Flange Faces types: Raised Face Flange vs. Flat Faced Flange

The flange face is the surface area that receives the gaskets. The material of the gasket seats over the flange face and creates an effective seal under pressure.

Designed in accordance with ASME B16.5 (for flanges up to 24 inches) and ASME B16.47 (for flanges 26 inches and larger), flange faces vary significantly to suit different applications. Choosing the wrong face can lead to catastrophic leakage.

In this guide, we break down the 6 most common flange face types, the differences between Raised Face and Flat Face, and how to select the right one for your project.

The 6 Standard Types of Flange Faces

The ASME standards categorize flange faces into six distinct types. Understanding these distinctions is vital for leak-free operations.

1. Raised Face Flange (RF)

The Raised Face (RF) is the most widely used flange type in the industry, particularly in oil and gas pipelines. As the name suggests, the gasket seating surface is raised above the bolting circle face.

Key Feature: The raised area concentrates contact pressure onto a smaller gasket area, significantly increasing the seal’s containment capability.
Application: Suitable for a wide range of pressures and temperatures.
Standards: Dimensions vary by pressure class. Standard RF flanges feature a surface finish of 125-250 RMS to bite into the gasket and prevent movement.

2. Flat Face Flange (FF)

Flat Face (FF) flanges feature a gasket surface that is in the same geometric plane as the bolting circle face. This design is simple but serves a critical mechanical purpose.

Key Feature: Prevents the “bending moment” (warping) that can occur when bolts are torqued.
Application: Primarily used in low-pressure, low-temperature environments (such as water treatment) and, most importantly, when mating with cast iron equipment or flanged fittings.
Crucial Tip: Never mate a Flat Face flange with a Raised Face flange, as the resulting stress can crack the cast iron flange.

3. Lap Joint Flange

A Lap Joint flange is unique because the flange itself does not provide the sealing surface. Instead, it is used in conjunction with a stub end.

Design: The flange slides over the pipe, and the stub end (welded to the pipe) provides the face.
Benefit:Because the flange creates a joint that accommodates the back of the stub end, it allows for easy alignment of bolt holes during installation.

4. Ring Joint Flange (RTJ)

Designed for extreme conditions, **Ring Type Joint (RTJ)** flanges are the go-to solution for high-pressure (Class 600+) and high-temperature (above 800°F/427°C) applications.

How it Works: The flange face features a precision-machined annular groove. A steel ring gasket is placed inside this groove. When bolts are tightened, the metal gasket deforms into the groove, creating a robust metal-to-metal seal.
Note: While RTJ flanges may appear to have a raised face, the raised portion does not act as the primary sealing surface; the groove does.

5. Male and Female Flange (M&F)

This type requires a matched pair of flanges to function correctly.

Male Face: Features a raised area (typically ¼” high) extending beyond the normal flange face.
Female Face: Features a matching machined recess (typically 3/16″ deep).
Function: The female face locates and retains the gasket, preventing blowout. This interlocking design also assists in accurate alignment during assembly.

6. Tongue and Groove Flange (T&G)

Similar to Male and Female flanges, Tongue and Groove faces must be matched.

Design: One flange has a raised ring (Tongue), and the mating flange has a matching depression (Groove).
Difference from M&F: The T&G design separates the gasket from the bolt holes entirely. The gasket is confined within the groove, protecting it from corrosion and erosion. These are commonly found on pump covers and valve bonnets.

How to Choose the Right Flange for Your Project？

Selecting the correct flange face is determined by three main factors: Pressure, Temperature, and Material Compatibility.

Go with Flat Face (FF): For low-pressure systems, water lines, or when connecting to cast iron fittings.
Go with Raised Face (RF): For standard oil and gas lines where higher pressure handling is required.
Go with RTJ: For extreme pressure and heat where gasket failure is not an option.

Need Expert Advice?

Navigating ASME standards and material specifications can be complex. Whether you need a standard Raised Face flange or a custom-machined solution, JSFITTINGS is here to help.

Our expert engineering and sales team can guide you to the perfect fit for your piping system, ensuring safety and longevity.

Contact JSFITTINGS Today for technical support or to request a tailored quote for your next project.

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