Conservatism of sizing methods

The likely conservatism of different sizing methods cannot be quantified without extensive large-scale experimental validation. (Some such validation at pilot-scale is currently being carried out by the Health and Safety Executive and as part of a collaborative EC project151) However, some qualitative guidance can be given. 

It is recommended that the chosen sizing method be evaluated for a range of overpressures. This will give an indication of the likely consequence of undersizing, in terms of how far the pressure may be expected to rise above the maximum permitted for the vessel. For moderate overpressures (typically 10-30% of the absolute relief pressure), the consequence of undersizing may sometimes be small. 

For untempered systems, the DIERS sizing equation (see 7.3) can be conservative, unless relief is of gas alone until close to the peak reaction rate. However, it is important to correctly account for any dissolved gas in the experimental test151. Methods which account for early loss of reactants from the reactor, by two-phase relief, should be regarded as best estimate methods rather than conservative calculations. 

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WORKBOOK FOR CHEMICAL REACTOR RELIEF SYSTEM SIZING A7.3 CONSERVATISM OF SIZING METHODS... 

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