A pipe flange connects piping and components in a piping system by use of bolted connections and gaskets. Most commonly used flanges are weld neck flange, slip on flange, blind flange, socket weld flange, threaded flange and lap joint flange (RTJ Flange). This type of connection in a pipe flange allows for ease of disassembly and separation for repair and regular maintenance. Most common specification for carbon steel and stainless steel flange is ANSI B16.5 / ASME B16.5.
A slip-on flange is connected to the pipe or the fittings by two fillet welds, one executed inside and one outside the cavity of the flange.
The bore size of a slip-on flange is larger than the outside diameter of the connecting pipe, as the pipe has to slide inside the flange to be connected by the execution of a fillet weld.
Slip-on flanges are also defined Hubbed Flanges and they are easy to recognize due to their slim and compact shape.
A welding neck flange (WN) features a long tapered hub that can be welded with a pipe.
This flange type is used, normally, in high-pressure and high/low temperatures applications that require an unrestricted flow of the fluid conveyed by the piping system (the bore of the flange matches with the bore of the pipe).
The absence of pressure drops prevents negative effects as turbulence and erosion/corrosion of the metals in the proximity of the flanged joints.
The tapered hub allows a smooth distribution of the mechanical stress between the pipe and the weld neck flange and facilitates the execution of radiographic inspections to detect possible leakages and welding defects.
The dimension of the flange (NPS and the pipe schedule) shall match the dimension of the connecting pipe.
Long weld neck flanges (LWN) are similar to weld neck flanges, with the exception that the neck (tapered hub) is extended and acts like a boring extension.
Long weld neck flanges are generally used on vessels, columns or barrels. These flange types are available also in the heavy barrel (HB) and equal barrel (E) types
Threaded flanges are joined to pipes by screwing the pipe (which has a male thread, generally NPT per ASME B1.20.1) onto the flange, without seam welds (in certain cases, though, small welds are applied to increase the strength of the connection).
Threaded flanges are available in sizes up to 4 inches and multiple pressure ratings, however, they are used, mostly, small size piping in low pressure and low-temperature applications, like water and air utility services.
Threaded flanges are also a mandatory requirement in explosive areas, such as gas stations and plants, as the execution of welded connections in such environments would be dangerous.
