What is a gasket?
The gasket is a material made of stainless steel, rubber or metal that is placed between the two planes to reinforce the seal, and a sealing member that is disposed between the static seal faces to prevent fluid leakage. It is usually used to prevent pressure, corrosion, and natural thermal expansion and contraction between two objects. Since the machined surface is not perfect, the gasket can be used to fill the irregularities. The gasket is usually made of sheet material such as mat paper, rubber, silicone rubber, metal, cork, felt, neoprene, nitrile rubber, fiberglass or plastic polymer (such as polytetrafluoroethylene). Gaskets for specific applications may contain asbestos.
|Materials of Gaskets||Types of gaskets|
|Standards of Gaskets||Gasket installation requirements|
|How to choose a gasket||How to get high quality gaskets|
Flange gaskets are made of various materials like:
- Cork and Cork Rubber
- Closed Cell Foam
- Cross-linked Polyethylene
- Plant Fiber
- Cellulose Fiber
- Stainless Steel
- Cast iron etc.
According to its material and structural characteristics, gaskets can be divided into non-metal gaskets, metal gaskets, metal-non-metal composite gaskets, and various types can be subdivided into several types.
- Compressed Non-Asbestos Fibre Gasket (CNAF)
- PTFE Gasket
- Rubber Gasket
Metal-non-metal Composite Gaskets
- Spiral Wound Gaskets
- Camprofile Gaskets
- Metal Jacketed Gasket
Non-metallic gaskets are usually composite sheet materials are used with flat-face and raised-face flanges in low Pressure Class applications. Non-metallic gaskets are manufactured from arimid fiber, glass fiber, elastomer, Teflon® (PTFE), graphite etc.. Full-face gasket types are suitable for use with flat-face flanges. Flat-ring gasket types are suitable for use with raised face flanges.
ASME B16.21 covers types, sizes, materials, dimensions, dimensional tolerances, and markings for non-metallic flat gaskets.
Non-Metallic gaskets types Source: wermac.org
Metal-non-metal composite gaskets are composites of metal and non-metallic materials. The metal is intended to offer strength and resiliency, while the non-metallic portion provides conformability and sealability. Often used semi-metallic gaskets are spiral wound and camprofile, and a variety of metal-reinforced graphite gaskets.
Semi-metallic are designed for almost all operating conditions and high-temperature and pressure applications, and are used on raised face, male-and-female, and tongue-and-groove flanges.
ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and semi-metallic gaskets.
Typical Spiral Wound gasket
Metallic gaskets are fabricated from one or a combination of metals to the desired shape and size. Often used metallic gaskets are ring-type-joint gaskets (RTJ). They are always applied to special, accompanying flanges which ensure good, reliable sealing with the correct choice of profiles and material.
Ring Type Joint gaskets are designed to seal by “initial line contact” or wedging action between the mating flange and the gasket. By applying pressure on the seal interface through bolt force, the “softer” metal of the gasket flows into the microfine structure of the harder flange material, and creating a very tight and efficient seal.
ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and composite gaskets.
Types of RTJ gaskets
The most common standards are:
- BS10:2009: British standard specification for flanges and bolting for pipes, valves and fittings.
- DIN Deutsches Institut für Normung): 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
- The sealing gasket and flange sealing surface should be cleaned, and there should be no defects such as scratches and spots that affect the connection sealing performance.
- The outer diameter of the gasket should be smaller than the flange sealing surface. The inner diameter of the gasket should be slightly larger than the inner diameter of the pipe. The difference between the two inner diameters is generally 2 times the thickness of the gasket to ensure the gasket after pressing. The inner edge does not protrude into the container or conduit to prevent interference with the flow of fluid in the container or conduit.
- The pre-tightening force of the gasket should not exceed the design regulations, so as to avoid the excessive compression of the gasket and the rebound ability.
- When the gasket is pressed, it is best to use a torque wrench. For large bolts and high strength bolts, hydraulic tensioners are preferred. The tightening torque should be calculated according to the compression of a given gasket. The hydraulic pressure of the hydraulic tensioner should also be determined by calculation.
- When installing the gasket, tighten the nuts in order. However, the design value should not be reached once. Generally, it should be circulated for at least 2 to 3 times so that the stress distribution of the gasket is uniform.
- For pressure vessels and pipelines with flammable and explosive media, safety tools should be used when replacing gaskets to avoid sparks caused by the collision of tools with flanges or bolts, resulting in fire or explosion accidents.
- 7. If the pipeline leaks, it must be replaced after the pressure reduction treatment or adjust the installation of the gasket. It is strictly forbidden to operate with pressure.
The choice of gasket material depends mainly on the following three factors: Temperature and pressure medium
A metal gasket material
- Carbon steel: The recommended maximum operating temperature does not exceed 538 ° C, especially when the medium is oxidizing. High-quality thin carbon steel sheets are also not suitable for use in equipment for the production of inorganic acid, neutral or acidic salt solutions. If carbon steel is subjected to stress, the equipment accident rate under hot water conditions is very high. Carbon steel gaskets are commonly used for high concentrations of acids and many alkaline solutions. The Brinell hardness is about 120.
- 304 stainless steel 18-8 (chromium 18-20%, nickel 8-10%), the recommended maximum working temperature does not exceed 760 °C. In the temperature range of -196~538 °C, stress corrosion and grain boundary corrosion are prone to occur. Brinell hardness of 160.
- The carbon content of 304L stainless steel does not exceed 0.03%. The recommended maximum operating temperature does not exceed 760 °C. Corrosion resistance is similar to 304 stainless steel. The low carbon content reduces the precipitation of carbon from the crystal lattice, and the resistance to intergranular corrosion is higher than that of 304 stainless steel. The Brinell hardness is about 140.
- 316 stainless steel 18-12 (chromium 18%, nickel 12%), adding about 2% molybdenum in 304 stainless steel, when the temperature increases its strength and corrosion resistance. It has higher creep resistance than other common stainless steels when the temperature is increased. The recommended maximum operating temperature does not exceed 760 °C. The Brinell hardness is about 160.
- The maximum continuous operating temperature of 316L stainless steel is not to exceed 760 ° C ~ 815 ° C. The carbon content does not exceed the superior stress resistance and grain boundary corrosion of 316 stainless steel. The Brinell hardness is about 140.
- 20 alloy 45% iron, 24% nickel, 20% chromium and a small amount of molybdenum and copper. The recommended maximum operating temperature does not exceed 760 ° C ~ 815 ° C. It is especially suitable for the manufacture of equipment resistant to sulfuric acid corrosion with a Brinell hardness of approximately 160.
- Aluminum aluminum (content is not less than 99%). Aluminum has excellent corrosion resistance and processing properties and is suitable for the manufacture of double-clamp gaskets. The Brinell hardness is about 35. The recommended maximum continuous operating temperature does not exceed 426 °C.
- The composition of copper copper is close to pure copper, which contains traces of silver to increase its continuous operating temperature. The recommended maximum continuous operating temperature does not exceed 260 °C. The Brinell hardness is about 80.
- Brass (copper 66%, zinc 34%), has good corrosion resistance under most working conditions, but does not adapt to acetic acid, ammonia, salt and acetylene. The recommended maximum continuous operating temperature does not exceed 260 °C. The Brinell hardness is about 58.
- Hastelloy B-2 (26-30% molybdenum, 62% nickel and 4-6% iron). The recommended maximum operating temperature does not exceed 1093 °C. Has excellent heat resistance and hydrochloric acid corrosion performance. It also has excellent resistance to wet hydrogen chloride gas corrosion, sulfuric acid, phosphoric acid and reducing salt solution corrosion. It has high strength under high temperature conditions. The Brinell hardness is about 230.
- Hastelloy C-276 16-18% molybdenum, 13-17.5% chromium, 3.7-5.3% tungsten, 4.5-7% iron, the rest are nickel). The recommended maximum operating temperature does not exceed 1093 °C. Has excellent corrosion resistance. It has excellent corrosion resistance for various tried cold nitric acid or boiling nitric acid with a concentration of 70%, good resistance to hydrochloric acid and sulfuric acid corrosion and excellent stress corrosion resistance. The Brinell hardness is about 210.
- Inconel 600 nickel-based alloy (77% nickel, 15% chromium and 7% iron). The recommended maximum operating temperature does not exceed 1093 °C. It has high strength under high temperature conditions and is usually used in equipment that needs to solve stress corrosion problems. It has excellent simultaneous processing properties under low temperature conditions. The Brinell hardness is about 150.
- Monel 400 (copper 30%, nickel recommended maximum continuous working temperature does not exceed 815 ° C. In addition to strong oxidizing acid, it has excellent corrosion resistance to most acids and bases. Easy to use in hydrofluoric acid, mercuric chloride, mercury medium Stress corrosion cracking occurs, and thus, it is not suitable for the above medium. It is widely used in equipment for manufacturing hydrofluoric acid, and has a Brinell hardness of about 120.
- Titanium: the recommended maximum operating temperature does not exceed 1093 °C. Excellent corrosion resistance at high temperatures. It is well known that it is resistant to chloride ions and has excellent resistance to nitric acid corrosion over a wide range of temperatures and concentrations. Titanium is used in most alkaline solutions and is suitable for use in oxidizing conditions. The Brinell hardness is about 216.
Non-metallic gasket material
- Natural rubber NR has good corrosion resistance to weak acids and bases, salt and chloride solutions, and poor corrosion resistance to oils and solvents. It is not recommended for ozone media. Recommended operating temperature -57 ° C ~ 93 ° C.
- Neoprene CR Neoprene is a synthetic rubber that is suitable for the corrosion of acid, alkali and salt solutions resistant to moderate corrosion. Good corrosion resistance to commercial oils and fuels. However, the corrosion resistance of strong oxidizing acids, aromatic hydrocarbons and chlorinated hydrocarbons is poor. Recommended operating temperature -51 ° C ~ 121 ° C.
- Butadiene Cyanide NBR Butadiene cyanide rubber is a synthetic rubber that is suitable for corrosion resistance to oils, solvents, aromatic hydrocarbons, alkaline hydrocarbons, oil and natural gas over a wide temperature range. Good corrosion resistance to hydroxides, salts and near-neutral acids. However, in strong oxidizing medium, chlorinated hydrocarbons, ketones and lipids, the corrosion resistance is poor. The recommended working temperature is 51 ° C ~ 121 ° C.
- The fluororubber fluororubber compound is prepared by mixing a binary and ternary fluorogenic rubber with a compounding agent and a vulcanizing agent. In addition to excellent heat resistance, medium resistance, and good physical and mechanical properties, it also has low compression set, good elasticity and long service life. Fluororubber has outstanding heat resistance (200 ~ 250 ° C), oil resistance, can be used to manufacture cylinder liner seals, plastic bowls and rotating lip seals, which can significantly improve the use time. Recommended operating temperature is -40 ° C ~ 232 ° C.
- Chlorosulfonated polyethylene synthetic rubber has good corrosion resistance to acid, alkali and salt solutions, and is not affected by weather, light, ozone, commercial fuels such as diesel and kerosene. It does not apply to aromatic hydrocarbons, chlorinated hydrocarbons, chromic acid and nitric acid. Recommended operating temperature -45 ° C ~ 135 ° C.
- Silicone rubber silicone rubber has outstanding high and low temperature resistance and can be used for ultra long-term use at 150 °C without performance change; it can be used continuously for 10,000 hours at 200 °C, and it can maintain its unique temperature within the operating temperature range of -70 to 260 °C. The use of elasticity, ozone resistance, weather resistance and other advantages, suitable for the production of sealing gaskets required in thermal mechanisms, such as sealing gaskets, valve gaskets, oil seals (for water media), etc., special silicone rubber can be used to make oil seals.
- Ethylene-propylene rubber has good corrosion resistance to strong acid, alkali, salt and chloride solutions. It does not apply to oils, solvents, aromatic hydrocarbons and hydrocarbons. Recommended operating temperature -57 ° C ~ 176.
- Graphite This material contains no resin or inorganic all-graphite material and can be classified into a metal with or without a metal element. The material can be bonded to make pipe gaskets with diameters in excess of 600 mm. It has excellent corrosion resistance to many acids, bases, salts and organic compounds and heat transfer solutions, even high temperature solutions. It does not melt, but it will sublimate beyond 3316 °C. The use of this material in strong oxidizing media should be prudent under high temperature conditions. In addition to being used for gaskets, this material can also be used to make non-metallic wound tapes in fillers and wound gaskets.
- Ceramic fiber and ceramic fiber molded into the strip are an excellent gasket material suitable for high temperature and low pressure working conditions and light flange conditions. The recommended working temperature is 1093 ° C, which can be used to make non-metallic winding tape in the wound gasket.
- PTFE concentrates on the advantages of most plastic gasket materials, including temperature resistance from -95 ° C to 232 ° C. In addition to free fluorine and alkali metals, it has excellent corrosion resistance to chemicals, solvents, hydroxides and acids. PTFE materials can be filled with glass to reduce the cold flow and creep of PTFE.
The charecteristics of Good Gasket
It will seal the fluid system.
A good gasket should be chemically inert to resists the system fluid to avoid chemical reaction with fluid system.
Should deform enough to flow into the imperfections on the gasket seating surfaces to provide intimate contact between the gasket and the seating surfaces.
Withstands system temperatures without serious damaging the fluid system properties.
Should be resilient and resists creep enough to maintain an adequate portion of the applied load.
A good gaskets has sufficient strength to resist crushing under the applied load, and maintain its integrity when being handled and installed.
Should not corrode the gasket seating surfaces.
There are limits on the degree of flange surface imperfection that can be sealed successfully with a gasket. Large nicks, dents, or gouges must be avoided, since a gasket cannot properly seal against them.
The surface finish of a flange is described as follows:
1. Roughness : Roughness is read in millionths of an inch (or meter) as the average of the peaks and valleys measured from a midline of the flange surface.
2. Lay – Lay : is the direction of the predominant surface-roughness pattern. Example: multidirectional, phonographic spiral serrations, etc.
3. Waviness : Waviness is measured in thousandths or fractions of an inch. Basically, it is the departure from overall flatness.
Typical roughness readings can be from 125 to 500 micro-inches for serrated flanges and 125-250 micro-inches for non-serrated flanges. Fine finishes, such as polished surfaces, should be avoided. Adequate “bite” in the surface is required to develop enough friction to prevent the gasket from being blown out or from extruding or creeping excessively.
The lay of the finish should follow the midline of the gasket. For example, concentric circles on a round flange, or a phonographic spiral. Every effort should be made to avoid lines across the face, such as linear surface grinding, which at 180º points will cross the seal area at right angles to the gasket, allowing a direct leak path.
Waviness is seldom a problem under normal conditions. There are two areas that must be watched, however, since excessive waviness is very difficult to handle.
The first area is glass-lined equipment where the natural flow of the fused glass creates extreme waviness. Often the answer here is to use thick and highly compressible gasketing.
The second area of concern is warped flanges. If warpage is caused by heat or internal stresses, re-machining is generally sufficient. However, warpage due to excessive bolt loads or insufficient flange thickness results in what is generally called bowing.
The solution is to redesign for greater flange rigidity. Sometimes backer plates can be added to strengthen the design without replacing the parts. Another step would be to add more bolts. When this is done, usually smaller bolt diameters are possible, thus adding more bolt stretch and better joint performance.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.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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