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tag value not getting to zero on removal of power from FF device

on delta v  as the power is removed from the FF device the last value is held and interlocks doesn't operate. How to make the field value Zero on removal of power from FF device so that every where zero is reflected including in trending.

baiju2p
  • Jonas Berge
    DeltaV function blocks use the status that comes with every PV. If comms is lost the status turns ‘Bad’. Function blocks have options for how to behave when the input status is Bad. Which block are you using for your interlocks? If you are coding, you can use the status in your code. For the historian you can make the trend pen red when the status is Bad to indicate that it is not valid. As per my notes you click: View \ Options: Status Display tab, Bad, Modify: Enable

    Note that for some measurements the value zero (0) may be valid also not triggering an interlock. It may be a good idea to use the status instead.

    Also, in many cases (but not all) triggering an interlock on lost communication or on loop current <4mA or >20 mA is not the best thing to do because in reality there is nothing wrong with the process; it would be a spurious trip. Many interlocks should only trigger on a true process issue. In this case using the status in the interlock also makes sense because the interlock can then tell the difference between a true process problem and some other issue with the comms/analog signal or a sensor failure. That is, status if used correctly can avoid spurious trips.

    Learn more about status and signal validity from this essay: www.linkedin.com/.../iiot-control-can-you-trust-your-sensors-jonas-berge
  • baiju2p

    In reply to Jonas Berge:

    Thank you Jonas Berge, for your specific reply, we were using a AI block for a pressure transmitter with an interlock for a quick shut off valve. Your view of interlock triggering only on a true process issue is valid but for an operator no signal on the HMI is Alarming. During a segment failure he will be mislead to believe that every thing is alright.We have used the" mod bad" signal to circumvent the issue ,but was looking for a more thorough solution reflecting the status at all places using the tag. It Was nice of you to respond on the issue and your suggestions enriched my knowledge, looking out for further interaction with you and learn from your huge experience. Thank You.
  • Jonas Berge

    In reply to baiju2p:

    When the status is Bad the background for the value in the operator graphics and on faceplates turns red - which is also alarming. Do you see this?
  • Rein
    You may use a CALC block after the AI block. In the expression, when status is BAD, OUT=0. Else, IN=OUT.
    For interlock, use the trip signal from SIS. Not advisable to use the signal from PAS as ESD or interlock.
  • baiju2p

    In reply to Rein:

    Thank You.
  • baiju2p

    In reply to Jonas Berge:

    Yes its true but its a very small change in a Graphic with something around 30 information.
  • John Rezabek

    In reply to Jonas Berge:

    I would ask the production supervisor, "Do you want the plant to trip when the instrument is bad?"
  • baiju2p

    In reply to John Rezabek:

    Definitely NO but the operator should have the real picture of Plant. May be Its safe to have a plant trip than to have no trip with alarms.

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