A variety of normal water distribution system operations will result in pressure fluctuations that can possibly lead to the occurrence of relatively brief (transient) low or negative pressures at various locations of the system, thus creating the potential for backsiphonage or backpressure of non-potable water from external sources into the distribution system, including contaminated water around leaking pipes. Extensive pressure monitoring of a single distribution system using seven electronic data loggers for 1.4 years found only nine occasions when distribution system pressures were less than 138 kPa (20 psi). No negative pressures were observed. Evidence showed that most low pressure events were caused by pump shutdowns. Distribution system modelling suggested that negative pressures (<0 kPa) were possible when the water treatment plant pumping station lost power. The model simulations suggested that protection against low pressures is possible by installing hydro-pneumatic tanks downstream of the pumping stations. Modelling results suggested that air/vacuum valves did not offer reliable protection against low pressures in the system.

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