A condition in which a fluid becomes limited in its flow or ‘choked’ and is not able to be increased further. For a fluid flowing through an orifice or small hole in a pipe, the increase in velocity corresponds to a decrease in pressure, known as the venturi effect. However, a point is reached in which the rate of flow will not result in any further decrease in pressure, thereby limiting flow. The choking of gases occurs when the velocity leaving the orifice approaches sonic velocity i.e. at a Mach number of one. This results in shock waves that restrict flow causing the choking effect. The deliberate choking of gases is sometimes useful for limiting the rate of flow to processes. For liquids, the decrease in pressure below the vapour pressure results in partial flashing and cavitation, with the formation of vapour effectively limiting flow. It is also known as the critical flow and is important in process safety, particularly in terms of the rate of release of material from a vessel or vent when depressurizing.