A safety instrumented system (SIS) consists of an engineered set of hardware and software controls which are especially used on critical process systems.
Safety instrumented systems are most often used in process (e.g., refineries, chemical, nuclear) 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.
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. A Safe State is a process condition, whether the process is operating or shutdown, such that a hazardous SH&E event cannot occur.
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 steam boiler. Critical parts of the process would include the lighting of the burners, controlling the level of water in the drum, and controlling the steam pressure.
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.
A formal process of hazard identification is performed by the project team engineers and other experts at the completion of the engineering design phase of each section of the process, known as a Unit of Operation. This team performs a systematic, rigorous, procedural review of each point of possible hazard, or “node”, in the completed engineering design. This review and its resulting documentation is called a HAZOP study.
A HAZOP study typically reveals hazardous scenarios which require further risk mitigating measures which are to be achieved by SIFs. Via a Layer of Protection Analysis (LOPA) or some other approved method, Integrity Levels (IL) are defined for the SIFs in their respective scenarios. The Integrity Levels may be categorised as Safety Integrity Level (SIL) or Environmental Integrity Level (EIL). Based on HAZOP study recommendations and the IL rating of the SIFs; the engineering (including the BPCS and the SIF designs) for each unit operation is finalized.
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).