Tightness of flange joints for large polyethylene pipes – Part 1 Numerical simulations

Tightness of flange joints for large polyethylene pipes – Part 1 Numerical simulations Leaks occasionally occur in flange joints in plastic pipelines, predominantly large dimension ones. Such pipelines are normally of importance for e g water supply, and repair is expensive. A better understanding is vital since a clear background is missing for the existing design and mounting recommendations, which also are differing. Analysis of plastic flanges is more complicated than for metal ones since the material is time dependent, and much softer than the backing rings and bolts. The aim of this work was to be able to assess on one hand if presently standardized flange geometries mean smaller safety margins when the size of the pipe is increased, and on the other hand if the instructions for mounting have to be improved. First, an analysis was made by manual calculations, without consideration of the time dependent properties of the material, in order to assess the stresses just after tightening of the joint but before pressurizing and start of service time of the pipeline. The manual analysis is of course misleading for assessment of the compression stresses in the flange surfaces over time, although it seems that such calculations often are used for design. The value of the manual analysis was mainly that it showed that the nominal stresses are similar for different sizes, except for the 630 mm pipe where they are significantly higher. Further it was established that pressurizing of the pipe means a moderate influence on the flange stresses and bolt forces, 10-15%, and that the bolt and backing ring are much stiffer than the plastic flange, meaning that it is mainly relaxation that is responsible for unloading of the joint over time. Computer simulations (FEM) were then made of both the tightening and the service phase for a set of geometries, and with a material model including time dependent properties of the plastic parts of the joint using material data from in-house experiments. Although the computer simulations are approximate too, they give a much better impression both of the principal function of the joint and of the magnitude of the stresses over time. It appears that the geometry of the flange joint means that the contact is lost over large parts of the flange surfaces already at pressurizing and that a triangular distribution of pressure covering a part of the flange surface corresponding to the width of the backing ring is developed over time which