Failure analysis of a PVC sewer pipeline by fractography and materials characterization

Abstract

Sewer rising mains are an integral part of wastewater network and convey sewage under pressure between gravity fed systems or from gravity systems to treatment plants. The failure of a rising main can result in significant volumes of sewage being discharged to sensitive environments, significant clean-up costs and severe indirect consequences being incurred.This paper describes the analysis of an Unplasticised Polyvinyl Chloride (PVC-U) pressure sewer (sewer rising main), nominal diameter 200mm, pressure class 12, that failed prematurely after 34years of service considering the design life of >100years. It was hypothesised that the root cause of pipe failure was either material degradation from exposure to an aggressive environment (the conveyed sewage) or an inherent defect in the pipe. To test this hypothesis, a thorough visual examination of the exhumed failed section and the fracture surface was undertaken, followed by Fourier Transform Infrared Spectroscopy, micro-scale examination of the fracture surface using Scanning Electron Microscopy and Energy Dispersive X-ray Spectrometry. Fourier Transform Infrared Spectroscopy showed no evidence of material degradation due to contact with sewage. Visual and micro-scale examination revealed that failure occurred in two major stages initiated at an inherent defect (foreign inclusion) in the pipe wall that was attributed to the original manufacture. In addition to the initiating defect in the pipe wall, numerous other potential stress risers were also identified. The application of an established crack growth model predicted pipe failure close to the observed lifetime. Application of the staged methodology presented is recommended to establish relationships between manufacturing eras of PVC pipes and severity of inherent defects.