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October 25, 2018
An elbow is installed between two lengths of pipe (or tubing) to allow a change of direction, usually a 90° or 45° angle; 22.5° elbows are also available. The ends may be machined for butt welding, threaded (usually female), or socketed. When the ends differ in size, it is known as a reducing (or reducer) elbow.
A 90º elbow, also known as a "90 bend", "90 ell" or "quarter bend", attaches readily to plastic, copper, cast iron, steel, and lead, and is attached to rubber with stainless-steel clamps. Other available materials include silicone, rubber compounds, galvanized steel, and nylon. It is primarily used to connect hoses to valves, water pumps and deck drains. A 45º elbow, also known as a "45 bend" or "45 ell", is commonly used in water-supply facilities, food, chemical and electronic industrial pipeline networks, air-conditioning pipelines, agriculture and garden production, and solar-energy facility piping.
Elbows are also categorized by length. The radius of curvature of a long-radius (LR) elbow is 1.5 times the pipe diameter, but a short-radius (SR) elbow has a radius equal to the pipe diameter. Short elbows, widely available, are typically used in pressurized systems, and in physically tight locations.
Long elbows are used in low-pressure gravity-fed systems and other applications where low turbulence and minimum deposition of entrained solids are of concern. They are available in acrylonitrile butadiene styrene (ABS plastic), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), and copper, and are used in DWV systems, sewage, and central vacuum systems.
A coupling connects two pipes. If their sizes differ, the fitting is known as a reducing coupling, reducer, or an adapter. There are two types of couplings: "regular" and "slip". A regular coupling has a small ridge or stop internally, to prevent over-insertion of a pipe, and thus under-insertion of the other pipe segment (which would result in an unreliable connection). A slip coupling (sometimes also called a repair coupling) is deliberately made without this internal stop, to allow it to be slipped into place in tight locations, such as the repair of a pipe that has a small leak due to corrosion or freeze bursting, or which had to be cut temporarily for some reason.. Since the alignment stop is missing, it is up to the installer to carefully measure the final location of the slip coupling to ensure that it is located correctly.
A union, similar to a coupling, allows the convenient future disconnection of pipes for maintenance or fixture replacement. In contrast to a coupling requiring solvent welding, soldering, or rotation (for threaded couplings), a union allows easy connection and disconnection, multiple times if needed. It consists of three parts: a nut, a female end and a male end. When the female and male ends are joined, the nut seals the joint by pressing the two ends tightly together. Unions are a type of very compact flange connector.
Dielectric unions, with dielectric insulation, separate dissimilar metals (such as copper and galvanized steel) to prevent galvanic corrosion. When two dissimilar metals are in contact with an electrically conductive solution (ordinary tap water is conductive), they form an electrochemical couple which generates a voltage by electrolysis. When the metals are in direct contact with each other, the electric currentfrom one to the other also moves metallic ions from one to the other; this dissolves one metal, depositing it on the other. A dielectric union breaks the electrical path with a plastic liner between its halves, limiting galvanic corrosion.
Rotary unions allow mechanical rotation of one of the joined parts, while resisting leakage.
A nipple is a short stub of pipe, usually male-threaded steel, brass, chlorinated polyvinyl chloride (CPVC), or copper (occasionally unthreaded copper), which connects two other fittings. A nipple with continuous uninterrupted threading is known as a close nipple. Nipples are commonly used with plumbing and hoses.
A reducer allows for a change in pipe size to meet hydraulic flow requirements of the system or adapt to existing piping of a different size. Although reducers are usually concentric, eccentric reducers are used as needed to maintain the top- or bottom-of-pipe level.
A double-tapped bushing is a fitting which serves as a reducer. It is a sleeve similar to a close nipple, but is threaded on both its inner and outer circumferences. Like a reducer, a double-tapped bushing has two threads of different sizes.
A double-tapped bushing is more compact than a reducer, but not as flexible. While a double-tapped bushing has a smaller female thread concentric to a larger male thread (and thus couples a smaller male end to a larger female), a reducer may have large and small ends of either gender. If both ends are the same gender, it is a gender-changing reducer.
There are similar fittings for both sweat and solvent joinery. Since they are not "tapped" (threaded), they are simply called reducing bushings.
A tee, the most common pipe fitting, is used to combine (or divide) fluid flow. It is available with female thread sockets, solvent-weld sockets or opposed solvent-weld sockets and a female-threaded side outlet. Tees can connect pipes of different diameters or change the direction of a pipe run, or both. Available in a variety of materials, sizes and finishes, they may also be used to transport two-fluid mixtures. Tees may be equal or unequal in size of their three connections, with equal tees the most common.
This specialized type of tee fitting is used primarily in pressurized hydronic heating systems, to divert a portion of the flow from the main line into a side branch connected to a radiator or heat exchanger. The diverter tee is designed to allow flow to continue in the main line, even when the side branch is shut down and not calling for heat. Diverter tees have directional markings which must be heeded; a tee installed backwards will function very poorly.
Crosses, also known as four-way fittings or cross branch lines, have one inlet and three outlets (or vice versa), and often have solvent-welded socket or female-threaded ends. Cross fittings may stress pipes as temperatures change, because they are at the center of four connection points. A tee is steadier than a cross; it behaves like a three-legged stool, and a cross behaves like a four-legged stool. Geometrically, any three non-collinear points can self-consistently define a plane; three legs are inherently stable, whereas four points overdetermine a plane, and can be inconsistent, resulting in physical stress on a fitting.
Crosses are common in fire sprinkler systems (where stress caused by thermal expansion is not generally an issue),but are not common in plumbing. One cross fitting is more expensive than two tees.
Caps, usually liquid- or gas-tight, cover the otherwise open end of a pipe. A cap attaches to the exterior of a pipe, and may have a solvent-weld socket end or a female-threaded interior. The exterior of an industrial cap may be round, square, rectangular, U- or I-shaped, or may have a handgrip. If a solvent-weld cap is used to provide for a future connection point, several inches of pipe must be left before the cap; when the cap is cut off for the future connection, enough pipe must remain to allow a new fitting to be glued onto it.
A plug fits inside the pipe segment or fitting.
A barb (or hose barb), which connects flexible hose or tubing to pipes, typically has a male-threaded end which mates with female threads. The other end of the fitting has a single- or multi-barbed tube—a long tapered cone with ridges, which is inserted into a flexible hose. An adjustable worm drive screw clamp (or other type of clamp) is often added, to keep the hose from slipping off the barbed tube. Barb fittings can be made of brass for hot-water applications, and plastic may be used for cold water; brass is considered more durable for heavy-duty use. The barb fitting may be elbow-shaped or straight.
Valves stop (or regulate) the flow of liquids or gases. They are categorized by application, such as isolation, throttling, and non-return.
Isolation valves are used to temporarily disconnect part of a piping system, to allow maintenance or repair, for example. Isolation valves are typically left in either a fully open or fully closed position. A given isolation valve may be in place for many years without being operated, but must be designed to be readily operable whenever needed, including emergency use.
Throttling valves are used to control the amount or pressure of a fluid allowed to pass through, and are designed to withstand the stress and wear caused by this type of operation. Because they may wear out in this usage, they are often installed alongside isolation valves which can temporarily disconnect a failing throttling valve from the rest of the system, so it can be refurbished or replaced.
Non-return or check valves allow free flow of a fluid in one direction, but prevent its flow in a reverse direction. They are often seen in drainage or sewage systems, but may also be used in pressurized systems.
Valves are available in a number of types, based on design and purpose: