The Basics of Flanges


Flanges connect pipes, valves, pumps and other equipment to form a piping system. They also provide easy access for cleaning, inspection or modification. As a standard, flanges are composed of stainless steel (316SS or 304SS) or carbon steel. Materials outside of these are considered exotics and specified due to process media requirements.

Flanges have three different mating faces – flat face, raised face, and groove face.

Flat face flanges have bolting and pressure retaining face in the same plane. Flat face flanges are the most economical flange type and used for non-critical, low or no specification piping.

Raised face flanges isolate the pressure retaining connection from the bolting connection. Raised face flanges are intermediate priced and are the most common type of flange used in process equipment.

The ring type joint flange, commonly referred to as RTJ flange, is made for high pressure, critical applications. The softer gasket material that seats in the groove area compresses against the harder metal material making an extremely efficient seal.

Commercially available flange ratings for forged steel are 300#, 400#, 600#, 900#, 1500#, and 2500#. Flange ratings are calculated based on the design temperature and design pressure of piping system. Use our calculator to determine the proper flange rating.

Flanges come in seven different types of connection configurations, each with distinct applications.


Slip-on Flange

Slip-on pipe flanges slide over the pipe. They are typically machined with an inside diameter slightly larger than the outside diameter of the pipe. This allows the flange to slide over the pipe but to still have a somewhat snug fit. Slip-ons are secured to the pipe with a fillet weld at the top and the bottom of the flange. 

Socket weld

Socket weld flange

Socket-weld  flanges attach by inserting the pipe into the socket and applying a weld around the top. Socket weld flanges are typically used on small pipe sizes.

Weld Neck

Weld Neck Flange

Weld neck pipe flanges are installed by welding the pipe to the flange neck. This allows for the gradual stress transfer from the flange to pipe. It also reduces high stress concentration at the base of the hub. Weld necks are often used for high pressure applications. 

Long Weld Neck

Long Weld Neck Flange

As the name suggests, long weld neck flanges are simliar to weld neck flanges with a longer neck. The longer neck eliminates the need for a pipe pieces and the subsequent butt weld connection between pipe and flange. Long Weld neck flanges are costly and may have long lead times, but are known to be high reliable and reduces fabrication efforts as compared to normal pipe to flange type nozzles.


Threaded flange

Threaded are designed like slip-on pipe flanges except the bore of threaded pipe flange is threaded. These flanges can be attached to piping without welding. While somewhat rare, these flanges are sometimes used for small diameter, high pressure requirements.

Lap Joint

Lap Joint Flange

Lap joint pipe flanges fit over the pipe and are most commonly used with stub ends. A pipe is typically welded to the stub end and the lap joint is free to rotate around the stub end. Lap joint flanges are used when  frequent dismantling may be required.


Blind Flange

Blind flanges bolt directly to the end of a pipe or a pressure vessel opening to prevent flow. This can be particularly useful to isolate a pipe or pressure vessel for hydro or pneumatic testing. Blind flanges can be used in lieu of t-bolt or quick opening enclosures to allow easy access and are frequently seen in high pressure applications.