The process industries often deal with large quantities of flammable, explosive and hazardous chemicals, and they have a long history of incidents resulting in lost lives, lasting injuries and environmental as well as property damage. Experiences gained from these have led to the use of safety instrumented systems (SIS), whose sole purpose is to maintain plants in safe condition. SISs have evolved over time, and numerous safety-related standards have been written to specify their design and implementation (Figure 1).

Safety instrumentation is not exclusively an instrument and control engineering subject. Successful implementation of an SIS project depends on knowledge of other disciplines, as well as a well-defined safety management system within the company. Without proper support structures and a good understanding by all involved in defining safety requirements, safety instrumentation on its own will be unlikely to deliver the levels of safety expected of it.

SIS structure

SISs are control systems that take the process to a safe state on detection of conditions that may be hazardous in themselves, or if no action were taken, could eventually give rise to a hazard. SISs perform safety instrumented functions (SIF) by acting to prevent the hazard or mitigate its consequences. Alternative names for an SIS include trip and alarm system, emergency shutdown system, safety shutdown system, safety interlock system and safety-related control system.

Note that the SIS is designed to be a separate control system that acts independently of any other controls or personnel, such as the basic process control systems (BPCS) or fire and gas (F&G) system (Figure 2).

SISs are normally regarded as being structured in three parts: sensors to measure, detect atmospheres, and determine process and equipment online conditions; a logic solver to evaluate the plant conditions, make decisions and output signals; and actuators to execute the required actions. SISs also have interfaces to users and other control systems to send shutdown and safety commands.