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PROCEEDINGS OF THE NINTH INTERNATIONAL CONFERENCE ON CIVIL AND STRUCTURAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Modelling of Ferrule Strap Connections to uPVC Pipes
F. Pozzessere+, N.A. Alexander* and R. Potter+
+School of Computing and Technology, Pipeline Technology Group, University of East London, United Kingdom
F. Pozzessere, N.A. Alexander, R. Potter, "Modelling of Ferrule Strap Connections to uPVC Pipes", in B.H.V. Topping, (Editor), "Proceedings of the Ninth International Conference on Civil and Structural Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 27, 2003. doi:10.4203/ccp.77.27
Keywords: ferrule strap, FEA, contact problem, cracking, uPVC, friction, fatigue, S-N curve.
A ferrule is a device that connects a customer to the mains water or gas supply. Typically the mains distributions pipe will be of 63 to 180 mm diameter located in roads or pavements. Ferrules are used to branch off to customer's premises with typically a 25 or 32 service pipe. On iron pipes, ferrules were commonly tapped directly into the pipe but since the introduction of other pipe materials the ferrule has been incorporated into a strap clamped onto the distribution pipe. In the water industry traditional designs of bolted ferrule strap were introduced about 70 years ago for the no longer used asbestos cement pipes and have not changed significantly in design despite the introduction of more flexible plastics pipe materials. It is over 40 years since the water industry introduced PVC distributions pipes. These pipes were designed to survive at least 50 years, but over the last 20 years there has been an increasing number of failures associated with leaks from ferrule straps. South West Water Ltd, a British water utility, has sponsored a research program at the University of East London to investigate this problem. The ferrule strap constrains the main pipe by a number of bolts that are normally tightened by hand to an undefined torque. The failed pipes reveal that the crack initiated from the hole and propagated along the longitudinal axis of the pipe . The major sources of problems have been found in the cutter design, the unequal tightening of the bolted clamps which produces a stress in the pipe wall  as well as the lack of installation standards .
The modelling of ferrule strap connections was achieved by using a combined approach of FEA and experimental measurement performed at the University of East London. The LUSAS 13.3 package was used for the FEA. The pipe was modelled with the ferrule strap contact problem. But due to the difficulties in specifying the exact initial geometries it was also modelled without considering the direct compliance of the ferrule strap. The system parameters that are investigated are (1) the influence of the applied torque on the tightening bolts, (2) the non-uniformity of contact pressures between the ferrule strap and the uPVC pipe. This includes the effect of the O-ring contact. (3), the role of internal mains pressures (4) the effect of contact friction. The density of the element meshing around the hole was increased to produce higher accuracy in the zones of greatest interest. The numerical modelling of a pipe constrained by a tapping band was carried out by Burn . In , the tapping band constraint forces were experimental determined by strain gauges at three points only. In contrast in the work presented here the contact pressure between the ferrule strap, O-ring and uPVC pipe was determined experimentally by placing a Pressurex sensitive film between the two surfaces. This produced a much more complete picture of the contact pressures. These thin films reveal the magnitude and distribution of the pressures between the contact surfaces. The colour intensity is directly related to the pressure applied. From the analysis of the film, it was observed that the contact pressure distribution is not at all uniformly distributed over the contact surface. This is due to the fact that the pipe, because of the manufacturing process, is not perfectly circular and nor is the ferrule strap. The cyclic internal pressure loading primarily induces an elastic stress state in the pipe hence a low amplitude stress based fatigue approach has been adopted. It was observed that the process of cutting of the hole in the pipe does introduce microscopic defects across the thickness of the pipe shell. These defects where uniformly distributed around the circumference of the hole. Due to the screwing action these defects are primarily orientated almost parallel to the pipe surface. Now it is almost certain that these defects play a part in the fatigue failure observed. However, experimental evidences show that there is a consistent location of the crack nucleation and growth, thus other factors significantly affect this behaviour. By a fusion of theoretical and experimental evidences, Wölher S-N fatigue curves were postulated. A modification and improvement to ferrule strap design are also suggested in this paper. Basically a layer of rubber is placed between the ferrule strap and the pipe. Thus attempting to increase the contact area and redistribute contact pressures in a more uniform way. A comparison in the performance of the standard ferrule strap with the modified one is made to comment on the efficacy of the proposed modification in design. The postulated S-N curves are used to assess the performance in fatigue of these modified ferrule straps.
Theoretical predictions show that optimised choice of a rubber interface layer and torque can increase the endurance of a ferrule strap by two orders of magnitude. This would imply a significant long term cost benefit for water companies.
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