Experimental Investigation of Leakage Flow Behavior in Pipes Subjected to Negative Pressures

An in-depth understanding of leak behavior in water distribution systems is essential for ensuring the long-term reliability and resilience of critical infrastructure. While previous research has extensively explored leak opening behavior under positive pressure conditions, this study aimed to investigate how leaks behave under negative pressures (e.g., caused by transients, fire flows, etc.). To achieve this, a series of experiments in a 110-mm diameter PVC pipe was conducted, examining various types of leaks under negative pressure conditions. Subsequently, modeling studies were conducted to assess the implications of the observed behavior on intrusion flows through longitudinal cracks considering common pipe materials and properties typical for real water distribution networks, i.e., PVC, PE, AC, and steel. The experimental tests showed consistent head-area slopes for leaks under both positive and negative pressures, with smaller discharge coefficients for intrusion compared to those for leakage flows. This finding suggests that the existing knowledge of leak behavior under positive pressures can be applied in negative pressure conditions. Although round holes and circumferential cracks showed negligible and minimal changes in leak area, respectively, the longitudinal cracks displayed large positive head-area slopes, meaning that their leak areas decreased under negative pressures and sometimes closed, limiting intrusion flows. The study further showed that a leakage exponent (N1) different from 0.5 obtained under positive pressure produces inaccurate results when applied to intrusion flows. The results of modelling studies showed that the intrusion flows in rigid pipe materials such as steel and AC have similar or slightly lower intrusion flows than in the fully rigid case. In contrast, more flexible materials, such as PE and PVC, showed a greater reduction in intrusion flows when subjected to negative pressures.