General comments about flow of viscous fluids

1 General comments about flow of viscous fluids 

The viscous systems that are of most concern are those which may give rise to laminar flow in the relief system. The level swell in the vessel may also be affected. These two topics are covered below. It should be noted that only moderately high viscosities, of 100 cP or more, may be sufficient to cause laminar flow or to change the level swell behaviour. Failure to take account of the effects of laminar flow could lead to the serious undersizing of the relief system. 

Viscous relief is best avoided, if at all possible. If the high viscosity occurs towards the end of a reaction (e.g. polymerisation ), a low relief pressure may help, by ensuring a relatively low conversion at the point when the relief system operates121. 

Viscous systems are the subject of continuing research by the US DIERS Users Group and in Europe. Research projects include the flow of high viscosity two-phase mixtures in safety valves, and the effect of bends and pipe fittings on high viscosity two-phase flow in pipes. 

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