lot of information is missing, L.O. and Hyd. Oil analysis. This pretty well all guessing but I wouldn.t run this machine again until a bigger investigation is made.

 

@ glenmorangie Sir the machine is shut down and investigation is under progress. The inspection window from the RGB was opened and one teeth was seen damaged and the shaft was rotated with manual ratcheting to check any other damage.
The RGB cover is to be opened in near future and Lube oil sample analysis is awaited.
How could tripping of mist eliminator cause contamination in lube oil? Would the higher tank pressure cause contamination?

 

Evaporated Oil which normally would be withdrawn could re-condense and change oil consistency, extra over pressure in tank could change oil flows. Were there any new oil leaks particularly around Accessory G/box seals? For me it is the very high RGB Temp. that is most concerning and the the probable cause of failure.
Loss of oil mist eliminator has caused me many strange things in the past. The old system of a large atmospheric vent was dirty but effective to relieve pressure

 

"... After 88QV was restored ..." What was found to be the problem with 88QV? Was any measurement taken of the current being drawn by 88QV? Is there any record of "normal" current drawn by 88QV to compare against?

Were the internal filters of the L.O. Mist Eliminator inspected? (I presume not because I presume the unit was running while the "troubleshooting" of the 88QV motor was underway.) Are the L.O. Mist Eliminator filters going to be inspected now that the unit is shut down?

What is the condition of the loop seal from the L.O. Mist Eliminator back to the L.O. Tank?

glenmorangie is correct: The hydraulic fluid is the same as the lube oil. BUT, there is no hydraulic fluid flowing to the RGB (Reduction Gear Box)--at least that I've ever seen on a GE-design Frame 6B heavy duty gas turbine driving a generator. An examination of the L.O. System and Hydraulic System P&IDs will, I believe, show that cooled and filtered L.O. is fed to the suction of the Main- and Auxiliariy Hydraulic Oil Pumps which increase the pressure of the fluid to that of the Hydraulic System. But, again, I'm not aware of any Hydraulic oil flowing to the RGB.

It's really difficult to make any direct correlation between L.O. Mist Eliminator operation and Hydraulic filter differential pressure.

I'm also confused by the temperatures listed for the RGB pinion gear temperature. When was it 130 deg C? And for how long was it 130 deg C? What's it's normal temperature: 76 deg C or 106 deg C? Journal bearing material (babbitt) usually begins to get soft/melt at around 300 Deg F (which is about 150 deg C or so) (if I recall correctly).

If the L.O. Mist Eliminator is the typical one where a blower, driven by 88QV, draws air/vapours from the L.O. Tank over a group of filter elements (which must be periodically replaced) and exhausts to the atmosphere an increase in current--which cause the TOL (Thermal OverLoad) relay of the motor's starter to actuate--would most likely be cause by plugged ("choked") L.O. Mist Eliminator filter elements. I have seen where the loop seal between the bottom of the mist eliminator and the L.O. Tank was not flowing properly and liquid oil began to fill the bottom of the L.O. Mist Eliminator causing an increase in suction pressure which caused the 88QV TOL to actuate.

There is usually a manual butterfly valve at the discharge of the 88QV motor-driven blower which is used to control the amount of vacuum on the L.O. Tank when the unit is at Base Load. Sometimes, that valve can be mis-adjusted, but it usually doesn't result in the TOL actuating; just incorrect L.O. Tank pressure/vaccum and/or a visible plume of oil vapour exiting from the L.O. Mist Eliminator.

Again, an examination of the Hydraulic System P&ID is probably going to show that it's hard to see how a non-working L.O. Mist Eliminator can result in an increase is the Hydraulic System Filter differential pressure.

As for what caused the RGB gear tooth to break--how old is the RGB? When was the last time the gear teeth back-lash was measured and recorded? What was the gear tooth backlash when it was measured and recorded--and what is the allowable limit of backlash for that gearbox?

The intent of the questions IS NOT to cause frustration--it's simply to offer ideas for possible causes of the problem(s). Think of them as 'food for thought'--though we would like to know the answers to the questions, we quite often don't get them. But, if they are helpful, that's what matters (even if we never hear back). It's not always either/or for problems. Sometimes, it's many possible things which can cause the same or very similar problems. Troubleshooting is often a process of elimination when there is more than one or two possible causes for a problem. But, relating 88QV overload and Hydraulic Filter differential pressure is not really possible--or relating either of them to a RGB gear tooth failure.

And, sometimes, age is just a factor. If the site experiences frequency and wild grid frequency fluctuations this can also cause excessive forces on RGB gear teeth. And, if this has been on-going for many years, well, things break. So, there's a lot of things we just don't know (including the condition of the L.O. Mist Eliminator filter elements).

Please write back to let us know what you find!

 

@CSA sir, Your presumption is right that mist eliminator filters were not inspected as the machine was running.
Now we will inspect the filters and loop seal as well.
The line from loop seal to the main oil tank was punctured a month ago and vacuum had reduced a lot but the punctured point was temporarily fixed. The line is now replaced (during this shut down).
The vacuum break valve's inner rubber seal was damaged and it has been now replaced as well.

The detailed inspection is to be done but the maintenance services group has not arrived yet at site.

The normal temperature of the RGB is less than 100 deg C and the alarm value is 120 deg C. It was at 130 deg C for around 15 minutes and then it reduced on its own.
We should have inspected the RGB then but as we are captive power plant, we didn't get the time to shut down the machine due to power constraints.
I think the RGB was checked last in 2016-17. I don't have the record of the gear teeth backlash. I will try to find it.

On checking P&ID I don't find any direct link between the mist eliminator and RGB damage but what I wanted to conclude is whether the tripping of mist eliminator caused something that led to rise in RGB side temperature or the the rise in RGB side temperature created conditions which led to the mist eliminator overload.

 

Mist Eliminator tripping can cause a pressure in the L.O. tank and maybe a lack of oil flow to RGB, could be cause of RGB damage by overtemp, 130 Deg. is way to high

 

glenmorangie, I can possibly envision a situation where the oil flow OUT of the RGB back to the L.O. tank could be impacted (to a small degree) by a higher than normal pressure in the L.O. tank. BUT, I can't understand why that would affect the oil flow INTO the RGB from the L.O. Bearing Header (the regulated pressure of the Main-/Aux. L.O. Pump discharge). The L.O. Bearing Header Pressure Regulator is going to regulate pressure (and, hence, flow) to the header--which feeds the RGB as well as the turbine and generator bearings. Why would low flow TO bearings only occur in the RGB?


The L.O. Mist Eliminator doesn't control L.O. Bearing Header temperature OR Bearing temperature. Bearing temperature is a function of L.O. flow AND load. L.O. Bearing Header temperature is controlled by VTR1-1, the L.O. Bearing Header Temperature Regulating Valve. And we haven't had any indication that VTR1-1 is not working properly--because if it weren't it would impact ALL the bearing temperatures, not just the pinion gear bearing in the RGB.

Based on the information provided, I think there is no correlation between 88QV tripping and the RGB tooth failure. But, that's just my opinion--and I have been wrong in the past, and I will be wrong in the future, too.

 

Hello I am new on this forum. We operate 2 Shaft Genset Frame 3 in Congo and we have issue on bearing n°3 ( Oil Leak). We did an overhaul but the problem is still present. Is someone know the Normal Lube Oil Tank Pressure ? we have 15 mm H2O. -> 1,5 mbarg

Regards

 

you don't have a P&ID (Flow diagram) or a Control Specification . Is your tank vented to atmosphere? I don't think that LO Tank pressure is causing your bearing leakage problem