Why and how are HDPE fabricated fittings de-rated? Are molded fittings de-rated?
Fabricated Fittings are de-rated due to their geometry, resin properties and wall thickness. Moulded fittings are designed with a thicker body wall (which can handle water hammer and surge pressures). Fabricated fittings do not have a thicker body, so these fittings must be made from thicker pipe in order to handle higher pressures. Industry standard derating factors for Fabricated Fittings (depending on the geometry of the fitting) have been used for some time now. Elbows were typically derated 25%, Tees at 50% and Wyes were only derated 40%. Recent modeling has led the American Society of Engineers (ASME) to assign increased de-rating factors according to fitting geometry, resin properties (Hydrostatic Design Stress – HDS) and even wall size. For example, Lateral Wyes were typically derated at 40%, but recent modeling confirms that Wyes are much weaker and must be de-rated at 67%. Moulded fittings are not de-rated. Consideration must also be applied to the ambient temperature and fluid media temperature when de-rating the PE pipeline system. As the temperature to which the PE pipe and fitting is exposed increases above the reference temperature (20°C) the long-term hydrostatic strength decreases. Consequently, the pipe and fitting pressure rating must be also be de-rated to suit the operating temperature as required.