The term safety often gets used in the context of occupational safety, namely slips, trips and falls. However, process safety and functional safety have a different focus.
Safety is a general term that has been defined as “Freedom from danger, risk, or injury”, or “Freedom from unacceptable risk”. It is the condition of being protected from harm, or any other event which could be considered undesirable.
Process safety involves keeping the process under control and keeping hazardous materials inside the pipes and vessels. It is the concern of many different disciplines including process experts, mechanical, electrical, control and instrumentation, as well as safety professionals.
Functional safety is a part of the overall plant Process Safety approach. When hazards occur, the correct operation of automated equipment such as sensors, logic solvers and valves, should bring the process to a safe state. Functional safety principles ensure that each hazard is prevented or mitigated by equipment designed with the correct integrity.
The need for Safety Instrumented Systems is driven by various factors, not least of which are process industry accidents, such as the one that occurred at Buncefield in 2005.
Operators of major accident hazard sites, look to reduce the cost of insurance, by demonstrating that SIS and other protection layers are well designed and maintained.
Standards are a third driver for SIS. Companies and organizations work globally to agree best practices, such as IEC61511, which is the global reference source for specifying, designing and maintaining safety instrumented systems.
A study of incidents involving control systems was conducted by the Health and Safety Executive, in the United Kingdom, with the first results published in the book "Out of Control: Why Control Systems go Wrong and How to Prevent Failure".
Although this is just one study, it's noteworthy that the hazard owner is responsible for setting requirements, as well as operating, maintaining and modifying an SIS after it has been placed in service. So, even if an end user contracts-out the design, this research shows that around 80% of problems get introduced either before or after a system is designed and installed.