Output of frame 6551B GE Gas turbine generator reduced to nearly 1 MW after Major Overhauling. Our's is a TMR Mark-V control operating in combined cycle. Unit runs with natural gas only. We checked and found all the parameters to be nearly as before. No major difference or abnormality in any behaviours or values. Except we experienced a little more vibrations during startup (happened in all of the 3 startups after overhauling is done).

Now vibration level is 3 to 4 mm per second which is normal & exactly as before. As far as I understand output is supposed to increase due to compressor was thoroughly cleaned & washed during overhauling. Want to know why output reduced.

Sheikh_Arif

 

Check BTU quality of the gas supply.

Bob

 

Fuel constituent change might be a possibility, but when something like happens after a major outage, it's an indication that something isn't right with the hot gas path section, like leaking side seals on transition pieces or something like that. Shrouds not sealing correctly or something similar.

Have you changed gas fuel suppliers?

Are you looking at data taken from after several hours at Base Load versus data taken from after running at several hours at Base Load before the outage?

What is the CPD now versus before the outage? Usually, someone takes some readings before a major inspection to have a "baseline" for the results of the hot gas path parts replacement.

What is the exhaust temperature?

What hot gas path parts components were replaced during the major?

Where were the parts purchased from?

What instrumentation was calibrated during the outage? CPD transducer(s)? P2 pressure transducer(s)? MW transducer(s)?

What instrument(s) are you taking your readings from?

 

Thanks to CSA for responsing. Here I try to provide the info you asked for:
1. We didn't change our fuel gas. It is natural gas from the national gas supply grid pipeline.
2. We checked data at steady Base load for before and after overhauling.
3. CPD is same 10.4 bar after and so do before.
4. TTXM & TTRF1 is 550 & 1109 degree celsius respectively and those are almost same as before overhauling.
5. We changed combustion liners, transition pieces, all the 3 stages' nozzles, shrouds & buckets, IGV blading with pinions etc.
6. The new parts are supplied by the company (a Dutch co.) involved in Long Term Service Agreement with our company. They also do refurbishments on some hot gas path parts.
7. Yes, most of the transmitters/transducers were calibrated including CPD/P2 pressure/MW transducers.

We use Mark-V speedtronic TMR control system to gather data on the (I) and most of the transmitters (pressure) are of "Rosemaunt" brand.
Hope to hear from you soon sir.

Sheikh_Arif

 

We are also experiencing a similar problem here as we have lost few MWs after comustion inspection of our three GE Frame 9 gas turbines. We have checked IGV angle, CPD and P2 calibration and other checks but no clue yet. After off-line WW, we have regained partially the lost MWs, but these will be gone in a few das as happens after WW. I think you should also check IGV angle to see if they are fully open.

 

You mentioned IGV Blading, pinion etc have been changed. Did you calibrate IGVs after this change? This could be one area you can investigate.

 

Many times transducers drift and indicate higher than normal output before they are calibrated. If the CPD transducer(s) were indicating higher than normal before calibration, that would make the output higher than normal (the unit would actually be slightly over-fired at Base Load, also).

If the MW transducer was calibrated, this could also be part of the problem. It may have been falsely indicating higher than normal output before calibration. And, now it's indicating normal output.

As CSD says, IGV LVDT calibration could also be the problem. Of all the LVDTs on the unit, these are the most critical to performance/output so the calibration of their feedback should be done carefully and verified. If the LVDT feedback is indicating 84 DGA when the IGVs are really at 82 DGA, the unit output will be decreased.

My times when instrumentation is calibrated, there is an as-found condition recorded as well as an as-left condition. You should be able to look at the instrument calibration data sheets to see the difference between the before and after conditions.

Also, when calibrating analog transducers (mA and voltage output in particular) on a Mark IV or a Mark V turbine control panel, one should leave the instrument in place and connected to the turbine control panel when making adjustments, using the indicated values on the operator interface display. Removing these devices and calibrating them on a "bench" and then re-installing them will usually result in a slightly different reading when verified in place. I've never been able to explain it, and it shouldn't be so, but it is. Makes calibration a little difficult, but it's the best method. So, if your CPD transducer(s) were calibrated in an instrument shop you should check the output with them connected to the Mark V and make any adjustments necessary (and I believe you will find adjustment will be necessary if they were removed and calibrated while not connected to the Mark V).

My suspicion is the MW transducer followed by the CPD transducer calibration, followed by the IGV LVDT calibration. It could be (and has been) a combination of all three. How long had it been before any of these devices had been calibrated? And, while most people believe calibration drifts low, it can and does sometime drift high.

 

You have calibrated your MW transducers, CPD during shutdown. Were they need any calibration / tunning during testing? i.e were any errors found during testing? It is possible that during testing these may have shown slightly positive error (which eventually would had shown you higher MW before shutdown)and the same had been fixed.

 

I have noticed what Mr. CSA has quoted in his reply to the above subject and I want to put my comment here about what I have come to know from the logic sequencing in the Mark-v about fuel control (temperature) at Base load under.

If CPD shows higher value than actual by the transmitter due to drifting or miscalibration problem, the unit is neither supposed to be overfired nor the unit would produce increased MW output. On the contrary, if CPD is really higher, it results in higher MW output because of cooling the exhaust temperature (TTXM) which pave the way for more fuel addition to match or rise of TTXM up to the value of CPD Bias Exhaust Temperature Reference (TTRXB).

But for both the situation above, TTRXB is lowered by the logic sequence (Rung “TTRXV6”) and that’s why we find TTXM (and surely TTRXB also) is low when CPD is really higher.
In the calculation we find ‘TTRXP = TTKn_I – (CPD - TTKn_C) * TTKn_S’ will be less in any case CPD is higher.

This TTRXP value eventually becomes TTRXB (after being ttr_min, then TTRX (or we can say the value of TTRXP contribute most to create the value of TTRXB) ; though there are other values which may contribute in changing the actual value of it more or less).
So, if Mr. Sheikh_Arif’s unit was showing higher CPD before overhauling, then the unit was producing less MW than it should before overhauling.

And after properly calibration MW was supposed to increase in that situation.
In addition to Mr. CSA’s wise comments on the problem, I would like to add some points here for Mr. Sheikh_Arif that after major overhauling your unit is experiencing i) Inter stage leakages of flue gas among the wheelspaces, ii) IGV is not really open at 86o iii) Loss of compressor efficiency iv) Leakage / decrease in of compressed air pressure(CPD) or loss of exhaust gas cooling efficiency due to increase of compressor discharge temperature for example.

M M Ahsan

 

Two similar experiences come to mind when units lost MWs after a major overhaul. One was outage debris (a used welding rod stub) left on top of the IGV (GE 7FA) hydraulic cylinder keeping it from going full open (90 deg in this case), but not enough to activate an out of position alarm. The other experience was larger tip clearences on the newly installed turbine buckets.

Greg Ponto
Lead Combustion Turbine Specialist
Elwood Operations
(815) 423-9883 x2232
Email: [email protected]