Transient-Based Leak Detection in Real Sewage Rising Main

Hydraulic transient-based methods were applied successfully for efficient diagnosis and condition assessment of water supply systems. However, the application of such methods to pressurized sewage lines of which there are more than 150km pipes in Hong Kong requires addressing challenges not encountered in water supply systems. For instance, sewage flows involve a large amount of entrained air, gases as well as solids with varying densities and sizes. The result is a highly variable wave speed which is an essential parameter for the application and accuracy of transient-based methods. This paper presents a case study of leak detection in real sewage rising main in Hong Kong using transient-based methods (TBDDMs). Field tests were conducted at Chung Hom Kok Beach rising main system where single leak of various sizes (ranging between 0.1 and 1 l/s) were simulated and localized. In each test, two transient generation mechanisms were used, namely a pump shutdown and a rapid closure of a side discharge valve. The resolution achieved by pump shutdown is quite poor in comparison to the valve closure. The model-free time reversal method is applied for the detection and localization of the leak. It is found that the knowledge of a baseline (defect-free) transient response signal is very critical to successfully detect the defects. Challenges confronted during field tests and insights for future practice are discussed.