Safety instrumented system

In functional safety a safety instrumented system (SIS) is an engineered set of hardware and software controls which provides a protection layer that shuts down a chemical, nuclear, electrical, or mechanical system, or part of it, if a hazardous condition is detected. An SIS performs a safety instrumented function (SIF). The SIS is credited with a certain measure of reliability depending on its safety integrity level (SIL). The required SIL is determined from a quantitative process hazard analysis (PHA), such as a Layers of Protection Analysis (LOPA). The SIL requirements are verified during the design, construction, installation, and operation of the SIS. The required functionality may be verified by design reviews, factory acceptance testing, site acceptance testing, and regular functional testing. The PHA is in turn based on a hazard identification exercise. In the process industries (oil and gas production, refineries, chemical plants, etc.), this exercise is usually a hazard and operability study (HAZOP). The HAZOP usually identifies not only the process hazards of a plant (such as release of hazardous materials due to the process operating outside the safe limits of the plant) but also the SIFs protecting the plant from such excursions. An SIS is intended to perform specific control functions to prevent unsafe process operations when unacceptable or dangerous conditions occur. Because of its criticality, safety instrumented systems must be independent from all other control systems that control the same equipment, in order to ensure SIS functionality is not compromised. An SIS is composed of the same types of control elements (including sensors, logic solvers, actuators and other control equipment) as a Basic Process Control System (BPCS). However, all of the control elements in an SIS are dedicated solely to the proper functioning of the SIS. The essential characteristic of an SIS is that it must include instruments, which detect that process variables (flow, temperature, pressure etc.

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Related concepts (3)

Industrial safety system

An industrial safety system is a countermeasure crucial in any hazardous plants such as oil and gas plants and nuclear plants. They are used to protect human, industrial plant, and the environment in case of the process going beyond the allowed control margins. As the name suggests, these systems are not intended for controlling the process itself but rather protection. Process control is performed by means of process control systems (PCS) and is interlocked by the safety systems so that immediate actions are taken should the process control systems fail.

Programmable logic controller

A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. Dick Morley is considered as the father of PLC as he had invented the first PLC, the Modicon 084, for General Motors in 1968.

Safety instrumented system