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.