Piping and plumbing

1. Hydraulic fittings

2. Elbow
Short radius 45° elbow or regular 45° elbow 
Long radius 90° elbow or sweep 90° elbow 

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 elbow, threaded (usually female) elbow, or socketed elbow, 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°
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 used to change the direction in piping and is also sometimes called a "quarter bend". A 90 degree elbow is available in many materials like copper, cast iron, carbon steel, stainless steel, galvanized steel. The main application of an elbow (90 degree) is to connect hoses to valves, water pressure pumps, and deck drains. 

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 elbows 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. 

3. Coupling 
Pipe coupling, 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. By convention, the term "expander" is not generally used for a coupler that increases pipe size; instead the term "reducer" is used.

4. Union 
A combination pipe union and reducer fitting, 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.

5. Reducer 
Reducer fittings, 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 reducers buteccentric reducers are used when required to maintain the same top- or bottom-of-pipe level.

6. Olets
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. The following are few configurations of olet connections:
Flanged Olet
Socket-Weld Olet
Threaded Olet
Lateral Olet
Elbow Olet
Nipple Olet
Butt-Weld Olet

7. Tee 
Pipe tee, 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 tee
Unequal tee
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. 

8. Cross
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 cross or female threaded ends cross.

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-colinear points define a plane" thus 3 legs are inherently stable.) Crosses are common in fire sprinkler systems, but not in plumbing, due to their extra cost as compared to using two tees.

9. Cap 
Pipe cap, 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 or a nipple. They have a solvent weld socket end cap or a female threaded end cap. 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. In plumbing systems that use threaded cap, the cap has female threads. Industrial caps can be round, square, rectangular, U-shaped, I-shaped and have a round hand grip or a flat hand grip.

10. Plug
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.

11. Nipple
A short stub of pipe, usually threaded steel, brass 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.