Hello Everyone

I am an operator working at a combine cycle power plant and we have 3 GE 9Es installed with Mark VIe control. My question is related to the operation of Turbine bearings sealing air supply transfer valve (VA-14).

Wondering when this valve switches its position, as there is no information available in GEs documentation about the conditions in which this valve would operate. I already know this valve is used to allow cooling and sealing air from stage 11 during start up, but am wondering when it changes its position to resume air flow from the 5th stage.

 

This valve is operated by Compressor Discharge Pressure (CDP). There should be a setting in your Control or Instrument Spec. which gives you the changeover value. Take a look at your Cooling & Sealing Air P&ID. Unfortunately I don't have a set of Frame 9 drawings to hand. Why do you need to know the value, are you having problems?

 

Just to add one thing--the valve's position is NOT controlled by the Mark*. As glenmorangie said, CPD (axial compressor discharge pressure) acts on the diaphragm in the actuator and causes it to move, shifting the source of the bearing sealing air. If you watch the valve during start-up/acceleration, it will begin to move as CPD increases. It's not like a solenoid-operated valve that opens or closes when the solenoid is energized or de-energized.

It's very similar to the False Start Drain Valves--they are closed simply by CPD; as CPD increases the False Start Drain Valves close (except on many F-class machine, of course they have to be different because they are spatial).

I think if you look at the sequencing for the alarm that says the valve is not in its proper position, it just checks for the proper position after the unit reaches 14HS--or sometimes after the generator breaker closes.

Please write back with what you find.

 

> I already know this valve is used to allow cooling and sealing air
> from stage 11 during start up, but am wondering when it changes
> its position to resume air flow from the 5th stage.

You are right. Purpose of this valve is to select between 5th and 11th stage discharge air to be used for sealing of bearings.

As explained by others, this is a self actuating valve, not controlled by GT control system. During the initial startup phase, when the speed of GT shaft is lower, the pressure at 5th stage of compressor will be a lower value than what is needed for the sealing. For this reason, air from 11th stage, which is at a higher pressure is used.

When the pressure at 5th stage reaches a desired value, the valve switches to accept the air from 5th stage. This scenario is applicable for a shutdown sequence as well. The set pressure for VA-14 valve is the parameter that defines this change over. This pressure value should be available in the device summary or instrument list. Depending on the life of the machine and ambient conditions, the change over can happen at different speed levels of the GT shaft.

Hope this helps.