RF vs FF Flange Valve Faces: Raised Face vs Flat Face Explained

The Short Answer

An RF (raised face) flange has a small raised ring around the bore that concentrates the gasket load, while an FF (flat face) flange is flat across the full face and is typically paired with a full-face gasket. RF flanges are the standard for most steel piping systems because the raised ring creates a more predictable sealing surface, while FF flanges are common in low-pressure and cast-iron systems where a full-face gasket protects the flange from cracking under bolt load. The two are not interchangeable without checking the mating flange and gasket.

What the Two Face Styles Look Like

On an RF flange, the sealing surface is the raised ring—typically 1.6 mm (1/16 inch) high in the ANSI-style system, with a larger height used in some high-pressure classes—and the gasket sits on that ring. The surrounding face is relieved, so the bolt load passes through the raised ring and the gasket sees higher unit pressure for the same bolt load. On an FF flange, the entire face is machined flat and the gasket covers the full face, including the bolt circle, which spreads the load over the whole area.

This difference sounds small, but it changes gasket selection, bolt load, and the mechanical behavior of the joint. An RF joint is designed around a gasket that fits inside the bolt circle, while an FF joint uses a full-face gasket with holes for the bolts. Putting the wrong gasket on either face type is a common field error that produces leakage or cracking.

Why RF Dominates Steel Piping

Most steel piping systems use RF flanges because the raised face gives a controlled sealing area. The gasket is compressed between two metal rings with predictable area, the bolt load required for a given gasket stress is lower than for a full-face design, and the joint is easier to align and inspect. Standards for steel flanges are built around the RF concept, with the raised face dimensions included in the flange standard. This is why an RF flange is the default in process plants and why most off-the-shelf gaskets are designed for it.

The raised face also concentrates the load where the gasket needs it. With a full-face gasket, part of the bolt load is spent compressing the gasket under the bolt circle and the outer face, which reduces the load available at the sealing line. The RF design avoids that waste, so a given bolt load produces a higher gasket stress.

Why FF Survives in Cast Iron

The FF flange survives in cast-iron and other brittle-material systems because it protects the flange itself. Cast iron is strong in compression but weak in bending and tension. A raised-face joint on a cast-iron flange can impose bending loads near the face, and the concentrated load around the raised ring can crack the flange under bolt tightening. A full-face gasket spreads the load across the whole face and reduces the bending stress, which is why FF flanges with full-face gaskets are the traditional choice for cast-iron equipment and low-pressure systems.

This is also why mixing face types in a cast-iron system is risky. Bolting a steel RF flange to a cast-iron FF flange without a spacer or an appropriate gasket can overload the cast-iron face. When the materials differ, the joint design should be reviewed as a system rather than assuming the gasket will sort it out.

Feature	RF (raised face)	FF (flat face)
Sealing surface	Raised ring around the bore	Full machined face
Typical gasket	Gasket inside the bolt circle	Full-face gasket with bolt holes
Gasket stress per bolt load	Higher (concentrated area)	Lower (spread over full face)
Typical material pairing	Steel piping and fittings	Cast iron and low-pressure equipment
Typical pressure classes	Wide range, including higher classes	Mostly low-pressure classes
Mixing risk	Check mating flange and gasket before pairing with FF	Brittle flange can crack if paired with RF loading

Application Judgment: The Mixed-Flange Joint

Imagine connecting a steel valve with an RF flange to a cast-iron pump suction nozzle with an FF flange. The straightforward answer is not to bolt them directly with a standard gasket. Options include a full-face gasket with an appropriate design, a spacer or adapter that converts the joint, or reviewing whether the cast-iron nozzle should be replaced or modified. The engineering judgment is to look at the bolt load path and the bending stress on the cast-iron face before deciding, because a leaking joint is cheaper to avoid than to repair, and a cracked cast-iron flange is more expensive to fix than either of the conversion options.

The gasket material and bolt torque matter in both face styles. The gasket must be rated for the fluid, pressure, and temperature, and the bolt load must produce the gasket stress the gasket manufacturer specifies. When either the face type or the gasket compatibility is uncertain, the safe move is to check the flange standards and the gasket data before assembly—the phrase “data needs further verification” applies to any joint where the exact face dimensions or gasket rating are not confirmed.

For the neighboring comparison, see our RF vs RTJ valve flange page for ring-joint faces, and the valve dimensions: face-to-face and end-to-end guide explains how face styles fit into overall valve dimensions.

Bottom Line

RF and FF are not cosmetic differences—they change the gasket, the bolt load, and the mechanical safety of the joint. Choose the face style that matches the piping standard and the flange material, and never mix face types without reviewing the full joint design.

This article provides general educational knowledge only and is not professional engineering advice.

Sources & Further Reading

ASME (flange standards and pressure-temperature ratings), MSS (valve flange standard practices), AWWA (flange standards for water service).

Tags: Valve Selection