Metal Bearing and High Drain of Turbine Journal Bearing

R

Thread Starter

rvs_gasturbine

We have 3, frame 6 gas turbine. In one of the gas turbine, a constant alarm persist i.e of lube oil drain differential temperature high (LTBRGHH_ALM).

On further analysis it was found that drain of turbine journal bearing -2 is on higher side (>22 than lube oil header temperature). Apart from this the metal bearing temperature probes of turbine journal bearing -2 is out.

seeing the above, kindly explain the gravity of the situation as these are for monitoring purposes only.
 
rvs_gasturbine,

Your post is unclear; specifically this sentence:

> Apart from this the metal bearing temperature probes of turbine
> journal bearing -2 is out.

What are you trying to say about the #2 bearing metal temperature values? Are they "normal"? Are they abnormal? If they are abnormal, how much higher or lower are they than normal?

The #2 bearing is located in the Load Tunnel of the gas turbine, which is surrounded by the exhaust area (diffuser). So, typically the temperatures in this area are a few deg C higher than, say, the #1 bearing housing (both bearing metal temperature and bearing drain temperature). And, the L.O. feed and drain to the #2 bearing passes through the Load Tunnel to the #2 bearing housing. And, typically, the #2 bearing drain T/C is located in the L.O. feed/drain pipe in the Load Compartment just outside the Load Tunnel. So, the L.O. drain temperature reading is probably 1 deg C or may two deg C higher than the actual drain temperature right at the discharge of the #2 bearing itself.

When the gas turbine is running bearing drain temperature is almost always higher than bearing metal temperature, because the oil is compressed under the load of the bearing in the oil wedge which raises the temperature of the oil, and because it absorbs some heat from the bearing (journal and housing), also.

Again, in the Load Tunnel the feed/drain piping is exposed to the high heat of the surroundings and will likely pick up another degree or two of temperature, but it's really not critical--or it should not be. If there is a leak of gas turbine exhaust which is impinging on the feed/drain piping that could explain an even higher bearing drain temperature reading which is NOT reflective of the actual bearing drain temperature because of the heat being picked up from the leak.

If your operators have been recording bearing metal temperatures and bearing drain temperatures periodically during operation (in years past these were called "rounds"--manually recording parameters on a log sheet) it should be possible to see when the L.O. drain temperature started increasing and how quickly it's been increasing. The key when these kinds of alarms come up is to know how fast the temperature is increasing (or decreasing, as the case may be) and what the normal temperature range is. That determines the severity of the situation--how fast the temperature is changing, and how much it deviates from the normal.

> seeing the above, kindly explain the gravity of the
> situation as these are for monitoring purposes only.

So, the gravity of the situation is a product of the rate of change and the magnitude of the change, as explained above. BUT, there is this false belief in today's world with digital turbine control systems and HMIs that the turbine control system (the "automation") will protect the turbine and auxiliaries and that operators don't have to do anything but push START and STOP and silence alarms and change the PRE-SELECT LOAD setpoint. This is simply not true. What is true is that the purchaser of this turbine and control system is that the option to trip the turbine based on bearing metal and bearing drain temperature wasn't purchased--so that means the operators, and their supervisors, are responsible for taking appropriate action based on the indications.

I've tried to explain how the #2 bearing drain temperature is sensed, and what might cause an erroneous reading of the #2 bearing drain temperature. What's not known is how long this problem has existed, or how fast the #2 bearing drain temperature has been increasing, nor how much it differs from the normal operating range of the #2 bearing drain temperature. (This is where data, either electronic (from some kind of historical or archived data system, or from manually recorded/logged temperature readings is important and necessary.)

There's one more thing about MOST of the temperature sensors used for the #2 bearing drain temperature: Most of them are dual element sensors, meaning that for a thermocouple there would be four leads coming out of the sensor--two for each element. Usually, only one element is connected to the turbine control system, so by moving the interconnecting wires to the unused element it could be that the original one is failing, or not. But, it's certainly worth a try.

Hope this helps! We need more information to be of more help.
 
R

rvs_gasturbine

Dear CSA

>> Apart from this the metal bearing temperature probes of
>> turbine journal bearing -2 is out.

ans :-the temperature probes of turbine journal bearing -2 are showing -17 which implies they are out.

> how fast the temperature is increasing (or decreasing, as the case
> may be) and what the normal temperature range is. That determines
> the severity of the situation--how fast the temperature is
> changing, and how much it deviates from the normal.

ans-:- the drain temperature ranges from 94 to 114 deg Celsius. the normal drain temperature being 74 to 79 degree Celsius. this change was observed when machine tripped on release of CO2,last year.
 
rvs_gasturbine,

So, both of the #2 bearing T/Cs are not working correctly. This is very odd. As with the bearing drain T/C, typically the T/Cs embedded in bearings are also dual element T/Cs. Have you tried reconnecting the interconnecting wires to the spare element of the two bearing T/Cs?

When did the bearing T/Cs go bad? After a maintenance outage? Did they both fail (go low) at the same time? GE heavy duty gas turbine control systems typically go low/negative when the T/C circuit is open? Have the circuits been checked to see if there is a wiring problem and that the T/Cs are properly connected to the control system?

It's not unusual for a GE-design heavy duty gas turbine to experience changes of parameters/readings after a trip from load. And if the unit experiences multiple trips, the changes can be worsened.

The bearing material ("babbit") used in most GE-design heavy duty gas turbine shaft bearings (journal bearings; tilting-pad bearings) begins to melt at approximately 300 deg F, which is approximately 150 deg F. If the oil temperature of the bearing drain temperature is approaching 150 deg F then it's very likely the actual bearing metal temperature is greater than that--which you can verify by looking at another journal bearing with working temperature and drain sensors. So, the higher the bearing drain temperature the higher the bearing metal temperature. I can't estimate how much the difference is, or when the bearing drain temperature indicates a very high bearing metal temperature. But, with the temperatures given in this thread, I would begin to be concerned.

Lastly, unless the CO2 directly sprayed on the T/Cs in question, I doubt very seriously that it had any effect on the T/C readings. Rather, I would suggest that the trip caused some shift of bearing housing, and that if the CO2 sprayed directly on the bearing housing that it could have contributed to the problem some misalignment. Usually, misalignment can be observed along with a simultaneous rise in vibrations. Have the vibrations for the #2 bearing increased in recent months/weeks?
 
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