In frame 6B gas turbine, fuel changeover was done (at FSNL condition) from 100 % NG to 100% IFO liquid fuel. As soon as machine came on 100% IFO, huge amount of liquid fuel was observed coming out of AA2 drain. Also huge amount of smoke accumulated in turbine compartment.
what can be the reasons?
Please describe significance of liquid fuel draining from AA2.
Please learn to use the P&IDs provided with the turbine and auxiliaries.
By looking at the P&IDs for the unit, probably the Liquid Fuel Purge P&ID, you will clearly see that if the liquid fuel purge check valve(s) are leaking in the reverse flow direction that liquid fuel will be allowed to flow to AA2, the tell-tale leak-off. The whole entire purpose of the tell-tale leak-off is to let a person know if the liquid fuel purge check valve(s) are leaking.
The liquid fuel purge check valves allow air to pass through them in for forward direction when running on gas fuel to purge the liquid fuel out of the liquid fuel portion of the fuel nozzles and to provide some cooling of the liquid fuel portion of the fuel nozzle and to prevent hot combustion gases from back-flowing into the liquid fuel portion of the fuel nozzle. Then when running on liquid fuel the purge air is blocked (usually through VA19-1, if I recall correctly) because the liquid fuel portion of the nozzles don't need purging at this point. The check valve is supposed to prevent liquid fuel from flowing through the purge piping and keep liquid fuel flowing into the fuel nozzle/combustor.
Your next question will be: How do I know which liquid fuel purge check valves are leaking? Use the manual selector valve at the discharge of the liquid fuel flow divider to check the liquid fuel pressures to each fuel nozzle combustor (when running on liquid fuel). The pressures should be fairly uniform, but any combustor with a pressure approximately 10% lower than the average pressure has some kind of problem. If liquid fuel is flowing out of the tell-tale leak-off, then the liquid fuel purge check valve is leaking. Very simple, really. (Another possibility is that the liquid fuel check valve is failing or has failed and the pressure drop across the liquid fuel check valve has decreased or is decreasing. Or that some portion of the liquid fuel nozzle has come apart or is severely worn. But, if liquid fuel is coming out of the tell-tale leak-off then it's a pretty good indication of a failed or failing liquid fuel purge check valve.)
P&IDs are absolutely critical to understanding and troublesho0ting GE-design heavy duty gas turbines. Regardless of the type or version of turbine control system. There should be A3 (B) size copies of the P&IDs in the Control Room for the operators, their supervisors and technicians to use. The copies in the Control Room should be readily available to every operator, and they should be laminated in plastic. Every control and instrumentation technician should have their own copies of the P&IDs, every maintenance technician should have their own copies of the P&IDs, and every maintenance department manager should have their own copies of the P&IDs, as should every plant manager and operations supervisor have their own copies of the P&IDs. Those with their own copies of the P&IDs should have made their own notes all over the P&IDs to help them understand the drawings.
In the Operations & Service Manuals provided with the unit, where the P&IDs will be found, there should also be another document called the 'Piping Symbols' drawing, and A4 (A) size document which describes the various symbols used on the P&IDs. This should also be be readily available to ever person operating, working on or supervising the operation, maintenance and repair of GE-design heavy duty gas turbines on the site.
During maintenance outages the operation supervisors and maintenance supervisors and technician supervisors should be taking their charges out to the unit and auxiliaries and helping everyone to find each of the elements on the P&IDs--to the extent possible. One or two systems per outage will do it. Everyone will get more familiar with the unit, it's auxiliaries, and how to read and understand the P&IDs.
The P&IDs should be animated and placed on EVERY HMI--the technology is simple enough. And that way the operators, their supervisors and anyone trying to troubleshoot or understand how the turbine and auxiliaries operate and work together can see the systems "in operation." This is a serious oversight on the part of the packagers of GE-design heavy duty gas turbines....
But, the fact remains that it's very difficult to understand and troubleshoot GE-design heavy duty gas turbine and auxiliary operation without a set of the P&IDs, and without being able to relate the drawings to physical devices in the field. Make your own copies of the P&IDs, the largest size possible, and then study them and make notes on them. Go out and find the devices on the P&ID (to the extent possible--some devices on the L.O. and Hydraulic P&IDs are located inside the L.O. reservoir). And I personally guarantee things will become a lot clearer and easier to understand and troubleshoot.
GE P&IDs are about the simplest of any original equipment manufacturer in the world. They are not overly complicated but are in fact very spartan (though that seems to be changing). All of the devices which the turbine control system controls and operates, and all of the parameters which the turbine control system monitors are shown on the P&IDs. The exception to this are the generator-related electrical devices, such as the protective relays and metering and synchronizing circuits--which are readily available on electrical schematic drawings. And, any instrumentation and control technician should have their own copies of these drawings, complete with notes, to go along with their copies of the P&IDs.
Is it possible to operate, troubleshoot and maintain GE-design heavy duty gas turbines without the P&IDs (and related electrical schematics)? You bet it is! But, the most successful plants are the ones that use and refer to the P&IDs (related electrical schematic drawings), and the ones where the operators and technicians and their supervisors and plant management all have access to the drawings and use them regularly.
Hope this helps! It's likely you have failed or failing liquid fuel purge check valves. And, it's likely there were high exhaust temperature spreads (because not all of the fuel that should be going into the combustor was going into the combustor--some, or a significant portion was flowing backwards through the liquid fuel check valve and out to AA2, the tell-tale leak-off). It's also probably likely that if someone were watching the exhaust stack during and after the change-over that there was some whitish haze coming out of the stack, at least for a short period of time. (I also wouldn't be surprised to learn that the unit tripped during or shortly after the change-over.)
And, the smoke in the turbine compartment--well that was probably the incomplete combustion of liquid fuel leaking out of fittings and components on to hot surfaces in the turbine compartment.