Wheelspace Temperature difference between 1st Stage Fwd & 1st Stage Aft Temperatures

Recently we have performed HGPI on one of our Hitachi H-25 Gas Turbines. Following that HGPI, a glaring observation regarding the wheel space temperatures was observed. The difference between 1st stage Fwd & 1st stage Aft temperatures is more than 60 degrees Celcius. Before the HGPI this difference was somewhere between 25 to 30 degrees.

Load5.8 MW9.8 MW
1st Stage FWD
427​
437​
1st Stage AFT
358​
366​
2nd Stage FWD
396​
406​
2nd Stage AFT
386​
406​
3rd Stage FWD
393​
414​
3rd Stage AFT
247​
268​

Following parts are replaced during HGPI:
- 1st stage bucket with OEM guidelines
- 1st stage nozzle
- 1st stage shroud blocks
- Complete set of combustion parts (i.e. Fuel Nozzles, Combustion Liners, Transitions pieces & CrossFire Tubes)

Experts opinion is requested on the extent of this abnormality & the consequences of running machine with this increased temperature difference. Thank you!
 
Also, please note that the machine is operating at higher than normal vibrations. Vibration readings are as following:
- Bearing#01 (Exhaust Side - Hot End): 1X=110 micron; 1Y=114 micron
- Bearing#02 (Gearbox Side - Cold End): 2X=70 micron; 2Y=78 micron

Alarm limit = 115 micron
Trip Limit = 155 micron
 
Dear @glenmorangie, we have increased the load of machine to 20MW. For 20MW, please find in the attached table:

WheelSpace Temperature20MW
(After HGPI)
20MW
(Before HGPI)
1st Stage FWD#1446464
1st Stage FWD#2445463
1st Stage AFT#1406432
1st Stage AFT#2376426
2nd Stage FWD#1394442
2nd Stage FWD#2426454
2nd Stage AFT#1432460
2nd Stage AFT#2431460
3rd Stage FWD#1443472
3rd Stage FWD#2425468
3rd Stage AFT#1286371
3rd Stage AFT#2317378

Observation:
1. During the last 04 years, the average difference between 1st Stage FWD – 1st Stage AFT is 32C
2. After recent HGPI, the subject temperature is 54C
3. Temperatures of 1st Stage FWD are lower by 18C while the temperature of 1st Stage AFT are lower by 40C
4. Difference between 1st Stage AFT#1 & 1st Stage AFT#2 is 30C
 
Did you remove/replace any wheelspace TC during the overhaul? 1st. Stage Aft2 & 2nd.Stage Fwd1 don't look like they are reading so well, it is quite easy if you removed any TC to not get it back fully in the tube
 
Usually it can be caused by the position of the nozzles in the turbine causing excessive or reduced air flows. I don't know how you are sure that the TC "are fine" , it is fairly easy to check their positions first before re-opening the machine
 
Can you give Exhaust Temp. Spread (TTXSPL!) at 20MW before and after. What I am trying to get an answer to is the difference in Temp between 1st. Stage Aft 1 vs 2. before and after overhaulthis could be :-
  1. Badly installed TC
  2. Badly installed 1st. stg nozzle
  3. High exhaust Temp. Spread (Bad Combustion)
Do you understand ? Can you check ?
 
Abubakar_Rashid_96,

glenmorangie is more than likely correct. Improper re-insertion of the wheelspace T/Cs OFTEN causes these kinds of strange readings. The next time the machine is open and the turbine shell is removed and the turbine rotor is removed, have a look at the guide tubes for the wheelspace T/Cs--from the outside of the turbine shell all the way to the nozzle segments. The wheelspace T/C has to be inserted in just the right way--which is very difficult to do.

Go out to the area where the old nozzle segments are presently being stored and have a look at the wheelspace T/Cs have to enter the segment and the little tip area (often called a "nipple") where the wheelspace T/C tip is to be residing. If the wheelspace T/C doesn't go back to exactly the same place as before the outage it's pretty difficult to see how readings can be compared before and after.

It's also possible that the re-assembly was not done correctly, meaning that the nozzle segments have shifted and are not in their proper places. Nozzle segments have been known to shift slightly which can result in leaks of hot gases where they shouldn't be. And it could be that excessive cooling and sealing air flows could be incorrect (and those are difficult to measure in the internals of the turbine).

This question has been asked and answered MANY TIMES before on Control.com. Why? Because it's pretty difficult to get the wheelspace T/Cs re-installed exactly as they were before the outage.

You are correct--it would appear the T/Cs are all working. However, are they reading the correct temperature because the wheelspace T/C tip is in the proper position? Is it touching nozzle segment metal instead of the nipple? Is it fully inserted in the nipple in the nozzle segment? Some times the wheelspace guide tubes have discontinuities in them (meaning they have two or more sections with gaps between the sections). I have seen wheelspace T/Cs which didn't go from one section to the other, and were not even in the nozzle segment nipple.

So, unless you can be entirely certain the wheelspace T/C tips are in exactly the proper position--and the same position as before the HGPI--it's a very safe bet there is either: 1) something wrong with the re-insertion; and/or 2) something wrong with the nozzle segment positioning.

Please try to get the data that glenmorangie is asking for. And, please try to understand how difficult it is to insert or re-insert wheelspace T/Cs properly every time they are removed and replaced.
 
Gentlemen,
The reason I feel the wheel space TCs are performing fine is that there are other observations as well on the machine. The machine is operating at high vibrations as mentioned earlier as well. The actual spread values are also on the higher side. So, I feel there is some mess-up in the combustion section as well as during installation of 1st stage turbine parts (i.e. Nozzles, Shroud blocks & Buckets). Please find the attached data.
Date07/07/202125/08/2021
ConditionBefore HGPIAfter HGPI
LOAD (MW)20.520.5
BEARING #01 VIBRATION-X29111
BEARING #01 VIBRATION-Y50135
BEARING #02 VIBRATION-X5363
BEARING #02 VIBRATION-Y1887
ALLOWABLE SPREAD88.890
ACTUAL SPREAD#151.965
ACTUAL SPREAD#251.561.4
ACTUAL SPREAD#349.658.9
EXHAUST TC#1582577
EXHAUST TC#2580570
EXHAUST TC#3597593
EXHAUST TC#4562564
EXHAUST TC#5590594
EXHAUST TC#6588597
EXHAUST TC#7596591
EXHAUST TC#8611559
EXHAUST TC#9589557
EXHAUST TC#10574596
EXHAUST TC#11576583
EXHAUST TC#12600622
EXHAUST TC#13585579
EXHAUST TC#14582589
EXHAUST TC#15581625
EXHAUST TC#16571617
EXHAUST TC#17557605
EXHAUST TC#18559585
 
Hi

You better have to check shroud installation or hole dedicated for air exctration /infiltation...see if it is ok..

Can you confirm that all equipements have been reinstalled in correct manner..

Is there kind of compressor "bore plug" checked...

I do not have documentation to review that would show the source of cooling air for the stage wheelspace area concerned, you would need to refer to your documents to understand the source of cooling air for this region.
 
So, is someone on your side trying to analyze what happened during your overhaul? Who conducted the overhaul?
A very experienced Machinery Team performed the overhauling. Currently, we are analyzing the vibration signatures, the data is collected using ADRE. In parallel, field parameters are also monitored & the trends are being analyzed.

The deviation in behavior is observed in wheelspace temperatures. So, I shared on this forum to have an expert's opinion on the matter.
 
Abubakar_Rashid_96,

I wish to point out one more thing about wheelspaces--not wheelspace T/Cs, wheelspaces. The turbine wheel rotates at several thousand RPM. The axial wheelspace area is just a few centimeters, at best, on each side of the turbine wheel. That means the air in the wheelspace (which is what the wheelspace T/C is really attempting to measure!) is being mixed at the rate of several thousand RPM. This means the wheelspace air temperature is going to be pretty uniform (similar) from one side of the machine to the other (on both sides of the machine, which is what the -1 and -2 wheelspace T/Cs measure, respectively).

So, if the wheelspace T/Cs are properly inserted into the nipples of the nozzle segments and are measuring air temperature--and not touching metal (nozzle segment metal; etc.) the temperature in the wheelspace--anywhere in a wheelspace--should be approximately the same. And any differences can be attributed to the T/C and/or its insertion depth.

The differences between -1 and -2 wheelspace temps in any wheelspace before the HGPI reflect this very, very well. The differences after--not so well.

It is conceivable that if hot air or cooler cooling and sealing air were blowing directly on a particular nozzle segment that if the wheelspace T/Cs (the -1 and -2 T/Cs) in any particular wheelspace might show a difference. But, AGAIN, the turbine wheel is spinning at several thousand RPM and mixing the air in the wheelspace. AND, the chances of a leak being directly in the area of one of the two wheelspace T/Cs are somewhere between slim and none.

Wheelspace T/C insertion is an imprecise procedure at best. It is best done after the nozzle segments are installed in the machine and before the rotor is installed--but no one EVERY does it that way. Why? Because the exposed portion of the wheelspace T/Cs are subject to being walked on and having tools and parts dropped on them by (uncaring) craftspeople (many of whom have little or not training or even experience; and their only metric is how quickly the job can get done--nothing else).

Most sites I am asked to visit or comment on which are experiencing the same issues as you have raised (and yours is NO exception, sir!!!) DO NOT want to, nor will they, disassemble the unit to inspect the nozzle segments and/or wheelspace insertion/guide tubes. In fact, they RARELY even agree to remove and inspect the wheelspace T/Cs from the machine, replace any found damaged, and reinsert them (because, of course, THAT requires a shutdown and lost production and revenue--and usually the outage ran past schedule anyway... so the immediate desire is to stay up and running and not shut down for ANY reason).

So, I just tell them to monitor the wheelspace T/Cs--specifically for any sudden changes in value, and if the changes are increasing quickly or decreasing quickly or jump to a new value then that's an indication of some kind of issue. I don't know to what extent or for how long the operators "monitor" the temperatures, but we rarely hear back from these people. Probably because they expected more (they certainly demand it in some parts of the world!) but can't get satisfaction (without shutting down the machine--and they WON'T do that!), and they don't have any other option. Especially if the performance of the machine seems to have improved--as expected--after the maintenance outage. (How's the performance of the machine at your site which is having wheelspace T/C issues??? I'm speaking of electrical output and fuel flow-rate, now--not vibrations.)

There is one thing which can be checked which is relatively easy. MANY people just think that T/Cs, which have two wires, are not polarity sensitive. But, they are, One wire is deemed to be the "positive" wire and the other the "negative" wire and the two wires have different colours. And, like colours have to be connected to like colours. And, in different parts of the world the SAME T/C can have wires with DIFFERENT colours and yet still be "positive" and "negative." I have found in a few cases that ensuring the T/C wires are PROPERLY terminated (meaning "positive" to "positive" and "negative" to "negative"--regardless of colour!) fixes a lot of problems like this. For BOTH wheelspace T/Cs AND exhaust T/Cs (and bearing metal T/Cs; T/Cs in general).

No, as to the vibration. It's not likely, in my personal opinion, that the increase in vibrations you are reporting are related to the wheelspace temperature issues you are experiencing. I would expect that if axial position was incorrect that wheelspace temperatures would be more unusual, but not radial vibration.

And, the exhaust temperature spreads you are reporting are high--for a natural gas-fired unit, after a maintenance outage, where fuel flow-rates of fuel nozzles were used to determine placement of new/refurbished fuel nozzles AND the fuel nozzle flow range for ALL the nozzles in the set being installed was in a very narrow band (say less than 10%, and less than 5% for a really good set of fuel nozzles!). But, you didn't say (or I missed it) what fuel was being burned. And, you didn't say what the fuel nozzle set flow range was, either. Fuel nozzles with widely differing flow-ranges should NOT be installed next to each other, just as turbine buckets with high weight differences should not be installed next to each other.

It seems as if not enough care might have been used in reassembly of the unit. And, that happens--it usually doesn't happen on the next outage in the series, and it usually doesn't happen if the same service/parts provider is used on the next outage. But, people change jobs, they retire, and Sourcing "organizations" are ALWAYS looking for the least expensive service and parts provider, and things like this happen.

But, understanding how the machine is designed and works, and how field devices (such as T/Cs) work, helps to ensure a better quality of outcome with good supervision as well as understanding how things like you are reporting can occur--AND what can (or can't) be done about it after the fact.

Hope this helps! I wish we could be of more help--but there's a LOT of information we don't have and you can't or wouldn't provide. We have offered what, in our experience and knowledge, are the common reasons for what might be happening based on the information provided. This is a free World Wide Web forum--those of us who respond get paid exactly the same amount as you paid to post your situation and question(s): nothing. If you want better help and information and direction, you need to contract with a firm who will ask for much more information in order to make a full analysis and recommendations. BUT, be prepared to shut the machine down to implement any fixes/resolutions. I would venture the most you can hope for would be a very expensive lesson about how to write the proposal for the next outage and how to supervise the next outage better for a better outcome.
 
Gentlemen,
The reason I feel the wheel space TCs are performing fine is that there are other observations as well on the machine. The machine is operating at high vibrations as mentioned earlier as well. The actual spread values are also on the higher side. So, I feel there is some mess-up in the combustion section as well as during installation of 1st stage turbine parts (i.e. Nozzles, Shroud blocks & Buckets). Please find the attached data.
Date07/07/202125/08/2021
ConditionBefore HGPIAfter HGPI
LOAD (MW)20.520.5
BEARING #01 VIBRATION-X29111
BEARING #01 VIBRATION-Y50135
BEARING #02 VIBRATION-X5363
BEARING #02 VIBRATION-Y1887
ALLOWABLE SPREAD88.890
ACTUAL SPREAD#151.965
ACTUAL SPREAD#251.561.4
ACTUAL SPREAD#349.658.9
EXHAUST TC#1582577
EXHAUST TC#2580570
EXHAUST TC#3597593
EXHAUST TC#4562564
EXHAUST TC#5590594
EXHAUST TC#6588597
EXHAUST TC#7596591
EXHAUST TC#8611559
EXHAUST TC#9589557
EXHAUST TC#10574596
EXHAUST TC#11576583
EXHAUST TC#12600622
EXHAUST TC#13585579
EXHAUST TC#14582589
EXHAUST TC#15581625
EXHAUST TC#16571617
EXHAUST TC#17557605
EXHAUST TC#18559585
You may right about Nozzles diaphragms shrouds /buckets cleareances may be not ok ...that s to be checked ..as CSA said it would need an outage..overhaul /borecscope inspection at least ..
something definetely missed during HGPI

James
 
Dear @CSA,

First of all, I am very grateful for your time & quite candid & valuable feedback.

I think you are very much right on the high probability of common mistakes during the handling & installation of the wheelspace thermocouples.

Regarding the fuel side, the machine operates on Natural Gas. As far as the installation of fuel nozzles are concerned, they are installed as per the Flow Balance Chart (after testing in the lab) shared by OEM with the fuel nozzles set.

The thermal efficiency of the machine is slightly improved (by a margin of 0.7%).

On the vibrations side, we have engaged the manufacturer's (Hitachi) diagnostics team as well as another team of experts. We are very close to reach the root cause & may also go for dual-plane field balancing. I'll make sure to keep you guys posted about any activity performed during this.

Thank you!
 
Dear @CSA,

First of all, I am very grateful for your time & quite candid & valuable feedback.

I think you are very much right on the high probability of common mistakes during the handling & installation of the wheelspace thermocouples.

Regarding the fuel side, the machine operates on Natural Gas. As far as the installation of fuel nozzles are concerned, they are installed as per the Flow Balance Chart (after testing in the lab) shared by OEM with the fuel nozzles set.

The thermal efficiency of the machine is slightly improved (by a margin of 0.7%).

On the vibrations side, we have engaged the manufacturer's (Hitachi) diagnostics team as well as another team of experts. We are very close to reach the root cause & may also go for dual-plane field balancing. I'll make sure to keep you guys posted about any activity performed during this.

Thank you!
Okay noted

Cheers
James
 
" The reason I feel the wheel space TCs are performing fine is that there are other observations as well on the machine "
" I think you are very much right on the high probability of common mistakes during the handling & installation of the wheelspace thermocouples. "


It took some time but now maybe you will believe that Wheelspace TC are a pain to install/replace. One of the biggest benefits from this forum is passing over our experiences and helping people to understand and correct problems.
 
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