PetroStreet

The concept of safety integrity levels (SILs) was introduced during the development of BS EN 61508 (BSI 2002) as a measure of the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function.

Following are 2 popular methods of determining SIL requirements to process industry installations:
– risk graph methods
- layer of protection analysis (LOPA)

But all these methods requires a lot of data, assumptions & calculations.

Is there any key avaiable to determine SIL requirement for any specific process / component?

Actually I need to determine SIL prior to design a protection system for an ammonia refrigeration loop which have ~15 Metric ton ammonia in it. Should it be SIL-1 or 2 or 3?

ANSI S84.04 requires that companies assign a target SIL for all Safety Instrmented Systems (SIS). As well, after a PHA study, the study team may determine that certain critical systems require that a SIL be assigned. The assignment is based on the amount of risk reduction that is necessary to mitigate the risk associated with the process to an acceptable level. All of the SIS design, operation and maintenance choices must then be verified against the target SIL.
The first step for assignment of Target SIL is to use your (updated) PHA’s or conduct new PHA’s to screen for the hazards. HAZOP is most commonly used methodology. If the risk is unacceptable then it is reduced or eliminated using non-SIS or SIS elements. You consider SIS only after all the non-SIS protection layers have been considered. HAZOP’s identify risks in terms of the likelihood and the severity of the hazards. Target SILs are assigned to SIF’s of the SIS identified in the PHA studies. Various methodologies are available for assignment of target SILs. As in the case with PHA studies, the assignment of Target SILs must involve people with the relevant expertise and experience. Methodologies used for determining SILs include, but are not limited to:

•Consequence only
•Risk Graph
•Layered Risk Matrix
•Risk matrix
•Layer of protection
•Fault tree analysis

Which ever tehnic is used the greatest increase in cost occurs when the decision is made that the SIL must be higher than SIL 1. The selection of SIL 2 or SIL 3 forces the SIS design toward device redundancy and diversity. With this recognition, many companies are taking the approach that "a safety system is a safety system and therefore should be SIL 3". This eliminates the arguments about whether escape is possible, someone will be injured or killed or the impact will be on-site and/or off-site. It saves time in the PHA process, reduces documentation in justifying the SIL choice, and ensures consistency across process units.

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Unfortunately, there is no easy answer when it comes to assigning SILs. The choice involves examining safety, community, environmental, and economic risks. Most importantly, tools must be developed at the corporate level to ensure that the choice of SIL is consistent with a company’s risk management philosophy and that the assignment method is congruent with the existing characteristics of the corporate risk assessment methodologies. Following can however be used as a conservative guide,
SIL 4 --- For hazards that can lead to Catastrophic Community Impact
SIL 3 --- For hazards that can lead to Employee and Community Impact
SIL 2 --- For hazards that can lead to Major Property and Production Protection. Possible Injury to employee
SIL 1 --- For hazards that can lead to Minor Property and Production Protection

You can study the papers available on the links below for determination of SIL levels.
http://www.hse.gov.uk/research/rrpdf/rr216.pdf
http://www.isa.org/Content/Microsites19 ... 8402p1.pdf
http://www.isa.org/Content/Microsites19 ... 8402p2.pdf

Thank you waseem for such an elaborated reply.
It'd be reallt helpful.
May i ask you to share the reference of the above excerpt?

The above is my understanding based on various references however the one i liked the most for an excellent concept of SIL is already attached above.

Thanx a lot Waseem.
So nice of you.

The topic has been updated with some attachments.

Very well done Waseem.
Your efforts are appreciated.