File 2_Chapter 1 and 2_GaASKETS

8/10/2019 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 #?%

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