What Does FF Flange Mean?

Alt-text: Illustration of three flat metal flanges with bolt holes, viewed from the front.

If you’re ordering piping components and come across “FF flange”, you might wonder what that actually means.

In flange terminology, FF stands for Flat Face flange. Instead of the raised gasket seat found in other types, an FF flange has a completely flat sealing surface that sits flush with the bolting circle.

Understanding FF Flange Face

FF, or flat face flanges, feature a sealing surface that lies completely even with the bolting area, with no elevated section.

Industries dealing with cast iron or other brittle materials often pick FF flanges to avoid bending stress during bolt tightening. The flat face distributes the torque evenly, lowering the chance of cracking.

When and Why to Use FF Flanges

FF flanges work best in systems running at lower pressures and temperatures, such as chemical dosing setups, municipal piping, or pump intakes. They’re commonly rated for classes 125 and 250, where gasket materials are often soft (Viton, EPDM), not metal.

These flanges are not meant for high-pressure pipelines—there, raised face (RF) or ring joint (RTJ) designs are better suited. Flat-face flanges help prevent stresses on brittle pipe components during assembly.

How FF Differs from RF and RTJ Flanges

While FF flanges are common in certain systems, it’s important to know how they differ from other face types like RF and RTJ. Each is designed for specific sealing performance based on pressure, temperature, and material compatibility.

Here’s how they compare:

Raised Face (RF ): These flanges have a slightly elevated sealing surface that compresses a smaller gasket area. This design creates a tighter seal and is widely used in mid- to high-pressure pipelines, especially in oil and gas.
Flat Face (FF): These flanges have a smooth, level surface that aligns directly with the bolting zone. They’re ideal for systems with softer gasket materials or brittle piping, where load must be distributed evenly to prevent cracking.
Ring Type Joint(RTJ): These use a grooved surface and a metal ring gasket for high-pressure sealing. Often found in offshore, petrochemical, and high-temp steam applications where leak prevention is critical.

Specifying FF Flanges: What You Need to Know

When choosing an FF flange, the right specs ensure long-term performance and easy integration into your system. These aren’t just low-pressure options—they need to match your project’s pressure class, material type, and flange standard to function as intended.

Pressure Class: FF flanges are typically used in ANSI Class 125 or 250 systems, which suit lower pressure operations like water treatment or HVAC. Choosing the wrong flange type can result in poor system performance or create unnecessary safety risks.
Gasket Type: A full-face gasket is required to cover the entire flange surface. These gaskets are usually made from soft, non-metallic materials like rubber or PTFE to maintain a leak-free seal under low torque loads.
Materials: Flat face flanges are commonly paired with cast iron or non-metallic piping to minimize stress during tightening. But if corrosion resistance is a priority, stainless steel FF options are available from Longan Flange, offering better durability in wet or chemically exposed environments.
Standards: Always confirm the flange meets ASME B16.5 or B16.47 specs. ASME flanges labeled as FF ensure you’re getting the correct face type for your design.