Raised-face flange and Flat-Face Flange are two common flange faces. When choosing between a Raised-Face Flange (RF) and a Flat-Face Flange (FF), it is important to consider application requirements, including pressure, temperature, and material type. In the following article, we will introduce the differences and applications of the two flange faces.
What is a flange face?
The Flange face refers to the part of the flange that contacts and forms a seal with the gasket, pipe, or equipment surface. The design and surface shape of the flange face directly affects the sealing performance and application of the flange connection. The flange face can have different design types depending on different usage requirements. The following are some joint flange face types:
Flat Face (FF): The entire flange surface is flat and is usually used in low-pressure applications, especially in connections with cast iron or other fragile materials, to prevent damage caused by uneven pressure.
Raised Face (RF): The flange’s sealing surface has a raised part and a smaller sealing area, allowing higher pressure to be applied to the gasket. It is usually used in medium and high-pressure applications.
Ring Type Joint (RTJ): The flange face has an annular groove specially designed to cooperate with metal ring gaskets. This joint provides extremely high sealing performance and is suitable for high-pressure and high-temperature environments.
Male-Female (M/F): A pair of flanges, one of which has a protrusion (convex face) and the other has a corresponding depression (concave face). This design helps maintain flange alignment while improving sealing performance.
Tongue-and-Groove (T/G): A pair of flanges where one flange has a raised tongue (Tongue) and the other flange has a corresponding groove (Groove). This design is also used to enhance sealing and flange alignment accuracy.
Let’s learn more about the difference between a flat face flange and a raised face flange.
Types of Flange Faces
What is a flat-face flange?
A flat-face Flange (FF) is a flange design whose sealing surface is flat and has no raised parts. The entire flange surface is in full contact with the surface of the connected pipe or equipment, ensuring that pressure is evenly applied when sealing. The design and manufacture of flat-face flanges usually meet the requirements of ASME B16.5 and EN 1092-1.
Features of flat face flange:
- Total flat sealing surface: The entire flange surface is in contact with the gasket, which is suitable for pipe connections with large diameters or fragile materials.
- Suitable for low-pressure environments: Due to its design, flat-face flanges are usually used in lower-pressure applications to avoid stress concentration caused by uneven bolt loads.
- Material compatibility: FF flanges are usually used with cast iron, copper, and plastics, and they are easily damaged by uneven pressure.
Applications of flat face flanges:
- Water treatment systems: such as water pipes and sewage treatment pipes.Heating, ventilation, and air conditioning (HVAC) systems: connecting air ducts and other equipment in low-pressure environments.
- Low-pressure steam, air, and natural gas pipelines: suitable for non-critical pipeline connections.
- Low-pressure equipment connections: These include the connections between pumps, valves, heat exchangers, and other equipment and pipelines.
What is a raised face flange?
The RF flange (Raised Face Flange) is a common flange type characterized by a slightly raised portion on the sealing surface. This raised portion is designed to increase the sealing effect, especially in medium-pressure and high-temperature applications. The design and manufacture of RF flanges usually follow the requirements of ASME B16.5, ASME B16.47, and other specifications.
For example:
Requirements for the raised height of raised face flanges in ASME B16.5:
- Small size flanges (NPS 1/2 to 12): The raised height is usually 1/16 inch (about 1.6 mm).
- Large-size flanges (NPS 14 to 24): The raised height is usually 1/4 inch (about 6.35 mm).
- Larger-size flanges (NPS 26 and above): The raised height varies according to the flange size and pressure rating and usually follows the design standards or engineering requirements.
Features of RF flanges
- Raised sealing surface:
The RF flange’s sealing surface (i.e., the connection surface of the flange) is slightly raised compared to the rest. This design provides a small and concentrated area for placing the gasket, thereby improving the sealing performance.
- Sealing performance:
Due to their raised design, RF flanges provide a more reliable sealing effect under high pressure. The gasket is subjected to higher pressure in the raised area of the flange, reducing the risk of leakage.
Application of RF flanges
RF flanges are widely used in industries such as oil and gas, chemicals, and power, especially in environments requiring high sealing and temperature resistance.
- High-pressure and high-temperature piping systems: RF flanges suit piping systems that withstand higher pressures and temperatures.
- Oil and gas industry: Pipeline connections for transporting and processing oil and gas.
- Chemical plants and power production: Widely used in process pipelines that require tight sealing.
- Mechanical and equipment connections: These are used for connections in various industrial equipment and systems.
What is the difference between Raised Face and Flat Face Flanges?
| Feature | Raised Face Flange (RF) | Flat Face Flange (FF) |
| Seal Design | Has a raised sealing face, slightly protruding | The sealing face is flat with no protrusions |
| Pressure Range | Suitable for medium to high pressure and high temperature environments | Typically used for low pressure and low temperature environments |
| Sealing Performance | Provides higher sealing pressure, reducing leakage risk | Lower sealing performance, suitable for use with brittle materials like cast iron |
| Material Suitability | Suitable for carbon steel, stainless steel, alloy steel, etc. | Typically used with materials like cast iron and copper |
| Application Areas | Oil and gas, chemical, power industries, high-pressure systems | Water treatment, HVAC systems, low-pressure applications |
| Manufacturing Standards | ASME B16.5, ASME B16.47, etc. | ASME B16.1, ASME B16.5, etc. |
| Installation Method | The raised portion of the flange must match the gasket and apply even pressure | The flat surface of the flange directly contacts the gasket |
References and Further Reading：
https://www.savree.com/en/encyclopedia/flange-faces
https://www.wermac.org/flanges/flanges_raised-face_flat-face_ring-type-joint.html
