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The control engineer begins the job of designing systems to automatically reduce risk after the risk analysts figure out how much risk reduction is required to meet company and regulatory goals. After considering the process design changes and inherently safe alternatives, many engineers begin the design of a safety-instrumented system (SIS), an automatic system to protect personnel, the environment, and company assets against process hazards. Like most control systems, it contains sensors, a controller (often called the "logic solver"), and final elements. SIS design requires all the knowledge and skill of normal control system design plus a good understanding of the special requirements and standards for safety.
Today, engineers perform SIS design work according to a set of commonsense national and international standards known as the safety life cycle (SLC), released in 1996 as ANSI/ISA-84.01. The Occupational Safety and Health Administration recognizes ANSI/ISA-84.01 as "good engineering practice," and it has become the expected way of doing business in many industries.
SIS design starts with safety requirement specifications, a document that details the automation functionality required to address the process hazards identified during the risk analysis. Each function, or safety-instrumented function (SIF), addresses a specific hazard. For each SIF the risk analysts have chosen an order of magnitude risk reduction target called a safety integrity level (SIL). For example, if a risk...