Safety Loops Identification and Targeting Method Compliant With IEC61511 Requirements
Safety Instrumented Systems are used in gas turbine (GT) systems design to mitigate potential hazardous situations that can arise from process deviations. International standard IEC61511, (released in 2003) sets out an approach for establishing the relative safety integrity of a safety-instrumented system to perform its prescribed action over the product life cycle (design, installation, and operation of safety instrumented systems). This unified approach is currently being adopted as an industry standard (e.g. UK HSE Guidance Note PM84 and ISO DIS 21789 dedicated to Gas Turbine Safety). Using this approach, each Safety Instrumented Function (SIF) must be evaluated to determine the level of risk reduction required for safe operation of the plant. Properly targeting the necessary level of risk reduction is critical. Underestimating the target risk reduction level can result in exceeding the plant tolerable risk threshold. Over-estimating the target risk reduction level can result in increased life cycle cost of the system design. This paper describes a scientific-numeric approach for targeting the safety integrity requirements that comply with IEC 61511. An Accident Scenario Review (ASR) for each major hazard identified is used to identify the SIF and associated maximum probability of failure on demand (for demand mode) or maximum probability of dangerous failure per hour (for continuous mode). This method can be used for systems previously designed or in operation. The process integrates field experience to validate and justify model assumptions.