Extracting air from Gas Turbine

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Thread Starter

DRP

We have GE make Frame-6 30 MW Gas Turbine. Normally there are three air extraction points from Frame-6 GT i.e from 5th, 11th and 17th stage of compressor.

Is is possible to have one more extraction (At @ 5 kg/cm2) so that it can be used as instrument air for plant use. i.e for pneumatic control valve operations.
 
Why do you need another extraction point when u already have the 5th, 11th and 17th stages?

Part of the 17th stage extraction can be taken through Air processing unit where it would be cooled and passed through air-water separators and stored in a compressed air tank.

Instrument air can then be taken from the tank through various pressure regulators where specific instrument operating pressures can be set for the various instruments.

You however need a standby compressor to cut in (for starting and shutting down purposes) when your CPD drops below a certain pressure.

I hope this helps
 
DRP,

Many GE-design Frame 6B heavy duty gas turbines have extractions from the compressor discharge casing to supply air to a plant for pneumatic control applications (instrument air).

It would necessary to review the drawings of the compressor discharge casing/combustion wrapper to see if there is flange with a bland that can be used as the extraction point.

Also, since the air is very hot coming off the compressor discharge (or anywhere along the compressor path) it will need to be cooled. And depending on the ambient humidity, and the possible presence of any type of evaporative cooling or fogging, the air will also need to be dried to be used as instrument air.

The problem with this scheme is that the air extracted from the axial compressor will decrease the output of the gas turbine (less air flowing into the combustors means less air for combustion and less mass flow through the gas turbine--both of which means less power will be produced by the gas turbine). Some plants find the lost air to the plant is too excessive and ultimately blind the system off permanently.

So, as is so often said, "There is no such thing as a free lunch!" If you want to extract air for use in a plant or process, you will need to 1), cool it; 2) dry it; and 3) you will lose power output from the gas turbine.

Hope this helps!
 
Dear DRP,

we operate 2 Frame 7EA machines and are using 17th stage extraction air for plant air service. We have the CTG feed in parallel with plant air compressors. We have a backpressure regulator valve on the feed from the gas turbines so that they are only supplying air when compressor discharge pressure is high enough for our system since CDP pressure from the turbine varies based on turbine load. We have been operating this way for over 2 years now with no problems.

A note for you is that we are also using a cooler to cool the CDP air prior to going into our instrument air dryers. The cooler is actually an old air to water cooler for liquid fuel atomizing air. The only other thing we added was an automatic blow down valve to remove some of the water that accumulates in the cooler.
 
Dear Sirs,

My first question is if at all we take additional air from 17 stage i.e. addition to the amount of air which at present we utilize for inlet air filter puffing will it not limit the quantity of air that is required for Gas Turbine Hot Gas Path components cooling?

Secondly reply to CSA's suggestion we have 30 MW GT and we operate it at part load like 20~24 MW. so it is possible to modulate IGV in such position that the compressor will supply air in reference to base load condition, so that additional air we can extract from required stage?
 
DRP,

Unless you have a very large plant with a lot of large pneumatic actuators that are constantly moving (remember: when actuators--air or hydraulic--are at a stable position there is virtually no air (or oil) flowing) then you will probably not use a lot of air.

You will likely not rob or starve gas turbine internal cooling--again, as long as there isn't a large consumption or lots of leaks.

The answer to your question about changing IGV angle is contained in the answer to another post you started:

http://www.control.com/thread/1374573091#1374591913
 
Dear MIKEVI,

Do you have any measure of the quantity of compressed extracted air? As explained by CSA sir there are also chances of loosing power output, have you noted that or experienced loss in power output of GT. Again our plant compressed air consumption is @ 34~35 Nm3/hr and it is connected to out utility department which supplies compressed air to our whole fertilizer cum petrochemical complex (@ 2500 Nm3/hr).
Can you suggest approximate quantity of air which can be extracted from GT compressor?
 
Dear DRP,

Unfortunately I do not have any measurement of the quantity of compressed extracted air we are using. I know that the amount supplied from the CTG varies as the load on the CTG is raised or lowered. Our instrument air source provides air to all control valves and instrumentation for the 2X1 power block as well to a water processing plant.

I have not really noted a decrease in output of the turbine, but I do not look at the very close numbers for our heat rate the way some of our engineers might. As CSA said you don't get something for nothing, I'm sure we have lost a small amount of power output for the extraction air being used for instrument air. But we decided it was worth it to not have an air compressor running all the time. As far as the turbine protection it will take care of itself, as you use more extracted air, the total flow through the compressor will be reduced, causing the machine to reduce fuel input to compensate.
My thought is if you are using extraction air through the 3/4" valve from the combustion wrapper that will limit you enough that it should not impact turbine output severely, but that is from a technical person like me, not an engineer.
 
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