A Safety Instrumented System (SIS) consists of a designed set of hardware and software controls which are especially used on critical process systems. A critical process system can be identified as one which, once running and an operational problem occurs, may need to be put into a "Safe State" to avoid adverse Safety, Health and Environmental(SH&E) consequences. Examples of critical processes have been common since the beginning of the Industrial Age. One of the more well known critical processes is the operation of a gas processing plant flare system. Critical parts of the process would include the lighting of the burners, controlling the level of flame of the flare and controlling the temperature.

A SIS is designed to perform "specific control functions" to failsafe or maintain safe operation of a process when unacceptable or dangerous conditions occur. 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. 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 specific control functions performed by an SIS are called Safety Instrumented Functions (SIF). They are implemented as part of an overall risk reduction strategy which is intended to eliminate the likelihood of a, previously identified, HS&E event that could range from minor equipment damage up to an event involving an uncontrolled catastrophic release of energy and/or materials.

A Safe State is a process condition, whether the process is operating or shutdown, such that a hazardous SH&E event cannot occur. The safe state must be achieved in a timely manner or within the "process safety time".

Sahoma Controlware has extensive experience in safety control systems for DCS and PLC based environments.  We have developed safety control systems for gas processing plants, compressor stations,,co2 recycle processing plants, material handling plants.