When 20CB failed in Frame 9E

I have gas turbine power plant frame 9.

WHEN THE UNIT IS RUNNING (GENERATING POWER)!!!

Frame 9E, 90 MW, Speedtronic MKV, TMR,(No DLN). When 20 CB (L20CB1X=0) failed (coil got burnt)

*Around midnight both the bleed valves opened. After opening bleed valves.
*GT power generation reduced !!!
*FSR increased !!

*If It Happened Didn't it trip TG ???

 

bil07,

>*If It Happened Didn't it trip TG ???

No; and because a large portion of the axial compressor flow was exiting through the compressor bleed valves the mass flow through the unit will decrease, which will decrease the CPD, probably slightly increase the FSR, and probably significantly decrease the power output of the unit.

If the unit had DLN-I combustors--yes, the turbine would trip.

Was it hard on the combustion hardware--yes.

But, should it necessarily trip the turbine? No.

Have a look at the L86CBT logic--I think you'll see that the only time the unit trips on bleed valve "failure" is if they fail to open when shutting down. That's pretty typical for Mark V-vintage machines with conventional (diffusion-flame) combustors. An alarm would be generated when they failed to close within 11 seconds of being commanded to close, or if they failed to open with 11 seconds of being commanded to open.

So, there should have been an alarm, "COMPRESSOR BLEED VALVE POSITION TROUBLE" (or something similar)--but not necessarily a trip.

If the unit was operating at Base Load when this occurred, it's possible that an Exhaust Overtemperature Alarm, and maybe even an Exhaust Overtemperature Trip, would occur. And, the unit should not be operated at Base Load (with the reduced power output) for an extended period of time as this would be hard on the axial compressor, primarily.

But, no; it's not necessarily a trip event.

Some of the early Mark V-vintage machines (and those commissioned prior to the Mark V generation--so if this machine was upgraded to Mark V from a previous version of Speedtronic turbine control system this will likely apply) used a compressor bleed valve solenoid with a mechanical switch which reduced the current drawn to hold the solenoid closed. The switch allowed a high inrush current, sometimes as high as 10- or 11 amps for a very short period of time (1 second or less) and then the mechanical switch, sensing the valve had closed, would reduce the current to something less than about 1 amp. The high inrush current is necessary to open the solenoid-operated valve to send air pressure/flow to the bleed valve actuators; but once the bleed valves are closed only a small amount of current is required to keep the solenoid-operated valve open.

There is supposed to be a fuse in one side of the 125 VDC supply (the positive leg, if I recall correctly) to protect the Mark V circuitry and wiring if the mechanical switch didn't operate fast enough to reduce the current from about 10-11 Amps to around 1 amp. The fuse, mounted in the <PD> core, was either a 2 amp "slow-blow", or a 3.2 amp "slow-blow" fuse. (2 amp fuses were used early in the production of the Mark V; and later models used 3.2 amp fuses.)

The "slow-blow" fuse would allow for a higher-than-rated current flow for a short time (a couple of seconds or so) before operating if the current remained above the fuse rating. If the coil burned, I'm surprised the fuse didn't blow.... Which makes me very suspect of the fuse condition/rating. Many times these circuits blow fuses and they get mistakenly replaced with a much higher fuse rating. Or, sometimes, if the fuse doesn't blow fast enough the circuitry of the Mark V gets damaged so the relay output gets moved to a circuit which doesn't have a fuse.... And, then when the current is high, the circuitry gets damaged again.

So, please have a look at the fuse for what should be the 20CB-1 relay output--it should be FU-27, sometimes FU-28. You should find the schematic of the circuits for CO16 and CO17 in the last pages of Appendix D of the Mark V Maintenance Manual, GEH-6195 (Rev. B or later). Usually CO16 is the output used most frequently for 20CB-1; CO17 is used for 20FD-1. If the compressor bleed valve solenoid output is on any other contact (relay) output, there is reason for concern.

Hope this helps!

 

thanks csa

So, i a look at the fuse FU-28. failed !!!!
bat can help me when 20cb d excite whate for tubin ????

 

bil07,

I think about what happened when the Bleed Valves opened WHITH 90MW!!!!

 

bil07,

I can't understand the question(s).

FU-28 is usually for 20FD-1 (does the unit also run on liquid fuel?).

A ground could cause the fuse to blow, and that could happen if the insulation in the solenoid gets wet or "burns"/shorts to ground.

 

hi
FU-27 is usually for 20CB !!!!!

 

Well said by CSA.

take care
g.rajesh

 

bil07,

> FU-27 is usually for 20CB !!!!!

Please explain this comment. Yes; FU-27 is usually for 20CB-1, and FU-28 is usually for 20FD-1 (if the unit has liquid fuel capability--this is the Liquid Fuel Forwarding Stop Valve Solenoid--another large solenoid-operated valve, usually made by the same manufacturer as 20CB-1, which requires a large in-rush current and a small holding current).

You indicated you found FU-28 blown. You were asked about FU-27, since your original post was about a "failure" of 20CB, and FU-27 is usually used for 20CB-1.

What's the status of this issue?

 

THANKS CSA
YES DIFAILLANCE FU-27 is usually for 20CB GAS TURBINE 9001E WITH FUEL GAS