During weekly test for control valves for CV2 and CV4, the valves already opened and close according to the sequence of the test but the feedback of the servo current not coming as usual.

I ask how can solve this problem?

Thanks,

 

Without understanding where the anomaly occurred during the test and the amount of "error" from a previous test, it's very difficult to say. But, I don't believe the servo current difference you are seeing is related to a Mark V problem. The possible exception would be if any related Diagnostic Alarms were annunciated during the testing. Were there any new and unusual Diagnostic Alarms annunciated during the control valve testing?

It's always been my understanding that one of the reasons for performing the weekly tests of steam turbine control valves was to remove any build-up which might accumulate on valve stems or seats or plugs which may prevent full closure of the valves in an emergency. To do so may require more servo current than with a clean valve stem/seat/plug. Do you have any records of boiler water chemistry which may indicate higher levels of silica or other contaminants during the previous week? Have you changed water treatment chemicals/suppliers recently? Has the load changed recently (were you running at part load prior to the last week and have now begun to run at higher loads or full power output)?

I'm not familiar with the exact mechanism the Mark V uses (constant position reference change or constant current reference change) but it sounds like it might be a constant position reference (constant rate of change of position reference while monitoring the servo current required to keep the position feedback changing at a rate equal to the change of position reference). If there was a build-up on the valve stems that required extra current to remove, then that would explain the difference you are trying to describe.

It could also be a problem with the actuator. If the unit was previously running at a lower load and recently has been running at a higher load, actuator cylinders and valve stems (where it passes through the packing) have been known to wear in one area when operated there for long periods of time and cause sticking when the piston/rings pass through the area.

But, again, without more information (which would be difficult to provide in this forum) the extra servo current is likely not a control system problem. Try to consider all the possible reasons for the testing and then eliminate them one by one as you troubleshoot. A control system problem would likely be indicated if there were Diagnostic Alarms, but not always. Again, we don't know enough about the "excursions" of servo current or when/where they occurred during the testing, and without being able to look at the print-outs of the graphs it's nearly impossible to offer any more information.