Emerson Exchange 365

DeltaV SIS logic solver as AO.

Hi,

Can I use DeltaV SIS logic solver as Analog Output?

• Control
• DeltaV

obi1987

• 10 May 2015 4:19 PM
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9 Replies

• Roshan Parte

  

  • 10 May 2015 5:17 PM

  Definitely, but it is actually HART two state output.

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• GeloCortez

  

  • 21 Oct 2015 5:45 AM

  only AI, DI, and DO for SLS

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• MC_Chow

  

  • 21 Oct 2015 1:23 PM

  In reply to GeloCortez:

  As Roshan pointed out, the AO on the DeltaV SIS is a two-state output, low (4mA) and high (20mA) with HART; the AO does not modulate and you therefore cannot use it for regulatory control.

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• Andre Dicaire

  

  • 21 Oct 2015 11:25 PM

  In reply to MC_Chow:

  The DeltaV SIS is certified for IEC 61511 Safety applications, which are not control, and by design offer On/Off signals to the SIS outputs. I've seen several applications where the user is asking for a modulated control valve on the safety system, which is counter to a safety application.
  
  An integral part of the safety function is the final control element. Using a Safety System for a modulation application means you need a modulating valve. I have not been able to find any supporting information for a safety rated modulating valve. ( my understanding is that the SIS DVC is off if the ma is below 12 and On if the mA signal is above 12ma.) A safety system is used for risk reduction based the SIL rating needed to get to tolerable risk. The sensors and final elements must also meet the SIL requirements, and ultimately, the availability of the SIF will be dictated by the final control element. If it is not safety rated, you will not be able to set a SIL rating on the SIF. So why use the Safety System for a modulating application?
  
  I'd like to understand what your application is. Please post more details.

  Andre Dicaire

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• Bill Reeson

  

  • 22 Oct 2015 12:51 AM

  In reply to Andre Dicaire:

  Riyaz Ali gave a presentation at Emerson Exchange on "Do I need a Safety-Certified Valve for my Final Control Element", Session 4-5263. This subject is broached in this presentation where the DVC6200 modulates the control valve via the BPCS but the solenoid valve between the DVC and control valve is controlled via the SIS. Therefore the SIS can close the control valve on demand. I would check with Riyaz for more details but I thought you would get credit using the control valve as a shutdown valve.

  Bill Reeson

  Global Project Pursuit & Strategic Account Manager | ASCO

  Emerson

  T +1 973 966 2097 | M +1 973 495 8561

  Bill.Reeson@Emerson.com

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• GeloCortez

  

  • 22 Oct 2015 3:28 AM

  In reply to Bill Reeson:

  i've seen control valve used as shutdown valve. DVC is used for Maintenance purposes only and is normally bypassed. so Solenoid Valve is always in control of the valve (which is on and off)

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• Dewey Kuchle

  

  • 22 Oct 2015 8:10 PM

  In reply to GeloCortez:

  Hi guys. I can't help but think we're missing the point here. Regardless of how one may choose to command the valve (solenoid vs. modulating positioner) we need to pay attention to how we're actually reducing risk. After all this is a safety function we're talking about right? In the case of a "shut down" one would normally assume a fail closed action is required. When the valve closes (by whatever means) does it actually have the ability to close tight/seal well enough to reduce risk to equipment and personnel? Chances are very good that a modulating control valve in constant service may not ever have been designed to provide a tight seal shut off when closed let alone perform that function after being in service for any amount of time. Imagine the possible repercussions of assuming tight shut-off or isolation has been achieved when in fact it has not. Just because we can command the valve closed doesn't mean our job as functional safety engineers is done. There is a good reason why ISA S.84 recommends against this practice and in anything other than perhaps a "fail open" scenario I'd be very wary of using control valves for shutdowns in a safety function.

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• Tadeu Batista

  

  • 22 Oct 2015 11:00 PM

  There are so many replies, did we answered your question? Could you explain why do you need AO on the DeltaV SIS Logic Solver?

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• Riyaz Ali

  

  • 23 Oct 2015 8:51 PM

  In reply to Bill Reeson:

  Bill,
  
  Thanks and appreciate your comments. A control valve when used in SIF loop as dual purpose for controlling through positioners by using DCS control signal and SOV taking valve to SAFE State during safety demand using Logic Solver DO, connected pneumatically in series is allowed per IEC61511 part 2, section 11.4.5 subject to analysis, If all Possible Failure modes of Control Valve do not put demand on any SIF. This practice is mostly used when there is lower SIL budget SIF loop by making use of not having additional "Final element" with proper assessments of safety function and hazard.

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