STANDARD TYPES OF FLANGES
1：WELDING NECK FLANGE
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.
2：LONG WELDING NECK
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.
3：SLIP ON FLANGE
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.
Slip-on flanges are also defined "Hubbed Flanges" and they are easy to recognize due to their slim and compact shape.
4：THREADED FLANGE
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.
5：SOCKET WELD FLANGE
Socket weld flanges are connected to pipes using a single fillet weld executed on the outer side of the flange (different from the slip-on flange type that requires two welds).
According to ASME B31.1, to execute a flanged connection using a socket weld flange, the pipe shall be at first inserted in the socket of the flange until it reaches the bottom of the flange, then it should be lifted by 1.6 mm and finally welded.
This gap shall be left to allow proper positioning of the pipe inside the flange socket after the solidification of the weld.
Socket Weld Flanges are used for small-size and high-pressure piping that do not transfer highly corrosive fluids.
This due to the fact that these flange types are subject to corrosion in the gap area between the end of the pipe and the shoulder of the socket.
Their static strength of socket weld flanges is similar to slip-on flanges', but their fatigue strength is higher due to the presence of a single, instead of double, fillet weld.
6：LAP JOINT FLANGE
Lap joint flanges feature a flat face and are always used in conjunction with a stub end.
A lap joint flange slips over the pipe and seats on the back of the stub end and the two are kept together by the pressure of the bolts.
The use of lap joint flanges in combination with stub ends is a cost-effective solution for stainless steel or nickel alloy pipelines, as the material of the lap joint flange can be of a lower grade (generally carbon steel) than the material of the stub end (which has to match the pipe grade, as in contact with the conveyed fluid).
This arrangement, therefore, has these two advantages:
A:reduces the overall cost of the pipeline's flanged joints, as the use of higher grade materials is minimized
B:bolting operations are simplified, as the lap joint flange can be rotated around the pipe to help with bolts alignment.
7:BLIND FLANGE
Contrary to all the flange types seen above, blind flanges do not have a center hole, and are used to blind or seal a pipeline, a valve/pressure vessel and block the flow of the fluid.
Blind flanges have to withstand remarkable mechanical stress due to the system pressure and the required bolting forces.
Blind flanges allow easy access to the pipeline, as they can be easily unbolted to let the operator execute activities inside the terminal end of the pipe (this is also the reason why the blind flange type is used as manhole for pressure vessels, at times).
It is maybe interesting to observe that, while this flanges type is easier to manufacture, they are sold at a premium average cost per kilogram compared to the other flange types.
