Types of flange, their material and standards

Flanges are used to mechanically connect two pipes together, or a pipe to be mechanically connected to a valve, instrument component or nozzle of equipment. The principle of a flange is to use a mechanical force exerted by the bolts to per-load the gasket sufficiently so that when internal pressure is applied, there is enough contact stress between the flanges and gasket to maintain a seal.

The flange itself needs to be connected to the pipe. This is usually achieved by welding, or though threaded connections. All flanges are sized according to their nominal bore size. For the majority of pipe sizes, this designation does not represent either their actual outside diameter or inside diameter

Types of flange

Slip-On Flanges: - The Slip-on type flanges are attached by welding inside as well as outside. These flanges are of forged construction. The slip-on flange has a low hub because the pipe slips into the flange prior to welding. It is welded both inside and out to provide sufficient strength and prevent leakage. They are ideal for low pressure application

Socket Weld flanges: - The Socket Weld flanges are welded on one side only. These are used for small bore lines only. These flanges have a bore and counter bore dimension. The counter bore is slightly larger than the O.D of the matching pipes, allowing pipes, allowing the pipe to be inserted into the flanges. The diameter of the smaller bore is the same as the I.D of the matching pipe. During assembly, the pipe is inserted in the larger diameter bore and is welded in place. This type of flange is often used with high-pressure systems that use smaller diameter pipes. These flanges application is in Chemicals, Hydraulics and Steams plants

Screwed Flanges: - The Screwed-on flanges are used on pipe lines where welding cannot be carried out. The threaded flange is similar to slip-on flanges, but the difference is that the bore is threaded. It can be fitted to the pipe without welding. It is use in low-pressure services at ambient temperatures, and in highly explosive areas where welding creates a hazard

Lap Joint Flanges: - The Lap Joint flanges are used with stub ends. The stub ends are welded with pipes & flanges are kept loose over the same. The lap joint flange is practically identical to a slip-on flange except it has a larger radius at the intersections of the bore and flange face. It is used with their associated stub end. The stub end is welded to the pipe and the lap joint then works as a backing ring. The advantage of this flange is that the bolt holes can be aligned with the matching flange after the weld has been completed. This flange is not suitable for areas with thigh external or dynamic loads.

Welding Neck Flanges: - These are the most common type of flanges used for high-pressure application. It is designed to transfer stresses concentrations of the base of the flanges. These are recognized by their long tapered hub. The hub provides an important reinforcement to the flange itself and acts to reduce rotation of the flange at bolt up. The smooth transition between flange and the hub combined with the strength of the butt weld joint, allows the flanges to be used in extreme conditions of cyclic loading, bending and temperature fluctuations. The Welding neck flanges are attached by butt welding to the pipe. These are used mainly for critical services where the weld joints need radio graphic inspection.

Blind Flanges: - The Blind flanges are used to close the ends which need to be reopened for modification, servicing or extending the header length for increasing the capacity. The blind flange is a flange without an opening or bore. It is used to close off the ends of a piping systems and/or a pressure vessel opening. They may often be supplied with NPT fittings to allow pressure test connections to be fitted.

Reducing Flanges: - The reducing flanges are used to connect between larger size pipes and smaller pipes sizes without using a reducer. In case of reducing flanges, the thickness of flange should be that of the higher diameter.

Integral Flange: - Integral flanges are those, which are cast along with the piping component or equipment.

Pressure temperature ratings of flanges

  •  Flange  Facing: There are four types of facing.
I.                   Raised Face
It is specified up to 600 (psi) rating pipe class.
II.                Ring Joint
It is specified from 900 rating (psi) & above pipe class.
III.             Flat Face
It is only used for 150 (psi) rating pipe class.
It is specified for utility fluids like Cooling Water and Low Pressure Nitrogen.
IV.             Tongue & Groove
Its use is mainly dependent upon the nature of fluid to be handled.
          It is specified to handle extremely hazardous fluids like Liquid Ammonia.
·         Design and Dimension Standard of Flanges: The dimensional standards generally used are
I.       ANSI/ASME B16.5 for size upto 24”
II.    ANSI/ASME B16.47 series A & B for size above 24”. Series B specifies compact design & is used when space and cost are the main constraints.
  • Flange Material: Flanges are manufactured from following forged materials.
o        Carbon Steel – ASTM A 105 (used most of the times)
ASTM A 181 (it is obsolete now)
o        Low Temperature Carbon Steel—ASTM A 350 Gr. LF 2
o        Low Alloy Steel – ASTM A 182 Gr. F 11 (generally used)
ASTM A 182 Gr. F 1/F22/F9
o        Stainless Steel – ASTM A 182 Gr. F 304
ASTM A 182 Gr. F 304 L
ASTM A 182 Gr. F 316
ASTM A 182 Gr. F 316 L
ASTM A 182 Gr. F 321
ASTM Sec. II A defines ferrous material. It gives detail properties of Ferrous Material
o        Chemical Analysis
o        Physical Properties (tensile strength,Yield strength, hardness, etc.)

Pressure Class (Rating)
Rating is maximum allowable non-shock working gauge pressure. There are 150, 300, 600, 900, 1500, 2500 ratings. To select a pressure class the following two steps are followed.
I.       Decide the group of material from Table 1A of ASME 16.5
II.      Refer Table 2 of ASME 16.5 for design condition.
Spectacle Blind
It is not exactly a flange but it is kept between two flanges and is used for the temporary isolation of a line.
Design & Dimension Standard----LS 423-06
Material----Same as Flange
Facing----Same as Flange
Generally it is used up to 10” & above 10” it is used in two separate pieces. One is known as Slip Plate (Blind Part) & second is known as Slip Ring (Hollow Part).

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