our MS 9001 fa gas turbine SRV has been stuck at 36.5% during load reduction. it is not going below that. so our SRV not closing below that. if we further reduce output load, our intermediate pressure increasing. if we again increase load, then its working properly its opening was proportionally to the load increasing. during load reduction the reference signal was not coming to dcs. if we want to reduce load or shut down, the turbine what we have to do.

 

I have seen this happening. In our case, it got stuck at ~40%.

The first time it happened, it got stuck at the 40% position. P2 Pressure increased, speedtronic commanded the servo to close. Since it didn't close, the reference kept decreasing till the point it was asking the valve to full close (servo applying full force to close the valve). In your case, despite the full force of the servo (reference=0), the valve is not moving.After a couple of seconds, the valve started to move (close) suddenly - under the full force of the servo. The dip in output was so great that mark-vi commanded the SRV to open quickly to compensate for the loss of power - the sudden increase of fuel flow caused an exhaust over-temperature trip. SRV is just a pressure reducing valve meant to maintain a nice and steady P2 for the GCV, its regulator is not designed to compensate for huge valve position fluctuations. When the GT was shutdown (with no high pressure hydraulic oil available, even then the valve continued to be stuck! We then applied the hydraulic oil pressure by forcing a few signals and turning the High pressure pump (88HQ) on, and then commanded the valve full open. It responded. Then we commanded the valve to close fully- and it did. We did find some scoring marks on the shaft. It was lightly smoothed out by rubbing emery paper. We then lubricated the guide rails on which the carbon bushing of the SRV actuator slide.

Then we stroked the valve in small steps, and progressively bigger steps. Once we were satisfied the valve was free, we started the unit and it worked fine. But the problem re-occurred about 2 months later. But at that time, it became unstuck itself. We changed over the GT to HSD, shutdown the unit, and did the same activity. It has been working fine since then. We have made it a practice to check the SRV in every planned (and unplanned) outage. There are many reasons of why an SRV actuator can become sticky at a particular point - but right now no one knows where the problem lies in your SRV.

Can you changeover to secondary fuel? Do you have liquid fuel firing capability? To your question of " if we want to reduce load or shut down, the turbine what we have to do", you can do one of these things:

1- Change over the liquid fuel, shut down the unit normally, and inspect the valve - or actuator. These could be many things that could be wrong with it, and there isn't much you can do in the way of diagnosing the problem if the GT is online.

2- Keep the load on GT as high as possible so that it stays out of the SRV's "sticky" region.

3- If you have only single fuel firing on your GT, then you can try and shutdown the unit. SRV does not have a "not following reference" trip when the unit is on load. So even if the SRV stays stuck, hopefully the GCV can control the load. But I guess you would have some sort of tripping before that, ultra high P2 maybe? Its worth a try - better than manually tripping the machine anyway. If you control over the pressure at which fuel is supplied to the GT, you can try and match that to the P2 reference. This would help you to achieve a smooth shutdown and avoid a trip.

4- Trip the unit - but there is no need to. Try step 3 and keep your fingers crossed!

At any rate, I would suggest that you shutdown the unit and inspect the valve, the actuator, the servo, the hydraulic oil circuit - there are just too many unknowns at this point. If this valve gets released and starts closing rapidly (as it did on our unit), and then opens again, you run the risk of over firing the GT. In our case, this was the final straw that exposed a combustion liner cooling problem, which became apparent in the boroscopic inspection we did right after the GT shutdown due to the sticky SRV.

 

Many F-class turbine installations have a gas fuel compressor. If the pressure output of the gas fuel compressor is variable, then you might be able to reduce the gas fuel pressure as you reduce load to try to keep the SRV above the sticking point. Once you get down to the point where the generator breaker opens, you might then just want to mash the emergency stop push-button and then set to work on resolving the sticking issue.

I don't really understand the statement about some signal not going to the DCS. Is the DCS using some information from the SRV function to control gas fuel pressure (say, to modulate the gas fuel compressor discharge pressure)?

Lastly, most F-class turbines have some kind of DLN combustion system. It's pretty important not to allow the P2 pressure to get too high (nor too low) in order to maintain the pressure drops across the fuel nozzles. So, just allowing P2 pressure to hit a really high value if the SRV doesn't close is not a good idea for the DLN combustion system.

And, as was said, it's pretty likely that once the valve does actually move it will "jump" to an undesirable position and then the Speedtronic will try to move it back to another more open position. A trip on loss of flame or exhaust over temperature is pretty likely, but if the unit doesn't trip it's likely that operation (output) will be unstable as the valve "hunts" around the sticky point.

Lastly, even if you succeed in getting smooth operation when the unit is shut down it's pretty likely that when there is fuel flowing through the valve and different pressures and forces are at work in the valve and on the plug and actuator that it will stick again. Better to just remove and replace the valve with a spare, or remove it and repair and/or refurbish it as quickly as possible. If it's one of the Woodward gas valves, better to just remove it and have it refurbished rather than to try to repair/refurbish it on site.

While it may be a servo problem, it's more likely an actuator problem or a valve stem/guide/packing/seal problem.

Just out of curiosity, is the Speedtronic a Mark V or a Mark VI or a Mark VIe?