TMCnet Feature
January 06, 2021

Your Guide to Piping & ANSI Flange Dimensions

To know everything about piping & ANSI flange dimensions, we first need to establish what a flange is. A flange is a plate or ring that forms a rim at the end of a pipe when it is fastened to the pipe. A blind flange is a plate used to cover or close the end of a pipe, while a flange joint is a connection of pipes where the two pieces connected have flanges where the parts are bolted together.

Flange Standards

There are different flange standards worldwide, but they have been designed to have standardized dimensions to allow easy functionality and interchangeability. Some of the most popular world standards include ASA/ASME (USA), PN/DIN (European), BS10 (British/Australian), and JIS/KS (Japanese/Korean). Despite most of these standards being interchangeable (as most local standards have been aligned to ISO standards), some local standards still differ. For instance, an ASME flange will not mate against an ISO flange and vice versa. Furthermore, flanges in each standard are categorized into pressure classes, which allow different flanges to take different pressure ratings, and they too are not interchangeable, meaning an ASME 150 will not mate with an ASME 300.

ANSI Flanges

ANSI Flanges are a particular standard of flanges that have been certified by ANSI (the American National Standards Institute). This is widely recognized as the American organization responsible for overseeing the national standards and conformity assessment system for products, services, processes, systems, and personnel and was established in 1918. They work domestically with different American government agencies and organizations, and also with international entities, to make ANSI standards useful around the world.

ANSI B16.5 -1996 is the official standard for ANSI Class Pipe Flanges and other Flanged Fittings in the United States. It is applied particularly to those made from cast or forged materials, as well as blind flanges and some reducing flanges crafted in the cast, forged, or plate. This standard covers evaluations and standards for pressure-temperature ratings, materials, tolerances, dimensions, marking, testing, and methods of designating opening noted in both metric and U.S. units of measure.

ANSI Flange Specifications

As mentioned earlier, ANSI flanges have to conform to a precise set of specifications, which are; Size (NPS 2", 4" etc.) - NPS is Nominal Pipe Size.

?Pipe Wall Thickness - where applicable (SCHEDULE 40, SCHEDULE 80, etc.)

??Rating Class (150#? or 150lb, 300# or 300lb, etc.)

?Flange Material (ASTM A350 LF2, ASTM A182 316L etc.)

Flange Facing (Raised Face, Ring Joint Face, etc.)

?Flange Type (Welding Neck Flange, Blind Flange, etc.)

Various Types of ANSI Flanges

There are a number of different ANSI flanges available in the market, and when purchasing, you have to primarily consider the intended purpose in order to get the right one. Here are the different types of ANSI flanges you can find in the market and their designated purpose.

Welding Neck Flange

Also called the WN flange, they are the easiest to recognize because of the long, tapered hub that goes gradually over to the wall thickness from a pipe or fitting. The tapered hub is very critical as it provides an important reinforcement for different applications that involve high pressures as well as sub-zero and elevated pressures. The taper also provides for a smooth transition from flange thickness to pipe or fitting wall thickness, which is extremely beneficial, especially under conditions of repeated bending. The WN flange is bored to match the inside diameter of the fitting it’s supposed to mate with to eliminate the restriction of product flow. This goes a long way in reducing turbulence, which in turn prevents erosion.

Slip-On Flange

As the name suggests, slip-on flanges are designed with a bore that is slightly larger than the matching pipe to make it possible for the flange to be slipped on to the pipe. This type of flange is normally attached to the pipe using 2 fillet welds (one on the inside (bore) and one on the outside of the flange). The calculated strength from a slip-on flange under internal pressure is around two thirds that of WN flanges, and their life under fatigue is about a third. It is worth noting that one main disadvantage of the slip-on flange is that the pipe must be welded and then fitted. It isn’t possible to combine the flange with a tee or flange and elbow because they don’t have a straight end that slides into the slip-on flange.

Socket Weld Flange

This type of flange is designed for use on a small but high-pressure piping. They match slip-on flanges in terms of static strength but have a fatigue strength that is 50% greater than that of double welded slip-on flanges. The pipe is connected to the socket of the flange and is secured with one fillet weld between the flange and the pipe. However, a small space must be created between the flange and pipe before welding, and this makes it not suitable for use on piping systems carrying highly corrosive fluids.

Blind Flange

Blind flanges are unique in the sense that they are the only ones that don’t have a hole in the middle. They are used to blank off the ends of pressure vessel openings, piping, and valves. Blind flanges are the most highly stressed type of flange when viewing from an internal pressure and bolt loading standpoint. They also don’t have a standard inside diameter, which makes them most suitable for higher pressure temperature applications.

Threaded Flange

They are also known as NPT flanges, and this is due to the female NPT thread in the bore. The NPT thread can then be screwed on to a pipe with a male NPT thread. These threads give the NPT flange a special advantage, which is that it doesn’t require welding and is therefore useful in areas without explosive gases. It is also worth noting that although they are still available in most sizes and pressure ratings, screwed fittings are used almost exclusively in smaller pipe sizes.

Lap Joint Flange

Lap joint flanges are unique in the sense that they are the only types of flanges that do not come into contact with the pipe because they are always used together with a stub end. This stub end is normally welded into the pipe, with the flange at the back. This trait also means that the flange can freely rotate for stud bolt assembly. These flanges almost perfectly match slip-on flanges with the exception of the bore to accommodate the flanged portion of the Stub End as well as a radius at the intersection of the flange face. Their pressure holding ability compared to slip-on flanges is only slightly better and has a fatigue life that is only one-tenth that of welding neck flanges. Despite that, they have a few special advantages, such as the fact that one can manufacture them from inexpensive (lesser quality) material as they don’t come into contact with the fluid. Also, one can re-use them when changing systems that corrode quickly because, again, they don’t come into contact with the fluid and therefore don’t get corroded.

Difference Between ANSI & ASME Flanges

As already noted, ANSI stands for American National Standards Institute, an organization focused on making uniformity in voluntary standards used in engineering disciplines. ASME, on the other hand, stands for the American Society of Mechanical Engineers, which was also created to bring uniformity in voluntary standards used in engineering and industrial disciplines. ANSI & ASME flanges, though almost similar, have a few differences:

  1. ANSI Flanges are manufactured based on standards developed by organizations, while ASME Flanges are based on codes and standards for mechanical devices.
  2. ANSI Flanges work to strengthen the position of the U.S. Market in the global market and ASME Flanges work to find solutions to real-time mechanical engineering problems.

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