Occurrence of Transient Low and Negative Pressures in Distribution Systems

The production of transient low and negative pressures in otherwise pressurized potable water supply distribution systems creates the opportunity for contaminated water to enter the system from external environments. This article documents the occurrence of transient low and negative pressures in distribution systems as a result of a variety of system features and operations. Fifteen surge events were observed that resulted in a negative pressure, with most of these caused by the sudden shutdown of all of the pumps at a pump station because of either unintentional (e.g., power outages) or intentional (e.g., pump stoppage or startup tests) circumstances. Smaller but still substantial surges were produced by shutdown of pumps while other pumps remained running as well as by operational occurrences such as flushing, hydrant use, and main breaks. The authors also compare the efficacy of different types of pressure-monitoring devices for detecting transient pressure surges.