We are running 2 GE frame 6/ Mark V DLN-1 gas fired machines. We had observed some variations in wheel space temperatures after MI. The data is shown below . Our preliminary conclusions are:

are swapping of CTDA2 with TTWS1FI2, TTWS2FO2& TTWS3AO2 malfunctioning. Since we are running machine in process industry shut down is extremely costly. We decided to run on continuous monitoring.

Also after running for about 4 weeks BB4 suddenly increased from 2.1 mm/sec to 29mm/sec. All bently readings normal, BB3 readings normal, bearing temperatures normal. So we decided to Force L39VT_4 ( for BB4) to "0" > We decided to attend in next opportunity.

We like to someone share/explain their experience in this spurious behaviour. Also we like to know the comments on our evaluation we had done which is mentioned

Data ( First is after MI, second before MI) <pre>
Wheeelsp. PostMI, Pre MI
Temp Deg C

Deg C
CT DA1 328, (346)
CT DA2 371, (348)
TTWS1FI1 371.4, (389.8)
TTWS1FI2 321.9, (389.6)
TTWS1AO1 377.2, (442.8)
TTWS1AO2 373.4, (418.5)
TTWS2FO1 396, (428.1)
TTWS2FO2 354, (422)
TTWS2AO1 426.7, (446.3)
TTWS2AO2 401.3, (440.9)
TTWS3FO1 429, (457.1)
TTWS3FO2 429, (459.6)
TTWS3AO1 278, (286.5)
TTWS3AO2 320, (291.5)
CPD 6.6 bar,(6.6 bar)
Load ~ 20MW, ( 20MW)</pre>

 

The first thing to ask on the wheelspace is whether you have replaced the t/c during MI. As the common practice is yes, my assumption is that the t/c is not inserted fully until the thermowell causing the increased reading. The best practice (which I know of) is actually to insert a brake cable (of a motorcycle) to the t/c insert to determine the exact length required to insert the t/c. After determining the length, then you could confidently insert the t/c

If you are confident that the t/c are inserted to the correct length, then the higher temp reading is actual and should be corrected during the next outage opportunity

As for the BB4, 24mm/s is unusually high and I wonder why is the unit not tripped on vibration protection. For that level of reading, have you physically experience the vibration level? I'm sure you could feel the high vibration when you are adjacent to the unit. Looking at Bently readings and concluding that the high BB4 is false reading is not a good practice. I have experience actual high vibration detected by the BB sensors but the Bently showing normal reading (as the nature of detection for both systems (prox and accelerometer are different). My advice would be to shutdown the unit, quick inspect the BB4 sensor connection (sometimes it gets broken and loose ). Maybe if you have a spare sensor, try to replace it and start up the unit again.

Good luck

 

Thank you for the good suggestion.

Of course T/C replacement is done and we suspect the points regarding T/C fixing .

Regarding BB4 we like to clarify that the BB3 reading is 2.1 to 2.4 mm/sec where as BB4 is nearing 29 mm/sec. Logically any change in vibration in BB4 should have its affect in BB3 also. Also variations should be there in BB7 also. But all the readings are OK. This is the basis of forcing BB4 logic. We expect further discussions/ suggestions and knowledge sharing on the topic which will be beneficial in trouble shooting. Thanks

 

Hello Over there...

Usually post MI these things can happen especially if the crew has some competency issues and not following the standard OEM's inspection and repair procedures.

Now back to reality;
WHEELSPACE

1. Did you plot also the exhaust temperatures? And what is your finding, such as spreads etc?
Answer:

2. Did you check the corresponding TTXM vs. CPD at your ambient conditions (use you control spec. and performance graphs?
Answer:

3. Did you verify the correct installation and the direction of the orifices in the cooling & sealing air lines?
Answer:

VIBRATION
1 Was the increase of vibration detected by BB4 step change or small increase over the period?
Answer:

2. Does your Load Gearbox have vibration protection (seismic etc.)?
Answer:

3. It appears that you have radial probes on the bearings. Are these readings below 75 microns (B#1 and#2)?
Answer:

Good Luck...
A.Oztas

 

Thank you for the explanations and notes

The clarifications are as under
General: Job by OEM. But young team

WHEELSPACE ISSUES

1. Answer: The exhaust temperature , spread is remaining the same ( ~16 deg C )

2. Answer: No appreciable variation in TTXM Vs CPD with post commissioning data . But as explained in the first thread CTD A2remains high , swaping with TTWS1FI2 is suspected

3 Answer: As per the checklist it is correct

VIBRATION ISSUES

1 Answer: The vibration reading had increased instantly and remains on that level. The pairing probe BB3 gives normal reading in the range of 2 mm/sec

2. Answer: Yes . The reading are in the range of 2-2.5 mm/sec

3. Answer: Yes. All radial proximity readings are well within the range . One axial probe is not working

All other parameters remaining within range .

We are exploring to change the vibration logic for the group voting to 2 O/O 2 till next shutdown opportunity so that the machine safety is taken care. Of course brainstorming in progress.

Expecting more discussion on the topic from the group

Thanks

 

Sunil,

Even do the OEM is having competency problems with their Field Engineers. Currently there is globally resources problem in particular with industrial rotating equipment (demand>resources).
I assume that this MI was contracted as time and material only.

Back again to reality:

As regard the seismic vibration on the bearing (BB4)it appears that, this problem is an isolated case. The vibration reading appears spurious, most likely there is a continuity issue in the loop. In order to mitigate this problem, you should monitor periodically the radial readings on the bearing #2 (<90 micron). Although the radial readings are damped readings but this will give you a good trend on that bearing. Accordingly you can intervene manually if required (unloading the GT etc.). In addition the seismic on the Load Gearbox (drive end side) will also give you a good reliable indication which can be referred back to bearing#2 condition.

As regard the wheelspace readings, they appear to be slightly on the high side. You confirmed the correct installation of the orifices in the C&S air lines? In this case, the most likely cause of high temperature will be due to inefficient cooling. Either there is a blockage in the lines or localized leakage of cooling air (loose flange, missing or broken gasket etc.).

Question: Are the nozzles replaced with new ones or did you use refurbished nozzles?

Since this job is performed by OEM, they should be able do a RCA and report their findings. Together with their findings, they should be able to give you the recommendations and the mitigation actions.
Question: Did you take also readings on base load?

As regard possible swapping of the CTDA2 TC, this will not lead directly to reliability issues of the GT. Most likely it will lead to efficiency issues of the GT. Verify the sequence if the logic is taking the max. value or the average temperature? Accordingly you can consider swapping? Prior swapping, proper loop check should be done in order to confirm the particular TC!

All the best...
No Matter What We Say Here, You are on your Own....

A. Oztas