Safety instrumented system

Safety instrumented system

A Safety Instrumented System (SIS) is a form of process control usually implemented in industrial processes, such as those of a factory or an oil refinery. The SIS performs specified functions to achieve or maintain a safe state of the process when unacceptable or dangerous process conditions are detected. Safety instrumented systems are separate and independent from regular control systems but are composed of similar elements, including sensors, logic solvers, actuators and support systems.

The specified functions, or safety instrumented functions (SIF) are implemented as part of an overall risk reduction strategy which is intended to reduce the likelihood 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 the process safety time. SIFs are focused on preventing hazardous events with a health&safety or environmental consequence.

The correct operation of an SIS requires a series of equipment to function properly. It must have sensors capable of detecting abnormal operating conditions, such as high flow, low level, or incorrect valve positioning. A logic solver is required to 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 (e.g. closing a valve) 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. This standard is based on IEC 61508, a generic standard for design, construction, and operation of electrical/electronic/programmable electronic systems. Other industry sectors may also have standards that are based on IEC 61508, such as IEC 62061 (machinery systems), IEC 62425 (for railway signaling systems), IEC 61513 (for nuclear systems), and ISO 26262 (for road vehicles, currently a draft international standard).


Other names

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

  • Critical control system
  • Safety shutdown system
  • Protective instrumented system
  • Equipment protection system
  • Emergency shutdown system
  • Safety critical system
  • Interlock (engineering)
  • Interlocking (railway signalling)

SIS reliability

What a SIS shall do (the functional requirements) and how well it must perform (the safety integrity requirements) may be determined from Hazard and operability studies (HAZOP), layers of protection analysis (LOPA), risk graphs, and so on. All techniques are mentioned in IEC 61511 and IEC 61508. During SIS design, construction, installation, and operation, it is necessary to verify that these requirements are met. The functional requirements may be verified by design reviews, such as failure modes, effects, and criticality analysis (FMECA) and various types of testing, for example factory acceptance testing, site acceptance testing, and regular functional testing.

The safety integrity requirements may be verified by reliability analysis. For SIS that operates on demand, it is often the probability of failure on demand (PFD) that is calculated. In the design phase, the PFD may be calculated using generic reliability data, for example from OREDA. Later on, the initial PFD estimates may be updated with field experience from the specific plant in question.

It is not possible to address all factors that affect SIS reliability through reliability calculations. It is therefore also necessary to have adequate measures in place (e.g., procedures and competence) to avoid, reveal, and correct SIS related failures.

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.

See also

External links