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karthik301
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Target SIL for ESD

09/28/2012 3:49 AM

Warning: It is a long story.......

We have an argument with our end user (customer) regarding the Target SIL Rating for an ESD function. This has a big impact on the ESD system in terms of design if we have to go ahead with the end user's advise.

Case-1:

Transmitter-A (2oo3) is causing a trip on Low Low Flow. Required SIL rating for this ESD function is SIL-3. To reduce the target SIL level, another IPL (Independent Protection Layer) has been added (addition of a 2oo3 Transmitter-XYZ which trips the system on High High temperature, hence a barrier of defense to prevent occurring of Low Low Flow trip).

End user's advise is that, the HH temp transmitter should be considered in a different ESD system all together (different IO card, cabinet, processor, etc).

Case-2:

Transmitter-A (2oo3) is causing a trip on High High Level. Required SIL rating for this ESD function is SIL-3. To reduce the target SIL level, another IPL (Independent Protection Layer) has been added (a new DCS transmitter considering an alarm which alerts the operator on high level of some tank which acts as a defense). As in case of most of the refineries, DCS-ESD communicates through Modbus over TCP/IP.

End user's advise is to consider DCS transmitter in a different controller than what is being used to communicate with ESD.

PS: Our Process Safety engineers dismiss the end user's recommendation saying it is verified to be SIL1 without segregation and no need of complicating the design. But the end user is adamant and asks us to do as per his command saying that if it is an "Independent Protection Layer", everything has to be "independent".

We are procuring a ESD system which is certified as SIL3. When we have considered the probability of failure of Control System already (0.1) in the target SIL analysis, should we still go ahead with the end-user's demands here?

I mean, the ESD system is supposed to be a centralized system for shutting down the plant/critical equipment. Just for 1 or 2 transmitters, how valid is it to consider a different cabinet,processor, field cable etc?? Has anyone experienced a similar situation?

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PWSlack
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#1

Re: Target SIL for ESD

09/28/2012 3:53 AM

What happens on power failure, according to the HazOp Study?

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karthik301
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#3
In reply to #1

Re: Target SIL for ESD

09/28/2012 4:18 AM

I did not understand your question. Is it related to a Utility "power failure" (overall), or is it specific to this loop, coz specific case of power failure for this loop hasnt been identified.

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PWSlack
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#5
In reply to #3

Re: Target SIL for ESD

09/28/2012 4:50 AM

OK: rephrase and extend: In the event of power being disconnected from these systems, such as in the case of a blown fuse in the instrumentation power supply for whatever reason, how inherently safe is the plant?

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karthik301
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#8
In reply to #5

Re: Target SIL for ESD

10/04/2012 9:48 PM

Similar to other plants (as a standard practice), loss of power to the ESD system will trip the outputs (send the valves to safe state) hereby stopping the process.

Ex: Feed valves will close, vent valves will open, etc.....

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pritam
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#7
In reply to #3

Re: Target SIL for ESD

09/29/2012 5:33 AM

I understand process but not so many abbrivations used like ESD, SIL, TCP etc. Only instrumentation people may understand.
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Tornado
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#2

Re: Target SIL for ESD

09/28/2012 4:15 AM

Were the end user's requirements in the specifications at the time of bidding, or are they now asking for a freebie?

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karthik301
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#4
In reply to #2

Re: Target SIL for ESD

09/28/2012 4:21 AM

Freebie.....!

The argument of our Process Safety team is this, "When it is not a contractual requirement, we are not bound to do this". However, the end user also says "Find some other solution to escape from this problem". Hence, the possibility of charging him is ruled out

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Allan Evans
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#6

Re: Target SIL for ESD

09/28/2012 6:36 PM

Your client is correct. It is obvious that this is a critical system. Live with it.

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karthik301
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#9
In reply to #6

Re: Target SIL for ESD

10/04/2012 9:53 PM

Allan, I too agree that it is a critical system. But I think all this work-around will make the system more complex...... Afterall, ESD is supposed to be 1 centralized system and not multiple systems :):)

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Allan Evans
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#10
In reply to #9

Re: Target SIL for ESD

10/08/2012 5:11 AM

Fair comment karthk301. I must admit that I am rather cynical about all of these "fancy safety systems". I have been designing safety systems on large power stations and pulp and paper for over thirty years, watching with some amusement, the development of the safety PLC, TUV approved systems and so on. It is an expanding industry based on fear. Has it improved our process safety? Hard to say. I know that all of the alarms that are now possible has created alarm flooding that has been responsible for some major industrial incidents. I have installed and commissioned a number of SIL 3 systems. Good money to be made. A good analogy would be the Y2K scam. Some large consulting companies are still talking about their achievement on this dreadful waste of money. I work on the principle that the logic should be designed such that if an instrument tech opened the page, it could be understood. KISS. Good luck with your client.

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