Improve control loop performance

Specification of control valves doesn’t adequately emphasize the very basic requirement that valve position respond in a timely manner or even at all — leading to process variability.

By Gregory K. McMillan, Emerson Process Management

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Particularly insidious on a rotary valve is a digital positioner feedback that uses actuator shaft position rather than closure member position because the positioner can think the valve moves when the ball or disk sticks. The response for Valve B in Figure 3 shows how such a positioner sees a change in position for step inputs of 1% when in fact the flow hasn’t changed until the step inputs are 5%. For some rotary on/off valves made into control valves by putting on a digital positioner, the actual deadband was 8% even though the smart positioner had extensive data showing the deadband was 0.5% [7].

Figure 3. Even at mid-throttle the flow through Valve B only responds to larger-than-normal changes in the input signals.

Figure 3. Even at mid-throttle the flow through Valve B only responds to larger-than-normal changes in the input signals.

Resolution and deadband add a dead time to the loop beyond that due to the actuator and positioner. This can be estimated as the resolution or half of the deadband divided by the rate of change of the controller output [1]. Resolution will cause a limit cycle (constant amplitude persistent oscillation) in any loop regardless of tuning [1]. For an integrating process such as level with a controller with integral action or a cascade control system where both the primary and secondary controller have integral action, deadband also will cause a limit cycle [8].

The amplitude of these cycles is the resolution or deadband multiplied by process gain for the process variable of interest [9]. For temperature and pH loops, this process gain can be 10 or more and can cause severe oscillations and process problems. Whereas problems from nonlinearity and response time are triggered by disturbances and tend to die out if the controller is properly tuned, limit cycles are continual. A digital positioner with good closure-member feedback that is tuned with a high gain and rate action can significantly reduce the amplitude of the limit cycles.

For pH control, the resolution of a reagent valve can determine the number of stages of neutralization needed. A small (fine adjustment) valve in parallel with a large (coarse adjustment) valve simultaneously manipulated by a model predictive controller can extend the sensitivity and rangeability of a reagent system enough to eliminate a stage [6].

Throttle your valve problems

A control valve package is only as good as its weakest link, whether it’s the actuator, positioner, feedback mechanism, packing or valve design. If the control valve and actuator are similar to those used for isolation valves, you’re a candidate for significant limit cycles (sustained variability) in your process. This is particularly a problem with packaged equipment (skids) where control valves are chosen based on piping specifications and lowest price rather than loop performance. Cost-effective solutions exist.

For example, a sliding stem valve designed for minimal seating friction and packing friction, coupled with a diaphragm actuator and a smart positioner, can reduce resolution and deadband to better than what you can achieve with a standard variable speed pump. Figure 4 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% (the smallest step listed in the ISA standard).

Figure 4. A sliding stem valve with a digital positioner can provide high resolution near seat.

Figure 4. A sliding stem valve with a digital positioner can provide high resolution near seat.

References

  1. Blevins, Terrence L., "Advanced control unleashed,” ISA, Research Triangle Park, N.C. (2004).
  2. McMillan, Gregory K., "The next generation — adaptive control takes a leap forward,” Chemical Processing (Sept. 2004). Online at www.chemicalprocessing.com/articles/2004/145.html
  3. "Test procedure for control valve response measurement from step inputs,” ISA standard ISA-75.25.01-2000 (R2006), ISA, Research Triangle Park, N.C. (2006)
  4. "Control valve response measurement from step inputs,” ISA technical report ISA-75.25.02-2000 (R2006), ISA, Research Triangle Park, N.C. (2006)
  5. McMillan, Gregory K., "A funny thing happened on the way to the control room,” www.easydeltav.com/controlinsights/FunnyThing/default.asp
  6. McMillan, Gregory K., "A fine time to break away from old control valve problems,” Control (Nov. 2005). Online at www.controlglobal.com/articles/2005/533.html
  7. McMillan, Gregory K., Plant design category, http://ModelingandControl.com
  8. McMillan, Gregory K., "Life is a batch," Control (May 2005). Online at www.controlglobal.com/articles/2005/379.html
  9. McMillan, Gregory K., "What is your flow control valve telling you?," Control Design (May 2003). Online at www.controldesign.com/articles/2003/164.html

Gregory K. McMillan, is a principal consultant for Emerson Process Management, Austin, Texas. E-mail him at Greg.McMillan@EmersonProcess.com.

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