A pipe fitting is defined as a part used in a piping system to change direction or function, which is mechanically joined to the system. Probably the simplest way to achieve this would be to bend the pipe in the direction required, but this process will stretch and thin the outer wall whilst thickening and wrinkling the inner wall. This results in flow resistance and accelerated wall erosion. A second method sometimes used is a miter joint, where pipes are cut to the correct angle and welded together to achieve the desired change.
A piping system using buttweld fittings has many advantages over other forms.

• Welding a fitting to the pipe means it is permanently leakproof.
• The continuous metal structure formed between pipe and fitting adds strength to the system.
• Smooth inner surface and gradual directional changes reduce pressure losses and turbulence and
minimize the action of corrosion and erosion.
• A welded system utilizes a minimum of space.