High Density Polyethylene (HDPE) is a common plastic used in a number of roles, including pipes used for drainage and low-pressure plumbing. HDPE is less common than polyvinyl chloride (PVC) piping in general residential use, but is used in places where crack resistance is a desired feature. Like PVC pipe, the interior and exterior walls of HDPE pipe can be quite smooth. HDPE pipe is also less likely to react chemically with the contents of the soil or the material going through it than PVC is. It's also used as the primary conduit for protecting cables in underground cable runs.
HDPE Pipe Fittings as Linings in Existing Pipes
One use where HDPE pipe is employed is as in situ replacement of existing steel or concrete conduit---rather than pull up all the steel pipe, the HDPE pipe is threaded down the steel pipe in lengths. In this instance the HDPE pipe is selected to have a larger interior diameter than the rated exterior diameter of the pipe that's being bolstered; the HDPE is slid down the outside of the existing pipe. . Because the HDPE pipe can be bent on site, and different lengths of HDPE pipe can be fused at the joints, some significant time savings can be had.
HDPE Pipe Wall Thickness to Interior Diameter Ratios
Manufactures of thermoplastic pipe fittings use a constant dimension ratio---depending on the pipe's intended use---whereby the thickness of the pipe wall remains at the same proportion to the interior diameter size. For example, if thermoplastic pipe fittings were designed with an interior diameter of 1 inch and an exterior diameter of 1.5 inch, the wall thickness would be 0.25 inch, and the dimension ratio would be 1:0.25---meaning the wall thickness would always be 1/4 of the interior diameter of the pipe; a 3-inch pipe made to the same thickness ratio would have 0.75-inch thick walls and an outer diameter of 4.5 inches.
Mechanical And Thermal Properties
While there are some variations in the resin used to make HDPE pipe fittings, most of the variations are fairly close to this: HDPE pipe typically has a density of 0.945 grams per cubic centimetre (water has a density of 1, steel has a density of about 7.8), and a melting temperature of 126 degrees C (about 52.8 degrees C C).
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