Column separation refers to the breaking of liquid columns in fully filled pipelines. This may occur in a water hammer event when the pressure drops to the vapor pressure at specific locations such as closed ends, high points or knees (changes in pipe slope). A vapor cavity, driven by the inertia of the parting liquid columns, will start to grow. The cavity acts as a vacuum, a low-pressure point, retarding the liquid columns, which finally starts to diminish in size when the liquid columns change flow direction. The collision of two liquid columns, or of one liquid column with a closed end, moving towards the shrinking cavity, may cause a large and nearly instantaneous rise in pressure. The large pressure rise travels through the entire pipeline and forms a severe load for hydraulic machinery, individual pipes and supporting structures. The situation is even worse: in one water- hammer event many repetitions of cavity formation and collapse may occur.
This report reviews water-hammer-induced column-separation from the discovery of the phenomenon in the late 19th century, the recognition of its danger in the 1930s, the development of numerical methods in the 1960s and 1970s, to the standard models used in commercial software packages in the late 20th century. A comprehensive survey of laboratory tests and field measurements is given. The review focuses on transient vaporous cavitation. Gaseous cavitation and steam-condensation are beyond the scope of the report. There are more than 300 references cited in this review report.