B. According to Function
There is an array of types of fittings used in piping systems, all of which perform specific functions. Majority of fittings can be categorized into one of these groups:
I. Fittings That Extend or Remove Pipe Lengths
An adapter connects two different pipes through soldering, threading, or solvent welding. It is generally used for PVC and copper pipes.
A coupling connects two similar pipes through soldering, threading, or solvent welding.
A sleeve connects two similar pipes using mechanical fasteners, such as anchors and screws. Generally, installation is faster and easier compared to a welded or soldered coupling connection.
A union is used as an alternative to flange connections in low-pressure small-bore piping where pipe detaching is needed often. It is made of three components – a nut, a male end, and a female end. Their connection, which is usually threaded or socket welded, promotes easy release.
A union is made of couplings that can be detached without cutting. There are three different types of coupling used in unions:
Full coupling can be a socket end or threaded type. It is widely used to connect small-bore pipes. It is used to attach pipe to pipe or pipe to nipple or swage.
Half coupling may have a thread or socket end on only one side. It is used for small-bore pipes branching from a large pipe or vessel.
Reduced coupling is used to attach pipes with different sizes. It is similar to a concentric reducer that maintains a similar centerline but smaller in size.
A cap attaches to a male connection through a thread, solvent, solder, or weld to cover the end of a pipe. It is used at the dead end of a pipeline as well as in piping headers for future connections.
A plug attaches to a female connection through a thread, solvent, solder, or weld to close the end of a pipe.
II. Fittings That Add or Change Direction
An elbow is one of the most commonly used pipe fittings. It changes the direction of a piping to different angles. The most common angles are 45° and 90°, but it can be cut to any degree. A 45° elbow is attached between the pipe to alter the direction of flow by 45 degrees. Meanwhile, a 90° elbow is attached to the pipe to change the direction of flow by 90 degrees. It is available in two forms:
In a long radius elbow, the centerline radius is 1.5 times the diameter of the pipe. A long radius elbow is more used in piping compared to a short radius elbow as there is less pressure loss.
The centerline radius of a short radius elbow is similar to the diameter of the pipe. It is often used when the application has limited space. However, this form has a higher pressure drop due to the sudden change in the flow direction.
A tee is used to connect three components of piping in a T-shaped intersection. This allows combining or splitting fluid flow. A pipe tee is a short piece of pipe with a 90 degree branch at the center. There are three types of pipe tees used in piping:
The diameter of the branch is similar to the diameter of the run (header) pipe.
The diameter of the branch is smaller than the size of the run (header) pipe.
A barred tee, also known as a scrapper tee, is used in “pigged” pipelines. The branch has a constraint bar internally welded to avoid the scrapper or pig from entering the branch. A constraint bar is welded in the branch in a manner that it restricts a free pathway of the pig from the run (header) pipe.
Also called a lateral, a wye combines three components of piping in a Y-shaped intersection. Similar to a tee, it combines or separates fluid flow but with less resistance. It typically has a branch at a 45-degree angle, but can also be in any angle other than 90 degrees. This type of pipe fittings decreases turbulence and friction that could block the flow.
A cross has 4-way connections to provide one entry and three outlets or vice versa. It is less stable compared to a tee and can create high stresses to the piping during temperature changes.
III. Fittings That Connect Pipes of Smaller Size
A pipe reducer includes all pipe connections that attach two or more pipes of different sizes. There are two types of reducers used in piping:
- Concentric or Conical Reducer
A concentric or conical reducer maintains the center elevation of the piping system as the centerlines of both of its ends are on the same axis. It is widely used in applications wherein the centerlines of a smaller pipe and larger pipe need to be on the same axis.
In an eccentric reducer, the centerlines of both ends are on different axes. It maintains bottom of pipe or BOP elevation of the piping system. This type of reducer is needed when one of the outside surfaces of the pipes must be maintained.
An olet is a reduction fitting connected to joined connections when proper reducing crosses or tees are not available.
IV. Fittings That Provide Special Functions or Connections
A valve connects pipes together and at the same time, controls the flow. Some valves are used for on/off applications, while others are used for controlling/throttling services. There are various types of valves available that varies on where they are used:
- Ball Valves – mainly used for on/off applications, but depending on the design, it can also be used for basic throttling.
- Gate Valves – mainly used for isolation.
- Globe Valves – widely used for throttling/controlling flow.
- Check Valves – also called as a non-return valve, it is used to prevent backflow
- Butterfly Valves – often used either for isolation or throttling services.
A standard nipple is a short stub of pipe with a male pipe thread at one end or both of its ends. It connects two different fittings at each end. It is common for attaching hoses, valves, and pipes.