GE-9E wheelspace temperature Abnormal

Dear all,
We are operating two GE-9E machines with dual fuel combustion facility(non-DLN burner; standard burner).These machines have been operated for seven years.Schedule maintenance(CI,HGPI) also carried out regularly.
One of our unit recently experiencing high differential temperature alarm for third stage forward outer wheel space sensors(TTWS3FO1-TTWS3FO2>>50~120 deg C).Half months ago Offline water wash was carried for this unit. After 14 days of operation TTWS3FO1 showing deviated value from TTWS3FO2 and generate differential alarm HI. Since all other parameters were found good we shutdown the unit and replaced the TTWS3FO1 with new thermocouple and restart.During loading the the difference well within the range but after 70% load the differential alarm high again appeared.This time TTWS3FO1 value was more higher than previous value(545~580 degC) and interesting thing is that below 70MW the differential alarm disappeared and greater than 70MW it appeared again.
Can anybody explain the reasons of the alarm and its impacts of running the unit with this alarm?
 
Did the temperature spread suddenly appear 14 days after the water wash or is that just when it finally alarmed?

I have seen an extraction valve accidently left closed after an offline water wash create your issue - That would be the first thing I would check - make sure all of the valves in the turbine compartment are in their correct position.
 
The turbine wheel rotates at 3000 RPM—it’s VERY difficult to get high differential temperatures in a particular wheelspace when the air all around the wheelspace is being mixed at that rate. Most problems are caused by improper insertion of the wheelspace, most likely a difference in the insertion depth between the two T/Cs in a particular wheelspace.

It is possible that the wheelspaces were higher than they should have been during the water wash, and this has caused a shift in the nozzle segments which is causing an air flow problem in the area of one particular T/C.

I always recommend painting the valve handles of manual valves which have to be moved to perform an off-line water wash and then returned to their normal positions after the off-line water wash. This way, they are easy to find. I usually use GREEN to indicate valves which are normally open and have to be closed during the wash, and RED for normally closed valves which have to be opened during the wash. This way, they are not only easy to find--but it's pretty easy to know which have be opened and which have to be closed, and when. Makes checking them easier too (because it's always a good idea to have a second person check valve positions before re-starting the unit).

For me, wheelspace temperatures are NOT about the value at any instant in time--they are about the change in value and the rate of change of the value. A sudden change is to be monitored, and if the rate of change is quick--then it bears even more attention. A slowly increasing value is to be monitored, but doesn't require as much attention. One also needs to consider how long it's been since the last maintenance outage, and the condition of the unit when it was reassembled after the last maintenance outage. And, whether there were issues with reassembly of any of the unit after the last maintenance outage (because problems in one area might mean to expect problems in other areas as well).

But, as MattyIce says, check valve positions. Also consider insertion depth differences--even with a new T/C. What did the other T/C, which was removed, look like? Was it bent, kinked, damaged--or did it appear to be in good condition? And, consider how long since the last outage, and how long to the next outage--and any issues with the unit since the last outage. It certainly seems like something has changed--it just difficult to know what at this point. Unfortunately.

Please write back if you find something soon, and details of outage schedule (past and future) would also be helpful.
 
Dear MattIce & CSA

There is no chance to remain a valve in closed condition since we apply LOTO for offline water wash and maintain it smoothly.
During offline water wash wheelspace T/Cs values were very close to each other and it continues smoothly for 13 days.At 13th days TTWS3FO1 fluctuation started and couple of hours later it was observed the fluctuation range were wide and generate differential temperature alarm. Unfortunately a unit trip was occurred on 14th day due to fuel side problem.After restart of the unit the deviation of 3FO1 & 3FO2 was closed(No alarm).Due to customer demand the unit was restarted without attending TTWS3FO1 at that time.
Again, on 17th day the unit was shutdown for fuel side problem and this time TTWS3FO1 replaced with new T/C.The unit was restarted after 30 hours of shutdown and upto 15 MW the temperature deviation was less between 3FO1 & 3FO2.When the unit load greater than 30 MW the deviation started to increase and at 70% load it was more than earlier.Now the TTWS3FO1 value is maintaining 545~578 degC at load 70~100MW.

Schedule outage information for this unit: November-2019 CI was performed and the next year April MI will be performed.

Regards,

Rokan
 
rokan_123,

I have seen a replacement wheelspace T/C also fail before. In my personal opinion it happens because some shift of the nozzle segments occurs which pinches the sheath.

I have also seen the T/C guide tubes be found to have broken free of their clips, which puts a twisting force on the sheath.

But, you’ll likely never know until the Major Inspection, and only then if care is taken when disassembling that area’s nozzle segments.

In my personal opinion, most analog values really only have meaning when they change—and it’s mostly about the rate of change not the instantaneous value at any point in time.

Just continue to monitor the two T/Cs in that area for change, and note the rate of change under similar operating conditions. If it has “jumped” to this new value and continues to hover in a small range it’s probably likely the new T/C has experienced similar damage because of the same reason. And if you have an opportunity to replace the new T/C you may find it is damaged in a similar way and place.

I don’t know of any endoscopes which could be used to try to examine the guide tubes or the nozzle segment, and the unit would have to be cool for any ‘scoping to occur. I suspect the damage occurs when the unit heats up and excessive shifting occurs for some reason(s). But that’s just a SWAG (Scientific Wild-Arsed Guess).

Best of luck!
 
rolan-123,

I have seen a replacement wheelspace T/C also fail before. In my personal opinion it happens because some shift of the nozzle segments occurs which pinches the sheath.

I have also seen the T/C guide tubes be found to have broken free of their clips, which puts a twisting force on the sheath.

But, you’ll likely never know until the Major Inspection, and only then if care is taken when disassembling that area’s nozzle segments.

In my personal opinion, most analog values really only have meaning when they change—and it’s mostly about the rate of change not the instantaneous value at any point in time.

Just continue to monitor the two T/Cs in that area for change, and note the rate of change under similar operating conditions. If it has “jumped” to this new value and continues to hover in a small range it’s probably likely the new T/C has experienced similar damage because of the same reason. And if you have an opportunity to replace the new T/C you may find it is damaged in a similar way and place.

I don’t know of any endoscopes which could be used to try to examine the guide tubes or the nozzle segment, and the unit would have to be cool for any ‘scoping to occur. I suspect the damage occurs when the unit heats up and excessive shifting occurs for some reason(s). But that’s just a SWAG (Scientific Wild-Arsed Guess).

Best of luck!
Thank you very much CSA for your nice feedback.
 
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