File 2_Chapter 1 and 2_GaASKETS
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Transcript of File 2_Chapter 1 and 2_GaASKETS
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CHAPTER-1
FUNCTIONS OF GASKETS & THE GASKET FORCES
1. INTRODUCTION
1.1 WHAT IS A 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
Thus, 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. askets allow "less-than-perfect"mating surfaces on machine parts
where they can fill irregularities.
!t is usually desirable that the gasket be made from a material that is to some degreeyielding such that it is able to deform and tightly fills the space it is designed for,
including any slight irregularities. To that extent it shall be flexible enough. Also it should
have ability to withstand"
#. $igh compressive bolt loads so that it will not get crushed.
%. $ydrostatic end load due to internal pressure that tends to separate the flanges
&. !nternal pressure acting on the portion of the gasket exposed to internal
pressure, tending to blow out gaskets
There are several other considerations when selecting a gasket which including,
'ompatibility with the fluid, Temperature, !nternal ressure. ther special conditions i.e.
vibration, erosive media, risk of the gasket contaminating the medium, corrosion of
flanges, integrity and economy.
http://en.wikipedia.org/wiki/Seal_(mechanical)http://en.wikipedia.org/wiki/Compression_(physical)http://en.wikipedia.org/wiki/Seal_(mechanical)http://en.wikipedia.org/wiki/Compression_(physical) -
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1.2 THE FUNCTIONS OF A GASKET
T$* +-'T!- + A A/*T is to create and maintain a static seal between two
stationary, imperfect surfaces of a mechanical system, designed to contain a wide
variety of li0uids or gases. The gasket must be able to maintain this seal under all the
operating conditions of the system including extremes of temperature and pressure.
The performance of the gasket is affected by a number of factors. All of these factors
must be taken into consideration when selecting a gasket.
THE FLANGE LOAD" All gasket materials must have sufficient flange pressure to
compress the gasket enough to insure that a tight, unbroken seal occurs. The flange
pressure, or minimum seating stress, necessary to accomplish this is known as the 1y1
factor. This flange pressure must be applied uniformly across the entire seating area to
achieve perfect sealing. $owever, in actual service, the distribution around the gasket is
not uniform. The greatest force is exerted on the area directly surrounding the bolts. The
lowest force occurs mid2way between two bolts. This factor must be taken into account
by the flange designer.
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2. FORCES THAT OCCUR IN A GASKETED JOINT:
THE INTERNAL PRESSURE" !n service, as soon as pressure is applied to the vessel,
the initial gasket compression is reduced by the internal pressure acting against the
gasket (blowout pressure) and the flanges (hydrostatic end force). To account for this, an
additional preload must be placed on the gasket material. An 1m1 or maintenance factor
has been established by A3* to account for this preload. The 1m1 factor defines how
many times the residual load (original load minus the internal pressure) must exceed the
internal pressure. !n this calculation, the normal pressure and the test pressure should
be taken into account.
TEMPERATURE:The effects of both ambient and process temperature on the gasket
material, the flanges and the bolts must be taken into account. These effects include bolt
elongation, creep relaxation of the gasket material or thermal degradation. This can
result in a reduction of the flange load. The higher he operating temperature, the more
care needs to be taken with the asket material selection. As the system is pressuri4ed
and heated, the joint deforms. 5ifferent coefficients of expansion between the bolts, the
flanges and the pipe can result in forces which can affect the gasket. The relative
stiffness of the bolted joint determines whether there is a net gain or loss in the bolt load.
enerally, flexible joints lose bolt load.
FLUID:The media being sealed, usually a li0uid or a gas with a gas being harder to seal
than a li0uid. The effect of temperature on many fluids causes them to become more
aggressive. Therefore, a fluid that can be sealed at ambient temperature, may adversely
affect the gasket at a higher temperature.
6oth the 1m1 and 1y1 factors will vary with the type of gasket and the thickness of the
gasket. Always consult with the manufacturer to determine the 1m1 and 1y1 factors for the
gasket material you are using.
!n any application, failure to meet the 1m1 or 1y1 factor will result in an imperfect seal and
will re0uire a change in the gasket design. This change can sometimes be made by
simply decreasing the gasket surface area or by using a thicker gasket. $owever, since
thinner gaskets are generally more effective, changing to a thicker gasket may not be
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the most satisfactory long2term solution. !n some cases, a revision to the flange design
may be re0uired.
-ew gasket design factorsbeing developed by A3* are for bolted joint designs where
it is important that a desired level of tightness be achieved. 131 and 1y1 factors do nottake fugitive emissions into account, whereas the new assumption is that all bolted joints
leak to some extent. This 1systems approach1 focuses on all the components of the
bolted joint not just the gasket. A tightness parameter (Tp), is a defined measure of
tightness of a joint. A higher value for Tp, represents a lower rate of leakage. ee
additional discussion under Other Considerations,in the section on asket election.
THE ABILITY TO SEAL
As stated previously, the purpose of a gasket is to create a static seal between two
stationary flanges. The seal itself is effected by achieving the proper compression on the
gasket causing it to flow into the imperfections on the surface of the flange. This results
in a tight, unbroken barrier, impervious to the fluid being contained.
!n many instances, a good seal is obtained through the limited 1swell1 caused by the
reaction of the inside edge of the gasket material with the fluid being contained.
A certain amount of swell is desirable, as long as it reaches e0uilibrium and does not
reach a condition of degradation where the gasket begins to breakdown. !n many
instances, the fluid being contained may 1cauteri4e1 the inside edge of the gasket and
1seal off1 the gasket from further fluid penetration
+7A-* +!-!$*
!t is recommend that metal flange faces be machined with a concentric2serrated finish of
for non2metallic gaskets. honographic serrations can also be used with our materials. !t
should be recogni4ed, however, that their continuous leak path makes them more
difficult to seal
The finish or the condition of the gasket seating surface has a definite effect on the
ability of the gasket to create a seal. heet gasketing is designed to have a seating
stress that allows the gasket material to 1flow1 into the serrations and irregularities of the
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flange face. This 1bite1 aids the gasket in resisting the effects of internal pressure, creep
and cold flow.
1mooth1 finishes are usually found on machinery or flanged joints other than pipe
flanges. 8hen working with a smooth finish, it is important to consider using a thinnergasket to lessen the effects of creep and cold flow. !t should be noted, however, that
both a thinner gasket and the smooth finish, in and of themselves, re0uire a higher
compressive force (i.e. bolt tor0ue) to achieve the seal.
Therefore, due to the flange design, one may have to resort to a thicker gasket, which
re0uires a lower compressive force to seat the gasket. Another way to seat the gasket,
when there is insufficient compressive force available, is to lessen the area of the
gasket.
Fla!" Fa#" F$$%
A flange face (raised face and flat face) has a specific roughness to ensure that this
surface be compatible with the gasket and provide a high 0uality seal as under
compression. The soft face from a gasket will embed into this finish, which helps create
a seal, and a high level of friction is generated between the mating surfaces. The most
used surfaces are smooth finish, 'oncentric errated, piral errated, and tock +inish.
S"''a(") Ra$%") Fa#"
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ASME B1*.+ Fa#" F$$%"%
Stock Finish:This is a continuous spiral or phonographic groove. uitable for practicallyall general service conditions, it is the most widely used of any flange surface finish. This
finish is suitable for gaskets that have a soft conformable face. nder compression, the
soft face will embed into this finish, which helps create a seal, and a high level of friction
is generated between the mating surfaces.
Spiral Serrated" This is also a continuous or phonographic spiral groove, but it differs
from the stock finish in that the groove typically is generated using a 9:2deg tool which
creates a 1;1 geometry with $?>3 finishes are typically
smoother than @
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,. BOLTING
6olting should be of sufficient strength to achieve proper compression of the gasket, tonot only seal the joint, but to maintain the seal without exceeding the yield strength of the
bolts being used. The tor0ue values in our tor0ue tablesare based on using AT3
A#9& rade 6B studs and %$ heavy hex nuts lubricated with never sei4e.
ince sheet gasket materials have micropores, they must be sufficiently compessed to
reduce porosity. 8ithout ade0uate compression the system pressure can force the
contained fluid into the gasket and degrade it.
Therefore, when installing the gasket it is important that good techni0ue be followed
including cleaning the flanges, inspecting the flange face and the bolts and bringing the
flanges together parallel and in stages. 3any field problems arise from improperly
installed gaskets.
roper gasket selection and installation should be based on minimi4ing tor0ue loss.
Tor0ue loss can be caused by the tendency of the gasket to relax or remold after it has
been compressed and?or by elongation of the bolts. This loss can be minimi4ed several
ways"
#) se of a thinner gasket" The surface of the gasket is actually the sealing surface. The
internal portion of the gasket is used primarily to insure that the imperfections in the
sealing surface are filled. ince it is this internal portion that is primarily affected by
creep relaxation, the thinner the gasket, the more effective the seal. $owever, if the
surface to be sealed is pitted or marred or is somewhat distorted, it may not be feasible
to switch to a thinner gasket.
%) se of a denser gasket" !n general, the denser the gasket
material, the less creep relaxation will occur. 8ith materials of similar composition,
greater density will re0uire greater seating stresses to seal. Therefore, some lighter
flanges may not be strong enough to use with a denser material.
http://www.durlon.com/InstallTorque.htm#BoltTorquehttp://www.durlon.com/InstallTorque.htm#BoltTorque -
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&) se of conical washer" The elastic effect of a conical washer helps to compensate for
some of the loss in gasket resilience. The washer also lengthens the bolt to a slight
degree, lessening the effect of bolt elongation.
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CHAPTER-2
GASKET SELECTION CONSIDERATIONS
1. INTRODUCTION:
The importance to the environment of selecting the right gasket for todayCs services
cannot be overstated. 8ith the emphasis on fugitive emissions gaining more and more
prominence, selecting the proper gasket involves many considerations.
D rocess safety
D *nvironmental concerns
D 7ife of service in the flange
D 3aintenance costs
D !nventory costs
ome things to consider when selecting a gasket are"
D 'hemical compatibility with the process fluid
D The pressure2temperature (xT +actor)
relationship of the gasket to the service
conditions
D hysical and mechanical properties of the
gasket material
D ther considerations such as fire safety,
and gasket design factors
2. CHEMICAL COMPATIBILITY
'hemical resistance of the gasket material is important because without it, the other
properties of the gasket are irrelevant. !t is also important to keep in mind the effect
temperature has on chemical resistance.
Achemical resistance chartcan be a helpful guide. This information is available but it
must be remembered that most chemicals become more reactive at higher
temperatures. This must always be considered when selecting the gasket.
http://www.durlon.com/CHEMICAL/DChem-A.htmhttp://www.durlon.com/CHEMICAL/DChem-A.htmhttp://www.durlon.com/CHEMICAL/DChem-A.htm -
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!n some instances it is only prudent to consider field testing in a controlled application
and we encourage this. amples are available for such purposes. +or samples
please re0uest, fill out and submit a sample re0uest form.
,. PRESSURE-TEMPERATURE RELATIONSHIP PT FACTOR/
!n all piping systems the flanges, valves and the piping itself have a pressure 2
temperature relationship. This xT factor is the result of multiplying the operating
pressure times the operating temperature to arrive at a numerical value. This value is
not constant and is different at each temperature and pressure combination.
!n the table below the xT factors for carbon steel piping per A-! 6#@.&< and saturated
steam are shown. The fact that xT values exists for piping should indicate that such
values also exist for gasketing, and just like piping, those values change with differences
in the pressure and temperature.
>*>*2T*3*>AT>* >*7AT!-$!
Temp.
('arbon teel) aturated
team'lass #=: 'lass &::
E+ psi ( x T) psi ( x T) psia ( x T)
#:: %F= (%F,=::) B
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-ow we can look at how sheet gaskets fit. As stated above just like piping, the xT
relationship for gaskets changes with each pressure 2 temperature combination and
therefore is not a constant.
The chart below shows compressed non2absestos and compressed asbestos gasketing
vs. three different pressure classes and saturated steam for reference. This chart shows
why, as a general rule, all sheet non2asbestos gasketing should be limited to 'lass &::
and below.
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0. PHYSICAL AND MECHANICAL PROPERTIES
AT3 +#:
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6! 2 +#%=A 2 $ot 'ompression
5!- 2 &=&= 2 as ermeability
+A 2 -32%:< 2 $igh ressure
aturated team Test
These tests are outlined in the test methods section.
+. OTHER CONSIDERATIONS
Additional considerations when selecting a gasket material may include"
Fire safe capability. There is no standard for 1fire safe1 gasket materials.
$owever, $urlon %&'' passed a modified A! @:B fire test that was done by an
independent lab.A! pec @6+, +ire Test for *nd 'onnections, and A! 6ulletins, @+#
and @+%, do discuss fire testing but for metal gaskets and A! rings, not soft gasket
material.
Gasket design factorsThe mand yvalues established by A3* and the newer
design factors being developed by the ;>' for fugitive emissions, are additional
considerations. The mand y values do not take fugitive emissions into account whereas
the newer tightness parameters (Tp) do.
These gasket factors recogni4e that all joints leak to some extent. Therefore, an
acceptable level of leakage is defined. A leak rate of #?%