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| Fig.1. |
Although the gasket material is considered the most
important element in the system, it may fail to provide seal, because the other
elements were not designed and constructed to make the best use of its
properties. It is the contact pressure between the gasket material and the flanges
that produce sealing. These pressure must be satisfactory for resisting the
movements caused by those agents previously discussed (temperature, fluid
pressure, fluid velocity and system vibration).
The flanges are important because they must transmit a compressive
load to the gasket. This load must be maintained above a critical pressure
called the minimum sealing stress. This stress is defined as the minimum stress
a gasket must experience to close its internal structure to the passage of the
sealed fluid and conform to the surface irregularities of the flange. The
minimum sealing stress is highly dependent on the gasket material used,
viscosity of sealed fluid, the width to thickness ratio of the gasket, the
gasket shape factor, surface finish of the flange, and the stiffness of flange.
For instance, a firm cork and rubber gasket requires a sealing stress of 600
psi to seal 800 psi internal pressure with stiff flanges (4).
FORCES ON A FLANGE ASSEMBLY |
| Fig.2. |
If the working pressure is too high for the design of the flanges, adverse flange and bolt distortions will occur. The major problem encountered with flanges is distortion. Distortions most commonly found are:bowing, bolt hole distortions, non-parallelism and surface roughness.
FLANGE BOWING
Distribution of stress on a flexible flange.
The area where this joint is most likely to leak is right at the center of the flange ( c ) where the smallest stress is produced by the bolts and where the maximum bending occurs from internal pressure. One way to improve this situation is to stiffen the flange to get the result shown in fig. 14. As obvious as this seems, the economics of producing stiff flanges often precludes this approach. The bowing of flange is the most common reason for leakage.
EFFECT OF STIFFENING THE FLANGES & STRESS DISTRIBUTION
The area where this joint is most likely to leak is right at the center of the flange ( c ) where the smallest stress is produced by the bolts and where the maximum bending occurs from internal pressure. One way to improve this situation is to stiffen the flange to get the result shown in fig. 14. As obvious as this seems, the economics of producing stiff flanges often precludes this approach. The bowing of flange is the most common reason for leakage.
EFFECT OF STIFFENING THE FLANGES & STRESS DISTRIBUTION
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| Fig.3 |
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| Fig.4. |
This occurs around the bolt holes in a flange. Figure 15 shows a stamped sheet metal flange before bolt loading (bottom) and deformation after loading (TOP). High stresses are transferred to the gasket material under the bolt head causing the gasket to crack, tear, rupture or extrude, as shown in fig 16. Bolt hole distortions do not necessarily lead to leakage but can cause bolt load losses that will result in fatigue failures if the system is subjected to dynamic forces.
NON - PARALLELISM
Flange cocking or non-parallelism. Cocking in itself does not create serious problems until the pressure in the area of low gasket compression falls below the minimum sealing stress. This type of distortion is usually caused by improper machining, improper heat treating, casting irregularities or improper tightening sequence of the bolts.
SURFACE ROUGHNESS
The various operations required to fabricate the surface of a flange face produce distortions or irregularities. A commercial finish is made by milling machines, grinders and abrades. The roughest surface normally found in flange joints is 250 micro inches. On the other hand, an exceptionally smooth or mirror finish is rarely found because expensive machining operation are required. There are some rough surface finishes that are planned and are defined as phonographic and concentric. They are not random irregularities but carefully dimension-ed in width, height and depth of cut. The serrated finish is almost always found in pipe flange assemblies and is used more or less as a stress riser. To eliminate leakage from surface irregularities, the minimum sealing stress must be achieved thereby ensuring that the gasket flows into all flange face imperfections.
BOLTS
The next element in the gasket joint is the bolts. The life of the gasket-ed joints can be greatly affected by the design of the bolting system. Almost all of the force for producing the minimum sealing stress is generated by the bolts (a small amount may come from gasket adhesion and gasket swelling from chemical and pressure effects). It is important that the bolts not only produce the design pressure initially on the gasket but maintain that pressure throughout the service life of the assembly.
Bolts lose their initial stress or clamp load in a variety of ways. Some of the more important reasons are listed as follows.
Gasket relaxation.
Bolt hole distortions of the flange.
Temperature effect.
Vibration loosening and,
System load changes.It is sometimes helpful to use a spring analogy in explaining the bolts specific function within a gasketed assembly. The bolts stretch or deform like a spring when a load is applied. The necessary clamp load cannot be exerted on the flange and gasket if this axial stretching did not occur. The amount of clamp load that can be applied depends on the bolts strengths, the rigidity of the flange and the compressive strength of the gasket material. After the assembly has been tightened, the bolts stretches in tension and the flanges and gaskets are in a state of compression. Assuming the flange is infinitely rigid and the gasket is thin so that its effects are negligible, the goal is to increase the amount of elongation that a bolt experiences helps to maintain the required clamp load.
There are two ways to increase bolt elongation:
By increasing the effective bolt length Decreasing the spring constant of the bolt
The rule of thumb is that, when the length of the bolt is five times greater than the diameter it can be elongated sufficiently to work as a spring between two flanges and will dampen vibration. The relationship between the effective bolt length and the clamp load might be understood more clearly with a hypothetical example. For instance, a bolt with an effective length of 1 inch elongates 0.004 inches under a given load. Now, what if the gasket material used relaxed in compression approximately 0.002 inches over some period? The original clamp load would be decreased by 50%. However, if the effective bolt length was increased to 2 inches under the same load, the bolt now would elongate 0.008 inches. The gasket relaxes about the same amount (the load has not changed) but this time only 25% of the original clamp load has been lost. Clearly a 25% load loss is more desirable if we are concerned with maintaining a clamping pressure above the minimum sealing pressure requirements.
Several things can be done to increase the effective bolt length:
Use a thicker washer under the bolt head
Design a boss on the flange
Use a thicker flange
Machine down the top portion of the engaged threads.The other method used to increase bolt elongation is to decrease the spring constant of the bolt. The spring constant (k) of the bolt is simply the ratio of the applied load (f) to the elongation (x) produced by that load:
K=F/X
EFFECTIVE BOLT LENGTH
 |
| Fig.5 |
Assemblies using bolts with low spring constants have a smaller clamp load loss than do high spring constant systems and provide longer gasket life in designs involving dissimilar metals with different coefficients of thermal expansion. Lowering the spring constant reduces the pounding of the flanges, causing the gasket to crush or fail. Low spring constant bolts elongate more than high spring constant bolts.
As mentioned earlier in the effective bolt length section, for a given clamp load, a gasket material will relax over a period of time. The low spring constant bolt loses less clamp load than the high spring constant bolt, thus adding more reliability to the sealing function.
The spring constant can be lowered by decreasing the number of bolts used, reducing the diameter of the bolts down to the root diameter, increasing effective bolt length, increasing the length of the threaded sections, or decreasing the size of the bolts. Decreasing the size of the bolts from 3/8 to 5/16 might involve going to a stronger grade of fastener to achieve the proper clamp load. Of course, any of these changes must be checked against the entire system to ensure proper performance and lasting service life.
GASKETS
The third and final element in the flange system is gasket. The main function of the gasket is to create a barrier against the transfer of fluids across two mating surfaces. There are, however, other reasons for a gasket’s existence, such as, cost and accessibility. Manufacturing and shipping costs are much lower when assemblies are composed of small elements that are joined later with a gasket to form a complete and leak tight unit. The assembled unit can at any time be disassembled for repair or maintenance. Without this accessibility, repair might be extremely difficult. If not impossible. Gaskets are also used to join dissimilar materials, dampen vibration, and insulate against the transfer of heat.
Materials used for gaskets fall into two categories: metallic and nonmetallic. The metallic gaskets can be steel, copper or metallic and organic composite. Materials such as asbestos, cork, cellulose, and rubber are non metallic gaskets. Other types of non metallic gaskets are sealants or formed in place of materials. These materials can be applied to any shape flange and generally considered to be pressure containing, but not load bearing, because of their fluid consistency upon application.
All gaskets, whether metallic or nonmetallic must perform four basic functions:
Must create a seal
Must maintain the seal
Must be impervious to fluid flow
Must be compatible with the environment
Each function has its own relative importance with respect to the integrity of the gasket assembly. A brief discussion of each one will point out their relative importance.
First, a gasket must create a seal between the flanges by conforming to all flange face irregularities.
On smooth and polishes flanges a relatively firm material may be used. When surfaces are rough or show excessive tool marks, either a thicker, softer gasket or heavier bolting pressures must be used. The effects of changing the gasket thickness on the other elements will be pointed out next.
Second, the gasket must maintain the seal throughout the life expectancy of the joint. Joints are sometimes subjected to considerable movements caused by vibration, mechanical strain, changes in temperature, pressure and velocity. Despite these movements, the gaskets and flange surfaces must remain in intimate contact. The major factor in maintaining contact is the elastic response of not only the gasket, but the bolts and the flanges.
Third, the gasket must be impervious to fluid flow, both internal and external. Material such as cork with rubber or straight rubber normally is impermeable even with small flange loads. Others such as fiber sheet packing’s and cork are impermeable only when compressed sufficiently to close their natural holes.
The fourth requirement is compatibility with the environment. A gasket material must be able to withstand the full range of temperature changes without deteriorating. Flange staining and corrosion, which is generally promoted by vulcanizing agents, accelerators and moisture present in the gasket material, should not be visible on the flange face. Gaskets must also be relatively inert to the effects of sealed fluids. Slight swelling is often beneficial, whereas deterioration of the material or contamination of the sealed fluid is not tolerable.
PHYSICAL PROPERTIES
Various physical characteristics have been defined to evaluate gaskets performance properties and to measure its ability to meet the four requirements previously discussed. The most important physical properties of a gasket material are:
COMPRESSIBILITY
This property indicates the degree to which the material is compressed and deformed, in thickness, by the application of a specific load. This property is expressed in terms on a percentage of the original thickness. Compressibility is used to determine whether a material can compress sufficiently to compensate for surface irregularities or non-parallel conditions in the flange.
COMPRESSIVE STRENGTH
Compressive strength can be described as the maximum compressive stress that a material is capable of withstanding without rupture or excessive extrusion. From a functional standpoint, this physical property is important because it is directly related to the ability of a gasket to resist flange loading without breaking down.
STRESS RELAXATION
This is a transient stress strain condition in which the stress decays as the strain remains
constant.
CREEP:
This is a transient stress strain condition in which the strain increases as the stress remains constant.
CREEP RELAXATION
This is a transient stress strain condition in which the strain increases concurrently with the decay of stress.
FLUID COMPATIBILITY
Immersion tests provide a simple means of measuring the effects of various liquids on a gasket material. The procedure consists essentially of immersing a prepared specimen in the desired fluid for a specified length of time at a specific temperature. The weight, thickness or volume increase is the properties most frequently measured after removal of the specimen from the fluid.
FLEXIBILITY
This property is usually determined by bending a specimen 1800 around a mandrel of a specified diameter. As applied to materials in their original condition, flexibility is of value in determining the handling qualities of the material. It does not directly correlate with other physical properties.
HEAT AGING
Heat aging properties consist of exposing the material at a specified material at a specified temperature in a circulating air oven for a specified length of time. Physical characteristics measured after removal of the test specimen may include flexibility, durometer hardness and elongation and are compared with those values for the same materials prior to testing.
There are many other considerations to be made before a suitable gasket can be selected, such as its ability to resist tearing, weather, fire, fungus and vermin. Most of the physical properties listed above pertain to pre-cut gaskets; however, formed-in-place materials must also meet many of these requirements, especially degradation from fluids, heat or other environmental constraints.
GASKET THICKNESS VERSUS STRESS DISTRIBUTION
Perhaps the most important requirements of any gasket material are to create and maintain the seal. In the section on flanges we defined minimum sealing stress as the minimum pressure the gasket must experience to close its internal structure to the passage of the sealed fluid and to conform to the surface irregularities of the flange. As we shall now see, the thickness of a gasket material has a profound effect on two of the major requirements of the gaskets, that is, creating and maintaining the seal. Figures 13 and 14 showed how stiff flanges improved the load distribution on a gasketed joint. To increase the minimum sealing stress between the bolts, figure 14 suggested stiffening the flange. The same effect can be obtained by making the flange bow more, through the use of a softer or thicker gasket.
Increasing the thickness of the gasket will enhance its capability to create the initial seal. Now it would be instructive to see exactly what effect increasing the gasket thickness has on maintain the seal. Relaxation is one of the major considerations for maintaining the seal, primarily because pre-cut gaskets creep or relax in service to some degree. The relaxation of the gasket as measured by torque loss in the bolt vs. thickness of the gasket. The solution to this effect is to use a harder or thinner material.
Formed in place material should also be considered with respect to creating and maintaining the seal. Flange loading was increased by using a thicker gasket. Now, if the stress distribution was redrawn for a non liquid formed-in-place material and non liquid material will not follow the movements of the flanges. What happens if the material is mildly liquid sealing agent? A mildly liquid sealant material will seal more flexible flanges, provided that liquid is also flexible enough to follow the flange bowing under internal pressure. Usually this liquid technique cannot be carried to the extreme because flange disassembly becomes very difficult. What can be done if the flanges are prestressed in such a manner that internal pressure does not exceed or add to the pre stress? This is accomplished by providing pre-bowed covers, or bosses on the mating surfaces. In other words, the gasket surface can be prestressed so that the stress distribution is more uniform as discussed.
We have seen that a sealed joint is not a static device. It moves under the influence of temperature, fluid pressure and velocity, and external vibration. If a joint leaks, it may not suffice to change only one element within the system. With any change, one must always keep in mind its effect on the minimum sealing stress.
SYSTEM RELIABILITY
The fatigue or dynamic loading characteristics of any assembly have not been fully discussed here. Their importance to the physical integrity of the joint cannot be overemphasized because the service life of an assembly is directly related to its ability to absorb and transfer these dynamic forces.
In most assemblies, the ratio of assembly rigidity to fastener rigidity is high enough to almost discount any addition to bolt tension produced by dynamic forces. In a flexible joint with a soft gasket between bolted flanges, the rigidities of the joint and the bolt are quite different; here, a much greater proportion of the externally applied tension is added to the bolt preload. The fatigue strength of a gasketed assembly must be evaluated in two ways: fatigue of the bolt, and fatigue of the bolted material. The properly tightened bolt will not fail in fatigue in rigid joint. Initial bolt tension will stay relatively constant until the external tension load on the joints exceeds the bolt load. If the service load is less than bolt preload, the bolt will experience no appreciable stress variations, and without stress variation, there can be no failure by fatigue. This is not the case where considerable flexibility is present. Variable stress in screw or bolt fastenings increases with the flexibility of the connected parts. If flexibly is too great, the variable stress present may be high enough to cause eventual fatigue failure of the fastener regardless of the initial bolt preload.
SUMMARY
Selecting rigid joint members and choosing flexible fasteners are important steps to successful design. Soft materials in joint members may spell trouble unless extra design precautions are taken. If gaskets are used, they should be as rigid and thin as practical. But this is not always possible because thick and soft gaskets are required to seal imperfections and produce the minimum sealing stress. Formed in place sealants (new liquid technology available worldwide) offer some advantages in this area. Here the load on the bolt was essentially the preload P1 (Since C is approximately 0) for stiff or hard gaskets. Liquid gaskets create metal to metal flange contact to form a rigid joint.
P= Pi +CFe Where P= final load on the bolt, lb.; Pi = initial preload or clamping load developed through tightening, lb.; Fe = external applied load, lb.; and the constant
C =EbAb /Lb /EbAb/Lb+EgAg/tg,
Where Eb = modules of elasticity of the bolt, psi;Eg =modules of elasticity of the gasket, psi; Ab = effective cross-sectional area of bolt, sq. in.; Ag = loaded area of gasket, sq. in.; Lb = effective length of bolt, in.; and tg = gasket thickness, in.
SYSTEM RELIABILITY - GASKET
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| Fig.6 |
The greatest single factor that can eliminate stress variations due to cyclic loading is proper preloading of the fastener. Test result indicates that rigid members bolted together by relatively elastic bolts after the best assurance for preventing fatigue failure.
Static seals are subjected to a variety of dynamic forces that cause movements and create stresses within the joint. Loss of fluid and catastrophic failure, such as, fatigue fracture of the bolts is the end result of poorly designed assemblies. The gasketed joint must be thought of as a system for affecting a seal. Each component of the joint plays an important role in creating a seal but more importantly, maintaining that seal throughout the expected life of the assembly.
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