ANSI/ISA-84 provides a framework for managing safety systems in the process industry to ensure the systems deliver sufficient risk reduction by reducing risk to acceptable levels.
The Safety Integrity Level (SIL) level is determined by a risk assessment. This SIL level specifies the amount of risk reduction (mitigation) needed to reduce the risk to acceptable levels. All processes must have some level of risk, as zero risk is impossible to achieve.
The SIL level required depends greatly on the consequences of failure of the installed product, process, or machine. If a catastrophic failure of a processes results in no injuries, no significant damage to the environment and little financial impact to the customer, then a relatively low SIL level (SIL 1 for example) may be acceptable. Risk is mitigated (reduced) through independent layers of protection. In the diagram below, process risk level is shown along with mitigations such as the control system, safety system, relieve valves, etc.
After the required SIL is determined, the processes and safety systems can be designed to meet the required SIL. The amount of risk reduction is determined by statistical models of the components. Ideally, these models are based on actual field data (failure rates). Otherwise, the models are based on standard databases of component failures. To increase the SIL, the probability of failure on demand (PFD) of the safety system must be lowered.