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Technical Library > FLANGES > INTRODUCTION TO FLANGES > What are Flanges?
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STEEL FLANGES


In this article, we give an answer to a recurring question: what are steel flanges? How to define flange?


 

WHAT ARE FLANGES?

In this article, we give an answer to a recurring question: what are steel flanges? How to define flange?

Flanges are steel products used to tightly connect mechanical devices (example: pipe to pipe, pipe to a valve, pipe to fittings, pipe to chokes, pipe to pumps, etc.). ASME B16.5 and ASME B16.47 are the most common specifications for flanges, followed by “API 6A” (flanges for upstream operations) and “EN 1092-1” (European flanges specification). They set dimensional ranges and tolerances for steel flanges for commercial use.

Flanges are available in multiple forged (and cast) material grades, the most common are ASTM A105 specification (carbon steel flanges for high-temperature service), ASTM A350 (CS flanges for low temperature), ASTM A694 (high yield carbon steel flanges for line pipes), ASTM A182 F304/F316/F321 (stainless steel flanges), ASTM A182 F51 (duplex steel flanges), and, ASTM A182 F53/55 (super duplex flanges).

For specific applications, example marine or aeronautical, non-ferrous flanges are also used (copper, bronze, cupronickel, aluminum flanges). As a general rule, the material of the flange should match the material of the piping system they are used for (see this article for more details).

External paints, and internal lining materials, can be applied to flanges that have to withstand the aggression of specific fluids.

 

Flanges for pipelines

Most used flanges for pipelines and for the petrochemical industry

 


FLANGED CONNECTIONS

A flanged connection (alternatively called “flanged joint”) requires two flanges (the “main” and the “companion”), a set of stud bolts and nuts (the dimension and the number of the required fasteners are defined by the flange bolting chart) and two gaskets (which can be either metallic, as spiral wound, ring joint, jacketed and Camprofile types, or non metallic, i.e. made of soft non-asbestos materials).

Flanged connections shall be made by trained personnel only, as badly executed connections impact the reliability of the piping arrangement (the standard TSE – TS EN 1591 Part 1-4, “Flanges and their joints”, sets the requirements in this field).

 

flanged joint connection

 

The two main types of flanged connections are the welded and the threaded types.

Welded connections are used for pipelines and piping systems with high pressures and temperatures, and with diameters above 2 inches.

Threaded connections are used for small diameter piping or when the piping arrangement is not subject to mechanical forces as expansion, vibration, contraction, oscillation (which would crack a threaded joint).

Flanged connection

The most common flanges for welded connections are:

WELD NECK CONNECTION

Weld-neck flanges have a tapered hub with a butt weld end that can be welded to a butt weld pipe. The quality of the welded joint may be examined visually or using radiography and/or ultrasounds (UT). The welded connection between a weld neck flange and a pipe has good fatigue and mechanical stress performance.

Weld neck connection

SOCKET WELD CONNECTION

Socket-weld flanges are often used for high-pressure pipeworks below 2 inches (DN 50). The pipe is fillet-welded to the hub of the socket weld flange. As a radiographic examination would not be easy to execute on the fillet weld, the welder experience is key for this type of connection. However, if a connection is critical, and visual examination is not sufficient, specific tests may be executed to determine the integrity of the connection, such as magnetic particle (MP), or liquid penetrant (PT).
Socket weld connection

SLIP-ON CONNECTION

Slip-on flanges are generally preferred to weld-neck flanges due to their lower cost and installation speed (and ease). The downside of slip-on flanges vs. a weld neck flanges is that their strength is about 1/3 lower. For this reasons, slip-on flanges are typically used for low-pressure, non-critical services such as fire water, cooling water, and similar nonchallenging pipeworks. The pipe is welded to both the hub and the bore of the flange and MP, PT, or a simple visual examination are the approaches used to check the integrity of a slip-on connection. Slip-on flanges are used, generally, for pipe sizes greater than NPS 2¹⁄₂ (DN 65).

 

 

Slip on flange connection

 

LAP JOINT CONNECTION (WITH STUB ENDS)

This type of connection is sometimes used for pipelines in high-cost materials (as stainless steel and nickel alloys) as it helps to reduce the overall cost of the required flanged connections.

A lap joint connection is made by the combination of a stub end (which is the part welded to the pipe) and a backing flange, or lapped flange.

A stainless steel stub end can be for instance used in combination with a carbon steel lap joint flange to have a reliable welded connection at a cheaper cost than a full stainless steel connection (less stainless steel is required, hence the cost is lower). Lap joint flanges have generally a raised face and are sealed with flat ring gasket.

 

Lap joint connection

1. Lap Joint flange 2. Stub End 3. Butt weld 4. Pipe or Fitting

 

HOW TO ORDER A FLANGE

The following parameters have to specify to order (or request estimates) for flanges:

  • Flange type (welding neck, slip-on, threaded, lap joint flange)
  • Specification (ASME/ANSI, JIS, EN, UNI, DIN, etc)
  • Diameter (generally between 1/2″ – 60″)
  • Rating: the rating of flanges (defined class) ranges from 150 to 2500 for ASTM, from 5k to 30k for JIS B2220 flanges and KS1503 flanges; from PN 6 to PN 100 for European flanges (DIN, UNI, EN 1092-1, GOST 12820 / 12821-80); Class 600/3, 1000/3, 1600/3, 2500/3, 4000/3 for SANS / SABS flanges (South African standard for flanges)
  • Schedule (only for welding neck/socket weld flanges)
  • Flange facing: according to ASME B16.5, the raised face is the standard facing for flanges (different facings, like RTJ or flat face FF, have to be ordered specifically)
  • Flange surface finish (smooth, stock, concentric serrated, etc)
  • Material grade (carbon, stainless, duplex, nickel-alloy, non-ferrous)
  • Quantity

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