Choke line friction

The method of calculating system vent flow is illustrated in Figure 10. The fluid flow model chosen consisted of a transfer line having a distributed impedance due to valves, line friction, etc, with an orifice at the downstream end. During pressurized cooldown, the downstream orifice operates at, or near, choked flow, and compressible fluid flow theory was used to calculate vent flow. 

When the line containing the jbursting disc is short and the disc itself has low frictional resistance after burst (total equivalent length to diameter ratio less than about 40), the friction associated with the inlet contraction from the vessel will dominate the overall frictional effect on flow capacity. Also, the critical pressure ratio can be reasonably approximated to that for a nozzle (see Annex 8, Figure A8.2). This is needed to check that flow is choked. 

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