Relief via a bursting disc

If relief is via a bursting disc, the flow capacity of the relief system will normally depend on friction and choke points in the relief system. The only exception is where friction is not important (LE/D less than about 40), where equation (A6.4) can be used.) Where friction is significant, an isometric sketch of the route of the relief system will be required to determine the capacity. If the system is to be of constant diameter, then using the sketch, the total equivalent length, LE, of the route, including the frictional resistance of bends and fittings can be determined111. This can also be expressed in terms of total frictional velocity head loss, K  

For a system of constant diameter (giving a single potential choke point at the end of the pipe), and if the gas is ideal, then the Design Charts for adiabatic flow of gases, given in Perry111 or the Omega method with = 1 (see Annex 8) can be used to determine the flow capacity. is a parameter within these charts. 

A number of proprietary computer codes for gas flow exist (see Annex 4). Many of these are intended for low velocity flow and do not handle choking. Before using such a code to evaluate relief system capacity, it should be checked that it is valid for high velocity choked flow (unless the use of equation (A6.5) indicates that flow will not be choked). 

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However, for tempered systems, relief via a bursting disc may give rise to two-phase relief due to flashing as the reactor depressurises. Although this does not affect the sizing of the relief system, it does increase the mass loss from the reactor and has implications for the disposal system design. Use of a safety valve, rather than a bursting disc, can prevent this. 

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