Water Hammer in a Pumping Main and its Prevention

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Abstract
An earlier investigation has been continued on the protection of a rising water main against the consequences of a sudden interruption in the supply. In place of the air bottle previously tried, an automatic air-inlet valve was placed at the lower end of the pipe-line. Experiments on a laboratory scale showed that the cushion of air thus introduced brought the returning water column to rest without sound or shock. Theoretical calculations of maximum pressure and of air drawn in were verified. The lift of the pipe-line being small compared with its length, it was found that, over the range of velocity usually employed, the maximum pressure set up by the returning column diminished as the initial velocity of the interrupted supply was increased. With no protective device in use, a series of violent impacts took place owing to the column striking and rebounding from the closed valve at the lower end of the pipe-line. An electronic gauge, having a very high natural frequency, was used to measure the magnitude and duration of the shock pressures. Its indications were generally in accord with an analysis based on the usual theory of pressure waves, but the maximum pressures actually recorded somewhat exceeded the theoretical estimates, owing to the existence of additional pressure fluctuations caused by reflections from bends, sockets, and other discontinuities in the pipe-line. The insertion of a spring-loaded relief valve only slightly reduced the shock pressures.

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