How to choose right gaskets?
Gasket is made of nonmetal, metal or metal nonmetal, placed between the two planes to strengthen the sealing material, in order to prevent fluid leakage, the sealing element is set between the static sealing faces.
Standard of gaskets
The most common standards of gaskets are:
- BS10:2009: British Standard specification for flanges and bolting for pipes, valves and fittings.
- DIN: German Institute for Standardisation (the German Institute for Standardisation).
- ANSI: the American National Standards Institute.
- ASA: the American Standards Association (now ANSI).
- ASME: the American Society of Mechanical Engineers The ASME B16.47 Series A and B took over from MSS SP-44 (Series A) and API 605 (Series B)
- ISO: International Organisation for Standardisation.
- JIS: Japanese Industrial Standards.
British and European Standards
The current standard for use in the UK and Europe is BS EN 1514-1:1997. This standard took over from BS 4865-1:1989 for PN flanges.
Other standards include:
- BS EN 12560-1:2001: Flanges and their joints. Gaskets for Class-designated flanges. Non-metallic flat gaskets with or without inserts.
- BS EN 12560-2:2013: Flanges and their joints. Dimensions of gaskets for Class-designated flanges. Spiral wound gaskets for use with steel flanges.
- BS EN 12560-3:2001: Flanges and their joints. Gaskets for Class-designated flanges. Non-metallic PTFE envelope gaskets.
- BS EN 14772:2005: Flanges and their joints. Quality assurance inspection and testing of gaskets in accordance with the series of standards EN 1514 and EN 12560.
Selection requirements of gasket manufacturer
- 1. The manufacturer of pressure vessel and pressure pipe gasket must obtain the special equipment manufacturing license issued by the State Bureau of technical supervision, have perfect manufacturing and inspection means, pass the ISO9000 series quality system certification, and have good performance in the petroleum and petrochemical industry.
- 2. The material purchasing department shall organize the quality inspection of sealing gasket after warehousing, including certificate of conformity, material certificate, specification and model, etc. For the sealing gasket of key equipment and sealing materials with special material requirements, the manufacturer shall put forward necessary re inspection requirements. The re inspection shall be carried out according to the corresponding manufacturing standards. The number of samples for acceptance inspection shall not be less than one piece per batch.
PTFE gasket can resist the corrosion of various chemical media including acid, alkali, solvent, etc., and has the characteristics of clean and pollution-free. Because of its cold flow performance and thermal creep characteristics, the service pressure of pure PTFE gasket is not more than 4.0Mpa, and the service temperature is generally not more than 150 ℃. The sealing surface type should adopt full plane, concave surface, convex surface, tenon surface and groove surface. However, PTFE coated gasket can overcome the shortcomings of ordinary PTFE gasket, such as non elasticity and easy cold flow, so its performance is better. Teflon coated gasket should not be used in vacuum or where the material of its embedded layer is easy to be corroded by medium. Generally, PMF and PMS are used to reduce the liquid retention in the pipe. PFT type is used in the case of nominal size greater than or equal to DN350.
(1) Application scope of spiral wound gasket
- ① The nominal pressure range specified by sh3407-2013 in petrochemical industry is PN 1.0 ~ 25.0mpa, and the temperature range is: stainless steel strip and special asbestos tape spiral wound gasket: – 50 ~ 500 ℃; stainless steel belt and flexible graphite belt spiral wound gasket: – 196 ~ 800 ℃ (oxidizing medium ≤ 600 ℃); stainless steel belt and polytetrafluoroethylene tape spiral wound gasket are – 196 ~ 200 ℃.
- ② The chemical industry standard hg20160-2009 stipulates that spiral wound gasket is suitable for nominal pressure PN 1.6 ~ 16.0mpa, and the temperature range is: the maximum temperature of stainless steel strip and special asbestos tape spiral gasket is 500 ℃; the maximum temperature of stainless steel strip and flexible graphite tape spiral gasket is 650 ℃ (oxidizing medium ≤ 450 ℃) ）The maximum temperature of stainless steel belt and PTFE tape wound gasket is 200 ℃.
(2) It is suggested that the hardness of metal belt used for spiral wound gasket is HV ≤ 150. When it is used for austenitic stainless steel flange sealing, the chloride ion content in non-metallic packing of spiral wound gasket should be controlled within 100ppm.
(3) The corresponding gasket type should be selected according to the type of flange. The spiral wound gasket with inner ring, outer ring or outer ring is recommended for flat and convex flange, and the spiral wound gasket with inner ring is recommended for concave convex flange.
(4) Generally, spiral wound gasket is only suitable for flange above medium pressure. If it is used on low pressure flange, the sealing effect may not be obtained due to insufficient bolt force. It is suggested that expanded graphite spiral wound gasket should be preferred.
(5) The inner ring material shall have the same or higher corrosion resistance as the metal belt material, and the stainless steel material shall comply with the provisions of GB / t3280 or GB / t4237. Unless otherwise specified, carbon steel can be used for middle ring, and GB / t11253 or GB / t912 can be used for carbon steel, and spraying, electroplating or other surface treatment can be adopted for carbon steel.
(6) The basic spiral wound gasket is only suitable for tenon and groove flange. The inner ring should be used for CLASS600, class900, class1500, class2500 and the gasket with PTFE as filling material.
The maximum working pressure of flexible graphite composite gasket is 6.3MPa, and the maximum working temperature depends on the material of metal core plate. For low carbon steel and stainless steel core plate, it is 450 ℃ and 650 ℃ respectively (450 ℃ for oxidizing medium).
Flexible graphite composite gasket is suitable for the following media: water, oil, solvent, acid, liquid hydrocarbon, low-temperature liquefied gas for liquid; air, hydrogen, oil and gas, high-temperature flue gas, steam, hydrocarbon and other gases with strong permeability.
Flexible graphite composite gasket has low requirements for flange surface processing, and can be used in corrosive occasions. It is suitable for welding steel flange with convex and concave convex face at low pressure and high temperature, especially it can be used to replace asbestos rubber plate used in common parameter occasions in the past.
Corrugated composite gasket is suitable for high and low temperature (- 200 ℃ ~ + 700 ℃), high, low pressure, vacuum (vacuum ~ 25MPa) and other occasions. It can maintain high sealing performance under the condition of low compression stress. It can be applied from dnl0 pipe flange to dn2500 vessel flange. It can also keep excellent sealing performance for a long time when the temperature and pressure are easy to fluctuate.
Metal clad gasket
The maximum service temperature of metal clad gasket is 450 ℃, and the maximum design pressure is 6.4 MPa. The maximum hardness Hb of coated metal shall not exceed the following values: L2<40, 08 steel<90, tinplate<90, 0Cr13<183, 0Cr19Ni9, 0cr18ni11ti, 00Cr17Ni14Mo2<187.
When asbestos board is used as filling material, splicing of asbestos board shall be prohibited. It must be clearly indicated when purchasing requirements are put forward by suppliers of metal clad gasket of our company.
Metal ring gasket
Metal ring gasket is mainly used in high pressure, high temperature equipment and pipe flange. The national standard and petrochemical industry standard are applicable to nominal pressure of 2.0 ~ 42.0mpa, chemical industry standard is applicable to 2.5 ~ 16.0mpa, and JB / t89-94 is applicable to 6.3, 10, 16 and 20MPa. The service temperature of metal ring gasket depends on the metal material used. The hardness of the metal ring gasket material should be hb30-40 lower than that of the flange sealing surface material, and its maximum value should meet the requirements of relevant standards.
The sealing surface of metal ring gasket shall be free of scratches, bumps, cracks and defects, and the surface roughness shall not be greater than Ra1.6 μ M.
The metal ring gasket of No. 10 steel or 08 steel shall be coated with anti rust oil after inspection.
Metal serrated gasket
The metal toothed gasket can be used for concave convex integral cast steel pipe flange and butt welding steel pipe flange with nominal pressure of 4.0 ～ 16.0 MPa. The maximum service temperature of 08 or 10 or 0 22cr17ni12mo2 is 450 ℃, the maximum service temperature of 0Cr13 is 540 ℃, and the maximum service temperature of 06cr19ni9 is 600 ℃.
The limit deviation of parallelism of two end faces of metal toothed gasket shall not exceed ± 0.05 mm per 100 mm diameter length. The surface roughness Ra of the tooth top plane of the metal tooth profile gasket is 1.6 μ M. The metal toothed gasket shall be made of a whole steel plate, and butt welding is not allowed.
- 1. The type and application scope of the gasket shall comply with the provisions of the design documents and the current effective standards of the state and relevant industries. The change of gasket material and use standard shall be approved.
- 2. There are many factors that affect the selection of gaskets. Generally, on the premise of ensuring safe operation, the gaskets with low price, easy manufacturing, easy installation and replacement should be selected as far as possible.
- 3. The thickness of the gasket depends on the specific situation. Generally speaking, if the sealing surface is well processed and the pressure is not too high, the thin gasket should be selected. However, when the internal pressure is high, the gasket is too thin, corresponding to the elongation of the bolt, and the rebound of the gasket is too small to achieve the necessary recovery amount, which is easy to cause leakage. Therefore, when the pressure is high, the thicker gasket should be selected.
- 4. In order to ensure the interchangeability of gaskets, the specification and material should be unified as much as possible in the selection. In consideration of meeting the use requirements, the specifications and materials should be merged as far as possible, and unnecessary diversification should be avoided.
- 5. In the case of gas medium and high pressure, it is not allowed to use high-strength graphite gasket without hemming, and corrugated gasket is not allowed for tenon and groove face flange.
- 6. To take care of special requirements, such as a certain medium does not allow micro fiber mixing, do not choose asbestos rubber plate and other fiber gasket. Some parts of the vibration is very large, it is necessary to choose a strong anti vibration, elastic, durable gasket.
- 7. Asbestos rubber gasket is not allowed to be used in highly toxic, explosive, strong corrosive and polluting media and harmful gases.
- 8. Spiral wound gasket with metal protection ring shall be selected for the valve and flange gasket of nozzle at the bottom of LPG spherical tank (the gasket of convex and concave flange shall be provided with inner reinforcing ring);
- 9. When selecting metal gasket, it should be used in fully annealed state, and soft metal material should be selected as far as possible. The hardness of gasket should be HB30-40 lower than that of flange sealing surface.
- 10. The replacement principle of gasket on imported equipment (including instruments) is to replace the original type and material. If the gasket form or material needs to be replaced due to special reasons, it shall be approved.
- 11. The type and material of gasket shall be selected according to the fluid, service condition (pressure and temperature) and sealing requirements of flange joint. The type and surface roughness of flange sealing surface shall be suitable for the type and material of gasket.
- 12. When asbestos or flexible graphite gasket is used for stainless steel or nickel base alloy flange, the chloride ion content in gasket material shall not exceed 50ppm.
- 13. When flexible graphite material is used in oxidizing medium, the maximum temperature should not exceed 450 ℃.
- 14. For flange with nominal pressure less than or equal to 1.6Mpa, semi metal gasket or metal ring gasket such as spiral wound gasket and metal clad gasket should be selected (very important).
- 15. When selecting gasket size, attention should be paid to ensure that the inner diameter of non-metallic flat gasket or inner ring of spiral wound gasket is not lower than the inner diameter of flange. If the inner diameter of gasket (or inner ring) is required to be equal to the inner diameter of flange, corresponding flange inner diameter shall be provided to gasket manufacturer as gasket inner diameter, but sufficient bearing width of gasket shall be ensured.
- 16. In the case of temperature and pressure change or vibration and impact, gaskets with good rebound characteristics should be used, such as corrugated composite gasket or spiral wound gasket (with inner and outer rings).
- 17. Asbestos rubber plate gasket: it is recommended that it is only used in non important equipment with low temperature and pressure. Asbestos rubber as gasket should be considered comprehensively and selected carefully. Asbestos gasket should not be used in extreme or highly hazardous media and high vacuum sealing occasions. The thickness of asbestos gasket for pipeline is generally 1.5-3 mm. The pressure of thin gasket is high, but the sealing pressure is large. The resilience of thick gasket is better than that of thin gasket, so it is easier to seal. The specific pressure value of gasket is also lower, but the thickness is more than 3 mm, so the leakage rate does not change much, and it is easy to extrude when the pressure is high.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.ugsteelmill.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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