A large power producer in the northeast US has a four unit power generation plant that operates on demand, based upon bid price to the area ISO. Plant efficiency and speed of response is crucial.

The Siemens PS2 digital valve positioner is being used on a feedwater and startup valve for Unit #3. The operations department fills the drum with the controls in manual mode. Startup procedures call for the operator to provide an output to the feedwater valve at a prescribed level. Repeatability and accuracy of the valve position in response to the controller output in “manual mode“ is critical to the drum fill procedure.   
The Siemens PS2 positioner replaces a competitor‘s analog I/P and a pneumatic input positioner. The PS2 unit includes a two-wire, valve position feedback transmitter to confirm the valve position to the operator in the control room. The previous analog I/P and pneumatic input analog positioner did not provide repeatable valve positioning with the controller in “manual.” The operations fill procedures called for an output to the valve of 50%. The plant experienced a slow fill that delayed their tie-in to the grid. Evaluation of the issue revealed that the valve was only at approximately 25% of travel. The existing analog devices in series providing input to the valve position was “drifting” with changes in temperature and vibration. Recalibration did not solve the control problem.