Tightness of flange joints for large polyethylene pipes – Part 2 Full scale experimental investigations

• Tightness of flange joints for large polyethylene pipes – Part 2 Full scale experimental investigations Leakage that sometimes occurs in flange joints in large size plastic pipelines for water supply is a serious problem. Research was undertaken in order to improve the knowledge about the function of such flange joints. The objectives were partly to find out the degree of sensitivity of the design, i e if small deviations from recommended practise is critical, partly to be able to suggest improved design and mounting procedures. One part of the investigation was a numerical (FEM) study of several geometries, which was reported in [2]. Use of a time-dependent material model made it possible to follow the development of deformations and flange surface pressures for long times. Although several important principal findings were made, the tightening procedure and exact material behaviour could not be modelled. Therefore a series of full-scale experiments were made on 630 mm pipes, for a number of combinations of flange dimension and gasket type. This second part of the research is reported here. Since four of the twenty bolts were instrumented, the bolt forces could be monitored and be related to the torque and to the pressure in the pipe over time. This resulted in novel, important information about the functioning of plastic flange joints. In short, the experiments were performed in the following way. First, the bolts were tightened in the recommended criss-cross fashion to pre-determined torque levels. Then the pressure in the pipe was increased until leakage occurred. For combinations of flanges, gaskets and torques where the joint was tight for pressures above 13 bars, the 13 bar pressure level was maintained for up to one week. The relationship between bolt force and torque does not agree at all with the frequently used rule of thumb formula, for the galvanized bolts used here. The bolt force was typically less than half the value obtained by the formula. Hence it is critical to verify the friction for the used combination of bolt material, surface treatment, and lubrication. The bolt forces also appear to be unevenly distributed, which is partly due to successive creep during the tightening procedure. For the high stresses in the flange creep is significant already for so short times as a few minutes, and the effect is increased by the fact that the bolts and backing rings are much stiffer than the plastic flange. For both the wide and the narrow (ISO) flange type used it appears that rubber gaskets perform better, in the sense that the joint is tight for lower applied torques. The hyper- elastic material properties help to smooth unevenness in the flange surface and to compensate for creep in the plastic. Use of SDR 17 pipes at pressure levels of 13 bars means considerable creep expansion of the pipe, although it is possible to obtain a tight joint. The expansion gives a wringing effect at the flange, also observed in [2], which contributes to concentrate the flange pressure to the outer parts of the flange surface. In summary, flange joints are possible to mount so that they are tight, also for 630 mm pipes. Since the design is a sensitive one it is vital to follow recommendations for 4 mounting of different combinations of flanges and gaskets. In particular, knowledge about the relationship between torque and bolt force is important.

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