# Compressor discharge temperature

H

#### Harish

We have issues in getting the compressor discharge temperature (CDT) of a gas turbine, may be bad data from the thermocouple. We are calculating compressor efficiency as a calculated output and it requires CDT. Is there a theoretical way to calculate CDT?

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F

#### fai

try the formula as per below

T2/T1 = (P2/P1)^(k-1/k)

where k = 1.4
T1 = ambient temp
P2 = compressor discharge press
P1 = ambient press
P2/P1 = CPR = compressor pressure ratio

T1 = unit in kelvin by adding 273 if the unit in your plant is using 'C. Later on T2 just minus back 273K again to get the value in 'C

H

#### Harish

Hi,

Do you mean the relation with polytropic efficiency (eta_p)

T2/T1 = (P2/P1)^(k-1/k*eta_p)?

IF so, this relation has efficiency term included.

To make it more clear, I'm looking if there is a way to find out the compressor discharge temperature of a gas turbine without any efficiency term included?

#### CSA

Harish,

Why can't you look at past (historical) data and use a typical value until such time as the issue(s) with the compressor discharge T/Cs can be resolved?

What is the function or application that requires such accuracy?

Do you run the unit at various loads meaning compressor discharge temperatures vary with load, or do you run the unit at Base Load where compressor discharge temperature will vary almost directly with ambient temperature?

Finally, why can't the issues with the values being reported by the compressor discharge thermocouples be resolved negating the need to find or calculate a compressor discharge temperature?

When did the problems with the values being reported by the compressor discharge temperature thermocouples start? After a maintenance outage? Are there multiple thermocouples? Or just a single, multi-element T/C?

I'd be extremely interested to know what function or application requires such a high degree of accuracy of compressor efficiency. Is there some downstream process that needs to know compressor efficiency? Are you using the compressor efficiency to determine when to do water washing (on-line and/or off-line)? What is driving the need for accuracy?

H

#### Harish

We have a real time performance monitoring tool which compares the actual data to the theoretical ones at any time. When the thermocouple fails, the tool is not capable of assessing the performance. I'm trying to find out the missing data with more engineering sense in order for the tool to proceed with the performance simulations. We also have considerable number of machines that runs on various loads. We use the actual performance data for health monitoring of machines. And of course, we are not interested in having any bad TCs but it takes time to replace the odds and we expect the monitoring tool to perform the simulations in the meantime.

Hope this answers all your questions. I would love to have such healthy discussions

#### CSA

How have you been able to increase power output, efficiency, and reliability using the real-time performance monitoring tool?

Most of the gas turbines I have worked on have redundant (at least two) compressor discharge temperature thermocouples, and they are not mulit-element thermocouples (meaning, they are independent thermocouples mounted in different locations). It's pretty unlikely more than one of these T/Cs will fail at the same time.

If your tool is so reliant on this one parameter it would seem that it would be beneficial to have multiple, redundant T/C inputs, possibly even using multi-element T/Cs, where one of the multiple elements is unconnected until the other element fails and then someone switches the wiring to the second element.

It's noted you didn't answer the question about when the problem started.

But, it would be very helpful if you could tell the forum how this tool has helped you increase the power output, efficiency, and reliability of the turbines at your site. Especially, if you can't run one turbine (it's presumed this is happening for a single unit; you haven't said specifically) without this tool.

Everyone here is very keen to learn about technologies which can help them operate their units better, more reliably, and more efficiently.

D

#### DABControl

You can't determine the real discharge temperature without assuming a value of isentropic efficiency (eta)less than one this efficiency is related to all forms of energy losses (brg friction, heat loss thru compressor outer casing, fouling blade, etc)that produce entropy and reduce isentropic efficiency,

these losses are more difficult to quantify, there is a way to use an old value of efficiency if u have calculated one before, but the accuracy is not guaranted

Disharge temperature in case of isentropic compression in less than disharge temperature in case of polytropic compression (real compression)and the difference is due entropy production (loss of energy,

Conclusion: real discharge temperature and efficiency are dependant parameters then u must know one to calculate the other, only discharge temperature in case of isentropic compression ideal case (eta: efficiency = 1) can be calculated

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#### Harish

Thank you!!!!!!

> You can't determine the real discharge temperature without assuming a value of isentropic efficiency (eta)less than one
> this efficiency is related to all forms of energy losses (brg friction, heat loss thru compressor outer casing,
> fouling blade, etc)that produce entropy and reduce isentropic efficiency,

---- snipped by moderator to save bandwidth ----

H

#### Harish

I haven't mentioned that the tool would help me in improving the power output or efficiency. The tool is health monitoring tool which requires data from the machine.

> How have you been able to increase power output, efficiency, and
> reliability using the real-time performance monitoring tool?

---- snipped by moderator to save bandwidth ----

#### CSA

Isn't the "health" of the machine directly related to performance and efficiency?

Even trying to predict parts life would involve some measure of degradation of efficiency and/or performance.

If the tool isn't being used to optimize performance and efficiency by "predicting" the health of the machine, then how is it being used?

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#### Harish

Yes, the health is predicted based on the performance. we are not improving the power output by any means. We do improve the reliability by reducing the downtime based on the performance over a period.

> Isn't the "health" of the machine directly related to performance and efficiency?