An Introduction To Introduction To gaskets and Its Types

An Introduction To Introduction To gaskets and Its Types

Gaskets

gasket is a material or combination of materials designed to clamp between the mating faces of a flange joint. The primary function of gaskets is to seal the irregularities of each face of the flange, preventing leakage of the service fluid from inside the flange to the outside. The gasket must be capable of maintaining a seal during the operating life of the flange, provide resistance to the fluid being sealed, and meet the temperatures and pressure requirements. Since it is expensive to grind and lap joint faces to obtain fluid-tight joints, a gasket of some softer material is usually inserted between contact faces. Tightening the bolts causes the gasket material to flow into the minor machining imperfections, resulting in a fluid-tight seal. A considerable variety of gasket types are in common

use. Soft gaskets, such as cork, rubber, vegetable fiber, graphite, or asbestos, are usually plain with a relatively smooth surface. The semimetallic design combines metal and a soft material, the metal to withstand the pressure, temperature, and attack of the confined fluid and the soft material to impart resilience. Various designs involving corrugations, strip-on-edge, metal jackets, etc., are available. In

addition to the plain, solid, and flat-surface metal gaskets, various modified designs and cross sectional shapes of the profile, corrugated, serrated, and other types are used. The object in general has been to retain the advantage of the metal gasket but to reduce the contact area to secure a seal without excessive bolting load. Effective gasket widths are given in various sections of the ASME Boiler andPressure Vessel Code.

Gasket Materials. 

Gasket materials are selected for their chemical and pressure resistance to the fluid in the pipe and their resistance to deterioration by temperature. Gasket materials may be either metallic or nonmetallic. Metallic ring-joint gasket materials are covered by ASME Standard B16.20, Ring-Joint Gaskets andGrooves for Steel Pipe Flanges. Nonmetallic gaskets are covered in ASMEStandard

B16.21, Nonmetallic Gaskets for Pipe Flanges. Typical selections of gasket materials

for different services are shown 

Gasket Compression.

 In the usual type of high-pressure flange joint, a narrow gasket face or contact surface is provided to obtain higher unit compression on the gasket than is obtainable on full-face gaskets used with low-pressure joints. The compression on this surface and on the gasket if the gasket is used, before internal pressure is applied, depends on the bolt loading used. In the case of standard raised face

joints of the steel-flange standards, these gasket compressions range from 28 to 43 times the rated working pressure in the Class 150 to 400 standards, and from 1 to 28 times in the Class 600 to 2500 standards for an assumed bolt stress of 60,000 psi (4200 kg/cm²). For the lower-pressure standards, using composition gaskets, a bolt stress of 30,000 psi (2100 kg/cm²) usually is adequate. The effect of applying the internal pressure is to decrease the compression on the contact surface, since part of the bolt tension is used to support the pressure load.

The initial compression required to force the gasket material into intimate contact with the joint faces depends upon the gasket material and the character of the joint facing. For soft-rubber gaskets, a unit compression stress of 4000 psi (280 kg/cm²) to 6000 psi (420 kg/cm²) usually is adequate. Laminated asbestos gaskets in serrated faced joints perform satisfactorily if compressed initially at 12,000 psi (850 kg/cm²) to 18,000 psi (1260 kg/cm²). Metal gaskets such as copper, Monel, and soft iron should be given initial compressions considerably in excess of their yield strengths.Unit pressures of 30,000 psi (2100 kg/cm²2) to 60,000 psi (4200 kg/cm²) have been used successfully with metal gaskets. Various forms of corrugated and serrated metal gaskets are available which enable high unit compression to be obtained without excessive bolt loads. These are designed to provide a contact area that will flow under initial compression of the bolts so as to make an initially pressure-tight joint, but at the same time the compressive stresses in the body of the gasket are sufficiently low as to be comparable to the long-time load-carrying ability of the bolting and flange material at high temperatures.

The residual compression on the gasket necessary to prevent leakage depends on how effective the initial compression has been in forming intimate contact with the flange joint faces. Tests show that a residual compression on the gasket of only 1 to 2 times the internal pressure, with the pressure acting, may be sufficient to prevent leakage where the joint is not subjected to bending or to large and rapid

temperature changes. Since joints in piping customarily must withstand both these disturbing influences, minimum residual gasket compressions of 4 to 6 times the working pressure should be provided for in the design of pipe joints.

Gasket Standards

There are a variety of standards that govern dimensions, tolerances, and fabrication

of gaskets. The more common international standards are

ASME B16.20-1997                           Metallic Gaskets for Pipe Flanges, Ring-

                                                            Joint, Spiral Wound and Jacketed

ASME B16.21-1990                            Nonmetallic Flat Gaskets for Pipe

                                                            Flanges

BS 4865 Part 1                                   Flat Ring Gaskets to Suit BS4504 and

                                                            DIN Flange

BS 3381                                              Spiral Wound Gaskets to Suit BS 1560

                                                            Flanges

API 6A                                               Specification for Wellhead and Christmas

                                                          Tree Equipment

Types of Gaskets

Gaskets can be defined into three main categories: nonmetallic, semimetallic, and

metallic types.

Nonmetallic Gaskets. 

Usually composite sheet materials are used with flat-face flanges and low pressure class applications. Nonmetallic gaskets are manufactured with nonasbestos material or compressed asbestos fiber (CAF). Nonasbestos types include arimid fiber, glass fiber, elastomer, Teflon (PTFE), and flexible graphite gaskets. Full-face gasket types are suitable for use with flat-face (FF) flanges. Flatring gasket types are suitable for use with raised faced (RF) flanges.

Semimetallic Gaskets. 

Semimetallic gaskets are composites of metal and nonmetallic materials. The metal is intended to offer strength and resiliency, while the nonmetallic portion of a gasket provides conformability and sealability. Commonly used semimetallic gaskets are spiral wound, metal jacketed, camprofile, and a variety of metal-reinforced graphite gaskets. Semimetallic gaskets are designed for the widest range of operating conditions of temperature and pressure. Semimetallic gaskets are used on raised face, male-and-female, and tongue-and-groove flanges.

      1-  Spiral Wound Gaskets. Spiral wound gaskets are the most common gaskets used on raised face flanges. They are used in all pressure classes from Class 150 to Class 2500. The part of the gasket that creates the seal between the flange faces is the spiral wound section. It is manufactured by winding a preformed metal strip and a soft filler material around a metal mandrel. The inside and outside diametersare reinforced by several additional metal windings with no filler.

For applications involving raised face flanges, the spiral wound gasket is supplied with an outer ring; for critical applications it is supplied with both outer and inner rings. The outer ring provides the centering capability of the gasket as well as the blow-out resistance of the windings and acts as a compression stop. The inner ring provides additional load-bearing capability from high-bolt loading. This is particularly advantageous in high-pressure applications. The inner ring also acts as a barrier to the internal fluids and provides resistance against buckling of the windings.

Spiral wound–ring gaskets are also used in tongue-and-groove flanges. Inner rings should be used with spiral wound gaskets on male-and-female flanges, such as those found in heat-exchanger, shell, channel, and cover-flange joints.

Marking Spiral Wound Gaskets

2-  Camprofile Gaskets. Camprofile gaskets are made from a solid serrated metal core faced on each side with a soft nonmetallic material. The term camprofile (or kammprofile) comes from the groove profile found on each face of the metal core. Two profiles are commonly used: the DIN 2697 profile

and the shallow profile. The shallow profile is similar to the DIN 2697 profile except that the groove depth is 0.5 mm (versus 0.75 mm for DIN 2697). This allows for a cost advantage for the shallow profile. The profile can be made from sheet metal or strip with a thickness of 3 mm instead of a thickness of 4 mm for DIN profile.For the original German Standard see 

The most common facing for camprofile gaskets is flexible graphite. Other facings such as expanded or sintered PTFE and CAF are also used. The camprofile gasket combines the strength, blowout, and creep resistance of a metal core with a soft sealing material that conforms to the flange faces providing a seal. Standard camprofile gaskets are available to suit ASME B16.5, BS1560, and DIN 2697. 

3- Jacketed Gaskets. Jacketed gaskets are made from a nonmetallic gasket material enveloped in a metallic sheath. This inexpensive gasket arrangement is used occasionally on standard flange assemblies, valves, and pumps. Jacketed gaskets are easily fabricated in a variety of sizes and shapes and are an inexpensive gasket for heat exchangers, shell, channel, and cover flange joints. Their metal seal makes them unforgiving to irregular flange finishes and cyclic operating conditions. Jacketed gaskets come in a variety of metal envelope styles. The most common style is double jacketed, 

Metallic Gaskets. Metallic gaskets are fabricated from one or a combination of metals to the desired shape and size. Common metallic gaskets are ring-joint gaskets and lens rings. They are suitable for high-temperature and pressure applications and require high-bolt loads to seal.

1- Ring-Joint Gaskets. Standard ringjoint gaskets can be categorized into three groups: Style R, RX, and BX. They are manufactured to API 6A and ASME B16.20 standards. Style R gaskets are either oval or octagonal. Style RX is a pressure-energized adaptation of the standard Style R ring-joint gasket. The RX is designed to fit the same groove design as the Standard Style R. Style BX pressure-energized ring joints are designed for use on pressurized systems up to 20,000 psi (138 MPa).

Flange faces using BX-style gaskets will come in contact with each other when the gasket is correctly fitted and bolted up. The BX gasket incorporates a pressure- balance hole to ensure equalization 

2- Lens Rings Gaskets. Lens rings gaskets have a spherical surface and are suited for use with conical flange faces manufactured to DIN 2696. They are used in specialized high-pressure and high-temperature applications. 

Other specialty metallic seals are available, including welded-membrane gaskets and weld-ring gaskets. These gaskets come in pairs and are seal-welded to their mating flanges and to each other to provide a zero-leakage high-integrity seal.

GASKET SELECTION

The proper selection of gasket is critical to the success of achieving long-term leak tightness of flanged joints. Due to their widespread usage, gaskets are often taken for granted. Industry demands for reduced flange leakage in environments of increasing process temperatures and pressures have led gasket manufacturers to develop a wide variety of gasket types and materials, with new gaskets being introduced on an ongoing basis. This rapidly changing environment makes, and will continue to

make, gasket selection difficult. It is highly recommended that the gasket manufacturer be consulted on the proper selection of gaskets for each application. Gasket manufacturers are familiar with the industry codes and standards and conduct extensive testing of their products to ascertain performance under a variety of operating conditions.

Flange design details, service environment, and operating performance guide the gasket selection process. Start with the flange design. Identify the appropriate flange standard, outlining size, type, facing, pressure rating, and materials (i.e., ASMEB16.5, NPS 4, Class 1500, RF, carbon steel). Identify the service environment of temperature, pressure, and process fluid. It is useful to highlight gasket-operating performance.

Calculation method

The ASME code has been used for many years to design flanges, though it has a number of recognized flaws when it comes to determining a suitable bolt load with regard to gasket sealing. Calculations are performed to determine the greater of either operating or initial forces using the following formulae.

Initial load requirement:

Operating load requirement:

The factors m and y are the gasket factor and initial seating stress values respectively. One problem is that the code does not use the whole gasket contact area in the calculation. In the formulae above the ‘effective width’ is b and the effective diameter is G. The system pressure is P. For a common raised face flange the contact width of the gasket element, from the raised face outside diameter to the gasket inside diameter, is designated N. The basic width is then calculated as being half this value and called b0. The effective width will depend on the value of b0 being greater or less than 6.3 mm (1/4 inch), though in the majority of cases this is likely. If b0 is greater than 6.3 mm then the

effective width is calculated as

Or in cases where b0 is equal to or less than 6.3 mm then b= b0. The effective diameter G is simply the outside contact diameter less 2×b0.

Therefore the actual gasket contact area is usually far greater in reality than the figure calculated by this method

Note also that the gasket factor m is effectively a multiplier of the system pressure as an operating stress.  the actual sealing performance of a gasket is more realistically a three-dimensional exponential decay curve, rather than a single number.

 the operational stress defines the gasket sealability.There seems to be no reason to try to link initial gasket seating loads (i.e. for compression purposes to take up flange flatness, etc.) with the m factor which isrelated to the operating stress requirement for a given system pressure. However, when originally devised in the 1930s and 1940s the relationship existed between the factors of (2m 1)² ×180 = y (using units for y of p.s.i. and rounding of the m factor to the nearest 0.25).

Thus for compressed fibre gaskets, a lower value of y was determined for the thicker materials presumably because they would deform more readily to make a crude seal against whatever flange distortion existed. Thus, from the relationship above, the thicker materials also gained a lower m factor, suggesting that they would give better sealing performance. However, it has been shown that thicker materials not only have a greater tendency to stress relax, but also have a greater number of micro-porosity channels where leakages can occur. In gas sealability tests 3 mm thick compressed fibre jointing material tends to leak at approximately twice the rate of a 1.5 mm sample of the same material at a given operating stress.

Gasket-Operating Performance

New flange and gasket designs are incorporating tightness factors in their calculations to reduce leak rates. Traditional ASME Section VIII code utilizes m and y gasket factors in the design calculations of flanges. These factors are useful to establish the flange design required to help ensure the overall pressure integrity of the system; however, they are not useful parameters to predict flange leak rates.

All flanges leak to a certain degree. Industry requirements are demanding reduction in leak rates along with predictable performance. This has lead to a more rigorous approach to establishing gasket factors and the associated methods for gasketed flanged-joint design. Significant progress has been made in the last six years in Europe by CEN and in North America by ASME’s Pressure Vessel Research Council (PVRC) to establish gasket test procedures and the development of design constants that greatly improve the gasketed flanged-joint design. Maximum allowable leak rates have been established for various classes of equipment. EPA Fugitive Emissions basic limits are shown below.

Component                                           Allowable leakage level

Flange                                                     500      ppmv

Pump                                                      1,000    ppmv

Valve                                                       500       ppmv

Agitator                                                  10,000  ppmv

PVRC has established a new set of gasket factors, Gb, a, and Gs and a related tightness parameter, Tp, which can be used in place of traditional m and y factors in determining required bolt load.

Gb and a (Part Atesting) represent the initial gasket-compression characteristics. 

Gb is the gasket stress at a tightness parameter (Tp) of 1; a is the slope of the line of gasket stress versus tightness parameter plotted on a log-log curve. This line shows that the tightness parameter (or leak tightness) increases with increasing gasket stress. That is, the higher the gasket stress, the lower the expected leakage. Gs is the unloading (Part B) gasket stress at a Tp 1.

A low value of Gb indicates that the gasket requires low levels of gasket stress for initial seating. Low values of Gs indicate that the gasket requires lower stresses to maintain tightness during operation and can tolerate higher levels of unloading, which maintain sealability. An idealized tightness curve showing the basis for gasket constants Gb, a, and Gs is shown in Fig.down

The data for many gasket styles and materials have been published in various PVRC-sponsored publications. Typical PVRC gasket factors for a variety of gasket  types are shown in Table down

PVRC Convenient Method. The PVRC Convenient Method provides an easy conservative method for determining bolt load (Wmo) used in flange and gasket design as an alternate to using m and y values.

Gasket operating stress

Seating stress

Design factor

Design bolt load

where 

a—The slope associated with  tightness data

 Ag—Area of gasket-seating surface, in² (mm²) = .7854(OD² - ID²)

 Ai—Hydrostatic area; the area against which the internal pressure is acting,

                in² (mm²) =.7854G²

  bo—Basic gasket seating width, in (mm)

                  bo = (OD- ID)/4

  b—Effective gasket seating width

               b =bo, when bo ≤ 1/4 in

                when bo > 1/4 in (mm)

 C—Tightness constant

          C 0.1 for tightness class T1 (economy)

          C 1.0 for tightness class T2 (standard)

          C 10.0 for tightness class T3 (tight)

e—Joint assembly efficiency; recognizes that gasket-operating stress is improved depending on the actual gasket stress achieved during boltup; also recognizes the reliability of more sophisticated bolting methods and equipment in actually achieving desired bolt loads

           e 0.75 for manual boltup

          e 1.0 for ‘‘ideal’’ boltup, e.g., hydraulic stud tensioners, ultrasonics

G—Diameter of location of gasket load reaction, in (mm), 

Gb—The stress intercept at Tp 1, associated with  tightness data psi (MPa)

Gs—The stress intercept at Tp 1, associated with  tightness data psi (MPa)

Pd—Design pressure, psi (MPa)

Pt—Test pressure (generally 1.5 Pd), psi (MPa)

Sm1—Operating gasket stress, psi (MPa)

Sm2—Seating gasket stress, psi (MPa)

Mo—Design factor

TC—Tightness class that is acceptable for the application, depending on the severity of the consequences of a leaker

      T1 (economy) represents a mass leak rate per unit diameter of 0.2 mg/ sec-mm

      T2 (standard) represents a mass leak rate per unit diameter of 0.002 mg/ sec-mm

      T3 (tight) represents a mass leak rate per unit diameter of 0.0002 mg/ sec-mm

Tp—Tightness parameter. Tp is a dimensionless parameter used to relate the performance of gaskets with various fluids, based on mass leak rate. Recognizes that leakage is proportional to gasket diameter (leak rate per unit diameter). Tp is the pressure (in atmospheres) required to cause a helium leak rate of 1 mg/sec for a 150 mm OD gasket in a joint. PVRC researchers have related Tp to other fluids through actual testing as wellas use of laminar flow theory.

Tpa—Assembly tightness; the tightness actually achieved at assembly

      = 0.1243 × C × Pt

Tpmin—Minimum tightness; the minimum acceptable tightness for a particularapplication 

       =0 .1243 × C× Pd

Tr—Tightness ratio; log (Tpa)/log (Tpmin)

Wmo—Design bolt load, lb (kN)

Critical Considerations

There are some critical considerations when using the m and y constants in these equations. First, these equations were originally derived to assist in the design of flanged joints. They do not specifically address joint tightness. They are often used to help determine minimum required bolt loads for assembly purposes. They currently do not take into account potential joint relaxation due to temperature effects, torque scatter and the inherent inaccuracies involved in assembly. For assembly purposes, they are more of an indication of minimum load required, and may not correspond to a bolt load required to achieve a certain tightness level under a given set of operating conditions.

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