Piping and plumbing fittings


From Wikipedia, the free encyclopedia

Copper fittings for soldered joints
Assorted pipe fittings: 1.Copper, solder; 2.Iron or brass, threaded; 3.Brass, compression; 4.Brass, transition from compression to solder; 5.Brass, special adaptors
PVC fittings

A fitting is used in pipe plumbing systems to connect straight pipe or tubing sections, to adapt to different sizes or shapes, and for other purposes, such as regulating or measuring fluid flow. The term plumbing is generally used to describe conveyance of water, gas, or liquid waste in ordinary domestic or commercial environments, whereas piping is often used to describe high-performance (e.g. high pressure, high flow, high temperature, hazardous materials) conveyance of fluids in specialized applications. The term tubing is sometimes used for lighter-weight piping, especially types that are flexible enough to be supplied in coiled form.

Fittings (especially uncommon types) require money, time, materials, and tools to install, so they are a non-trivial part of piping and plumbing systems.[1] Valves are technically fittings, but are usually discussed separately.



There are certain standard codes that need to be followed while designing or manufacturing any piping system. Organizations that promulgate piping standards include:

For example, pipes need to 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

The B31.3 / B31.4 code has requirements for piping found in petroleum refineries; chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants; and related processing plants and terminals. This code specifies requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of piping. This Code is applicable to piping for all fluids including: (1) raw, intermediate, and finished chemicals; (2) petroleum products; (3) gas, steam, air and water; (4) fluidized solids; (5) refrigerants; and (6) cryogenic fluids.


Display of a variety of threaded cast iron fittings

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

  • Carbon Steel (CS)
  • Impact Tested Carbon Steel (ITCS)
  • Low Temperature Service Carbon Steel (LTCS)
  • Stainless Steel (SS)
  • Malleable iron (malleable iron)
  • Non-Ferrous Metals (Inconel, Incoloy, Cupro-nickel etc.)
  • Non-Metallic (ABS, GRE, PVC, HDPE, tempered glass, etc. )
  • Chrome-molybdenum steel (Alloy steel) — Generally used for high temperature service

The bodies of fittings for pipe and tubing are most often of the same base material as the pipe or tubing being connected, for example, copper, steel, polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), or acrylonitrile butadiene styrene (ABS). However, any material that is allowed by the plumbing, health, or building code (as applicable) may be used, but must be compatible with the other materials in the system, the fluids being transported, and the temperatures and pressures inside and outside of the system. For example, brass- or bronze-bodied fittings are common in otherwise copper piping and plumbing systems. Fire hazards, earthquake resistance, and other factors also influence choice of fitting materials.


Gaskets are used for sealing of flange joints. In general, gaskets should not be reused.[citation needed][2] Various types of gaskets are available depending upon their construction, materials, and features. The following are the type of gaskets commonly used:

  • Non-Metallic Gaskets (ASME B 16.21)
  • Spiral-Wound Gaskets (ASME B 16.20)
  • Ring Joint Gaskets (ASME B 16.20)

Non-Metallic Gaskets are used with flat face or raised face flanges. Spiral-Wound Gaskets are used with raised face flanges. They are available with an inner ring and outer ring, which is also known as the cantering ring. Ring Joint Gaskets are used with Ring Type Joint (RTJ) flanges. They are available in octagonal or oval cross sections.

A very high surface stress is developed between an RTJ gasket and the flange groove when RTJ is bolted up in a flange. This leads to plastic deformation of the gasket. Thus, the hardness of the gasket is kept less than the hardness of the groove to achieve coining i.e. bringing two metal surfaces of different hardness so tightly together that the softer surface deforms to match harder surface exactly in shape and finish.[further explanation needed] Hence RTJ gaskets are not recommended for reuse.[3]

Gender of fittings

Piping or tubing are usually (but not always) inserted into fittings to make connections. To avoid confusion, connections are conventionally assigned a gender of male or female, respectively abbreviated as "M" or "F". An example of this is a "3/4 inch female adapter NPT," which would have a corresponding male connection of the same size and thread standard (in this case, NPT).

Common fittings for both piping and plumbing

While there are hundreds of specialized fittings manufactured, some common types of fittings are used widely in piping and plumbing systems. Fittings - ASME B 16.9


See also: Street elbow
Short radius or regular 45° elbow (copper sweat)
Long radius or sweep 90° elbow (copper sweat)

An elbow is a pipe fitting installed between two lengths of pipe or tubing to allow a change of direction, usually a 90° or 45° angle, though 22.5° elbows are also made. The ends may be machined for butt welding, threaded (usually female), or socketed, etc. When the two ends differ in size, the fitting is called a reducing elbow or reducer elbow.

Elbows are categorized based on various design features as below:

  • Long Radius (LR) Elbows – radius is 1.5 times the pipe diameter
  • Short Radius (SR) Elbows – radius is 1.0 times the pipe diameter
  • 90 Degree Elbow – where change in direction required is 90°
  • 60 Degree Elbow – where change in direction required is 60°
  • 45 Degree Elbow – where change in direction required is 45°

A 90 degree elbow is also called a "90 bend" or "90 ell". It is a fitting which is bent in such a way to produce 90 degree change in the direction of flow in the pipe. It is used to change the direction in piping and is also sometimes called a "quarter bend". A 90 degree elbow attaches readily to plastic, copper, cast iron, steel and lead. It can also attach to rubber with stainless steel clamps. It is available in many materials like silicone, rubber compounds, galvanized steel, etc. The main application of an elbow (90 degree) is to connect hoses to valves, water pressure pumps, and deck drains. These elbows can be made from tough nylon material or NPT thread.

A 45 degree elbow is also called a "45 bend" or "45 ell". It is commonly used in water supply facilities, food industrial pipeline networks, chemical industrial pipeline networks, electronic industrial pipeline networks, air conditioning facility pipeline, agriculture and garden production transporting system, pipeline network for solar energy facility, etc.

Most elbows are available in short radius or long radius variants. The short radius elbows have a center-to-end distance equal to the Nominal Pipe Size (NPS) in inches, while the long radius is 1.5 times the NPS in inches. Short elbows are widely available, and are typically used in pressurized systems.

Long elbows are typically used in low-pressure gravity-fed systems and other applications where low turbulence and minimum deposition of entrained solids are of concern. They are readily available in acrylonitrile butadiene styrene (ABS plastic), polyvinyl chloride (PVC) for DWV, sewage and central vacuums, chlorinated polyvinyl chloride (CPVC) and copper for 1950s to 1960s houses with copper drains.


Pipe coupling (copper sweat)

A coupling connects two pipes to each other. If the size of the pipe is not the same, the fitting may be called a reducing coupling or reducer, or an adapter. By convention, the term "expander" is not generally used for a coupler that increases pipe size; instead the term "reducer" is used. There are two different types of couplings: slip and regular couplings.


A combination pipe union and reducer fitting (brass threaded)

A union is similar to a coupling, except it is designed to allow quick and convenient disconnection of pipes for maintenance or fixture replacement. While a coupling would require either solvent welding, soldering or being able to rotate with all the pipes adjacent as with a threaded coupling, a union provides a simple transition, allowing easy connection or disconnection at any future time. A standard union pipe is made in three parts consisting of a nut, a female end, and a male end. When the female and male ends are joined, the nuts then provide the necessary pressure to seal the joint. Since the mating ends of the union are interchangeable, changing of a valve or other device can be achieved with a minimum loss of time. Pipe unions are essentially a type of flange connector, as discussed further below.

In addition to standard, simple unions, other types of union exist:

  • Dielectric unions are unions with dielectric insulation, used to separate dissimilar metals (such as copper and galvanized steel) to avoid the damaging effects of galvanic corrosion. When two dissimilar metals are in contact with an electrically conductive solution (even tap water is conductive), they will form a battery and generate a voltage by electrolysis. When the two metals are in direct contact with each other, the electric current from one metal to the other will cause a movement of ions from one to the other, dissolving one metal and depositing it on the other. A dielectric union breaks the electric current path with a plastic liner between two halves of the union, thus limiting galvanic corrosion.
  • Rotary unions are unions that allow for rotation of one of the united parts.


Reducer fittings, bronze threaded (left) and copper sweat (right)
Main article: Reducer

A reducer allows for a change in pipe size to meet hydraulic flow requirements of the system, or to adapt to existing piping of a different size. Reducers are usually concentric but eccentric reducers are used when required to maintain the same top- or bottom-of-pipe level. Material - ASTM A234 WPB


Whenever branch connections are required in size where reducing tees are not available and/or when the branch connections are of smaller size as compared to header size, olets are generally used[further explanation needed]. The following are few configurations of olet connections :

  • Flanged Olet
  • Socket-Weld & Threaded Olet
  • Lateral & Elbow Olets
  • Nipple Olet
  • Butt-Weld Olet
  • Swage Nipples


Pipe tee (copper sweat)

A tee is the most common pipe fitting. It is available with all female thread sockets, all solvent weld sockets, or with opposed solvent weld sockets and a side outlet with female threads. It is used to either combine or split a fluid flow. It is a type of pipe fitting which is T-shaped having two outlets, at 90° to the connection to the main line. It is a short piece of pipe with a lateral outlet. A tee is used for connecting pipes of different diameters or for changing the direction of pipe runs. They are made of various materials and available in various sizes and finishes. They are extensively used in pipeline networks to transport two-phase fluid mixtures. They are categorized as:

  • Equal
  • Unequal

When the size of the branch is same as header pipes, equal tee is used and when the branch size is less than that of header size, reduced tee will be used. Most common are tees with the same inlet and outlet sizes. Some of the industrial tees are Straight Tee, Reducing Tee, Double Branch Tee, Double Branch Reducing Tee, Conical Tee, Double Branch Conical Tee, Bullhead Tee, Conical Reducing Tee, Double Branch Conical Reducing Tee, Tangential Tee, and Double Branch Tangential Tee.

The above tees are categorized on the basis of their shapes and structure. They can also be classified on the basis of the application they are required to perform.[further explanation needed]

The three outlet sizes should be named in order (e.g. left, middle, right; measuring 15-22-15).[further explanation needed] The three sizes of a tee are end x end x center, so a tee that is 1" on both ends and 3/4" in the center it would be 1" x 1" x 3/4".[contradiction]


Cross fittings are also called 4-way fittings. If a branch line passes completely through a tee, the fitting becomes a cross. A cross has one inlet and three outlets, or vice versa. They often have solvent welded socket ends or female threaded ends.

Cross fittings can generate a huge amount of stress on pipe as temperature changes, because they are at the center of four connection points. A tee is more steady than a cross, as a tee behaves like a three-legged stool, while a cross behaves like a four-legged stool. (Geometrically, "any 3 non-collinear points define a plane" thus 3 legs are inherently stable.) Crosses are common in fire sprinkler systems,[why?] but not in plumbing, due to their extra cost as compared to using two tees.


Pipe cap (copper sweat)

A type of pipe fitting, usually liquid or gas tight, which covers the end of a pipe. A cap is used like plug, except that the pipe cap screws or attaches on the male thread of a pipe. A cap may have a solvent weld socket end or a female threaded end and the other end closed off. In plumbing systems that use threads, the cap has female threads. Industrial caps can be round, square, rectangular, U-shaped, I-shaped and may have a round hand grip or a flat hand grip.

If a solvent weld cap is used to provide for a future connection point, several inches of pipe must be left before the cap. This is because 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 closes off the end of a pipe. It is similar to a cap but it fits inside the fitting it is mated to. In a threaded iron pipe plumbing system, plugs have male threads. Some of the popular types of plugs are:

  • Mechanical pipe plug
  • Pneumatic disk pipe plug
  • Single size pneumatic all rubber pipe plug
  • Multi-size pneumatic pipe plug
  • Multi-size flow-through pipe plug
  • High pressure pipe plug


Main article: Nipple (plumbing)

A short stub of pipe, usually threaded steel, brass, chlorinated polyvinyl chloride (CPVC) or copper; occasionally just bare copper. A nipple is defined as being a short stub of pipe which has external male pipe threads at each end, for connecting two other fittings. Nipples are commonly used for plumbing and hoses, and second as valves for funnels and pipes.[clarification needed]


Hose barb fittings made of brass

A "barb" or "hose barb" fitting is used to connect flexible hose or tubing to pipes. A barb fitting typically has a male-threaded end used to mate with female threads. The other end of the fitting has either a single- or multiple-barbed tube having a tapered stub with ridges, which is inserted into a flexible hose to secure it. An adjustable worm drive screw clamp or other type of clamp is often added, to help to keep the hose from slipping off the barbed tube. Barb fittings can be made of brass for hot water applications, while plastic may be used for cold water; brass is considered more robust and durable for heavy-duty use. The barb fitting can be either elbow-shaped or straight.


Water shutoff valves below a kitchen sink
Main article: Valve

Valves are equipment designed to stop or regulate flow of any fluid (liquid, gas, condensate, stem, slurry, etc.) in its path. Valves are categorized depending on their applications like isolation, throttling, and non-return. Various type of valves are available depending upon the type of construction as follows:

  • Gate valve - used for isolation only
  • Plug valve - used for isolation only
  • Globe valve - used for throttling
  • Butterfly valve - used for isolation as well as throttling
  • Check valve - used for preventing reverse flow (non-return)
  • Diaphragm valve - used for isolation as well as throttling
  • Ball valve - used for isolation only
  • Needle Valve - used to control flow rate to a desired amount

Drain-waste-vent (DWV) and related fittings

Cast-iron and copper DWV installation

Because they operate at low pressure and rely on gravity to move fluids (and often entrained solids), drain-waste-vent systems use fittings designed to be as smooth as possible on their interior surfaces. The fittings may be "belled" or expanded slightly in diameter, or otherwise shaped to accommodate insertion of pipe or tubing, without forming a sharp interior ridge that might catch debris or accumulate buildup of material and cause clogging.[4] The absence of interior snags also makes it much easier to "snake out" or "rod out" a clogged pipe using long flexible tools made for this purpose.

Underground piping systems for landscaping drainage, or disposal of stormwater or groundwater, similarly use gravity flow at low pressure, often with entrained solids. Piping fittings used for these systems bear a strong resemblance to DWV fittings, though often at a larger scale.[5] When high peak flow volumes are involved, the design and construction of these systems are closely inter-related to sewer design.

Fittings for central vacuum systems are very similar to DWV fittings, though usually of thinner and lighter construction, since the weight of the materials conveyed through the system is much less. Vacuum system designs share with DWV designs a concern about eliminating internal ridges, burrs, sharp turns, or other obstructions to smooth flow that might cause build-up of material into pipe blockages.

Sweep elbow

DWV elbows are usually long radius or sweep types, to reduce flow resistance and solids deposition when the direction of flow is changed. A well-designed system will often employ multiple 45° elbows in preference over 90° elbows (even sweep elbows), to reduce flow disruption as much as possible.

Central vacuum system inlet fittings are intentionally designed with a tighter radius of curvature than any other bends in the system. This is done to ensure that if any vacuumed debris becomes stuck, it will jam right at the inlet, where it is easiest to discover and to remove.

Closet flange

The closet flange is the drain pipe flange to which a water closet (toilet) is attached. It is a specialized type of flange connection designed to sit flush with the floor, allowing a standard toilet to be installed above it.


Clean-outs are fittings with removable elements that allow access to drains without requiring removal of plumbing fixtures. They are used for allowing an auger or plumber's snake to clean out a plugged drain. Clean-outs should be placed in accessible locations at regular intervals throughout a drainage system, often including outside the building, because clean-out augers have limited length. The minimum requirement is typically at the end of each branch in piping, just ahead of each water closet, at the base of each vertical stack, and both inside and outside the building in the building main drain/sewer. Clean-outs normally have screw-on caps or screw-in plugs. Clean-outs are also known as rodding eyes from the eye-shaped cover plates often used on external versions.

Trap primers

Trap primers regularly inject water into traps so that "water seals" are maintained, as necessary to keep sewer gases out of buildings. The trap primer must be installed in a readily available place for easy access for adjustments, replacement, and repair. Strictly speaking, a trap primer is a specialized valve, and it is usually connected to a clean water supply, in addition to a DWV system.[6] Because of this dual connection, the design usually must be certified to resist accidental backflow of contaminated water.


A combination tee (combo tee) is a tee with a gradually curving center connecting joint. It is used in drain systems to provide a smooth, gradually curving path to reduce the likelihood of clogs, and to ease pushing a plumber's snake through a drain system. The "combo" is a combination of a wye and a 1/8 bend or 45° elbow.

Sanitary tee

A sanitary tee is a tee with a curved center section. In drainage systems it is primarily used to connect horizontal drains ( including fixture trap arms) to vertical drains. (It is against all major codes to use a sanitary tee to connect a vertical drain to a horizontal drain.) The center connection is generally connected to the pipe which leads to a trap (the trap arm).

Double sanitary tee (sanitary cross)

Similar to a cross. This fitting differs from a standard cross in that two of the ports have curved inlets. The fitting has been used in the past for connecting the drains of back-to-back fixtures (such as back-to-back bathroom sinks). Some current codes (including the 2006 UPC) prohibit the use of this fitting for that purpose, instead requiring a special "double fixture fitting" (double combination wye).[further explanation needed]

Wye ("Y") fitting

A fitting with three openings, a wye is used to create branch lines. It is a type of waste fitting tee which has the side inlet pipe entering at a 45° angle, or an angle other than 90 degrees. A standard wye is a "Y" shaped fitting which allows one pipe to be joined to another at a 45 degree angle.

Wyes are similar to tees except that the branch line is angled to reduce friction and turbulence that could hamper the flow. The connection is typically at a 45-degree angle rather than a 90-degree angle. If a branch turns out further at the end to be perpendicular to the through line, the fitting becomes a "tee wye" (TY).

PVDF Corrosive Waste Piping Systems utilize wye fittings that feature a smooth inside diameter without any irregularities, for uninterrupted flow path. These fittings should be able to withstand acids, bases, and solvents. Flame-retardant systems withstand intermittent corrosive fluids up to high temperatures. Polypropylene and PVDF Double Wyes and Double Reducing Wyes come in various sizes and are largely used for this purpose.

A wye branch allows splitting a branch line equally in two directions. The opening sizes can vary for different situations, for instance in situation where a large main line needs to be split into two smaller branches.

Low-priced wyes are often spot-welded together, whereas industrial wyes have a continuous weld at each seam. In long-distance pipeline applications, a true Wye fitting is also engineered for closed system instrumentation pigging configurations[further explanation needed] or wherever a smooth pipe branch is used.

Double-tapped bushing

A double-tapped bushing is a fitting that has opposing threads on the inside diameter of the bushing.

Hydraulic fittings

Hydraulic systems use extremely high fluid pressures to create useful work, such as in the hydraulic actuators for powered machinery such as bulldozers and backhoes. Therefore, hydraulic fittings are designed and rated for much greater pressures than those experienced in general piping systems, and they are generally not compatible for use in general plumbing. Hydraulic fittings are designed and constructed to resist high pressure leakage and sudden explosive failure. More information on hydraulics and their specialized fittings can be found in the hydraulic machinery article.

Connection methods

Much of the work of installing a piping or plumbing system involves making leakproof, reliable connections. In addition, most piping requires mechanical support against gravity and other forces (such as wind loads and earthquakes) that might disrupt an installation.[7] Depending on the connection technology used, basic skills may be sufficient, or specialized skills and professional licensure may be required.


A fastener is a hardware device that mechanically joins or affixes two or more objects together. They are usually used to attach pipe and fittings to mechanical supports in buildings, but not to interconnect the actual pipes to each other. The following are the type of fasteners commonly used with piping:

  • Stud bolt with nut — Usually the stud bolts are used with full threading and with two heavy hexagonal nuts.
  • Machine bolt with nut
  • Powder-actuated tool (PAT) fastener — Usually a simple nail, or a threaded stud, driven into concrete or masonry

Threaded pipe

MIP pipe and FIP elbow fitting (steel threaded)

A threaded pipe is a pipe with a screw thread at one or both ends for assembly. Steel pipe is often joined using threaded connections, where tapered threads (in the US/Canada see National Pipe Thread, in Commonwealth Nations such as the UK, Australia, New Zealand see British standard pipe thread) are cut into the end of the tubing segment, sealant is applied in the form of thread sealing compound or thread seal tape (also known as PTFE or Teflon tape), and it is then threaded into a corresponding threaded fitting using a pipe wrench.

Threaded steel pipe is still widely used in many homes and businesses to convey natural gas or propane fuel, and is a popular choice in fire sprinkler systems due to its high heat resistance. Threaded brass pipe was once used in a similar fashion, and was considered superior to steel for carrying drinking water, but is now effectively obsolete.

Assembling threaded steel pipe takes some skill, plus careful planning to allow lengths of pipe to be screwed together in proper sequence. Most threaded pipe systems require occasional use of pipe union fittings to allow final assembly.

Solvent welding

Polypropylene fittings for solvent welding

A solvent is applied to PVC, CPVC, ABS, or other plastic piping, to partially dissolve and fuse the adjacent surfaces of piping and fitting. Solvent welding is usually used with a sleeve-type joint, to connect pipe and fittings made of the same (or closely compatible) material.

Unlike regular welding of metals, solvent welding is relatively easy to perform, although care is still needed to produce reliable joints. Solvents typically used for plastics are usually toxic, may be carcinogenic, and may also be flammable, requiring adequate ventilation.


Brass fitting soft-soldered to copper pipe

To make a solder connection, a chemical flux is applied to the inner sleeve of a sleeve type joint, and the pipe is inserted. The joint is then heated using a propane torch or MAPP gas torch, solder is applied to the heated joint, and the melted solder is drawn into the joint by capillary action as the flux vaporizes. Sweating is an alternate term sometimes used to describe soldering of pipe joints.

In situations where many connections must be made in a short period of time (such as plumbing of a new building), solder offers much quicker and much less expensive joinery than compression or flare fittings. A degree of skill is needed to rapidly make large numbers of reliable soldered joints. If flux residues are thoroughly cleaned, soldering can produce a long-lasting connection at low cost. However, the use of open flames for heating joints can present fire and health hazards to occupants of a building being worked on, and requires adequate ventilation.


Brazing is a thermal joining process in which the two pieces of the base metal are joined when a molten brazing filler metal is allowed to be drawn into a capillary gap between them. It is used to join most metals and alloys commonly used in engineering. Brazing filler metals have very high melting points, but always below the melting point of the metals being joined. It is closely related to soldering, but uses harder materials and higher temperatures. Brazing can be used to join pipes, rods, metal pieces or any other shape as long as the pieces fit neatly against each other without leaving large gaps. It is also capable of joining tungsten carbide, ceramics and similar non-metallic materials.

Successfully brazed joints are as strong as the parent metal pieces being joined and can withstand demanding service conditions i.e. they are strong and ductile. Well brazed joints have smooth, neat fillets and they offer good electrical conductivity.


Partially-welded steel pipe joint

Welding of metals differs from soldering and brazing, in that the connection is made without adding a special low-melting-point material (e.g. solder) to complete a joint. Instead, the material of the pipe or tubing is itself partially melted in a carefully controlled manner, and the fitting and piping are directly fused together. Generally this requires that the piping and the fitting be made of the same (or closely compatible) material. Proper ventilation is essential to remove dangerous metallic fumes from welding operations, and special personal protective equipment must be worn.

Properly welded and inspected joints are considered to be very reliable, robust, and long-lasting. Pipe welding is a specialized skill, and is often performed by specially licensed workers who are tested periodically for the quality of their work. For critical applications, every joint is tested using non-destructive testing methods. Because of the expensive skilled labor required, welded pipe joints are usually restricted to high-performance applications, such as in shipbuilding, chemical reactors, and nuclear reactors.

Compression fittings

Compression connectors on an isolating valve. The red metal is a copper compression ring.
Main article: Compression fitting

Compression fittings (sometimes called "lock bush fittings") consist of a tapered concave conical seat, a hollow barrel-shaped compression ring (sometimes called a "ferrule"), and a compression nut which is threaded onto the body of the fitting and tightened to make a leakproof connection. Fittings are typically made of brass or plastic, but stainless steel or other materials may be used.

Compression connections do not typically have the long life that sweat connections offer, but are advantageous in many cases because they are easy to install using basic tools. A disadvantage of compression connections is that they take longer to make than sweated joints, and sometimes require retightening to stop slow leaks that develop over time.

Push-to-pull compression fittings

Flare fittings

Main article: Flare fitting
Flare connection: 1 Screw thread, 2 O-ring, 3 Body, 4 Nut, 5 Seal interface, 6 Support ring (sleeve), 7 Flared tubing.

Flared connectors should not be confused with compression connectors, with which they are generally not interchangeable. Flared connectors lack a compression ring, but do use a threaded nut. A special flaring tool is used to enlarge tubing into a tapered "bellmouth" shape that matches the tapered projecting conical shape of the flare fitting. The flare nut, which has previously been installed over the tubing, is then tightened onto the fitting. Fittings are typically made of brass or plastic, but stainless steel or other materials may be used.

Flare connections are a labor-intensive method of making connections, but are quite reliable over the course of many years. Flared fittings are sometimes thought to be more secure against leaks and sudden failures, and are often preferred for safety-critical connections, such as in hydraulic brake systems.

Flange fittings

Flange connection, using a gasket

Flanges are generally used when there is a connection to valves, in-line instruments and/or connection to equipment nozzles is required. Flange fittings generally involve pressing two surfaces to be joined tightly together, by means of threaded bolts, wedges, clamps, or other means of applying high compressive forces.[8] Often, a gasket, packing, or an O-ring is installed between the flanges to prevent leakage, but it is sometimes possible to use only a special grease, or nothing at all, if the mating surfaces are precisely formed. Flanges are designed to the following pressure ratings: 150 lb, 300 lb, 400 lb, 600 lb, 900 lb,1500 lb and 2500 lb or 10 Bar, 15Bar, 25Bar, 40Bar, 64Bar, 100Bar and 150Bar. Various types of flanges are available depending upon the type of their constructional features. The following are types of flanges generally used in piping. These flanges are available with different facing like raised face, flat face, ring joint face etc.

  • Orifice
  • Threaded
  • Slip-On
  • Blind
  • Weld Neck
  • Socket
  • Lap Joint
  • Reducing

Slip On flanges are slipped over the pipe and then welded from both inside and outside to provide sufficient strength and prevent leakage. This flange is used instead of weld necks by many users because of its lower cost and also the fact that it requires less accuracy when cutting pipe to length.

Blind flanges do not have a bore and are used to shut off a piping system or vassal opening. Its design permits easy access to vassal or piping system for inspection purpose. It can be supplied with or without hubs at the manufacturer's option.

Weld neck flanges are designed to be joined to a piping system by butt welding. They are expensive because of its long neck, but is preferred for high stresses to the pipe, reducing stress applications. The neck, or hub, transmits stress concentration at the base of the flange. The gradual transition of thickness from the base of the hub to the wall thickness at the butt weld provides important reinforcement of the flange. Turbulence and erosion are reduced due to the matching bore size of the pipe and flange.

Socket Flanges are similar to a slip on flanges in outline, but the bore is counter-bored to accept pipe. The diameter of the remaining bore is same as the inside diameter of the pipe. A fillet weld around the hub of the flange attaches the flange to the pipe. An optional interval weld may be applied in high stress applications. Its biggest use is in high pressure system such as hydraulic and steam lines.

Lap Joint flange is again similar to a slip flange, but it has radius at the intersection of the bore and the flange face to accommodate a lap stub end. The face on the stub end forms the gasket face of the flange. Its applications are where sections of piping systems need to be dismantled quickly and easily for inspection or replacement.

Flanged connections tend to be more bulky than other connections, but can perform well in demanding applications, such as large water mains and hydroelectric power systems.

Mechanical fittings

Multiple mechanical sleeve clamps connecting piping

Manufacturers such as Victaulic or Grinnell produce special sleeve clamp fittings that are increasingly replacing classic flanged connections. They typically attach to the end of a pipe segment by using circumferential grooves pressed (or cut, in older designs) around the end of the pipe to be joined. Mechanical connectors are widely used on larger steel pipes, but can also be used with other materials.

The chief advantage of these newer connectors is that they can be installed in the field after cutting the pipe to length, which is much faster than traditional flanged connections, which must be factory-welded or field-welded to pipe segments. However, mechanically fastened joints are sensitive to residual stress issues and thickness stresses caused by dissimilar metals and temperature changes.

A grooved fitting, also called a "grooved coupling", has four elements:

  • the groove pipe
  • the gasket
  • coupling housing
  • the nuts and bolts

The groove is made by cold forming or machining a groove into the end of a pipe. A gasket encompassed by the coupling housing is wrapped around the two pipe ends, and the key sections of the coupling housing engage the grooves. The bolts and nuts are tightened with a socket wrench or impact wrench. In the installed state, the coupling housing encases the gasket and engages the grooves around the circumference of the pipe to create a leak-tight seal in a self-restrained pipe joint.

There are two basic grooved coupling styles:

  • Flexible grooved couplings- allow a limited amount of angular movement
  • Rigid grooved couplings- do not allow movement and can be used wherever immobility in the pipe joint is needed, similar to a flanged or welded joint.

Crimped or pressed fittings

Fittings for crimping

Crimped or pressed connections use special fittings which are permanently attached to tubing with a powered crimper. The special fittings, manufactured with sealant already inside, slide over the tubing to be connected. High pressure is used to deform the fitting and compress the sealant against the inner tubing, creating a leakproof seal.

The advantages of this method are that it should last as long as the tubing, it takes less time to complete than other methods, it is cleaner in both appearance and the materials used to make the connection, and no open flame is used during the connection process. The crimped fitting process is basically designed with both hard and half-hard copper tube, without using fluxes or filler metals. The possibility to get the connection done, even when the line is "wet" gives the process an added advantage. These are ideal for drinking water pipes and other hot and cold systems that include central heating as well. The disadvantages are that the fittings used are harder to find[citation needed] and cost significantly more than sweat type fittings.