SIS Rules

[C6RAPTOR]

I agree with JLSeagull in thread 830-276979 about SIS & DCS separation. You would think that Code Compliance /Rules for the subject industry would dictate this very important subject. I cannot find a rule on this anywhere for the LNG industry. I can find in NFPA 85 Boiler and Combustion Systems Hazard Code 4.6.3.2.5 (A) Requirement for the burner management system (SIS) to have independent logic, I/O & power supplies.
I have been in the automation industry for over 40 years and the rule I have always gone by with SIS design is the separation of safety systems from the control systems is always the rule. This strategy also applies for sensors such as boiler furnace pressure and drum level transmitters or switches that input the SIS. In this case the rules always has been separate sensors as well as separate sensing points. In most cases of critical process sensing for SIS, more than one sensor is required in a voting arrangement .
At this time reading up on safety compliance rules for systems used for SIS and DCS in the LNG industry.
I have read the LNG Industry related CFR 49 193 & NFPA 59A and cannot find any reference to SIS requirements. I am about to get more documentation on the subject from EN 1473, CSA Z276 , AS 3961 & NOM-013-SECRE-2003.
Does anyone have any other references of SIS rules?

 

[controlnovice]

The decision to implement an SIS system is dependent on your process. You (your company) has to determine first the level of acceptable risk. Then, you will determine if your existing protection layers meet that acceptable level. If not, then you must design another layer (often an SIS, but not always).

It is you (your company) that must determine whether or not an SIS system is required. There is no documentation to tell you that it is required.

You can try ISA84 for further information or the book "Safety Instrumented Systems: Design, Analysis and Justification" by Gruhn and Cheddie. I think there is another book from AIChE that may help.

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This is normally the space where people post something insightful.

 

[ICLL]

This all comes back to the operating company and their acceptance of risk (or not) by insisting on industry standards. The usual standards for design of safety systems are IEC61508 (for equipment) and 61511 (for process systems). If your plant is being built to those standards, then there are quite specific rules for separation of process control and safety shutdown.

My experience has been that there has not usually been too much reluctance to applying those standards to new plant designs. Most process engineers developing P&IDs will tend to separate out the control and shutdown anyway, so the control systems engineer has a relatively easy task. Where it gets messy is when trying to apply the standards to older plants that have not been designed with separation.