Our is 6B gas turbine in which the IGV refused to open from the closing position of 34 degrees. During many trial runs as the unit attains 82% speed, it will trip. the following components had been checked.

Main filter-replaced.
88HR and FH6-1 replaced.
Voltage across 90TV and 20TV checked and found okay. The discharge pressure indicates 80bar.

WE have stroke the IGV (logic force) and the actual equal the reference i.e 34, 57, 87 degrees

Kindly help/explain.
my regards.
dottey.

 

Can you give some more detail on the trip during start-up. What Alarms do you get? Do you see the IGVs attempt to move ? Are you sure it is at 84% speed?

 

When did this problem start? After the servo was replaced? After some card was replaced? After some maintenance outage?

We don't know what kind of control system is being used on the machine. Is the control SIMPLEX or TMR? If the control system is a Speedtronic Mark IV, Mark V, Mark VI or Mark VIe, are there any Diagnostic Alarms present before the trip? What Process- and Diagnostic Alarms are present before and after the trip? Please list all the alarms, Process and Diagnostic, from 60 seconds prior to the trip to 10 seconds after the trip.

We don't know what the voltages across the servo coils are (90TV); we would need to know what the voltage across each coil is. The servo-valves are bi-polar devices, so the polarity of the voltage (and current) being applied is important. The coils are usually about 1K ohm resistance, and the applied current should be between -10 mA and +10 mA, so the voltage should vary between approximately -10 VDC and +10 VDC. When the unit is at steady state condition, the voltage should be approximately -0.267 VDC per coil, for a TMR system, and approximately -0.8 VDC for each coil of a SIMPLEX system.

Again, polarity is very important. Look at the wiring drawings to be sure you are measuring the voltages correctly with the voltmeter leads. Be sure to measure the voltage across each coil identically.

When the unit is at rest, usually the IGVs are at some position less than 34 DGA, and during starting up to the point at which they begin to open they should be at approximately 34 DGA. Is this happening at your site? Or, have the IGVs been calibrated such that when they are fully closed against the minimum mechanical stop (usually the actuaor stop, not the IGV ring stop) they are at 34 DGA?

When you say the voltage across 20TV is correct, what is it? Are you sure there is trip oil pressure at the IGV actuator when the unit is running? Refer to the IGV Piping Schematic drawing (P&ID) for information. You may need to install a pressure gauge to be certain of the presence of trip oil at the IGV actuator "after" 20TV to allow high-pressure hydraulic oil to flow to the actuator to open it.

What is the IGV control reference (usually signal name CSRGV) doing when the unit is accelerating and reaches the point at which the IGVs should open to the minimum modulating position? When CSRGV is increasing above 34 DGA, the servo current from a Speedtronic turbine control panel should be negative. (Negative servo current opens the IGVs; positive servo current closes the IGVs.) If the IGVs are not moving when the reference is increasing above 34 DGA, then the magnitude of the servo current should be increasing in the negative direction to try to make the IGVs move to follow the reference.

But the biggest question is, and always should be when troubleshooting: What has changed? When did this problem start?

And, please, try to provide more information about the unit at your site when posting. A Frame 6 gas turbine is pretty much a Frame 6, from a mechanical perspective. But, the packaging (auxiliaries and devices and instruments and generator) and the control system can be very very different from one Frame 6 to any other. We aren't at your site, and your site is not exactly like any other site. We can speak in generalities, but without knowing something about the unit at your site, that's all we can do.