We have Servo of rexroth and control system of BHEL for high pressure control valve in STG governing control. We are facing issue related to deviation in command to servo and feedback of vale opening. The deviation remains constant for some time and automatically matches after sometime. During this transition, the load of turbine also fluctuates, indicating opening of valve.
Anyone faced similar situation kindly comment.

 

Is the valve linear? I mean, if you send a command for 25% flow rate, does the valve also open 25%? Many valves are non-linear or have an equipotential flow curves.

 

The following steps can be taken to troubleshoot Servo valve actuation in STG (assuming you have servo electrohydraulic actuators for HP, LP valves as the case maybe for extraction type or condensing type turbines):
1. While unit is shutdown, Static test of servo valves should be done by accessing calibration mode in said governing system and forcing the analog output to the servo valves, and plotting a curve against given command % and actual lift in field in mm, and also noting the secondary oil pressure as each command is issues first in ascending and then in descending order. The curve has to be linear and should approximately match with the OEM control curve
2. Check for hunting in pressure in the Oil pumps (MOP or AOP) which supply governing and secondary oil pressure for the movement of the servo Valves as any fluctuation in discharge or header pressure of the oil can cause unwanted valve actuation.
3. Calibrate the valves in the governor configuration settings, changing the PI, PD and Gain constants can impact the valves performance. The valves can be tuned by tweaking the PID constants so that it is more strict or lenient in damping the offshoots.
4. I2H convertor currents (usually in milli amperes) can be checked to eliminate faulty converters
5. If else fails, the Turbine HP valve should be checked by Mechanical maintenance. Sometimes the cones which control the amount of steam going into the turbine chest is faulty and doesnt lift/gets stuck so it has to be mechanically repaired.


I would like some clarification regarding your query as well. Is the valve in question, HP side valve? if it is having deviation, what is the effect on the turbine observed? RPM fluctuation is also there I pressume. Is is running on Droop or Isochronous mode?