Pipe Fittings

Pipe Fittings are a general term for components used in piping systems for connection, branching, reducing diameter, changing direction, and sealing. There are various ways to classify pipe fittings, usually by connection type, function and application, material, and manufacturing standard. The detailed classification is as follows:

I. Classification by Connection Type

This is the most common classification, determining how fittings are assembled with pipes.

Welded Fittings

Features: Both ends of the fitting are beveled and connected to the pipe by welding (e.g., TIG welding, arc welding).Advantages: Good sealing, high strength, resistant to high pressure and temperature, suitable for long-distance transmission and industrial pipelines.Common types: Welded elbow, welded tee, reducer, cap, etc.

Threaded Fittings

Features: Threads are machined on the inner or outer surface of the fitting, connected to the pipe by screwing.Advantages: Easy installation and disassembly, no welding equipment required.Disadvantages: Relatively low pressure resistance, prone to leakage at threads, not resistant to high temperatures.Common types: Female elbow, male coupling, tee, union.Applications: Home decoration, low-pressure water supply, gas pipelines.

Flanged Fittings

Features: One or both ends of the fitting have a flange disc, connected by bolting two flange faces with a gasket in between.Advantages: Easy disassembly, convenient for maintenance, good sealing.Disadvantages: Relatively high cost, large size and space occupation.Common types: Flanged elbow, flanged tee, flanged spool.Applications: Large-diameter pipelines, high-pressure pipelines, areas requiring frequent disassembly (e.g., both ends of valves).

Socket Weld / Socket Fittings

• Socket weld: The fitting has a socket; the pipe is inserted and then welded. Commonly used in small-bore, high-pressure, high-temperature piping systems (e.g., instrument tubing).
• Rubber ring socket: Common in plastic pipes (PVC, PE) or ductile iron pipes, sealed by elastic deformation of rubber gaskets, with very fast installation.

Press / Clamp Fittings

• Press type: Common in thin-walled stainless steel pipes, using special tools to crimp the fitting and pipe together.
• Grooved clamp type: Grooves are formed at the ends of pipes and fittings, sealed with rubber gaskets, fastened with external clamps and bolts. Widely used in fire pipelines for extremely fast installation.

II. Classification by Function and Shape

Classified according to the role of the fitting in the piping system.

Direction Changing Fittings

• Elbow: Used to change the direction of the pipeline.
  • 45° Elbow: For small-angle direction change.
  • 90° Elbow: For vertical direction change.
  • 180° Elbow (Return Bend): For U-turn bends.
  • Special: Long Radius (LR) and Short Radius (SR) elbows.

Connection / Extension Fittings

• Coupling: Used to connect two pipes of the same diameter for extension.
• Nipple: Short pipe threaded at both ends.
• Union: Composed of three parts, used for easily detachable straight-line connections.

Diameter Changing Fittings

• Reducer: Commonly known as a “size head”, used to connect two pipes of different diameters.
  • Concentric Reducer: Centers are aligned, often used in vertical pipelines.
  • Eccentric Reducer: Centers are offset (usually flat bottom or flat top), often used in horizontal pipelines to prevent air or liquid pockets.

Branch Fittings

• Tee: Used to branch a lateral pipe from the main line, forming a “T” shape.
  • Straight Tee: Three outlets of the same diameter.
  • Reducing Tee: Branch diameter differs from the main line.
• Cross: Used to split into two branches, forming a cross shape, commonly used for fluid distribution.

Sealing / Plugging Fittings

• Cap: Welded at the pipe end to close the pipeline.
• Plug: Threaded plug for blocking pipe openings.
• Blind Flange: Flange-type blank plate for closing flange ends.

Special Function Fittings

• Flange: Often listed separately despite being a connecting component.
• Reinforcing Pad: Welded at openings to increase structural strength.
• Expansion Joint: Used to absorb displacement caused by thermal expansion and contraction of pipes.
• Strainer: Used to filter impurities in fluids.

III. Classification by Material

Fitting materials are usually matched with pipe materials to ensure consistent weldability and corrosion resistance.

• Metal Fittings: Carbon steel (20#), stainless steel (304/316), alloy steel, cast iron, copper, aluminum, etc.
• Plastic Fittings: PVC, PE, PPR, ABS, PB, etc.
• Composite Fittings: Steel-plastic composite, aluminum-plastic composite, etc.

IV. Classification by Manufacturing Standard

Different countries and industries have different manufacturing standards, which determine the dimensions, wall thickness, and tolerances of fittings.

• GB (Chinese National Standard): e.g., GB/T 12459
• ASME/ANSI (American Standard): Widely used internationally, e.g., B16.9
• EN/DIN (European/German Standard)
• JIS (Japanese Standard)

Summary

In actual engineering, describing a pipe fitting usually combines the above classifications, for example:DN100, 90° Long Radius, Sch.40, 20# Carbon Steel Welded Elbow