Vapour pockets formed in a pipeline when the pumps were tripped, causing the pipe to burst. A theoretical study revealed that two vapour pockets formed. Collapse of a large pocket at the pumps caused watehammer over-pressures. The size of vapour pockets and time to collapse were calculated using rigid column theory. Theoretical pressures agreed well with those recorded. A reflux valve installed in the pipeline eliIninated water hammer over-pressures.