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 Assembly says:In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16″ (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.The purpose for the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal. The image shows you the X measure for the expansion gap.The disadvantage of socket weld flange is right the gap, that must be made. By corrosive products, and mainly in stainless steel pipe systems, the crack between pipe and flange can give corrosion problems. In some processes this flange is also not allowed.
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 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.
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 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.