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 "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 fills 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 gaskets are made entirely of metal and rely on a seating surface to accomplish the
seal; the metal's own spring characteristics are utilized.
This is typical of some "ring joints" or some other metal gasket systems such as those made
by Grayloc.
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 or a plastic polymer.
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 range or higher.
Generally speaking, there are several truisms that allow for best 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 The premise is simple in that a sheet of material
has the gasket shape "punched out" of it.
This leads to a very crude, fast and cheap gasket.
In previous times the material was compressed asbestos, but in modern times a fibrous material
such as high temp graphite is used.
These gaskets can fill many chemical requirements based on the inertness of the material used
and fit many budgetary restraints.
Common practice prevents these gaskets from being used in many industrial processes based
on temperature and pressure concerns.
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 wound outwards in a circular spiral with the filler material
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.
[2] 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, 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 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 if the flange sealing surfaces are capable of attaining a seal, the sealing elements
are compatible with the process fluid and application, and 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 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.
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 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:
Sheet gaskets Corrugated metal gaskets
Ring gaskets 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.
Reasons for failure Uneven distributed pressing force
Uneven pressing force is caused by a variety of factors, first is the human factor: asymmetric
construction of the preload bolt, this factor can eliminate construction; theory on the
flange pressed, the sealing surface is absolutely parallel to the practice, however, the centerline
of a pipeline can not be absolutely concentric, 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 is reduced, the running load, prone to leakage.
Third, the density of bolt arrangement on the pressure distribution 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 decreased and other factors, the working process
of the spiral wound gaskets stress relaxation, the bolt tension will be gradually decreased,
resulting in loss of torque, causing a leak.
In general, long bolts, the remnants of the torque, the smaller the diameter the more
advantageous to prevent the loss of torque, with long, thin bolt 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 is thinner and smaller the loss of torque.
In addition to prevent the strong vibration of the machine and the pipe itself, and exclude
the impact of adjacent equipment vibration, the impact of the sealing surface is not meaningless,
not to beat the bolts tightened, can prevent the loss of torque.
Surface not smooth It is important to make the sealing finish
proper otherwise it will cause leakage Temperature
Flanged leak often occurs in the cooling, because the cooling rate of cooling when the
flange and bolts do not After cooling, the pressing force of the metal gaskets stress
relaxation, combined with the cold contraction 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 flexible gasket; 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-ring
Ozone cracking Polymer degradation
Vacuum flange Washer
Sources ^ [1]
Bickford, John H.: An Introduction to the Design and Behavior of Bolted Joints, 3rd
ed., Marcel Dekker, 1995, pg. 5 Latte, Dr. Jorge and Rossi, Claudio: High
Temperature Behavior of Compressed Fiber Gasket Materials, and an Alternative Approach to
the Prediction of Gasket Life, FSA presented Paper, 1995, pg. 16
External links Gasket Fabricators Association
Fluid Sealing Association, gasket division Gasket Selection
http:europeansealing.comaboutdivisions/flange-gaskets-division Gasket Materials Informative Resource
Expanded PTFE Joint Sealant User Guidance Informative Source
/gasket failure reasons Informative Source: Gasket Materials
Informative Source: PTFE Gaskets