1. Choose your calculation method

1.1 The different methods

Assessing the probability of failure under load, which is essential for determining the SIL level of low-load RIS, requires an unavailability calculation. Various methods can be used for this purpose:

• 1 - "Approximate" calculation formulas ;

• 2 - fault tree ;

• 3 - Markov graphs ;

• 4 - Petri dishes.

These methods vary in complexity. Methods -1- and -2- are frequently used to calculate SIL levels, as they can be applied by industrial risk specialists. Methods -3- and -4-, on the other hand, are more suitable for operating safety specialists, and are therefore less widely used in...