Critical flow and pressure: When a compressible gas is expanded through an orifice, nozzle, or pipe end, its velocity and sp. vol increases with decreasing downstream pressure. For a given upstream conditions, mass flow rate increase until a limiting velocity is reached. This limiting velocity is the velocity of sound in the flowing fluid at that location. The flow rate corresponding to limiting velocity is known as critical flow rate. The absolute pressure at the exit of nozzle, pipe, orifice etc. at sonic velocity is the critical pressure. Under critical flow conditions, actual exit pressure cannot fall below critical pressure even if the downstream pressure is much lower. Sizing of relief valve use different equations depending on if the critical flow or sub-critical flow (exit pressure higher then critical flow pressure) exist.
Posted on: Fri, 26 Jul 2013 13:33:50 +0000
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