Dear Sir,
We are operation two GE 7FA DLN 2.6 units

When checking the daily log I found almost 10 DEGC difference in CTD (compressor discharge temperature) between two units

Here are the data of GT#01 & GT#02

GT#1

MW# 159, CTD#405.42 degc, CPD# 14.528 Psig, Exhaust mass flow# 425.33 Kg/sec

GT#02

MW# 162.94 MW, CTD# 396.67 degc, CPD# 14.587 Psig, Exhaust mass flow# 427.75 Kg/sec

I wonder as per the gas laws, if your CPD is on higher side your CTD also proportionally Increase. but you can see the above data, GT#01 CPD is less than GT#02 but still GT#01 CTD is 10 degc higher than GT#02.

I have put the trend as per the trend I observed that when exhaust mass flow on higher side CTD reduces & when exhaust mass flow lower side, CTD tends to increase

Could you please advice

Regards
KKK

 

My first guess would be that GT#02 IGV's are slightly more open than GT#01.

 

I would attribute the very slight (10 deg C) differential in temperatures to differences in how the CTD T/Cs are inserted into the machines. I would venture a guess that if T/C "rakes" were used in the compressor discharge of both machines that the differential would be much less than that presuming both machines were running at the same load (Base Load) and the IGVs were both open to the same physical angle (i.e., the IGV LVDTs were accurately calibrated for both machines, which, is in itself highly unlikely for any two machines), and that the mechanical conditions of the two machines were virtually identical (meaning that the hot gas path components were roughly of the same vintage, same part numbers, the compressors were both similarly clean (or dirty, as the case may be), the inlet filter differential pressure drop was the same for both machines, and the exhaust duct back pressure was the same for both machines).

There are a lot of variables that affect compressor performance, and measuring the parameters is one of them. Inserting T/Cs into GE-design heavy duty Frame gas turbines is not an easy task, and to do it exactly the same on both machines would be very unusual.

And, as mentioned above, for the two machines to have exactly the same operation parameters they would have to be physically identical, down to compressor cleanliness and exhaust duct back pressure and hot gas path components and inlet filter condition (cleanliness). And, that just usually isn't the case, even for two machines sitting side by side, since the inspection outages are usually staggered somewhat.

Lastly, trying to link one small parameter differential to machine performance, especially on a machine equipped with DLN combustors, is very difficult. Because of the way the IGVs are used during loading and unloading of a DLN 2.x machine, and because of all of the other conditions mentioned above, and with such a small differential as mentioned, it's very difficult.

I wouldn't even try to relate mass flow and performance on machines with DLN combustors to one single parameter like CTD. There are just too many other variables which can cause slight differences. Not to mention the T/C insertion issue (even from the factory).

Lastly, machines are machines. You can have two identical machines, built at the same time using the same components, and they will not behave exactly the same--not even during initial operation. There are just too many variables in machines as complex as these to cause two machines to behave identically, and especially over time as the machines undergo maintenance and repair.

There may be some mathematical formulae that could be used to explain CTD as a function of CPD and mass flow, but to relate two machines on this one parameter, well, I don't even think physicists would try that without being to control every variable. Which would be prohibitive.

 

Dear CSA,

As per the gas laws when the pressure increases the temperature will also increase, in Gas turbine compressor. the CPD (compressor discharge pressure) & CTD (Compressor discharge temperature) would behave proportionally i.e CPD increases. CTD will also increase. over a period of time, compressor fouls hence CPD will fall. I wonder why CTD increased in fouled compressor ( i.e not following proportional ratio )

Please help me to understand the cause

best regards
KKK

 

KKK,

Axial compressors are odd beasts; they don't always behave as expected.

As anyone who follows my posts here on control.com knows and will attest to, I am not about the maths of most processes. So, I'm not going to try to pull out my university thermodynamics texts to analyze the situation.

In my experience, as axial compressors get fouled (dirty) the amount of air flowing through the compressors is reduced, and the temperature of the discharge air increases. My personal theory is that the fouled blades (stationary and rotating) create turbulence that increases the temperature of the air. The rotor spins at the same speed (presuming a stable grid frequency) and the work being done actually increases even though the air flow decreases, and some of that energy goes into increased compressor discharge air temperature. And 10 deg out of 400 is a very small percentage increase.

While it's difficult for many novice- and managerial personnel to understand and accept, two "identical" machines installed at the same time and operated similarly <b>will NOT</b> operate identically. Many parameters, including power output and efficiency, will be slightly different. And, for this particular parameter differences in T/C insertion can cause the differential you previously cited.

There are just too many intangibles in machines of this size for them to operate with identical parameters--and outputs--at all times. That's just a fact.