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from the POWER GEN department...
GTG exhaust temperature alarm
Power generation equipment control. topic
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Posted by Ayed on 7 January, 2013 - 3:17 am
we have GTG which is frame 7EA and MARK V controlled. Recently during the unit started up after the unit reached speed of 2682RPM before the bleed valve closed, we observed that the temperature shown 636 degree Celsius the exhaust temperature alarm activated. Maintenance has changed the following:

1- Intake air filters.
2- Some cards in the MARK V which belongs the three processor .

Would you please tell me are there some factor must be checked rather than the above mentioned?


1 out of 1 members thought this post was helpful...
Posted by CSA on 7 January, 2013 - 1:17 pm
sanjay,

The real problem is: Why did the compressor bleed valves (plural) close at 2682 RPM? (Because the compressor bleed valves are all supposed to close at the same time.)

It's not normal for the compressor bleed valves to close below approximately 95% speed (which would be about 3420 RPM for a Frame 7EA). And on some units, they are closed at 100% speed, and on an even smaller percentage of units they are closed at 5 MW (after the generator breaker is closed).

The purpose of the bleed valves is to reduce the air flow through the compressor during low speed operation (start-up and shutdown) to prevent surge/stall. Surge/stall occurs when air flow through the compressor actually "stops" and then suddenly starts again. This can cause compressor blades (stationary and rotating) to break off, which is not very good for the machine. So, by extracting ("bleeding") some of the flow through the compressor at an intermediate stage surge/stall is prevented. Closing the compressor bleed valves--or even just one valve--can be very damaging to the unit.

It's conceivable that if a surge/stall condition occurred when the bleed valve(s) closed early the exhaust temperature would increase even if the fuel were held constant because of the reduced air flow through the compressor. But, that's a SWAG (Scientific Wild-Arsed Guess).

In any case, the real problem--which won't be solved by replacing inlet air filters or printed circuit cards--is: Why did the bleed valve(s) close much earlier (2682 RPM would be approximately 74.5% speed) than they should have closed? Then the high exhaust temperature will probably not occur.

Please write back to let us know what you determine and how you resolve the issue.


1 out of 1 members thought this post was helpful...
Posted by SalehSaeed on 12 January, 2013 - 2:10 am
I am colleague of Ayed. our situation GE 7EA heavy duty first start unit proceed normally to FSNL, but when stop unit and start again on fuel pick-up or on GAS trip with High temp, and High vib, at 2700 rpm. There is time delay during start up if unit start again (hot start)?,


Posted by Ayed on 12 January, 2013 - 3:38 am
Dear;

The main problem why did we receive exhaust over temperature alarm?
We do not have any problem with the bleed valve. One time the unit was started up on light fuel oil and reached the FSNL but while it was started up on gas we usually receive this alarm.

Maintenance changed the air inlet filters and checked the MRAK V.

The below some massage from the operation

Unit was started on Gas, fire, attend FSNL, in presence of I/C & ME, at 72% little vibration Unit shutdown. Speed stack up at 83% RPM, Emergency stop command was given. Unit was Re-started on LFO, fire, but Emergency stop command was given @ 73% RPM due to Excessive vibration and received "Exh .Over Temp. Trip." Unit was Re-started as per I/C request on Gas, fire, but tripped @ 73% RPM by "Exh .Over Temp. Trip" alarm.


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Posted by CSA on 12 January, 2013 - 1:12 pm
DEARS

I try as hard as I can to understand the information that is provided, and sometimes I make a mistake. As in with the description of 2682 before bleed valves closed; I understood that to mean that the bleed valves had closed at 2682 RPM--which would be problematic. Apparently, that was not the correct understanding.

You are now telling us that there is high vibration at about 70-72% speed and the operators are "shutting down" the unit because of the high vibration. This could be one of the two critical speeds of the turbine-generator. We don't know if this problem started after a maintenance outage (HGPI or Major, for example), or after a trip from high load (when the shaft was hot), or if you are attempting to start the unit without having the unit on cooldown (Slowroll, or Hyd. Ratchet, or "turning gear") for several hours before attempting to start the unit.

What are the maximum vibrations being experienced during the start attempts?

Describe the machine conditions in the eight hours prior to a start attempt. (Is it on cooldown (ratchet; turning gear; etc.) for four hours, or two hours, or one hour, or no hours? Is the shaft at ambient temperature?)

There should be no programmed "delay" in start-up, but stranger things have happened. You should be using VIEW2 or VIEW2T to try to gather information and data about the start-up to try to determine what is happening.

What SHOULD be happening is that the unit should be accelerating on a acceleration rate reference (TNHAR) that is compared to the actual acceleration rate (TNHA), and fuel is increased or decreased as necessary to try to keep TNHA as close to TNHAR as possible during start-up. It's hard to imagine a condition where the unit would be "stuck" and acceleration control would increase the fuel to the point at which it trips on overtemperature, because even if it increases the fuel to try to increase the acceleration rate it will hit the isothermal exhaust temperature limit and will not be allowed to increase any further.

Has anyone been adjusting Control Constants lately? If so, which one(s)?

Has anyone "calibrated" Gas Control Valve LVDTs recently?

The last theory I have is that when the operators are "shutting down" the unit when it is at 70-72% speed, they are selecting STOP to do so and the Mark V is shifting from Start-up Control to Shutdown control and the Shutdown FSR is too high for the turbine speed at that point and because it's simply a setpoint (open loop) that it's too high and is causing an exhaust overtemperature trip.

Based on the information provided, I have no other theories.

Please write back with answers to all of the above questions if you want more assistance with the issue. There's just not enough information to be of any further help.


Posted by Ahmad on 13 January, 2013 - 7:44 pm
Dears

I have been started the unit 2 times on both fuels (LFO-Gas) & unit tripped at 72% speed by "Exh.Over Temp.Trip" alarms.

My observation as the following:-

1- Unit vibration & Exhaust Temp. normal from 0 speed to 70% speed.

2- CPD Increasing normally & at 70% speed start fluctuating & dropping.

3- Vibration on bearing-2 start increasing at 70% speed to 15mm/s with abnormal sound.

4. At 72% speed Exh.Temp.Increased from 530C to more than 600C & unit tripped.

5. SRV & TNH Increasing normally.

Regards


Posted by CSA on 13 January, 2013 - 11:21 pm
DEARS,

Are you absolutely 100% certain all the compressor bleed valves are closed?


Posted by Ahmad on 14 January, 2013 - 9:09 am
Compressor bleed valves were checked by maintenance during start up till unit trip & they found open all the time.


Posted by CSA on 14 January, 2013 - 11:40 am
DEARS,

CPD doesn't really increase very much until the axial compressor speed gets pretty close to rated (100%) and the IGVs are at minimum operating position (approximately 57 DGA (DeGrees Angle)) and the compressor bleed valves close. This is characteristic of axial compressors. So, if you are seeing an abnormal increase of CPD, I would suggest one or more of the compressor bleed valves is not fully open.

The vibration increase you are reporting may be characteristic of compressor bleed valves which are not fully open or are closed when they should be open. The axial compressor bleed valves MUST be 100% open during start-up (and shutdown) to prevent compressor surge/stall, which will initially manifest itself as high vibration, though it usually increases very rapidly.

As was said in the original reply, if the bleed valve(s) were not fully open during start-up it's conceivable that a surge/stall condition could result in excessive exhaust temperature.

Around 70% speed, the IGVs are commanded to stroke open slowly from the closed position (usually about 34 DGA for a Frame 7EA) to minimum modulating position (usually 57 DGA). If the IGVs are not opening or are opening too quickly this could cause problems with the start-up.

You have been advised to use one of the Mark V software "tools" for gathering data to assist with understanding what is happening. That recommendation still stands. Other posters have copied the file to a web-hosting site and then posted a URL to the file location so we can analyze the data and make some recommendations or suggestions.

Signals for which data should be gathered include (presuming a START on Natural Gas Fuel):

TNH
CPD
CSGV
CSRGV
L20CB1X (or possibly L20CBX)
L33CB10
L33CB20
L33CB30
L33CB40
TTXM
TTRX
BB_MAX
TTXSP1
TTXSP2
TTXSP3
FPG2
FPRG
FSGR
FSG
FSR
FSRACC
FSRN
FSRMIN
FSRSU
FSRSD

However, one question which has gone unasked and unanswered is the most important question when a problem like this occurs:

WHEN DID THIS PROBLEM START?

After a maintenance outage (a CI, or a HGPI, or a Major)?

After a trip from load (when the shaft and axial compressor was hot)?

Please list EVERY Process Alarm which is annunciated prior to the START and during the start-up/acceleration up to the exhaust overtemperature trip. You can get the list of alarm which are annunciated during the START attempt from the Mark V Trip History function. (It can be saved as an ASCII text file, and the alarm portion can be copied to your reply to control.com). The list of current and active alarms present before the START is initiated must be manually put in your reply.


Posted by CSA on 14 January, 2013 - 8:16 pm
DEARS,

The more I re-read the posts to this thread, the more I am convinced there is some problem with the air flow through the axial compressor. Either the compressor bleed valves are NOT in the proper position, OR there is something wrong with the axial compressor (rotor and/or stator), OR there is something wrong with the IGVs opening too early or too quickly which is causing the fluctuating CPD reading, the abnormal noise, and the increased vibration at approximately 70% speed.

This is likely NOT a controls-related problem, but a mechanical problem. Either something is causing an axial compressor surge/stall during acceleration, or there is truly an imbalance causing the high vibration. But, the coincidence of the fluctuating CPD, the increasing vibration and the abnormal sound (presumably coming from the axial compressor) is very indicative of a compressor surge/stall which can occur if the compressor bleed valves are not open or fully open, or the IGVs are opening too fast and/or too much. Combine this with the fact that exhaust temperature is increasing quickly when fuel flow is not increasing quickly (per the information provided), and this just sounds like a compressor problem of some kind. And, as the most recent reply from site indicates this problem is occuring on both fuels at approximately the same speed during acceleration is even more indicative of some problem with air flow through the compressor, either caused by improper positioning of the compressor bleed valve(s), a problem with IGV positioning, or a compressor problem (bent/broken blades, stationary and/or rotating).

This does not sound like a problem with the Mark V or the fuel control valves. Unless there are other Process Alarms alluding to IGV Position Trouble that we aren't aware of, it's not likely the IGVs are opening too fast or too much.

It might be a good idea to get someone to borescope the axial compressor.... And if the start attempts continue without some resolution in the hopes that the problem will fix itself, then it's very likely that any damage which presently exists will get even worse, or that damage may occur which already hasn't.

Please write back to let us know what you find.


0 out of 1 members thought this post was helpful...
Posted by Ayed on 16 January, 2013 - 7:06 am
We experienced a tripping on Exhaust Over Temperature High at 74 to 78% Speed for GTG Unit No. 9 GE Machine with very High Vibration on BB3 with a surge sound from air Inlet side: the following were checked and found ok: -

1. Compressor bleed valves found opened.
2. IGV found 34 and remained 34 till the trip.
3. Unit was tested on Gas and LFO but we have same problem in both cases.
4. Gas pipe line and strainer is ok.
5. AIH Filter New.
6. 96 TV-1 & 2 were checked and calibrated found ok.
7. 96 CD-1,2,3 checked found is ok.


0 out of 1 members thought this post was helpful...
Posted by Ahmad on 17 January, 2013 - 2:07 pm
Today maintenance checked the main compressor and they found pittings & cracks in compressor blades and there are some blades very dirty.


Posted by Ayed on 5 March, 2013 - 2:40 am
IGV and first row of compressor were cleaned by maintenance and then the unit was started up. during the unit starting I observed that the pressure reached to 2.2 Bar which never happen before. and the exhaust temperature increased rapidly then the unit tipped. So i would like to know what is the minimum pressure can compressor build up during starting till the bleed valves close. Also it has been observed by the Bore scope that the compressor blades are having fouling. Also I would like to know about the air fuel ratio. Is it can contribute in this problem? We need to check the required fuel rate at 74% of speed how much but for the air there is no measuring.

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