Oscillation of SRV and GCV in MS5002 C turbine with MKV.

When starting the gas turbine and completing the starting sequence, it is evident that the SRV and GCV gas valves are constantly oscillating. For this reason, impedance in the servo valves is checked, verifying the calibration of LVDT´S and the vote of the processors. What other factor could I rule out, thanks.
Is the unit reaching rated speed and producing power? Is it taking too long to reach rated speed and produce power? Or, is it taking less time than usual to reach rated speed and produce power?

When did this problem start? After a maintenance outage?

What is the hydraulic supply pressure doing when the SRV and GCV are oscillating?

The issue to understad is: What is causing the oscillations--is it the SRV or the GCV? In some cases, it's an unnatural interaction with the fuel gas supply pressure regulator and the SRV (a harmonic oscillation). What is happening to the gas fuel supply pressure when the SRV and GCV are oscillating during starting and acceleration? If the gas fuel supply pressure is also fluctuating, then something may be wrong with the gas fuel supply pressure regulator/control system.

The secondary purpose of the SRV is to maintain a pressure upstream of the GCV which is a function of HP speed. If the HP speed signal(s) are not stable, then there will be a problem with the SRV control.

The SRV outlet pressure (GCV upstream pressure) is called the P2 pressure. (It's also called the "interstage fuel pressure.") Some machines have a single P2 pressure transmitter, and others have three, redundant P2 pressure transmitters. If the P2 pressure transmitters are not all producing the same output signal that has caused similar problems in the past on many machines. On many Mark Vs, the P2 pressure feedback (input) signal is FPG2. And, it can be checked using the Pre-Vote Data Display to see how all three signals are doing (even if there's only one transmitter). The P2 pressure reference signal (again, derived from the HP speed signal(s)) is FPRG.

During starting (firing and warm-up) the GCV position is strictly a position reference--it should go to a stable position and stay there during firing and warm-up (I think Firenze uses the typical starting sequence for the two-shaft machines). In this case, if the SRV is unstable and the P2 pressure is unstable.

Once warm-up is complete (usually one minute after flame is established), then the Mark V opens the GCV to admit fuel to the machine based on an HP acceleration rate reference (signal name TNHAR). So, it tries to match TNHA (the actual HP acceleration rate) to TNHAR by modulating the GCV. Again, if the HP speed signals are not stable, then the HP acceleration signal will not be stable, and the GCV position will not be stable.

If the SRV is having trouble maintaining P2 pressure during firing, warm-up and acceleration then the HP speed will probably also be unstable, which could be contributing to the problem and making any other problem worse.

You have to try to understand what is the stimulus for the oscillation. Have you used the manual control feature of AutoCalibrate to slowly increase and decrease the SRV and GCV positions and see what happens to servo current as you do so?

Have you tried to use 'Verify Current' or 'Verify Position' of AutoCalibrate to check to see what is happening (look at the trend graphs produced when this happens to see if the device is not moving when it should be, or the LVDT feedback is unstable at some position(s)). REMEMBER: You have to run an AutoCalibrate on a device's LVDTs BEFORE you try to use 'Verify Current' or 'Verify Position.' And, if you then want to switch to another device, you have to run an AutoCalibrate on that next device's LVDTs to run 'Verify Current' or 'Verify Position' on that next device. You can just re-boot the processors when you're finished with both devices to return them to the EEPROM values from a previous AutoCalibrate.

Using 'Verify Current' and 'Verify Position' have been covered before on control.com. As has SRV/GCV instability.

Please write back to let us know what you find and how you fare in resolving the problem.