Refer to the attached dwg. what's the function of VA18-1 in the atomizing air circuit of GE frame 9E?

http://tinypic.com/2mply807

 

Can anybody help me out here; because I heard that VA18-1 is normally closed and opens when unit runs on gas. But here, I see the atomizing air, all the way passes through this valve.

We're on a construction stage, installing a GE PG9171E turbine with conventional burners.

 

Black,

The valve appears to be a fail-open, air-operated control valve that is used to limit the air flow to the Main Atomizing Air Compressor (the Accessory Gear-driven Atomizing Air Compressor).

Before GE took over the Belfort, FR, manufacturing/engineering facility and gave them carte blanche to change long-standing control concepts and philosophies, VA-18 was used to reduce the air flow through the Atomizing Air Compressor when a dual-fuel GE-design heavy duty gas turbine was operating on gas fuel (reduces energy consumption off the turbine shaft when operating on gas fuel). It was a simple open/close, solenoid-operated valve, that reduced the air flow through the Main Atomizing Air Compressor when open. No muss; no fuss. (Some valves on larger turbines were air-operated via solenoid, but they were still open/close--not infinitely positionable).

Now it's a pneumatic control valve, with an I/P positioner, which requires an analog control signal (probably 4-20 mA), extra wire and conduit, calibration, and a clean source of dry instrument air through a pressure regulator. This means more expensive spare parts, more complicated control schemes, more I/O, more maintenance, and just MORE money and time and aggravation, which increases the cost to the buyer.

 

Dear CSA

Thank you for your kind response. I understand that this valve is essentially limiting the air flow that is to be drawn at main compressor suction; perhaps with much more control than conventional designs to maintain certain pressure ratio. Do you think that this infinitely positioned valve serves to maintain definite pressure ratios at different loads?

Secondly, if the valve is on line all time; no matter unit being operated by gas or liquid; shouldn't it be a fail close type just as gas valves VSR or VGC?

 

Black,

I wouldn't think of second-guessing the Belfort Bunch. It was only my interpretation of the notation FO on the drawing that led me to believe it was a fail-open valve, which may also be incorrect.

Most dual-fuel machines these days are designed to be primarily operated on natural gas, particularly DLN combustor-equipped units. Also, since it's important to get as much atomizing air to the fuel nozzles as possible during starting on liquid fuel, I think the fail open concept follows that philosophy. Since we can't determine where the air source for the VA-18 actuator is coming from the drawing (maybe if I could read French...) it still kind of makes sense for the valve to be fail open.

If you want to question the Belfort Bunch design, then question the GE engineers on site and have them get a response from their engineering support group.

As I understand it, there have been some changes the DLN system recently and this may be one of them in order to maintain emissions.

As for the control scheme used to position this valve, I would say the presence of the differential pressure transmitter across the Main Atomizing Air Compressor would suggest that the control system is monitoring the dp and possibly adjusting the valve position to maintain a certain dp depending on the operating conditions. Again, this would be a question for the GE personnel, or which could be answered by reviewing the application code in the Mark VIe for the turbine.

Please write back to let us know what you learn!