In this article, we illustrate the different types of flanges available on the market (ASME) and their key features.
WELD NECK FLANGE
A weld neck flange (“WN”) has a long tapered hub that can be welded with a pipe or a fitting and is normally used in high-pressure and high/low temperatures applications. The dimension of the flange (NPS and schedule) shall match the dimension of the pipe or the dimension of the fitting to be connected. Pipes and WN flanges are welded together by a single full penetration V-shaped butt weld. Welding neck flanges are also available with extended collars (called: long weld neck flange).
LONG WELD NECK FLANGE
Long weld neck flanges (“LWN”) are similar to weld neck flanges, with the exception that the neck is longer. LWN flanges are used as noozles for columns or barrels. Long welding neck flanges are available also in the heavy barrel (HB) and equal barrel (E) types.
SLIP ON FLANGE
A slip-on flange is connected with pipes or fittings by two fillet welds, inside and outside the flange. The diameter of a slip-on flange is larger than the diameter of the connecting pipe so the flange can slide onto the pipe and fit it smoothly. The lifetime of a slip on the flange is shorter than the lifetime of a weld neck flange, under the same mechanical stress conditions. While fittings can be welded directly on WN flanges, only pipes can be welded directly to a slip-on flange. Slip-on flanges are also defined “Hubbed flanges” and they are easy to recognize due to their thin look.
Threaded flanges are joined to pipes by screwed connections, without welding activities (in certain cases, though, small welds are applied to increase the strength of the connection). Threaded flanges are available in most sizes and pressure ratings, however, they are practically seen only for smaller pipe sizes (below 2 inches) having a not too thin wall thickness. Threaded flanges are largely used in hazardous environments, i.e. in presence of combustible fluids, as steel welding would be dangerous in such conditions.
SOCKET WELD FLANGE
Socket weld flanges are connected to pipes using a fillet weld which is executed on the outer of the flange. Socket Weld Flanges are used for small-size and high-pressure piping. Their static strength is equal to slip on flanges, but their fatigue strength is stronger than double-welded slip-on flanges.
LAP JOINT FLANGE
Lap joint flanges do not have a raised face and are used in conjunction with stub ends. Lap joint flanges are similar to slip-on flanges except for the radius at the crossing of the flange face and the bore to accommodate the flanged portion of the stub end. These flanges are intended to slip on the pipe and are not welded or fastened to it by other means. The pressure of bolts is passed to the gasket by the pressure of the flange against the back of the stub end. Lap joint flanges used in connection with stud ends are cost-saving solutions, especially for stainless steel or higher grades pipelines (exotic materials, such as nickel alloys).
Blind flanges do not have a center hole, differently from all other types of flanges seen above, 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 system pressure and bolting. 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.
SWIVEL RING FLANGE
As with the composite lap-joint flange, a hub will be butt welded to the pipe. A swivel ring sits over the hub and allows the joint to be bolted together. Swivel-ring flanges are normally used for subsea service as the rotating ring facilitates the alignment of the flanges during the bolting operations. The flange is sealed using a ring-type joint (RTJ) metal gasket. For more details about swivel flanges, read this article.
Video: Types of flanges commonly used in piping (source: Technical Piping)