We’ve recently observed an unusual behavior at our power plant concerning the Fuel Gas Control Valve (GCV) position. At a constant load, the GCV position has increased from 32% to 47%, and it is currently around 50%, as shown in the attached parameter trend.

• The GCV appears to be following the FSR value correctly,
• However, such a shift in GCV position has not been observed previously under similar load conditions.
• In the past, we noticed that changes in the CV had some impact on GCV behavior, but that pattern no longer seems to apply.

We would appreciate if anyone can share:

1. Whether similar issues have been encountered in your experience,
2. Possible reasons behind such a GCV position increase at steady load,
3. Recommended checks or corrective actions we can perform to identify or resolve the issue.

 

There are numerous conditions that can cause increase in GCV position.

From compressor condition, air filter condition, ambient temp, IGV position etc....

The FSR is either temperature or speed controlled.

The controller is driving the valve to a position to maintain either required speed or temperature.

Firstly

Locate your Control Specification which should provide the FSR and expected GCV position for different loads.

Perhaps there is no problem.

 

@uneeba Ghazanfar,

The position of the GCV is a function of Speed Control (Droop or Isochronous) AND the P2 pressure--the gas fuel pressure between the SRV (Stop-Ratio Valve) and the GCV. IF the gas fuel supply pressure UPSTREAM of the SRV has decreased for some reason (low gas fuel supply pressure due to dirty/choked gas fuel filters or a partially blocked/choked y-strainer upstream of the gas control valve the and the SRV position has increased and is still unable to maintain the P2 Pressure Reference (usually signal name FPRG) then the P2 pressure upstream of the GCV will be lower than normal and to achieve the same load the GCV will have to open further. If there is a gas fuel compressor on the site it could be that the site gas fuel compressor isn't maintaining the proper supply pressure to the turbine, or it could be if the site has a pressure regulating valve to reduce the gas fuel pressure from the supplier it may be that the pressure regulator isn't controlling the proper pressure presuming the gas fuel pressure from the supplier is within normal range. But any of these conditions could be restricting the pressure/flow of gas fuel to the SRV, and the SRV will eventually move to the full open position as the pressure/flow isn't sufficient. Again, this will cause the operators/Pre-Selected Load Control will keep opening the GCV to try to maintain the desired load.

As @J OP wrote, you need to use the Expected Gas Fuel Characteristics section of the Control Specification and compare it to the actual running conditions--especially if you don't have any records of operation at a similar load. The Mark* turbine control system calculates the P2 pressure reference, FPRG, based on turbine speed (we presume the turbine speed is at or very near 100.0%) and compares it to the actual P2 pressure based on the signal(s) from the 96FG-2 pressure transmitters and adjusts the position of the SRV to try to make the actual P2 pressure reference equal to the P2 pressure reference as long as the gas fuel supply pressure to them machine can support the required flow to the machine.

You haven't told us what the other Process Alarms are that are being annunciated when this problem is occurring. You haven't told us when this problem started, such as after a maintenance outage when the GCV LVDTs may have been "calibrated" (incorrectly), or because the P2 pressure transducer valves weren't returned to the proper position after some maintenance procedure. We can only offer generalized possibilities based on the information provided. We also don't know if the machine is running on Droop Speed Control or Isochronous Speed Control or even what load the machine is running at.

That's all we can offer at this point based on the information provided. Please write back with more information if you want further assistance, and to let us know what you discover and how the problem was resolved.

Blessed day.