Lars Jacobsson, Hans Andersson, Daniel Vennetti and Sven-Erik Sällberg
Papers # 2012 Barcelona
Leakages are reported in flange joints of large pipelines for water supply although mounted according to recommended practise. FEM analyses and full scale experiments have been performed in order to better understand the functioning of the joints. The objectives were to find out about the sensitivity to mounting and service conditions and, if possible, to suggest improvements in today’s practise. A FEM script was developed, where arbitrary joint geometries can be simulated with a time hardening visco-elastic behaviour. In-house relaxation tests on a PE 100 material were used to assess the parameters of the material model. Simulations were performed for 400 and 630 mm flange joints with various geometries, gaskets and loading conditions. Principally, bolts were given forces corresponding to recommended torques and then a pressure of 10 bars was applied, leaving the system to relax for up to 10,000 hours. The main results are that the pressure in the joint surfaces is concentrated to the outer parts, that it decreases significantly and that most of the decrease, relaxation, occurs for short times. It is also possible to study the influence of gaskets, flexible backing rings and re-tightening. Experiments were performed with 630 mm joints, which were tightened with different torque levels and then pressurized until leakage occurred. Since strain gauges were applied on some bolts the real bolt force resulting from the nominal torque could be registered with time. Configurations which remained tight up to 13 bars were tightness tested for up to seven days. Among the important findings were that the bolt forces are much lower than foreseen from the formula usually used to convert torque to force. Further, the forces are very unevenly distributed, partly since much of the relaxation occurs already during the tightening rounds. Soft rubber gaskets and re-tightening significantly improve the performance of the flange joints.