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.