Quantification and location of leaks by internal inspection

Abstract

Quantification and location of leaks within distribution gas mains has posed a considerable problem for many years. Leak location is currently achieved by means of gas detectors from above ground, but this method proves to be inefficient when pinpointing leaks and is also incapable of detecting small leaks. Accurate leakage measurements have required pressure decay tests on isolated sections of main, requiring interruption of gas supply and consequently high costs. An alternative method is presented in this paper. Leakage is detected and measured by using a prototype pig that can travel inside the gas main. The pig travels incrementally and examines the pipe in sections to locate and measure small leaks of the order of 0.0028 SCMH (standard cubic metres per hour) up to 1.0 SCMH. The system is designed to operate under live gas conditions in conjunction with top entry access equipment, and can be adapted to leak test services from the main. It is also possible to examine dead mains by using a compressed air supply. A fully functional 200 mm (8 in) prototype system, together with a laboratory test rig including deliberate leaks, and with the facility for testing at up to 200 kPa (2 bar), has been built. Consideration of pigs ranging in diameter from 150 mm to 915 mm is being made. Software has been developed to analyse all data returning from the pig and to fully automate the system. The software also provides the user with on-line real-time leak reports, displaying the measured leaks as they are found.