The flange is the core connectors in the connection between piping systems and pressure vessels. Its sealing performance directly determines the safety and stability of the whole system. The flange sealing surface is the key part of sealing. Different structural forms apply to different working conditions. If the choice of the flange sealing surface is improper, it may cause media leakage, equipment damage, or even a safety accidents. In this paper, the common forms of flange sealing surfaces are combed systematically, the structural characteristics, application scenes, advantages and disadvantages of flange cover are analyzed, and professional reference is provided for practical application.
Analysis on the Shape and Characteristics of Common Flange Sealing Surface Forms
1.Flat Face Sealing Surface (FF)
Flat cover is the simplest form of structure. It is a flat circular surface with no grooves or protrusions. Easy to handle, inexpensive and easy to align during installation. Commonly used in low pressure and low temperature applications of non-corrosive media, such as water supply and drainage pipes, low pressure compressed air pipes, etc.
Advantages: convenient processing, good installation alignment, compatible with various gaskets (e.g. asbestos gaskets, rubber gaskets, non-asbestos fiber gaskets).
Weaknesses: Large sealing area, high bolt pretension required to ensure sealing. The smoothness of the sealing cover has a great influence on the sealing performance. Scratches or deformations on surfaces can easily lead to leaks.
Applicable Applications: Nominal pressure PN ≤ 1.6 MPa, temperature ≤ 200°C, medium such as water, air and steam (low pressure).
Typical applications: civil building piping and low voltage equipment interfaces.
2. Protruding face (RF)
Raised surface seal is on the flange body (usually 1.6mm or 3.2mm high, depending on the pressure class). The gasket only touches the raised ring, not the entire flange surface. This design reduces the sealing area and allows for better sealing with lower bolt pretensioning. It is one of the most widely used types in industry.
Strengths: Small sealing area, low bolt preload requirements, better superior sealing performance flat sealing surfaces, compatible with a variety of gasket types (such as metal-jacketed gaskets, graphite composite gaskets, spiral wound gaskets).
Weaknesses: Bulging rings are easily deformed on impact and require careful installation and protection.
Applicable Applications: Nominal pressure ≤ 4.0 MPa, temperature ≤ 400°C, oil, gas, vapour and light corrosive chemical liquids.
Typical application: Low voltage piping and general mechanical equipment interface (e.g. pumps and valves) in petrochemical plants.
3. Male and female faces (M&F)
The public cover and the mother cover consist of a pair of mating flanges: one flange with a a raised "male" sealing surface and the other flanged with the other a recessed "female" sealing surface. " The gasket is located in an annular groove on a concave surface, effectively preventing displacement during installation or operation.
Advantages: precise positioning of gasket, resistance to subsidence or displacement, stable sealing performance, suitable for medium pressure.
Weaknesses: Requires High machining precision and must match male and female surfaces accurately. Mismatch of specifications with flange (e.g. groove depth and projection height) may result in seal failure.
Applicable Applications: Nominal pressure 1.6 MPa < PN ≤ 10.0 MPa, temperature ≤ 350°C, medium such as crude oil, lubricants and medium corrosive chemical media (e.g., dilute sulfuric acid and diluted hydrochloric acid).
Typical application: connection of oil refining pipe to medium pressure vessel.
4. Tongue Trench Cover (T&G)
Tongue and groove seal is similar to an uneven surface and consists of a tongue surface (narrow convex band) and groove surface (corresponding groove surface). However, the tip of the tongue is narrower and the groove is deeper, allowing the gasket to be fully embedded in the groove, touching only the side and bottom of the groove. This structure minimizes the movement of the gaskets and distributes contact pressure evenly between the sealing covers, resulting in excellent sealing performance. Advantages: The gasket is fixed firmly, strong vibration resistance, high sealing reliability, even in the case of pressure fluctuation can be leakproof. Weaknesses: Difficult processing, high cost, precise alignment of grooves during installation. Residues easily accumulate in ditches and are difficult to clean.
Applicable Applications: Nominal pressure 4.0 MPa < PN ≤ 16.0 MPa, temperature ≤ 450°C, medium such as high pressure vapour, flammable and explosive liquids (e.g. propane and ethylene), strong corrosive medium (e.g. concentrated nitric acid and sodium hydroxide solution).
Typical applications: highpressure chemical pipeline, oil and gas pipeline, high temperature, autoclave interface.
5. Ring joints (RTJ)
The public cover and the mother cover consist of a pair of mating flanges: one flange with a a raised "male" sealing surface and the other flanged with the other a recessed "female" sealing surface. " The gasket is located in an annular groove on a concave surface, effectively preventing displacement during installation or operation.
Advantages: precise positioning of gasket, resistance to subsidence or displacement, stable sealing performance, suitable for medium pressure.
Weaknesses: Requires High machining precision and must match male and female surfaces accurately. Mismatch of specifications with flange (e.g. groove depth and projection height) may result in seal failure.
Applicable Applications: Nominal pressure 1.6 MPa < PN ≤ 10.0 MPa, temperature ≤ 350°C, medium such as crude oil, lubricants and medium corrosive chemical media (e.g., dilute sulfuric acid and diluted hydrochloric acid).
Typical application: connection of oil refining pipe to medium pressure vessel.
Key considerations in Flange Sealing Surface Selection
Pressure Rating: For low pressure applications, it is better to have flat or raised surfaces; for medium-and high-pressure applications, it is better to have concave-convex surfaces or tongue grooves; and for high pressure and extreme pressure applications, it is better to have circular seams.
Temperature range: Flat or raised surfaces are acceptable for low temperatures (≤200°C); concave-convex or tongue groove surfaces are acceptable for moderate temperatures (200°C -400°C);and ring seam surfaces are required for high temperatures (>400°C).
Media Characteristics: Corrosive media require a corrosion-resistant sealing surface material (e.g. stainless steel), preferably tongue grooves or circular seams (to reduce the contact area between the medium and the sealing surface). For viscous or easily crystallizing media, deep grooves (such as tongues and groove surfaces) should be avoided to prevent debris accumulation.
Installation and maintenance: When field installation conditions are limited, choose flat or raised surfaces (for alignment). The use of ring seam surfaces (to increase disassembly) should be avoided when frequent maintenance is required for long-term operation.
Common Selection Misconceptions and considerations
Myth 1: Assuming the cover is flat. In fact, flat sealing surfaces requires a significant bolt preload under high pressure, which can lead to flange deformation and leakage.
Myth 2: Concave and convex surfaces are confused with tongue groove surfaces. The height of the bump and the depth of the groove are different, so mating flanges must be strictly matched and not used interchangeably.
Note: Check the smoothness of the the sealing surface before installation (surface roughness generally requires ≤ 3.2 microns). Fixed any scratches or deformations (e.g. through tie-ups). The gasket material must match the type of sealing cover (e.g., metal ring gaskets must be used on ring seam surface; non-metallic gaskets are not accepted).
The selection of the flange sealing surface type is a systematic process that requires consideration of operating conditions, costs and maintenance. There is no "one-size-fits-all" type, only "suitable" options. Whether it is low voltage civil pipe or high voltage industrial equipment, only by fully understanding the characteristics of different sealing surfaces and making comprehensive assessment according to actual needs can the sealing reliability of flange connections be guaranteed, and the safety hazards and economic loss caused by improper selection can be avoided.