Emerson Exchange 365

Alarm on an AO block

Does anybody have a good suggestion for alarming on an AO block?  As you know, an AO block can be configured to have a READBACK signal.  In this case, I would like to set it up to alarm on deviation (difference between SP and PV) and absolute values.  I tried connecting an AO block to an ALM block (AO1/OUT to ALM1/SP, AO1/PV to ALM1/IN), however, there is an issue with the ALM block, where a changing SP will cause the deviation alarm not to work.  You will get it to work once, following a download, but will not work again after that when it goes in and out of deviation range.

• AO Block with ALM Block

Lance Dusing

• 10 May 2016 1:45 PM
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3 Replies

• neilrbrown

  

  • 10 May 2016 2:39 PM

  Lance,

   

  There may be a neater way to do this but I suspect you will need a SUB or CALC block to calculate the difference between the AO OUT and READBACK signals and then apply an alarm to that.  I would strongly suggest that you apply a filter to the calculated difference as subtracting two similar values always increases the ‘noise’ in the result, and another nuisance/repeating alarm is the last thing you want.  You may want an on-delay on the alarm as well – the READBACK signal comes via HART which introduces some latency, as do the mechanical components of the valve itself.  Also, only add this alarm if the operator is realistically able to do something about it, and the impact of not responding is significant.

   

  Best regards,

   

  Neil Brown

   

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• GeloCortez

  

  • 11 May 2016 3:14 AM

  Here's the set-up in 1 of our client:
  
  Input parameter (discrepancy SP = 6%) ======> CALC/IN1 (if the difference > IN1, OUT1 := TRUE) OUT1=======> ON DELAY (10s) ========> Out Parameter (alarm)

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• Alex Lutz

  

  • 28 Jan 2019 6:23 PM

  has this been submitted to product development at all? I'm experiencing the same thing and this sounds like an oversight of the alarm block functionality to not be able to have a variable setpoint in the logic.

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