Determining a Safety Integrity Level

If a PHA (process hazard analysis) concludes that an SIS is required, ANSI/ISA S84.01 and IEC 61508 require that a target SIL be assigned. The assignment of a SIL is a corporate decision based on risk management and risk tolerance philosophy. Safety regulations require that the assignment of SILs should be carefully performed and thoroughly documented.

Completion of a HAZOP determines the severity and probability of the risks associated with a process. Risk severity is based on a measure of the anticipated impact or consequences.

On-site consequences include:

• Worker injury or death
• Equipment damage

Off-site consequences include:

• Community exposure, including injury and death
• Property damage
• Environmental impact
• Emission of hazardous chemicals
• Contamination of air, soil, and water supplies
• Damage to environmentally sensitive areas

A risk probability is an estimate of the likelihood that an expected event will occur. Classified as
high, medium, or low, a risk probability is often based on a company’s or a competitor’s
operating experience.

Several methods of converting HAZOP data into SILs are used. Methods range from making a
corporate decision on all safety system installations to more complex techniques, such as an IEC
61508 risk graph.

Reference: Triconex