I am an operator of GE 9E gas turbine and want to know why the temperature reference ttrxb while starting the unit and after synch is constant about 610 c? Related to thermodynamic relations it shouldn't be constant while firing temperature changes? and after transferring to temp control it decreases gradually until it equals ttxm. what are the parameters affecting ttrxb before reaching temp control and after that??

 

Turboget,

There have been many posts about exhaust temperature control on control.com. There is a cleverly-hidden 'Search' field at the far right of the Menu bar of every control.com webpage. (It's recommended to use the Help function to learn the syntax of search entries.)

TTRXB is <b>NOT</b> Base Load Exhaust Temperature Control--contrary to popular belief. Is is Speed-Biased Exhaust Temperature Control, used mostly only for two-shaft GE-design heavy duty gas turbines. It just happens that when the speed bias is not enable, TTRXB is exactly the same as TTRX, which <b>IS</b> Exhaust Temperature Control Reference.

The exhaust temperature control reference is actually a limit on the maximum allowable exhaust temperature for any operating condition. And, the limit is a straight line, or sometimes two or more straight lines, with negative slopes. The maximum allowable exhaust temperature for ANY operating condition is what's termed the Isothermal Limit, TTKn_I, or TTKI_[n]. Usually that value is approximately 1100 deg F for most GE-design Frame 9E heavy duty gas turbines. When CPD (or CPR for newer units) exceeds TTKn_C, or TTKC_[n], the exhaust temperature value begins to decrease from TTKn_I (TTKI_[n]).

So, for all operating conditions up to TTK0_C, or TTKC_[0], the maximum allowable exhaust temperature will be TTK0_I, or TTKI_[n]. And, then TTRX will start to decrease as CPD (or CPR for newer units) increases.

Actually, TTXM is increased until it equals TTRX as the unit is loaded. For units with IGV Exhaust Temperature Control (including DLN combustor-equipped units), the IGVs are held closed to maximum exhaust temperature (but never above TTRX), and when the IGVs are at maximum operating angle (CSKGVMAX) and TTXM equal TTRX the unit is deemed to be on Exhaust Temperature Control (CPD-, or CPR-, biased exhaust temperature control).

Hope this helps!

 

> what are the parameters affecting ttrxb before reaching temp
control and after that??

TTRXB>>>>>>>>Speed biased temp control reference.

C^2

 

I am very thankful for you csa for your perfect answer. if you please I still have some confusion. when the unit reached temp control, do the controller try to make firing temp const?? Does If cpd increase or decrease due to changing in ambient like in summer and winter, firing temp will change??

 

Turboget,

The sloped portion of the exhaust temperature control "curve" represents constant firing temperature. So, when the unit is operating at Base Load (on the curve with the IGVs at maximum operating angle) firing temperature is constant. For a GE-design Frame 9E heavy duty gas turbine, firing temperature is the temperature of the hot gases leaving the first stage turbine nozzle.

As ambient temperature changes CPD (and CPR) will also vary, which will cause TTRX to vary--but firing temperature will remain constant (when operating at Base Load, with the IGVs at maximum operating angle).

 

Yes it does, you missed this bit in CSA's answer "When CPD (or CPR for newer units) exceeds TTKn_C, or TTKC_[n], the exhaust temperature value begins to decrease from TTKn_I (TTKI_[n]).

So, for all operating conditions up to TTK0_C, or TTKC_[0], the maximum allowable exhaust temperature will be TTK0_I, or TTKI_[n]. And, then TTRX will start to decrease as CPD (or CPR for newer units) increases."

So,as CPD changes, TTRX changes trying to linearise Firing Temp. Remember as CPD changes, cooling air flows will change and slightly change the relationship between Exhaust Temp. and Firing Temp.

 

Thank you very much CSA, Glenmorangie for your reply and your time.

but i hope you patient with my questions. in our plant we have 8 units, 4 with standard combustion and the other 4 with dln upgrade. already igv control is different from standard to dln. in dln it's atemp control. so in state of preselect load suppose 100Mw igv closes to roughly 60 trying to make ttxm very close to ttrx, with comparison to standard we find igv fully open 84 at the same load 100mw.

so the question is how the turbine can give the same load at standard and dln while the mass flow of air is less in dln? i think in order to make this fuel consumption in dln should be more than standard at the same load in order to make up the effect of igv closing. because work from turbine = m*cp*delta T (ttrf-ttxm), ttrf in dln at the same load should be more higher to make delta T higher.

So our gentlemen is my perception about this point as i explain above right??

 

Turboget,

The IGVs are used for very different purposes on the two types of machines--those with conventional combustors and those with DLN combustors. It's incorrect to assume GE is not correctly controlling the two types of GE-design Frame 9E heavy duty gas turbines simply because the IGVs appear to operate differently.

Ge-design Frame 9E heavy duty gas turbines with conventional combustors may use the IGVs to maximize exhaust temperature for steam production at part load, while those with DLN combustors use the IGVs to control air flow to maximize the load range available in Premix Steady State combustion mode.

In reality, the IGV control schemes are very similar when being used to maximize exhaust temperature in GE-design Frame 9E machines--but because of the very different types of combustors it can appear to be different.

But, again: It's incorrect to assume GE is not aware of the different requirements of the two types of combustion systems used on Frame 9Es and is not properly controlling the IGVs for each type of combustor because the operation appears to be different.

 

Thanks a lot csa i am really thankful for you