Delta T calculation in gas turbines

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Thread Starter

njagan2001

We have mark V controlled frame 5 GE gas turbines. Can anybody tell me how the exhaut delta T is calculated and how the exhaust temp set point varies with load. And also what is the relation between Compressor discharge pressure and exhaust temp?
 
Have you searched control.com for "exhaust temperature control"? Try http://www.control.com/1026226909/index_html for one. There are some other good threads on that subject, too. If you still need clarification, ask your specific questions here.

By "delta T" it is presumed you are referring to exhaust temperature spreads. If you have a GE-packaged turbine, you will have some very good information in the Instruction Books provided with the units--in the 'Control and Protection Systems' tab.

Reading BBLs (Big Block Language blocks, or algorithms) can be a daunting task at first, but if you're responsible for the troubleshooting and maintenance of a gas turbine equipped with Speedtronic turbine control systems, well, there's no time like the present to begin.

The function you are referring to is commonly called the "combustion monitor", which looks at exhaust temperature differentials and checks to see if the hottest- and coldest temperatures are adjacent, or close to each other--indicating a problem in one or more of the combustors or the hot gas path parts (combustion liners, transition pieces, side seals, nozzles, etc.).

A hint: look for logic signals L60SP1, -2, -3, -4, -5, and -6. See how those signals are determined and what they do in the rung which drives L86TXT (the typical CDB (Control Signal Database) pointname for the logic which trips the turbine on high exhaust temperature spreads.

If you have questions about the BBL or the sequencing, we'd be happy to help. Many people could benefit from the exercise.

Happy hunting!

markvguy
 
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Rahul P Sharma

Exhaust Temp Control aims to maintain a Constant Firing Temperature - Repeat A Constant Firing Temperature...

Now GE might have researched and emperically determined that to achieve this objective, the Exhaust Temp Setpoint must be decreased Linearly with an increasing PCD (The Compressor Discharge Pressure).... How did they arrive at this conclusion, as a user, it's beyond our scope to know.... But it was observed that if the Exhaust Temperature Setpoint is linearly reduced with an increasing PCD value, it's possible to achieve A Constant Firing Temperature...

So a linear relationship of the form
Exhaust Temp (T) = -m*(PCD) + C was established.

Now say, if the ambient temp falls, the PCD will increase, hence the Exhaust Temp Setpoint MUST decrease as per the above relationship inorder to MAINTAIN A Constant Firing Temperature....!! This topic was discussed earlier too, and I've understood this fact a hard way...

au revoir
Rahul
 
Good to see you back and contributing, Rahul!

Only thing is, you don't sound convinced at all. You correctly say the right things, but it's obvious you're not convinced. And it's not beyond the ability of anyone to comprehend or understand--it's not "smoke and mirrors." It just doesn't seem to follow what one would presume would happen.

So, here comes "the rest of the story..." which may help you understand what's going on.

When the unit is being operated at a constant "firing" temperature (which is defined as combustion gas temperature at the first-stage nozzle--not flame temperature in the combustor), and the ambient temperature increases which causes air density to decrease, the mass flow of air through the compressor and turbine decreases. When the air flow through the unit decreases two things happen: first, the compressor discharge pressure decreases. Second, the exhaust temperature increases. That's right--with less air flowing into the combustor through the cooling and dilution slots and vents in the body of the combustor, there is less of a cooling and dilution effect on the combustion gases, and therefore on the exhaust temperature.

The opposite happens when the ambient temperature decreases. The air density increases, the mass flow through the unit increases, the compressor discharge pressure increases, and the net effect of the increased air flow is to decrease exhaust temperature.

Again, this is all true when the unit is being operated on CPD(CPR)-biased exhaust temperature control--which, is, exactly as you said, to maintain a constant "firing" temperature.

Now, to really confuse the issue--when CPD goes down as the unit is being operated on exhaust temperature control fuel is actually reduced--very slightly, but it is reduced. Less air means a higher "firing" temperature if fuel were to be held constant, so fuel is decreased as CPD decreases. And the opposite happens when CPD increases while the unit is being operated at a constant "firing" temperature.

If one were to monitor operation of a gas turbine-generator while it is at part load on droop speed control (i.e., NOT while on Preselected Load Control or Automatic Governor Control or External Load Control--just plain old droop speed control) at a steady turbine speed reference which corresponds to a relatively constant fuel flow-rate, one would see that during the day as ambient conditions changed the power output of the unit would change as well, and so would the exhaust temperature and CPD.

markvguy
 
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