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Flange Facing Options and How to Choose

When choosing flanges, flange facing type and finish—in addition to flange material and class—are important considerations. Without the correct facing type and related options, your piping system may not meet the minimum requirements, leading to leakage or failure. How do facing type and finish affect flange performance? Explore this guide to flange facing types to learn more.

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What is a Flange Face?

The flange face is the contact surface area where the gasket and flange mate to create a seal. There are a number of flange face types designed to meet your needs for a range of piping system types and applications.

 

What Are the Most Common Flange Facing Types?

 

Raised Face Flange Drawing

Raised Face Flange

The most common flange facing type is raised face (RF), which includes a gasket seating area that is slightly raised above the flange face—generally, 1/16 inch for Class 150 and 300 flanges and 1/4 inch for flanges between Class 400 to 2500. The raised face decreases the surface area where the gasket and flange meet, which creates a stronger seal. When pressure is applied, the serrated flange face surface bites into the gasket for a strong grip and tight seal.

 

Flat Face Flange Drawing

Flat Face Flange

Flat face (FF) flanges are machined flat and do not have a ridge on the mating surface, so the gasket has full contact. Flat face flanges are most often used for relatively low-pressure, low-temperature applications, most often for 150 through 300 psi rated flanges.

 

If your piping system requires a facing that is not FF or RF, we can custom manufacture these in our machine shop.

 

How to Choose the Appropriate Flange Facing

When ordering pipe flanges, connecting flanges should be compatible. Flange face types should match—such as flat to flat and raised to raised. Pipe flange materials should match as well, pairing stainless steel to stainless steel or steel to steel. Additionally, the flange facing should meet the required specification for the application, such as ANSI or AWWA.

 

How to Choose Flange Face Finishes

A full face gasket spans the surface area of the flange, from the inner diameter (ID) to the outer diameter (OD), whereas a ring gasket covers the raised portion of a raised face flange. Machined finishes aid in a proper seal and the optimum roughness. These surface finish options, which indicate the roughness or smoothness of the flange face, include smooth, serrated, mill or rolled, and machined finish.

Two common flange face serration types are continuous and concentric. Continuous serrations are a spiral on the face of the flange, from the outside edge to the inner bore, or to the center in the case of a blind flange. Concentric serrations are circular grooves spaced evenly across the entire face, like a bullseye. Refer to the standards to choose the appropriate finish—usually, the requirement is 30 to 55 grooves per inch with between 125 to 500 microinches surface roughness. The AWWA standard finish is between 24 to 55 serrations per inch.

Flange face finish is not a standalone feature. Most flange types are available in any facing, so ensure that you designate your other flange requirements—including connection type, flange material, and class—when placing an order.

When flanges are shipped, they’re packed carefully to protect the serrations on the faces, and they should always be transported and stored with care to prevent damage which helps ensure the best seal.

 

Flange facing and finish are important considerations for your piping system. Choose your flange facing based on temperature-pressure needs, and consider the standards when selecting your flange face, finish, and features. If our in-stock items don’t suit your project, we can create custom solutions in our Custom Machine Shop.