Dear Friends,
I am working @ CCPP, 01 Gas Fired Turbine, 01 HRSG and 01 Steam Turbine (a simple config with 90s technology)
i need to understand which mode of Gas Turbine is efficient in terms of Gas consumption and minimum stress production
i request all of you specially dear CSA to give your expert opinion

thanks & regards
Ashiq

 

Have you tried the search function?

Unfortunately, CSA has retired and moved onto greener pastures. His last post is here:
https://control.com/forums/threads/mark-counter-and-timer.50432/

But there are some others who might well answer your question.

 

I might take a run at answering the question IF I understood the abbreviations. People need to realize that all abbreviations are not universally used in all CCPPs (Combined Cycle Power Plants). One of the things I appreciated about CSA was that he almost always explained any abbreviations he used and actually seemed not to use them very much at all (though he NEVER explained what CSA meant.?.?.!). (Many people like to use abbreviations and acronyms because it makes them appear to be more intelligent.)

OTC and TETC seem to be related to some kind of operating modes in that particular CCPP. Some kind of Temperature Control (hence the TC suffixes), but that’s about as much as I am willing to guess. (I detest guessing about technical issues.)

 

Exhaust temperature control vs Load Control
these are both GT controllers

 

Okay; I'm going to try to follow CSA's example and answer an extremely broad question as best as possible.

In terms of overall gas turbine, steam turbine and plant efficiency it's best to operate in exhaust temperature control mode with Base Load selected (I'm presuming the gas turbine is a GE-design heavy duty gas turbine...). The definition of Base Load exhaust temperature control is operating with the IGVs (Inlet Guide Vanes--see how I did that?) at maximum operating position, allowing as much air as possible to enter the axial compressor which is spinning at a relatively constant, stable speed if the plant is synchronized to a well-regulated grid of any size. In this mode, the turbine control system looks at the axial compressor discharge pressure (or axial compressor discharge pressure ratio--the ratio of axial compressor discharge pressure to atmospheric pressure (a fancy method of factoring in atmospheric pressure that can vary throughout the day and the year)) and calculates the maximum allowable gas turbine exhaust temperature and puts in as much fuel as possible to keep the (average) actual gas turbine exhaust temperature equal to the maximum allowable exhaust temperature for the present operating conditions (ambient temperature; atmospheric pressure; etc.). This the most efficient mode of gas turbine operation, full stop, period.

And in a combined cycle power plant it's also the mode that generally produces the maximum amount of high-temperature steam to be supplied to the steam turbine, which is usually always operating with the steam inlet control valve fully open to maximize steam turbine-generator power output. (I am NOT including any auxiliary duct burners in operation as while they can increase steam temperature and to a certain extent pressure it is not a very efficient means of increasing power output depending on the cost of fuel.)

There is another type of gas turbine exhaust temperature control--IGV exhaust temperature control--that is possible in a combined cycle power plant. And it can be used at Part Load operating conditions to maximize gas turbine exhaust temperature by restricting the amount of air flowing into the axial compressor (which increases the gas turbine exhaust temperature above the temperature which would be produced if the IGVs were not partially closed). (Part Load is defined as loads less than Base Load, when the IGVs are NOT at maximum operating position.) This is sometimes called "Combined Cycle Mode" as opposed to "Simple Cycle Mode" in which the IGVs are only modulated based on stable operating parameters. BUT by restricting the air flow through the gas turbine to maximize gas turbine exhaust temperature the efficiency of the gas turbine is reduced (because to make the same power if the IGVs weren't being closed slightly requires more fuel).

If the gas turbine is a GE-design heavy duty gas turbine with a DLN combustion system, it uses IGV exhaust temperature control to help maintain stable combustion in the gas turbine combustors, so the air flow is reduced to maintain stable combustion by modulating the IGVs which increases the gas turbine exhaust temperature. But, AGAIN, this is not the most efficient operating mode for the gas turbine. IGV exhaust temperature control increases the efficiency of the overall plant (amount of electrical power for the amount of fuel being consumed). (Again, I'm not including any auxiliary duct burners or fuel heating or any additional means of affecting overall plant efficiency, just a straight combined cycle power plant.)

So, the best efficiency for the overall plant (amount of electrical power for the amount of fuel being consumed) is when the gas turbine is operating at Base Load with the IGVs at maximum operating position. The most fuel is being burnt in the gas turbine combustors and produces the maximum exhaust flow and best possible gas turbine exhaust temperature for the current ambient and atmospheric conditions.

If the gas turbine were operating in Simple Cycle Mode with the IGVs on a fairly linear operating scheme the gas turbine exhaust temperature would be slightly lower at loads less than Base Load which means the steam production would not be great and the overall plant efficiency (amount of electrical power for the amount of fuel being consumed) would not be very good but the gas turbine efficiency would be a little better (than in Combined Cycle Mode). BUT, by using IGV exhaust temperature control at Part Load the gas turbine exhaust temperature would be maximized which would produce more steam (than in Simple Cycle Mode for the same operating conditions) and that increases the overall plant efficiency (amount of electrical power for the amount of fuel being consumed).

Most heavy duty gas turbines operate similarly (since combined cycle power plant physics is the same pretty much regardless of the gas turbine manufacturer). If the combined cycle power plant has to be operated such that the gas turbine IS NOT operating at Base Load with the IGVs at maximum operating position then overall plant efficiency can be maximized by restricting the air flowing into the axial compressor to increase the gas turbine exhaust temperature to produce more steam (IGV exhaust temperature control).

If the gas turbine has DLN (Dry Low NOx--see how I did that?) combustors then it's highly likely it uses some kind of IGV exhaust temperature control at Part Load to improve combustion stability while maintaining low exhaust emissions. But some plants use other means to limit emissions (SCR (Static Catalytic Reduction--see how I did that?) and ammonia injection. Some use a combination of IGV modulation and exhaust system methods to achieve low exhaust emissions.

So, the question was not clear, and still the OTC and TETC descriptions are not fully explained. We don't know if the gas turbine has DLN (Dry Low NOx--see how I did that?) combustors or if the gas turbine exhaust temperature is used to help warm up the steam turbine inlet piping/valves during start-up (yet another form of exhaust temperature control, sometimes called Temperature Matching). We don't know if the plant has auxiliary duct burners in the exhaust duct (HRSG--heat recovery steam generator, or boiler (see how I did that?)). We don't know if the plant is operated at full power output all the time, or if the plant output has to be limited at times by the utility or host (often called Load Control or Tie-line Control). There's just a LOT we don't know, and this is the best answer we can provide with such little information and an unclear definition of what efficiency is in question (gas turbine; steam turbine; overall combined cycle power plant efficiency; or ???). If we had more information and a clearer understanding of what efficiency was being asked about we could maybe have provided more concise information.

But, as CSA showed us, we can answer questions without asking 20 questions to get to a proper answer--it's just that the amswer might be long-winded and/or it might not be exactly what the questioner was referring to. But, still, people reading the thread can find useful information even if the questioner didn't get the precise answer desired.

 

And I may not have been clear about exhaust temperature control versus Load Control. We already know what exhaust temperature control does--at both Part Load and Base Load. When a machine is operating at Part Load on Load Control the gas turbine control system is adjusting fuel flow to make the actual load equal to the load setpoint. And, the exhaust temperature will be what it will be. Some plants (wrongly) set a very high Load Control setpoint to drive the machine to Base Load (and beyond, but that's for another thread). It accomplishes the goal of getting the machine to Base Load (IGVs at maximum operating position), but it's not good for at least one other reason. (It's a much better choice to just select Base Load and be done with it; much fewer complications.)

If the machine is operating at Part Load on Load Control, AND IGV exhaust temperature control is enabled and active, then the turbine exhaust temperature will be maximized for the present operating conditions by restricting air flow into the axial compressor with the IGVs. But, as we learned above, this is NOT an efficient mode for the gas turbine even though it is overall a more efficient operating mode for the overall plant efficiency. Load Control is also BAD for grids that can be unstable (unless there is special control software to counter Load Control when grid frequency is not at design).

That should answer the broad question fully.

 

Dear WTF
Thank you very much you explained it beautifully, actually we were analyzing two conditions where two controllers of gas turbine in action 1) Part Load on Load Control 2) Base Load (machine achieved TETC (Turbine Exhaust Temp: Control) setpoint in our case 540 C and IGV is full open. Secondly we have not installed the additional burners at our HRSG (Heat Recovery Steam Generator)
Our concern was to run the system efficiently and we had two options 1) TETC Control 2) Load Control ,,,,, idea was given to run the machine always on Load Control by changing TETC Setpoint. so that machine never reach TETC controller.
Attached snapshot is taken when plant is low load.
Conclusion from your opinion: Machine is operating at Part Load on Load Control, AND IGV exhaust temperature control is enabled and active, then the turbine exhaust temperature will be maximized for the present operating conditions by restricting air flow into the axial compressor with the IGVs. But, as we learned above, this is NOT an efficient mode for the gas turbine even though it is overall a more efficient operating mode for the overall plant efficiency.

 

Losing some efficiency in the gas turbine (it's not a lot--and, no, I don't know how much the efficiency of the GT decreases when the air flow through the machine is throttled in order to increase gas turbine exhaust temperature) to gain efficiency in the overall plant is probably why "combined cycle mode" of GT operation was developed. Someone ran the numbers and decided in the larger scheme of things (in other words, the overall plant efficiency) it was more economical to sacrifice some GT efficiency in order to produce more power for the same amount of fuel in the combined cycle where steam is produced by the GT exhaust heat.

I don't think engineers and physicists would have developed combined cycle mode (and by that I'm referring to the process of throttling air flow at Part Load in order to maximize GT exhaust temperature) if the numbers didn't support the process. The overall electrical production in combined cycle mode must be such that by losing a little efficiency in one part of the combined cycle (in this case the GT) was outweighed by the increase in electrical production in the other part of the combined cycle (the steam turbine).

That's how it was explained to me (without all the formulae--just the principles and the concepts)--and judging by the number of combined cycle power plants that operate in this mode at Part Load it must be sound economic practice. Meaning the revenue produced by using combined cycle mode exceeds the revenue produced by NOT using combined cycle mode. Even if that means the GT efficiency isn't being maximized at Part Load--the watts produced by the overall plant for the BTUs burned by the GT is higher than if combined cycle mode wasn't used. That ultimately means more money in the owners/investors pockets (bank accounts).

Again, I think the GT efficiency loss isn't very much in the overall scheme of things--and it must be offset by the overall plant efficiency.

Right?

 

Gas turbines can operate in different modes, and the choice of mode depends on the application and the requirements of the power system.
https://www.linkedin.com/pulse/gas-turbine-modes-operation-ahmed-hamdy-abd-elrahman/

 

I always assumed (and you know what CSA would say about assuming something) that CSA was short for Customer Service Agent. I have no idea if my assumption is correct.

 

Hi Gents, If the unit has overshooting in MW up to set point in pre select , for example the set point is 30 MW , but the unit goes to 45 MW then back to 30 MW .What is the reason ?
Thank you

 

@Lu RPP,

1) What kind of combustion turbine are you encountering this problem on (the manufacturer and machine size/model)?

2) What kind of combustion system does the machine have (low emissions (DLN-I, for example) or conventional combustors?

a) If the machine is a GE machine with DLN-I combustors, does it have IBH (Inlet Bleed Heat) and is the IBH system enabled and running when this problem occurs?

b) At what load is the pre-selected load control mode enabled?

c) Does the machine change combustion modes during the loading, and if so, what combustion modes does the machine go into and/or out of during the loading when it overshoots the pre-selected load control reference?

3) What kind of fuel is being burned when this problem occurs, and is the fuel supply pressure and flowrate up to the machine stable?

4) When did this problem start--very recently or has it been persistent since commissioning, or after some maintenance outage (what work was performed during the maintenance outage)?

5) What other alarms are present when this problem occurs, and did any of the alarms start at about the same time as this problem? (If the machine has a GE Mark* turbine control system, what Process Alarms and Diagnostic Alarms are existing or annunciated when this problem occurs? (Please list ALL alarms (Process and Diagnostic) even you think the alarms are irrelevant--they may, or may not, be irrevelant; let us be the judge of that by providing a comprehensive and complete list of alarms (Process and Diagnostic).)

Without more information, we can't be of any help.

 

Hi Sir,
1- GE MS6001 / Frame 6B.
2-DLN
3-when IBH is disable.
4-if the operator want to increase load from 10 MW(Primary) to go 30 MW by 5 MW step (preselect).
5- yes , combustion chamber mode has been changed from Primary , Lean to Lean , Extended L-L , Secondary and Premixed .
6-Row Gas fuel , and pressure 24 bar stable .
7-the problem happened after do HGPI and DLN tuning for machine.
8-actually , no alarm also the frequency of grid is stable during that .
I appreciate your help

 

@Lu RPP,

When IBH is disabled the typical loads at which DLN-I combustion mode transfers occur when the ambient temperature is not excessively high or low (compared to the machine's nameplate ambient temperature rating) is about 15-20% of rated load for Primary-to-Lean Lean combustion mode and at about 70-8-% of rated load for Lean Lean-to-Premix combustion mode. For a typical GE Frame 6B machine that would make the approximate load at which the machine transferred from Primary to Lean Lean combustion mode around 6 MW, and the approximate load at which the machine transfers from Lean Lean to Premix around 31-32 MW (assuming an ISO rated Base Load power rating of 42 MW).

Sometimes when a machine is getting very close to a combustion mode transfer point (which is a function of TTRF (or TTRF1--NOT a specific load setpoint) when the machine initiates the combustion mode transfer the Mark* will issue a continuous load RAISE command until the machine completes the combustion mode transfer, and then if the actual load is above the Pre-Selected Load Setpoint it will reduce the load to the Pre-Selected Load reference. Sometimes if the operator chooses a Pre-Selected Load Reference that causes the TTRF (or TTRF1) value to hover very near the transfer setpoint (which, again, is a temperature--NOT a load--setpoint) the machine will complete the transfer and then when reducing load it will transfer back into the previous combustion mode, and so on. It is incumbent on the operator to understand what the combustion mode transfer setpoint TTRF (or TTRF1) temperatures and to monitor the actual TTRF (or TTRF1) value and make slight adjustments in the Pre-Selected Load reference as necessary to prevent the transfer from going into and out of combustion modes to try to hold a Pre-Selected Load reference.

To give you any further assistance we would need to have comma-separated value data from a loading sequence that is acting as you describe. It would need to include ambient temperature, IGV angle, Pre-Selected Load reference value(s), MW values, and combustion mode status for a loading sequence.

It should be noted that occasionally if DLN-I tuning is done at an extreme of the range of typical ambient temperatures for the year (such as at or near the lowest possible ambient temperatures, or at or near the highest possible ambient temperatures) then at other times of the year the machine will have difficult transferring combustion modes during loading. And, if the machine is "cold" when being started and if it is being loaded quickly then it will also have difficulty successfully completing combustion mode transfers just because the machine internal temperatures are not near normal.

I would imagine that after a HGPI the IGV LVDT feedback was calibrated (or should have been!) and it's possible that the IGV LVDT feedback was not correctly calibrated and that can also negatively impact combustion mode transfer success--especially when the machine is "cool" and hasn't had time to warm up.

AND, it might also be that the gas fuel control valves were also "calibrated"--and not done very well or verified very well which would negatively affect combustion mode transfers.

Also, sometimes DLN tuning is rushed and not fully completed and/or verified and that can mean that sometimes the machine has difficulty making combustion mode transfers successfully.

It's not clear when you describe all the various combustion modes--because the list you provided is a list of possible combustion modes, and you haven't described if the machine, when transferring from Lean Lean to Premix actually goes into Extended Lean Lean and then has to be manually unloaded to Lean Lean Positive combustion mode and then reloaded to attempt another transfer into Premix. This would likely indicate something is amiss with IGV LVDT calibration or operation and/or DLN tuning wasn't done properly or properly verified or was done at an ambient temperature extreme.

GE likes to say (and write) that DLN-I is a "mature technology" and if we're simply talking about how long it's been in production and used that might be true. But the reality is that it is a work in progress and given the complexities of DLN combustion (premix operation of a DLN-I combustion system is, after all, an inherently unstable process that can be made (tuned) to work properly for most ambient- and operational conditions, but not always). GE is continually amassing operational data and finding that for some machines reliable operation, while achievable, can actually be somewhat difficult to achieve. For the most part, they do have some very good troubleshooting diagrams and procedures, but they all assume that maintenance activities were carried out properly (especially LVDT feedback verification and calibration, if necessary at all!). Changes in fuel constituency can also negatively impact DLN reliability.

But, the data you have provided is anectodal--meaning it is subjective and not accompanied by any actual operating data during the supposed problems. To troubleshoot such a problem, actionable data is required--actual values of operating parameters over time need to be reviewed and analyzed to make any kind of definite statements or recommendations. I have attempted to cover all the bases--not knowing any operational, actionable data--so that should allow you to methodically work through all of the various possibilities as you troubleshoot the problem, eliminating those that don't solve the problem until you reach a point at which the problem is resolved or more targeted actions need to be taken. The mechanical department is ALWAYS going to say they maintenance outage was completed without any errors--and they believe that when they say it. The reality can be very different without good and proper supervision of the various crews and people actually performing the work. Because the Mark* is an electronic control system with LOTS of wires and flashing LEDs they will always say it's the Mark* that's causing the problem. The Mark* is only as good as its inputs (garbage in; garbage out), and if there aren't a lot of alarms (Process and/or Diagnostic) then it's most likely something is amiss with the reassembly or the hot gas path hardware which was installed in the machine wasn't installed properly OR has some clearances or openings which aren't to proper specification (dilution hole diameters, for example, or fuel nozzles with under- or over-sized orifices). These things are outside the control of the Mark*--it presumes all of the combustion hardware is as per specification and was installed correctly, and that any calibrations were performed and/or verified properly. That's all it (the Mark*) can do.

So, if you have better, actionable data, in csv (comma-separated value format) that you can attach to this thread then we might be able to offer some more concise recommendations. If not, that's all we got. You also might try asking GE for assistance (if you haven't already).

 

Thank you for your support , very important information , sure will do the best .