Influence of Line Motion on Waterhammer Pressures

The large pressure increases following rapid valve closures in liquid flow pipe systems produce large forces which must be resisted by the system structure. These forces invariably produce motion of the pipe line. The motion which is usually of small amplitude is generally ignored in waterhammer analysis. It is shown, however, that the motion may produce pressures that differ significantly from those predicted using rigidly supported pipelines. Initial pressure surges with magnitudes greater than 20% above the commonly used maximum for instantaneous valve closure may result under certain combinations of structural parameters and closing conditions and subsequent surges can be much greater than the initial surge. Under other conditions, it is shown that the pressure buildup is significantly retarded and the line motion can be utilized to control the waterhammer pressures. The line motion associated with the waterhammer surge can also produce characteristics such as pressure overshoot and jagged portions of the pressure trace which have often been observed experimentally.