A study of water pressure influence on failure of large diameter water pipelines

Abstract

This research is part of a large research project investigating how, when, and where critical pipes (diameter≥300mm) fail in water supply networks. The failure of large- diameter pipe is critical, as it may have devastating consequences for both the public and the water utility. Factors contributing to pipe failure, such as internal pressure, external load, and corrosion, are examined in this research. The objective of the study is to understand the contribution of internal water pressure and external loads (i.e., traffic load) to pipe failure, and to examine the failure mechanisms of large-diameter cast iron pipes with corrosion patches. To achieve the first objective, a pressure monitoring program and pressure-transient modelling were conducted in four selected network sections. A buried pipe section was instrumented under heavy traffic load to measure the performance of the pipe during the movement of traffic. In order to examine the possible pipe failure modes, a new pipe testing facility was designed and developed at Monash University. The new experimental set-up is capable of pressurizing until failure old cast iron pipes collected from the field. Natural and artificial corrosion patches were created in the test pipe specimens. Pipe sections were pressurized to investigate the failure mode under internal water pressure. The study conducted on the magnitude of internal and external loading indicated that the main contributing factor causing failure in the pipe barrels of large-diameter pipes is the internal water pressure (i.e., the stresses in monitored pipe sections were several times higher under internal water pressure than those due to external loading). It was found that the pressure changes during a pressure transient event can change the stresses in the pipe abnormally. Therefore, pressure transients can be considered as the main factor that triggers many failures in deteriorated water pipelines. Accurate information about the magnitude of transient pressure for pipe failure prediction was obtained using validated pressure-transient numerical models. Pipe burst experiments indicated that large-diameter cast iron pipes in good condition (without large corrosion patches) can sustain 5 to 7 times higher pressure than typical operational pressures (600 to 700kPa). Therefore, the evidence gathered from the field and experiments indicated that a significantly large corrosion patch with maximum corrosion at the base of the patch in the order of 80% or more is required to cause failure in the barrel of a large-diameter cast iron pipe. In addition, such failure in the first instance is most likely to be a crack that could lead to water leakage, provided the cement lining and graphitisation is weakened. Such leakage is likely to remain for a period of time until the crack has grown to a critical length, at which time spontaneous crack growth may occur as a catastrophic burst. This behaviour is termed “leak-before-burst” (LBB) for cast iron pipes. This finding means that utilities need to place more emphasis on leak detection to identify pipes that may be close to failure. In this context, this approach provides a means to prevent the failure of critical pipes. However, further research is needed on the LBB concept to examine further its general applicability to field conditions and on how to evaluate the window of time from leak to break.