Risk assessment in the chemical industries. Quantitative risk assessment is of little practical value to plant managers or design engineers. A German view

Abstract

In the case of simple technical facilities, existing hazards uite evident, and necessary countermeasures are ever, the safety roblems are rather complex and therefore, as a rule, setailed risk assessments are required. There are various approaches to the desired aim, i.e., to design and construct a safe plant, to operate it safely, and to assess the risks associated with it.In the present paper we should like to critically a praise the method of quantification of risks, i.e., proba \ ilistic risk assessment, and to offer a few remarks as to how far this com aratively new instrument is suited to the require-As a matter of fact, accidents do ha pen, and the industry dents. We wish to state quite clearly that engineers in this industry would gladly use the new method extensively-in addition to the other available techniques-if it could contribute to the safety of process plants.As a matter of fact, a quantitative approach in general and to safety in particular is not foreign to the chemical industry. For every rocess the kinetics of the chemical reacfluid dynamics, the structural strength of vessels, pipes, and other equipment, as well as other similar items are determined quantitatively by experiment or calculation, drawing on a vast body of experience.In regard to the potential effects ofan incident, the quantitative evaluation of such items as dispersion of pollutants, effects of fire, explosion, or toxic substances on materials, structures,or human beings is taken into account during the design of lants posing certain hazards. Unthis area; knowledge is still too incomplete to permit sufficientl reliable assessments. ical industry adopts an extreme1 reserved and cautious at-There is no argument about the great benefit of a design team undertaking a formal safety analysis by applying one of the hazard-identification methods. That is a usual practice in the chemical industry. However, safety analyses are rarely pushed to the point of establishing logic diagrams (fault or event trees), because the effort is considerable and worthwhile only for really critical areas.Quantitative analysis begins when y b a b i l i t y figures are inserted in the logic diagram. An here is the point where so far we do not see any advantage in employing this kind of analysis. We feel that the advocates of this method are too enthusiastic about it and may be admiring the technique more than appreciating the realities. There are imortant limitations which do not appear to have always Eeen fully appreciated: are equa 3 ly obvious. In the chemical process industry, howments o P the chemical industry.makes every effort to limit the num . & er and effects of incition, the heat an if mass transfers, the corrosion rates, the doubtedly a number o t! problems remain to be resolved in As r ar as probabilities are concerned, the German chemtitude towards quantitative ana i ysis.The probability of an event calculated in an analysis is no more than an estimate-based on the pathways considered. And only i f d l the important pathways have been taken in...