Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutiveAppendix four:Emergency pressure reliefor vent design1 The design of emergency pressure relief systems for exothermic runaway iscomplex. It requires a thorough understanding of the reaction during runaway,including any side reactions or unintended reactions that may occur, and reliefsystem sizing methodology. You need information on:(a) the credible maloperations and system failures that might occur during reaction;(b) the kinetics of the reaction under runaway conditions;(c) whether the reaction pressure is from vapour or non-condensable gas (or both);(d) the flow regimes, both in the vessel and the relief system, during pressure relief;and(e) the design and layout of the relief system.Unless you use such information and apply it properly in the pressure relief systemdesign, then the system may not be correctly sized.2 The identification and characterisation of worst case scenarios have beendiscussed in paragraphs 149-152 of the main text.3 State-of-the-art techniques for vent design have been developed by theDesign Institute for Emergency Relief Systems (DIERS). A number of relief-systemsizing methods are available. 59 HSE has published a workbook 36 that givesinformation on methods available.What happens during relief?4 During an exothermic runaway the reaction temperature increasesexponentially. As the temperature increases the pressure in the reactor may alsoincrease. This can occur in three ways:(a) The pressure generated is due solely to the increasing vapour pressure of thereaction mass as the temperature increases - vapour pressure systems.(b) The pressure generated is due to a permanent gas - gassy systems. This canoccur in both synthesis and decomposition reactions.(c) The pressure generated is due to both the vapour pressure and the permanentgas generation - hybrid systems.5 Vapour pressure systems and some hybrid systems are ‘tempered’. Providingthe relief system is large enough, the vapour generation allows sufficient latent heatof vaporisation to moderate the runaway. This holds the temperature constant andkeeps the reaction rate approximately constant.6 Gassy systems are ‘untempered’ because there is no available mechanism forthe removal of significant amounts of heat from the vented system.Designing and operating safe chemical reaction processes Page 51 of 64