Comparison of safety integrity level selection methods and utilization of risk based approaches

摘要

The method used for the selection of a Safety integrity Level (SIL) for a Safety Instrumented System (SIS) of a hazardous process should use a risk based approach to evaluate potential consequences and likelihoods,as well as the effectiveness of process safeguards.Process Safety Management,under OSHA 29 CFR Part 1910,requires an assurance of mechanical integrity for all emergency shutdown systems and safety critical controls.The International Society for Measurement and Control (ISA) promulgated an industry standard, S84.01 to assist in compliance with the PSM regulation in the United States,and the International Electrochemical Commission (IEC) created a similar standard for Europe.these standards specify that all electrical,electronic,and programmable electronic systems,used in SISs,are required to be designed such that they achieve a psecified SIL.These standards are performance based,giving guidance on how to select and achieve an appropriate SIL for a given SIS,but are not prescriptive to any particular methods or equipment.The SIL selectionh is critical since in many cases the SIS is the last 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 standards.Since neither standard specifies requirements for integrity level selection,the determination of a SIL a SIS is based solely on reducing the risk of a process to a level acceptable or tolerable to each individual company.