Abstract
The aim of this investigation was to study the hydraulic behaviour of small circular openings in plastic water pipes. According to the theoretical orifice equation, the flow through an orifice is proportional to the square root of the pressure. However, a number of field studies have shown that this exponent can be considerably larger than 0.5, and typically varies between 0.5 and 2.79 with a median of 1.15 (Farley and Trow 2003). The implication is that water losses are substantially more sensitive to pressure than originally thought.
Van Zyl and Clayton (2005) proposed four categories of factors that may be responsible for the observed behaviour: leak hydraulics, pipe material behaviour, soil hydraulics and water demand. The aim of this study was to investigate the hydraulics of small circular holes in water distribution systems to determine their behaviour under various conditions. Variables studied in this investigation include pipe material, leak size, surrounding media and pressure fluctuations.
Boundary conditions need to be established to accurately simulate the conditions that a pipe in a water network experiences. The author designed and built the apparatus to provide these constant boundary conditions. The apparatus consists of six major components. These are the frame, casing, sample, pressure vessel, constant pressure regulators and measuring equipment. The frame provides structural stability to the apparatus during testing. The casing’s purpose is to house the material that surrounds the sample while being tested; it also creates the constant boundary conditions necessary for the experiments. 1mm and 2mm holes are drilled into the uPVC and HDPE pipes. This serves as the test samples. The pressure vessel is used to enable constant pressure with better pressure control. Constant pressure regulators help to provide the required conditions within the casing. The measuring equipment records that data from the experiments. The data is then processed into interpretable information.
It was found that leaks discharging into air show good correlation with the theory, but differ significantly from the behaviour of leaks discharging into water or glass beads. Other findings of the study include that pressure fluctuations do not have a significant effect on the leak behaviour, the classification of the opening (as an orifice, tube or pipe) is important and that a discontinuity in the pressure leakage relationship occurs in holes classified as tubes. It is suspected that the discontinuity is caused by separation of the fluid stream from the tube wall, effectively changing it into an orifice.
Prof. J.E. Van Zyl
Prof. C.R.I. Clayton