A flange is a protruded ridge, lip, or rim, either external or internal, that serves to increase strength, for easy attachment/transfer of contact force with another object, or for stabilizing and guiding the movements of a machine or its parts. Flanges are used to connect pipes with each other, to valves, to fittings, and to specialty items such as strainers and pressure vessels. They are the second most used joining method after welding and add flexibility when maintaining piping systems by allowing for easier disassembly and improved access to system components.

Flanges can come in multiple materials as well as types, and they are most commonly made up of carbon steel but can also be stainless steel, cast iron, aluminum, brass, bronze, plastic, etc. . A typical flanged connection is comprised of three parts: pipe flanges, gasket, and bolting. Flanges can be bolted together between pipes, and sealing is often completed with the use of gaskets or other methods.

There are different types of flanges, including:

• Weld Neck Flanges: Used for terminating or connecting pipes, their long necks are butt-welded to a pipe. The flanges bore matches that of the pipe, reducing turbulence and erosion.

• Slip-on Flanges: Featuring a two-piece design, lap joint flanges require butt welding of the stub end to the pipe or fitting with the use of a backing flange to create the flanged connection. This design makes this style popular for use in systems with limited physical space or systems that require frequent dismantling and maintenance.

• Socket-Weld Flanges: Ideal for smaller pipe diameters in low-temperature and low-pressure scenarios, socket-weld flanges feature a connection in which you place the pipe into the flange and then secure the connection with a single multi-pass fillet weld.

• Threaded Flanges: Also known as a screwed flange, this style has a thread inside the flange bore that fits with the matching male thread on the pipe or fitting. The threaded connection means you can avoid welding in many use cases.

• Lap Joint Flanges: Slides over the pipe and used with a stub end. Also known as loose-ring flanges and back-up flanges, these flanges are used on piping fitted with lapped pipe or with lap joint stub ends. With the stub end, the lap joint flange is typically used in systems requiring frequent dismantling for inspection and cleaning. Another advantage is its ability to swivel and align with bolt holes.

Flanges are joined by bolting, and sealing is often completed with the use of gaskets or other methods. Mechanical means to mitigate effects of leaks, like spray guards or specific spray flanges, may be included.