Flow establishment in helical corrugated pipe

Abstract

Corrugated steel pipe suppliers have recently switched from annular rivetted corrugated pipe to helical lock seam corrugated pipe. This was done as a manufacturing expedient, but there has been an unexpected benefit in terms of reduced hydraulic resistance. Studies on full scale pipes have shown that the friction factor for full flow is lowered substantially as the helix angle of the corrugations is reduced. In fact at a helix angle of 52.5 ° the pipe resistance was found to be almost as low as for smooth pipe. However, these tests were concerned exclusively with determining rates of head loss for fully established flow. Drainage culverts for highways, grid roads, and farm crossings are usually relatively short structures. In some cases the length to diameter ratio may be as low as 10, and in most cases would not exceed 50. This immediately introduces the question of the length of culvert required to develop fully established helical flow, because before the flow becomes established the rate of head loss will be greater. In effect the friction factor will be highest at the culvert inlet and decrease along the length of the culvert, becoming a minimum after the flow is fully established. In theory the accumulated loss could be calculated by using a variable friction factor over the establishment length. This would be cumbersome and impractical. It would be simpler to calculate the pipe friction on the assumption that it is constant, at the minimum value, for the whole length of the pipe and then add a "development head loss" term to account for the extra loss. Experiments to determine the development head loss were conducted at the Hydraulics Laboratory, Department of Civil Engineering, University of Saskatchewan. The model pipes were 107 mm inside diameter and 2.13 m long. Standard corrugations were simulated and helix angles of 61.0 and 71.4° were tested. It was found that about 12 diameters of pipe length were required to obtain fully established flow, and a value for the development head loss of 0.2 times the velocity head was indicated. Key words: helical corrugated pipe, head losses, n values, flow establishment.