Our advice
The event tree: a risk analysis method

In the case presented below, we consider on the one hand the dreaded event Gas leak on a pipeline with a frequency of occurrence of 10 -² /year, and on the other hand a Safety Instrumented System (SIS) composed of :

• a gas detector with a PFD equal to 10 -¹ /sollicitation;

• a treatment plant with a PFD equal to 10 -² /sollicitation;

• an automatic isolation valve with a PFD equal to 10 -¹ /sollicitation.

Based on the above data, the PFD of the SIS is evaluated as follows:

PFD (SIS) = PFD (Gas detector) + PFD (Logic processing) + PFD (Isolation valve) = 10 -¹...