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I I I I I I I I I I I I I I I I I I I e. Select the desired factor of safety (Fl. n. Establish the hydrostatic design stress (HDS or Sl by dividing the HOB by F. E. Select the desired dimension ratio (SOR or DRl. F. Select the surge allowance (PSI if desired. G. Calculate pressure rating or pressure class. PVC is considered a flexible pipe and relies on the protection of a zoned backfill material for installation to a height six inches above the outside diameter. The principal disadvantage of PVC pipe, when considered for use in the Towaoc water supply 1 i ne, is its i nabi 1 i ty to handl e the hi gh pressures that wil 1 exist over much of the line. PVC pipe, in 1Ii and 14-inch sizes, ..ated for service up to 23'l pounds per square inch (psi I, is available. In the l?-inch size, PVC pipe, a pipe rated for service up to 200 osi is available (with a safety factor of either 2.0 or 4.0\. Consequently, if PVC pipe is used in the Towaoc water line, it must be limited to lower pressure portions of the line. Perhaps PVC cou1 d be used in areas havi ng corrosi ve soi 1 s where some other pipe materials would require extensive corrosion protection. 3.5.3 Fiberglass Pipe Fiberglass pipe, like PVC, is inert and not subject to corrosion. IJnlike PVC, fiberglass pipe can be obtained in pressure classes suitable for the Towaoc water line. In sizes up to 12 inches, a fiberglass pipe lined with PVC called "permastran", is manufactured by the J-M Pipe Company. Permastran is available in pressure classes of 250 and 350 psi. Permastran has the flow characteri sti cs of PVC, but has 1 arge i nsi de di ameters in compari son to the nOMinal diameters, similar to ductile iron. Fiberglass pipe in sizes greater than 12 inches is available, but each size is of special manufacture and is prohi bi ti ve 1y expensi ve in the pressure classes requi red by the Towaoc water line. 3-20 1060c