Types of Flanges - A Thomas Buying Guide

Flanges are used to connect pipes to pipes and other pipeline components such as valves, pumps, etc. They are used in various plumbing applications also, such as water-closet flanges. Of particular importance are vessel flanges, whose design and manufacture are guided by the ASME’s Boiler and Pressure Vessel Code. The code covers simple unfired pressure vessels as well more complex designs and discusses in detail subjects such as certification, materials of construction, and non-destructive testing.

Although there is some overlap, this article will summarize the various flange types by both material and attachment method. The primary types of flanges are:

  • Steel (ferrous) Flanges
  • Aluminum (non-ferrous) Flanges
  • Threaded Flanges
  • Plastic Flanges
  • Welded Flanges

Steel (ferrous) Flanges

Essentially, three standards govern pipe flanges. ASME 16.5 defines the ANSI flange, the most commonly-used flange. ASME B16.47 covers two series, A and B, which represent large diameter applications. Series A flanges are heavier and thicker than Series B for the same pressure and size. Series B flanges are normally selected for refurbishment work. ASME B16.1 defines the AWWS flange, but it is only for flanges used in potable-water service at atmospheric temperatures. Then, there is the so-called Industry Standard flange which is not defined by a governing body but instead reflects historical practice. The dimensions for these flanges are covered by ASME B16.1, the standard for 25, 125, and 250 class cast-iron-pipe flange and flange fittings.

Flanges used forhydraulic systems are typically governed by SAE standards and are sometimes referred to as four-bolt flanges.

Flanges can incorporate several different methods to seal adjoining faces, including O-rings, seal rings, and gaskets. Seal rings provide an especially tight joint and for the same bolt stress applied to a flat-face gasket, can resist a higher pressure. Faces of adjoining flanges may be flat, recessed, raised, or tongue-and-groove.

Steel flanges are available in carbon and stainless steels and may be forged or rolled.

Aluminum (non-ferrous) Flanges

Aluminum flanges are typically cast. They are available in many of the same forms as steel flanges. Welding of aluminum flanges usually requires a MIG or TIG process. Aluminum pipe is popular for handrails and a host of mounting flanges are available for these structural applications, both weldable and clamp-on varieties. For specialty applications, flanges of titanium, nickel and nickel alloys, brass and bronze, as well as the various superalloys are also available.

Threaded Flanges

The threaded, or companion flange, is suitable only for low- to medium-pressure applications. An advantage of threaded flanges is their ease of field installation as pipe-threading machines are low in cost and portable.

Plastic Flanges

Flanges for plastic pipe are available in similar sizes and shapes as those used for steel piping, including some large-diameter designs used for water. They are typically threaded or solvent welded although some of the largest HDPE flanges may be fusion welded in the field using specialized equipment. Flanges for PVC pipe will correspond to Schedule 40 and 80 dimensions. Some plastic flanges are fiber-reinforced. Plastic pipes are usually dry fitted, then marked, as the solvent used to connect them is especially fast-acting.

Welded Flanges

Weld-on flanges are available in a handful of styles, rated by pressure and temperature. These styles include lapped, weld neck, socket weld, ring-type joint, and slip-on. The various weld-on flanges permit higher pressures than threaded varieties. Lapped flanges are often used where disconnections will be frequent as the flange can spin freely, simplifying bolt-hole alignment. A special case is the so-called blind flange, which is used to seal the end of a pipeline but allows connection to another pipe or piece of equipment later.

Flanges are welded in place by either butt-weld or socket-weld methods. Socket-weld flanges, usually forged, are restricted to smaller pipe diameters (up to NPS 4, but usually NPS 2 or smaller) and are available in 3000, 6000, and 9000 class pressure ratings, corresponding to Schedule 40, 80, and 160 pipe. Socket flanges are welded into place with fillet welds, which makes them weaker than butt-welded flanges, but still preferable to threaded flanges for high-end work.

Other Flanges

Some flanges are application specific and identified as such. Roof flanges, for example, are used where vent stacks must penetrate the shingles of a roof.  Likewise, wheel flanges, microwave flanges, etc. are identified by their specific uses. Some flanges are specific to industries, such as automotive or aircraft flanges.


This article presented a brief summary of some of the common types of flanges. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

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