Why exhaust Temperature curve is negative

A

Thread Starter

aliya

Dear all,

I am working with GE gas turbine having Mark VI control system.
As I have gone through various threads here regarding exhaust temperature control.

I understand that as CPD increases exhaust temperature decreases and vice versa.

this is for maintaining constant firing temperature. but i am not able to understand that how this thing is being done.
As per normal understanding, more CPD => more fuel =>more exhaust temperature.

But how it is maintaining constant firing temperature by reducing exhaust temperature reference upon CPD increase.

Kindly reply.
 
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Bob Johnston

As CPD increases, cooling air flows increase, so relationship between Firing Temperature & Exhaust Temperature changes slightly. To maintain constant Firing Temperature as this CPD increases requires a slight reduction in Exhaust Temp.
 
Thanks for your reply Bob sir,

But that is what i am asking, what is relation is between firing and exhaust temperature. And how exhaust temperature affects firing temperature.

And by lowering exhaust temperature HOW firing temperature is being maintained at constant value.

Hope my question is clear.
 
B

Bob Johnston

What we are really trying to control on a Gas Turbine is Firing Temp. On industrial turbines particularly, it is difficult to measure Firing temp. and have thermocouples that would be easy to replace so we measure Exhaust Temp. and use this to control Firing Temp. There is a known relationship between the two which we use as the calculation but Cooling Air flows (which comes directly from CPD)will change this relationship. As CPD increases at the same Firing Temp. the Exhaust Temp will become slightly lower due to the increased cooling flow. As Exhaust Temp. is the basis for Temperature Control, this would allow us to increase load to come back to the same Exhaust Temp. point but the increase in load would allow the firing temp to increase. We don't want this to happen as we would be overfiring the machine so , as the CPD increase, we reduce the Exhaust Temp. control setpoint to maintain a constant Firing temp.

Maybe the concept is difficult to understand for you, I hope this helps to clarify.
 
S
Dear Aliya,

Whenever there is increase in CPD, exhaust temp control should issue the lower Set point for exhaust Temp to control the firing Temp within the limit. If exhaust temp remains same, there will be increase in firing temperature with same exhaust Temp.

Pl. refer to the basic brayton cycle with Pressure and Entropy coordinate.If there in rise in pressure at compressure discharge, turbine inlet point will shift upward with same exhaust temp (identical operating point for Turbine outlet/discharge), so firing temp. will be high.

To control the firing temp, there is shift of turbine outlet point downwards.

Hope explanation above will help you to understand.

Shailendra
 
Thanks shailendra and Bob Sir,

Now i can understand why exhaust temperature comes down upon CPD increase.

Yet I have two questions in my mind -

1. How firing temperature can be increased by increasing "CPD bias corner TTK_C(0)" and "FSR bias corner". as we did this for gas turbine under uprate scheme.

2. In temperature control curve CPD bias, before corner point does firing temperature remains at constant value or does it start to stay at constant point only after corner point.

Also before corner point does firing temperature reaches the max limit or remains at lower value?
 
dear sirs,

I am writing again here if i failed to put correctly my above question.
I want to know that, does firing temperature remain constant from the beginning of the temperature control curve? i mean at lower CPD values OR before corner point in graph, "firing temperature remains constant" or only after corner point it remains constant?

Before corner point does firing temperature increases with increase in CPD?
 
Before the "corner" point (on the horizontal part of the control curve) the firing temperature is less than the limit. The horizontal part, also called the isothermal limit, is there to protect the exhaust system components. If you study the software code for the control, you will see that the control limit is a "low select" between the isothermal limit and the calculated firing temperature limit.
 
Ok that means on horizontal line before corner point, the firing temperature remains less than limit firing temperature.
Is it correct till now?

Secondly, on horizontal line does firing temperature increase from with increase in CPD?

From very left value on X-Axis(CPD axis)to corner point for CPD, firing temperature increases?

Thanks in advance.
 
kindly reply to this post, as i need the answer urgently.

How does firing temperature behave for entire graph.
From CPD zero value to higher value?
Does firing temperature remains same for entire graph?
 
B

Bob Johnston

We all try our best on this self help forum to give answers whenever we have the time to spare. For this question people have given so much help already, when you ask for an urgent answer, it kind of puts me off replying.

It seems that you don't really understand the relationship between CPD and firing temp yet. As has been stated already, on the horizontal section of the graph, the firing temp will start lower than the limit, at the beginning it will be at it's lowest and, until the knee point, it will increase until it is at it's maximum allowed at the knee point. From there it will be held constant by the decreasing exhaust temp. control set-point.
 
F

Recently our OPs department needed a more detailed explanation of how the exhaust temperature is controlled during baseload operation. We looked at the site specific logic & control constants and developed an excel sheet that mimics the logic. Use the following link to download it:

https://rapidshare.com/files/2922102090/GT_Exhaust_Temperatures_setpoint.xlsm

https://www.yousendit.com/download/T2dkQndDSWVrWTlsQXNUQw

(Remember to remove any spaces in the links above that might be inserted by the forum software; both links are active at the moment but will be removed by the websites soon as I only have the free account on the above 2 file sharing services, and that means all uploads come with an expiration date).

The sheet is in xlsm format because it contains a macro for the double linear interpolation function (it is not available in excel in a simple formula, so a macro was needed). Allow any macros if prompted.

At the top left you will find the inputs that are needed to determine the exhaust temp setpoint. You enter the conditions there, and the sheet will tell you the setpoint (TTRXB) that the GT tries to match by varying fuel (via temp FSR, FSRT).

The sheet also makes a graph of TTRXB vs CPR to give you an idea of variation of TTRXB for different CPD. Note that the curve changes as well based in the inputs.

Ignore the combined cycle options.

Please note that this is a very site specific sheet. The control constants and logic might be different on your site, and there is no way of telling whats going on at your site without looking at the m6b/tcw . But the differences would be small and the sheet will help you get a very good idea for what you are looking for. Of course, I am assuming that you will be able to look at the formulas in the sheet and work it out.

Hope this helps you and others.
 
Sir,

I am not able to download the file. Can you please send it on my email id.

You send me your mail id, i will respond the same.
 
F
I just check the links, they are both working. In the first link, there is an extra space (between set and point)... you need to remove that manually.

The second link is working as it is.

You are expecting people who are helping you to post their email addresses instead of taking the time to create your own private email address which you would feel safe giving out. I took the time to upload the files, and I expect that you post your email address if you are unable to download from the above two links.
 
Thanks for your reply freelanceng Sir,

and I am extremely sorry, if unknowingly i have wrote the wrong statement.
Actually reason was that, somehow i was not able to remember my password, that why i asked to give your id and i will reply on that from my friend's mail id. I just didnt' want to write other persons id here.

Sorry again for my mistake.

my id is - [email protected]

Sir you can send on this id. Thank You.
 
Dear Bob,

plz correct me if you feel so.

A gas turbine has generally two modes of operation (LCM & BASE, though Float is also there but I’m ignoring that) in a co-generation plant. The moment we put the turbine in BASE mode it comes to PCD biased Temperature control mode. But before that it is in LCM mode (below its rated value) and the limit acting is isothermal limit for firing temperature. Now in LCM, when we have fixed the generation amount that means (TNR-TNH) is always going to remain constant i.e. amount of fuel will remain constant. PCD at any point of time will remain constant and Exhaust temperature will have some fixed value. Now suppose ambient conditions favour and PCD value increases and the LCM is same as before then the firing temperature will reduce(just because more cooling will be available) and vice versa.

So i believe the in the beginning of the curve the firing temperature will be maximum (for the horizontal line portion only) and it will go on reducing as PCD increases till the time operature puts the machine in temperature control mode.

thanks n regards.
 
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