In a steel plant there are large numbers of pipe networks. These pipe networks use a large number of pipe fittings. These pipe fittings are of various types, shapes and sizes.
Pipe fittings allow pipes to be joined or installed in the appropriate place and terminated or closed wherever necessary. They can be expensive, require time, and different materials and tools to install. They are an essential part of piping systems. There are thousands of specialized fittings manufactured. Each type of pipe or tube requires its own type of fitting, but usually all pipe fittings share some common features.
The basic purpose of using pipe fittings in pipe systems is to connect the bores of two or more pipes or tubes. Pipe fittings are used in piping systems normally (i) to connect straight pipe or tubing sections, (ii) to adapt to different sizes or shapes, (iii) to branch or re-direct the piping system, (iv) if necessary to provide a jointing method if two dissimilar piping materials are used in the one system, and (v) for other purposes, such as regulating, measuring or changing the direction of the fluid flow or to connect up threaded pipe and equipment. They are also used to close or seal a pipe.
Fittings for pipe and tubing are most often made from the same base material as the pipe or tubing being connected, e.g., stainless steel, steel, copper or plastic. However, any material that is allowed by code 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.
Fittings (especially uncommon types) require money, time, materials, and tools to install, so they are not a trivial part of the piping systems. Valves are also technically fittings, but are usually considered separately.
Pipe fittings are usually manufactured from pipe, bar, hollow bar, castings or forgings. The fittings are used in non-critical, low pressure applications where welding is not possible or required. They therefore provide a relatively low cost method of connection. Pipe fittings are usually fitted with a sealant (paste or tape such as PTFE) and are considered to be permanent pipe-work. Butt weld or socket weld fittings are a type of fittings used for forming circumferential butt weld joints in pipework systems. They are used for critical systems and in areas where pipe-work is permanent and are designed to provide good flow characteristics.
Fittings for piping systems can be expensive and require a proportionally large labour element to install, therefore correct selection and use is of vital importance to a well installed piping system. Every type of piping material has a range of fittings that can be used with it and some piping materials can have multiple different ranges of fittings that can be used. For example copper piping systems can be installed by bending the pipe and therefore using no elbows, using soldered copper fittings or compression brass fittings depending on the type of service being transferred in the copper pipe. Fittings are generally available with ends to match the piping installation.
While there are thousands of specialized pipe fittings manufactured, some common types of pipe fittings (Fig 1) which are used widely in piping systems are (i) elbow, (ii) tee, (iii) cross, (iv) reducer, (v) cap or plug, (vi) union, (vii) coupling, (viii) nipple etc.
- Elbow – It is a pipe fitting which is installed between two lengths of pipe or tube allowing a change of direction, usually 90 degrees or 45 degrees. The ends may be machined for butt welding, threaded (usually female), or socketed, etc. When the two ends differ in size, it is called a reducing or reducer elbow. Most elbows are available in short radius or long radius of types. The short radius elbows have a center to end distance equal to the nominal diameter, while the long radius is 1.5 times the nominal diameter. Elbows used on powder transfer systems have a much longer radius (radius of bend can be 10 times the nominal diameter of the pipe) to ensure smooth flow, reduce wear to both product and piping and to reduce the chance of getting blockages.
- Tee – A tee is used to either combine or split a fluid flow. Most common are equal tees which have the same body and branch diameter but there is also a wide range of reducing tees where either the branch or the body is of a different diameter relative to each other. A swept tee is where the branch enters the body at an arc and is used to minimize the frictional losses and promote flow in the system. Wye (Y) pipe tee fittings are with three openings and are used to create branch lines. A wye tee is where the branch is stabbed into the body at an angle usually 45 degrees and is usually used where the branch is a smaller diameter than the main pipe. Wye tees are similar to tees except that the branch line is angled to reduce friction and turbulence that could hamper the flow.
- Cross – A cross has one inlet and three outlets, or vice versa and like tees come in equal and reducing forms. A cross is more expensive than two tees but has the advantage of reduced space and requires less labour to install.
- Reducer – Reducers are used to join two different pipe sizes together. They can be either concentric or eccentric which refers to the relative position of the center lines of the outlet and inlet. Special attention must be given when using reducers in a horizontal orientation as the slope prevents free draining of a system if not installed correctly.
- Cap or plug – It is a type of pipe fitting which is liquid or gas tight, and is used to cover the end of a pipe. A cap has a similar function to a plug. For screwed systems the cap is having female threads where a plug is having male threads.
- 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, or an adapter. By convention, the term ‘expander’ is not generally used for a coupling that increases pipe size; instead the term ‘reducer’ is used.
- Union – A union is similar to a coupling except that it is designed to allow quick and convenient disconnection of pipes for maintenance or fixture replacement.
- Nipple – A short stub of pipe usually of threaded steel, brass, chlorinated polyvinyl chloride 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 hoses.
Fig 1 Types of fittings
Bushings are special type of pipe fittings normally make to use the diameter of a pipe fitting smaller. They differ from reducers in that they make abrupt changes in diameter and take very little space. Two examples of galvanized steel bushings are face bushings, which take the least amount of space, and hex bushings which can be tightened with an adjustable wrench.
Adapter fittings are used to change the end of a non-threaded pipe to male or female threads as needed. Both male and female adapters are common.
Types of pipe fittings
Depending on the purposes served, pipe fittings can be categorized as given below.
- Pipe fittings to extend or terminate pipe runs. Examples are couplings, adapters, unions, caps and plugs.
- Pipe fittings to change a pipe’s direction such as elbows.
- Pipe fittings to connect two or more pipes. Examples are tees, cross, side-inlet elbows, and wyes.
- Pipe fittings to change pipe size. Examples are reducers, bushings, and couplings.
- Pipe fittings to manage or regulate flow such as valves.
- Pipe fitting tools such as pipe fasteners.
- Pipe flanges.
Piping or tubing are usually (but not always) inserted into fittings to make connections.Pipe fittings are used to connect pipes or tubes in the following two ways.
- By threading – Threaded pipes are screwed together to connect or join. Generally metal pipes are threaded and they have threaded fittings.
- By slip fit – Slip fit pipes use sleeves that slip into one another. The plastic pipes are either threaded or slip fit.
Pipe fittings are either male fittings or female fittings. In threaded pipe fittings, female threads are on the inside while male threads are on the outside. Pipe fittings that have one female end and one male end are called street fittings.To avoid confusion, connections are conventionally assigned a gender of male or female, respectively abbreviated as ‘M’ or ‘F’. Accordingly pipe fittings are organized as follows.
- Male threaded – These pipe fittings have exterior threads. They are screwed into the inside of pipe end of a larger diameter with internal threading.
- Female threaded – These pipe fittings have interior threads. They are either screwed on the outside of pipe end of a smaller diameter with external threading or they receive male threaded pipe fittings.
- Male slip fit – In these pipe fittings there are no threads. They slip fit into a slightly larger diameter sleeve.
- Female slip fit – In these pipe fittings there are no threads. They slip fit into a slightly smaller diameter sleeve.
Selection criteria for pipe fittings
Pipe fittings are to be selected considering certain factors. These factors are given below.
- Connection types – When using pipe fittings, one is to be aware of the fact that a fitting can have two different connector types. Both the ends can have female threads or one end of the fitting can have female threads while the other end can have male threads. One end may have male slip while the other end can have threads. They can also have matching ends which can accommodate any requirement.
- Materials of construction – As a rule, the pipe fitting should be of the same material as the material used in the making of the pipe in which it is to be fitted. However, in some cases, materials conforming to certain codes or standards can also be used in pipes of another material.
- Check for flow – To keep the flow of the fluid consistent in the piping system, the ends of pipe fittings are to be slightly larger than the rest of the pipe so that they can accommodate connections without narrowing the inner diameter (ID) of the pipe.
- Type of fitting – Besides pipe materials, pipe fittings are identified by the type of fitting such as threaded or slip, male or female.
- Size – When measuring the size of pipe fittings, it is to be noted that the male threaded fittings are measured to the outside edge or outer diameter (OD), while female fittings are measured to the inside edge of the inlet or ID.
- Thickness – The pipe fittings are available in a number of different thicknesses or ‘schedules’ in a same way as the pipes are available.
- Design – Each pipe or tube is designed to carry certain specific types of fluids, liquids, gases, chemicals under varying conditions. Accordingly, the pipe fittings are also available in variety of designs.
- Standards and codes -There are certain standards and codes set by various standard organizations by which the different pipe fittings are graded. Pipe fittings are selected and used satisfying these standards.
Materials for pipe fittings
The material with which a pipe is manufactured often forms as the basis for choosing pipe fittings. Pipe fittings are made of the following materials.
- Carbon steel
- Impact tested carbon steel
- Low temperature service carbon steel
- Stainless steel
- Malleable iron/ductile iron
- Non-ferrous metals
- Non-metallic materials
- Chrome-molybdenum steel (alloy steel) normally 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. However, any material that is allowed by the code 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 piping system. Fire hazards, earth quake resistance, and other factors also influence choice of pipe fitting materials.
Hydraulic systems use extremely high fluid pressures to create useful work, such as in the hydraulic actuators for hydraulic powered machinery. 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 general use. Hydraulic fittings are designed and constructed to resist high pressure leakage and sudden explosive failure.