Abstract
Every day, water networks across the developed world are relied on by billions of people to provide them with a fresh supply of water. Many of these networks are comprised of pipes made from grey cast iron and may have been in service for up to 150 years. Despite their age, some parts of these networks continue to operate with little degradation, whereas in other areas they degrade rapidly: more recently laid pipes are being outlived by their forerunners. In such networks, it is the trunk mains (pipes between 12-60” [300 mm to 1500 mm] in diameter) that are of great concern, since they pose the greatest risk of failure and are already bursting more frequently. Accurate NDE is required to enable the mains in poor health with the highest risk of failure to be identified and replaced before they burst. A review of the published literature has shown that whilst there are many NDE techniques to choose from, many are not practical for application to the mains. The review process also highlighted the kinds of defects present in grey cast iron and an initial stress analysis using strength models and material data published in the literature has suggested defect sizes approaching 5 mm must be able to be detected to prevent catastrophic pipe failure. Ultrasonic inspection has been investigated and shown to work effectively on uncorroded cast iron. Speed of sound values between 4100 – 4600 m s-1 have been observed across several pipes. A speed of sound of 2950 ± 80 m s-1 has been measured for graphitic corrosion, however, inspection on corroded main has not been possible. A complementary magnetic technique, with the potential to scan pipe rapidly in order to identify mains in need of further investigation, as well as providing supplementary condition data, has been trialled and shown to detect corrosion layers up to 6 mm thick. A methodology using a 3D scanner to accurately determine the “ground truth” pipe condition has been developed. This methodology proved to be successful and provided corrosion measurements that were in-keeping with those obtained through standard pit depth measurements. Further, the data showed that traditional pit depth measurements do not always find the deepest external corrosion pits, particularly where the surrounding geometry is complicated. This methodology was used in a live comparison exercise of two, commercially available techniques. This comparison highlighted problems with the surface preparation required by some techniques, which can be quite damaging, and with some proprietary post-processing algorithms – the raw data can be more useful. From this assessment process, it has been possible to specify very detailed schedule for the testing of new NDE techniques in the future.