Petrosadid: Flange

A pipe flange is a disc, collar or ring that attaches to pipe with the purpose of providing increased support for strength, blocking off a pipeline or implementing the attachment of more items. They are usually welded or screwed to the pipe end and are connected with bolts. A gasket is inserted between the two mating flanges to provide a tighter seal. These flanges are either custom with dimensions provided by the customer or they are manufactured according to published specifications. Several organizations and associations have published specifications that provide dimensional information as well as pressure specifications at different temperatures.

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Welding Neck

This group of flanges is Designed with a hub on the backside tapering to a diameter that will match the pipe to which it will be welded. These flanges are bored to match the inside diameter of the mating pipe so there will be no restriction of product flow. Generally machined from a forging, these flanges are typically butt welded to a pipe. The rim has a series of drilled holes that permit the flange to be affixed to another flange with bolts. Such flanges are suitable for use in hostile environments that have extremes of temperature, pressure or other sources of stress.[4] The resilience of this type of flange is achieved by sharing the environmental stress with the pipe with which it is welded. This type of flange has been used successfully at pressures up to 5,000 psi.

1. Weld Neck flange
2. Butt Weld
3. Pipe or Fitting
Long Weld Neck

Long Weld Necks are shaped like a slip on flange with a piece of pipe already attached but are formed and machined in a one-piece construction. As the name suggests, Long neck flange has long neck than regular weld neck flange. Other than neck, all the dimensions are same as the regular flanges. You can see the difference in the image. It is used as a nozzle in the pressure vessel and heat exchanger. you can see that Long neck has two advantages over the normal pipe to flange connection. First, it does not require pipe piece and second it eliminates a but weld between pipe and flange that required in case of conventional nozzle connection. These types of flanges are normally used in high pressure and temperature application. It is costly but gives high reliability and reduces fabrication efforts as compared to normal pipe to flange type nozzle.

Long Weld Neck flange Section Detail
Socket Weld

These flanges are developed for use in small diameter, high pressure lines. Internally welded socket type flanges are typically used in chemical process, hydraulic lines and steam distribution lines. They have a counterbore from the hub side slightly larger than the outside diameter of the pipe.

The socket weld flange is similar to a slip-on flange, except only one fillet weld is made to the flange and pipe. These pipe flanges are attached by inserting the pipe into the socket end and applying fillet weld around the top. This allows for a smooth bore and better flow of the fluid or gas inside of the pipe.

1. Socket Weld flange
2. Filled weld
3. Pipe
X = Expansion gap
Slip On

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.

The connection with the pipe is done with 2 fillet welds, as well at the outside as also at the inside of the flange.

This space is necessary, to do not damage the flange face, during the welding process.

A disadvantage of the flange is, that principle always firstly a pipe must be welded and then just a fitting. 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.

1. Slip On flange
2. Filled weld outside
3. Filled weld inside
4. Pipe

Threaded flanges are threaded in the bore to match an external thread on the pipe. The threads are tapered to create a seal between the flange and pipe as the tapers approach the same diameter. These flanges are normally designed for low pressure, non-cyclic applications.

hreaded Flanges are used for special circumstances with their main advantage being that they can be attached to the pipe without welding. Sometimes a seal weld is also used in conjunction with the threaded connection. Although still available in most sizes and pressure ratings, screwed fittings today are used almost exclusively in smaller pipe sizes.

1. Threaded flange
2. Thread
3. Pipe or Fitting

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.

1. Blind flange
2. Stud Bolt
3. Gasket
4. Other flangePipe
Figure 8 (Spectacle Blind) – Spade and Spacer

SPECTACLE BLIND and SPADE and SPACERD are the same stuff. All together assembled they make a spectacle blind (which is also called Figure 8 since it looks like 8). Spade and Spacers are basically the same as Spectacle Blinds, except that both are not attached to each other.

The Spacer is made from a metal ring in different type of material, with a welded handle. The spacer function is to allow the passage of the fluid (or gas) in the line); the Spade is made from a round piece of different type of material, with a handle furnished in a different form, according to the dimensional standard. In the middle there is a welded handle. See below a drawing for your easy reference and a pic.

Left: Spade
Right: Spacer
Special Flanges

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Special Flanges

Highlights of the Flanges


ASTM A105/ A105 M-98, ASTM A181 M-95 b Class 60, ASTM A181 M-95 b Class 70,

ASTM A694/ A694M (F42, F46, F48, F50, F52, F56, F60, F65, F70)

ASTM A515/ A515M (Gr.55, 60, 65, 70)

ASTM A350/ A350M-99(LF1, LF2, LF3, LF5, LF6, LF9, LF787),

ASTM A707/ A707M(L1, L2, L3, L4, L5, L5, L6, L7, L8)

ASTM A516/ A516M (Gr. 60, 65, 70)

ASTM A182/ A182 M-98ª(F 304, F 304H, F 304L, F 304N, F304 LN, F 309H, F310, F 310H, F 310 MoLN, F 316, F 316H, F 316L, F 316N, F 316LN, F 317, F 317L, F 347, F 347 H, F 348, F 348 H, F 321, F321 H)

ASTM A240/ A240M( 304,310, 316)

ASTM A182/ A182 M-98a( F 1, F 2, F 5, F 5a, F 9, F 91, F 92, F911, F11 Class 1, F11 Class 2, F11 Class 2, F12 Class 1, F12 Class 2,F 21 F3V, F21 3VCb, F22 Class 1, F22 Class 3, F 23, F 24, F R)

ASTM A387/ A387M(Gr.2, 12, 11, 22, 22L, 21, 21L, 5, 9, 91)

ASTM A182 (F 50, F 51, F 52, F 53M F 54, F 55, F 57, F 59, F 60, F 61, F 904L)

SB 564(UNS N02200, UNS N04400, UNS N06022, UNS N06025, UNS N06035, UNS N06045, UNS N06058, UNS N06059, UNS N06110)


ASME B16.5, ASME B16.47 (Serie A and Serie B), API 6A ASME B16.36, ASME B16.48 and MSS SP44 are the key specifications covering the dimensions and the manufacturing tolerances of Flanges. ASME B16.25 covers the preparation of butt weld connections between pipes and Weld Neck Flange. MSS SP25 covers the marking system.

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