Safety Considerations Guide for Triconex General ... - ICEWeb

6 Chapter 1 Safety Concepts
As a required SIL increases, SIS integrity increases as measured by:
• System availability (expressed as a percentage)
• Average probability of failure to perform design function on demand (PFD avg )
• Risk reduction factor (RRF, reciprocal of PFD avg )
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.
Safety Considerations Guide for Triconex General Purpose v2 Systems