Flanges are used when the piping joint needs dismantling. These are used mainly at equipment’s, valves and specialties. In certain pipelines where maintenance is a regular feature, breakout flanges are provided at definite intervals. A flanged joint is composed of three separate and independent although interrelated components; the flanges, the gaskets and the bolting. Special controls are required in the selection and application of all these elements to attain a leak-proof joint. Classification of flanges is done in several alternate ways as follows;

Based On Pipe Attachment

Flanges can be classified based on the method of attachment to the piping as below;

  • Slip On Flange –The Slip On type flanges are attached by two fillet welds, inside as well as outside the flange. The calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges, and their life under fatigue is about one-third that of the latter. Normally, these flanges are of forged construction and are provided with hub. Sometimes, these flanges are fabricated from plates and are not provided with the hub.The disadvantage of the flange is that a combination of flange and elbow or flange and tee is not possible because named fittings have not a straight end that complete slid in the Slip On flange.
  • Socket Weld Flange –The Socket weld flanges are attached by only one fillet weld, only on outside, and are not recommended for severe services. These are used for small-bore lines only. Their static strength is equal to Slip On flanges, but their fatigue strength is 50% greater than double-welded Slip On flanges. The thickness of connecting pipe should be specified for this type of flanges to ensure proper bore dimension.In socket weld flange, before welding, a space must be created between flange or fitting and pipe. ASME B31.1 Preparation for Welding (E) Socket Weld
  • Screwed Flange –The Screwed or Threaded flanges are used on pipe lines where welding cannot be carried out. A threaded flange or fitting is not suitable for a pipe system with thin wall thickness, because cutting thread on a pipe is not possible. Thus, thicker wall thickness must be chosen.ASME B31.3 Piping Guide says:
    Where steel pipe is threaded and used for steam service above 250 psi or for water service above 100 psi with water temperatures above 220° F, the pipe shall be seamless and have a thickness at least equal to schedule 80 of ASME B36.10.Socket welding and threaded flanges are not recommended for service above 250°C and below -45 C.
  • Lap Joint Flange –The Lap joint flanges are used with stub ends when piping is of a costly material. For example, in a stainless steel pipe system, a carbon steel flange can be applied, because flange will not come in contact with the product in the pipe. The stub ends will be butt-welded to the piping and the flanges are kept loose over the same. The inside radius of these flanges is chamfered to clear the stub end radius.These flanges are nearly identical to a Slip On flange with the exception of a radius at the intersection of the flange face and the bore to accommodate the flanged portion of the Stub End. Their pressure-holding ability is little, if any, better than that of Slip On flanges and the fatigue life for the assembly is only one tenth that of Weld Neck flanges. Thus this flange connections are applied in low-pressure and non critical applications.
  • Weld Neck Flange –Welding Neck Flanges are easy to recognize as the long tapered hub, that goes gradually over to the wall thickness from a pipe or fitting. The long tapered hub provides an important reinforcement for use in several applications involving high pressure, sub-zero and / or elevated temperatures. The smooth transition from flange thickness to pipe or fitting wall thickness effected by the taper is extremely beneficial, under conditions of repeated bending, caused by line expansion or other variable forces.These flanges are bored to match the inside diameter of the mating pipe or fitting so there will be no restriction of product flow. This prevents turbulence at the joint and reduces erosion. They also provide excellent stress distribution through the tapered hub.The Weld neck flanges are attached by butt-welding to the pipes. These are used mainly for critical services where all the weld joints need radiographic inspection. While specifying these flanges, the thickness of the welding end also should be specified along with flange specification.
  • Blind Flange –Blind Flanges are manufactured without a bore and used to blank off the ends of piping, Valves and pressure vessel openings.From the standpoint of internal pressure and bolt loading, blind flanges, particularly in the larger sizes, are the most highly stressed flange types.However, most of these stresses are bending types near the center, and since there is no standard inside diameter, these flanges are suitable for higher pressure temperature applications.
  • Reducing Flange –The Reducing flanges are used to connect between larger and smaller sizes without using a reducer. In case of reducing flanges, the thickness of the flange should be that of the higher diameter. These flanges normally come in blind, slip-on, threaded and weld neck flanges. They are available in all pressure classes and provide a good alternative to connecting two different sizes of pipe. This type of flange should not be used if an abrupt transition would create unwanted turbulence, such as at a pump.
  • Integral Flange –Integral flanges are those, which are cast along with the nozzle neck or the vessel or pipe wall, butt-welded thereto, or attached by other forms of arc or gas welding of such a nature that the flange and the nozzle neck or vessel or pipe wall is considered to be the equivalent of an integral structure. In welded construction, the nozzle neck or the vessel or pipe wall is considered to act as a hub. Thickness of integrally cast flanges and welded on flanges differ in certain sizes.

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.

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