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10 The term dimension ratio is commonly used to describe and categorize various aspects of thermoplastic pipe. It is abbreviated by the acronym DR and is defined by the formula in which... DR = d/t DR = dimension ratio d = outside diameter of the pipe t = minimum wall thickness The ASTM F-17 hydrostatic pressure rating of pipes is based on the use of a service design factor of 0.5 of the results of a sustained long-term (1,000 hour) pressure test (ATSM D-2827) that must result in at least 1,600 psi. In addition a short-term pressure test must result in a 2,900 psi fiber stress before rupture. The design stress for HDPE is 800 psi or 56.24 kg/cm2 at 23O C. In the ISO pressure ratings the allowable hydrostatic design stress (S) for HDPE resin is 50 kg/cm2. All pipes of the same class of material and the same DR will have the same pressure rating regardless of pipe diameter. Table 4 summarizes the DR-pressure relationship for the more common ratings used in ocean outfalls. Dimension ratio Pressure rating in psi and (kg/cm2) 41 32.5 40 50 (2.8) (3.6) Table 4 26 21 64 80 (4.5) (5.6) (7.0) 17 15.5 13.5 11 100 110 130 160 It is very important to select the appropriate DR for HDPE pipe to be used in an ocean outfall in order to obtain a trouble free long useful life. Selection of the proper DR is dependent upon a number of factors. • External hydrodynamic forces in the ocean • Stresses imposed by construction and handling • Internal hydraulic pressure • Spacing of the ballast weights or anchors Generally, the section of the outfall that is situated in the surge-surf zone is subjected to the greatest external punishment. It is also the area most subject to undermining. This zone is usually shallower than 15 meters when facing the open ocean but in locations subject to hurricanes it can be greater. In some cases, the forces exerted at shallower depths are sufficiently great that the outfall must be buried and in others the outfall can be sufficiently anchored by use of ballast weights or mechanical anchors. The shallow entry area is usually the section the submarine outfall that sustains the greatest internal pressure. In addition, this area is frequently subject to impact stress from placement of rock ballast. For all of these reasons it is common practice to use a thicker pipe wall (a smaller DR) in the entry zone in order to offset the additional impact, flexing, shear, and hoop stresses. A DR of 11 to 21 is the range commonly used in the surge-surf zone and a range of 19 to 26 is commonly selected for the section outside this zone. The section of the outfall that is exposed to the greatest external hydrodynamic stress should be determined by wave dynamics and current analysis. The hydrodynamic forces, especially the horizontal and vertical forces that are exerted by waves and currents, should be taken into consideration when selecting the dimension ratio and determining the method of stabilizing the outfall.PDF Image | SMALL DIAMETER (HDPE) SUBMARINE OUTFALLS
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