Properties - Gasket Design:
Sheet Gaskets - Solid Material Gaskets - Spiral Wound Gasket - Double Jacketed Gaskets - Kammprofile Gaskets - Flange Gaskets Explained
Spiral Wound in Detail - Rubbers - O-rings
Gaskets Explained
A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression.
Gaskets allow for "less-than-perfect" mating surfaces on machine parts where they can fill irregularities. Gaskets are commonly produced by cutting from sheet materials.
Gaskets for specific applications, such as high pressure steam systems, may contain asbestos. However, due to health hazards associated with asbestos exposure, non-asbestos gasket materials are used when practical.
It is usually desirable that the gasket be made from a material that is to some degree yielding such that it is able to deform and tightly fill the space it is designed for, including any slight irregularities. A few gaskets require an application of sealant directly to the gasket surface to function properly.
Some (piping) gaskets are made entirely of metal and rely on a seating surface to accomplish the seal; the metal's own spring characteristics are utilized (up to but not passing σy, the material's yield strength). This is typical of some "ring joints" (RTJ) or some other metal gasket systems. These joints are known as R-con and E-con compressive type joints.
Properties
Gaskets are normally made from a flat material, a sheet such as paper, rubber, silicone, metal, cork, felt, neoprene, nitrile rubber, fiberglass, polytetrafluoroethylene (otherwise known as PTFE or Teflon) or a plastic polymer (such as polychlorotrifluoroethylene).
One of the more desirable properties of an effective gasket in industrial applications for compressed fiber gasket material is the ability to withstand high compressive loads. Most industrial gasket applications involve bolts exerting compression well into the 14 MPa (2000 psi) range or higher. Generally speaking, there are several truisms that allow for better gasket performance. One of the more tried and tested is: "The more compressive load exerted on the gasket, the longer it will last". There are several ways to measure a gasket material's ability to withstand compressive loading. The "hot compression test" is probably the most accepted of these tests. Most manufacturers of gasket materials will provide or publish the results of these tests.
Gasket design
Gaskets come in many different designs based on industrial usage, budget, chemical contact and physical parameters:
Sheet gaskets
When a sheet of material has the gasket shape "punched out" of it, it is a sheet gasket. This can lead to a crude, fast and cheap gasket. In previous times the material was compressed asbestos, but in modern times a fibrous material such as graphite is used. These gaskets can fill various different chemical requirements based on the inertness of the material used. Non-asbestos gasket sheet is durable, of multiple materials, and thick in nature. Material examples are mineral, carbon or nitrile synthetic rubber. Applications using sheet gaskets involve acids, corrosive chemicals, steam or mild caustics. Flexibility and good recovery prevent breakage during installation of a sheet gasket.
Solid material gaskets.
The idea behind solid material is to use metals which cannot be punched out of sheets but are still cheap to produce. These gaskets generally have a much higher level of quality control than sheet gaskets and generally can withstand much higher temperatures and pressures. The key downside is that a solid metal must be greatly compressed in order to become flush with the flange head and prevent leakage. The material choice is more difficult; because metals are primarily used, process contamination and oxidation are risks. An additional downside is that the metal used must be softer than the flange — in order to ensure that the flange does not warp and thereby prevent sealing with future gaskets. Even so, these gaskets have found a niche in industry.
Spiral-wound gaskets
Spiral-wound gaskets comprise a mix of metallic and filler material. Generally, the gasket has a metal (normally carbon rich or stainless steel) wound outwards in a circular spiral (other shapes are possible) with the filler material (generally a flexible graphite) wound in the same manner but starting from the opposing side. This results in alternating layers of filler and metal. The filler material in these gaskets acts as the sealing element, with the metal providing structural support.
These gaskets have proven to be reliable in most applications, and allow lower clamping forces than solid gaskets, albeit with a higher cost. (see Spiral-wound gaskets in details, below)
Constant seating stress gaskets
The constant seating stress gasket consists of two components; a solid carrier ring of a suitable material, such as stainless steel, and two sealing elements of some compressible material installed within two opposing channels, one channel on either side of the carrier ring. The sealing elements are typically made from a material (expanded graphite, expanded polytetrafluoroethylene (PTFE), vermiculite, etc.) suitable to the process fluid and application. Constant seating stress gaskets derive their name from the fact that the carrier ring profile takes flange rotation (deflection under bolt preload) into consideration. With all other conventional gaskets, as the flange fasteners are tightened, the flange deflects radially under load, resulting in the greatest gasket compression, and highest gasket stress, at the outer gasket edge.
Since the carrier ring used in constant seating stress gaskets take this deflection into account when creating the carrier ring for a given flange size, pressure class, and material, the carrier ring profile can be adjusted to enable the gasket seating stress to be radially uniform across the entire sealing area. Further, because the sealing elements are fully confined by the flange faces in opposing channels on the carrier ring, any in-service compressive forces acting on the gasket are transmitted through the carrier ring and avoid any further compression of the sealing elements, thus maintaining a 'constant' gasket seating stress while in-service. Thus, the gasket is immune to common gasket failure modes that include creep relaxation, high system vibration, or system thermal cycles. The fundamental concept underlying the improved sealability for constant seating stress gaskets are that (i) if the flange sealing surfaces are capable of attaining a seal, (ii) the sealing elements are compatible with the process fluid and application, and (iii) the sufficient gasket seating stress is achieved on installation necessary to affect a seal, then the possibility of the gasket leaking in-service is greatly reduced or eliminated altogether.
Double-jacketed gaskets
Double-jacketed gaskets are another combination of filler material and metallic materials. In this application, a tube with ends that resemble a "C" is made of the metal with an additional piece made to fit inside of the "C" making the tube thickest at the meeting points. The filler is pumped between the shell and piece. When in use, the compressed gasket has a larger amount of metal at the two tips where contact is made (due to the shell/piece interaction) and these two places bear the burden of sealing the process. Since all that is needed is a shell and piece, these gaskets can be made from almost any material that can be made into a sheet and a filler can then be inserted.
Kammprofile gaskets
Kammprofile gaskets are used in many older seals since they have both a flexible nature and reliable performance. Kammprofiles work by having a solid corrugated core with a flexible covering layer. This arrangement allows for very high compression and an extremely tight seal along the ridges of the gasket. Since generally the graphite will fail instead of the metal core, Kammprofile can be repaired during later inactivity. Kammprofile has a high capital cost for most applications but this is countered by long life and increased reliability.
Flange gasket
A flange gasket is a type of gasket made to fit between two sections of pipe that are flared to provide higher surface area.
Flange gaskets come in a variety of sizes and are categorized by their inside diameter and their outside diameter.
There are many standards in gasket for flanges of pipes. The gaskets for flanges can be divided in major 4 different categories:
1. Sheet gaskets
2. Corrugated metal gaskets
3. Ring gaskets
4. spiral wound gaskets
Sheet gaskets are simple, they are cut to size either with bolt holes or without holes for standard sizes with various thickness and material suitable to media and temperature pressure of pipeline.
Ring gaskets also known as RTJ. They are mostly used in offshore oil- and gas pipelines and are designed to work under extremely high pressure. They are solid rings of metal in different cross sections like oval, round, octagonal etc. Sometimes they come with hole in center for pressure.
Spiral wound gaskets are also used in high pressure pipelines and are made with stainless steel outer and inner rings and a center filled with spirally wound stainless steel tape wound together with graphite and PTFE, formed in V shape. Internal pressure acts upon the faces of the V, forcing the gasket to seal against the flange faces.
Improvements
Many gaskets contain minor improvements to increase or infer acceptable operating conditions:
• A common improvement is an inner compression ring. A compression ring allows for higher flange compression while preventing gasket failure. The effects of a compression ring are minimal and generally are just used when the standard design experiences a high rate of failure.
• A common improvement is an outer guiding ring. A guiding ring allows for easier installation and serves as a minor compression inhibitor. In some alkylation uses these can be modified on Double Jacketed gaskets to show when the first seal has failed through an inner lining system coupled with alkylation paint.
Spiral-wound in detail
Spiral wound gaskets are widely used in the oil & gas industry, particularly in high operating pressure and critical applications. Spiral wound gaskets are manufactured by spirally winding a preformed metallic strip and a filler on the outer periphery of a metallic mandrel. The metal strip/metal wire (most often 304SS or 316SS) is wound outwards in a circle and the filler material (most often graphite) is wound starting at the opposite side in the same direction.
This results in a growing circle of alternating layers of filler material and metal strip. The guide ring of a spiral wound gasket allows for the centering of the sealing element on the flange face, as well as delivers additional radial strength, and acts as a compression limiter. The outer metal ring of a spiral wound gasket holds the windings of the gasket in place and centers the gasket between the flange bolting, aligning the sealing surfaces.
The finished product will resemble the spiral wound gasket image below, with the various components discussed here indicated with labels
Pressure Class
Corresponding to flange pressure classes, spiral wound gaskets are available in 150, 300, 600, 900, 1500, and 2500 pressure classes. A low-pressure class (I.E. 150#) is made to a low-density construction, with a low number of metallic windings per unit width. High density applications require a more resilient design and a higher number of metallic windings per unit width must be incorporated into the gasket design. Therefore, data sheets do not exist to cover all variations of the spiral wound gasket design.
Material (Winding and Filler)
Material is identified by the color on the outer edge of the gasket. The stripe along the outer edge denotes filler (graphite, Teflon, etc.). MSI's Spiral Wound Gasket Color Chart shows the different material colors for the outer edge and fillers that the gaskets identify by.
Inner Ring (optional)
A spiral wound gasket with an inner ring is common because the inner ring helps prevent what is known as spooling. Spooling can be caused when pressure is applied during bolting, causing the winding to uncoil in the ID of the gasket, causing a lot of problems. This can also happen during the pigging process used when cleaning a pipeline.
Series A vs Series B
Large bore flanges (above NPS 24") to ASME B16.47-Serie A or MSS-SP 44 are heavier/stronger than the ASME B16.47-Series B in the same size and pressure rating. In most end-users piping specifications the Series A is specified, which can withstand more external loadings than the Series B type. For both series, the dimensions are different. From a commercial point of view, the series B type may be selected for piping with a non-critical application.
Dimensional Data
NPS
d1
d2
Class 150
Class 300
Class 400
Class 600
Class 900
1/2
21
48
54
54
54
64
3/4
27
57
67
67
67
70
1
33
67
73
73
73
79
1¼
42
76
83
83
83
89
1½
48
86
95
95
95
98
2
60
105
111
111
111
143
2½
73
124
130
130
130.2
165
3
89
137
149
149
149
168
3½
102
162
165
162
162
...
4
114
175
181
178
194
206
5
141
197
216
213
241
248
6
168
222
251
248
267
289
8
219
279
308
305
321
359
10
273
340
362
359
400
435
12
324
410
422
419
457
498
14
356
451
486
483
492
521
16
406
514
540
537
565
575
18
457
549
597
594
613
638
20
508
606
654
648
683
699
24
610
718
775
768
791
838
NPS
d1
Class 150
Class 300
Class 400
Class 600
Class 900
d2
• d1 = Inside diameter.
• d2 = Outside diameter.
• Dimensional tolerances outside diameter NPS 12 and smaller: +0 / -1.5 mm.
• Dimensional tolerances outside diameter NPS 14 and larger: +0 / -3.0 mm.
• Dimensional tolerances inside diameter NPS 12 and smaller: ± 1.5 mm.
• Dimensional tolerances inside diameter NPS 14 and larger: ± 3.0 mm.
• Thickness (t) specified by customer
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RUBBER TYPES
- NATURAL RUBBER (NR) - BUTYL (IIR) - EPICHLOROHYDRIN (ECO) - ETHYLENE ACRYLIC (AEM) - ETHYLENE-PROPYLENE (EPDM) - FLUOROCARBON (FPM) - FLUOROSILICONE (FSi) - HYDROGENATED NITRILE (HNBR) - NEOPRENE / CHLOROPRENE (CR) - NITRILE (NBR) - POLYACRYLATE (ACM) - POLYETHYLENE (CSM) - POLYURETHANE (AU) (EU) - SILICONE (Si) - STYRENE BUTADIENE (SBR) -
NATURAL RUBBER (NR)
Natural rubber is a product coagulated from the latex of the rubber tree, hevea brasiliensis. Natural rubber features excellent compression set, high tensile strength, resilience, abrasion and
tear resistance, good friction characteristics, excellent bonding capabilities to metal substrate, and good vibration dampening characteristics.
Material advantages:
» excellence compression set
» good resilience and abrasion
» good surface friction properties
Material disadvantages:
» poor resistance to attack by petroleum oils
» poor resistance to ozone and UV ray
Temp. Range : Low -60ºF High 220ºF
(example)
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SILICONE (Si)
Silicone is a semi-organic elastomer with outstanding resistance to extremes of temperature with corresponding resistance to compression set and retention of flexibility. Silicone elastomers
provide excellent resistance to ozone, oxygen, and moisture.
It grant high abrasion resistance that, combined with high friction properties limit, makes it excellent in static seal applications.
Silicone utilizes a flexible siloxane backbone rather than a carbon backbone like many other elastomers and has very low glass transition temperatures.
Material advantages:
» excellent extreme temperature properties
» excellent compression set resistance
» Hygienic, neutral odor and taste
Material disadvantages:
» typically not good for dynamic seals due to friction properties and poor abrasion resistance
» higher price than other rubber compounds
Temp. Range : Low -75ºF High 450ºF
(example)
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FLUOROSILICONE (FSi)
Fluorosilicones combine most of the attributes of silicone with resistance to petroleum oils and hydrocarbon fuels.
Low abrasion resistance but high adhesion limit to surfaces. Fluorosilicones are used primarily in aircraft fuel systems.
Material advantages:
» excellent extreme temperature properties
» excellent compression set resistance
» Hygienic, neutral odor and taste
Material disadvantages:
» typically not good for dynamic seals due to friction properties and poor abrasion resistance
Temp. Range : Low -75ºF High 450ºF
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BUTYL (IIR)
The distinctive properties of IIR rubber materials are very low gas and moisture permeability, excellent insulating properties, good ozone and weathering resistance, and resistance to a great
many organic and inorganic media. IIR elastomer materials can be polymerised with various halogens (e.g. Chlorine / Bromine) to improve resistance to certain chemical media, but at the
expense of electrical insulation and moisture resistance. They can be used from –40 to +120°C typically, and are mostly used in the production of tyre inner tubes, seals and gaskets, vacuum
seals and membranes, and pharmaceutical goods.
Material advantages:
» excellent ozone resistance
» excellent water resistance
Material disadvantages:
» poor oil resistance
» poor flame resistance
Temp. Range : Low -40ºF High 275ºF
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EPICHLOROHYDRIN (ECO)
ECO is generally resistance to high temperatures, oils, ozone, and flame with gas resistance comparable to NBR’s. The temperature range for continuous use is –40 to +120°C, but are
generally unsuitable for rubber to metal bonding (they are corrosive to metals). ECO elastomers are suitable for use in seals, gaskets, diaphragms, cable jackets, belting etc, for a wide range
of media. However, they are unsuitable for use with ketones and esters, alcohols, phosphate ester hydraulic fluids, sour gas, water and steam.
Material advantages:
» excellent oil resistance
» good ozone resistance
» good compression set
Material disadvantages:
» poor abrasion resistance
» poor water resistance
Temp. Range : Low -40ºF High 250ºF
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POLYETHYLENE (CSM)
Chlorosulfonated polyethylene or CSM is a synthetic rubber based on polyethylene. This rubber is a material with Neoprene Polychloroprene plus other qualities.
Chlorosulfonated polyethylene is known for its excellent resistance to atmospheric conditions and good resistance to chemicals. Chlorosulfonated polyethylene is used in a variety of industrial
and automotive applications that require high performance and have to withstand extreme weather conditions.
Material advantages:
» excellent oil resistance
» abrasion resistance
» good water resistance
Material disadvantages:
» poor fuel resistance and poor compression set
Temp. Range : Low -60ºF High 320ºF
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ETHYLENE-PROPYLENE (EPDM)
Ethylene-propylene compounds are prepared from ethylene and propylene (EPM) and usually a third monomer (EPDM). These compounds are used frequently to seal in brake systems, and
for sealing hot water and steam. Ethylene propylene compounds have good resistance to mild acids, detergents, alkalis, silicone oils and greases, ketones, and alcohols. They are not
recommended for applications with petroleum oils, mineral oil, di-ester lubricants, or fuel exposure.
EPDM compounds are typically developed with a sulfur or peroxide cure system.
Material advantages:
» excellent weather resistance
» good low temperature flexibility
» excellent chemical resistance
» good heat resistance
Material disadvantages:
» poor petroleum oil and solvent resistance
Temp. Range : Low -60ºF High 300ºF
(example)
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HYDROGENATED NITRILE (HNBR)
HNBR is created by partially or fully hydrogenating NBR. The hydrogenating process saturates the polymeric chain with accompanying improvements to the ozone, heat and aging resistance
of the elastomer and improves overall mechanical properties.
HNBR, like Nitrile, increasing the acrylonitrile content increase resistance to heat and petroleum based oils and fuels, but decreases the low temperature performance.
Material advantages:
» excellent heat and oil resistance
» improved fuel and ozone resistance (approximately 5X) over Nitrile
» abrasion resistance
Material disadvantages:
» decreased elasticity at low temperatures with hydrogenation over standard nitrile
Temp. Range : Low -22ºF High 300ºF
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NITRILE (NBR)
The popularity of nitrile is due to its excellent resistance to petroleum products and its ability to be compounded for service over a temperature range of -30°C a 100°C.
Nitrile is a copolymer of butadiene and acrylonitrile. Variation in proportions of these polymers is possible to accommodate specific requirements. An increase in Acrylonitrile content
increases resistance to heat plus petroleum base oils and fuels but decreases low temperature flexibility.
Nitrile provides excellent compression set, tear, and abrasion resistance. The major limiting properties of Nitrile are its poor ozone and weather resistance and moderate heat resistance, but
in many application these are not limiting factors.
Material advantages:
» excellent compression set
» superior tear resistance
» abrasion resistance
Material disadvantages:
» poor weather resistance
» moderate heat resistance
Temp. Range : Low -22ºF High 212ºF
(example)
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POLYACRYLATE (ACM)
Polyacrylates are copolymers of ethyl and acrylates which exhibit excellent resistance to petroleum fuels and oils and can retain their properties when sealing petroleum oils at continuous
high temperatures up to 150°C. These properties make polyacrylates suitable for use in automotive automatic transmissions, steering systems, and other applications where petroleum and
high temperature resistance are required.
Polyacrylates also exhibit resistance to cracking when exposed to ozone and sunlight.
Polyacrylates are not recommended for applications where the elastomer will be exposed to brake fluids, chlorinated hydrocarbons, alcohol, or glycols.
Material advantages:
» petroleum fuel and oil resistance
» good ozone resistance
» good heat resistance
Material disadvantages:
» fair compression set performance relative to NBR
» water resistance and low temperature performance
Temp. Range : Low -60ºF High 300ºF
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NEOPRENE / CHLOROPRENE (CR)
Neoprene homopolymer of chlorobutadiene and is unusual in that it is moderately resistant to both petroleum oils and weather (ozone, UV, oxygen). This qualifies neoprene uniquely for
certain sealing applications where many other materials would not be satisfactory. Neoprene is classified as a general purpose elastomer which has relatively low compression set, good
resilience and abrasion.
Neoprene has excellent adhesion qualities to metals for rubber to metal bonding applications.
It is used extensively for sealing refrigeration fluids and ammonia.
Material advantages:
» moderate resistance to petroleum oils
» good resistance to ozone, UV, oxygen
» excellence resistance to refrigeration fluids and ammonia
Material disadvantages:
» moderate water resistance
» not effective in solvents environments
Temp. Range : Low -40ºF High 250ºF
(example)
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FLUOROCARBON (FPM)
Fluorocarbon exhibits resistance to a broader range of chemicals combined with very good high temperature properties more so than any of the other elastomers.
Fluorocarbons are highly resistant to swelling when exposed to gasoline as well as resistant to degradation due to expose to UV light and ozone. When exposed to low temperatures,
fluorocarbon elastomers can become quite hard.
Fluorocarbons exhibit low gas permeability making them well suited for hard vacuum service and many formulations are self-extinguishing. FKM materials are not generally recommended for
exposure to hot water, steam, low molecular weight esters and ethers, glycol based brake fluids, or hot hydrofluoric or chlorosulfonic acids.
Material advantages:
» excellent chemical resistance
» excellent heat resistance
» good mechanical properties
» good compression set resistance
Material disadvantages:
» poor low temperature flexibility
» poor resistance to hot water and steam
Temp. Range : Low 5ºF High 390ºF
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POLYURETHANE (AU) (EU)
Millable polyurethane exhibits excellent abrasion resistance and tensile strength as compared to other elastomers providing superior performance in hydraulic applications with high
pressures.
Fluid compatibility is similar to that of nitrile at temperatures up to approximately 80 °C. At higher temperatures, polyurethane has a tendency to soften and lose both strength and fluid
resistance.
Material advantages:
» excellent strength and abrasion resistance
» good resistance to petroleum oils
» good weather resistance
Application Disadvantages
» poor resistance to water
» poor high temperature capabilities
Temp. Range : Low -60ºF High 175ºF
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STYRENE BUTADIENE (SBR)
Styrene Butadiene (SBR) is a copolymer of styrene and butadiene, has properties similar to those of natural rubber. SBR is used in hydraulic brakes system seals and diaphragms, with the
major of the industry usage coming from the Tire Industry. SBR features excellent resistance to brake fluids, and good water resistance.
Material advantages:
» good resistance to brake fluids
» good resistance to water
Application Disadvantages
» poor weather resistance
» poor petroleum oil and solvent resistance
Temp. Range : Low -50ºF High 212ºF
(example)
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ETHYLENE ACRYLIC (AEM)
It exhibits properties similar to those of Polyacrylate, but with extended low temperature range and with enhanced mechanical properties.
Ethylene-acrylic offers a high degree of oil, ozone, UV and weather resistance.
Material advantages:
» excellent vibration dampening
» excellent heat aging characteristics
» good dynamic property retention over a wide temperature range
» resistance to transmission fluids, water, glycol mixtures, and alkalies
Application Disadvantages
» not recommended for exposure to fuel, brake fluid, aromatic hydrocarbons or phosphate esters.
Temp. Range : Low -40ºF High 300ºF
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O-ring
Typical O-ring and application
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.
The O-ring may be used in static applications or in dynamic applications where there is relative motion between the parts and the O-ring. Dynamic examples include rotating pump shafts and hydraulic cylinder pistons.
O-rings are one of the most common seals used in machine design because they are inexpensive, easy to make, and reliable and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) of pressure.
Manufacturing
O-rings can be produced by extrusion, injection molding, pressure molding or transfer molding.
History
The first patent for the O-ring, is dated May 12, 1896 as a Swedish patent. J. O. Lundberg, the inventor of the O-ring, received the patent. The US patent for the O-ring was filed in 1937 by a then 72-year-old Danish-born machinist, Niels Christensen. Soon after migrating to the USA in 1891, he patented an air brake system for streetcars (trams). Despite his legal efforts, his intellectual property rights were passed from company to company until they ended up at Westinghouse. During World War II, the US government commandeered the O-ring patent as a critical war-related item and gave the right to manufacture to other organizations. Christensen received a lump sum payment of US$75,000 for his efforts. Litigation resulted in a $100,000 payment to his heirs in 1971, 19 years after his death.
Theory and design
O-ring mounting for an ultra-high vacuum application. Pressure distribution within the cross-section of the O-ring. The orange lines are hard surfaces, which apply high pressure. The fluid in the seams has lower pressure. The soft O-ring bridges the pressure over the seams.
O-rings are available in various metric and inch standard sizes. Sizes are specified by the inside diameter and the cross section diameter (thickness). In the US the most common standard inch sizes are per SAE AS568C specification (e.g. AS568-214). ISO 3601-1:2012 contains the most commonly used standard sizes, both inch and metric, worldwide. The UK also has standards sizes known as BS sizes, typically ranging from BS001 to BS932. Several other size specifications also exist.
Typical applications
Successful O-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the O-ring. This introduces a calculated mechanical stress at the O-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the O-ring, leaking cannot occur. Fortunately, the pressure of the contained fluid transfers through the essentially incompressible O-ring material, and the contact stress rises with increasing pressure. For this reason, an O-ring can easily seal high pressure as long as it does not fail mechanically. The most common failure is extrusion through the mating parts.
The seal is designed to have a point contact between the O-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials accommodates imperfections in the mounting parts. But it is still important to maintain good surface finish of those mating parts, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline. Surface finish is also especially important in dynamic applications. A surface finish that is too rough will abrade the surface of the O-ring, and a surface that is too smooth will not allow the seal to be adequately lubricated by a fluid film.
Vacuum applications
In vacuum applications, the permeability of the material makes point contacts quite useless. Instead, higher mounting forces are used and the ring fills the whole groove. Also, round back-up rings are used to save the ring from excessive deformation [6][7][8] Because the ring feels the ambient pressure and the partial pressure of gases only at the seal, their gradients will be steep near the seal and shallow in the bulk (opposite to the gradient of the contact stress [9] See: Vacuum flange#KF.2FQF. High-vacuum systems below 10−9 Torr use copper or nickel O-rings. Also, vacuum systems that have to be immersed in liquid nitrogen use indium O-rings, because rubber becomes hard and brittle at low temperatures.
High temperature applications
In some high-temperature applications, O-rings may need to be mounted in a tangentially compressed state, to compensate for the Gow-Joule effect.
Sizes
O-rings come in a variety of sizes British Standard (BS) which are imperial sizes or metric sizes. Typical dimensions of an O-ring are internal dimension (id), outer dimension (od) and thickness / cross section (cs)
Metric O-rings are usually defined by the internal dimension x the cross section. Typical part number for a metric O-ring - ID x CS [material & shore hardness] 2x1N70 = defines this O-ring as 2mm id with 1mm cross section made from Nitrile rubber which is 70Sh
BS O-rings are defined by a standard reference.
Material
Some small O-rings
O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, durometer, size and cost.
Synthetic rubbers - Thermosets: Thermoplastics:
Chemical Compatibility:
• Air, 200 - 300 °F – Silicone
• Beer - EPDM
• Chlorine Water – Viton (FKM)
• Gasoline – Buna-N or Viton (FKM)
• Hydraulic Oil (Petroleum Base, Industrial) – Buna-N
• Hydraulic Oils (Synthetic Base) – Viton
• Water – EPDM
• Motor Oils – Buna-N