Non-SIS IPL Testing Frequency
Layer of Protection Analysis (LOPA) is a method widely used in the process industry to evaluate whether safety measures are adequate and to assign Safety Integrity Levels (SILs) to Safety Instrumented Functions (SIFs). A central concept in LOPA is the Independent Protection Layer (IPL), a safeguard that can prevent hazardous consequences. When credited in LOPA, an IPL is considered safety critical equipment, requiring inspection and testing per OSHA PSM standards, though these do not specify testing frequency.
While SIS equipment testing is well-regulated (via IEC/ISA 61511 and ISA TR 84.00.02), non-SIS safety critical equipment (e.g., alarms, relief valves, emergency block valves) lacks consistent testing practices. To apply a quantitative approach, a performance target (expressed as Average Probability of Failure on Demand (PFDavg) must be defined, using the concept of “partial” IPLs to allocate a fraction of the typical PFDavg budget.
Although non-SIS devices often form only part of an IPL (e.g., just detection or action), existing methods from IEC/ISA 61511 can still be applied. Once performance targets are established, simplified formulas from ISA TR 84.00.02 help calculate appropriate test intervals based on failure rate and target PFDavg.
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