In one of Gas Turbine (Frame 6B) running on Liquid Fuel load is not increasing beyond 25 MW. In gas same machine was giving 30 MW. Up to 24 MW, FSR is in control. As soon as we increase preselect load to 25 MW, FSR starts increasing but MW does not increase even with FSR increase to max limit.

We have checked Liquid fuel system for choking, Liquid fuel bypass valve calibration etc.

Now what is left is changing with new LFBV and new liquid fuel pump.

Can there be any other cause for this?

 

vinayak,

I've read some of your previous posts and it seems the liquid fuel may include naphtha--which is very corrosive.

You said you check the calibration of the LFBV. I would be most interested to know how this was accomplished on a GE-design Frame 6B heavy duty gas turbine. Can you give me some details (I'm not looking for a step-by-step procedure--just the basics, thank you very much.)

If FSR is increasing, what is happening to the LFBV servo current?

If FSR is increasing and servo current is increasing in the negative direction--less than approximately -4.0% per processor (presumption is the turbine control system is TMR) and does not decrease to less than approximately approximately -1.33% then it certainly seems like something is not allowing flow to increase--and it could well be the high-pressure liquid pump internal tolerances have increased to the point the pumping efficiency is down significantly.

There is also a relief valve in the bypass around the high-pressure liquid fuel pump (usually in parallel with the LFBV), and if it's leaking when the pressure gets above a certain value than that could be the source of the loss of flow to the fuel nozzles.

Have you checked the liquid fuel pump clutch to be sure it's not slipping? The best way to do this is with a photo-tachometer pointed at the coupling between the output of the clutch and the shaft of the high-pressure liquid fuel pump. If it's slipping then you won't be able to "freeze" the coupling with the photo-tach strobe and it will be unsteady. If you can "stop" the rotation of the coupling by adjusting the photo-tach strobe and it stays "stopped" then the clutch is not slipping.

It's not likely a problem with the servo-valve (could be, but unless you have poor quality lube oil (which is used for the hydraulic system) and/or a plugged last-chance filter or a clogged pencil filter on the servo (all of these are not probably with a well-maintained lube oil system and periodic testing and "polishing" routine), the servo should really be the last thing you check--but it should not be forgotten in the process.

What is the null bias current being applied to each control processor? (I can't recall if this machine has a Mark V or Mark VI, or even a Mark IV, turbine control system--so could you please clarify this, also? Thanks!)

Most GE-design Frame 6B heavy duty gas turbines have a type of LFBV that does not allow visual indication of the movement of the valve stem/plug, and does not have LVDTs. So, it's difficult to tell if the plug moves through it's full range of travel when the turbine is operating.

That's about it. If you're certain there are no restriction in the liquid fuel supply (plugged pump suction strainers; non-working pressure regulator valve; etc.), and the filters (low- and high-pressure) are not plugged, then it's the pump or the pressure relief valve or the clutch or the servo. It could even be something is preventing the LFBV plug from moving and it's sticking at some point. That could even be the hydraulic actuator--so that's even another thing to examine. Use the Liquid Fuel P&ID to check and eliminate all the possible issues and you'll usually find the problem.

And, something to remember always: Many times there can be more than one problem combining to make it look like only one problem (multiple things can have the same effect!), so a process of elimination is a good thing, but sometimes it has to be taken a step or two further to fully resolve the issue.

Best of luck, and write back to let us know how you fare, please!

(Oh, and don't forget to give us a few details about how you checked the LFBV calibration.)

 

Are you sure that your Liquid Fuel LP supply pressure is OK. If you can increase it, maybe 0.5 Bar, and see if that helps.

 

This problem was solved once we replaced warren pump and LFBV. Warren pump internals checked and found OK, hence LFBV was giving the problem.
Though we are unable to find root cause, our fuel quality is causing problem to LFBV operation over time.

For LFBV, we are checking only stroke length and nothing else.