Although water column separation and collapse is not common in large water distribution networks, it can have a significant effect on the transient behavior and this effect must be considered. Cavitation in water distribution systems occurs when the local pressure is decreased to the value of the vapor pressure at the ambient temperature. All the gases within the water are released, the water starts to evaporate, and pockets of vapor appear. Except for large pipelines, the low pressure seldom poses a problem and does not normally threaten the structural integrity of the piping system. What does cause problems is the collapse of the vapor cavity when the water columns rejoin at high speed producing a large pressure surge. In this case, both vacuum and strong pressure surges are present which may result in substantial damage. This problem is most common following a pump trip in a system pumping to an elevated storage tank. If the water column separates downstream from the pump, the elevation difference will eventually cause a cavity collapse and, sometimes, a very large pressure surge. The best design is one which prevents negative pressures and the formation of a vapor cavity.
InfoSurge can simulate water column separation and collapse with time and space to allow the user to assess the potential consequences and the need for remedial surge protection devices. The occurrence of cavitation due to low pressures that fall below vapor pressure and the associated growth and decay of cavity pockets are automatically computed and monitored. A vapor pocket can form on either or both sides of a network component (e.g., pipe junction, pump) if the pressure drops below the vapor pressure. The growth of the cavity (vapor cavity volume) is determined based on the magnitude of the velocity of the liquid adjacent to the cavity and the time over which given conditions persist. Subsequent actions can accelerate the growth of the vapor cavity or can produce a condition that causes a net flow toward the network component and the subsequent collapse of the vapor pocket. A negative pressure wave impinging on a vapor cavity will accelerate its growth while a positive pressure wave will decrease the growth rate and may initiate collapse of the vapor pocket. Significant pressure surges can be produced upon collapse of a vapor pocket and these are calculated based on the net flow velocity in the line (toward the cavity) just prior to the collapse. InfoSurge tracks the growth or collapse of the vapor cavity at each time step during the transient simulation so it can be determined whether or not a vapor cavity will be present when the next action takes place. This monitoring process continues until the end of the simulation period is reached.