Power spectral analysis of a forcemain failure caused by waterhammer

Abstract

The 14 in. (0.36 m) forcemain followed an overall convex pipeline profile for 4000 ft (1200 m) and would be subject to negative surge problems associated with pump shutdown. The forcemain couplings failed three times near the pumphouse and the failure was thought to be related to waterhammer effects. The sequence of breakages is reviewed. A series of pressure recordings were then made over 14 days on the repaired forcemain, leading up to and including the final failure. These pressure recordings were digitized and subjected to power spectral analysis. The power spectra pointed out several significant events that were not evident from the pressure record alone.These included the fact that the original breakdown in the forcemain occurred several days prior to its ultimate failure and discovery on the surface. It was also determined that the break in the system was due to the apparent merging of the primary waterhammer wave with an existing but gradually changing lower frequency wave. This second wave was associated with rigid column motion and gradually increased its frequency. The resultant wave superposition collected sufficient energy at one point to cause the ultimate failure of the evidently already damaged forcemain system.Power spectral analysis proved useful as a method for analyzing waterhammer effects in a forcemain complicated by column separation, leakage, and vapour pocket collapse, and may be a useful way of monitoring the performance of longer pipelines.