Reduced Gas Turbine Power Output

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Thread Starter

Kyle Gardner

My plant has experienced a drop off in power output over the last few months. I have compared all relevant data from different time periods. Compressor is not fouled etc. The only difference in all relevant parameters is reduced current output, which has resulted in a net loss of MW production. (P=V*I) the I variable is significantly lower. Everything else is the same. I am not an I&E Tech, I am a Control Room Operator... just trying to figure out what is wrong with our unit?
 
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bob peterson

I would suggest that the reduction in current you have noticed is the result of the reduced power output, and not vice versa.

--
Bob
 
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Bob Johnston

I still think that something is limiting the GT output rather than the Generator output. You say that you have checked against past data, are you bringing everything back to ISO to get a true comparison?

Compressor fouling is only one of the many things that could reduce output
 
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Kyle Gardner

Looks like we may have leak by from the compressor through our 11th stage AIr Extraction. If so this would be piped to Turbine Exhaust. It just doesn't seem like that would be enough to cause a power drop off. We're talking leak by at 80 - 90 psi. Our CPD is 174 psi. Normally its 150. Does this make any sense?
 
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Bob Johnston

If you had a Bleed Valve leak it could cause a drop off in output depending how big was the leak, but you should see a drop in CPD. 150 psi is about correct, are you sure all instruments are OK as 175 is high.Can you give us a bit more detail on your machine

(Model, age, maintenance history)
 
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Kyle Gardner

Well we rebuilt Bleed Heat Valve last Sunday. Not really leaking by and no change in MW output. We have a Bleed Heat System on this unit (GE Frame 6B) and we are thinking maybe it is leaking by. Infra Red shows some leak by. The big issue here is CPD is running at 180+ psi up from a norm of 150. Also Our Exhaust temp is down by a good 30-50 degrees. Restriction?we are shutting down again this weekend to check out Bleed Heat Leak By and also take a look in the HRSG to see if there is anything restricting Flow. Mass Flow is not up or down but the CPD is definitely an issue... calibrated it also last Sunday. Any thoughts? Drop off in power is at least 2 MW from 1 month ago.
 
Kyle,

this is a very interesting situation to say the least. Things just don't add up in my mind. If CPD is truly 20 psi higher than usual then I would not be looking at IBH valve or CBV's. They could not have this much effect on CPD if they wanted to.

The fact that CPD is up and exhaust temp is down is also interesting. It sounds like the normal path of combustion air is restricted. The fact that CPD is up and exhaust temp is down makes me think that there is less air entering the combustors but a larger volume than normal getting through to the exhaust.

Have you noticed decreased wheelspace temperatures?

I wouldn't suspect anything from the HRSG. Typically exhaust duct pressures run in the 10inH2O area, any restriction of the HRSG should raise that quickly, and most logic code I have seen trips the unit at around 20inH20.

I would want to boroscope the unit. I would be looking strongly at the aft section of the compressor around the discharge of the last stage. It sure seems like the compressor is developing pressure, but its not getting to the combustors in the volume it should be. Also wondering if you are seeing any change in your normall emission concentrations?

Please keep us informed as to what is found, very curious!
 
> Mass Flow is not up or down but the CPD is definitely an issue... calibrated it also last Sunday.
> Any thoughts? Drop off in power is at least 2 MW from 1 month ago.

Hi Kyle,

You wrote that CPD transmitters were calibrated and are OK but you see on <I>/HMI apr 180psi instead of 150psi. At the same time you observe TTXM dropped and MW output reduced.

Below are my hypothesis:

The reading of CPD 180psi is unrealistic high.
It seems that the scale in Speedtronic for CPD transmitters does not match the calibration range of the installed transmitters.
On some systems, the Speedtronic input has settings: offset: 0V and gain 60psi/volts so transmitter should be calibrated 0...5V/0...300psi. On some newer systems GE changed the calibration range for the transmitter and it is 4...20mA/0...300psi so the Speedtronic input should have settings: offset 1V and gain 75psi/volt and resistor 250 Ohms included in the loop by inserting the proper jumper.

With turbine running close to Base Load the correct CPD readings should be around 150psi what should produce 12mA and should give 3V drop on the 250 Ohm resistor at the Speedtronic input. This 3V converted to psi in Speedtronic with wrong scale setup (offset 0V and gain 60psi/V) will give false reading 180psi instead of 150psi.

As consequence of this wrong, high CPD readings, compressor pressure ratio (CPR) calculated by Speedtronic is extremely high what as result limits TTXM by reducing FSR what as result gives reduced power output.

It seems that transmitters are calibrated 4...20mA / 0...300psi and scale in Speedtronic instead to be: offset 1V (actually, GE has setup 1.0005) and gain 75psi/V it is setup: offset 0V and gain 60psi/V.

I suggest to confirm that scale setup in Seedtronic for CPD transmitters match with the calibration range of the transmitters.

Regards
 
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Kyle Gardner

Actually our tech did not calibrate transmitters... This is what he did:
"We applied known calibration gas pressure to the CPD sensing line and verified that the transmitters were sending the corresponding signal to the MKIV controls at 0-200psi in 50psi increments."

Would you describe this as accurate enough?
btw Our transmitters 96cd-1, 1b & 1c are Rosemount 0-5 volts 300psi

thanks
 
> "We applied known calibration gas pressure to the CPD sensing line and
> verified that the transmitters were sending the corresponding signal to the
> MKIV controls at 0-200psi in 50psi increments


Hi,

Writing my suggestions about the possible error in Speedtronic settings for pressure transmitters I assumed that this is MKV: one setting is common for three transmitters.
In your reply you stated: ' ...MKIV '. Is this typo?

Verification done by your IC dept. are sufficiently accurate if applying the pressure, readings were taken also on HMI, not only looking at mA or voltage output from CPD transmitters.

Anyhow, with turbine running, in order to prove that readings of CPD are correct (if no local dial gauge is available) compare them with CPBH1 (upstream pressure on the IBH valve). Readings should be the same.

Maybe IGV blades are opened more than indicated on HMI. LVDT sensor is fixed to the actuator not on the IGV blades. Stroke IGV and measure the angle by protractor and verify for backlash of blades and driving mechanism.

As writes MIKEVI, leaks through IBH or CB valves can reduce output but can not increase CPD.

As asked Bob above: do you bring ambient parameters to ISO making your comparison?

Was this reduced power output and increased CPD observed as gradual process or it was a jump?
Were done any maintenance jobs on any of the turbine systems before the issue appeared?

Regards
 
When was the last off line Water Wash of your compressor?

The CPD increment affects the Exhaust Temp, reducing it, in accordance whit the Temperature Control Curve, In your Control Specifications.
This can be a compressor fouling trouble.
 
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