Some seals and gaskets1. o-ring2. fiber washer3. paper gaskets4. cylinder head gasket

Polytetrafluoroethylene (PTFE) gasket

GasketFrom Wikipedia, the free encyclopedia

A gasket is a mechanical seal which fills the space betweentwo or more mating surfaces, generally to prevent leakagefrom or into the joined objects while under compression.

Gaskets allow "less-than-perfect" mating surfaces on machineparts where they can fill irregularities. Gaskets are commonlyproduced by cutting from sheet materials.

Gaskets for specific applications, such as high pressure steamsystems, may contain asbestos. However, due to healthhazards associated with asbestos exposure, non-asbestosgasket materials are used when practical.

It is usually desirable that the gasket be made from a materialthat is to some degree yielding such that it is able to deformand tightly fills the space it is designed for, including any slightirregularities. A few gaskets require an application of sealantdirectly to the gasket surface to function properly.

Some (piping) gaskets are made entirely of metal and rely on aseating surface to accomplish the seal; the metal's own springcharacteristics are utilized (up to but not passing y, thematerial's yield strength). This is typical of some "ring joints"(RJ) or some other metal gasket systems such as those madeby Grayloc (an Oceaneering International company). Thesejoints are known as R-con and E-con compressive type joints.[1]

Contents

1 Properties1.1 Gasket design1.2 Sheet gaskets1.3 Solid material gaskets1.4 Spiral-wound gaskets1.5 Constant seating stress gaskets1.6 Double-jacketed gaskets1.7 Kammprofile gaskets1.8 Flange gasket

2 Improvements

http://en.wikipedia.org/wiki/File:Gaskets.jpghttp://en.wikipedia.org/wiki/O-ringhttp://en.wikipedia.org/wiki/Washer_(mechanical)http://en.wikipedia.org/wiki/Cylinder_headhttp://en.wikipedia.org/wiki/Head_gaskethttp://en.wikipedia.org/wiki/File:PTFEseet.JPGhttp://en.wikipedia.org/wiki/Seal_(mechanical)http://en.wikipedia.org/wiki/Compression_(physical)http://en.wikipedia.org/wiki/Asbestoshttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Oceaneering_International

Compressed fiber gasket

3 Reasons for failure3.1 Uneven distributed pressing force3.2 Stress relaxation and torque loss3.3 Surface not smooth3.4 Temperature

4 See also5 Sources6 External links

Properties

Gaskets are normally made from a flat material, a sheet suchas paper, rubber, silicone, metal, cork, felt, neoprene, nitrilerubber, fiberglass, polytetrafluoroethylene (otherwise knownas PTFE or Teflon) or a plastic polymer (such aspolychlorotrifluoroethylene).

One of the more desirable properties of an effective gasket inindustrial applications for compressed fiber gasket material isthe ability to withstand high compressive loads. Mostindustrial gasket applications involve bolts exertingcompression well into the 14 MPa (2000 psi) range or higher.Generally speaking, there are several truisms that allow forbest 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 "hotcompression test" is probably the most accepted of these tests. Most manufacturers of gasket materialswill provide or publish the results of these tests.

Gasket design

Gaskets come in many different designs based on industrial usage, budget, chemical contact and physicalparameters:

Sheet gaskets

The premise is simple in that a sheet of material has the gasket shape "punched out" of it. This leads to avery crude, fast and cheap gasket. In previous times the material was compressed asbestos, but in moderntimes a fibrous material such as high temp graphite (http://macrosealinc.com/high-temperature-graphite-gasket-materials/) is used. These gaskets can fill many chemical requirements based on the inertness ofthe material used and fit many budgetary restraints. Common practice prevents these gaskets from beingused in many industrial processes based on temperature and pressure concerns.

http://en.wikipedia.org/wiki/File:Compressed_fiber_jointing.JPGhttp://en.wikipedia.org/wiki/Paperhttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Siliconehttp://en.wikipedia.org/wiki/Metalhttp://en.wikipedia.org/wiki/Cork_(material)http://en.wikipedia.org/wiki/Felthttp://en.wikipedia.org/wiki/Neoprenehttp://en.wikipedia.org/wiki/Nitrile_rubberhttp://en.wikipedia.org/wiki/Fiberglasshttp://en.wikipedia.org/wiki/Polytetrafluoroethylenehttp://en.wikipedia.org/wiki/Plastichttp://en.wikipedia.org/wiki/Polymerhttp://en.wikipedia.org/wiki/Polychlorotrifluoroethylenehttp://en.wikipedia.org/wiki/Compression_(physical)http://en.wikipedia.org/wiki/Screw#Bolthttp://en.wikipedia.org/wiki/MPahttp://en.wikipedia.org/wiki/Pound-force_per_square_inchhttp://en.wikipedia.org/wiki/Truismhttp://en.wikipedia.org/w/index.php?title=Hot_compression_test&action=edit&redlink=1http://en.wikipedia.org/wiki/Punch_(metalworking)http://macrosealinc.com/high-temperature-graphite-gasket-materials/http://en.wikipedia.org/wiki/Inertness

Solid material gaskets

The idea behind solid material is to use metals which cannot be punched out of sheets but are still cheapto produce. These gaskets generally have a much higher level of quality control than sheet gaskets andgenerally can withstand much higher temperatures and pressures. The key downside is that a solid metalmust be greatly compressed in order to become flush with the flange head and prevent leakage. Thematerial choice is more difficult; because metals are primarily used, process contamination and oxidationare risks. An additional downside is that the metal used must be softer than the flange in order toensure that the flange does not warp and thereby prevent sealing with future gaskets. Even so, thesegaskets 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 theopposing side. This results in alternating layers of filler and metal. The filler material in these gaskets actsas 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 solidgaskets, albeit with a higher cost. [2] (http://www.bing.com/images/search?q=spiral+wound+gaskets&id=8C264F3FFBAA6AC593C155D7075E811633CDF20F&FORM=IQFRBA)

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 twoopposing channels, one channel on either side of the carrier ring. The sealing elements are typically madefrom a material (expanded graphite, expanded polytetraflouroethylene (PTFE), vermiculite, etc.) suitableto the process fluid and application. Constant seating stress gaskets derive their name from the fact thatthe carrier ring profile takes flange rotation (deflection under bolt preload) into consideration. With allother 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 whencreating the carrier ring for a given flange size, pressure class, and material, the carrier ring profile can beadjusted 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 carrierring, any in-service compressive forces acting on the gasket are transmitted through the carrier ring andavoid any further compression of the sealing elements, thus maintaining a 'constant' gasket seating stresswhile in-service. Thus, the gasket is immune to common gasket failure modes that include creeprelaxation, high system vibration, or system thermal cycles. The fundamental concept underlying theimproved sealability for constant seating stress gaskets are that (i) if the flange sealing surfaces arecapable 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 thepossibility of the gasket leaking in-service is greatly reduced or eliminated altogether.

http://en.wikipedia.org/wiki/Flangehttp://en.wikipedia.org/wiki/Oxidationhttp://en.wikipedia.org/wiki/Stainless_steelhttp://www.bing.com/images/search?q=spiral+wound+gaskets&id=8C264F3FFBAA6AC593C155D7075E811633CDF20F&FORM=IQFRBA

Copper flange gaskets used for ultrahighvacuum systems

Double-jacketed gaskets

Double-jacketed gaskets are another combination of filler material and metallic materials. In thisapplication, a tube with ends that resemble a "C" is made of the metal with an additional piece made to fitinside of the "C" making the tube thickest at the meeting points. The filler is pumped between the shelland piece. When in use the compressed gasket has a larger amount of metal at the two tips where contactis 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 canbe made into a sheet and a filler can then be inserted. This is an effective option for most applications.

Kammprofile gaskets

Kammprofile gaskets are used in many older seals since they have both a flexible nature and reliableperformance. Kammprofiles work by having a solid corrugated core with a flexible covering layer. Thisarrangement 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 laterinactivity. Kammprofile has a high capital cost for most applications but this is countered by long life andincreased reliability.

Flange gasket

A flange gasket is a type of gasket made to fit between twosections of pipe that are flared to provide higher surface area.

Flange gaskets come in a variety of sizes and are categorizedby their inside diameter and their outside diameter.

There are many standards in gasket for flanges of pipes. Thegaskets for flanges can be divided in major 4 differentcategories:

1. Sheet gaskets2. Corrugated metal gaskets3. Ring gaskets4. spiral wound gaskets

Sheet gaskets are simple, they are cut to size either with bolt holes or without holes for standard sizeswith 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 designedto 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 andinner rings and a center filled with spirally wound stainless steel tape wound together with graphite andPTFE, formed in V shape. Internal pressure acts upon the faces of the V, forcing the gasket to seal againstthe flange faces.

http://en.wikipedia.org/wiki/File:Used_copper_flange_gaskets_for_ultrahigh_vacuum_systems.jpghttp://en.wikipedia.org/wiki/Pipe_(material)http://en.wikipedia.org/wiki/Screwhttp://en.wikipedia.org/wiki/Pipeline_transporthttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/PTFE

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 flangecompression while preventing gasket failure. The effects of a compression ring are minimal andgenerally 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 andserves as a minor compression inhibitor. In some alkylation uses these can be modified on DoubleJacketed gaskets to show when the first seal has failed through an inner lining system coupled withalkylation paint.

Reasons for failure

Uneven distributed pressing force

Uneven pressing force is caused by a variety of factors, first is the human factor: asymmetric constructionof the preload bolt, this factor can eliminate construction; theory on the flange pressed, the sealingsurface is absolutely parallel to the practice, however, the centerline of a pipeline can not be absolutelyconcentric, and thus tighten the bolts on the flange moment, so that the flange discontinuity.Asymmetrical connection, the sealing surface more or less deformed, so that sealed the pressing force isreduced, the running load, prone to leakage. Third, the density of bolt arrangement on the pressuredistribution more obvious impact, the closer the bolts, the more uniform the pressure.

Stress relaxation and torque loss

Tighten bolts on the flange, due to the vibration of the bodies, the temperature increased or decreasedand other factors, the working process of the spiral wound gaskets stress relaxation, the bolt tension willbe gradually decreased, resulting in loss of torque, causing a leak. In general, long bolts, the remnants ofthe torque, the smaller the diameter the more advantageous to prevent the loss of torque, with long, thinbolt is an effective way to prevent torque loss. Heating a certain period of time to make it stretch the bolt,and then to maintain a given torque, is very effective to prevent the loss of torque. There is a gasket isthinner and smaller the loss of torque. In addition to prevent the strong vibration of the machine and thepipe itself, and exclude the impact of adjacent equipment vibration, the impact of the sealing surface isnot meaningless, not to beat the bolts tightened, can prevent the loss of torque.

Surface not smooth

It is important to make the sealing finish properly otherwise it will cause leakage.

Temperature

Flanged leak often occurs in the cooling, because the cooling rate of cooling when the flange and bolts donot After cooling, the pressing force of the metal gaskets stress relaxation, combined with the coldcontraction of the pipe, resulting in toward the bolt tensile direction force, this force will lead to leaks in,where the low-temperature media occasions when gasket should be noted: 1) low temperature flexiblegasket; 2) shim thickness should be as much as possible to take a small flange gap as small as possible ; 3)high strength bolts, so that the strain is small.

See also

O-ringOzone crackingPolymer degradationVacuum flangeWasher (mechanical)

Sources

1. ^ [1] (http://www.oceaneering.com/subsea-products/grayloc/)

1. Bickford, John H.: An Introduction to the Design and Behavior of Bolted Joints, 3rd ed., MarcelDekker, 1995, pg. 5

2. Latte, Dr. Jorge and Rossi, Claudio: High Temperature Behavior of Compressed Fiber GasketMaterials, and an Alternative Approach to the Prediction of Gasket Life, FSA presented Paper, 1995,pg. 16

External links

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Categories: Seals (mechanical) Engine technology

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