Safety instrumented system

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Safety Instrumented Systems (SIS) are designed to respond to industrial plant conditions that themselves may be hazardous or if no action is taken could eventually result in a hazardous condition and to respond to a detected hazardous condtion by initiating pre-defined actions that either prevent the hazard or mitigate the hazards consequences.

Safety instrumented systems are separate and independent from control systems and are composed of sensors, logic solvers, final elements and support systems. The SIS performs specified functions to achieve or maintain a safe state of the process when unacceptable process conditions are detected.

The specified functions or safety instrumented functions (SIF) are implemented as part of an overall risk reduction strategy which is intended to reduce the risk of identified hazardous events involving a catastrophic release. The safe state is a state of the process operation where the hazardous event cannot occur. The safe state should be achieved within one-half of the process safety time. Most SIF are focused on preventing catastrophic incidents.

The correct operation of an SIS requires a series of equipment to function properly. First, the sensors must be capable of detecting abnormal operating conditions, such as high flow, low level, or the valve is in the wrong position. Secondly, the logic solver must receive the sensor input signal(s), make appropriate decisions based on the nature of the signal(s), and change its outputs according to user-defined logic. The logic solver may use electrical, electronic or programmable electronic equipment, such as relays, trip amplifiers, or programmable logic controllers. Next, the change of the logic solver output(s) results in the final element(s) taking action on the process to bring it to a safe state. Support systems, such as power, instrument air, and communications, are generally required for SIS operation. The support systems should be designed to provide the required integrity and reliability.

International standard IEC 61511 was published in 2003 to provide guidance to end-users on the application of Safety Instrumented Systems in the process industries.

[edit] Other names

Other terms often used in conjunction with and/or to describe safety instrumented systems include:

Critical control systems
Safety shutdown systems
Protective instrumented systems
Equipment protection systems
Emergency shutdown systems
Safety critical systems
Interlocks

[edit] SIS examples

Safety instrumented systems are most often used in process (i.e., refineries, chemical, nuclear, etc.) facilities to provide protection such as:

High fuel gas pressure initiates action to close the main fuel gas valve.
High reactor temperature initiates action to open cooling media valve.
High distillation column pressure initiates action to open a pressure vent valve.