The main types of flanges are welding neck, long welding neck, lap joint, threaded, and blind flanges, as well as slip-on, socket weld, and in addition to these standard flanges, there are some special flanges, rotating flanges, expansion/reducer flanges, and hole flanges.

1. Standard Flange

The type of flange used in piping applications mainly depends on the strength required for the flange joint. Using flanges or welded connections can simplify operation and maintenance (flanged connections can be quickly and easily disassembled).

2. Welding Flange

Welding flanges ("WN") have a long tapered hub that can be welded to the pipe. This type of flange is typically used in high-pressure and high-temperature/low-temperature applications. These applications require the fluid transported by the piping system to flow unrestricted (the flange hole matches the pipe hole), with no pressure drop to prevent negative effects, and turbulence and corrosion can occur in the metal near the flange joint. The tapered hub allows mechanical stress to be evenly distributed between the pipe and the welding neck flange, and it aids in radiographic inspection to detect possible leaks and welding defects. The size of the flange (NPS and pipe surface) should be consistent with the size of the connecting pipe.

3. Long Welding Neck Flange

The long welding neck flange ("LWN") is similar to the welding neck flange, but the neck (tapered hub) extends and serves as a boring extension. Long welding neck flanges are typically used for vessels, columns, or barrels, and these types of flanges can also be used for heavy barrels (HB) and the same barrel (E) types.

4. Slip-On Flange

Slip-on flanges are connected to the pipe or fitting by two fillet welds, one used internally and the other externally on the flange cavity. The hole size of the slip-on flange is larger than the outer diameter of the connecting pipe because the pipe must slide into the flange to be connected by the fillet weld. Slip-on flanges are also defined as "hub flanges." Due to their thin and compact shape, they are easy to identify. In the long run, flange connections formed by slip-on flanges are more brittle than welded flange connections (under similar conditions). Welded flanges have a tapered hub that is not present in slip-on welding flanges. The mechanical stress between the pipe and the flange can be distributed more evenly in the welded joint, requiring only one welding area instead of two (socket welding flanges). Another advantage of welding neck flanges is that they can be connected to pipes and fittings, while socket welding flanges are only suitable for pipes.

5. Threaded Flange

Threaded flanges connect to pipes by screwing the pipe threads onto the flange without the need for seam welding. The maximum size of threaded flanges is 4 inches, and they come with various pressure ratings, but they are mainly used in low-pressure and low-temperature applications for small pipes, such as water and air facilities.

6. Blind Flange

Blind flanges have no center hole, and they can seal pipe valves and prevent fluid flow. Due to system pressure and the required bolt connection force, blind flanges must withstand significant mechanical stress. Blind flanges can be easily installed in pipes and are easy to disassemble, allowing construction personnel to install or maintain them at the end of the pipe (which is why blind flanges are used as inspection holes for pressure vessels).