Metal Gaskets for Flanges
I. Introduction
In the modern industrial system, in equipment with strict requirements for sealing performance, such as high-pressure hydrogenation units, flange connections are a widely used and crucial connection method. The sealing performance of flange connections is directly related to the safe and stable operation of the equipment. When the RJ surface is used for the flange sealing surface, the metal ring gasket, as a specially adapted sealing gasket, plays an indispensable role. According to the cross-sectional shape, metal ring gaskets are mainly divided into elliptical metal ring gaskets (elliptical gaskets) and octagonal metal ring gaskets (octagonal gaskets). Both types have the characteristic of metal elastic line contact sealing and show unique application advantages under high-pressure working conditions.
II. Standards for Metal Ring Gaskets
(I) Standard Types of Metal Ring Gaskets for Steel Flanges
The main standards followed by metal ring gaskets for steel flanges include the PN series, corresponding to the standard GB/T 9128.1 - 2023 Metal Ring Gaskets for Steel Pipe Flanges - Part 1: PN Series; and the Class system, corresponding to the standard GB/T 9128.2 - 2023 Metal Ring Gaskets for Steel Pipe Flanges - Part 2: Class Series. These standards strictly regulate various performance indicators, dimensions, and specifications of metal ring gaskets to ensure their applicability and reliability under different working conditions.
(II) Requirements in the Standards
Structural Integrity Requirements: Metal ring gaskets are strictly prohibited from being spliced. Splicing will lead to non-uniform force distribution of the gasket on the sealing surface, and stress concentration points are likely to form at the splicing joints, seriously affecting the sealing effect. Under internal pressure or external loads, the splicing joints are very likely to become weak points for leakage. At the same time, the sealing surface of the metal ring gasket should not be repaired by welding. Welding repairs on the sealing surface will cause changes in the metal structure, possibly leading to changes in hardness and metallographic structure, thereby reducing the flatness and smoothness of the sealing surface and significantly decreasing the sealing performance.
Sealing Surface Quality Requirements: The sealing surface of the metal ring gasket must be free of defects such as scratches, dents, cracks, and pits. These defects will form microscopic leakage channels between the sealing surfaces. Under pressure, the medium is very likely to leak through these channels. For example, scratches will locally reduce the effective contact area of the sealing surface and destroy the smoothness of the sealing surface, resulting in a significant decrease in the sealing effect.
Hardness Requirements: The hardness of the metal ring gasket is usually required to be lower than that of the flange to ensure a tight connection. The principle of this hardness difference design is that during the flange connection process, the relatively softer ring gasket can better adapt to the microscopic shape irregularities of the flange sealing surface, fill the gaps, and achieve a tight fit. For octagonal gaskets, due to their complex cross-sectional shape, Brinell hardness is preferably used for testing. Brinell hardness testing can effectively reflect the ability of the material to resist deformation in a relatively large area. For elliptical gaskets, since their shape is relatively regular, Rockwell hardness is preferably selected. Rockwell hardness testing has advantages in measurement accuracy and efficiency.
Storage and Transportation Requirements: Metal ring gaskets must be placed horizontally during storage and transportation. Vertical placement may cause the gaskets to deform under their own weight. Especially for thinner metal ring gaskets, long-term vertical placement may cause permanent deformation, which will have an adverse impact on their sealing performance.
(III) Marking Examples
PN Series: Take the octagonal gasket as an example. If the nominal diameter is DN100, the nominal pressure is PN63, and the material is 316, the marking is: GRO GB/T 9128.1 DN 100 PN 63 316. This marking method clearly and explicitly identifies key information such as the type of the metal ring gasket (GRO represents an octagonal gasket), the applicable standard (GB/T 9128.1), the nominal diameter, the pressure rating, and the material, which is convenient for accurate identification and selection in engineering applications.
Class Series: For an elliptical gasket with a nominal diameter of DN100, a nominal pressure of class 150, a ring number of R36, and a material of 06Cr17Ni12Mo2, the marking is: GRV GB/T 9128.2 R36 316 or GRV GB/T 9128.2 100 - 150 316. This marking provides multiple combinations of necessary information to meet the requirements for the identification and maintenance management of metal ring gaskets in the Class system.
III. Application of Metal Ring Gaskets in High-Pressure Hydrogenation Units
(I) Sealing Requirements of High-Pressure Hydrogenation Units
High-pressure hydrogenation units operate in harsh environments with high temperature, high pressure, and the presence of special media such as hydrogen. Under such working conditions, the sealing performance of the flange connections is of vital importance. Due to the extremely high internal pressure, any slight leakage may lead to serious safety accidents and also cause a waste of hydrogen resources. Therefore, a sealing gasket with the ability to withstand high pressure, good elasticity, and excellent corrosion resistance is required to meet the sealing requirements.
(II) Advantages of Metal Ring Gaskets
High-Pressure Sealing Performance: Both elliptical gaskets and octagonal gaskets belong to metal elastic line contact seals. Under high-pressure environments, their line contact characteristics can provide reliable sealing effects. Take the octagonal gasket as an example. It has a surface contact (bevel) on the tapered surface of the groove. This contact method can generate a relatively uniform compressive stress after assembly. As the internal pressure increases, the contact stress will increase accordingly, effectively compensating for the gaps that may be generated due to pressure changes and preventing the leakage of the medium. The line contact (a line on the arc surface) of the elliptical gasket also performs excellently under high pressure. Its curved surface design enables the contact part to adaptively adjust the tightness of the fit under pressure, just like a natural elastic self-adaptive mechanism.
Elasticity and Adaptability: Metal ring gaskets are made of metal materials and have good elasticity. During the assembly process of the flange connection, the ring gasket can undergo a certain amount of elastic deformation to adapt to the machining errors of the flange sealing surface. Even if there are microscopic unevenness on the flange sealing surface, the metal ring gasket can fill these gaps through its own elastic recovery force, ensuring the reliability of the seal.
Corrosion Resistance: For the hydrogen medium in high-pressure hydrogenation units, the material selection of metal ring gaskets usually takes their corrosion resistance into full consideration. For example, metal ring gaskets made of 316 stainless steel contain alloy elements such as chromium and nickel, which can significantly improve their oxidation resistance and corrosion resistance. In a medium environment with relatively strong activity like hydrogen, they can maintain chemical stability for a long time and effectively extend the service life of the gaskets.
IV. Characteristics of Elliptical Gaskets
(I) Cross-Sectional Shape and Contact Characteristics
The elliptical cross-sectional shape of the elliptical gasket determines that it has a line contact at the part where the sealing surface is an arc surface. This line contact method can form a uniform initial sealing pressure around the contact line during the initial assembly. During operation, when the pressure in the system changes, the elastic deformation of the elliptical gasket will be transmitted along the contact line, enabling the contact stress to dynamically adjust to adapt to the pressure changes. Compared with octagonal gaskets, the elliptical shape of the elliptical gasket makes it easier to achieve precise centering under certain specific flange shapes and installation conditions. For example, in some connections with curved flange sealing surfaces, the elliptical gasket is more likely to fit the sealing surface and reduce local stress concentration.
(II) Hardness Testing and Applicable Scenarios
As mentioned above, Rockwell hardness is preferably used for testing elliptical gaskets. Rockwell hardness testing is relatively simple and fast to operate and can accurately reflect the uniformity of the hardness of the elliptical metal ring gasket at different parts. This uniformity is crucial for ensuring the stability and reliability of the elliptical gasket during the sealing process. Elliptical gaskets are suitable for some flange connection scenarios where the requirements for the shape accuracy of the sealing surface are relatively high, the pressure fluctuations are relatively small, but quick installation and disassembly are required. For example, in the branch pipe flange connections of some small high-pressure hydrogenation units that require frequent maintenance, the quick installation and disassembly characteristics of the elliptical gasket and its good sealing performance make it an ideal choice.
V. Characteristics of Octagonal Gaskets
(I) Contact Method and Force Distribution
The octagonal gasket has a surface contact (bevel) on the tapered surface of the groove. This surface contact method makes the pressure distribution on the contact surface relatively uniform after assembly. Under high-pressure environments, this uniform pressure distribution can more effectively disperse the internal pressure to the entire contact surface, thereby enhancing the sealing effect. The shape design of the octagonal gasket enables it to provide in multiple directions when matched with the octagonal groove. This multi-directional has significant advantages in resisting axial and radial deformations during the flange connection process.
(II) Hardness and Applicable Situations
Brinell hardness testing is preferably used for octagonal gaskets. Brinell hardness can better reflect the overall hardness of the octagonal metal ring gasket with a complex cross-sectional shape. By accurately measuring the Brinell hardness, it can be ensured that the octagonal gasket has appropriate hardness at different parts to meet different sealing requirements. Octagonal gaskets are suitable for flange connections that bear large axial and radial stresses, have a high pressure rating, and have extremely high requirements for sealing. For example, in the main pipe flange connections of large high-pressure hydrogenation units, the surface contact characteristics and uniform pressure distribution of the octagonal gasket can meet their high sealing requirements.
VI. Matching of Metal Ring Gaskets and Flange Sealing Surfaces
(I) Groove Design and Machining Precision
For the groove design of the flange sealing surface where the metal ring gasket is installed, there are strict technical requirements. Key parameters such as the dimensional tolerance and angular accuracy of the groove will all affect the installation and sealing performance of the metal ring gasket. For example, during the installation of the octagonal gasket, the accuracy of the tapered surface of the groove must be strictly matched with the shape of the octagonal gasket. If the deviation of the tapered surface angle is too large, it will cause uneven force on the octagonal gasket after installation, seriously affecting the sealing effect. At the same time, the surface roughness of the groove is also of great importance. Excessive surface roughness will prevent the metal ring gasket from forming an effective sealing surface when in contact, and too low a surface roughness may affect the initial installation and positioning of the metal ring gasket.
(II) Precautions During the Installation Process
Cleanliness Requirements: Before installing the metal ring gasket, it is necessary to ensure the cleanliness of the flange sealing surface and the metal ring gasket. Any impurities, oil stains, or rust will affect the contact between the gasket and the sealing surface, resulting in sealing failure. For example, in high-pressure hydrogenation units, if there are tiny rust particles on the sealing surface, under pressure, these particles may be embedded between the metal ring gasket and the sealing surface, scratching the sealing surface and destroying the sealing effect.
Installation Tools and Methods: Appropriate installation tools should be selected to avoid damaging the metal ring gasket during the installation process. For example, special fixtures and guiding tools are used to ensure the correct installation position of the metal ring gasket. According to the different shape characteristics of the elliptical gasket and the octagonal gasket, different installation methods and guiding measures are required. When installing the octagonal gasket, due to its special shape, more attention needs to be paid to its centering situation in the groove.
VII. Maintenance and Replacement of Metal Ring Gaskets
(I) Regular Inspections
During the operation of high-pressure hydrogenation units, regular inspections should be carried out on the metal ring gaskets. The inspection content includes whether there are signs of deformation, corrosion, and the contact situation of the sealing surface. For example, external non-destructive testing methods such as ultrasonic testing or magnetic particle testing can be used to check whether there are tiny cracks or corrosion defects inside the metal ring gasket. For metal ring gaskets installed in positions that are difficult to directly observe, sensor technology can be used for condition monitoring. For example, pressure sensors and temperature sensors are installed near the flange, and the sealing state of the metal ring gasket is indirectly judged by monitoring the pressure and temperature changes at the sealing location. If there are abnormal fluctuations in the pressure at the sealing location or the temperature is too high, it may indicate that the metal ring gasket has a leakage or local sealing failure.
(II) Replacement Timing
When the metal ring gasket shows obvious signs of wear, corrosion, deformation, or other irreparable defects, it should be replaced in a timely manner. In high-pressure hydrogenation units, even a slight leakage may lead to serious consequences. Therefore, the replacement timing of the metal ring gasket must be strictly controlled. During equipment maintenance, even if the metal ring gasket appears normal on the surface, it should be re-evaluated. Because during long-term operation, the internal structure of the metal ring gasket may have suffered invisible damage. It is necessary to comprehensively judge whether the metal ring gasket needs to be replaced based on factors such as the operating time of the equipment, the operating environment, and previous leakage records.
VIII. Conclusion
Metal ring gaskets play a crucial and irreplaceable role in flange sealing, especially in the flange sealing of high-pressure hydrogenation units. Through in-depth research on the standards of metal ring gaskets, their applications in high-pressure hydrogenation units, the respective characteristics of elliptical and octagonal gaskets, their matching with flange sealing surfaces, maintenance, and replacement, we can have a more comprehensive understanding of the importance and application key points of metal ring gaskets. In the future industrial development, with the continuous improvement of requirements for sealing performance and the continuous progress of technology, the design, manufacturing, and application of metal ring gaskets will also continue to develop and improve to meet more complex and stringent industrial needs.
EATHU Flange Factory, as a leading enterprise in the industry, has many remarkable advantages. In terms of the production process, it has advanced automated production lines and can precisely manufacture metal ring gaskets in strict accordance with various international standards to ensure the stability and consistency of product quality. In terms of material selection, it has established long-term and stable cooperative relationships with high-quality raw material suppliers and can obtain high-quality metal materials, ensuring the performance of products from the source. At the same time, EATHU Flange Factory has a professional R & D team, continuously invests in R & D resources for the development of new products and technological innovation, and can provide customized solutions according to the special needs of customers. In terms of after-sales service, it has established a complete after-sales service system, can respond to customer needs in a timely manner, and provide customers with comprehensive technical support and service guarantees.