FORCES THAT OCCUR IN A
GASKETED JOINT
| THE FUNCTION OF A
GASKET is to create and maintain a static seal between
two stationary, imperfect surfaces of a mechanical system, designed to contain a wide
variety of liquids 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
"y" factor. This flange pressure must be applied uniformly across the entire
seating area to achieve perfect sealing. However, 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 mid-way between two bolts. This factor must
be taken into account by the flange designer. |
|
 |
THE INTERNAL PRESSURE: In 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 "m" or maintenance factor has been established by ASME to account
for this preload. The "m" factor defines how many times the residual load
(original load minus the internal pressure) must exceed the internal pressure. In 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 pressurized and heated, the joint deforms.
Different 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. Generally, flexible
joints lose bolt load.
FLUID:
The media being sealed, usually a liquid or a gas with a gas being harder to seal than a
liquid. 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.Both
the "m" and "y" factors will vary with the type of gasket and the
thickness of the gasket. Always consult with the manufacturer to determine the
"m" and "y" factors for the gasket material you are using.
In any application, failure to meet the "m" or
"y" factor will result in an imperfect seal and will require 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. However, since thinner gaskets are generally more
effective, changing to a thicker gasket may not be the most satisfactory long-term
solution. In some cases, a revision to the flange design may be required.
New gasket design factors being developed
by ASME are for bolted joint designs where it is important that a desired level of
tightness be achieved. "M" and "y" factors do not take fugitive
emissions into account, whereas the new assumption is that all bolted joints leak to some
extent. This "systems approach" 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. See additional discussion under Other Considerations, in the section on Gasket Selection. |
| We recommend that
metal flange faces be machined with a concentric-serrated finish of 125-500 AARH, with 250
AARH being the optimum for non-metallic gaskets. Phonographic serrations can also be used
with our materials. It should be recognized, 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. Sheet gasketing is designed to have a seating stress that allows the
gasket material to "flow" into the serrations and irregularities of the flange
face. This "bite" aids the gasket in resisting the effects of internal pressure,
creep and cold flow. |
|
"Smooth" finishes are
usually found on machinery or flanged joints other than pipe flanges. When working with a
smooth finish, it is important to consider using a thinner gasket to lessen the effects of
creep and cold flow. It should be noted, however, that both a thinner gasket and the
smooth finish, in and of themselves, require a higher compressive force (i.e. bolt torque)
to achieve the seal. Therefore, due to the flange
design, one may have to resort to a thicker gasket, which requires 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. |
FLANGE TYPES
|
RAISED FACE to
FULL FACE. We do not recommend mating a full face
flange to a raised face flange especially when the full face flange
is cast or ductile iron. Due to the potential for warping the
flange, or in the worst case cracking it, the utmost care should be
taken.
Even if a spacer that fits on the
raised face flange outside the raised face area is used, damage to
the flanges can still occur and
great care should be taken. |
|
FULL FACE FLANGES.
In a
bolted joint using ANSI full face (or flat face) flanges it must be
remembered that the same
bolts used in the corresponding raised
face joint are now being asked to seal
3 to 4 times the
gasket area with full face flanges. It is almost impossible to
create an effective seal and
high strength bolts should be
considered.
ANSI Class 150 Full face bolted
joints are poor design and should only be used for non-critical
fluids. |
It must be strongly stated that
the use of metal based anti-seize compounds in general is not recommended. There are
two issues to consider:
1) Under heat and pressure, the metals in the compound can
adhere to the flange surface, causing distortion of the flange and/or fill in the
serrations. After a period of time, when this condition has been allowed to
progress, no amount of additional torque will allow the gasket to seal.
2) Anti-seize lubricates the gasket. This isn't a
problem
|
|
until gasket compression is lost for some reason.
Then the lubricated gasket can either be extruded, or forced out of the flange by
the internal pressure. Here the friction created by the flange serrations play a
role.
For these reasons the use of anti-seize is not recommended.
However, if the application of anti-seize is not too liberal and the flange
serrations are thouroughly cleaned each time a gasket is changed to maintain their
integrity, it doesn't present as much of a problem. The important thing is keeping
the serrations intact, getting good compression on the gasket, and a minimum bolt stress
of at least 40% of bolt yield. |
THE 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. In
many instances, a good seal is obtained through the limited "swell" 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 an equilibrium and does not reach a condition of
degradation where the gasket begins to breakdown. In many instances, the fluid being
contained may "cauterize" the inside edge of the gasket and "seal off"
the gasket from further fluid penetration |
BOLTING
| Bolting should be of sufficient
strength to achieve proper compression of the gasket, to not only seal the joint, but to
maintain the seal without exceeding the yield strength of the bolts being used. The
torque values in our torque tables are based on
using ASTM A193 Grade B7 studs and 2H heavy hex nuts lubricated with never seize. Since sheet gasket materials have micropores, they must be
sufficiently compessed to reduce porosity. Without |
|
adequate 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
technique be followed including cleaning the flanges, inspecting the flange face and the
bolts and bringing the flanges together parallel and in stages. Many field problems arise
from improperly installed gaskets. Refer to the section on gasket installation. |
MINIMIZING TORQUE LOSS
Proper gasket selection and
installation should be based on minimizing torque loss. Torque 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 minimized several ways:
1) Use 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. Since it is this internal
portion that is primarily affected by creep relaxation, the thinner the gasket, the more
effective the seal. However, 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.
2) Use of a denser gasket: In general, the denser the
gasket
|
|
material, the less creep relaxation will occur. With
materials of similar composition, greater density will require greater seating stresses to
seal. Therefore, some lighter flanges may not be strong enough to use with a denser
material.
3) Use 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.
4) Greater bolt load: The use of stronger bolts or more
bolts can also help in the reduction of torque loss. Care should be taken to insure that
the maximum loads on the bolts are not exceeded.
|
GASKET SELECTION
The importance to the environment
of selecting the right gasket for today's services cannot be overstated. With the emphasis
on fugitive emissions gaining more and more prominence, selecting the proper gasket
involves many considerations.
| · Process safety |
| · Environmental
concerns |
| · Life of service in
the flange |
| · Maintenance costs |
| · Inventory costs |
|
|
Some things to consider when
selecting a gasket are:
| · Chemical compatibility with the process fluid |
· The pressure-temperature (PxT Factor)
relationship of the gasket to the service
conditions |
· Physical and mechanical properties of the
gasket material |
· Other considerations such as fire safety,
and gasket design factors |
|
CHEMICAL COMPATIBILITY
| Chemical resistance of the gasket
material is important because without it, the other properties of the gasket are
irrelevant. It is also important to keep in mind the effect temperature has on chemical
resistance. A chemical
resistance chart can 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. In some instances it is only prudent to consider field testing in a
controlled application and we encourage this. Samples are available for such purposes.
For samples please request, fill out and submit a sample
request form. |
PRESSURE-TEMPERATURE
RELATIONSHIP (PxT FACTOR)
| In all piping systems the
flanges, valves and the piping itself have a pressure - temperature relationship. This PxT
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. |
|
In the table below the PxT
factors for carbon steel piping per ANSI B16.34 and saturated steam are shown. The fact
that PxT 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. |
|
|
|
PRESSURE-TEMPERATURE RELATIONSHIPS |
Temp.
|
(Carbon Steel) |
Saturated
Steam |
| Class 150 |
Class 300 |
°F |
psi |
(P x T) |
psi |
(P x T) |
psia |
(P x T) |
100 |
285 |
(28,500) |
740 |
(74,000) |
1 |
(100) |
200 |
260 |
(52,000) |
675 |
(135,000) |
12 |
(2,400) |
300 |
230 |
(69,000) |
655 |
(196,500) |
68 |
(20,400) |
400 |
200 |
(80,000) |
635 |
(254,000) |
250 |
(100,000) |
500 |
170 |
(85,000) |
600 |
(300,000) |
680 |
(340,000) |
600 |
140 |
(84,000) |
550 |
(330,000) |
1550 |
(930,000) |
700 |
110 |
(77,000) |
535 |
(374,500) |
3100 |
(2,170,000) |
The chart below graphically represents
the information presented above
|
| Now we can look at how sheet gaskets fit. As stated
above just like piping, the PxT relationship for gaskets changes with each pressure -
temperature combination and therefore is not a constant. The chart below shows compressed non-absestos and compressed asbestos gasketing
vs. three different pressure classes and saturated steam for reference. This chart
shows why, as a general rule, all sheet non-asbestos gasketing should be limited to Class
300 and below.
|
 |
PHYSICAL AND MECHANICAL
PROPERTIES
| ASTM F104,
the Standard Classification System for Nonmetallic Gasket Materials includes a line
call-out encompassing ASTM test methods for evaluating the physical and mechanical
properties of nonmetallic gasket materials. Some of
these ASTM tests are:
F
36 - Compressibility and Recovery
F 37 - Sealability
F
38 - Creep-relaxation
F
146 - Fluid Resistance
F1574 - Compressive Strength |
|
In addition to ASTM tests, we
also do testing to BSI (British Standards), DIN (German Institute for Standardization) and
FSA (Fluid Sealing Association) standards. These tests include: BSI - F125A - Hot Compression
DIN
- 3535 - Gas Permeability
FSA
- NMG-204 - High Pressure
Saturated Steam 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 "fire safe" gasket materials. However, Durlon 8500 passed a modified API 607 fire test that was
done by an independent lab.
API Spec 6BF, Fire Test for
End Connections, and API Bulletins, 6F1 and 6F2, do discuss
fire testing but for metal gaskets and API rings, not soft gasket material.
Gasket design factors. The m and y values established by ASME and the newer design factors being
developed by the PVRC for fugitive emissions, are additional considerations. The m and y values do not take fugitive emissions into account
whereas the newer tightness parameters (Tp) do. |
|
These gasket factors recognize
that all joints leak to some extent. Therefore, an acceptable level of leakage is
defined. A leak rate of 1/2480 lb/hr per inch of OD has been defined as a
"standard" acceptable leak rate and is known as T2. Tp classes and their associated leak rates are as follows:
T1 - Economy - 1/25 lb/hr per inch of OD
T2 - Standard - 1/2480 lb/hr per inch of OD
T3 - Tight - 1/248,000 lb/hr per inch of OD
torque values for Durlon products are calculated using a
tightness parameter of T3. |
|
