i have used only gas fuel in my frame 6 gas turbine. i want to remove the atomizing air compressor. when i did it, the fuel nozzles have got over heat. so i need how to do without any problems

 

Without atomizing air flowing through the system (at a reduced rate because of the Atomizing Air Compressor Bypass Valve being open), there is no cooling air flow through the atomizing air system, and it has been proven that without a positive air flow from the AA compressor when running on gas fuel that hot combustion gases can backflow into and circulate through the AA system.

AA is also used to purge, and cool, the liquid fuel nozzles when running on gas fuel.

Yes; the AA compressor creates a load on the turbine, reducing the power output of the turbine by the amount required to run the AA compressor. But, when running on gas fuel the AA compressor throughput should be reduced with the AA Compressor Bypass Valve open.

You need to remove all of the liquid fuel components, and I believe there is a gas fuel tip without an opening for the liquid fuel nozzle and with no AA passages.

But, I believe this should all be engineered, because the deletion of the AA used for purging and cooling will likely have an adverse effect (is it 'affect' or 'effect'; I can never remember!) on the gas air/fuel ratio, making the fuel/air ratio slightly richer and therefore hotter.

However, the air supply for the AA compressor comes from an extraction off the axial compressor, and that air is now flowing into the combustor. So, it might not be a problem at all. I'm not a combustion engineer, so I can't say for sure.

But, if you want to remove the AA Compressor without damaging the liquid fuel nozzles and AA air tips, you're going to need to remove all of the components and use nozzle components that prevent backflows and do not require purge air for cooling.

It's not as simple as just removing the AA Compressor. You need to review the Piping Schematics (P&IDs) to understand all of the flows and the possibilities for backflows.

 

I think there is no solution removing the AA compressor because the various fuel supply lines which are on off position during gas fuel operation need cool down which provided by AA compressor and also it may be needed During start up when main compressor does not provide the major air.

 

Hi all

Atomizing air compressor carry a great role for Duel fuel operating machines also.

When a Gas turbine operating on fuel gas operation, Fuel oil and water injection lines are cooled by using the sealing air and cool air system by bypassing the AA compressor.

Basically AA compressor, compress the compressor discharge air and useful to provide the atomization of fuel oil for proper combustion while fuel oil operation. But at the time of start up it is useful to purge the Water injection and fuel oil lines while machine starting with Fuel gas.

While machine start-up with Fuel gas, compressor discharge pressure is not sufficient to provide the proper purging to the Fuel and water injection lines.

As per the GT frame-7FA logic,

Purge air pressure should be greater than 96% of the compressor discharge pressure, otherwise GT will trip on purge pressure ratio low while start-up. That means 4% of the pressure losses allowed from compressor discharge (seal air and cool air header) to respective purge header.

But while start-up this condition will not satisfies due to low compressor discharge pressure which is in sufficient to purge the fuel and water injection lines. While start-up AA compressor will start and take the suction from Gas turbine compressor and compress it to 1.5 times of the compressor discharge pressure to provide the sufficient quantity of air flow for purging to satisfies the above logic.

GT start-up sequence with fuel gas operation, AA compressor will start. If AA compressor not running after purge sequence, GT will not go to firing speed i.e. it will stay at purge speed only due to non running of AA compressor.

After a particular compressor discharge pressure, AA compressor automatically cut off i.e. compressor discharge which is AA compressor bypass is sufficient to maintain the purge flow.

while fuel oil operation, seal air and cool air header will supply purge air to the fuel gas lines with out involvement of AA compressor. Same seal air and cool air header supply suction to the AA compressor to provide the atomizing air while fuel oil operation.

So, AA Compressor is very important mounting for a Gas Turbine.

Without diagram (GE Purge P&I)) it is difficult to explain, but I hope it is useful

Waiting here for immediate reply

 

Devudu,

There are several variations of Atomizing Air and Purge systems for F-class machines, some that don't even use the AA compressor during natural gas operation any more for energy savings (called, interestingly enough, 'Compressorless Purge'). So, to make a blanket statement about how purge systems operate on F-class units without identifying which F-class unit(s) and which combustion systems they are equipped with that you have experience with is kind of misleading. The pressure ratios vary depending on many factors, through the ones you list are pretty generic for most F-class units without Compressorless Purge.

But, you are still correct, in that purge air flow, from the AA compressor (in the case of a Frame 6), is used for purging and cooling of several systems when operating on gas fuel. So, removing only the AA compressor without removing the systems that require the purging and cooling air flows is going to lead to problems such as the originator reported.

You are also correct in that the AA Compressors on dual fuel units do more than just provide atomizing air for improved atomization (and combustion) of the liquid fuel.