Flanges: Buying guide

A flange is a protuberant rim or ridge that serves to increase the strength, easily attach one object with another by means of contact force, and guide and stabilize a machine’s movements or its parts. “Flange” is also a term used for a tool used to form flanges.

Flanges are piping components bound together using a gasket as a sealing material in between two flanges. They are used to connect pipes with other pipes, flanged valves, flanged fittings, flanged piping specialty items like strainers, and segregate piping sections using a blind flange. Flanges are also used in plumbing applications- for example, water-closet flanges that provide the mount for a toilet.

If flanges are not fixed correctly, they result in a gap through which undesirable substances can flow out of the system, causing harm to humans and the environment.

Flanges materials

Steel (ferrous) flanges

Stainless steel

Stainless steel is distinguished for its ability to resist corrosion, durability, and ease of sanitation. It is best suited for any system that is required to remain clean throughout its service life- for example, a drinking water system.

Furthermore, the clean and polished finish of the flange is unparalleled. However, these rewarding benefits come with a comparatively higher price tag.

When using stainless steel flanges, ensure that the hardware you use to connect the pieces come with long service life. Also, they should be replaced at the appropriate time, even though the flange is in good working order.

Carbon steel

Carbon steel is best when toughness is the priority rather than sanitation. For example, carbon steel flanges are used in your homes to deliver the natural gas supply. Carbon steel flanges will stand up to heat and vibration very well.

During installation, some finishing may be required on the flange faces to ensure a tight connection.

Aluminum (non-ferrous) flanges

Hardened aluminum is frequently used on the face of steel (stainless) flanges.

If an entire flange is made of aluminum, it can slash the weight of the component. If a lighter component puts comparatively less pressure on the system’s structure or saves fuel, aluminum is a good choice.

Plastic flanges

Plastic flanges are even lighter than aluminum ones. They will not be able to bear excessive pressure or heat unless you invest in a highly durable material. Buying heavy-duty plastic flanges may get expensive. However, it still won’t provide the service life of carbon or stainless steel.

Types of flanges

Threaded flanges (Screwed flange)

It consists of a thread inside the flange bore that fits with a matching male thread on the pipe. With a threaded connection, you can avoid wielding in some cases.

Socket-weld flanges

You simply have to place the pipe into the flange and secure the connection with a multi-pass fillet weld. Such flanges are ideal for pipes with smaller diameters in low-pressure and low-temperature scenarios.

Slip-on flanges

In this, you have to match the flange to the intended pipe’s outer diameter and secure the connection using fillet weld on both sides. Slip-on flanges are available in different sizes to easily accommodate systems with higher throughput and flow rates.

Lap joint flanges

These flanges require butt welding of the stub end to the pipe using a backing flange to generate the flanged connection. Lap joint flanges are best suited for systems that require frequent dismantling and maintenance or systems with limited physical space.

Weld neck flanges

Weld neck flanges require butt welding for a secure connection, just like lap joint flanges. Such flanges are the best bet for process piping due to their performance, integrity, and ability to be used in high-temperature and high-pressure systems.

Blind flanges

These flanges are materially boltable blank discs used for isolating or terminating pipe systems. When combined with correct gaskets and appropriately installed, bling flanges can achieve an exceptional seal that is easy to remove whenever required.

Flange facing types for connection

Flange designs and flange materials are only the beginning when considering an ideal flange for your system. Face types are another characteristic that has a significant impact on your flanges’ service life and final performance.

Facing types determine the gaskets required to install the flange and create a more robust seal. Typical face types are:

Flat face

A flat face flange features an even, flat surface. The flat surface enables the gasket to have complete contact with most of the flange surface. Its purpose is to eliminate the bending moment that is put on the flange when the bolts are torqued.

Raised face

It features a small raised section above the bolting section where a circle gasket is placed.

Ring joint face

The ring joint face flange features a groove where a metal gasket is placed to maintain the seal. It is used in high-temperature and high-pressure processes.

Tongue and groove

It features raised surfaces and matching grooves, making the installation process easier as the design enables the flanges to self-align and provide a repository for gasket adhesive.

Male and female

These flanges use raised sections and a matching pair of grooves to secure the gasket. The gasket is retained on the female face, providing accurate placement and more gasket material options.

Flange dimensions for proper fit

Besides the functional design, flanges’ dimensions also impact the system. Common considerations to ensure proper fit include:

  • Outside diameter- Distance between the opposite edges of a flange face.
  • Thickness- Width of the outer attaching rim.
  • Bolt circle diameter- Distance between the opposite bolt holes, measure from the center.
  • Pipe size- The size of the pipe with which the flange correlates.
  • Nominal bore size- Measurement of the inner diameter of the flange connector.

Flange markings and standards

Flanges fall under the standards established by ASME (American society of mechanical engineers).

If you are looking forward to verifying or replacing existing parts, ensure that the flanges include essential markers (usually on the outer perimeter). The markers are-

  • Manufacturer code or logo
  • ASTM material code
  • Material grade
  • Service rating
  • Size
  • Thickness
  • Heat number
  • Special designations (if any- like W for repair by welding, QT for quenched and tempered)


This guide provides thorough and detailed information about the flange types and designs, which will enable you to select the ideal flange for your system. Before investing in any flange, make sure you consider the points mentioned above.

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