Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

6 Hazard and Risk AnalysisSafety Integrity LevelsThe figure below shows the relationship of DIN V 19250 classes and safetyintegrity levels (SILs). As a required SIL increases, SIS integrity increases asmeasured by:• System availability (expressed as a percentage)• Average probability-to-fail-on-demand (PFD avg )• Risk reduction factor (RRF, reciprocal of PFD avg )The relationship between AK class and SIL is extremely important and should notbe overlooked. These designations were developed in response to serious incidentsthat resulted in the loss of life, and are intended to serve as a foundation for theeffective selection and appropriate design of safety-instrumented systems.Risk99.99999.990.000010.0001>10,000SIL 4AK 8AK 7Standards andRisk MeasuresReductIon99.9099.0090.00PercentAvailability0.0010.010.1PFD avg10,000–1,0001,000–100100–10RRFSIL 3SIL 3SIL 2 SIL 2SIL 1ANSI/ISAS84.01SIL 1IEC61508AK 6AK 5AK 4AK 3AK 2AK 1DIN V19250Risk MeasuresRisk StandardsDetermining a Safety Integrity LevelIf a PHA concludes that an SIS is required, ANSI/ISA S84.01 and IEC 61508require that a target SIL be assigned. The assignment of a SIL is a corporatedecision based on risk management and risk tolerance philosophy. Safetyregulations require that the assignment of SILs should be carefully performed andthoroughly documented.Tricon Safety Considerations Guide