Pipe fittings technology overview

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  • Production of fittings: Injected, thermoformed or fabricated
  • Heat fusion fittings – simple and established
  • Electrofusion fittings – the innovative way

The lowest pressure-rated component in a pipeline determines the operating pressure of the whole piping system. For this reason proper design of a pipe system has to take into consideration the type of fittings used to join the piping components as well as the durability of the resulting joints. The wide range of fittings available, such as couplers, elbows, tees, reducers, end caps and saddles, such as branch or repair, also help to increase the overall use of piping systems.

Material

In most cases the fitting material follows the piping material, for instance, polyethylene (PE), polyvinyl chloride (PVC) and polypropylene (PP) fittings are the most common, followed by acrylonitrile-butadiene-styrene (ABS), polyoxymethylene (POM) and other thermoplastic materials. For multilayer pipes, for instance crosslinked polyethylene (PEX) aluminium composite pipes, the fittings are usually made of brass. There are also composite fittings available, for example, made of plated brass and polyphenylsulfone (PSSU) plastic material which are used in press and compression fitting applications.

Production of fittings: Injected, thermoformed or fabricated

Most of the smaller and middle-sized fittings are produced by using the injection moulding process. If injection moulded, usually large moulds with projecting cores and a relatively low clamping force are employed. Larger diameter fittings, that exceed the capabilities of the injection mould machine are typically fabricated and made by joining sections of pipe, machined blocks, or moulded fittings. The joining methods for fabricated fittings, especially if employed as in pressure pipe systems, can be either butt or socket heat fusion or electrofusion. In this case the fittings are designed in such a way that additional material is available in regions of sharp geometrical changes and regions that are subject to high localised stress. This can be done by increasing the wall thickness in highstress areas by fabricating fittings from heavier wall pipe sections. Other joining methods for fabricated pipes include hot gas and extrusion welding. But as the resultant joint strength of hot gas and extrusion welded joints is significantly lower, it is not recommended for the fabrication of pressure pipes. Electrofusion couplings and fittings are either injection moulded or fabricated for butt and socket fusion fittings or manufactured from pipe stock.

Larger PE fittings, for instance, for corrugated drainage piping applications, can also be produced by using the rotomoulding process. Thermoforming is another possible production process that is able to produce fittings such as sweep elbows, forged stub ends and reducers. Here a section of a pipe is heated and then reshaped by the thermoforming tool into the desired fitting shape.

Joining process – the integral part of a fitting

An integral part of any fitting is the method used to join the system components. Plastic pipes and fittings can be joined by numerous methods. Depending on the raw material, there are several different appropriate methods such as heat fusion, electrofusion, threaded joints, solvent welding, flanges, rolled grooves, mechanical compression joints, reinforced over-wraps to concrete encasement with steel reinforcement or rebar. The reason for such a variety of joining methods available is mainly because the pipe joint has to be adapted to different field conditions. Additionally, very large diameter fittings need special care during transportation, handling and installation. For protection purposes, these fittings are sometimes wrapped with a reinforcement material, for instance, fibre glass.

PVC pipes and fittings, especially if employed in a pressure pipe system, are usually connected by solvent welding. The solvent welding process uses PVC “cement” to create a connection that is significantly stronger than an unwelded joining, thus allowing the piping to handle higher fluid pressure. It is normally a one-way process: the fitting can be installed, but cannot be removed again. For pressureless applications fittings are not necessarily welded. An example is the union fitting which is able to create a leak proof seal between two pipes without the use of glue or welding.

When it comes to PE fittings there are also many different types and styles available, and even a larger variety of joining methods. In general, PE parts can be joined to each other by heat fusion, electrofusion or with mechanical fittings. And PE pipes can be joined to other pipe materials also by means of compression fittings, flanges, or other types of manufactured transition fittings. Each offers its particular advantages and limitations for each joining situation the user may encounter.

Heat fusion fittings – simple and established

Here both the external surface of the pipe and the internal surface of the socket fitting are heated simultaneously until the material reaches the required fusion temperature. Then the pipe is inserted into the socket and held in place until the joint cools. The three most common types of heat fusion joints currently used in the industry are butt, saddle and socket fusion.

Electrofusion fittings – the innovative way

Electrofusion fittings are manufactured with a coil-like integral heating element. During the electrofusion process, the two pipe ends are inserted into the electrofusion fitting. The integrated wires of the continuous coil act like a heating element when an electric current is passed through it. It is mainly used for socket and saddle-type joints. The final assembly can also include internal components such as steel stiffeners, o-rings, gripping collets, and other components depending upon the design. A wide variety of coupling configurations are available including tees, ells, caps, reducers, and repair couplings.

There are many different approaches to manufacturing the electrofusion component. As a result every major pipe supplier offers its own electrofusion system, for example Simona offers its “Simofuse”, George Fisher its “Elgef Plus” and Rehau “Fusapex”. However, all the electrofusion methods of joining PE plastic fittings have one common idea: the simple and rapid method of joining plastics. Electrofusion can be considered as the most advanced joining process for PE pipes.

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