Bursting discs with short relief lines

1 Bursting discs with short relief lines 

The capacity of the relief system can be obtained from a two-phase flow calculation for nozzle flow. If the flow is not choked, then the Omega method (see Annex 8) or suitable computer code must be used to calculate flow capacity. For choked flow a larger range of methods may be applicable, e.g. ERM for vapour pressure systems (see 9.4.2) or Tangren et al. s method for gassy systems (see 9.4.3), together with the application of a discharge coefficient. The capacity can then be obtained from  

Suitable values of discharge coefficient for different situations are given in BS 2915[13] for single phase liquid or gas flow. CCPS1141 indicate that a gas discharge coefficient should be used for two-phase flow provided the flow chokes, otherwise a liquid discharge coefficient should be used. 

In the absence of a discharge coefficient, the most accurate way of estimating a flow reduction factor is to use the Omega method (see Annex 8). Alternatively, a discharge coefficient can be estimated from the following equation which applies for single-phase non-choked flow  

Typical values of number of velocity heads lost, 8, and equivalent lengths, L, of fittings which may sometimes be found in relief lines are given by CCPS1141. 

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