Hi All,

We have two westinghouse gas turbine of 50 MW each working in combined cycle mode. (driving 65 MW steam turbine). Ambient temperatures are same.

I understand that there is a direct relationship between CTD and CPD. Now both the machines have same CPD but CTD of one of the machine is 10-15 degrees higher and hence it loses 2-3 MW compared to the other machine.

--One of the hypothesis is that last section of compressor blades are fouled or rubbing which increases the discharge temperature.

--Second hypothesis is that back pressure of combustion turbine is causing the loss of load (which is beyond my understanding).

My question is what might be other reasons which are causing loss of load.

Thanx

 

Are you 100% certain the sensors measuring the CTD are installed identically in both machines?

Are you 100% certain the pressure transducers measuring the CPD of both units are calibrated identically?

Are you 100% certain that the amount of air entering the compressors of both units is exactly the same (inlet air filter cleanliness is identical; any variable inlet guide vanes are calibrated and operating identically; exhaust/boiler back-pressure on the units is identical; etc.)?

Because, I don't think you can ascribe a 2-3 MW power output differential to a 10-12 deg CTD differential. There are far too many factors affecting turbine performance of machines, even two "identical" machines sitting side-by-side, to try to pin a 2-3 MW differential on a 10-12 deg CTD differential alone.

Besides, a higher CTD would likely mean less fuel would be required for the same firing temperature. Some combustion turbines have "regenerators" which are used to heat the axial compressor discharge air with exhaust gases, just to reduce the amount of fuel required to achieve the same load, just like air heaters in a large boiler. Or feedwater heaters in a large boiler.

And rubbing compressor blades would likely cause some kind of noise and would quickly "rub out". And reduced clearances in compressors are usually better than increased clearances.

Something else is amiss.

 

Thnax for the reply

Sensors for CTD are identical in both machines and the PTs measuring the CPD are also identically calibrated. Flow transmitter before IGVs are also showing almost same value. IGVs are also operating identically. I am looking for the exhaust and boiler back pressure values and will let you know.

One another thing the temperatures after third turbine blades are also high on this unit and there is slight leakage of lube oil around the exhaust tunnel, its temperature increases beyond normal limits during startups while it remains high during operation.

 

Napster,

I didn't ask if the CTD sensors were identical. The question asked if the CTD sensors were <b>installed</b> identically.

I have very little experience with Circle-bar-W machines, but in other combustion turbines the sensors measuring axial compressor discharge temperature can be improperly installed causing differing readings. Sometimes the sensor tips are touching metal when they shouldn't be; sometimes they are not properly inserted into the thermowells.

Again, the output difference is likely NOT related to any of the issues you have described (CTD differences; L.O. leak). There is likely some mechanical internal difference (internal air leaks; problems with turbine nozzles; problems with blade tip seals; improper internal cooling; etc.) that's causing the difference in power output between the two machines.

This topic has been written about many times on control.com; there are many things that can cause two seemingly identical machines to have differing power outputs. The list is long. But, a 10 deg differential in axial compressor discharge temperature, which is most likely the result of some installation difference or wiring problem, is not one of them.

The high temperatures you are citing in the third stage exhaust area are probably due to some internal mechanical problem which may be related to the power differential.

Write back and let us know what you find.

 

Sorry for diverting the topic, but CSA can you please give some model nos. of machines using regenerators so that i can google them for further info.

Regards.

 

Hi

Investigations are still underway. I am making a report of the problem. It is in crude form now but will give a summary of actions and research been done.

(I will post a complete report when the solution is found.)

Scenario:
CPD(P2C) of two identical machines is same, while there is a difference of 10-15 F in the CTD (T2C). The machine with high T2C is generating 2 MW less than the machine with lower T2C.

Formula for compressor efficiency;<pre>
[( P2C/P1C)^(k-1/k) - 1]
Eff= ___________________________

[( T2C/T1C) -1]

Where P2C = compressor discharge pressure
P1C = compressor inlet pressure
T2C = compressor discharge temperature
T1C = compressor inlet temperature
K = 1.4
Units of pressure should be ‘psi’
Units of temperatures should be ‘Rankins’ (F+460)</pre>
Factors affecting the P2C:
Ambient conditions:
Both machines are located side by side and humidity, air density, temperature and pressure etc. are identical.

Air filters:
Cleanliness of air filter will affect the overall compressor efficiency; dirty filters will result in decrease in P2C and increase in T2C.

Evaporative coolers:
Again density and temperature of inlet air will affect overall efficiency; inefficient coolers will result in decrease in P2C and increase in T2C.

IGVs:
Modulation of IGVs affects the P2C directly. Difference in angle of IGVs in both machines will result in different values of T2C and P2C. However by physical inspection very slight difference was observed between the positions of IGVs in both machines.

Instruments:
Instruments measuring the values of P2C and P1C along with their calibration dates were checked. Their trends were also observed and showed no deflections or errors. Values of P2C and P1C were almost identical for both machines.

Proposed actions:
Based on above findings following conclusions can be made:

1) Issue with the sensitivity of T2C sensor of the machine showing higher value. (Being investigated)
Sensor = K type thermocouple (redundant pair available).

2) Issue with the last stages of the compressor blades. Fouling and reduced clearings due to thermal expansion in last stages can increase T2C while having a little effect on P2C.
(Refer: GT engineering handbook. Meherwan P Boyce)

Further Investigation:
Effect of Combustor back pressure and temperature on the P2C and T2C (Problem in the combustion or hot gas path resulting in the increase T2C and loss of load)

 

Frame 3 two-shaft (MS-3002)

 

> Frame 3 two-shaft (MS-3002)

Also Frame 5 two-shaft (MS-5002A and MS5002B). There are many more MS3002 regenerative units out there than MS5002.

 

Thanks a lot all you there for giving the model numbers.

 

this is the bad thing to revert the topic from CPD and CTD problem.....please share knowledge about that so that Napster may resolve his issue.