Safety factors in the design and use of pressure equipment

A general perception is that safety factors are there to provide confidence in the safe use of an engineering component, assembly or system. Pressure equipment by its nature is potentially hazardous and needs factors of safety to provide a margin against failure from uncertainties in design, materials, manufacture, and in operation including repair, replacement and life assessment. Do the current factors of safety for pressure equipment provide this confidence, are they consistent with the level of hazard, and are there grounds for changing the balance? It is possible to identify three different types of safety factor. The first concerns uncertainties in the technology where a full understanding or necessary data is lacking. Secondly, variability in human performance and error is always present and usually sets the limit to which safety factors could be reduced, even with a full knowledge of the technology. In addition to these more tangible aspects, misadventure, possibly defined as a remote but conceivable set of circumstances, needs to be accommodated. In summary, safety factors cover uncertainties in technology, the human element and misadventure. The public perception of Factor of Safety is often seen as a Factor of Ignorance. It indicates the extent to which the designer or user does not have reliable data on the properties and performance of materials or on the validity of the calculations or on the behaviour of the system in operation. This definition seems to be crude and incomplete as far as the engineer is concerned since the reality is always more complex and subtle. In the context of pressure equipment, it may be observed both that the rate of failures in the UK is at a very low but not negligible level, and that many of the failures are the result of human error. Does this mean that the factors of safety are not yet optimised? On closer examination, it can be concluded that there is an inherent difficulty in defining and determining Factors of Safety for pressure equipment and that the perception of a safety factor varies considerably in the public eye and between different sectors of industry.