Hai all,

I read that GT base load operation is based on the primary operating curve i.e. CPD biased Exhaust temperature control set point(TTRXP) and TTRXP>TTRX. Both statements are conflicting. TTRXP is from primary operating curve, TTRX is calculated set point from existing conditions of the machine. So, Which one is the actual operating reference set point to the FSR to maintain Gas turbine exhaust temperature at base load operation.

any body can help me,
Waiting for reply

 

The only written reference that you should trust when trying to understand GE heavy duty gas turbine control for your machine is the sequencing or application code running in the Speedtronic turbine control panel at your site. The sequencing or application code running in the Speedtronic panel at your site is the definition of how your turbine operates.

TTRX should be the minimum selected value of either TTRXP (the primary exhaust temperature control reference, usually CPD-biased) or TTRXS (the secondary exhaust temperature control reference, either FSR- or load-biased).

The blocks (algorithms) used in Mark IV, Mark V, Mark VI, and most Mark VIe Speedtronic turbine control panels are depicted using graphical elements in a kind of relay ladder fashion. Some of the blocks in later Mark VI and Mark VIe systems use virtually the same descriptions, but available through the 'Block Help' function (right-click on the block and select 'Block Help' and a Help screen will usually pop up).

It's been said many times before on control.com: Redraw the block or the rung in a manner that you can understand, one element at a time, and usually the function will quickly become very apparent. Just follow one signal at a time and draw it on a separate sheet of paper, adding other signals as necessary.

I'm going to refer to the block TTRXV5; the other versions should be similar. Working backwards from TTRX, you should be able to see the main input to the calculation of TTRX is an internal signal (internal to the TTRXV5 block), ttrmin. Working backwards from ttrmin, you will see that it is the output of a MIN SEL gate of with the two main inputs are TTRXP and TTRXS.

Now, if one doesn't like reading blocks, then straight reasoning should work fine for understanding this. There are two exhaust temperature control references: the primary- and secondary references (TTRXP and TTRXS, respectively). Only one is to be controlling the exhaust temperature reference at a time, though both are being calculated at all times. So, there has to be some selection method. A minimum select function makes sense here since we are trying to limit the *firing temperature* (which is what CPD- or FSR- or load-biased exhaust temperature control is really doing).

 

Thanks for your immediate response Mr. CSA and really I appreciate your explanation.

Sir, please help me to clarify the following items,
At part load operation:

Exhaust temperature is controlled by the IGV and speed/Droop is controlled by the FSR (FSRN).FSRN will increase while increasing the GT load and FSRT will drop. While part load operation how the FSRN will modulate to maintain the constant speed (according to Grid frequency) irrespective of the load?

How the exhaust temperature reference set point will come to the IGV to maintain the exhaust temperature at this part load. Normally to maintain this exhaust temperature, IGV will modulate by the MW correction curves and Compressor inlet temp. correction curves and feed back of the actual exhaust temperature (TTRXM). But how the Exhaust temperature reference set point will generate to IGV at part load?

Is TTRXS useful for exhaust temperature control reference set point to IGV at part load operation?

At base load operation:

Is TTRXS calculation only useful for back up? if CPD transducer fails. What is the purpose of back up controller (TTRXS) for DLN machines?

If 1. TTRXISO >TTR_MIN

2. TTRXP>TTRXS (Normally it will not happen, if all systems and controller constants are Ok)

3. IGV full open

If above 3 conditions are satisfied, TTRXS as the TTR_MIN to operate the FSR ?

This condition is advisable to operate the Machine?

Waiting here for your reply

 

Dev,

So many questions and assumed relationships.

You failed to tell us that the unit(s) at your site has(have) DLN combustors; it makes something of a difference when responding to questions.

When a unit is operating at Base Load on CPD- (CPR-) biased exhaust temperature control, the IGVs should be at their maximum operating angle. By definition, Base Load is considered to be operation with the IGVs full open, so they should not be modulated when the unit is operating at Base Load.

On DLN units, the IGVs are used to control air flow through the machine and into the combustors. However, there is a limit to how much the IGVs can be closed, and that limit is the exhaust temperature control reference, TTRX. You should never see the IGVs closed such that TTXM is greater than TTRX. The IGVs should never close such that the exhaust temperature exceeds the exhaust temperature reference at any load.

Again, on a DLN machine the IGVs aren't being used to control exhaust temperature (though it appears that way), they are being used to control air flow but the way the Speedtronic is programmed is that the air flow is basically known for given values of exhaust temperature so the IGVs are modulated as a function of exhaust temperature in order to control air flow. Kind of complicated, but on a DLN machine, that's how it works.

On non-DLN machines, the IGVs are used to control exhaust temperature during Part Load to maximize exhaust temperature for combined cycle applications. So they are basically held closed as much as possible to maximize exhaust temperature, but still only up to the exhaust temperature reference and not any higher.

On DLN machines, you are correct; if the CPD transducers fail (there should be more than one!) then the unit will be tripped and the back-up exhaust temperature control is really not ever used, except as something of a protection function should there be problems with CPD feedback or other parameters. On DLN machines, back-up exhaust temperature contro, when programmed properly, represents a kind of "last line of defense" for normal operation, but, as you say, it can't be used in the event of loss of CPD feedback to keep the unit running. (In units with conventional combustors, in the event of loss of CPD feedback the unit could continue to run on TTRXS.) But, TTRXS and TTRXP are feeding the min select block for TTRX all the time.

Exhaust temperature reference is always being calculated, all the time, even when the unit isn't running on exhaust temperature control (when the IGVs are not fully open). There is a minimum select block that chooses the lesser of the various FSR inputs, including FSRN and FSRT.

I don't understand "MW correction curves".... Not familiar with them at all.

Droop Speed Control has been covered ad nauseum on control.com. Use the 'Search' feature of control.com to find lots of explanations of how the turbine speed in controlled when operating on Droop Speed Control. Suffice it to say that the Speedtronic is not controlling turbine speed when it's connected to a grid in parallel with other machines.

I don't understand the last part of your question at all, the three "conditions". On most Speedtronic panels supplied with DLN machines there is a process alarm to notify the operator of the condition when TTRXS is less than TTRXP (or when TTRXP is greater than TTRXS). It's something like 'Back-up Exh Temp Control Active'; I don't recall the text message exactly. But, it's purpose is to tell the operators and technicians that something is amiss with the operation of the machine. Because, presuming the back-up exhaust temperature control parameters are calculated correctly, the primary exhaust temperature control reference (TTRXP) should *always* be less than TTRXS, the back-up exhaust temperature control reference. Basically, if the unit is operating on back-up exhaust temperature control reference, then it's probably not producing as much power as it might be capable of.

Or, there's something amiss with the calibration of the CPD transducers. Or, possibly, the IGV LVDT feedback isn't calibrated properly.

But, something's not quite right, and, no, it's not normal to run the unit for extended periods of time on TTRXS. The condition that has resulted in TTRXP being greater than TTRXS should be investigated and resolved. It shouldn't hurt the machine to run it on the back-up exhaust temperature control reference curve, but it's not the intended reference and it indicates something is not right.

Hope this helps!

 

Dear CSA,

Please clarify the following statement

This is your statement given on older post that

"on a DLN machine the IGVs aren't
being used to control exhaust
temperature (though it appears that
way), they are being used to control air
flow but the way the Speedtronic is
programmed is that the air flow is
basically known for given values of
exhaust temperature"

I am talking about the Part load operation only. At base load operation IGV will full open,there is no air flow control at this operation. Now our discussion is for part load operation which is at IGV not full open. I am working in DLN machine.

1. As per your above post, IGV will controls the air flow for DLN machines for a given value of exhaust temperature. What is the criteria to change the exhaust temperature(Value giving by the speedtronics) while increase the GT load.

2. How the speedtronics will generate exhaust temperature set point.
TTRX value is based on compressor discharge pressure. without opening the IGV,compressor discharge pressure will not change. So,TTRX value will not change.

While increase the GT load, how the speedtronic will generate exhaust temperature set point which is command to operate the IGV. After received the command, IGV will start to open which increases the compressor discharge pressure cause to drop TTRX value. all these are consequences after receive the command from speedtronics only.

I think, "increase the air flow in DLN machines" or "increase the IGV opening in non DLN machines" are to maintain the exhaust temperature only. Even though the way of explanation is different.

Waiting here for your reply

 

Dear all,

Ok, for DLN combustion machines, amount air flow to the combustion chamber also very important to maintain the pre-mix operation at any load condition to maintain the lower emission levels without effecting the flame intensity and also to maintain the exhaust temperature

for Non DLN machines, IGV opening is to maintain the exhaust temperature without effecting the flame intensity.

but normally Gas turbine IGV operation is seems to maintain the exhaust temperature only.

 

As was said before, the IGVs "seem" to control exhaust temperature in DLN machines, but really the exhaust temperature control is the factor which controls the IGV angle, and the designers know (approximately) what the air flow is at various IGV angles so that's how they "control" the fuel/air mixture at various loads/IGV angles.

For non-DLN machines, IGVs are used in combined cycle applications to maximize exhaust temperature to maximize steam production at low loads when the exhaust temperature would typically be low. In non-DLN machines, there is always "excess" combustion air and a lot of the air entering the liner does so through cooling and dilution slots, not through the fuel nozzle end of the combustor. So, fuel/air mixture isn't *as* important as it is in DLN combustors, where lean is the name of the game.

In simple cycle applications, the IGVs are not modulated to control exhaust temperature; there's no reason to maximize exhaust temperature in simple cycle mode. The exhaust heat is just going up the stack, wasted, so it doesn't make any difference if it's high or low.