An abnormal accident with no flow in a pumping system occurred with a large displacement of the pipe system. A slow closure of an isolation valve installed in a large suction pipe was the main action. As soon as the valve was closed the 1.6 m diameter pipe moved and the pipe supports were broken, presenting an expansion joint almost fully opened and the main pipe almost broken. This was a critical event that took place in one of the most important pumping stations of the water supply system in Lisbon. In order to avoid future accidents, a detailed analysis was developed as a priority in order to identify the cause of this event. The paper describes the event and the methodology followed in order to fully validate the diagnostic and the proposed chain of events that caused the pipe movement. The process included a careful, in situ observation of the system, hydraulic and structural analyses and some site tests. Ironically, the explanation of abnormal events can give the opportunity to better understand the operation of water supply systems. Weak points in the design and in the operation can then be detected and future accidents can also be avoided.

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