A growing level of attention has recently been given to the automated control of potentially hazardous processes such as the overpressure or containment of dangerous substances. Several independent protection methods provide measures to reduce the risk from such hazards to personnel, the environment and assets. A significant level of this risk reduction is allocated to safety instrumented functions (SIF). The integrity of the safety instrumented system (SIS) to perform these functions (known as functional safety) is therefore critical and the requirements for determining and achieving functional safety are given in IEC 61511-1[REF 1]. This standard is now adopted as the predominant worldwide standard for such systems in the process industry. The integrity requirements of the SIS have implications on all the elements that comprise the system such as sensors, interfaces, controllers, logic solvers, actuators and valves. All the connections that make up the complete control loop are also taken into consideration. One of the key instruments in this loop is the logic solver (decision maker), which initiates the final element to make the process safe if the need arises. The aim of this paper is to explore some of the possibilities available to the SIS designer of an overpressure protection system for the logic solver and to show examples of straightforward system topologies and their associated safety integrity level (SIL) calculations. A general step-by-step procedure to define and evaluate an SIS is suggested in the Appendix. The examples used in this paper illustrate how the procedure is applied in specific cases.
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