April 2013 MI was performed. IGV blades, gears, pinion and actuator were replaced. Since then during start up at @ 85 % of GT speed IGV starts opening and not limiting at 57 Deg A but reaches upto 72 DEG A for a while and then come back to 57 GEG A before FSNL. Exhaust temp reaches 576 deg C max. before IGV starts opening. What could be cause?

 

Jigs,

The problem started when the IGV actuator was replaced during the outage. It's not clear if the electro-hydraulic servo-valve was also replaced at the same time, and if the proper servo-valve was used for the replacement, and if the polarity of the current applied to each coil was verified during the commissioning/start-up activities.

It is also presumed that the IGV LVDTs were "zeroed" properly and calibrated properly after the outage.

Have you tried manually positioning ("stroking") the IGVs using AutoCalibrate's Manual Position feature to see how the IGVs are behaving? If so, what did you observe?

Have you used the 'Verify Position' or 'Verify Current' features of AutoCalibrate, and if so, what were the results?

Since the IGV actuator was replaced, it's likely the IGV LVDTs were calibrated. Was the Null Bias Current value changed during the calibration procedure, and if so, what is the current value? Was the regulator gain changed during the commissioning/start-up, and if so, what was it changed to?

Even if the Null Bias Current- and IGV Regulator Gains weren't changed during commissioning/start-up, what are they?

From the information provided, it would seem that no Control Constants were changed. So, presuming the reference is being calculated correctly (and there's no reason to believe changing the IGV actuator would change the way the IGV position reference is being calculated) then the most likely cause is some problem with the IGV assembly (actuator, bushings, etc.) binding, or the actuator isn't properly bolted down, or the servo-valve isn't the right one, or the actuator isn't working properly. But, from the information provided, it would seem something is causing the actuator/IGV assembly to overshoot (travel past) the reference setpoint and then the Mark V has to change servo-current to make it return to the proper value.

If the Mark V has to keep putting out more current to make the IGVs move because they are either not moving or are sluggish to move (mechanically sticking or binding) and then when they do move they travel past the reference setpoint and the Mark V has to change servo current to get them to come back to reference setpoint, it's not a control system problem--it's a mechanical problem.

Please write back and let us know how you fare, or if you need more help/information.

 

Thanks CSA,

All data pertaining IGV calibration, gain etc. are with C&I dept, i'm from operation, i will get it and post here. We also believe its mechanical, problem. Anyway thanks for details and reply.