Determination of the head loss coefficient of closely spaced pipe bends

Abstract

Space limitation in ships and the complex pipe layouts in chemical, mineral and food processing plants lead to the employment of closely spaced bends. The limited information regarding the head loss coefficient of pipe bends orientated as bend-spacer-bend has led pipeline designers to treat them as isolated bends with the same loss coefficient. Thus, to calculate the head loss in the piping system would simply involve summing the head loss coefficient of bends and neglecting their configuration. This practice causes inaccurate computation of head losses in the system. In this study a computational model is developed for the head loss coefficient of closely spaced pipe bends. This is then supported by experimental verification. A more accurate but still simple and easy to use empirical correlation is derived. The empirical correlation is established and the data presented under isothermal conditions for turbulent flows in a range 7.3x104 ≤ Re ≤ 5.8x105 and a spacing ratio of 1D ≤ L/d ≤ 10Dand curvature ratio of 3 ≤ rc/d ≤ 5. Using ANSYS® CFX® 11, a commercial computational fluid dynamics (CFD) package, the fluid domain representing two 900 smooth pipe bends separated by a short pipe was solved and the mechanisms causing the head loss coefficient were explored by using the CFD results to visualise the fluid flow structure/pattern. The computational model was validated by comparing the head loss coefficient of a single bend and the model was found to be sound. The experiments conducted in the built test facility using smooth pipes showed similarities in the trends between the CFD work and the published data and they were to be found have a similar trend. The experiment had shown results that agree to the findings from literature.