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We have a Frame VI GE Gas turbine duel fuel on NG and HSD. Recently we are facing problems on one of our machine, in which atomizing air temperature quickly drops to around 60 degC from 105 degC and pressure starts hunting between 12 to 15kg/cm2 whenever fuel is transferred to HSD or NG/HSD mix. Precooler flow regulating valve is robertshaw make which we operate manually to adjust AA temperature, but when this valve just slightly opened/closed there is an unusually huge drop/rise in AA temperature, say for eg: 20degC drop. We checked the check valve parallel to booster compressor to see if the air flow in circuit is restricted because of it but the check valve was found working fine. Another interesting observation is that the discharge temperature of main AA compressor was measured using a temperature gun and found to be around 170degC, about 30degC higher than on our other machines.

Someone please reply, if some data is missing/needed please do let me know. Recently one of the AD-8 piping from compressor discharging casing got ruptured in the said machine.

GTG,
It's very difficult to relate Main AA Compressor inlet temperature differentials to AA pressure fluctuations on your Frame 6GE-design heavy duty gas turbine.
There is an air-actuated bypass valve around the Main AA Compressor on dual-fuel Frame 6B that is controlled by a solenoid. You didn't mention having checked the operation of that valve and/or its actuator or the solenoid. Have you reviewed all the components in the AA and Purge Air systems, per the Piping Schematics (P&IDs)? If so, what were the results of the review/analysis/testing?
60 deg C is VERY low for Main AA Compressor inlet air temperature. The temperature should be above boiling (to prevent condensation) and below approximately 135 deg C (which is the alarm setpoint; the regulator should be controlling the temperature to approximately 107 deg C when operating properly). A slug of water from the Precooler entering the Main AA Compressor won't do the compressor any good. And, if there is moisture condensing in the AA Precooler shell, you can tell by opening the Precooler shell drain when the unit is operating and seeing if there is water/moisture coming out of the shell drain (again--refer to the Piping Schematic (P&ID) which depicts the AA Precooler).
And large swings in temperature would be also be very difficult to understand, since a properly operating Robertshaw valve is very slow to respond. Very slow. Manually operating the valve could be part of the problem, but it's difficult to conceive how unless the actuator was devoid of fluid and/or the spring was broken.
So, there are just several things here which don't seem to add up (be related). Which might explain why you

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