Study of Dynamic Stresses in Pipe Networks and Pressure Vessels Using Fluid-Solid-Interaction Models

A lack of understanding of the fluid-structure interactions has resulted in a number of infamous structural failures in the past. For example, the collapses of the Tay Bridge in Scotland in 1879, the Tacoma Bridge in 1948 and three tall cooling towers in Ferrybridge/England in 1965 have been intrinsically related to fluid forces acting on the structure. Flutter, flow-induced vibration, divergence and related phenomena may be studied using the Fluid-Solid-Interaction (FSI) approach. This paper gives three examples of the FSI approach and shows the innovative application of state-of-the-art computational methods to improve realism and accuracy in engineering analyses. Case 1: Study of Hydrodynamic Sloshing Loads: The sloshing of liquid in large vessels under seismic loads is a timely topic. The movement of the free surface of the liquid is simulated using a two-phase volume of fluid model at various liquid heights. The transient forces generated by the fluid on the vessel wall and internals are superimposed as loads on a dynamic non-linear calculation and the fatigue and stresses are computed in an explicit finite element analysis. This approach calculates the local sloshing effects on internals as opposed to the traditional approach of using spring-mass elements. Case 2: Bending of Large Pipes due to Temperature Differentials: Pipe temperature differentials can be caused by either extremely cold liquids or hot liquids flowing at the bottom of a piping system while the top is exposed to atmospheric conditions. Differential expansion can cause pipe deformation resulting in pipe lift-off at its supports and failure at the weld locations and T-joints. Heat transfer from complex multi-phase flows was simulated using CFD. The predicted pipe wall temperatures were then input to an FEA grid and analyzed for heat transfer and thermal stresses. These stresses were compared to ASME standard allowable limits. Based on this analytical approach, a design guide for various diameters of flare header pipes, supports and tees has been established. Details of this paper were previously published in [Ref 1] and are not described in this paper. Case 3: Establishing velocity limits and line sizing criteria in pipes: The original guidelines in Fluid Flow Manuals were developed over the last fifty years based on project experience and economic and best practices technology of the time. The criteria have proven out as good, but overly conservative with regards to line size. Compressor discharge guidelines are based on the erosion velocity limits. Based on a dynamic analysis approach — using unsteady flow rates from compressors — stresses due to flow-induced vibration, noise and fatigue, hydraulic transients such as waterhammer effects for long lines (greater than 1000 feet), flashing and control valve cavitations may be studied. FSI was used to determine if the velocity limit guidelines hold in the current designs and use a parametric approach to mitigate the bottlenecking by supplying a simple fix to the problem. Furthermore the approach was used to define the correct velocity limit and establish optimal layout for the piping network.