Safety instrumented system justification

Once the need for risk reduction has been identified there is usually not much argument about the basic idea of installing a safety related system and often this is a safety instrumented system. Justification issues arise when the scale of investment has to be decided. .. is it to be a cheap system with high running costs or a more expensive model that repays its cost in reduced operating and maintenance costs Searching out the links between SIS costs and true running costs of the plant may be a tricky job. 

ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) requires that equipment used in safety instrumented systems be chosen based on either lEC 61508 certification to the appropriate SIL level or justification based on "prior use" criteria (ANSI/ISA-84.00.01-2004 (lEC 61511Mod), Part 1, Section 11.5.3). However the ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) standard does not give specific details as to what the criteria for "prior use" means. Most agree however that if a user company has many years of documented successful experience (no dangerous failures) with a... 

It gives application specific guidance on the use of standard products for the use in safety instrumented systems using the proven-in-use justification. The guidance allows the use of field devices to be selected based on proven in use for application up to SIL 3 and for standard... 

Gruhn P, Cheddie HL. Safety instrumented systems design, analysis, and justification. 2nd ed. Research Triangle Park, NC ISA 2006. 

Gruhn, P., Cheddie, H., Safety Instrumented Systems Design, Analysis, and Justification, The Instrumentation, Systems, and Automation Society, Research Triangle Park, North Carolina, 2006. 

Chapter 13 Justification for a safety instrumented system. In practice... 

Gruhn, P., and Cheddie, H., Safety shutdown systems, design, analysis and justification, Instrument Society of America, Research Trirngle Park, NC, 1998. 

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