Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/721218
26 WET NEWS SEPTEMBER 2016 If the flow through a drainage pipeline is to perform as intended, accurate hydraulic analysis is critical in determining the choice of pipe material and configuration, says the CPSA's Stuart Crisp. Performance management INSIGHT Pipes & drainage • T he correct estimation of hydraulic performance in drainage system design is critical if a scheme is to perform as intended. Accurate hydraulic analysis is a critical factor in determining the size, configuration and material to ensure the intended pipeline flow. Precise analysis will help ensure a pipe is sized correctly in terms of performance and in optimising its cost. If the analysis is wrong, or it fails to address appropriate hydraulic factors, then the end result could be a pipeline that fails to perform as intended, leading to the possibility of damage to both property and the reputation of the designer or installer. Key hydraulic criteria that designers and specifiers should consider when selecting materials and deciding on a pipeline's configuration include: Internal diameter The scale of variations in a manufactured pipe's internal diameter might come as a surprise to some. It is a worthwhile exercise to check that the internal diameter of the pipe delivered is the same as that used in the design calculations. A variation in a pipe's diameter of as little as 3% can have a significant impact on its capacity. This is best illustrated using the example of a 300mm nominal diameter storm water sewer. Using Hydraulic Design Research Tables (based on the Colebrook-White equation), and assuming a hydraulic roughness (Ks) of 0.6mm, a 300mm diameter pipeline laid to a gradient of 1:60 should have a capacity of 140l/s. If, however, the actual pipe diameter is 3% less at 291mm, then its carrying capacity will be reduced by 4l/s to 136l/s. This capacity shortfall will be more significant for larger diameter pipes, particularly where a reduction in diameter is accompanied by other capacity-decreasing factors. Armed with just a CCTV camera, the Picote Coating Pump, the Picote Mini Miller, the necessary accessories (brushes hoses etc.) and the two-part, the contractor is ready to complete the lining operation. First the pipeline is cleaned and if necessary degreased and abraded with Picote's Smart Cutter, or other suitable cleaning tools, to provide a good rough pipe surface to which the resin can adhere. Then the application hoses and the Picote Coating pump are prepared. It is recommended that the resin delivery hose is no longer than 15m from the pipe access to the farthest point of application. Pipe shape Hydraulic design assumes a pipe with a circular cross section remains circular over its service lifetime. As a consequence, when the ovalisation, or flattening, of a flexible pipe occurs a pipeline's hydraulic efficiency can be reduced. For surface water sewers the majority of rainfall events lead to flow rates in the pipeline that are significantly less than the maximum design value. Under these circumstances the depth of flow in the pipes can be less than 50% and the average velocity at a specific flow rate in a flattened, ovalised pipe will be less than would have been the case had the pipe remained circular. This can reduce the opportunity for a pipeline to achieve self-cleansing flow and increase the possibility of sedimentation and detritus accumulation. To see the how ovalisation can impact the capacity of a pipeline try using hydraulic design soœware with a 6-20% ovalisation factor, the lower figure allowed by many client authorities and the upper figure recorded in problematic cases. And, for an even more realistic assessment of the impact, try adding the ovalisation figure to the figure for flow reduction produced by the (negative) variance in nominal diameter mentioned earlier. In addition to variations in a pipe's cross-sectional area, additional hydraulic losses can occur as the result of variations