A fitting or adapter is used in pipe systems to connect sections of pipe (designated by nominal size, with greater tolerances of variance) or tube (designated by actual size, with lower tolerance for variance), adapt to different sizes or shapes, and for other purposes such as regulating (or measuring) fluid flow. These fittings can manipulate the conveyance of fluids such as water for potatory, irrigational, sanitary, and refrigerative purposes, or other liquids like gas, petroleum, and liquid waste.

Fittings allow multiple pipes to be connected to cover longer distances, increase or decrease the size of the pipe or tube, or extend a network by branching, and make possible more complex systems than could be achieved with only individual pipes. Valves are specialized fittings that permit regulating the flow of fluid within a plumbing system.

Standards

Standard codes are followed when designing (or manufacturing) a piping system. Organizations which promulgate piping standards include:

ASME: American Society of Mechanical Engineers A112.19.1 Enameled cast-iron and steel plumbing fixtures standards A112.19.2 Ceramic plumbing fixtures standard ASTM International: American Society for Testing and Materials API: American Petroleum Institute AWS: American Welding Society AWWA: American Water Works Association MSS: Manufacturers Standardization Society ANSI: American National Standards Institute NFPA: National Fire Protection Association EJMA: Expansion Joint Manufacturers Association CGA: Compressed Gas Association PCA: Plumbing Code of Australia

Pipes must conform to the dimensional requirements of:

ASME B36.10M: Welded and seamless wrought-steel pipe ASME B36.19M: Stainless-steel pipe ASME B31.3 2008: Process piping ASME B31.4 XXXX: Power piping

Materials

The material with which a pipe is manufactured is often the basis for choosing a pipe. Materials used for manufacturing pipes include:

Carbon (CS) and galvanized steel Impact-tested carbon steel (ITCS) Low-temperature carbon steel (LTCS) Stainless steel (SS) Malleable iron Chrome-molybdenum (alloy) steel (generally used for high-temperature service) Non-ferrous metals (includes copper, inconel, incoloy, and cupronickel) Non-metallic (includes acrylonitrile butadiene styrene (ABS), fibre-reinforced plastic (FRP), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), high-density polyethylene (HDPE), Cross-linked polyethylene (PEX), and toughened glass; polybutylene has also been used, but is now banned in North America because of poor reliability)

The bodies of fittings for pipe and tubing are often the same base material as the pipe or tubing connected: copper, steel, PVC, CPVC, or ABS. Any material permitted by the plumbing, health, or building code (as applicable) may be used, but it must be compatible with the other materials in the system, the fluids being transported, and the temperature and pressure inside (and outside) the system. Brass or bronze fittings are common in copper piping and plumbing systems. Fire resistance, earthquake resistance, mechanical ruggedness, theft resistance, and other factors also influence the choice of pipe and fitting materials.

Gaskets Gaskets are mechanical seals, usually ring-shaped, which seal flange joints. Gaskets vary by construction, materials and features. Commonly used gaskets are non-metallic (ASME B 16.21), spiral-wound (ASME B 16.20) and ring-joint (ASME B 16.20). Non-metallic gaskets are used with flat- or raised-face flanges. Spiral-wound gaskets are used with raised-face flanges, and ring-joint gaskets are used with ring-type joint (RTJ) flanges. Stress develops between an RTJ gasket and the flange groove when the gasket is bolted to a flange, leading to plastic deformation of the gasket.

Gender

Piping or tubing is usually inserted into fittings to make connections. Connectors are assigned a gender, abbreviated M or F. An example of this is a "-inch female adapter NPT", which would have a corresponding male connection of the same size and thread standard (in this case also NPT).

Common piping and plumbing fittings

This section discusses fittings primarily used in pressurized piping systems, though there is some overlap with fittings for low-pressure or non-pressurized systems. Specialized fittings for the latter setups are discussed in the next major subsection.

Adapter

In plumbing, an adapter is generally a fitting that interfaces two different parts. The term commonly refers to: any fitting that connects pipes of different materials, including: expansion adapters which have a flexible section to absorb expansion or contraction from two dissimilar pipe materials mechanical joint (MJ) adapters for joining polyethylene pipe to another material bell adapters which are like mechanical joint adapters but contain a stainless steel backup ring to maintain a positive seal against the mating flange flange adapters which attach to a polyethylene pipe with butt fusion to stiffen a junction and allow another flanged pipe or fitting to be bolted on a fitting that connects pipes of different diameters, genders, or threads adapter spools (also called crossover spools), used on oilfields and pressure control, have different diameters, pressure ratings or designs at each end adapters to convert NPT to BSP pipe threads are available a fitting that connects threaded and non-threaded pipe

Elbow

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.

Clarity on the difference between plumbing terminologies and geometric angles:

In plumbing, the term "45-degree elbow" for example, refers to the angle of bend from the original straight pipe position (0 degrees) to the new position (45 degrees), not the actual angle formed by the joint.

On a protractor, the actual angle of the above joint is 135 degrees, an obtuse angle.

This naming convention applies to other plumbing elbows, such as:

- "88 degree elbow" = 92 degrees on a protractor. Visualise bending the left end of the pipe up 88 degrees. Now turn the piece of pipe around so the horizontal piece of pipe is in line with the zero degrees line on the protractor. The protractor will read 92 degrees.

The key point is that the plumbing term focuses on the degree of bend from the original straight pipe, not the resulting angle.

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. Wide available short elbows are typically used in pressurized systems and 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.

Coupling

A coupling connects two pipes. The fitting is known as a reducing coupling, reducer, or adapter if their sizes differ. There are two types of collars: "regular" and "slip". A regular coupling has a small ridge or stops internally to prevent the 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.

Union

A union also connects two pipes but is quite different from a coupling, as it allows future disconnection of the pipes for maintenance. 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, 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 current from 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.

Nipple

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.

Reducer

A reducer reduces the pipe size from a larger to a smaller bore (inner diameter). Alternatively, reducer may refer to any fitting which causes a change in pipe diameter. This change may be intended to meet hydraulic flow requirements of the system or adapt to existing piping of a different size. The reduction length is usually equal to the average of the larger and smaller pipe diameters. Although reducers are usually concentric, eccentric reducers are used as needed to maintain the top- or bottom-of-pipe level.

A reducer can also be used either as a nozzle or diffuser, depending on the mach number of the flow.

Bushing A double-tapped bushing, commonly shortened to 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 common type of this style fitting is a "hex bushing" with a hex head for installation with a pipe wrench.

A double-tapped bushing is more compact than a reducer but not as flexible. While a double-tapped bushing has a more minor 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.

Tee

A tee combines or divides fluid flow. Tees can connect pipes of different diameters, change the direction of a pipe run, or both. Available in various 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.

Diverter tee