Using the ISA84 / HAZOP / LOPA Process to Design a Safety Instrumented System (SIS) for a Fumed Silica Burner

A Cabot team used the ISA 84-2004 (IEC61511-1) standard to evaluate the Safety Instrumented System (SIS) for an existing fumed silica burner and made design changes to reduce the risk. Years prior to this initiative, the layers of protection review used was subjective and resulted in assigning unreasonably high Safety Integrity Levels (SIL) on safety critical interlocks. The team turned to the ISA84 approved LOPA process and found it to be a more realistic and a less subjective risk evaluation that resulted in a practical, defensible process safety system design. The process steps followed were to: 1) Perform a HAZOP, 2) Use Process Hazard Analysis (PHA) software to generate LOPA spreadsheets, 3) Populate LOPA spreadsheets for each risk scenario and compare to the Cabot Risk Matrix, 4) Design an SIS/SIL to bridge the LOPA-identified risk gaps, and 5) Implement and document the changes. After completing the PHA/LOPA analysis, two scenarios were identified where the process safety risk was marginal and required design improvements. The team struggled on how to design proper SILs and, with the help of a certified consultant, completed Step 4. Both SIS design improvements were implemented in 2009, reducing the inherent control-related risks with a practical solution.