Wheelspace temperature reading now is abnormally high for 2 of the thermocouples at location 2nd stage aft, and 3rd stage forward. Others are normal.
Unit is GE Frame 6 single shaft coupled to Alsthom Generator for power generation.

Experts, please share possibilities and share experiences.

 

Sounds like a cooling air problem. More information would help. Was the problem sudden or occur over time?

Since more than one TC is involved, I believe the problem is real but check the TCs anyway. How many hours and starts does the turbine have? When was the last hot gas path inspection? I apoligize for attempting to answer a question with more questions.

 

CTTech, quit 'cher apologizing. This is the second post you've felt it necessary to make an apology in. One thing you should have learned from your experience here is that people provide little to no information about their condition, and we have to ask for more. Sometimes we get it, sometimes we don't. But, you shouldn't be apologizing for a lack of information provided with a request for help.

One thing I wish people who post here would learn from reading other posts is that the more information they provide the better and more timely their response is going to be. I also wish people would tell us what they've done to troubleshoot the problem. I know from years of field experience, people tell you about this problem or that one and wait for a response without providing any details about what they've done, and you ask about checking this or that and they always respond, haughtily, "We've already done that!" I think they're looking for validation of their troubleshooting efforts (prowess), and then get offended with the simplicity of the response or that they've already done the obvious things and it should be obvious they've already done the obvious things. All I know is, it's very frustrating, but you shouldn't be apologizing for a lack of information or clarity.

*Most* GE-design heavy duty gas turbines have two (2) thermocouples per wheelspace, a TTWS2A1 and TTWS2A2, for the Second Stage Aft left- and right T/Cs, respectively. You haven't told us how high the temperature is, nor what the alarm setpoint is. You haven't told us if both the T/Cs in the wheelspace are reading high, or if one T/C in each wheelspace is reading high. You also haven't told us if this just began occurring while the unit was running, or, as is usually the case, after a major maintenance outage in which the T/Cs were removed then re-inserted, so we can't make an appropriate comment because we don't have enough information.

As has been said *many* times before, an instantaneous reading is only slightly indicative of the problem; sure it might be higher than an alarm setpoint, but, has it been trending high (or low) for some period of time, how quickly has it been increasing (or decreasing, as the case may be), and what about the other parameters which are or might be related. Troubleshooting is as much about *trends* and the rate of change as it is about instaneous problems. We have no indication about how the readings were trending over time before they exceeded the alarm setpoint.

Wheelspace T/Cs measure the temperature of the area near the axis of the shaft, which should be "protected" from the combustion gases passing through the turbine sections by seals, and cooled/pressurized by extraction air from the axial compressor discharge pressure. If *both* T/Cs in a wheelspace read similarly and high, it's a safe bet that there's some kind of issue. If the wheelspace T/Cs are properly installed, it's virtually impossible for one T/C in a wheelspace to read significantly higher (or lower), unless there is air or hot gases blowing directly on the T/C or metal surrounding the thermowell, which would also indicate a problem. But, generally, if one T/C of a pair in a particular wheelspace is indicating significantly higher or lower than the other in the same space, it's an installation problem.

However, since the unit has "high temperatures" in two related spaces (2nd stage aft and 3rd stage forward) it's possible there's a physical problem.

Or, it might be a control problem; more than one reading has been affected by damaged T/C leads.

But, we don't have enough information to say anything for sure.

 

Hi,

This problem occurs after Major Inspection. Any chance for installation error? Anyone heard about 2nd stage nozzle wheelspace thermowell misalign?

Thanks.

 

You *still* haven't answered *all* of the questions. Do *both* of the T/Cs in *each* wheelspace indicate a high temperature or just one in each wheelspace? How high or low are the readings in each space?

It's pretty clear that the unit is running and producing power even though there's some issue with the wheelspace T/Cs. And, it's pretty clear that the unit will continue to run and produce power because you haven't provided all the answers to the questions indicating that the initial fears have subsided. And, the unit is probably not going to be shut down to try to fix the problem.

You should continue to monitor the temperatures, trending them to see if they increase or decrease over time. Again, that's the most important factor, not the instantaneous value of any parameter but the value over time and how fast the value is changing that will really tell you if there's a problem and how bad the problem is.

The real problem is that most people don't look at trends; they wait until the instantaneous value exceeds the alarm (or trip) setpoint and then they get concerned. It's not until they start an investigation into the problem (if they have historical data) that they discover the value has steadily been changing over time. And rather than learn from that experience and begin to monitor all parameters to detect trends, they wait until the next "emergency" to start an investigation to find that the parameter was changing steadily over time and went undetected until it exceeded an alarm/trip setpoint.

There is beginning to be some condition monitoring software make its way onto the market (very expensive) that does nothing more than trend parameters and alarm on change. This software needs to be manually tweaked to prevent a lot of nuisance alarms which are annunciated until the adjustments are done. But it's encouraging to see that the power of computers is being used for more than just monitoring values to see if they exceed a setpoint. However, there will need to be some manual adjustments done to get the systems configured to plant operating characteristics, and because that will take time most operators will just continue to ignore the alarms. Which is why the software is necessary to begin with, because operators don't pay attention to trends and wait until a parameter goes into alarm before taking action (which is usually just to call a technician!).

 

Sorry for the lack of info.

TTWS2A01 & TTWS2A02 approximately 550DegC
TTWS3A01=530DegC TTWS3A02=450DegC
The others are around 420DegC.

This happens after inspection, and we did replace the 2nd stage nozzle (refurbished not new). Are there any specs (dimensions) for the thermocouple guide tube? If the the thermocouple is not fully inserted, can the temp reading differ by 200DegC (compared to normal)?

I already checked the loop by injecting temp/mv from JB and observed reading at Mark5.

Thank you.

 

A few answers and questions.

Yes, wheelspace thermocouples can read high particularly if they are not fully inserted as they can touch the tube and be reading metal temperatures rather than wheelspace.

We are running the same machines as you and the control spec. alarm values for 3FO=480 Deg C and 3AO= 430 Deg C, I think you have a problem if these are real temperatures. I guess the 2nd. Stg. Nozzle was installed OK and all the clearances were OK? Thermocouple tubes should all be OK, Look in your parts manual, there are some nice drawings of them there.

 

Can you say what the wheelspace temperatures were before the maintenance outage?

It is *very* common for wheelspace T/Cs to not be properly and/or fully inserted into the guide tubes which direct the T/C tips into the thermowell tips of the nozzle assemblies after a maintenance outage. It's one of the most common problems after a maintenance outage, after leaking flanges and/or fittings and improperly re-connected wiring. One of the things which can happen is that the stainless steel sheath can be bent in one or more places or the locating SwageLok ferrule not be placed in exactly the proper position to allow the T/C tip to be properly positioned in the thermowell.

On a Frame 5 or 6, the turbine wheel/shaft spins at approximately 5100 RPM, which means the gases in the wheelspace are being mixed by the rotating turbine wheels very quickly. That's why it's virtually impossible for there to be a high differential between the two readings in the same wheelspace *if the T/Cs are properly inserted*. If the T/C tips are touching metal they shouldn't be touching or they are both not properly inserted in the thermowell, well, then the readings will be different.

Thermocouple guide tubes are usually just lengths of stainless steel tubing, approximately 0.3125 to 0.375 inches in diameter. They usually have bends, and sometimes "a" guide tube is made up of more than one length of tubing, and it's possible for the T/C to miss the second tube when being inserted. You should be able to look at the drawings in your Parts Manual to see the tubes and their location. The tubes just direct the T/C into the thermowell, and it's also possible for the T/C to completely miss the opening to the thermowell. I'll bet that if you talk to the instrument tech who installed the T/Cs, that person probably had a difficult time inserting the T/C all the way to the locating ferrule.

YES! The differential can be very high if the T/C is not properly inserted.

Unfortunately the only way to know if the T/C is inserted properly is to disassemble the machine, which is unpractical. Again, *trend* the readings; if they're fairly stable over time then it's probably just an alignment/positioning problem. If they are increasing or decreasing with time, then it's most likely a seal or nozzle/diaphram positioning problem, in other words a mechanical problem.