Dear Sir,

We are operating GE frame 7FA DLN 2.6 gas turbines in Kingdom Of saudi Arabia,

We have two units of GE frame 7FA DLN 2.6

we did Unit#01 Major Inspection in OCT-12 and UNIT#02 MI in NOV-12

After major inspection initially both machine delivering the more of less same output (165 MW)

But since JAN-13 the Unit#01 performance was degrading. we did Unit#01 water wash in MARCH-13. after water wash Unit#01 output crossed Unit#02 (almost 5 to 6 MW higher than unit#02)

but what we observed that unit#01 output again degrading just after 3 days of offline water wash.

After water wash of unit#01, the exhaust temperature was found only 584 degc and TTRF1# 1300 degc. we traced the past trend and found that exhaust temp remained around 615 degc and the reference TTRF1# 1317 degc

there was an alarm just after the offline water wash that AFPAP difference high between the units

AFPAP (atmospheric pressure) reading was found lower than unit#02 after Unit#01 off line water wash

1. GT#01, Exhaust thermocouple TTDX#13 reading# 548 degc and the TTXSP1# 70 degc. we may have to parallel TTXD#13 with healthy thermocouple

2. Also “AFPAP difference high between the units” alarm appeared and persisting

GT#01 AFPAP # 27.7 In/In

GT#02 AFPAP# 29.77 in/in

If difference goes above 0.5 in/in, high difference between units alarm will appear

3. Exhaust temperature (TTXM) came down from 624 degc to 600 degc

4. CPD increased from 13.55 bar to 14.17 bar


5. CTD decreased from 397.35 degc to 395.74 degc

6. Unit#01 MW increased from 150 MW to 157 MW

We did Unit#01 offline water wash on 3rd march-13, and on 05th march-13 we observed unit#01 MW was 3 MW lower than Unit#02

GE recommended to do calibration of unit#01 AFPAP transmitter. transmitter was calibrated and the reading back to normal same as GT#02

after AFPAP calibration Unit#01 MW crossed 4 MW higher than Unit#02

Following parameters were increased after GT#01 AFPAP calibration

1. TTRF1 increased from 1300 to 1317 degc
2. FSR increased from 70% to 72%
3. MW increased from 155 MW to 161 MW

Can you please share your expert knowledge regarding above subject?

Why the Reference temperature gone down after the offline water wash?

what are the inputs to calculate the TTRF1?

Your help in this regard is much appreciated

Thanks
Parag

 

Paragkumar,

The Atmospheric Pressure inputs (there are usually three of them) are very critical to performance. The median selected value is used to calculate CPR: Compressor Pressure Ratio. CPR is a fancy CPD calculation.

CPR is used for calculating exhaust temperature, and I believe it's also used in the TTRF1 calculation. So, if the feedback is inaccurate, it can cause problems with both calculations.

TTRF1 is <b><i>calculated</i></b> firing temperature. It's only real usefulness is in the determination of when to switch combustion modes. As long as combustion mode transfers are occurring without problems, the TTRF1 calculation is working fine. While it is an indication of what firing temperature might actually be, it is only a calculated value--and, again, it's only used for making combustion mode transfers.

I had a difficult time following the sequence of events in your post, but I'm confident in saying that the problem most likely lies in an inaccurate Atmospheric Pressure input.

The Atmospheric Pressure transmitters have to be open to atmosphere, and GE or its packagers usually use some kind of sintered metal or "spongy" material in the atmospheric end of the tubing to the transmitters. This can cause problems if allowed to get excessively dirty and/or plugged. They must be kept clean and unobstructed for the Atmospheric Pressure transmitters to work properly.

People also mistakenly remove the sintered iron covers/spongy material, which then allows dirt (like sand) to get into the transmitter taps/ports and cause problems. The covers/spongy material are very important--and it's equally important to keep them clean as well.

The biggest problem with Off-line Water Washes is the lack of a written procedure for doing them. The second biggest problem with Off-line Water Washes is the valves which must be manually opened and closed to start the procedure, and then closed and opened (respectively) to complete the procedure are never identified properly by the packager (GE). Many times people have a "name" for a valve, but since that valve isn't tagged with that name (stamped on a stainless steel tag and tied to the valve with a chain or stainless steel wire) people can't find a valve and so it doesn't get moved to the proper position before and after the procedure. This is a VERY BIG problem with Off-line Water Washes, and it's potentially destructive to DLN combustion hardware if water is allowed to get into passages it shouldn't be allowed into.

Also, there are many low-point drains--also not tagged by the packager (GE)--which must be opened and water allowed to drain out of piping before re-starting, or else hardware problems can occur on DLN machines after an Off-line Water Wash.

If the same people don't do the washes all the time, the chances of improper valving and improper draining are very high. So, the chances of hardware problems are also very high. I'm not saying that's what occurred in this case, but I am saying it has caused many problems, up to and including, premature loss of performance and combustion hardware failure.

Hope this helps!