Safety Integrity Level (SIL)

SIL Classification

Safety Instrumented Functions (SIF’s) are important elements within many industries for the protection and mitigation of risks. Therefore, it is important to identify the protection requirements of each SIF in respect to its function. The requirement is often described in terms of Safety Integrity Level (SIL). A SIL can be one of four levels, each within a target band known as the Probability of Failure on Demand (PFD), where SIL1 is considered the lowest and SIL4 is considered the highest.

Concept of Risk Tolerability

The purpose of determining the tolerable risk for a specific hazardous event is to state what is deemed reasonable with respect to both the frequency of the hazardous event and its specific consequences.

The tolerable risk will depend on many factors. For example, the severity of the consequences or injury, the number of people exposed to danger, the frequency and the duration of the exposure. Important factors will be the perception and views of those exposed to the hazardous event. Risk reduction is achieved by a combination of all the safety protective features, including any associated SIFs. The necessary risk reduction to achieve the specified tolerable risk, from a starting point of the risk presented by the Equipment Under Control (EUC), is shown below.

Safety integrity applies to the Electrical / Electronic / Programmable Electronic (E/E/PE) SIFs, other technology safety instrumented systems and external risk reduction facilities and is a measure of the likelihood of those systems satisfactorily achieving the necessary risk reduction. Once the tolerable risk has been set, and the necessary risk reduction estimated, the safety integrity requirements for the SIFs can be allocated in terms of the PFD.

Methods of SIL Determination

There are various methods in achieving SIL targets qualitatively by Risk Graphs, semi-quantitatively by Layer of Protection Analysis (LOPA) or fully quantitatively by Fault Tree Analysis (FTA).