A change of flow velocity induces pressure fluctuations in piping systems, where flow of a liquid is under pressure. Liquid column separation occurs when the pressure drops to the level of the liquid vapour pressure. For the requirements of the graduation thesis, the computer program for water hammer calculation, which does not take into account a vaporatization process at the liquid vapour pressure, was upgraded to the discrete vapour cavity model, where the condition of the liquid column separation is taken into account. In addition to the primary upgrade, the program was further improved with the method of improved timing of opening and collapse of a vapour cavity. The validation of three versions of the program, which was carried out with the comparison between numerical results and results from laboratory measurements, clearly showed advantages of the improved numerical model.