Water hammer pressure in piping is modified by the consequent motion of the piping. In general, accurate estimates of dynamic pressure and pipe displacement must account for this interaction. One approach is to formulate and solve the coupled equations of motion for the liquid and pipe structure. Implementation for practical pipe systems would require a computer code comparable in scope to a structural finite element program combined with a hydrodynamics program. This paper presents an alternative approach that utilizes any available finite element program to compute natural frequencies and mode shapes of the piping, and then uses those modes to modify a hydrodynamic analysis and to predict motion of the piping. An example analysis demonstrates application of the method to assess the consequence of removing a brace intended to restrain pipe motion caused by water hammer. Results are compared to those given by analyses that neglect the effect of pipe motion on pressure.

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