Empirical study on drainage stack terminal water velocity

Abstract

A building drainage network is an essential provision of each developed city all over the world. The terminal velocity at discharging drainage stack is a crucial issue as it provides the permitted flow rate of a building drainage system. The National Plumbing Code (NPC) of the US has specified the permitted flow rate for drainage systems. Mathematical expressions for terminal velocity were reported in research studies since the 1920s but discrepancies among calculations remained. This study proposes an empirical approach in determining the terminal velocity in a drainage stack using the air pressure distribution mechanism. New experimental observations of the stack terminal velocity using a digital high-speed video camera were presented as a validation database. The proposed model was validated in terms of the measured terminal velocities and the locations of the maximum air pressure in a discharging stack at a steady water flow rate (1.0, 2.0, 3.0 and 4.0 L/s). This article offers an empirical approach and validation for terminal velocity instead of using the existing deductive approach. Practical applications: This article proposes an empirical approach with experimental validation for terminal velocity at a drainage stack. New evidence presented would lead to a review of the terminal velocity in drainage stacks in future drainage network designs. Due to the importance of terminal velocity, it is anticipated that this result would be used to lead the new technical development of building drainage system.