FACTORS IN GASKET DESIGN

Gaskets are used to create and maintain a seal between two separable flanges. In theory, if the flanges were perfectly smooth, parallel and rigid, they could be bolted together and would seal without a gasket. But in practice, flanges have rough surface finishes and limited rigidity. They are not perfectly parallel and may be secured by bolts of varying lengths that may not be uniformly spaced around the flanges. Reflecting these conditions, flange loading is often non-uniform. The gasket must compensate for this non-uniform flange loading and distortion. It must also conform to surface irregularities.

Once it is realized that the insertion of a gasket between two flanges is necessary, a host of design problems must be met.

1. The first is the recognition that the medium being sealed may be corrosive to the gasket material.
2. In addition, the pressure of the medium being sealed exerts radial forces on the gasket, tending to blow it out. This pressure can also exert forces on the assembly, tending to open the flanges md reduce the sealing stress on the gasket.
3. Furthermore, the gasket and its environment are likely to be subjected to large variations in temperature, and thermal distortions ultimately will therefore occur.
4. Finally, under the influence of the sealed medium, the operating temperature, and the pressure the gasketing material may change dimension, because of its creep-relaxation properties, and lower bolt torque and sealing stress.

Various factors that affect the gasket design are listed below:

• Sealing Stress: The basic factor in the creation of the seal is sufficient stress on the gasket to ensure its conformation to the flange surfaces. This blocks the passage of the media between the gasket and the flange. In addition, this stress must be high enough to close any voids in the basic material if it is to block the passage of sealed media.
• Temperature: A gasket most often is a viscoelastic material. It will change its load-deformation properties with temperature. When a gasket undergoes permanent relaxation, sealing stress on the gasket is lowered. If this occurs in a poorly loaded joint or in a flange where there is non-uniform bolt spacing and an inherently poor bolt loading pattern, the sealing stress may well fall below that minimum stress required to seal the medium. The joint will then leak.
• Hydrostatic End Force: The load remaining on the gasket during operation must remain high enough to prevent blowout of the gasket. During operation, the hydrostatic end force, which is associated with the internal pressure, tends to unload the gasket and could result in leakage or blowout.
• Flange and Fastener Details: Design details such as number, size, length, and spacing of clamping bolts; and flange properties such as thickness, modulus, surface finish, waviness, and flatness are important factors. In particular, flange bowing is a most common type of problem associated with the sealing of a gasketed joint.
• Surface Finish: Different gasket materials and types require different surface finishes for optimum sealing. Soft gaskets such as rubber can seal very rough surface finishes in the vicinity of 500 µin. Some metallic gaskets may require finishes as fine as 16 to 32 µin for best sealing. Most gaskets, however, will seal in the surface finish range of 60 to 125 µin.
• Fasteners: The function of the fastener in a gasketed joint is to apply and maintain the load required to seal the joint. The fastener device must be able to produce a spring load on the gasket to compress it to its proper thickness and density for sealability. The fastener must also be able to maintain proper tension to maintain this compression of the gasket material throughout the life of the assembly. Addressing the question of how many bolts or other fasteners can be used involves space available, economic limitations, and flange flexibility considerations as well as getting the required initial load. Approximately 80 percent of the load applied by the fastener may be distributed out along the flange to the midpoint between the bolts. It is difficult to provide useful “rule of thumb” guidelines, but cutting the distance between bolts by half will reduce the bowing effect to 1/8th its original value. Conversely, stiffening the flange is frequently more cost effective than increasing the number of fasteners.

OVERLOADING THE GASKET

It is essential to avoid overloading the gasket. Gasket materials will crush owing to a combination of compression-, shear-, and extrusion-type displacements of the material. The maximum unit load is a function of the type of material, operating temperature, thickness, and section width, among the principal factors.

FLANGE THICKNESS

Flanges must have adequate thickness. Adequate thickness is required to transmit the load created by the bolt to the mid-point between the bolts. It is this midpoint that is the vital point of the design. Maintaining a seal at this location is important and should be kept constantly in mind.

Adequate thickness is also required to minimize the bowing of the flange caused by the bolt loads. If the flange is too thin, the bowing will become excessive and low bolt load will exist at the midpoint. See Figures 1 and 2.

Figure 1: Illustration of Flange Bending or Bowing

Figure 2: Midpoint Loading between Bolts

INTERNAL PRESSURE

Internal pressure also can create loads on both bolts and flanges to create another type of distortion. For instance, bolts might elongate. This would be elongation in addition to that caused by the initial tightening torques. Yielding of the bolt and unloading of the gasket could result. Also, the flange might deflect or reveal a bowing in addition to the bending caused by the imposition of initial bolt loads. Another force or load created by internal pressure is blowout. Figure 3 depicts this blowout as acting on the inner edge of the gasket, tending to push it out from between the flanges.

Figure 3: Blowout Pressure in Gasketed Joint

TEMPERATURE

The effects of temperature on gasket performance are very complex and not too well understood. In low-pressure applications, moderate temperatures appear to favor the initial seal; that is, it is improved so the joint becomes a little more impermeable to sealed fluids. This can be attributed to the softening effect produced in the gasket by initial heating. The gasket, being softened under loading conditions, will more than likely flow into the surface imperfections on the flange, thereby completing the conformation between gasket and flange. This is called “settling in.”

Prolonged exposure to higher than ambient temperature will cause many gasket materials to harden. Fortunately, the hardening does not appear to seriously affect either the initial seal or the slight improvement in sealing caused by the initial heating.

While moderate temperatures promote sealing, abnormally high temperatures will result in a complete breakdown of the gasket. These are the temperatures which normally cause burning or charring in nonmetallic materials. Hence temperature can have both beneficial and detrimental effects on the initial seal.

Gasket materials, in general, have somewhat higher coefficients of thermal expansion than most of the metals from which flanges and bolts are made. In certain situations involving wide and rapid temperature fluctuations this factor of relative expansion and contraction due to such temperature changes may require special design considerations. The gasket must be able to seal when exposed to changing temperatures.

PENETRATION OF INTERNAL FLUID INTO GASKET

Even in joints where the flange pressures are high enough to produce initial seals, the internal fluid will penetrate the gasket to a slight degree. Such penetration, or edge effect, is perfectly normal and has little or no effect on the gasket’s sealing ability. If anything, it aids sealing. Moderate swell may be highly beneficial even in assemblies where flange pressures are lower than those required for sealing. The swell will compensate for the lack of gasket loads and produce acceptable seals in joints which otherwise would exhibit leakage. On the other hand, excessive swell can be detrimental, particularly if the gasket becomes too soft and tends to disintegrate in the sealed liquid.

REDUCTION OF STRESS ON GASKET

After the initial sealing stress is applied to a gasket, it is necessary to maintain a sufficient stress for the designed life of the unit. All materials exhibit, in varying degrees, a decrease in applied stress as a function of time, commonly referred to as stress relaxation. The reduction of stress on a gasket is actually a combination of two major factors, stress relaxation and creep.

In a gasketed joint, stress is applied by tension in a bolt or stud and transmitted as a compressive force to the gasket. After loading, stress relaxation and creep occur in the gasket, causing corresponding lower strain and tension in the bolt. This process continues indefinitely as a function of time. The result is a loosening of the gasketed joint and a tendency for leakage.

ASME CODE, SECTION VIII, DIVISION 1

The ASME Code for Pressure Vessels, Sec. VIII, Div. 1, App. 2, is the most commonly used design method for gasketed joints. An integral part of the ASME code centers on two gasket factors:

1. m factor, often called the gasket maintenance factor, which is associated with the hydrostatic end force and the operation of the joint.
2. y factor, which is the minimum seating stress associated with particular gasket material. The y factor is concerned only with the initial assembly of the joint.

The m factor is essentially a multiplier on pressure to increase the gasket clamping load to such an amount that the hydrostatic end force does not unseat the gasket to the point of leakage. The factors were originally determined in 1937, and even though there have been objections to their specific values, they have remained essentially unchanged to date. The values are only suggestions and are not mandatory.

This method uses two basic equations for calculating required bolt load, and the larger of the two calculations is used for design. The first equation is associated with the required bolt load to initially seat the gasket:

The second equation states that the required bolt operating load must be sufficient to contain the hydrostatic end force and simultaneously maintain adequate compression on the gasket to ensure sealing:

Wm1 =  required bolt load for maximum operating or working conditions, lb

      Wm2 =  required initial bolt load at atmospheric temperature conditions without internal pressure, lb

      G     =  diameter at location of gasket load reaction, generally defined as follows:

When bo is less than or equal to ¼ in, G = mean diameter of gasket contact face, in;

When bo is greater than ¼ in, G = outside diameter of gasket contact face less 2b, in

           P     =  maximum allowable working pressure, psi

           b     =  effective gasket or joint contact surface seating width, in

           2b   =  effective gasket or joint contact surface pressure width, in

           bo    =  basic gasket seating width. (defined in terms of flange finish and type of gasket, usually from one-half to one-fourth gasket contact width)

           m    =  gasket factor. (m for different types and thicknesses of gaskets ranges from 0.5 to 6.5)

           y     =  gasket or joint contact surface unit seating load, psi (values range from 0 to 26,000 psi)

Taken from Gaskets – Design, Selection and Testing by Daniel E. Czernik