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May
10

Control Valves on the Skids

Packaged equipment is notorious for using the lowest priced instrumentation when customers base their selection solely on lowest bid. If the user was to consider the cost of maintenance and variability, the same type of consistency, intelligence, and performance demanded for their process installations would be required. When I worked on projects for Monsanto’s premier plants in Texas light years ago, bids for packaged equipment had to meet rigorous specifications and an acceptable vendor list for automation. These requirements were eventually made a corporate standard.

The benefit of Hart or Fieldbus smart instrumentation and digital valve positioners is becoming more commonly recognized as important everywhere. Still overlooked for skids is the core performance of the sensor or valve. Diagnostics provide a lot of data but may not give the most important variables of interest to reduce variability (repeatability for measurements and resolution for control valves). Particularly insidious is a common practice of using rotary on-off valves that employ actuator shaft rather than ball or disk position for feedback. In these cases, the digital positioner can be storing and sending gobs of data showing the valves resolution is 0.5% when in fact the resolution in terms of movement of the actual ball or disk is 8% (see April 9 entry “Control Valves that Can Turn on a Dime, or at Least a Quarter – Part 1”). Cost effective solutions exist. For example, a sliding stem valve designed for minimal seating friction and packing friction, with a diaphragm actuator and a smart positioner can provide exceptional resolution. The following test result shows that even operating near the seat a sliding stem control valve with a digital positioner can respond to changes as small as 0.1%. Note that the ISA technical report ISA-75.25.02-2000 (R2006) “Control Valve Response Measurement from Step Inputs” suggests that a resolution of 0.1% to 0.5% is a good valve. If you also consider that data on resolution supplied by valve manufacturers is for a mid throttle position where seating and sealing friction are minimal, the results shown here are particularly noteworthy. Besides reducing the limit cycle in the controller output from the resolution to the noise level in the process, such performance extends the rangeability and sensitivity of the valve, which for pH control can eliminate a stage of neutralization.

Valve with Exceptional Resolution Near Seat

Not commonly recognized is that resolution is more important than repeatability for the control valve, because the process control loop will correct for changes in the magnitude of the valve response. If the valve’s plug, ball, or disk doesn’t move, there is nothing the loop can do. Strangely enough, valve specifications do not normally require the valve actually to move in response to the size of the controller output changes seen during the execution of the loop.