In the industrial field of fluid transportation and mechanical connection system, flanges are like “bridges” between precision running industrial gears, and their performance advantages and disadvantages directly affect the safety and stability of the whole system. As two widely-used members of the flange family, raised face flange and flat flange each play a key role in different working conditions by virtue of their unique structural design and performance characteristics. From high-temperature and high-pressure pipelines of petrochemical industry to the daily transportation network of water supply and drainage of buildings, the figures of the two are everywhere.In this paper, under the title of “Raised Face VS Flat Face Flange”, we will analyze the structural differences, cost factors, sealing principles and application areas.
Structural Differences:
| Item | Raised Face Flange (RF) | Flat Face Flange (FF) |
| Sealing Surface Structure | Slightly raised sealing surface above the flange body | Sealing surface flush with the flange end face |
| Protrusion Height | Typical protrusion height: 1.6mm-6.4mm | No protrusion, completely flat |
| Suitable Gaskets | High-performance gaskets like spiral wound or composite gaskets | Generally uses full-face gaskets such as rubber or non-metallic gaskets |
| Sealing Performance | Concentrated sealing pressure provides superior sealing | Evenly distributed sealing pressure but relatively weaker sealing capability |
| Application Conditions | High-pressure, high-temperature, or systems requiring superior sealing | Medium-low pressure systems, suitable for cast iron equipment or brittle connections |
| Connection Notes | Compatible with most flange standards for interconnection | Must connect with another flat face flange to avoid damage from uneven pressure distribution |
Sealing Principles：
Raised Face Flange：
The sealing surface of a raised face flange has a ring-like raised structure (usually 1.6-6.4mm in height), and its sealing depends on high local contact pressure and plastic deformation of the gasket. When the bolts are tightened, the raised part concentrates the load on the gasket, causing it to undergo elastic-plastic deformation, filling the microscopic unevenness of the flange surface and forming the initial seal. Under medium pressure, the gasket is further compressed, and the seal is maintained by the gasket rebound effect. Since the convex structure increases the pressure per unit area, it is suitable for medium and high pressure working conditions.
Flat Face Flange：
The sealing surface of the plane flange is completely flat, and its sealing relies on the elastic deformation of soft gaskets (e.g., rubber, PTFE) by the uniformly distributed bolt load, which is completely affixed to the surface of the flange. Due to the lack of concentrated load on the convex surface, the sealing relies on the compression resilience of the gasket, and the sealing is realized by filling microscopic defects. However, due to the low contact pressure, it is only suitable for low-pressure working conditions, and the use of metal gaskets should be avoided (easy to cause leakage due to high hardness). Cast iron flanges are often used with FF faces, which cannot withstand the high localized stresses of RF flanges due to their brittle material properties.
Application Scenarios:
Below is a table comparing Raised Face Flange and Flat Face Flange in different application scenarios, covering industry usage, connection methods, applicable pressure ratings, gasket usage, and more:
| Item | Raised Face Flange (RF) | Flat Face Flange (FF) |
| Main Application Industries | Oil & gas, chemical, power, high-pressure piping systems | Water treatment, fire protection, low-pressure steam, building pipelines |
| Suitable Pressure Class | Medium to high pressure (Class 150 and above) | Low pressure (typically Class 150) |
| Sealing Surface Structure | Raised surface concentrates pressure, enhancing sealing | Entire flange face is flat, less sealing performance |
| Typical Gasket Type | Spiral wound gaskets, metal-graphite composite gaskets | Non-metallic gaskets (rubber, fiber, etc.) |
| Mating Surface Requirement | Can match with raised, male-female, or tongue-and-groove faces | Must mate with flat surfaces to avoid stress concentration |
| Deformation Sensitivity | Less prone to distortion under bolt load | More prone to deformation, affecting sealing |
| Typical Temperature Range | Suitable for high-temperature applications (e.g., boilers, hot gas) | Used in ambient or low-temperature systems (e.g., potable water) |
| Common Installation Points | Pumps, valves, pressure vessel nozzles, high-temp/high-pressure systems | Cast iron equipment, low-pressure outlets, general piping connections |
| Cost Efficiency | Higher cost, but suitable for demanding environments | More economical for low-demand applications |
| Common Standards | ASME B16.5, EN 1092-1, etc. | Also applicable to ASME B16.5, EN 1092-1 (FF type) |
Cost Considerations:
RF Flange:
Due to the design of a protruding area on the sealing surface, the protruding face flange requires additional machining processes, such as precision turning and surface finish control, which increases the production man-hours and equipment requirements. At the same time, in order to ensure the sealing performance in medium and high pressure environments, it is usually necessary to use metal-wound gaskets or other high-performance sealing materials, resulting in an increase in the overall cost of the package. In addition, the installation process on the flange surface and gasket fit requirements are higher, the construction precision also has certain requirements. Therefore, cornice flanges are generally higher than flat flanges in terms of manufacturing costs, accessory costs and installation labor costs, but their advantages in pressure tolerance and sealing reliability make them the first choice for high-pressure systems in the industrial field.
FF Flange:
Flat flange structure is simple, sealing surface and the whole flange surface in the same plane, processing is relatively fast, usually only need to rough machining and plane grinding, manufacturing cost is low. It is often used with flexible material gaskets (such as rubber, asbestos or PTFE), gaskets are inexpensive, the installation process is relatively loose, the pipeline neutral and fastening force requirements are not high. As a result, flat flanges have low material costs, processing costs and accessory costs, making them an economical solution for low-pressure, low-temperature or non-critical sealing situations. It should be noted, however, that their limited pressure resistance makes them unsuitable for use in pressurized or systems with significant thermal expansion and contraction.
International Standards and Material Options:
ZIZI‘s RF and FF flanges are manufactured in accordance with international standards such as ASME B16.5, EN 1092-1, and DIN, and are available in a wide range of materials, sizes, and pressure ratings.
We are professional in manufacturing and exporting flanges in various materials.
