Types of Fittings
Pipe fittings are distinguished based on the connection types and functions they perform.
Fittings are attached to pipes via a number of different connection methods, each with its own conveniences and advantages.
Compression fittings are fittings which connect pipees using compression on a gasket, ring, or ferrule. The compression is typically made by tightening a nut onto the fitting over the piping and ferrule, compressing, and securing the piping inside. Standard compression fittings do not require tools to assemble, making them convenient for quick, simple installation. However, they cannot withstand very high pressures and do not have as much flexibility as soldered fittings, making them less functional in systems with vibration, thermal cycling, and other dynamic forces. They also come in a limited number of materials (most commonly brass) and are best suited for metal on metal connections.
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Bite-type fittings are compressive fittings with a sharpened ferrule that "bites" the pipe when compressed and provides the seal. Bite-type fittings, like standard compressive fittings, require no special tools to assemble, but provide a stronger, higher pressure connection.
Mechanical grip fittings are two-ferrule assemblies. The back ferrule grips the piping while pressing up against the front ferrule, which spring-loads the front ferrule and creates a seal between the piping and fitting body. These fittings can be reassembled multiple times without damaging components or piping. They have good resistance to mechanical vibration.
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Flare fittings consist of a body with a flared or coned end. Special flaring tools are used to install the pipe inside the flared end, providing a deep seal. Flare fittings can handle higher pressures and a wider range of operating parameters than standard compression fittings.
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End fittings provide specific surfaces for connecting pipes.
Clamp ends are fittings which allow pipes to be clamped to the connection.
Plain ends are fittings which allow pipes to be connected by adhesive, solder, welding, or other means.
Flange fittings are rims, edges, ribs, or collars with flush surfaces perpendicular to the attached pipe. These surfaces are joined and sealed via clamps, bolts, welding, brazing, and/or threading. For more information on flanges, visit the Pipe Flanges Selection Guide on GlobalSpec.
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Threaded fittings have screw threads (built-in grooves) on their inner (female) or outer (male) surfaces designed to accept piping with matching threads. Threads which provide a simple connection but no seal are called straight threads. Tapered threads are designed to provide a tight seal for gases or fluids under pressure. Seal reliability can be improved by adding a coating or seal tape (Teflon). Especially precise threads are called "dry fit", meaning they seal without the need for an additional sealant, which is important in applications where sealant addition could cause contamination or corrosion.
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The thread size is measured and based on the inside of the pipe. Common thread size standards include NPT (National Pipe Thread) and BSP (British Standard Pipe), though other standards exist and usage often varies by country and industry. Each standard corresponds to a particular number of threads per inch (TPI). Plumbingsupply.com provides an excellent overview on pipe thread sizing. Below is a chart depicting nominal thread sizes and their corresponding values.