(155b) Safety Integrity Level Determination and Categorization

Andreea Elena Catana, Anca Elena Costea, Andreea Iordache

Different risks originate from plants or plant components. As the risk increases, the demands on the safety related system (SRS) also increase. The standards IEC 61508 (umbrella standard, the first standard from which the others are derived) and IEC 61511 (standard for process industry) define a measure for the reliability of the safety systems, known as the Safety Integrity, which comprises four levels. These four levels of Safety Integrity are named SIL 1, SIL 2, SIL 3 and SIL 4, SIL 1 being the lowest and SIL 4 – the highest level.

Certain safety functions, such as those that are commonly found in the chemical and petrochemical industry (e.g. overfill protection system) are generally classified as low demand ones. This is because it is expected that the demand would be less than one per year. There is another category of Safety Functions that are found in places where the demand rate is very high and sometimes even continuously present. These are called high demand applications.

Safety instrumented functions are related to a specific hazardous event in which a process may be compromised: shutdown in a hazardous process; excessive opening of pressure relief valves; on/off tank overflow control; adding coolant to arrest exothermic runaway; automatic shutdown when manual control is not present or available; closing a feed valve to prevent tank overflow; fire suppressant release; evacuation alarm alert.

The four levels of Safety Integrity are used in a HAZOP study for instrumentation analysis. The whole system is assessed to determine the level of protection during any type of scenario. Each type of instrumentation (e.g.: level indicators, temperature alarms, pressure alarms) is compared to other instrumentation. The SIL level is then determined, and as long as the overpressure protection comes mostly from the safety functions and not from the pressure relief devices, the safety integrity level increases and reduces the occurrence of catastrophic events.