Understanding Flange Face Types: RF vs FF vs RTJ
In the world of industrial piping, the integrity of a system relies heavily on its connections.
A flange face is the exact surface area that hosts the gasket and ensures a leak-proof seal between two flanges.
Choosing the wrong flange face type can lead to catastrophic leaks, equipment damage, and severe safety hazards.
While there are several flange face configurations, three dominate the industry: Raised Face (RF), Flat Face (FF), and Ring Type Joint (RTJ).
This guide breaks down the design, applications, and core differences between these three essential flange types to help you make informed engineering and procurement decisions.
What is a Flange Face?
The flange face is the mating surface where two flanges meet, separated only by a gasket.
The specific geometry of this face determines the type of gasket required, how the bolting force is distributed, and the maximum pressure and temperature the connection can safely withstand.
Flange Face Types Explained: FF, RF, and RTJ
1) Flat Face (FF) Flanges
As the name suggests, a Flat Face flange has a mating surface that is completely flat and sits on the same plane as the bolting circle.
How it Works
FF flanges require a full-face gasket, which covers the entire surface of the flange, including the bolt holes.
Because the contact area is very large, the bolting force is distributed across a wide surface, which prevents the flange from cracking when the bolts are tightened.
Common Applications
Flat Face flanges are almost exclusively used for low-pressure and low-temperature applications. They are highly common when mating with cast iron equipment (like pumps or valves) or fragile materials like fiberglass or plastic (PVC/CPVC).
Why use it?
The primary advantage is safety for brittle materials. If you were to bolt a fragile cast iron flange to a Raised Face flange, the bending moment caused by tightening the bolts over the raised gap could easily snap the cast iron.
2) Raised Face (RF) Flanges
The Raised Face is the most widely used flange face type in process plant applications. It features a small raised section around the bore where the gasket is seated.
How it Works
By concentrating the entire clamping force of the bolts onto a smaller raised area, the RF flange creates a much higher seating pressure on the gasket. This allows the joint to hold significantly higher pressures and temperatures than a Flat Face flange.
- Gasket Types: RF flanges typically use flat ring gaskets (which sit entirely inside the bolt circle) or spiral wound gaskets.
- Dimensions: The height of the raised face depends on the pressure class. For standard ASME B16.5 Class 150 and 300 flanges, the raised face is typically 1/16 inch (1.6 mm). For higher pressure classes (Class 400 to 2500), the raised face height increases to 1/4 inch (6.4 mm).
- Common Applications: Oil and gas, chemical processing, power plants, and general high-pressure piping systems.
3) Ring Type Joint (RTJ) Flanges
Ring Type Joint flanges are designed for the most extreme, high-pressure, and high-temperature environments.
Instead of a flat mating surface, an RTJ flange has a deep, precisely machined groove cut into its face.
How it Works
RTJ flanges do not use standard soft gaskets. Instead, a solid metal ring (often made of soft iron, low carbon steel, or stainless steel) rests inside the grooves of the two mating flanges.
When the bolts are tightened, the metal ring is crushed into the grooves, creating an exceptionally strong, metal-to-metal seal.
- Benefits: Because the sealing mechanism relies on the deformation of metal under high compressive loads, RTJ connections are incredibly durable and blow-out proof. Furthermore, as system pressure increases, it actually pushes the metal ring tighter against the groove walls, self-energizing the seal.
- Common Applications: Deepwater subsea piping, high-pressure oil extraction, petrochemical refineries, and applications exceeding temperatures of 800°F (427°C) or pressures above Class 600.
Comparison Guide: RF vs FF vs RTJ
To summarize the operational differences, here is a quick reference table.
| Feature | Flat Face (FF) | Raised Face (RF) | Ring Type Joint (RTJ) |
|---|---|---|---|
| Face Profile | Completely flat | Raised lip around the bore | Deep machined groove |
| Gasket Type | Full-face soft gaskets | Ring gaskets, spiral wound | Solid metal ring gaskets |
| Sealing Force | Low (widely distributed) | High (concentrated) | Extreme (metal-to-metal) |
| Pressure Rating | Low (Typically Class 150/300) | Moderate to High (All Classes) | Very High (Class 600 to 2500+) |
| Typical Mating Material | Cast iron, plastics, fiberglass | Carbon steel, stainless steel | High-grade steels, alloys |
| Cost | Low | Moderate | High (due to precision machining) |
Conclusion
Selecting the correct flange face is not just about making a simple connection; it is about ensuring the long-term safety and efficiency of your entire piping system.
By understanding the distinct roles of Flat Face, Raised Face, and Ring Type Joint flanges, engineers can prevent costly leaks and catastrophic equipment damage.
Always remember to never mix flange faces and to carefully align your choice with the specific pressure and temperature demands of your project.
Making the right choice upfront guarantees a secure, reliable, and compliant industrial operation.
Frequently Asked Questions (FAQs)
1. Can I connect a Raised Face (RF) flange to a Flat Face (FF) flange?
No, you should never mix these two. Bolting an RF flange to an FF flange creates a bending gap that can easily snap or crack the flat-faced component, especially if it is made of brittle materials like cast iron or plastic.
2. What is the main advantage of an RTJ flange?
The primary advantage of a Ring Type Joint (RTJ) flange is its ability to handle extreme pressures and temperatures. It uses a solid metal ring to create a highly robust, blow-out proof metal-to-metal seal.
3. Why do Flat Face flanges use full-face gaskets?
Full-face gaskets cover the entire surface of the flange, including the bolt holes. This distributes the bolting force evenly across a wide area, which prevents damage to delicate mating materials during tightening.