The Classification of Gaskets
Detailed Explanation of the Classification of Gaskets

Flanges

A flange is a component that serves as a connection on a process pipeline, which is between pipelines and process equipment, as well as the connection of flange valves and accessories on pipelines. The use of flange connections provides both flexibility in installation and disassembly, as well as reliable sealing.​ The medium transported by process pipelines. It has a wide variety of types. The temperature and pressure are also different. Therefore, different requirements are put forward for the strength and sealing of flanges. In order to meet the needs of process pipeline installation engineering, many types of flanges with different structures and pressures have emerged.

Flange gaskets

Leakage is the main form of pipe flange failure, which is related to many factors such as the sealing structure type, the stiffness of the connected parts, the performance of the sealing parts, operation, and installation. Gaskets are the main sealing components for flange connections, so the correct selection of the gasket is also the key to ensuring that flange connections do not leak. There are many types of flange gaskets selected based on the corrosiveness, temperature, pressure, and sealing surface form of the medium transported by the pipeline. There are non-metallic gaskets, semi-metallic gaskets, and metal gaskets for pipe flanges.

Rubber asbestos pads

The rubber asbestos pad is mainly made of asbestos fibers and rubber as the main raw materials, supplemented by rubber compounding agents and fillers. It is made through mixing and stirring, hot roll forming, vulcanization and other processes. It can be applied to many media, such as steam, gas, air, salt water, acid, and alkali. The thickness of rubber asbestos pads varies among different professions. They usually use a thickness of 3mm. For flanges with a nominal diameter less than 100mm, the thickness of gaskets shall not exceed 2.5mm. When using pressure for smooth sealing surface flange connections, it should not exceed 2.5MPa.

There are two types of rubber asbestos pads commonly used in industry. One is an oil resistant rubber asbestos pad. It is suitable for conveying general oil products, liquefied hydrocarbons, propane, acetone and other media at temperatures below 200 ℃ and nominal pressures below 2.5 MPa. High temperature and oil resistant rubber asbestos pads can be used at temperatures up to 350 to 380 ℃. Another type is the medium pressure rubber asbestos pad, which can be used in media such as steam, condensate, water, air, etc. at 200 ℃ and below PN2.5MPa.

Rubber pads

The rubber gasket is a gasket made of rubber plate, which has a certain degree of corrosion resistance. It is commonly used for flange connections in pipelines that transport low-pressure water, acid, alkali and other media at temperatures below 60 ℃ and pressures not exceeding 1.0 MPa. The characteristic of this gasket is to use the elasticity of rubber to achieve good sealing effect. Therefore, it is also commonly used in the installation of cast iron flange valves.

Spiral wound gaskets

The spiral wound gasket is made by winding metal steel strip and non-metallic filler strip. It mainly includes metal wound gaskets, graphite wound gaskets, graphite metal wound gaskets, metal graphite wound gaskets, inner and outer ring wound gaskets, metal asbestos wound gaskets, metal PTFE wound gaskets, etc. This type of gasket has the advantages of low price, full utilization of materials, and good sealing performance. It is suitable for nominal pressures below 4.0 MPa and is suitable for temperature ranges. The temperature of 08 steel can reach 450 ℃. The winding gasket made of 0Cr13 steel strip can be used at a temperature of 540 ℃. 
  
This type of gasket is mostly used for smooth flange connections, and its sealing surface does not require a water line. Some wound gaskets also come with locating rings to prevent the gasket from deviating from the center of the flange. The thickness of the gasket is generally 4.5mm. When the diameter is greater than 1,000mm, the thickness of the gaskets is 6 to 7mm. The thickness of the positioning ring is about 3mm. Metal steel strip materials include 08 steel, 0Cr13 steel, and 1Cr18Ni9Ti steel, etc. Non-metallic strip materials include characteristic asbestos, flexible graphite, polytetrafluoroethylene, etc.

Toothed cushion
 
Toothed pads are made of various metals, including ordinary carbon steel, low alloy steel, and stainless acid resistant steel, with a thickness of 3 to 5mm. It uses the tooth-shaped dense pattern of concentric circles to contact the flange sealing surface, forming multiple seals, thus achieving good sealing performance. It is commonly used for the connection of concave convex sealing surface flanges. The maximum nominal pressure can reach 16.0MPa, which is suitable for areas with high working temperatures, such as the toothed gasket made of 0Cr13 material, which can be used at temperatures up to 530 ℃.

Metal washers
  
There are many types of metal washers, divided by shape into metal flat washers, elliptical, octagonal metal washers, and lens type washers. According to manufacturing materials, there are low-carbon steel, stainless acid resistant steel, copper, aluminum, and lead.
  
Metal flat washers are commonly used for smooth surface flat welding flanges, which can withstand lower temperatures and pressures.
 
Elliptical and octagonal metal gaskets are commonly used for trapezoidal groove butt welding flanges, with a nominal pressure range of 6.4 to 22.0MPa. 
 
Although the sealing is good, the manufacturing is complex and requires high precision. Lens gasket, named after its cross-sectional shape resembling a lens, has good sealing performance. In various high-temperature and high-pressure pipeline flange connections in petrochemical production, this type of gaskets is widely used, with a common nominal pressure range of 16.0 to 32.0MPa.
  
The use of metal gaskets follows a principle that the surface hardness of the gasket must be lower than the hardness of the flange sealing surface.
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