How to choose the right gasket?
In the production, processing, storage, transportation and even sales of petroleum, petrochemical, chemical and other industries, there are often risk factors such as flammability, explosion, high temperature, high pressure, toxic and harmful, corrosion and so on. The “running, emitting, dripping and leaking” of working medium of machines and equipment in use bring great harm to production. Leakage of working medium in equipment will cause waste and environmental pollution.
Gasket seal is the most important type of static seal for pressure vessel, process equipment, power machine and connecting pipe in process industry. They contain a wide range of fluid media, and preventing the leakage of liquid or gas through these joints is the most important and difficult task faced by the factory.
With the large-scale production equipment, the production process is developing towards high temperature, high pressure and high speed, there are more and more opportunities for leakage, the probability of accidents is increasing, and the economic loss is also increasing. Often, the leakage of a flange may lead to the shutdown of a unit or even the whole plant, and it is also very likely to cause fire, explosion, casualties and other major accidents. In case of leakage, environmental pollution, product loss and even accidents, the importance of gasket sealing is not obvious. Therefore, the problems existing in the selection, design and manufacture of gasket have gradually attracted people’s attention.
Gasket and gasket seal
Gasket is a kind of material or a combination of materials clamped between two independent connecting parts (mainly refers to flange). Its function is to maintain the seal between two connecting parts within a predetermined service life.
The gasket must be able to seal the joint surface and ensure that the sealing medium is impervious and not corroded, and can withstand the effect of temperature and pressure. Gasket seal is generally composed of connecting parts (such as flange), gasket and fastener (such as bolt and nut) (see Figure 1). Therefore, when determining the tightness of a flange, the whole flange connection structure must be considered as a system. Whether the gasket works normally or fails depends not only on the performance of the gasket itself, but also on the stiffness and deformation of the system, the roughness and parallelism of the joint surface, and the size and uniformity of the fastening load.
Figure.1 flange connection structure of gasket seal
Five elements of gasket selection
Knowing the structure of gasket seal, we can choose a suitable gasket from five aspects of temperature, application, medium, pressure and dimensions (tamps).
The selected gasket should have a reasonable service life under the highest or lowest working temperature, and the highest working temperature of different gasket materials is very different.
Fig.2 maximum operating temperature of common non-metallic materials
In addition to the maximum and minimum operating temperature that can be tolerated in a short time, the allowable continuous operating temperature shall also be considered. The gasket material should be able to resist creep to reduce the stress relaxation of the gasket, so as to ensure that the gasket can be sealed under working conditions. With the increase of temperature, the creep of most gasket materials will become serious. Therefore, an important index of gasket quality is the creep relaxation performance of the gasket at a certain temperature.
It mainly refers to the information of the connection system where the gasket is located. It is necessary to select the appropriate gasket material and type according to the flange material, flange sealing surface type, flange roughness and bolt information.
For non-metallic flange, the gasket with small preload requirement must be selected, otherwise it is likely that the gasket has not been pressed and the flange has been crushed in the process of flange tightening (as shown in Figure 4).
Figure.3 non metallic flange crushed
For different flange sealing surface types, the corresponding gasket types are also different. For example, the spiral wound gasket type corresponding to raised face flange (RF) is spiral wound gasket with inner and outer ring, the spiral wound gasket type corresponding to tenon groove flange (TG) is basic spiral wound gasket, and the spiral wound gasket type corresponding to concave convex flange (MFM) is spiral wound gasket only with inner ring (such as Fig. 5).
Fig.4 different types of flange sealing face correspond to different types of spiral wound gasket
Different gaskets have different requirements for flange roughness.
The gasket shall not be affected by the sealing medium under the whole working condition, including high temperature oxidation resistance, chemical corrosion resistance, solvent resistance, permeability resistance, etc. obviously, the chemical corrosion resistance of gasket material to medium is the primary condition for gasket selection. For non-metallic materials in non-metallic gasket or composite gasket, refer to the chemical resistance table of non-metallic materials provided by gasket manufacturer. For the metal materials in direct contact with the medium, the gasket shall have the same or higher corrosion resistance as the flange material.
The gasket must be able to withstand the maximum pressure, which may be the test pressure, which may be 1.25-1.5 times of the normal working pressure. For the gasket of non-metallic materials, the maximum pressure is also related to the maximum working temperature. Generally, the value of the maximum temperature multiplied by the maximum pressure (i.e. pxt value) has a limit value. Therefore, when selecting the maximum working pressure, the maximum pxt value that the gasket can bear should also be considered.
When meeting the conditions of flange roughness and unevenness, choose the thin gasket as much as possible. The thickness of gasket is related to its type, material, diameter, processing condition of sealing surface and sealing medium. For most of the non-metallic plate gasket, the ability of thin gasket to resist stress relaxation is also relatively large. Because the internal contact area of the thin gasket with the medium is also small, so the leakage along the gasket body is also reduced. In this case, the blow out force that the gasket bears is also small, and the gasket is not easy to blow out. Figure 9 shows that the blow out resistance of the thin gasket is stronger than that of the thick gasket. Of course, you can also refer to the gasket standard to select gasket thickness and other dimensions.
Figure.7 schematic diagram of radial force of gasket with different thickness under the same working condition
In addition, the gasket usually adopts the bolt flange connection structure, so the bolt should be pre tightened to meet the requirements of sealing. In addition, the gasket in this structure is a sealing element affected by many factors, so how to control the pre tightening force is a very difficult problem. All kinds of gaskets need different compression loads. For gasket seals with special requirements, they do not have standard connection dimensions, such as flange thickness, bolt size, bolt spacing, etc., which requires special design.
Source: China Gaskets Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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