Abstract
The method used for the selection of a Safety Integrity Level (SIL) for a Safety Instrumented System (SIS)of a hazardous process shoud we a risk based approach to evaluate potential consequences and likelihoods, as well as the effectiveness of process safeguard. Process Safty Management, under OSHA 29 CFR Part 1910, requires an assurance of mechanical integrity for all emergency shutdown systems and safty critical controls. The International Society for Measurement and Control (ISA) promulgated an industy standard S84.01, to assist in compiance with the PSM regulation in the United States, and the International Electrochemical Commission (IEC) created a similar standard for Europe. These standard specifi that all electrical electronic, and rogrammable electronic system, used in SISs, are requiread to be desiped such that they, achieve a specified SIL. These standard are perfomance baed giving guidunce on how to select and achieve an appropriate SIL for a given SIS, but are not prescriptive to any particular method or equipment. The SIL selection is critical since in many cases the SIS is the ht layer of protection for the prevention of hazardous events. The probability of failure on demand of a SIS are quantitatively categorized into discrete integrity levels (i.e. SIL) by both industry standard. Since neither standard specfies requirements for integrity level selection, the determination of a SIL for a SIS is based solely on reducing the risk of a process to a level acceptable or tolerable to each individual company.
In this paper, current industry practices for the selection of SILs are evaluated to illustrate how each method provides an estimation of risk reduction that the SIS will provide for a given process. The level of effort, required expertise, and accuracy of the methods are also compared to assist the reader in choosing the appropriate SIL selection methodology.