We have 106 MW, GE Frame 9E gas turbine of control system mark-IV.Before 2 months our GT tripped at 60% speed due to IGV trouble. Because IGV open fully due to cable fault and it creates tripping due to difference between IGV Reference and actual IGV angle become > 7.5 DGA.

After Tripping C&I dept. change the servo v/v & cables. Other than this we did wet washing in compressor and IGV cleaning also.

When GT again started it starts giving 7-8 MW more load at base load with same IGV opening 84 DGA at base load. However CPD at base load is around 0.3 bar higher than previous CPD and FSR is slightly higher 1.5% compare to other GT'S.

When we reduced the load to 70 MW (Technical min. load), exhaust temp come down to 530 C, with IGV angle is minimum 54 DGA. However in other GT'S it is maintaining 540 C. FSR is also around 2% less compare to other GT'S at 70 mw load.

Interesting thing is that we checked the IGV physical calibration in the recent shutdown but it is found ok.

Now what may the reasons of GT giving 7-8 MW higher load at base load mode, although IGV calibration is ok .

 

You say you checked the IGV physical calibration in the recent shutdown but it is found ok. How did you verify the accuracy of the calibration? Did you measure the IGV angle with a machinist's protractor? Did you use the crude gauge on the compressor casing?

What shutdown was it that you checked the IGV calibration on? Before the trip on IGV control trouble? After the trip on IGV control trouble?

We have no way of knowing how dirty your compressor and IGVs were. I would suspect that if you manually cleaned the IGVs in addition to a "wet wash" (it's presumed you are talking about an off-line compressor water wash, with a soak and rinse), that the compressor was likely fairly dirty. What is the condition of the turbine inlet air filters and when was the last time they were changed?

People can be amazed at how much power output will improve with a good (meaning a proper) off-line compressor water wash. Manually cleaning dirty IGVs and bellmouth area will also help if they are excessively dirty.

When was the last time you did an off-line water wash on any of the unit?

When was the last time you calibrated the IGV LVDT feedback? It's possible that the previous calibration (at the lower power output) was off slightly, and in conjunction with a dirty compressor and IGVs this could cause output to be low.

But we don't have enough information about how you verified calibration, when you verified calibration, how dirty the compressor and inlet (bellmouth and IGVs) was, etc.

A clean compressor will produce a higher compressor discharge pressure. A higher compressor discharge pressure will allow more fuel to flow for the same firing temperature (at Base Load).

I'm not clear what happened when you reduced load; it's not clear. Exhaust temperature would likely be lower with lower fuel flow and higher compressor air flow, such as with a clean compressor and inlet.

 

Provided that the LVDT calibrations were as good as the recently done. my only bet is that you have done a good offline washing. I'm having the same frame 9e as well. Sometimes after the wash the load can really go very much higher (but just for some days only )

Don't be confused and worried about the higher output. Just sit back and enjoy the moment while its still there

 

How do they calibrate the position feed back on IGV? is there a offset the put in, Or do they close or open them and zero the position?

 

Dear CSA thanks for reply,

I would like to share with you that we had done IGV calibration in the recent shutdown. after 2 month of machine tripped on IGV trouble during startup. means our machine was running continuously with 7-8 MW higher load. then we decided to go for shutdown & do IGV calibration.

Secondly IGV calibration was done through machinist's protractor only. but here i would like to mention that we did IGV physical calibration on 8 to 10 sample blades. so whether is it possible out of 64 blades few blades open to higher angle & increase the flow.

The offine wet washing operation which we did after tripping on IGV fault was done after around 2500 hrs of running, so it was not very long period. and if this load increase is due to wet washing than it should come down to normal value in 2 month of time but it is still giving higher load by 7-8 MW.

As far as CPD is concern it increased by around 0.3 bar at base load from 9.32 bar previously to 9.59 bar.which is a average gain from wet washing.

But my question is that if the load is increased actually by wet washing than at 70 mw (technical min.) load also it should give higher CPD compare to other. here in my case at 70 MW it is giving around 0.2 TO 0.5 bar lesser CPD compare to other, but FSR is also less by around 2% compare to other GT at 70 MW.

So clearly there is something wrong is happening.

What may be the reasons of it.

 

I'm not following your logic, or understanding the sequence of events.

I'm also not understanding this whole "IGV cable" thing that's causing the IGVs to open during start-up.

If the <b>ONLY</b> thing that was done prior to the large increase in power output was to perform an off-line water washing (you still haven't said if that's what you mean by "wet washing"), then on the face of what you are telling us it would seem there is some kind of problem.

Depending on the condition of the IGV mechanism (gears and bushings, etc.) a representative sample of 8 blades, at roughly equal intervals around the bellmouth, should provide a pretty good indication of the IGV angle. But, how much different were the highest and lowest values at each measured angle?

When you "calibrated" IGV LVDT feedback, did you first document the as-found condition of calibration? If so, what was the difference in the before- and after calibration? In other words, how much out of calibration was the IGV LVDT feedback before the calibration?

 

Dear all,

after i read all post before, i think there's no wrong with your power plant, it's caused by the mechanical bias adjustment of servo valve. mechanical bias define how much oil flow to actuator and how much it drained. it might be your old servo drain too much oil than it should be..

actually you can do adjustment. but be careful, you need to know first the time response open-close of the actuator..

hope these help...

 

nanang,

The originator of this thread asked about an abnormally high power output sometime before or after a "wet wash". The Speedtronic turbine control panel, a Mark IV I believe, will not allow the actual exhaust temperature to exceed the calculated exhaust temperature reference/limit when at Base Load even if one or more of the servo-valves is not operating correctly. If the actual exhaust temperature is greater than the exhaust temperature reference/limit, then something else is wrong, and if it's more than 40 deg F above the reference/limit then the Speedtronic would trip the turbine.

The calculated exhaust temperature reference/limit is a function of compressor discharge pressure (CPD), and CPD is a function of IGV angle. The amount of current applied to the IGV servo is a function of the difference between the IGV reference and the IGV LVDT feedback. The originator said the IGV LVDT feedback has been accurately verified with a machinist's protractor measuring 8 of 64 IGVs, and likely using the average of the readings. It's presumed the IGV position during running is very close to the IGV reference, so that would mean that the mechanical null bias spring is likely adjusted properly and that the CPD feedback of the 96CD-1 transducer(s) is accurate (presuming the transducers are accurately calibrated).

Presuming that the IGVs are in the proper position at Base Load and the CPD transducer(s) are calibrated properly and the exhaust T/Cs are working properly, the Speedtronic will adjust the fuel control valve to make the exhaust temperature feedback equal to the exhaust temperature limit (reference) at Base Load.

I'm having a very difficult time understanding how a mis-adjusted servo-valve mechanical null bias spring can have an effect on the Base Load power output of a GE-design heavy duty gas turbine. At Base Load, the reference is exhaust temperature (TTRX) and for a properly running machine that should be TTRXP (the "primary" exhaust temperature control reference, the one derived from compressor discharge pressure). The Speedtronic will try to put as much fuel in the machine as possible to make the actual exhaust temperature equal to the calculated exhaust temperature reference, regardless of the adjustment of any servo-valve mechancial null bias spring. But even if the servo-valve weren't working properly, the Speedtronic won't allow any more fuel into the machine which would make the actual exhaust temperature greater than the exhaust temperature reference/limit, meaning that power would not be increased above the normal expected value due to a malfunctioning servo-valve mechanical null bias spring.

I guess we haven't asked the originator if TTXM was greater than TTRX when the power output was higher than expected, and if so by how much. Can you provide that information, HSB?

If you could help us to understand how a misadjusted servo-valve mechanical null bias spring would affect Base Load Power output, that would be very helpful.

It's great that you have the knowledge and ability to make adjustments to servo-valve mechanical null bias springs. We are all keen to know how to determine if a mechanical null bias spring requires adjustment, how to make such an adjustment, and how to verify if the adjustment has been made correctly.

Would you please answer the following questions, concisely and in simple terms that everyone can understand?

1) When do you recommend adjusting the mechanical null bias spring of a servo-valve? Specifically, what observed problems that can be directly attributed to an incorrectly adjusted mechanical null bias spring require adjustment of the spring?

2) Is there some method for checking to see if the mechanical null bias spring of a servo-valve is properly adjusted before making any adjustments? Would you recommend checking the adjustment of a servo-valve's mechanical null bias spring to see if it's correct before deciding if any adjustment is required?

2) What test equipment is required for the adjustment of servo-valve mechanical null bias springs?

3) What information is required for the adjustment of servo-valve mechanical null bias springs?

4) Where does one obtain the necessary information for the adjustment of servo-valve mechanical null bias springs?

5) What is the general outline of a procedure for adjusting servo-valve mechanical null bias springs?

6) Can you provide an example of a step-by-step procedure for adjusting servo-valve mechanical null bias springs?

7) How does one determine if the adjustment has been made correctly?

Thanks in advance!

 

Dear nanang,
I have forwarded all ur queries to our C&I cell, since i belong to opeartion dept.& mechanical stream so i am not very much confident about the answers. Thanku

 

What I understand is that under any given circumstances it is not normal for the actual exhaust temperature to overshoot the temperature control reference other than the frequency excursions with machine on rated load considering the fact that 25 & 40 degree F incremental values would mean an indication followed by a protective function.

What is confusing me is the magnitude of load that went up post wet washing and IGV cleaning. I would not deny that this could have happened but simply a wash and cleaning would not restore the output to this extent considering the fact that compressed air pressure went up by only 0.3 bar. Would this mean that the earlier operation was with a heavily fouled compressor and if so the requirement of a wash would had been adjudged by comparing the actual parameters at rated load vis a vis reference parameters recorded during a newly tried machine (Especially CPD, CTDA & TTXM).

As for IGV calibration, I understand that the CAGV values would be changing in response to the values entered for stoke check comparing the LVDT postion feed back and the indicator on the compressor casing (pl. mention if there is more to this). For the reduced temperature owing to a reduced load (that too is marginal) could be purely on account of higher mass flow since the wash would have been on a fairly fouled compressor. You could use the mnemonic PCT Load to check this but you can compare if you have the same data for pre-wash.

Frankly speaking, the logic of reduced CPD as compared to a pre-wash CPD is not clearly understood, I mean to say than what good did the wash do. The originator needs to be more clear in mentioning on this. Hope the frequency in each of the cases of comparison has been the same.