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The ANSI/ISA S84.01-1996 standard on the Application of Safety Instrumented Systems for the Process Industries has introduced many new requirements for the assessment, evaluation, design, installation, operation, and maintenance of safety instrumented systems (SIS). As with any new standard, one of the most difficult compliance aspects to address is the consideration for existing systems. The standard includes a clause for existing equipment, in which the owner/operator must only demonstrate that the SIS is “designed, maintained, inspected, tested, and operating in a safe manner.” The process industries are under continuous pressure to improve the operation of their facilities. This optimization results in an environment of constant change, in terms of upgrades and expansions. The “grandfather” clause is for existing SIS only. At some point, the modifications of the process unit and the SIS will be significant enough to trigger the full requirements of ANSI/ISA S84.01-1996. This paper will address the various criteria that are being used throughout the process industries to define the boundaries and limitations of this clause. It will also provide recommendations on how to make sure that a process facility is protected in regard to the “grandfather” clause.
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