Methods that provide layers of protection should be:
• Designed for the specific safety risk
This figure shows how layers of protection can be used to reduce unacceptable risk to an acceptable level. The amount of risk reduction for each layer is dependent on the specific nature of the safety risk and the impact of the layer on the risk. Economic analysis should be used to determine the appropriate combination of layers for mitigating safety risks.
Figure 1 Effect of Protection Layers on Process Risk
When an SIS is required, one of the following should be determined:
• Level of risk reduction assigned to the SIS
• Safety integrity level (SIL) of the SIS
Typically, a determination is made according to the requirements of the ANSI/ISA S84.01 or IEC 61508 standards during a process hazard analysis (PHA).