Emerson Exchange 365

DI Card Input Signal Loss?

Hello, 

We have an on/ff valve that occasionally loses the input 120V signal and Operations see  "Confirmation Closed Lost" lost. 

The valve is energized to close, so we have a PV - SP mismatched is what the DCS is alarming. 

Technicians checked the loop electrical components. 

It it possible the DI input channel is losing the valve closed input signal intermittently?

Thank you

DCS Newbie

• 28 Sep 2015 9:36 PM
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11 Replies

• Andre Dicaire

  

  • 29 Sep 2015 12:15 AM

  It's possible. I've heard that with High Side sensing of the AC powered DI signal, the DI Channel can have issues when the contact is open. The wire from the card to the contact is subject to coupling and depending on the wiring, length, and environment can result in issues on the DI. We found that by moving to an Isolated DI wired to the low side of the contact, the same installation worked reliably.
  
  I don't know if this is what you are experiencing, but it is worth investigating. If you are using a system powered DI card on the DeltaV system, it is a high side sense circuit. if you can use an Isolated channel and wire the isolated DI on the low side of the contact, and that solves the problem, let us know.
  
  There is always the use of an isolation relay, which requires more electromotive force on the part of the signal to activate the relay and the coupled wire is unlikely to have enough energy to activate the relay. But you might try a test circuit to see if this solves the issue.
  
  I believe some customers have used load resistors in the circuit to thwart the induced signal, but I'm not sure how that was setup. ( who uses the word thwart these days?)

  Andre Dicaire

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• Bruce Brandt

  

  • 29 Sep 2015 11:20 AM

  In reply to Andre Dicaire:

  Emerson has released a new CHARM that has a built in resistor to address this issue. The problem is caused by induced current on the slgnal wires that is just high enough to cause the CHARM to sense it as being indicative of a closed contact depending on what is inducing the current it is entirely possible that it is intermittent, a large motor that is it used continuously for instance. A dropping resistor will fix the problem.

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• DCS Newbie

  

  • 29 Sep 2015 2:46 PM

  In reply to Andre Dicaire:

  Thank you so much Andre,

  I am still learning, so I may need some help on explanation.

  1.  We are using DI Card, 8 Ch., 120 VAC, Isolated. Only channels 1 thru 5 are in use.

  2.  What do you mean by "High Side sensing of the AC powered DI signal" ? We are using channel 4 (terminals +7 and -8). Is +7 the high side sensing?

  3. " the DI Channel can have issues when the contact is open. " Did you mean to say issues with the limit contact is closed?

  4. What means "moving to an Isolated DI wired to the low side of the contact" ? How do I accomplish this?

  5. The DI card powered from the DeltaV system 12V power supply.

  Thank you kindly

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• DCS Newbie

  

  • 29 Sep 2015 3:10 PM

  In reply to Bruce Brandt:

  Bruce Brandt said:

  Emerson has released a new CHARM that has a built in resistor to address this issue. The problem is caused by induced current on the slgnal wires that is just high enough to cause the CHARM to sense it as being indicative of a closed contact depending on what is inducing the current it is entirely possible that it is intermittent, a large motor that is it used continuously for instance. A dropping resistor will fix the problem.

  Thank you Bruce, 

  I need to mention that the distance from DI card to the field device is a bit far then when normally used. So we use a relay for induced voltage. The sensing signal is 120V.

  The device contacts is normally closed and opens intermittently causing trip or Closed Confirm alarm on the DCS for the valve position. So 120V is sensed to the DI card at all times during normal operation.

  Will induced voltages/currents cause the contacts to open? Maybe over current (from the induced current) on the DI card fuse cause the circuit to open?

  What means "  dropping resistor"

  Thank you. This is good challenge for me.

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• DCS Newbie

  

  • 29 Sep 2015 3:47 PM

  In reply to Andre Dicaire:

  Hello,
  
  Also, we have a .25Amp fuse on the DI wires before wires connect to the DI card. If overcurrent, the fuse would have blown and cause continues open circuit.
  
  Thanks

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• Bruce Brandt

  

  • 29 Sep 2015 3:52 PM

  In reply to DCS Newbie:

  I just realized from your other post that you aren't using the CHARM I/O. I also misread the post that you are actually losing the signal when the field contacts are closed. The issue I've dealt with most recently is when the field contacts are open yet the channel reads them as closed because of induced current so not your issue.
  
  It's easy to say the issue is with the I/O cards but my experience is that more likely then not there are issues in the field since solid state detection devices tend not to fail intermittently. You could have too much voltage drop so check that first. Also check the voltage at the terminals of the I/O card when it is working properly and when it is showing open but is actually closed. If the voltage has dropped then the problem is external to the DCS. Also, are you certain that the limit switch is set correctly? Is it a physical pair of contacts or a proximity switch? Is the valve subject to a lot of vibration? Is the wiring exposed or in conduit? Are all the connections tight?
  
  Good luck in finding the real cause.

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• DCS Newbie

  

  • 29 Sep 2015 4:41 PM

  In reply to Bruce Brandt:

  Thank you Bruce,
  
  We pulled the entire valve assembly (solenoid, valve, and limit switches) and bench test each part. I saw the limit switches operate correctly and all mechanical parts tightly connected. Technicians confirmed the connections for the DI loop is tight. They also did an Ohm test on the wire.
  
  It is proximity switches. No valve vibration.
  
  We did have voltage drop, so we installed a relay on the loop to confirm true 120V.
  
  The fuses on the DI card are not blown either.
  
  I am not sure what to do now. I even called Emerson support.
  
  Whats even worst, is this valve is tied to our Flare unit. When signal is loss on the DI card, the entire unit trips off.

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• DCS Newbie

  

  • 29 Sep 2015 4:48 PM

  In reply to Andre Dicaire:

  Andre Dicaire said:

  
  
  There is always the use of an isolation relay, which requires more electromotive force on the part of the signal to activate the relay and the coupled wire is unlikely to have enough energy to activate the relay. But you might try a test circuit to see if this solves the issue.

  Thanks,

  Yes, we are using an isolation relay in the circuit to confirm the 120V back to DI card. 

  Maybe the relay is failing at times.

  Thanks

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• jimgraves

  

  • 29 Sep 2015 4:53 PM

  I have seen this exact problem more than once due to improper grounding.

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• DCS Newbie

  

  • 12 Oct 2015 3:59 PM

  In reply to jimgraves:

  Thank you for replying.

  How do you resolve improper grounding for DI card?

  The limit switch loop on a 120V isolated loop with a isolation relay for induce voltages.

  Thanks,

  Darrell

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• JunieBass

  

  • 21 Oct 2015 5:38 PM

  The condition you are discussing is possible. As per my experience with this the High Side sensing of the AC powered most of the time the DI signal the DI Channel have issues when the contact is open.
  The wire from the card to the contact is subject to coupling and depending on the wiring, length, and environment can result in issues on the DI.
  If you are using a system powered DI card on the DeltaV system, it is a high side sense circuit.
  Also most of the time the use of Relay is there.

  quick-turn pcb assembly

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