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When selecting flanges for an industrial piping system, the facing may seem like a small detail. However, the surface where the gasket sits can influence how the joint seals, how the load is distributed during bolting, and whether the connection is suitable for the equipment around it.
Raised face (RF) and flat face (FF) flanges are two commonly used configurations. Both provide a surface for sealing a flanged joint, but they differ in geometry, gasket arrangement, and typical applications. Choosing between them requires more than looking at the flange face alone.
Let’s examine the differences in raised face vs flat face flange and what to consider when selecting the right type for an industrial application.
What Is a Raised Face Flange?
A raised face flange has a small, elevated sealing surface around its bore. This raised area sits above the surrounding flange face and provides the contact surface for the gasket.
The raised face concentrates the compressive load over a smaller gasket area than a full-face gasket arrangement. This helps establish the contact pressure required to create a seal when the joint is assembled correctly.
Raised face flanges are widely used in industrial piping, including oil and gas, petrochemical processing, power generation, and other process applications. They are available in different materials, sizes, and pressure classes, depending on the applicable flange standard.
The height and dimensions of the raised face are governed by the relevant standard and pressure class. The gasket must be suitable for the flange facing, operating conditions, and joint design.
Key features of raised face flanges
- An elevated gasket seating surface.
- Commonly used with ring-type or spiral-wound gaskets, depending on the joint specification.
- A concentrated gasket loading arrangement.
- Availability across a range of materials, sizes, and pressure classes.
- Suitability for many industrial process piping applications when correctly specified.
An important point is that the raised face alone does not determine the pressure capacity of the connection. The flange rating, material, temperature, gasket, bolting, and installation procedure all influence joint performance.
What Is a Flat Face Flange?
A flat face flange has a sealing surface that remains in the same plane as the surrounding flange face. Unlike an RF flange, it does not have an elevated gasket seating area.
Flat face flanges are often used where the mating component has a flat face, particularly in applications involving cast iron equipment or other components for which the joint design requires a flat mating surface.
A flat face arrangement commonly uses a full-face gasket that extends across the flange face and includes the bolt holes. This helps distribute the gasket contact and bolting load over a wider area.
Flat face flanges are found in water distribution, water treatment, and various equipment connections. Their suitability depends on the material, pressure-temperature requirements, mating flange, and applicable design standard.
Key features of flat face flanges
- A level sealing surface without a raised section.
- Commonly paired with full-face gaskets.
- A wider gasket contact area when used with a full-face gasket.
- Often specified for connections involving cast iron equipment.
- Available in configurations suited to particular piping and equipment requirements.
Flat face flanges should not be considered limited to low-pressure applications in every case. Their permissible service depends on the specific flange standard, material, design, and operating conditions.
Raised Face Flange vs Flat Face Flange: Key Differences
Although both types create bolted connections, the geometry of their sealing surfaces affects gasket selection and compatibility with mating equipment.
| Comparison factor | Raised face flange (RF) | Flat face flange (FF) |
| Facing geometry | Raised sealing surface | Flat sealing surface |
| Gasket arrangement | Often a gasket contained within the bolt circle | Commonly a full-face gasket |
| Gasket loading | Concentrated over the gasket seating area | Distributed over a wider area with a full-face gasket |
| Mating surface | Generally paired with a compatible raised-face arrangement | Often paired with flat-faced equipment |
| Cast iron equipment | Requires careful compatibility assessment; an RF-to-FF joint may be unsuitable | Commonly specified where a flat face is required |
| Typical applications | Process piping, oil and gas, petrochemical and power systems | Water systems, equipment connections and other specified services |
| Selection considerations | Facing dimensions, gasket type, pressure class and mating flange | Full-face gasket, bolting load, mating equipment and material compatibility |
These are typical arrangements rather than universal rules. The approved piping specification and equipment requirements should determine the final configuration.
FF Flange vs RF Flange: How Does Gasket Selection Differ?
The gasket is an essential part of a flanged joint. Even when the flange dimensions are correct, an unsuitable gasket or poorly prepared sealing surface can lead to leakage.
With a raised face flange, the gasket generally sits on the raised sealing area. The smaller contact area concentrates the compressive load, helping the gasket develop the required sealing stress. The gasket material and design must be suitable for the fluid, temperature, pressure, and flange facing.
A flat face flange commonly uses a full-face gasket. Because the gasket extends across the flange face, the contact area is larger. This arrangement is particularly important when connecting to equipment that requires a flat mating surface.
The gasket should never be selected based on facing type alone. Material compatibility, chemical exposure, temperature limits, bolt load, surface finish, and the manufacturer’s recommendations also matter.
Why Use a Raised Face Flange?
Raised face flanges are commonly selected because their sealing geometry suits a wide range of industrial piping arrangements. The raised surface provides a defined gasket seating area, allowing the joint to develop the required gasket contact pressure when assembled correctly.
They are frequently used in process systems where operating pressure, temperature, fluid properties, and maintenance requirements must be considered together. Depending on the design, raised face flanges can accommodate different gasket constructions and material specifications.
However, the choice should not be based on an assumption that RF is always better than FF. A raised face flange is appropriate when the piping specification, mating equipment, and joint design call for it. If the equipment requires a flat face, selecting RF simply because it is common can create an avoidable compatibility problem.
How to Choose the Right Flange Facing
The right choice begins with the piping specification and the equipment to which the flange will connect. Before placing an order, consider the following factors.
1. Check the mating flange
Confirm the facing of the adjoining flange or equipment nozzle. The two sides must form a compatible joint, with the correct gasket arrangement and loading requirements.
2. Review pressure and temperature
Check the design pressure and temperature against the flange standard and material rating. A pressure class alone does not establish suitability for every temperature or service condition.
3. Select the correct gasket
Choose a gasket designed for the facing arrangement and compatible with the process fluid. Verify its temperature range, chemical resistance, dimensions, and required seating conditions.
4. Consider the flange material
Material selection should account for the fluid, corrosion exposure, temperature, mechanical loading, and adjoining equipment. Extra care is necessary when connecting flanges to cast iron or other materials that may be sensitive to concentrated bolt loads.
5. Verify standards and dimensions
Confirm the nominal pipe size, pressure class, facing dimensions, bolt pattern, and applicable standard. Depending on the application, specifications such as ASME B16.5 or ASME B16.47 may apply to the flange.
6. Follow the correct assembly procedure
The gasket, bolting, flange alignment, and tightening procedure all contribute to joint integrity. Uneven tightening, damaged faces, incorrect gaskets, or misalignment can compromise sealing even when the correct flange type has been selected.
Conclusion
The main difference between raised face and flat face flanges lies in their sealing-surface geometry and the way the gasket interfaces with the joint. RF flanges concentrate gasket loading over a raised seating area, while FF flanges provide a level surface and are commonly used with full-face gaskets. Neither type should be selected without checking the mating equipment, material, pressure-temperature conditions, and applicable standards.
At KAFCO Asia, we manufacture and supply flanges in a range of materials, sizes, and pressure classes for industrial requirements. If you are looking for a reliable flanges manufacturer in India, contact our team at info@kafcoasia.com to discuss your specifications and project requirements.
Frequently Asked Questions
Can a raised face flange be converted to a flat face flange?
Yes, machining may be possible, but it must be approved for the specific flange design, pressure class, and applicable standard. Removing material can affect flange dimensions and suitability, so consult the responsible engineer before making any modification.
How can I identify RF and FF flanges on an engineering drawing?
Check the flange-facing designation in the piping specification, equipment drawing, or bill of materials. RF typically identifies a raised face, while FF identifies a flat face. Confirm the actual flange dimensions before ordering or installation.
Does the flange facing affect stud bolt length?
It can. The required stud bolt length depends on flange thickness, facing dimensions, gasket thickness, nut engagement, and the joint configuration. Verify the dimensions against the applicable standard and project specifications.
Can the same gasket be used for RF and FF flanges?
Not necessarily. Gasket suitability depends on the facing arrangement, gasket design, material, and operating conditions. A gasket suitable for an RF joint should not automatically be used in an FF connection without confirming the joint specification.
