Firing temperature of Gas Turbine


Thread Starter


Dear sir's

What the 'Firing Temperature'(TTRF1) of a gas turbine indicates? weather the actual combusion temp or TIT (Turbine inlet temp)? as, the TIT is less than the 'combustion chamber' temp due to mixing of 1st stage nozzle cooling air.

Thanks in adv
In GE terminology, Firing temperature is the temperature just on the downstream of S1N. TIT on the other hand refers to the temp of the hot gases just upstream of the S1N. Thus, as per GE, the TIT is much more than the firing temp coz the S1N cooling air lowers the temp of gases to bring it down to Firing temperature. So, TTRF is the temp of the hot gases just downstream of S1N.

TTRF1 is the calculated value of "firing temperature", which in GE-speak is the temperature of the combustion gases at the trailing edge of the first-stage turbine nozzle. As GT MECHANIC says, air-cooled turbine nozzles decrease the temperature of the combustion gases entering the first-stage turbine nozzle, and what GE is trying to control or limit is the temperature of the combustion gases which are being directed on the first-stage turbine buckets.

TTRF1 is used as the variable in several COMParators which compare TTRF1 to setpoints to determine when DLN combustion modes should be switched during loading and unloading. There is no thermocouple or pyrometer or temperature sensing device (though laser devices have been tested by some companies) used on GE-design heavy-duty gas turbines to monitor the firing temperature--it's just a calculated value.

When all the inputs to the TTRF1 calculation are working correctly, it's a very good approximation of the temperature of the combustion gases at the trailing edge of the first-stage turbine nozzles. And, in a DLN combustor is proportional to the combustion gas temperatures in the combustors.

But, that's all TTRF1 is: a calculated value of the temperature of the combustion gases at the trailing edge of the first-stage turbine nozzle. And, usually, the only use for TTRF1 is to determine when to change combustion modes as a DLN combustor-equipped unit is loaded or unloaded.
The Temperature Control System will limit fuel
flow to the gas turbine to maintain internal operating temperatures within design limitations of turbine hot gas path parts. The highest temperature in the gas turbine occurs in the flame zone of the combustion chambers. The combustion gas in that zone is diluted by cooling air and flows into the turbine section through the first stage nozzle. The temperature of that gas as it exits the first stage nozzle is known as the “firing temperature” of the gas turbine; it is this temperature that must be limited by the control system.

From thermodynamic relationships, gas turbine
cycle performance calculations, and known site
conditions, firing temperature can be determined as a function of exhaust temperature and the pressure ratio across the turbine; the latter is determined from the measured compressor discharge pressure (CPD). The temperature control system is designed to measure and control turbine exhaust temperature rather than firing temperature because it is impractical to measure temperatures directly in the combustion chambers or at the turbine inlet. This indirect control of turbine firing temperature is made practical by utilizing known gas turbine aero– and thermo–dynamic characteristics and using those to bias the exhaust temperature signal, since the exhaust temperature alone is not a true indication of firing temperature.

Firing temperature can also be approximated as a
function of exhaust temperature and fuel flow (FSR) and as a function of exhaust temperature and generator output (DWATT). Either FSR or megawatt exhaust temperature control curves are used as back–up to the primary CPD–biased temperature control curve.The lines of constant firing temperature are used in the control system to limit gas turbine operating temperatures, while the constant exhaust temperature limit protects the exhaust system during start– up.

I'm working with 2 GE Frame 9E gas turbines. Recently during a planned shutdown, we conducted the offline wash and replaced all the air filters for both GTs. Everything seem to be working fine and the machines were able to reach base load as usual. However after about 5-6 hours, the CPD of one of the GT started to reduce followed by the loss of load. The second GT which undergone the same washing and filter replacement seems to be working fine.

My question is, what can be the reason if the CPD is low and the exhaust temperature is at maximum 565C and still the machine couldn't pick up load? Can it be caused by some defect in the fuel nozzles which in turn result in overburning? Im saying this because the rate of fuel gas entering the problem turbine is lesser than the normal turbine and the load is back to normal when the online washing is done.
Your problem is like related to some problem with the CPD transmitter(s) (most larger units have three CPD transmitters).

If you look at the CPD-biased exhaust temperature control curve, it has a negative slope, which means that as CPD decreases the exhaust temperature reference will increase, to what's known as the "isothermal" temperature value, which is near 565 C for many Frame 9Es.

So, if the CPD transmitter output decreases, the exhaust temperature reference will increase, and the load will decrease (which, has been noted elsewhere on, is not what one would expect, but it's the way it works).

So, use the Prevote Data Display to check the value(s) of CPD you will likely see that one or more of the feedback signals is low or decreasing.

Since this just occurred after a water wash, it's possible there is a valve problem or water someplace there shouldn't be.
Compressor rotating stall can affect both, CPD and exhaust temperature. CPD will decrease and exhaust temperature will increase due to air flow chocking by stall cell, and as a result the turbine load will decrease. on line washing may correct the problem because we can assume it as a type of surge/stall protections (inlet air injection) the washing water injection may work, couldn't it??
If an axial compressor experiences a stall condition, it's not very likely that it will keep running for very long, long enough to degrade performance and try to initiate on-line water washing.

There will likely be high vibration and eventually the compressor blades (rotating and stationary) will likely start to break off if it was allowed to continue to operate for very long.

If water was injected in to the compressor that would increase the mass flow through the compressor which would likely exacerbate the stall condition.

No compressor should be operated at or near any stall limit for any appreciable period of time. If a turbine control system is "detecting" and annunciating a stall, then the first thing to be done is to verify the condition of the instrumentation.