Gas turbine speed and bleed valves

N

Thread Starter

Napster

We have two Westinghouse W251B11 gas turbines with iso ratings of 50 MW. Rated synch speed is 5418 rpm.

Both machines have LP and HP bleed valves on 7th stage and 13th stage respectively.

During start up ramp as soon as machines reach 5000 rpm "bleed valve close command" is given to HP valves which closes in 5sec and as soon as its closing is confirmed command to LP valve is given which takes further 5 sec. During this period compressor discharge pressure understandably increases along with the actual speed while the speed command or reference remains steady. Ideal scenario according to my experience is that after closing of bleed valves difference between actual and reference should not be grater then 57 (actual speed leading the reference speed).

Now the problem with one of the machines is that during this time actual speed to reference speed sometimes increases to 60~70, which gives a sudden close command to pneumatic governor. IF the close command to governor is less then 8~9% machine speed decreases to 4940 rpms and from here governor is unable to attain the speed reference (now 5150+) with in time and ultimately machine trips.

If the difference between actual and reference rpm remains under 60 close command to governor remain above 13% and drop in rpm remain above 5000 which results in successful start.

What is causing the speed to jump above 60 rpms as before 5000 difference between reference speed and actual speed remains 1-2 rpms?
Demand from temperature controller remains above speed controller this whole time (override control).
 
> "Check your fuel control valve for this variation".

It was the first thing that was checked. It behaved normally offline. The problem is not the valve but sudden decrease in the demand due to speed difference. As i said if speed difference remains with in limit demand remains normal and if it exceeds that limit demand to fuel valve decreases suddenly. This fuel valve behaves very precisely on other speeds.
 
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