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Keywords:

  • safety instrumented system;
  • safety integrity level;
  • layer of protection analysis;
  • hazard and operability study;
  • process hazard analysis;
  • mechanical integrity

Abstract

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 practical design changes to reduce the control-related process safety risks. Prior subjective layers of protection reviews and poor definitions for “tolerable risk” had resulted in assigning unreasonably high and impractical safety integrity levels (SIL) on safety critical interlocks. The Team turned to the ISA 84-approved layers of protection analysis (LOPA) procedure 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 Team's procedure focused on the following steps within the SIS life cycle:

  • 1
    Perform a hazard and operability (HAZOP) process hazards analysis (PHA),
  • 2
    Use PHA/HAZOP software to generate LOPA spreadsheets,
  • 3
    Populate the LOPA spreadsheets for each risk scenario and compare the risk levels to the corporate risk matrix,
  • 4
    Design an SIS/SIL to bridge the risk gaps identified using the LOPA, and
  • 5
    Implement and document the changes.

After completing the PHA/LOPA/tolerable risk matrix analysis (Steps 1–3), two scenarios were identified where the process safety risk was marginal and required design improvements. However, the Team struggled with Step 4, the SIS/SIL design, and progress essentially stopped. The Team's key learning, overcoming this roadblock, focused on locating a Certified Functional Safety Expert to complete and verify the SIS/SIL design integrity. Once Step 4 was completed, the Team implemented both identified SIS design improvements (Step 5), reducing the inherent control-related process safety risks with a practical solution. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011