Lifetime Prediction of PVC Push-fit Joints [thesis]

A. Marques Arsénio
Despite the role played by joints in the failures occurring in drinking water networks, scientific literature has focused on barrel failure. Therefore, it can be assumed that the significance of these appurtenances has been disregarded. The objective of this work is to create a procedure to predict the remaining lifetime of PVC push-fit joints. PVC was selected due to it being the most used material in the Netherlands and resistant to corrosion which facilitates condition assessment due to the
more » ... bsence of tuberculation. Finally, working with PVC, for example, in the laboratory, does not incur the inherent safety/health risks as with, for example, AC. Failure modes of push-fit joints In this thesis, failure is defined as leakage; a joint fails when it begins leaking. Seven failure modes for push-fit joints are presented with joint bending and axial pull-out being the most relevant for PVC push-fit joints. Failures due to physical-chemical degradation, although significant, are not considered in this thesis. This decision was made because non-destructively characterizing the physical/chemical condition of PVC in the field is difficult to achieve. Non-destructive evaluation of PVC push-fit joints The alignment of both pipes inside the joint is assumed to be a surrogate measurement for the joint's condition. When two pipes are connected with a double-socket PVC joint, the two pipes are separated by a gap. The alignment of a joint can be determined if the gap is measured at four different locations: 3 h (springline), 6 h (invert), 9 h (springline) and 12 h (crown). Two angles are calculated, one for the 12h-6h pair of gap values and a second for the 9h-3h pair. This can be achieved using a non-destructive evaluation (NDE) tool. v vi Summary Several NDE tools were surveyed whereby the most promising for the assessment of PVC push-fit joints were selected and tested in the laboratory including ultrasound, CCTV and Panoramo ® with CCTV consistently considered as the best, delivering both accurate and reproducible results. Real-time pipe monitoring This component of the work was projected as an alternative to inspections employing NDEs, for example, in the case of important assets where the risk of failure is minimal, i.e., consequences of failure are exceptionally high * . For this, a PVC 250 mm drinking water pipe supplying water to approximately 1,250 customers was continuously monitored from September 2011 until June 2013. The aggregated data encapsules strains registered on pipes and joints in operation; soil temperatures next to the pipe; and strains registered on non-loaded coupons of isolated PVC and also installed next to the pipe. The data exhibit an expected positive correlation between temperatures and strains. Daily water demand patterns were ascertained with the strain gauges affixed to the pipes and joints. Two confirmed episodes of water-hammer were also detected by the sensors. This demonstrates the accuracy of the strain gauges and their potential in detecting dynamic loads that can be detrimental to a pipe. This work also demonstrates that a pipe can be continuously monitored for expansion/contraction when other assessment methods are not available. Destructive laboratory tests with PVC Only minimal information is available regarding the limit conditions of joints in operation (e.g. the limit bending angle before leakage). Therefore, laboratory tests with PVC were planned. Pipes and joints of two diameters, 110 and 315 mm, were tested under two different conditions: water pressure (4 bar) and air sub-pressure (0.2 bar). Both joint bending and axial pull-out tests were performed. To monitor the behavior of the joint, its stiffness (bending moment vs. bending angle) was monitored in real-time employing a force sensor. First, it was concluded that joint stiffness increases with bending angle, insertion, diameter and inner-pipe pressure. An increase in stiffness is considered dangerous because the joint becomes less able to bend. However, joint stiffness does not increase linearly with the bending angle (for the same diameter and pressure). The joint becomes stiffer after a threshold angle of 3°for 110 mm and 5°for 315 mm joints. These results are, for 10 m barrel pipes, more conservative than the limit defined by the AWWA (34°for 110 mm; 12°for 315 mm) and nearer to the limit defined by the Dutch PVC manufacturers of 6°for both diameters. Second, the role played by insertion in the increase of joint stiffness demonstrates that the pipes should not be installed completely inserted inside the joints. * An example is a pipe installed in a dyke. Summary vii Finally, for a rubber ring in good condition, leakage/intrusion can only be expected at bending angles above 10°and with complete pull-out of the pipe from the joint. Such extreme angles have not yet been detected in the field (Chapter 3).
doi:10.4233/uuid:18a79a31-abd9-4f24-81f5-15935e3523d0 fatcat:rgy6lswn2bdnldlr2m3vawd6iy