Safety classes

All failures can be categorized according to frequency of occurence and consequence of failure. The figure shows the classification of consequence levels for chemical plants. Frequency and consequence levels can be described by a table of safety classes where A is the most dangerous state and D is the safest state.

Besides levels of consequence and frequency, the numerical values of the parameters define zones of acceptable and unacceptable risks.

For complex engineering systems, for nuclear power plant piping for example, the classification can made by consequence and failure mechanism. This classification defines priority in the identifying of inspection regions. The highest priority is given to large leakages in pipe, nozzles, and welds with severe consequence to the safety of the plant and personal - A.

Safety classes can be categorized by stress state (from unidirectional to three-dimensional + residual stress), stress / yield strength ratio, materials or temperature. Safety class of a structure is the minimum rating after considering all the factors.

The risk - based design consists of 9 steps. Identification of failure scenarios is the first step of the analysis.