Sizes for Self-Cleaning Pipes in Municipal Water Supply Systems

Historically, the minimum diameter of a pipe in a municipal water distribution system has been governed by fire flow requirements. In most cases across North America, the minimum allowable pipe size is six inches (150 mm), even on branching stubs that feed 100 or fewer homes. These peripheral regions of the pipe network often experience low velocities, long residence times, poor water quality and accumulation of settled deposits. As a consequence, many water utilities routinely flush these lines in an effort to cleanse them. In recent years, several water utilities (most notably in the Netherlands) have introduced the concept of a "self-cleaning" network where, for several minutes each day, a critical pipe velocity occurs and resuspends particles that settled during the low flow periods. Self-cleaning networks are a radical departure from the typical looped distribution system. Self-cleaning networks have a branch-type arrangement with downstream declining diameters and unidirectional velocities that reach at least 0.4 m/s for several minutes each day. Peak flows in the self-cleaning network are estimated using an empirical "q square root N" method, where q is the tapping unit demand usually taken as 0.083 litre per second per unit and N is the number of tapping units in the neighborhood. The problem of estimating the peak flow and, hence, the required pipe diameter can also be investigated using basic principles from the Poisson Rectangular Pulse (PRP) model for residential water demands. The objective of this paper is to apply theoretical results from the PRP model to generate reliability based estimates of the peak flow and the corresponding pipe size needed to assure a critical self-cleaning velocity as a function of neighborhood size. Results from the PRP model are compared against the conventional but conservative"q square root N" method.