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What is FSR gagging and when it should be done and what cares should be taken before enable and disable.
FSR Gag is primarily used for troubleshooting, especially when one is trying to determine the source of fuel control valve instability and load and/or speed instability. It's typically never used during normal operation (though I'm sure some sites have found unique reasons to do so for questionable purposes).
Let's say a turbine is burning natural gas and running at about 80% of rated power output and there is a pretty significant load swing of approximately +/-2 MW, for a total of four MW. Let's also say that because FSR is varying at roughly the same rate as the load is changing, that means the GCV (Gas Control Valve) is opening and closing similarly. And the SRV (Stop-Ratio Valve) is also opening and closing at a similar rate. And, the fuel supply pressure is also changing, increasing as the load decreases and decreasing as the load increases (so inversely to the load swings).
Is the load fluctuation (which means the fuel flow is fluctuating) caused by the GCV, or the SRV, or the fuel supply pressure?
To try to eliminate the GCV, one can use FSR Gag. Enable FSR Gag, and start lowering the Gag value from 100% down to very close to the current maximum FSR being experienced during the load swings (let's say that is about 55%). As you lower the Gag value below 55%, the GCV should begin to stabilize, FSR should not increase above the FSR Gag value.
If it were my site, I would lower the FSR Gag value (which would also lower the load!) to the point that, hopefully, the load swings would stop almost entirely. If they did and the SRV stabilized and the gas fuel supply pressure stabilized, then I would suspect a problem with the GCV or the GCV servo or the GCV actuator.
If, however, the load swings only decreased somewhat and the SRV did not stabilize and the gas fuel supply pressure also kept changing, I would first suspect a problem with the gas fuel supply flow-rate and/or pressure, second a problem with the SRV or the SRV servo or the SRV actuator. If the P2 pressure was reasonably stable but the gas fuel supply pressure was not stable, I would suspect a problem with the gas fuel supply flow-rate. Such as a problem with a filter or strainer or a pressure regulator upstream of the SRV. If the P2 pressure was unstable, then I would suspect a problem with the SRV or the SRV servo or the SRV actuator, or possibly a problem with the P2 pressure transducer.
But, without being able to eliminate at least one of the valves/possibilities, the troubleshooting could be quite difficult. So, FSR Gag can be very useful under this circumstance. It's not always the best choice for troubleshooting, but it can be an option.
When one has finished gathering data, then one needs to increase the FSR Gag value back to 100% to ensure that it isn't limiting fuel/power output. And disable FSR Gag, also. Usually, there is a Process Alarm to alert the operator that FSR Gag is enabled and/or if the FSR Gag value is less than 100%.
It can also be used for the liquid fuel control valve, really for any fuel control valve that's driven by FSR.
Let's say a turbine is burning natural gas and running at about 80% of rated power output and there is a pretty significant load swing of approximately +/-2 MW, for a total of four MW. Let's also say that because FSR is varying at roughly the same rate as the load is changing, that means the GCV (Gas Control Valve) is opening and closing similarly. And the SRV (Stop-Ratio Valve) is also opening and closing at a similar rate. And, the fuel supply pressure is also changing, increasing as the load decreases and decreasing as the load increases (so inversely to the load swings).
Is the load fluctuation (which means the fuel flow is fluctuating) caused by the GCV, or the SRV, or the fuel supply pressure?
To try to eliminate the GCV, one can use FSR Gag. Enable FSR Gag, and start lowering the Gag value from 100% down to very close to the current maximum FSR being experienced during the load swings (let's say that is about 55%). As you lower the Gag value below 55%, the GCV should begin to stabilize, FSR should not increase above the FSR Gag value.
If it were my site, I would lower the FSR Gag value (which would also lower the load!) to the point that, hopefully, the load swings would stop almost entirely. If they did and the SRV stabilized and the gas fuel supply pressure stabilized, then I would suspect a problem with the GCV or the GCV servo or the GCV actuator.
If, however, the load swings only decreased somewhat and the SRV did not stabilize and the gas fuel supply pressure also kept changing, I would first suspect a problem with the gas fuel supply flow-rate and/or pressure, second a problem with the SRV or the SRV servo or the SRV actuator. If the P2 pressure was reasonably stable but the gas fuel supply pressure was not stable, I would suspect a problem with the gas fuel supply flow-rate. Such as a problem with a filter or strainer or a pressure regulator upstream of the SRV. If the P2 pressure was unstable, then I would suspect a problem with the SRV or the SRV servo or the SRV actuator, or possibly a problem with the P2 pressure transducer.
But, without being able to eliminate at least one of the valves/possibilities, the troubleshooting could be quite difficult. So, FSR Gag can be very useful under this circumstance. It's not always the best choice for troubleshooting, but it can be an option.
When one has finished gathering data, then one needs to increase the FSR Gag value back to 100% to ensure that it isn't limiting fuel/power output. And disable FSR Gag, also. Usually, there is a Process Alarm to alert the operator that FSR Gag is enabled and/or if the FSR Gag value is less than 100%.
It can also be used for the liquid fuel control valve, really for any fuel control valve that's driven by FSR.
