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5 Computer programs can be used to develop a family of head loss-discharge curves for different diameters of pipe to facilitate the selection of an optimal diameter. In their absence the nomographs in Figures 1 (Metric Units) and 2 (English Units) based on the Hazen Williams formula can be used to determine the pressure losses and velocities for various flow rates in different internal pipe diameters. HDPE plastic pipe is described by a specified exterior diameter and by a minimum wall thickness needed to obtain the pressure rating of the pipe. Tables 2 and 3 present the external and internal diameters, the unit weight of the pipe for various pressure ratings and the respective dimension ratio. 6. CONSIDERATION OF TIDES AND DENSITY OF SEA WATER For small communities it is almost always important to keep operating costs as low as possible. This usually means that the designer of the outfall will try to use static gravity head and avoid pumping the sewage to be discharged. For this reason a few words of precaution are in order. It must be remembered that the times of tidal fluctuations change daily and that the magnitude of 0tidal changes varies throughout the year as well as the lunar month. For this reason, it may well be assumed that the highest tide and peak sewage flow can probably occur simultaneously. The submarine outfall and appurtenances such as flow equalizers of pumps should be designed accordingly, so as to avoid undesirable surcharge of gravity sewers that have service connections. The fact that seawater has a density that is approximately 2.5 percent greater that the density of sewage must also be taken into account. This static head must be overcome by the gravity head available or by the pumping facilities. This head can be significant, especially in deep outfalls. For example, an outfall 60 meters deep would have a "density difference" static head of one and one half meters. 7. SELECTION OF THE PIPE PRESSURE RATING AND DIMENSION RATIO (DR) The pressure rating of the pipe is given by the formula in which... P=2S/[(d/t)-1)] P = pressure rating of the pipe S = hydrostatic design stress d = average outside diameter of the pipe t = minimum wall thicknessPDF Image | SMALL DIAMETER (HDPE) SUBMARINE OUTFALLS
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