Selection of worst case scenarios by-screening

At this stage, a number of credible maloperations will have been defined that can lead to vessel over-pressurisation. In order to cope with all the credible runaway scenarios, the relief system will need to be sized for the worst case runaway reaction that can occur, and this is normally the maloperation that will give rise to the highest rate of temperature and/or pressure rise over the relief range. 

Determination of the worst case is by no means straightforward. It is important that selection is on the basis of rate of reaction rather than heat of reaction, even though heat of reaction is more easily measured in a screening apparatus. i 

Alternatively, with a knowledge of the reaction kinetics, it may be possible to rule out certain scenarios as being unlikely to produce the worst case. However, the reaction kinetics used to do this must be based on the reaction that actually occurs under runaway, rather than the reaction that is theoretically expected. A degree of testing of the reaction under, runaway is usually needed and, for. the information to be sufficiently reliable, this should normally have been obtained using a purpose built vent sizing calorimeter (see Annex 2). It is essential that personnel who are experienced in carrying out these tests are involved in the assessment. 

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