The Weld-Neck flange is butt-welded to the pipe. Weld-Neck flanges are typically used on extreme duties such as high pressures and/or hazardous fluids. The butt weld may be inspected by radiography or ultrasonic as well as Magnetic particle Inspection (MPI) or Dye Penetrant Inspection (DPI) during fabrication. There is therefore a high degree of confidence in the integrity of the weld. A butt weld also has good fatigue performance and its presence does not induce high local stresses in the pipe work.
Slip-On Weld Flange
Used typically on low pressure, low hazard services such as fire water, cooling water, etc. The pipe is “double-welded” both to the hub and the bore of the flange and again radiography is not practical. MPI or DPI will be used to check the integrity of the weld. Where specified, the Slip-On flange will be used on pipe sizes greater than 1.5 inches with a preference for the Socket Weld flange for sizes up to and including 1.5 inches.
Threaded flange is similar to that of Slop-On flanges except the bore of threaded pipe flange has tapered threads. Threaded flanges are often used for small diameter, high pressure requirements. The benefit of threaded flanges is that it can be attached to piping without welding.
Socket Weld Flange
Socket weld flanges are often used on high pressure, hazardous duties but will be limited to a nominal pipe size (NPS) of 1.5 inches. The pipe is fillet welded to the hub of the SW flange. Radiography is not practical on the fillet weld and correct fit-up and welding is therefore crucial. The fillet weld will normally be inspected by MPI or DPI.
Lap Joint Flange
Comprises of a hub or “stub end” welded to the pipe and a backing flange or capped flange which is used to bolt the joint together. This type of flanged joint is typically found on cunifer (Cu/Ni/Fe) and other high alloy pipe work. An alloy hub with a galvanized steel backing flange is cheaper than a complete alloy flange. The flange has a raised face and sealing is with a flat gasket such as a compressed asbestos fiber (CAF) sheet gasket.
Blind flange is round plate without a bore and used to blank off the ends of piping, valves, and nozzles on pressure vessel openings. Blind flange is used for testing the flow of gas or liquid through a pipe or vessel and also allow easy access in case work must be done inside the piping or vessel.
Swivel Ring Flange
Similar to the 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 found on subsea services where the swivel ring facilitates flange alignment. The flange is sealed using a Ring Type Joint (RTJ) metal gasket.
Types of Flange Facings
Raised Face (RF)
Sealing on a RF flange is by a flat non-metallic gasket (or a flat metallic gasket for special applications), which fits within the bolts of the flange. The facing on a RF flange has a concentric or phonographic groove with a controlled surface finish. If the grooves are too deep (or a rough surface finish), then high compression is required to flow the relatively soft gasket material into the grooves. Too shallow (exceptionally smooth surface finish) and again high compression is required as a leak path then becomes more possible. It is important to always check the flange surface finish for imperfections which would make sealing difficult. A radial groove for example is virtually impossible to seal against.
The surface finish on the flange facing depends on the type of gasket being used.
Flat Face (FF)
Sealing is by compression of a flat non-metallic gasket (very rarely a flat metallic gasket), between the phonographic/concentric grooved surfaces of the mating FF flanges. The gasket fits over the entire face of the flange. FF flanges are normally used on the least extreme duties such as low pressure water drains and in particular when using cast iron, cunifer or bronze alloy, where the large gasket contact area spreads the flange loading and reduces flange bending.
Both ANSI B16.5 and BS 1560 specify Flat Face Flanges and Raised Face Flanges as well as RTJ Flanges. API 6A is specific to RTJ flanges only.
Ring Type Joint (RTJ)
Typically found on the most severe duties, for example high pressure gas pipe work. Ring type metal gaskets must be used on this type of flange facing.
- RTJ to API 6A Type B, BS 1560 and ANSI B16.5: The seal is made by metal-to-metal contact between the gasket and the flange groove. The faces of the two opposing flanges do not come into contact and a gap is maintained by the presence of the gasket. Such RTJ flanges will normally have raised faces but flat faces may equally be used or specified.
- RTJ to API 6A Type BX: API 6A Type BX flanges seal by the combined effect of gasket compression and flange face-to-face contact and will therefore always have raised faces. The flanges also use special metal ring joints. A Type BX flange joint which does not achieve face-to-face contact will not seal and should not be put into service.