Pressure surges create axial forces on a pipe between two elbows due to unbalanced pressures. These axial forces depend on the rates of pressure change, lengths and diameters of pipes, sonic speeds in the transported medium, and pressure wave propagations and attenuations. The magnitude of axial forces can vary significantly from one pipe to another, even within proximity. Differing from the worst case for overpressure, the worst case for axial forces depends mainly on the steepness of the surge pressure waves and the lengths of the pipes.
To demonstrate the effect of these two key parameters, hydraulic transient models are developed for three case studies. These include a pump trip at an origin pump station of a crude pipeline, a valve closure at a delivery terminal of a crude pipeline and an emergency valve closure in an LNG rundown line for feeding LNG storage tanks. The modeling results show the resultant maximum force diagram for each of the piping systems is essential to locate and design the pipe supports. The modeling results also reveal the surge wave propagation patterns and the maximum axial force locations for three distinguished surges caused by a check valve slam after the pump trip, a vapor cavity collapse downstream of the delivery valve following closure, and an instant flow stoppage due to the emergency shutdown valve closure.
The paper proposes a methodology to identify the worst force case, to estimate the highest axial forces, and to determine the variation in axial forces along the piping system. The paper shows that the worst force case could be different from the worst pressure case. The paper also demonstrates that both upsurge and down-surge are equally important in estimating the maximum forces.
