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Reduced Gas Turbine Base Load Output
Baseload output of our GE frame 9E has reduced from 110MW to 95MW in spite of the fact that we have carried out air inlet filter change and an off-line compressor water wash.

The baseload output of our GE frame 9E has reduced considerably from 110MW to 95MW in spite of the fact that we have carried out air inlet filter change and an off-line compressor water wash. We only had a Major Inspection on this unit about 8months ago. All other parameters look normal. What might be cause of this problem?

Please be mindful of the fact that we are in temperature control (hence Baseload), but fuel flow has reduced from the usual 8.2kg/s to 7.6kg/s with a cpd of 10.6, ctd of 363, ttxm of 560 degrees Celsius.

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Do you have comparative data when at 110 Mw? You don't mention Inlet Temp. where are you located? Has the Inlet Temp. changed? Do you have data before and after the Major and before and after the offline wash?

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kwabenaasare,

What Process- and Diagnostic Alarms are active when the unit is running at Base Load? (List ALL alarms--even if they seem irrelevant to you or you believe you know whey they are active!)

Do the operators click on BASE LOAD to load the unit to Base Load, or do they put in a Pre-Selected Load Control setpoint that is higher than the normal Base Load value to load the unit to Base Load?

What was the CPD at Base Load before the Major, or when the load was "normal" at Base Load after the Major?

What is the value of TTXSPL (Turbine Temperature, Exhaust Spread--Allowable/Limit)? What are the values of TTXSP1, TTXSP2 and TTXSP3 (the three actual turbine exhaust temperature spreads)?

Does the unit have DLN-I combustors or conventional, diffusion flame combustors?

What is the indicated IGV angle, from the IGV LVDT feedback/input when the unit at Base Load on Exhaust Temperature Control? (The signal name is usually CSGV, Control Stroke-Guide Vane.)

Have you physically measured (using a machinist's protractor) the actual IGV angle versus the IGV LVDT feedback? If so, how recently? Have you compared the IGV LVDT feedback to the IGV angle indicator on the side of the axial compressor casing (which is a coarse indication, and can be not properly adjusted--which is why it's important to verify LVDT feedback versus actual angle using a protractor, and adjusting the indicator if necessary to be as accurate as possible).

Does the unit use CPR (Compressor Pressure Ratio) or CPD for the primary exhaust temperature control bias? (If it has DLN combustors, it will use CPR.)

If the unit uses CPR it will have one or three atmospheric pressure transmitters, 96AP-1, -2 and -3 (if it has redundant atmospheric pressure transmitters). The signal AFPAP is derived from the atmospheric pressure transmitter(s), and it's not uncommon for one or all of the pressure transmitters to be out of calibration, or for the low pressure sensing port to be clogged, or for the valves in the sensing lines to be in the wrong position, or for there to be water/condensate in the sensing lines. Have you checked the atmospheric pressure transmitter sensing line tubing connections for tightness?

AFPAP should be VERY close the actual atmospheric (Barometric) pressure. If not, then something is amiss with the tubing or the valves or the transmitters. If the unit uses CPR this signal is VERY important as it impacts the Compressor Ratio Calculation which biases (determines) the primary exhaust temperature control reference.

What is the value of TTRXP (Turbine Temperature Exhaust Reference-Primary) and what is the value of TTRXS (Turbine Temperature Exhaust Reference-Secondary) when the unit is operating at Base Load, on exhaust temperature control?

Have you verified the calibration of the CPD transmitter(s), 96CD-1, -2 & -3 (if it has redundant CPD transmitters)? (For a machine of this size, there are usually three, but older machines may only have one.)

What control system is used on the turbine?

Do you know how to view the Prevoted values of the input signals from redundant transmitters (all of the ones listed above)?

The most important questions if you still want/need help are: What are ALL the Process- and Diagnostic Alarms active when the unit is at Base Load? What is the indicated IGV angle when the unit is at Base Load on exhaust temperature angle? What are the values of TTRXP and TTRXS? What is the value of AFPAP versus the actual site atmospheric (Barometric) pressure? Have you verified IGV LVDT indication versus actual position, and if so, how (using a machinist's protractor, or the indicator on the axial compressor casing)?

Please write back to let us know what you find--and if you need additional assistance you will need to provide the answers to all the questions, particularly the active Process- and Diagnostic Alarms. This may seem like a lot of information, but we aren't there and we can't see what you can and there are a LOT of factors that affect Base Load--these are ONLY most of the control-related factors. Axial compressor condition/clearances, turbine nozzle/bucket condition/clearances, and excessive exhaust back-pressure can also affect the power output of a gas turbine. Even if a Major was done recently, re-assembly may not have been entirely correct and if non-OEM parts were used it's possible they may not have been the best quality (though even OEM parts can sometimes fail prematurely, though usually only if the thermal stresses on the hot gas path parts are extreme--such as can occur if the unit has a high number of emergency trips (especially from high loads) and starts, and failed starts, and even a high number of starts--though a few months shouldn't normally result in a lot of trips and/or starts, but some sites have more problems or need to start frequently for some reason).

Also, proper valve positioning (often called valve "line-up") before and after an off-line compressor water wash is VERY important to preventing water wash fluid and rinse water from getting into the sensing lines/tubing. Have you checked the CPD transmitter sensing lines/tubing to make sure there is no liquid (detergent; water; condensate) in the lines which could be negatively affecting the reading(s)?

Looking forward to the answers to the questions, and to hearing how you progress in resolving the issue.

Dear Sir,

Thank you very much for your prompt response. At the moment, I am not in the office and therefore can not re-verify some of the things that you asked me to check. However, I'll attempt to answer some of your questions.

There is no process or diagnostic alarm on the alarm display of the HMI and event logger. The three atmospheric barometric transmitters were calibrated during the Major inspection, in addition to the three CPD transmitters. Their readings have been normal. Yes, i have verified them on Mark VI toolbox. Nonetheless, I will check them again when the unit comes down.

Yes, the unit uses CPR for calculating the Bias. The control system is Speedtronic Mark VI. IGV calibration was done using Lvdt Calibration function in Toolbox. We also used the mechanical protractor to physically measure the angle. Presently, the IGV is physically measuring 84degrees and electronically measuring same.

It was the problem that led us to carry out the offline compressor water wash and there is a standard procedure for that. More over, the unit experience marginal improvement for a very short while and reverted to its reduced baseload output.

I was very glad when you touched on the fact that there is the possibility of turbine bucket clearance problems even though we just under went a major inspection. I really suspect that that is our problem. Albeit, I am not able to pinpoint it. This is because, perusing the data that we have for previous years' baseload runs, they suggest strongly that there is no problem with both the compressor and air inlet area, which leaves me to suspect the turbine capital parts which we procured from a third party LTSA contractor. The second stage turbine buckets were brand new from the third party and the remaining parts were refurbished by the third party.

Unfortunately, it is very difficult to explain these issues to my mechanically biased colleagues and bosses to understand and appreciate the problem. We also don't have an instrument that measures the exhaust back pressure.

If my suspicions are accurate and true, what are the indications that pinpoint to the turbine capital parts clearance problem. Do you think that a borescope inspection will pick up the increase in clearance? This is because my mechanical colleagues refuse to carry out the porescope inspection. They believe that it would be unclear in the video.

If my suspicions are wrong, then what exactly is the problem or the cause?

Thank you and looking forward to your prompt response once again.

Dear Kwabenaasare,

The use of non-OEM genuine CAPITAL parts is not a good practice. But in some cases we do it! Like when the OEM decide to not more manufacture a specific part and the new replacement part, which cost more, you also need to upgrade the entire mechanism to fit the new part.

Using boroscope to estimate (or measure) clearances is not an easy task (say impossible). But in your case and because you are suspecting the non-OEM BUCKETS, I think it is critical to perform at first opportunity a boroscopic inspection just to get a look to your buckets, honey combs, etc.

GT under temperature control is a normal situation that means GT is supporting the maximum power/torque to handle. So you may need to check your driven generator, coupling and accessories for power lost or sticking.

If something is going wrong within the hot gas path usually, not all time, you should be able to read it on the exhaust temperature thermocouples (exhaust temperature spread, combustion trouble and combustion trip).

What about start-up and shutdown phases and are they happening smoothly? Is there any sticking during start-up?

What about fuel? Are using the same fuel before and after your MI?

Regards
Karim

By Kwabena Asare on 22 March, 2017 - 7:11 am
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Dear Sir,

I am now in the office and I have checked all the signals that you asked me to check again and I am glad to say that they are all reading well.

I checked the following: cdp1, cdp2 and cpd3, afpap1, afpap2, afpap3, IGV angle is 84degrees. TTXSP1 is 28, TTXSP2 is 28 and TTXSP3 is 29. TTXSPL is 80. All 24 thermocouples are reading temperatures between 545degrees Celsius and 564degrees Celsius, but for numbers 12 and 13 which are reading 576degrees Celsius and 578degrees Celsius respectively. The exhaust temperature, TTXM is 563degrees Celsius. We are at base load and on Light Crude Oil.

I suspect the turbine bucket clearances, especially between the second stage buckets and the honeycomb shroud blocks have increased and it is resulting in hot gas leakages or escape, thereby culminating in the high exhaust temperatures at a lower load, hence base load operation (FSRT limiting fuel flow in order to protect the capital parts).

Is there a way to check the clearances and ascertain the veracity of my claim or suspicion?

Thank you. Looking forward to your prompt response.

So, you only want to pursue the cause you believe to be the reason for the loss of output?

You did not provide all of the information requested--the two most important signals were not included in your response: TTRXP and TTRXS.

Someone who is very skilled with a borescope can check tip clearances and look for cracks and nozzle/bucket damage.

Best of luck.