Machinery's Handbook, 31st Edition
Pipe Fittings 2771 Pipe Fittings.— Pipe fittings are used to connect pipes together either permanently or non-permanently. Pipe systems should be designed to minimize the number of fittings because they cause pressure loss, require pressure derating, and are a potential source of leaks. Materials: Pipe fittings are available in a variety of materials, and should be selected for compatibility with the pipe material and the substance being transported. Dissimilar metals should be avoided to prevent galvanic corrosion. Plastic fittings are also available for use with plastic pipe. Types: Pipe fittings may be threaded, slip, or interference type connections. Interference connections, like compression or stab fittings, are used with plastic pipe and with metal and plastic tubing. PEX pipe is typically connected using either a copper crimp ring or ex- pansion fittings with outer expansion rings. In the crimping method, a copper crimp ring is put over the pipe near the end, a copper fitting is inserted into the pipe end, and a crimping tool is used to crimp the ring over the pipe and fitting. In the expansion fitting method, an expansion tool is used to open the diameter of the pipe, into which an expansion fitting is inserted. The pipe shrinks over the fitting as it cools, and then a plastic expansion ring is pressed over the pipe and fitting. Compression fittings can be used as an alternative; that method is discussed in the section Tube Fittings on page 2776. The most common types of pipe fittings are illustrated in Table 16. These examples are by no means exhaustive of the types and shapes available. Table 16. Common Pipe Fittings
Union
Nipple
Bushing
Adapter
Coupling
90 ° Elbow
45 ° Elbow
Male Elbow
Tee
Wye
Cross
Street Elbow
Cap 180 ° Bend Pressure Considerations: Pipe fittings have pressure ratings that must be considered during the selection process. Low pressure pipe fittings are generally rated to 150 psi. Medium pressure fittings are commonly found rated to 300 psi. High pressure fittings are rated as high as 3,000 psi, and extreme pressure fittings can be found rated to 6,000 psi or even higher. Pressure rating depends on schedule, size, material, and connection type. Metal Tube.— Metal tube is generally smaller and thinner than metal pipe. The same considerations in terms of material properties, mechanical stresses, and environmental factors apply to both tube and pipe. Plug Flange
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