Hi all,
following the CI by almost month we started getting high spread alarm mainly on spread1. To fix the situation we replaced all suspected TC and at the same time we checked CPD transmitter calibration as shown below:<pre>
ISS data:
0 psi o VDC
200 psi 5 VDC

As found readings:
0 psi 1.23 VDC
200 psi 6.32 VDC

After calibration
0 psi 0.85 VDC
200 psi 5.64 VDC</pre>
do you agree with me that the as found readings was letting the combustion to consume more fuel in order to compensate the faulty CPD readings?

then the whole engine eff will go down

Note: our machine is GTG frame6b with Mark IV control

 

First, did replacing the suspect T/Cs resolve the spread condition?

Second, what kind of transmitter is being used to sense CPD: one with a 0-5 VDC output, or one with a 4-20 mA output?

Third, was the power output <b>at Base Load</b> with the 'as-found' calibration values higher or lower than rated? Please quantify any power output differential at Base Load when the unit was running with the 'as-found' values.

Fourth, was the power output at <b>Base Load</b> with the 'after calibration' values higher or lower than rated? Please quantify any power output differential at Base Load when the unit was running with the 'after calibration' values.

Fifth, how is efficiency related to a discussion of exhaust temperature spreads caused by unusual CPD transmitter calibration values?

Sixth, what does ISS stand for?

Seventh, and last, how do you think the Mark IV will respond to CPD feedback which is higher than expected? Can you quantify your thought on the matter of CPD feedback being higher than expected for a given operating condition (say, Base Load)?

Thanks!

 

> First, did replacing the suspect T/Cs resolve the spread condition?
Answer: no

> Second, what kind of transmitter is being used to sense CPD: one with a 0-5
> VDC output, or one with a 4-20 mA output?
Answer: 0-5 VDC

> Third, was the power output <b> at Base Load</b> with the 'as-found' calibration
> values higher or lower than rated? Please quantify any power output
> differential at Base Load when the unit was running with the 'as-found' values.
Answer: lower than rated by 4MW

> Fourth, was the power output at <b> Base Load</b> with the 'after calibration'
> values higher or lower than rated? Please quantify any power output
> differential at Base Load when the unit was running with the 'after calibration' values.
Answer: far away from rated, lower by 14MW

> Fifth, how is efficiency related to a discussion of exhaust temperature
> spreads caused by unusual CPD transmitter calibration values?
Answer: eff is dropping

> Sixth, what does ISS stand for?
Answer: instrument specification sheet

> Seventh, and last, how do you think the Mark IV will respond to CPD feedback
> which is higher than expected? Can you quantify your thought on the matter of
> CPD feedback being higher than expected for a given operating condition (say, Base Load)?
Answer: mark IV reading is higher than another local gauge. Therefore, the fuel ration/flow rate will try to balance with Mark IV CPD readings.

 

First, if you are finding "burning" of nozzle components, then replacing exhaust T/Cs probably wouldn't resolve the spread problem.

Second, if the transmitter is 0-5 VDC, why can't you adjust the zero down to 0.0 VDC and the span (gain) to 5.0 VDC? How did the transmitter come to be calibrated thusly and why can't it be calibrated properly?

The odd thing here is that the span (range) for both the 'as-found' and 'after calibration' values is approximately 5 VDC, but for the Mark IV to work properly (assuming there's no "offset" in the Mark IV) it needs to be 0-5 VDC, not 1.5-6.5, or 0.6-5.5, but 0-5 VDC. Because most Mark IVs expect 0.0 VDC to be equal to 0.0 psig and 5.0 VDC to be equal to 200 psig (in your case and in many cases, actually). So, something seems amiss. I don't recall the Mark IV having an offset and a gain, only a gain, and that gain for this application would have been 40 psig/VDC. So, if the actual CPD was 120 psig and the CPD transmitter was calibrated properly, then the output of the transmitter (and the input to the Mark IV) should be 3.0 VDC.

Third & Fourth, I would have expected exactly the opposite--that output was much lower with a highly elevated CPD feedback, and a slightly lower output with a less elevated CPD feedback.

So, hmmmmm.....

Fifth, I don't agree that efficiency is the problem here. Sure, if the machine is at part load when it should be at rated load then extra work is being lost to compression, but the fuel flow will be lower, also, at Base Load, and when I think of gas turbine efficiency I think of heat rate and I don't know how an elevated CPD feedback value would affect heat rate.

Sixth, thanks. I've never heard a Device Summary called an ISS before. (Maybe that's what NP called the "Device Summary"????)

Seventh, in your original posting you seemed to be implying that by having an elevated CPD feedback the unit would suffer damage because of over-firing (too much fuel). But, I would believe that an elevated CPD feedback (i.e., CPD feedback greater than actual) will cause power output to be reduced when the unit is being operated at Base Load (CPD-biased exhaust temperature control). And the data you provided seems to agree.

One word of caution: Do not try calibrating any transmitter being used to supply a Mark IV (or Mark V) on a "bench" with a standard power supply. You can get it perfect on the bench, but when you connect it to the Mark IV (or Mark V) there will be a difference that will need to be compensated for or adjusted out. It's just easier to calibrate the transmitter in place being powered by the Mark IV (or Mark V). yes, you have to drag a pressure source and fittings out to the unit, but you'll probably have to do that anyway to get rid of the error. So why not just recognize that bench calibration isn't going to work well and do it one time at the device while it's being powered by the Mark IV (or Mark V)?

 

I agree with what you are saying/replying to question but I am reluctant to accept the fact of bench calibration and field calibration. Again I am not arguing but it seems weird although I have never worked on GE so I may be completely wrong but I understand that every Transmitter/instrument functions properly with in acceptable voltage/power range. Why it needs to be calibrated in Field? Does it mean that in fact they have an issue with the electronic module/system which is supplying voltage/power to the device? Just curious.

Thanks for your immense help to control.com community and others.