Layer of Protection Analysis (LOPA) utilizes Initiating Event Frequencies (IEF) and Independent Protection Layer (IPL) probabilities of failure to predict hazardous event frequencies in a process. Non-SIS/SIF protection layers, outside the scope of IEC 61508/61511, are not subject to the same rigor in design verification which requires consideration of Demand Rate and Mode of Operation. The risk of a non-SIF protection layer not responding on demand may not be properly evaluated if not identified based on the following: 1. Rate of demand is not taken into account during the PHA/LOPA review of the interlock, 2. All interlock components are not reviewed for failure, 3. Low/incomplete documentation on what is a “dormant” safety interlock vs “active” BPCS control function, and 4. The possibility of a misclassified control function.
This paper is a case study of the Chlorine Unloading area interlocks for the La Porte Oxy Facility. IPL demand rates were compiled and reviewed to determine the interlocks that may need further study through LOPA. The LOPA method was applied to the Interlock component failure scenarios to determine the control function of the area interlocks.
See more of this Group/Topical: Global Congress on Process Safety