Gaskets – Materials and Types The primary purpose of a seal is to contain a fluid and protect the immediate environment from contamination. A gasket is used to create and retain a static seal between two relatively stationary parts. A static seal aim to provide a complete physical barrier against the fluid contained within by blocking any potential leakage path. In this article information is give on gasket, gasket materials and gasket categories (types). Gasket A gasket is a compressible material, or a combination of materials, which when clamped between two stationary members prevents the passage of the fluid across these members. To prevent passage of fluid, the gasket must be able to flow into (and fill) any irregularities in the mating surfaces being sealed, while at the same time be sufficiently resilient to resist extrusion and creep under operating conditions. The seal is effected by the action of force upon the gasket surface (usually by bolts), which compresses the gasket, causing it to flow into any surface imperfections. Gasket Materials Wide varieties of materials are used in the manufacture of gaskets. This section is aimed at providing a brief overview of the common materials. For simplicity they are divided into 4 parts. • Elastomeric materials • Fibrous materials • Other materials • Metallic materials Elastomeric materials They are the “entry level” to sheet sealing products. More commonly, they act as the binder when compounded with various fibres and fillers. They are made in various composition (hence performance) and are available in specification grade and commercial quality. Butyl rubber (IIR, also known as isobutylene,isoprene) An elastomer offering good resistance to ozone and gas permeation. Suitable for mild acids, alkalis and esters, but little resistance to oils and fuels. BS 3227 Grades B60, B70. Chlorosulphonated polyethylene An elastomer with excellent chemical resistance against acids and alkalis. Good oil resistance. Outstanding fire protection properties. Ethylene propylenediene (EPDM) Elastomer which offers good resistance to ozone, steam, strong acids and alkalis, but is not suitable for solvents and aromatic hydrocarbons. BS 6014 Grades EP60S, EP70S, EP80S. Fluoroelastomer A fluorinated hydrocarbon which offers excellent resistance to acids, aliphatic hydrocarbons, oils and many corrosive applications. Not suitable for amines, esters, ketones or steam. Natural rubber (NR) Excellent for recovery properties. Good resistance to most inorganic salts, mild acids and alkalis. Not recommended for oils and solvents, or where exposure to ozone, oxygen or sunlight is prominent. BS 1154 Grades Z40, Z50, Z60, Z70, Z80 Neoprene (chloroprene, CR) Excellent resistance to oils, ozone and weathering. Suitable for moderate acids, alkalis, salt solutions, petroleum, solvents, oils and fuels. It is not recommended for strong acids or hydrocarbons. BS 2752 Grades C40, C50, C60, C70, C80 Nitrile (NBR) Improved chemical resistance and temperature capabilities over neoprene. Good resistance to hydrocarbons and oils. Not suitable for chlorinated hydrocarbons, esters, ketones and strong oxidizing agents. BS 2751 Grades BA40, BA50, BA60, BA70, BA80, BA90 BS 6996 Grades BO60, BO80 Silicone Excellent temperature properties, and unaffected by ozone and sunlight. Not suitable for many hydrocarbons and steam. Styrene butadiene (SBR) Suitable for use with weak organic acids and moderate chemicals. Not suitable for strong acids, most hydrocarbons or ozone. Fibrous materials Aramid Aromatic amide fibre, offering high strength and stability, with medium temperature suitability. Raw fibres can fibrillate. Asbestos Since the 1890’s, the most common material used for sealing flanges, because of its ability to seal effectively over a broad range of service conditions. Now increasingly replaced by asbestos-free substitutes (mandatory in many locations). ‘Asbestos’ is the term applied collectively to various classes of fibrous minerals used in the industry characterized by their resistance to heat, strength and flexibility of their fibres. Chrysotile (white) asbestos is by far the most important variety. It is a hydrated silicate of magnesium. It may also contain small traces of aluminium and iron, and dependent on the quantities of these traces the colour of chrysotile asbestos in the crude rock form varies from pure white to greyish-green. Individual chrysotile asbestos fibres are silky and very flexible with a diameter smaller than that of any synthetic fibre. Asbestos is incombustible and is a poor conductor of heat. It is unaffected by temperatures up to approximately 450 deg. C, when it begins to lose its chemically combined ‘water of crystallisation’; this process is completed at about 700 deg. C, but the residue which remains fibrous, does not fuse until temperatures of 1450-1500 deg. C are reached. Asbestos is inert and is not toxic to touch, smell or ingestion. Asbestos fibre is not a health hazard unless its dust becomes airborne and such dust is continuously inhaled in large amounts over a prolonged period. Chrysotile fibres are the least harmful of all varieties of asbestos due to their curvi-linear nature. International Labor Organization (ILO) has recommended not to use Crocidolite (blue asbestos) variety of asbestos fibres, which is the most harmful due to its needle like structure. Carbon fibre High thermal conductivity ensures rapid heat dissipation and allows high temperature capability (except in oxidizing atmospheres). It has wide chemical resistance and may be used in the pH range 0 – 14. It is not suitable for oxidizing environments. Cellulose Natural fibre, suitable for low temperature and medium pressure applications. Raw fibres can fibrillate. Glass Inorganic complex of metal silicates. It offers good strength and moderate chemical resistance. Suitable for medium to high temperature applications. The fibres do not fibrillate. Man made mineral fibre (MMMF) Also referred to as “mineral wool”. Inorganic fibres consisting of metal silicates, with a wide range of diameters. Suitable for medium to high temperature applications. Fibres do not fibrillate. Other materials Flexible graphite Following processing into its exfoliated form, the material is essentially pure graphite, typically over 95% elemental carbon. The material has a wide chemical resistance. It is suitable for exceptionally wide temperature range from up to 400 deg. C in oxidizing environments and under certain circumstances, to 2500° deg. C in inert conditions. It has excellent resistance to stress relaxation, even at elevated temperatures. Mica (vermiculite) Naturally occurring, complex aluminium silicates, characterized by laminar morphology and near-perfect basal cleavage. The structure possesses a high degree of flexibility, elasticity and toughness. Excellent thermal stability and chemical resistance. Cork It compresses readily with negligible lateral flow, recovers speedily, and is relatively inert. It lacks flexibility and mechanical strength. PTFE Extremely wide chemical resistance (PTFE is attacked only by molten alkali metals and fluorine gas), with excellent anti-stick and dielectric properties. Material has high compressibility, which allows it to conform well to flange surface irregularities. Easy to handle. Low permeability. Extremely low coefficient of friction. Susceptible to degradation by radiation. It can be prone to relaxation and creep Metallic materials Various metallic materials used for making gasket are: Carbon steel, 316, 316L, 304, 304L, 321, 347, 410, Titanium, Alloy 600, Alloy 625, Alloy 800, Alloy 825, Alloy 200, Alloy 400, Alloy B2, Alloy C276, Alloy 20, Alloy x-750, Aluminium and Copper. Standard classification for non-metallic gasket materials ASTM Designation F104 provides a means for specifying or describing pertinent properties of commercial non-metallic gasket materials. Materials composed of asbestos, cork, cellulose, and other nonasbestos materials in combination with various binders or fillers are included. Materials normally classified as rubber compounds are covered in Method D2000. Gasket Categories (Types) Depending on construction, gaskets can be classified into three main categories (types): • soft (non-metallic) • semi-metallic • metallic The mechanical characteristics and performance capabilities of these categories will vary extensively depending on the type of gasket selected and the materials from which it is manufactured. Soft gaskets (non-metallic): Often they are composite sheet materials, suitable for a wide range of general and corrosive chemical applications. Generally they are limited to low to medium pressure applications. They are available either in sheet form or as gaskets cut accurately to any reasonable shape and size. Types include: Elastomers, compressed asbestos fibre (“CAF”), asbestos-free (non-asbestos) compressed fibre materials, graphite, PTFE, cork, mica, etc. Elastomer (rubber) sheet gaskets Elastomers are incompressible, extensible, highly impermeable and elastic. Incompressible—can be deformed, but can never be reduced in volume. Extensible—can be assembled over a projection or shoulder and snap tightly within a groove. Highly impermeable—can serve as a tight barrier against the passage of gases or liquids. Elastic—little flange pressure required to effect intimate contact with gasket, allowing it to move with the flange surfaces, always maintaining a seal. Elastomer gaskets are used for relatively low pressure applications as at high seating stresses a rubber gasket may extrude from between the flanges. They are available in a wide range of specification (premium) grades for industrial and military requirements as well as “commercial” grades for general purpose applications. Neoprene sheets are widely used as gasket material. Physical properties of premium grade (ASTM) rubber gaskets made by Garlock are reproduced below for ready reference. Typical physical properties of Commercial grade Neoprene (made by James Walker brand number 264 C) is as under. Hardness, IRHD: 55 to 70 Density, Mg/m3: 1.4 ±0.2 Tensile strength, MPa: 5 (1 MPa = 1 N / square mm) Elongation at break, %: 200 Operating temperature range: –20 deg. C to +100 deg. C. Insertion sheets Elastomer sheet is called insertion sheet when it is reinforced by a cloth / fabric to give it additional strength and resistance to spread under compression. Kalrez® Perfluoroelastomer FFKM by DuPont This high performance elastomeric material made by DuPont combines the resilience and sealing ability of rubber with almost universal chemical resistance and temperature capabilities up to 316° C. Various grades of Kalrez® are available for critical and/or high purity sealing applications. Compressed asbestos fibre (“CAF”) sheets Historically, compressed asbestos fibre sheet material has been the material of choice for “soft” gasket materials. It is regarded as easy to use and very tolerant of abuse, for which it is recognized as very “forgiving”. The material is used to seal almost all common applications, and usually gave reasonable performance. Jointing sheets are manufactured by calendering process in which the mixture of asbestos fibre, filler and binder is compressed between two rollers under load. The overall characteristics are influenced by asbestos quality and the nature of binder (usually elastomer). Tensile strength of the material depends on length of asbestos fibre and chemical properties are decided by type of binder. They are available either in sheet form or as gaskets cut accurately to any reasonable shape and size. Compressed asbestos-free (non-asbestos) fibre sheets More recently, with the tendency away from the use of asbestos fibres, a new generation of non asbestos fibre jointing material substitutes has been developed by the sealing industry. The sheets are made by calendaring process typically using carbon, glass and aramid or a mixture of these fibres. Overall, these new materials can outperform their asbestos equivalent, but are usually less forgiving and handling of these materials require more care. The maximum temperature capabilities are however slightly reduced compared to asbestos. For a given material, the maximum temperature limit also reduces with increasing thickness. In view of this wherever possible use the thinnest gasket. Graphite sheets Graphite sheets contain more than 95% pure exfoliated graphite. An Ultra High Purity (99.8%) grade is available for nuclear industry applications. More care is required in handling and storage of these sheets as they get easily damaged. PTFE sheets PTFE is generally used because of its outstanding chemical resistance. As it can be prone to relaxation and creep, filled grades are often employed to overcome some of these effects. Fillers are also used to improve wear resistance and thermal conductivity. Glass fibre, graphite, molybdenum disulphide, bronze, etc. are used as filler materials. They are widely used for food and pharmaceutical service. Tape / cord expanded PTFE (also known as joint sealant) Usually on a spool or roll, this high compression material is very flexible and is available with adhesive on one side to aid installation. The material can be rolled out onto the flange mating surface, cut off, overlapped and compressed between the flanges. It is often referred to as “form in place”, an ideal do-it-yourself gasket material for easy field installation. Generally used for less severe pressures and temperatures, especially where flanges are lightly loaded or of relatively flimsy construction. Cork / corkelastomer sheets Elastomer-bonded cork sheets are generally used as cork lacks flexibility and mechanical strength. They are mainly used on low pressure duties such as oil covers (transformers) and applications where the available bolting is relatively low (on glass, porcelain, etc.). Their expanded nature is also found beneficial in inhibiting the transmission of noise and machinery vibration and are used for air ducts in air conditioning. Many grades are available for covering applications in electrical (high voltage switchgear), marine, aerospace (aircraft fueling equipment), automobile and mechanical engineering. Semi-metallic gaskets: They are composite gaskets consisting of both metallic and non-metallic materials. The metal generally provides the strength and resilience to the gasket. They are suitable for both low and high temperature and pressure applications. Types include: Kammprofile, metal eyelet, metal jacketed, metal reinforced soft gaskets (tanged graphite, wire reinforced compressed asbestos fibre materials, etc.), corrugated metallic and spiral wound gaskets.
Posted on: Tue, 23 Jul 2013 16:24:04 +0000
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