The phenomenon of choking in compressible flow

Choking is a phenomenon that occurs in high speed compressible flow (e.g. in relief systems). It occurs because, as the pressure falls along a pipe or through a nozzle, the fluid density decreases. This, means that the volumetric flow rate and, hence, the velocity increases (because the mass flow is constant). Choking occurs when the downstream pressure is reduced to the point where the velocity cannot increase any more. This effectively limits the maximum velocity and, hence, flow rate of the fluid. 

Two-phase flow models allow the calculation of both the two-phase mass flow rate per unit area (G) and also the critical pressure for choking. DIERS recommend the homogeneous equilibrium model (HEM, see 9.4.1) for this calculation. 

In relief systems of uniform diameter, the choke point (if choking, occurs) will be at the downstream end of the pipe. However, if the relief system comprises more than one diameter, then multiple choke points are possible and it will be necessary to determine the position of the choke point that limits the flow. This can be a complex calculation, for which there are two common cases  

For safety valve systems, the flow-limiting choke is always in the safety valve nozzle. See 9.7.1. 

Bursting disc systems with multiple diameters require iterative calculations to find the position of the flow limiting choke. See 9.6.3 and A8.4.5. 

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