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A Safety Integrity Level (SIL) Analysis is the initial step in the Safety System Design Process. Where the HAZOP process normally discovers potential hazards and provides general recommendations, the SIL is a specific analysis which defines the Safety Criteria and Mitigation of hazards which can lead to a significant economic, safety and environmental consequences.

There are three SILs utilized by ISA S84 and four by IEC 1508/1511 for Risk Classification, as defined in terms of Probability of Failure on Demand (PFD). This paper provides a methodology to evaluate and classify risk in terms of Consequences, and to determine the SIL for the process under consideration based on these Consequences and the Process Demand Rate. The methodology can be customized to comply with existing company standards, and should satisfy many of the critical OSHA 29 CFR - 1910.119 requirements.

In addition, the paper addresses the configuration of the Safety Instrumented System (SIS) and the impact of field devices on the system SIL, including the necessity of redundancy and testing of field devices to achieve and maintain SILs of 2 or higher.

In the functional safety world different standards exist for safety equipment and systems. These standards all have different ways of rating or classifying safety systems. The purpose of this paper is to describe the differences and similarities between SIL levels and AK levels.

The purpose of this paper is to show the influence of design parameters on the performance of safety systems. Four basic safety system architectures are examined and compared, i.e., 1oo1, 1oo2, 2oo3, and 2oo4. The performance is measured in terms of two attributes, i.e., the Probability of Failure on Demand (PFD) and the Probability of Fail-Safe (PFS) or spurious trip. Both attributes are important in the safety world as their values represent respectively a measurement for the safety introduced and financial loss caused by the safety system because of spurious trips. The required PFD is expressed in national and international standards as the safety integrity level [1,2]. These safety integrity levels (SIL) represent discrete levels of reliability depending on the severity of the process or the equipment under control (EUC). For the PFS no such levels exist at the moment.

The purpose of this paper is to show the effect online diagnostic and periodic proof testing have on the performance of the safety function in terms of the PFD. For three different architectures (1oo1, 1oo2, and 2oo3) the influence of the diagnostic coverage, the proof test coverage, and the proof test interval on the PFD are determined. A performance indicator is used to express this influence and show the effect.