Providing Operators Process Control Valve Position Feedback

Control systems provide signals to control valves to regulate their position per the requirements of the control strategy. It is good engineering practice, especially for critical control valves to have valve travel feedback from independent devices such as position transmitters, limit switches, or positioner output feedback.

Emerson's Riyaz AliEmerson’s Riyaz Ali, whom you may recall from earlier posts, shared ways this position information can be fed back to the operators at their operator workstations.

Actual travel feedback of control valve can be viewed in the DCS by following means:

  1. Installing independent limit switches sent to the control system as a digital input
  2. Installing independent position transmitter to pass an analog input signal to the control system
  3. Installing microprocessor-based digital valve controllers (smart positioners) using the option to provide 4-20mA analog signal feedback, which is independent of control signal passed back to the control system
  4. Installing HART-based smart digital valve controller positioners if the control system has HART pass through, where HART command 3 is used to read travel feedback
  5. For IEC 62591 wireless installations:
    1. Using wireless position monitor
    2. Using HART-based wireless THUM adapter installing on existing smart positioner

Riyaz noted that new, greenfield project could make the decisions upfront on which technology to use for this valve travel feedback. Existing brownfield installations may have challenges installing these devices on the existing control valves. If the control system has either HART pass-through capabilities or the Foundation fieldbus communications protocol, the option likely exists to get travel feedback.

If the existing control valve with has a smart, HART-based positioner, there is a possibility that a position feedback option exists or you can add HART TriLoop or HIM (HART Interface Module) to extract travel information. If none of these options are available another path would be to install independent position transmitters, limit switches, or wireless position monitors.

Wiring can be a challenge in existing plants. To install the position feedback option to existing smart, HART-based positioner or to install a position transmitter or limit switch, the following steps must occur: laying wire, cable tray or conduit from field to the control system, using spare or inserting new I/O card for digital or analog signals, conduit connections, changing drawings to show wiring and I/O, barriers, and power supplies. Also, costs include the labor for installation and commissioning these feedback devices.

Riyaz explained that wireless is a smart field device communications technology that has been established over the past several years and is straight forward, reliable and overcomes the constraints faced with wired approaches.

Wireless field device networks require:

  • No wires
  • No I/O or I/O cabinet space
  • No Conduit
  • No Power line (Battery Operated)
  • No Barriers
  • No change in existing drawings / documentation
  • No connection with existing wired infrastructure

These networks support organic growth over time as monitoring needs dictate and they can overlay over any installed infrastructure. Here are links to the Fisher 4320 wireless position monitor and HART-based wireless THUM adapter for additional information.

You can connect and interact with Riyaz and other experts in the Valve Controllers & Positioners track of the Emerson Exchange 365 community.

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  • Corey

    It’s unclear to me whether position feedback from a smart positioner is truly independent of the reference signal from the control system, as the positioner ostensibly uses that same information as a measurement in its own local position feedback loop (for which the reference signal is the setpoint). I’m guessing it’s not in most cases (and note that this trait is probably not unique to Emerson devices).
    If you’re driving the valve to a certain position with the reference, and then using the position feedback to verify that the valve is actually at the position you drove it to, there is a potential common-cause failure in the position sensing and processing. For independence I’d think you would have to either use other means to drive the valve (e.g., a dump solenoid valve), or have position sensing distinct from that used by the positioner.

    • http://www.linkedin.com/in/JimCahill Jim Cahill

      Thanks for your thoughts on this Corey. I’ve shared your comments with Riyaz and have asked him to provide his thoughts.

  • riyaz Ali

    Common cause factor is main concern when using position transmitter within positioner as done by various other vendors.

    In case of DVC6200 SIS position transmitter is uniquely designed to completely isolate positioning function, making completely independent of positioner, should input signal or power to positioner fails or any issue related to positioner ceases positioner function. Position transmitter function in built but completely independent will still provide valve position.

    This position transmitter function of DVC6200 SIS (independent of positioner) is certified up to SIL2 by Exida.

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