ROSSI FLANGE TYPES
Regardless to Flange Type, A flange is used to join pipe, valves, or a vessel within a system. Two commonly used flange standards are ANSI (American National Standards Institute) and BS 1560 (British Standards). A third standard, API 6A (American Petroleum Institute) specifies flanges for Wellhead and Christmas Tree Equipment.
Flanges of different standards are not normally joined. If necessary to do so, engineering advice must first be sought to ensure the compatibility of the mating flanges.
Weld-Neck Flange (WN)
This flange is normally used for high-pressure, cold or hot temperatures and hazardous fluids.
Slip-on Flanges (SO)
The pipe slips into the flange prior to welding both inside and outside to prevent leaks. SO typically is used on low pressure, low hazard services such as fire water, cooling water, etc.
Lap Joint Flanges
The lap joint flange has a curved radius at the bore and face to accommodate a lap joint stub end. The lap joint flange and stub end assembly are normally used in systems requiring frequent dismantling for inspection
Socket Weld Flange
This flange is similar to slip-on flange, except they have a bore and a counter bore. The counter bore is slightly larger than the OD of the matching pipe, allowing the pipe to be inserted. A restriction is built into the bottom of the bore, which acts as a shoulder for the pipe to rest on, and has the same ID of the matching pipe. The flow is not restricted in any direction.
Threaded Flange
This type of flange is used in systems not involving temperature or stresses of any magnitude.
Blind Flange
A blind flange, which has no bore, and is used to close ends of piping systems. A blind flange also permits easy access to a line once it has been sealed. The blind flange is sometimes machined to accept a pipe of the nominal size to which the reduction is being made. The reduction can be either threaded or welded
Long Weld-Neck Flange
This is a special flange used for nozzles on pressure vessels. The hub is always straight, and the hub thickness is greater than the diameter of any piping that may be bolted to the flange.
FLANGE BORE
Weld-Neck and Socket-Weld flanges are drilled (machined) with the wall thickness of the flange having the same dimensions of the matching pipe. The lighter the pipe is, the larger the bore; conversely, the heavier the pipe, the smaller the bore.
Other flanges are drilled to match the outside diameter of pipe sizes, and do not have bore markings to indicate a pipe schedule.
Ring Type Joint (RTJ)
Typically found on the most severe duties, for example high pressure gas pipework. 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.
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. Note that the surface finish on the flange facing depends on the type of gasket being used.
Flat Face (FF)
Sealing is also 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 arduous of 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.
NOTE: Both ANSI B16.5 and BS 1560 specify Flat Face Flanges and Raised Face Flanges as well as RTJ Flanges. API6A is specific to RTJ flanges only.