Please i need detail explanation on working principle of limit switches 33cb on compressor bleed valve VA2-1 and VA2-2, and logic state of the limit switches when in open and close position.
Don't you have a Device Summary, Control Spec. or a set of drawings? The limit switch is closed when the bleed valves are Open (de-energized).
You didn't say what Frame unit you are referring to, nor who the packager of the unit was.
IN GENERAL, the axial compressor bleed valve limit switches are installed as normally open switches--which are actuated by a lever on the bleed valve shaft to close the switch when the bleed valve is fully open.
In other words, the switch contacts are to be held closed when the compressor bleed valve is fully open. To protect the axial compressor during starting and acceleration the compressor bleed valves must be fully open prior to initiating a START--and the closed contact (actuated by the lever) provides the indication to the turbine control system that bleed valves are fully open.
Also the bleed valves must be fully open during shutdown to protect the axial compressor. If the fuel stop valve is still open during shutdown and the compressor bleed valves are not fully open the turbine control system will trip the turbine by immediately closing the fuel stop valve.
Hope this helps!
We had a problem with the bleed valve twice before during shut-down.
During turbine shut-down, the bleed valve should be opened when the turbine speed reaches 95% (14HS - Operating Speed). The problem was that the mechanism that pushes the limit switch didn't fully reach out. The bleed valve opened at 95% but the feedback signal didn't go to the Mark V, so the control 'thought' it was still closed and tripped the turbine without executing the shut-down sequence. We just needed to replace the adjustment screw with a longer one.
Put your e-mail and I will send you some photos.
A good wiring diagram guide for reference
Bleed Valves in heavy duty gas turbine are used to protect the axial compressor during start-up and shutdown against stalling and surging conditions. These valves are typically open during start-up, acceleration, normal stop & shutdown.
L20 CB is the signal assigned to the Solenoid valve which control the air taken from axial compressor (stage).
the bleed valves are spring return valves, normally open. during the start up when the machine accelerate & just before complete sequence, the SOV will be energized allowing the air taken from the axial compressor stage, to actuate the bleed valves than L33CB1, 2 change status feedback.
during shut down the SOV de-energizes, cutting the air from the bleed vales actuators, than both bleed valve will open.