Guidance for Good Dynamic Control Loop Performance

If ever there were a need for dynamic control, it would be for the global bond and equity markets this week. Instead of tackling that global challenge, with today’s post I’ll highlight Emerson senior process control consultant Mark Coughran‘s Emerson Exchange presentation. He presented, “Equipment Selection and Installation for Dynamic Loop Requirements” last week.

The purpose of Mark’s presentation was to share ways to extract more value from the control system by designing it for the process dynamics or to diagnose and improve control in an existing installation by better understanding the process dynamics.

A key dynamic control requirement for the loops is that process variable (PV) changes should move to a new setpoint (SP) without oscillation. Recovery from process disturbances should also occur without oscillation. The loops should run in a cascade or automatic mode without the need for operator intervention. And finally, the loops need to respond at the speed required to meet the process objectives.

Mark stressed the importance of properly selecting, installing and maintaining the final control elements (like control valves), transmitters and controllers. No amount of tuning can overcome improperly sized or malfunctioning loop elements.

Given Mark’s experience in working with process manufacturers, here’s his top-ten list of mistakes that he encounters:

  • Oversized control valves
  • Inappropriate valve flow characteristics
  • Non-communicating, low-performance valve positioners
  • Positioners not tuned for the damper/actuator
  • Inadequate signal conditioning (thermocouples)
  • Incorrect range/resolution of transmitters
  • Poor physical location of transmitters
  • Oscillatory tuning of loop controllers
  • Unit controller tuning ignores interactions
  • Inappropriate control logic for switching block valves

Mark’s presentation included a number of specific examples including refinery boiler feedwater control, reactor jacket temperature control, Olefins plant reactor temperature control, boiler O2 control, acetylene reactor temperature control, beverage-quality CO2 heat exchanger gas pressure control, pulp mill lime mud density control, Lysine plant resin bed level control, and chiller level control. This is a wide-ranging list of control challenges, but the common thread is to start with an understanding of the process dynamics by measuring them.

The solutions Mark discussed ranged from control valve trim changes, to Lambda loop tuning, to thorough operating training, to application of advanced control strategies like model predictive control.

Some of the common threads of Mark’s guidance were:

  1. Use high-performance smart positioners
  2. Specify the control valve/damper dead band, response time (ANSI/ISA-75.25.01-2000)
  3. Select the valve size and flow characteristic for installed gain
  4. Use a signal characterizer or PID gain scheduling if process gain varies greatly
  5. Tune the positioner for the valve/actuator
  6. Install the sensor for minimum dead time
  7. Use a transmitter to amplify low-level signals
  8. Select the appropriate transmitter hardware span
  9. Apply Lambda tuning for non-oscillatory response
  10. Apply Lambda tuning to coordinate response of interacting loops
  11. Check other control logic outside the PID block

None of this might help with the roiling financial markets, but it can certainly help your process run more smoothly and efficiently.

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