what is the relationship of exhaust setpoint with firing temp in case of 501f? GE has calculated TTRF (based on several para like CTIM CTD CPR) and m/c operates based on this calculation but confused with 501F as don't know relationship. Searched everywhere and no details about 501f Please help.

Thx

 

Hi there..

To put it simply... Bearing in mind my simple answer is not exhaustive, the exhaust setpoint is basically the reference for the firing temperature..

The fuel used to fire up the gas turbine, is controlled such that, for a particular moment in time, the amount of fuel use will produce a firing temperature that subsequently will produce an exhaust temperature that cannot exceed the exhaust temperature setpoint for that particular moment of operation,..

As mentioned GE uses the parameter TTRF, which stands for Turbine Temperature Reference Firing, which the turbine controller uses as a reference for the GE machine firing temperature.. At any moment in time for the unit load operation, the measured firing temperature cannot exceed the reference temperature given by the TTRF.

 

thanks, true but looking similar calculation of GE.

 

Quick correction: Calculated firing temperature in GE heavy duty gas turbine control systems is used for one thing--combustion mode changes ("switches"). When TTRF (or TTRF1) reaches certain values, that initiates combustion mode changes on units equipped with DLN combustion systems.

Calculated firing temperature is not (has not been) used for exhaust temperature control or combustion protection in GE heavy duty gas turbine control systems. It's simply a calculated value that seems to be intended to prevent others from easily reverse engineering the calculation of combustion mode changes.

One would presume if exhaust temperature is part of the exhaust temperature control scheme of 501F GTs that either axial compressor discharge pressure or "blade path temperature" is also used in the algorithm--but that's a SWAG.

And one would think it's also somehow visible in the sequencing/logic used in the control system (either the printout or the operator interface used to monitor unit operation). Again, this is also a SWAG.

One would also presume there would be some description or explanation in the unit or control system instruction manuals?

 

sorry to acknowledge very late.

Now I have more questions. Isn't TTRF1 is almost same as firing temp? how much we deviate? have seen different TTRF1 for 6E 6B n 7FA and i was believing that it is firing temp but now

Different control and mechanical persons reading this and if anyone would like to comment and explain difference between TTRF1 -combustion reference temperature and firing temp? how closely it is matching? and what about 501F whether Siemens or Mitsubishi how this exhaust temp setpoint generated using discharge pressure only?

 

GTT,

If all the inputs to the combustion reference temperature algorithm are working correctly, there's probably very little difference between actual firing temperature and the combustion temperature reference.

I believe that GE stopped calling TTRF/TTRF1 the "Firing Temperature Reference" and started calling it the Combustion Temperature Reference is because too many people were claiming their turbine wasn't making rated power (or was making more than rated power) because TTRF/TTRF1 didn't match the firing temperature rating of the machine. Since it's only used as the "switch" for combustion mode changes, it doesn't have to be exact.

Exhaust temperature control and firing temperature are only related in that the exhaust temperature control "curve" represents a constant firing temperature--and that firing temperature is determined from empirical data that compares exhaust temperature and axial compressor discharge temperature to known firing temperature (measured on machines with lots of temporary test equipment, or on combustors in a laboratory environment). Actual firing temperature is NOT measured on a GE-design heavy duty gas turbine; there is no instrumentation to do so. If there were, it wouldn't be necessary to use axial compressor discharge pressure to calculate an exhaust temperature control reference--the desired firing temperature would be calculated as a reference and the actual firing temperature would be used as the feedback, and fuel would be adjusted to make actual equal to reference and that would be that.<pre>

| Constant
| Firing Temperature
|******* /
TTXM | * /
or | * /
TTRX | *
| *
| *
------------------------
CPD</pre>
In the graph above, the negatively-sloped line represents a constant firing temperature that is achieved when the actual exhaust temperature (TTXM) equals the exhaust temperature control reference (TTRX) at a particular CPD (axial compressor discharge pressure).

When the unit is operating at Base Load, usually CPD is high enough to be to the right of the "corner" where the sloped line starts going down and to the right of the flat portion of the exhaust temperature control "curve." The IGVs must be at their maximum operating angle, and when the actual exhaust temperature for a given CPD equals the exhaust temperature reference then the unit is operating at Base Load. <b>AND</b> the firing temperature is very near rated. (Firing temperature is defined to be the temperature of the hot gases leaving the first stage turbine nozzles--<i>not</i> the temperature of the "flame" in the combustor(s).)

As long as the unit is operating at Base Load, as CPD changes throughout the day (and it does, with ambient temperature and -humidity) the Speedtronic will sense the change in CPD and change the exhaust temperature reference (TTRX) and change the fuel flow-rate to make the actual exhaust temperature (TTRX) equal to the exhaust temperature reference. Load will vary (as fuel flow varies). And CPD will vary as ambient temperature (and humidity) varies. But, the firing temperature will remain <b>constant.</b>

It's extremely important to note that firing temperature is <b>NOT</b> measured. Based on decades of empirical data GE knows that for the hot gas path components in that machine and that for the defined relationship between CPD and exhaust temperature that the firing temperature will be [x]. And they program the Speedtronic with the necessary parameters to maintain a constant firing temperature when operating at Base Load.

TTRF/TTRF1 was developed as a means to approximate combustion temperature, and was inadvertently called "firing temperature." In a DLN combustor combustion temperature is very close to actual firing temperature--but, firing temperature is a little bit lower than combustion/combustion reference temperature because of the cooling air from the first stage nozzles which enters the hot gas as it flows through the first-stage turbine nozzles.

TTRF/TTRF1 is <b>NOT</b> used as a reference for the total amount of fuel flowing into a GE-design heavy duty gas turbine equipped with DLN combustors. It's <b>ONLY</b> used to determine when to switch combustion modes. And as long as the unit is switching combustion modes without a problem, then TTRF/TTRF1 is working "good enough."

It's almost coincidental when TTRF/TTRF1 is equal to a machine's rated firing temperature. But, it's not used as a part of the total fuel flow-rate reference calculation. (It is sometimes used for DLN fuel splits, but not in every case, and not on every type of DLN combustion system--and there's a bunch of DLN combustion systems, isn't there?)

So, if you're thinking that TTRF/TTRF1 is somehow used for exhaust temperature control, you're wrong. I would concede that as a unit with DLN combustors is loaded and nears rated load with the IGVs below maximum operating angle that TTRF/TTRF1 and TTRX are sometimes equal or very nearly equal--but, again, that's almost coincidental. (By that I mean, the planets have to be in alignment.) Yes; it does happen on some units--but not every hour of every day of every year. Unless the site is obsessive/compulsive about keeping their instruments in calibration and their turbine control system out of alarm--and that's rare these days.

Most gas turbine manufacturers use a very similar method of calculating exhaust temperature control or limits. Almost no manufacturer actually measures firing temperature (yet; infrared sensors are being developed that still aren't reliable and capable of averaging the stratification that naturally occurs through turbine nozzles). So, if firing temperature (again, defined as the temperature of the gases leaving the first stage turbine nozzles) isn't actually known, <b>but</b> it can be very closely approximated using two parameters that are easily measured--then that's how most manufacturers are going to approximate firing temperature.

(Any sensor made today that could withstand the temperature of the hot combustion gases leaving the first stage turbine nozzle is not known to have a long life, and generally disintegrates (breaks apart) after a short period of time. Stray pieces of sensors striking moving turbine buckets tends to cause a LOT of damage. So, it's just not done--currently--except on test machines, and in combustion laboratories.)

I HOPE this answers your questions--at least about GE-design heavy duty gas turbines. I sense some frustration, and it shouldn't be so frustrating.

 

Thanks for nice explanation!!!

Good to know about TTRF/TTRF1 & firing temp coincidental. like the way GE has calculated.

Hope some 501 expert will shed some more lights as have seen several times firing curve changed and do not know how they come with new no