At 60 MW load when GT fuel changeover initiated from Gas to LFO and kept at 10% LFO, and GT trip with exhaust over temperature alarm trip. Found exhaust temperature went really high. LFO bypass valve went fully closed very fast before trip even though only 10% LFO. Speed pick up replaced for precautionary measure, cables of the speed pick up checked. Then again when GT fuel changeover initiated from GAS to LFO at 60 MW then at 10% LFO load shot up from 60 MW to 95MW, and found LFO bypass went fully closed very fast even at 10% LFO. This time GT was not tripped as immediately it was brought back to NG.

Could any one give me some suggestion what could be the problem?

 

You're on the right track, since flow divider feedback from the flow divider speed pick-ups closes the loop in the liquid fuel bypass valve servo-valve output (the reference is a flow-rate, and the speed feed-back is calibrated/converted to flow-rate). So, a sudden, momentary loss of feedback from the flow divider would cause the bypass valve to close suddenly. If, there was a sudden increase in flow while the bypass valve was closing, then you could experience the problems you are describing.

If you're confident it's not the speed pick-ups, the speed pick-up gaps are correct, and the wiring is okay, then it's likely that something is causing a sudden decrease in actual fuel flow which is causing the feedback to decrease which is causing the bypass valve to close--and then something is causing the flow to suddenly increase and the bypass valve servo-valve output either isn't responding quickly enough to the change or it can't respond quickly enough.

Likely culprit: air in the piping.

Next likely culprit: a malfunctioning fuel forwarding skid pressure regulator.

Not so likely culprit (but shouldn't be discounted): slipping liquid fuel pump clutch.

Least likely culprit (working in combination with one or both of the above): a servo-valve output gain that's not set correctly. (Unless, bypass valve LVDTs were recently calibrated, or someone's been mucking with the system!)

There's so many components in the liquid fuel system which aren't directly controlled by the Speedtronic or whatever turbine control system is in use. And they all have to work together properly. And good troubleshooting is a process of elimination.

In this case, the first thing to do is to make sure there's no air in the piping. If the unit is only run occasionally on liquid fuel, there's probably air in the piping, the fuel filter canisters; any high point in the piping. And, I'll bet big bucks there aren't any high-point piping vents which can be used to bleed the air out of the piping, either; most plant designers don't put them in.

If you're sure there's no air in the piping, then just start making sure the supply pressure to the turbine is stable and not fluctuating. Then, start trying to determine if there's a problem with the clutch (easier said than done; you'll probably have to have someone watching or videotaping (recorders are just so cheap these days!) the input shaft to the high pressure liquid fuel pump to see if the speed is constant or "slips").

 

When we are going for fuel change over at that time we slowly observe the process of changeover and see that there is no load fluctuation. Even if we are only at 10% LFO, and there is no feed back from speed pick up of flow devider, the by pass valve will close in order to send more fuel to turbine. It will keep on opening till there is some feedback from the flow devider.

Now if the flow devider has become sluggish or has jammed then in that case when the by pass valve has made sufficient pressure in the inlet of flowdevider by closing more and more, flowdevider will suddenly move and will send excessive liquid in the turbine and this will cause sudden shootup of exhaust temprature or sudden rise in load. This is a very dangerous condition and should be avoided. To prevent the recurrence of such event flow devider must be changed as this will eliminate possibility at least at this front.