i just want to know in the FR-6 ALSTOM GTG during starting from where the air comes for purging, i am just confused either from 17 stage compressor or atomizing air compressor?

 

me,

And we just want to know which "purge" you are referring to: the start-up, pre-fire purge, or the purge of fuel systems?

 

i am just want to say the purging what happened for 5 minutes in the time of start-up? i am wondering what is pre-fire purge or fuel system purge?

 

i let you know one another thing that i as far only heard that the purging is done only in the combustion chamber to carry away the unburnt products.

 

me,

One of the wonderful things about the English language is that words and terms can mean different things in different contexts.

The turbine combustors, turbine section and exhaust (including any waste heat recovery steam generator ("boiler") in the gas turbine exhaust) must be purged of any possible combustible gases prior to energizing any spark generator (ignitor; spark plug; etc.). This is pretty much the law in most parts of the world, and if not legislated by governments and ruling bodies then it is usually required by insurance companies.

In the case of GE-design heavy duty gas turbines the gas turbine axial compressor is used for the source of air to move or displace three to five times the volume of the turbine combustors plus the turbine section plus the turbine exhaust plus any stack volume or boiler and stack volume. While there may not be much pressure during the low-speed operation of the gas turbine axial compressor there is <i>considerable flow</i>--even with the variable IGVs (Inlet Guide Vanes) in the "closed" position (usually around 34 DGA, DeGrees Angle).

During full speed operation of multi-fuel GE-design heavy duty gas turbines the fuel passages of the fuel not being burned are usually "purged" to prevent the back-flow of hot combustion gases from the combustors into the fuel passages, and also to provide some cooling air flow to the passages.

So, the turbine and exhaust (including any boiler and/or stack) is purged during starting before the energization of any sparking or flame source, and the source of air for that is the gas turbine axial compressor. It doesn't go through any special valves or passages; the air just flows through the turbine and exhaust in the same way it does during full speed operation.

If the unit is multi-fuel, and/or in some cases DLN combustors which only run on gas fuel also have some purging/cooling flows, the source of purging air is the main atomizing air compressor or the gas turbine axial compressor discharge. For the details of fuel purging at your site, if present, refer to the P&IDs for the fuel system and the fuel purge system(s), if present.

Hope this helps!