The second set of piping benchmark problems is solved using current technology pipe elements PIPE289 and ELBOW290. The straight and curved piping segments are discretized with PIPE289 and ELBOW290 respectively. In order to better model the ovalization carry over from the curved pipe elements to the straight pipe elements, the ELBOW command is used to automatically convert some straight PIPE289 elements near the pipe bends into ELBOW290 elements. A finer mesh compared to the NUREG model is used with PIPE289 and ELBOW290 elements so that the subtended angle in each ELBOW290 element does not exceed 45 degrees.
PIPE289 element is based on the extended Timoshenko beam theory with the inclusion of pipe wall expansion degrees of freedoms. PIPE289 supports both plane stress (thin pipe) and 3D stress (thick pipe) states and is effective for modeling straight pipes. Based on 3D finite-strain shell theory, ELBOW290 element uses Fourier series for explicitly modeling the cross-section deformation (that is, non-uniform radial expansion, ovalization, and warping). These arbitrary section deformation modes are commonly observed in curved pipes under loading.
In contrast, PIPE16 and PIPE18 adopt legacy technologies that limit their ability to model piping systems accurately. For instance, PIPE16 and PIPE18 have no way to model the section ovalization between them in a continuous manner; PIPE18 may fail to capture the stiffness and localized stress distribution on a pipe bend under general loading, due to its outdated flexibility factor technology method (published in 1959). The NUREG results come from a computer program that employs a flexibility factor technology method, and the results are not based on analytical calculations. For these reasons, the results obtained from current technology elements are not compared against NUREG results.
Piping Benchmarks using Current Technology Elements:
The results obtained using current technology elements are more accurate and match more closely to the results obtained from the equivalent 3D model. To confirm this, three demonstration problems are included, where the results obtained from the piping model meshed with current technology pipe elements are compared against the piping model meshed with 3D, 8-node SHELL281 elements.