Gaskets can be segregated into three (3) main gasket categories:
- Non-metallic (soft)
It should be noted that gaskets may be categorized differently in other documents i.e. ASME categorizes gaskets into two basic groups, metallic (i.e. ASME B16.20 “Metallic Gaskets for Pipe Flanges which includes spiral wound gaskets”) and non-metallic.
The mechanical characteristics, performance and capabilities of these gasket categories will vary extensively, depending on the type of gasket selected and the materials from which it is manufactured. Obviously, mechanical properties are an important factor when considering gasket design, but the selection of a gasket is also influenced by:
- temperature and pressure of the fluid to be contained
- chemical nature of the fluid, compatibility with the operating fluid
- mechanical loading affecting the gasket
- variations of operating conditions ( i.e. during cycling)
- type of joint involved
This Handbook is focused on maintenance engineers and fitters and it is assumed that, in general, the selection of gasket type and material(s) will be decided by the plant operator or designer in the first instance. Hence, the maintenance operator may have little flexibility to choose the sealing material. Consequently, this section provides only brief guidance notes about the majority of sealing materials and gasket types available.
Each gasket category has an associated set of materials to consider which is dependent upon the specific gasket style and operating conditions. In the following sections, is a brief review of considerations to be aware of, what materials are required when selecting a specific gasket style from a given category and is a summary of each category including typical application parameters where each is utilized, gasket characteristics, and features.
• A word of caution; despite the similarity of many materials, the properties of the seal and performance achieved will vary from one manufacturer to another. Always consult the manufacturer for detailed guidance on specific products.
• It is important to always use a good quality gasket from a reputable supplier, because the cost of a gasket is insignificant when compared to the cost of downtime or safety considerations.
A. Non-metallic (soft) Gaskets
Non-metallic materials are suitable for a wide range of general and corrosive chemical applications. These gaskets are suitable for low and high temperature applications depending on materials. Their use is generally limited from low to medium fluid pressure applications. Non-metallic gaskets are also typically the least expensive of the three (3) gasket categories, however, specialty materials used in them can be an exception. Types include: compressed fiber materials (“CNF”), flexible graphite, polytetrafluoroethylene (PTFE), and mineral based (i.e. vermiculite, mica).
Non-metallic (soft) gaskets are available as “ring type” (ID/OD gaskets), full face (ID/OD with bolt holes) and can be supplied in custom shapes for specialty flanges (square, rectangular, etc…with and without bolt holes). Technology today has given rise to various automated means of cutting non-metallic (soft) gasket material such as automated knife cutters, laser cutters and punches in just about any shape required. Typical material sheet width is sixty inches (60”), while the sheet length varies by manufacturer. It is not uncommon for non-metallic (soft) gaskets to be segmented for sizes larger than available sheet dimensions.
There are typical tests to help define and compare characteristics of non-metallic (soft) gasket materials. This may be found listed on data sheets from the manufacturers for the product, to help guide in selection of material(s) for a particular application. These standardized tests are defined by ASTM International (United States), CEN (European), and DIN (German), BSI (British), AFNOR (French), JIS (Japan), ISO (Global), among others and are summarized in Appendix A.
Another consideration when selecting soft gasket material(s) is the gasket thickness. Typically nominal non-metallic (soft) gasket thicknesses are, in North America, 1/32″, 1/16″, 1/8″ and, in Europe and Asia, 0.75mm, 1.0mm, 1.5mm, 2.0mm, and 3.0mm. Thicker and thinner gaskets are also available. Contact the manufacturer for specific tolerances on their gasket thickness and thickness variations within a sheet. It should be noted that thickness can affect the pressure/temperature rating of a gasket (refer to Chapter 3 Section A). Gasket thickness should also be noted by the end user when reviewing standardized test data and what particular material thickness was utilized to perform the test compared to the thickness being considered for an application.
For gaskets cut from sheets, it is recommended to use the thinnest material that the flange arrangement will allow. But thick enough to compensate for unevenness of flange surfaces, their parallelism, surface finish, rigidity, etc. The thinner the gasket, the higher the bolt load the gasket can withstand and the less loss of bolt stress, due to relaxation. Also, the thinner the gasket material, the lower the gasket area which will be exposed to attack from the internal pressure and aggressive fluid.
Non-metallic (soft) gaskets are typically either homogeneous (i.e. flexible graphite sheet, virgin PTFE) or they are a composite of several materials each serving a specific purpose (i.e. CNF, mineral based, filled PTFE). Of course, for each material within a gasket, consideration has to be given in terms of compatibility with fluid and temperature.
Generally, a composite gasket has the following main components:
Fiber – added for increased mechanical properties such as
tensile and compression (i.e. aramid, cellulose, ceramic, glass)
- Binder – added to increase flexibility and act as a binding agent for the other materials utilized (i.e. NBR, SBR, EPDM)
- Filler – added for various reasons such as reducing cold flow, creep and cost reduction (i.e. silica, clay, mica, powdered graphite, barium sulfate)
- Coatings – added to both faces to facilitate easy release of gasket from flange face. (i.e. PTFE, silicone)
Current non-metallic (soft) gaskets encompass a broad spectrum of materials with a wide range of physical properties, which are suitable for various temperature and pressure ranges. New materials continue to appear in the market, as do variations of conventional products. The total number of materials on the market is extensive and arduous to list. A practical approach is to comment generally on materials commonly in use, which for the most part offer the gasket user a complete enough range to make a proper selection. The user should consult the manufacturer’s literature for proper material selection.
ASTM F104 provides one framework for characterizing gasket material properties. The F104 call-out is an alphanumeric sequence which defines specific properties of the material. An end user may use a call-out to specify a gasket material for a particular application. The call-out may not be comprehensive, therefore, for critical applications it is recommended that further investigation of the material properties and suitability take place.
Please note that in the listings which follow, operating limits are indicative only. Many of the gasket materials are composites, containing a variety of binders, fillers, etc., the inclusion of which will modify the performance envelope of the gasket. Operating limits and suitability may vary significantly, dependent upon material constituents and specific operating conditions; under these circumstances, the advice of the gasket manufacturer is vital! Always consult the gasket manufacturer for guidance on suitability for specific applications and limits which may be achieved under specific operating conditions. Whichever gasket material or type is selected, ensure it is correct for the application.
1. Gasket Materials
Beater Addition Compressed Fiber Sheet Gasketing
The Beater Addition process incorporates a range of natural or synthetic fiber (i.e. cellulose, aramid, etc.) for strength. Additionally, binders (i.e. NBR, SBR, etc.) and resins are added for flexibility, strength, heat or chemical resistance. Gaskets manufactured per this method can also be laminated to support materials such as stainless steel for increased strength. Fluid compatibility and acceptable application temperature range will be a function of the material utilized and material thickness.
Compressed Fiber Sheet Gasketing
Compressed sheets have been around since the 1890’s. It is a composition of fibers, elastomers and fillers that is formed into sheets of finite dimensions with the process dictating maximum sheet width and maximum sheet length.
Fluid compatibility and acceptable application temperature range will be a function of the material utilized and material thickness. Different compressed sheets are available that can function over an extensive range of fluids, temperatures and pressures. End users should ensure they provide complete application service conditions to optimize in-service effectiveness and life.
Although there are many new materials available today, cork gaskets may continue to be used in some applications where minimal bolt load is available such as stamped metal flanges or easily damaged materials (i.e. glass or ceramics). Cork is impervious to water, lubricating oil and other petroleum derivatives.
Cork gaskets are primarily used for low internal pressures up to 345 kPa (50 psi), where sustained fluid temperature does not exceed 120°C (250°F). Cork has a tendency to stick to flanges and also has limited shelf life due to humidity. It should not be used to seal inorganic acids, alkalis or oxidizing solutions.
Flexible graphite refers to natural graphite flake that has been expanded and then compressed. It is a material with the essential characteristics of graphite and complementary properties of flexibility and resilience, as well as an ability to compress and conform.
A sheet density of 1.12 g/cm3 (70 lbs/ft3) is often typical of the processed material in the US, while a density of 1.0 g/cm3 (62.4 lbs./ft3) is typical in Europe and is widely used for the majority of industrial gasket applications. While this density is approximately fifty percent (50%) of the theoretical maximum density of graphite, the through-thickness sheet permeability to fluids, as measured by the helium admittance test, is extremely low. Characteristics of the flexible graphite can be tailored for specific gasket applications simply by changing the starting density of the sheet.
Effective sealability is inherent in the flexible graphite by virtue of its conformability to the flange surfaces under load and once in place because of its low creep relaxation and stability under a wide range of compressive load/temperature conditions. Since the tensile strength of flexible graphite is significantly lower than that of binder containing products, a reinforcement material is commonly employed to improve handleability of the flexible graphite for many applications. Gasket reinforcement can be either metallic or non-metallic material. There are many variables and options for consideration when selecting the gasket reinforcement and construction of flexible graphite gaskets.
Considerations for Reinforcements:
- Non-metallic materials, such as fiberglass cloth and polyester film, can be used to produce the laminate or composite gasket material. The benefits of these inserts are chemical resistance and ease of cutting.
- Metallic materials, such as stainless steels, carbon steels and other alloys, can be used to reinforce the laminate or composite gasket material. The insert material can be in the form of flat sheet (foil), woven wire screen, perforated foil or tang. Generally tang and wire screen inserts require higher compressive loads to achieve the same level of sealability as the unreinforced or flat metal laminates. The benefits of these inserts are increased handling robustness and the metallic reinforcing layers can be selected to address specific chemical resistance requirements for special applications.
Considerations for Construction:
- Flexible graphite gaskets can be composed of multiple layers of flexible graphite sheet and reinforcements, in order to achieve the desired thickness or application requirements.
- Adhesive is used to bond polyester to flexible graphite. The polyester film itself, is the non-metallic reinforcement. Joining the materials occurs by applying a double-sided adhesive polyester tape to the flexible graphite. This bonding technique provides enhanced tensile strength and toughness for handling robustness. Plastic layers can also be thermally bonded by heating the thermoplastic material to its softening point and adhering the softened material directly to the flexible graphite.
- Chemical Bonding is used to join metallic reinforcements to the flexible graphite. The use of thermally activated contact adhesives is an effective method of construction that minimizes the glue layer thickness. The benefit of this technique is improved gasket sealability under low gasket loads and sheet cutting ease, compared to gaskets and sheet constructed via tanging.
- Tanging is a process of perforating the metal reinforcement and creating tangs (hooks) in the metal. The finished laminate is then made by clinching the flexible graphite facing material onto the tanged metal. One benefit of this technique is high sheer strength between layers which provides enhanced blowout resistance. Another benefit is the absence of adhesive, which eliminates related concerns of creep relaxation and outgassing of the adhesive layer, chemical compatibility and process contamination.
- When using woven wire mesh the flexible graphite sheet is bonded to the mesh with adhesives.
Phyllosilicates (Mica & Vermiculite)
The main drawback with graphite gaskets is that they can oxidize at high temperatures/ when in the presence of oxygen or other oxidizing agents. Phyllosilicates are a group of minerals based on the mica family which can be used to make non-oxidizing, high temperature gasket materials like sheet, tanged metal reinforced, flat foil reinforced, perforated reinforced, spiral wound and kammprofile gaskets. The two (2) main phyllosilicates used are mica and vermiculite; both have the same temperature and chemical resistance properties. Vermiculite is different from mica, in that it can be exfoliated due to “water” being between the layers.
Polytetrafluoroethylene (PTFE) Gasketing
Polytetrafluoroethylene (PTFE) Gasketing is a material with unique chemical resistance and physical properties. PTFE is chemically inert to most chemicals, with the exception of molten alkali metals and free fluorine and can withstand a wide temperature range. PTFE also has excellent anti-stick, dielectric and impact resistance properties.
However, most PTFE gaskets are subject to cold flow or creep relaxation under compression, which means that gaskets lose thickness, expand in width and length under applied loads. Creep results in the loss of bolt load and ultimately gasket stress in an application. And frequently requires additional efforts to regain that lost stress in the field, which results in increased maintenance costs. Generally, initial seating stress decreases significantly in the first 4 to 24 hours after installation. Also creep increases as temperatures increase.
While the property of creep is inherent in this material, the degree of creep in the PTFE material differs significantly amongst the three (3) largest types of PTFE gaskets; virgin, filled, and expanded. There are other, less common, PTFE variations commercially available such as envelope, microcellular, and filled biaxial sheets.
Virgin PTFE gaskets are made from unaltered PTFE, which preserves the full chemical compatibility of PTFE but has the inherent high cold flow characteristic of PTFE. Virgin PTFE provides the basis for improvement of creep relaxation for other PTFE variations. As a result, this product is generally used in low temperature non-critical applications.
With the addition of a filler material to virgin PTFE, filled PTFE gaskets reduce the cold flow; thereby improving the creep relaxation performance. The fillers act as “speed bumps” slowing the cold flow. However, the addition of a filler reduces the range of media for which the gasket may be suitable. For example, the addition of Silica or Barium Sulfate filler would limit the use of the gasket to either acid or caustic service. Multiple fillers are available to ensure the product is chemical compatible for a particular application.
The most common filler materials for PTFE gaskets are:
Expanded PTFE (ePTFE)
Cold flow is most effectively reduced in expanded PTFE gaskets (ePTFE) by physically altering the PTFE microstructure. Because no fillers are used, the complete chemical compatibility of the virgin PTFE is retained.
The ePTFE microstructure is a network of interconnected nodes and fibrils that increases the tensile strength of the material, decreases the density of the material and increases the compressibility of the material. Because of the high compressibility, ePTFE gaskets conform well to surface irregularities. Unlike virgin and filled PTFE gaskets, ePTFE materials are more easily applied in glass-lined steel, fragile plastic flanges and other fragile flanges. In addition, ePTFE is available in sheet, tape or cord (joint sealant) form. The tape and cord forms are solid form-in-place products.
Because of rubber’s many beneficial features and advantages as a sealing material, it has become a primary material used in the production of gaskets.
Today, a wide variety of rubber polymers and compounds, each possessing unique features and advantages, have been developed to produce the highest quality gaskets and sealing materials for many industries.
Basic Features of Rubber:
There are seven (7) basic features of rubber which establish its advantages as an ideal gasket and sealing material.
- Rubber is a naturally resilient material. It is elastic and squeezes into joint imperfections under relatively light bolt loading. As such, it provides effective sealing properties even under difficult conditions.
- The availability of various rubber polymers provides a wide range of physical properties:
• Tensile strength
• Compressive Modulus
• Compression Set
- A variety of desired properties can be combined into a single compound to meet specific application needs.
- Rubber can be reinforced with fabric or steel inserts to add strength and prevent creep, rupture or blowout.
- Rubber can be compounded to resist the effects of temperature, oil, chemicals, ozone, weathering, aging and abrasion. The result is longer gasket life and reduced maintenance.
- Rubber sheeting can be produced in an infinite variety of thicknesses, widths, lengths, surface finishes and colors to meet user needs and requirements.
- Rubber can be specially formulated to meet specific requirements. For example, some Natural Rubber compounds are Food & Drug Administration (FDA) approved, using only ingredients generally recognized as safe and listed in the FDA Federal Register for Food Handling Materials.
Rubber gasket and sealing material applications involve all types of gases, liquids and solids. In sealing or separating these materials, a wide range of service conditions, such as temperature and chemical exposure may be encountered. It is, therefore, essential that all factors carefully be considered to ensure selection of the grade and type of rubber gasket or sealing material, which will deliver optimum performance as well as economy.
As a result of rubber’s excellent features and advantages, the actual uses of rubber sealing devices can range dramatically from very general, non-critical applications, such as plumbing; to very demanding and critical high technology service, as encountered in the aircraft industry. Rubber gaskets and sealing materials provide equally optimum results.
Woven and Folded Impregnated Cloth
Fabrics such as glass fiber fabrics are woven in such a manner as to allow sufficient impregnation of elastomers or PTFE, whereby all voids are filled and fibers are properly coated to prevent or minimize wicking.
Fabrics measure 0.8mm (1/32″) or 1.5mm (1/16″) in thickness. Thickness over 1.5mm (1/16″) are obtained by plying additional layers.
Where additional reinforcement is required, wire strands of brass, high nickel alloys and occasionally, stainless steel are used. Such fabrics are able to withstand higher pressures and temperatures. The wires are tightly interwoven into the yarns with no looping of the wire observed. A uniform coating of the impregnating material is necessary for optimizing properties in the final product.
Folded Cloth Gaskets
Texturized or blended fiberglass cloth gaskets are folded to form endless seals, used for hand hole and manhole gaskets. They can be round, oval, square or rectangular and may also be die cut into various configurations.
Tape is formed by slitting and folding cloth. It can be made in widths of 6mm (1/4″) to 254mm (10″) and in thicknesses of 1.5mm (1/16″) to 13mm (1/2″).
Groove or Door Packing
Any of the rubberized fabrics can be rolled or folded to form round, square or rectangular cross sections.
This tapes consists of fabrics wrapped around various core materials which include:
- Glass or ceramic rope
- Rubber cords – silicone, chlorosulfunated polyethylene (CSM), Chlororoprene (CR)
- Wire mesh cables – high nickel alloys, stainless steel
The selection of core and cover materials is based on heat resistance, pressure, environment and compressibility. The cloth extends beyond the core to form a flat lip. This lip can be punched or drilled to accept bolts and the extension or “tail” is then bolted to the mating surface. This allows the bulb portion to be compressed and provides a good seal for irregular surfaces that must be subjected to opening and closing.
Non-impregnated cloth with various cores may also be used and the tadpole shape is formed by using a high temperature thread.
A popular product in less severe service, is the liquid form-in-place (F.I.P.) gasket made of silicone, polyester-urethane or other polymer based substance. On assembly of the flange a ribbon of paste-like polymer is applied to the flange area. Then the F.I.P. material is compressed between the mating surfaces to conform to all the surface irregularities.
Application of the liquid F.I.P. gasket can be done manually from a tube, caulking gun or automatically using a high speed programmed machine which is commonly used in automotive assembly plants.
Gaskets of flexible graphite tape; PTFE rope; folded cloth or tape, can be formed-in- place to create a gasket. It is an ideal do-it-yourself gasket material for easy field installation.
The solid F.I.P. is furnished on a roll with adhesive backing in various sizes. The material is rolled out onto the flange mating surfaces, cut off, overlapped and compressed between the flanges.
B. Semi-Metallic Gaskets
Semi–metallic gaskets are made by combining soft materials such as fillers, facings or insertions together with a metallic component to optimize the characteristics of the composite material as a gasket. In a composite or semi-metallic gasket, the metal generally provides the strength structural requirements and, depending upon configuration, increased resilience. Non-metallic material may also provide resilience and enhanced sealing characteristics. Many synthetic composites and/or mineral-based materials are used as well as elastomer based compounds. These soft materials may be inserted in a specifically designed metallic profile, may be applied to a metallic face or carrier, or may be partially or completely encapsulated by metal. Semi-metallic gaskets can be suitable for both low and high temperature and pressure applications, depending on the materials and configuration used. Types include: grooved metal gaskets with covering layers, metal eyelet, metal jacketed, metal reinforced non-metallic (soft) gaskets (including tanged metal or wire reinforced “sheet” materials), corrugated metallic and spiral wound gaskets among others. Each gasket style has a particular set of materials which must be defined based upon the components making up the gasket. The following is a brief description of the more popular semi-metallic styles and the components involved and considerations when choosing a material for each component.
1. Gasket Styles
Metal Reinforced Non-metallic Gaskets
Typically made with a thin stainless, alloy core or wire mesh approximately .002″/.004″ thick, it acts as a carrier and support for the soft sealing material. The material used is typically 316L stainless, however, nickel and other materials are also utilized.
A=Metal Insert; B=Non-metallic sheet material
Concerning the .002″ /.004″ thick cores, they can be either flat metal, perforated or tanged cores. If the core is flat metal, the soft sealing material is usually adhesively attached to the core, while if the core is perforated or tanged, the soft sealing material is mechanically pressed and clinched onto the core. Soft sealing materials such as flexible graphite, PTFE and mineral-based materials (i.e. vermiculite) are often used. Both the soft sealing material and metallic core material should be considered when looking at operating temperature, pressure and fluid.
These gaskets consist of a thin metal that is corrugated or embossed with concentric rings and faced with a soft material such as flexible graphite.
A=Metal Corrugations; B=Soft Facing Material
Corrugated gaskets utilize the substrate’s geometry to achieve conformability to flange irregularities and promote recovery over the life of the seal, they are essentially a line contact seal. Multiple concentric corrugations provide a labyrinth effect, along with mechanical support for the facing material.
Corrugations provide resilience, depending on pitch and depth as well as the type and thickness of the metal used. Again, both the thin metal and the facing material must be considered and be suitable for the fluid and operating conditions.
Jacketed gaskets consist of a soft compressible filler, partially or wholly encased in a metal jacket. In some instances, corrugated metal is used in place of soft filler material and also may have a soft surface layer of material such as flexible graphite. The primary seal against leakage is the inner metal overlap, where the density of the gasket is the greatest when compressed.
A=Metal Jacket (Outer Layer); B= Filler Material
This area cold flows it creates a seal. The entire outer lap, if any, provides a secondary seal between the flanges when compressed. Any intermediate corrugations, if they exist, may contribute to the labyrinth effect. These gaskets are used for circular as well as non- circular applications and for applications at temperatures up to those which limit the filler and metal endurance. These gaskets are normally specified in thicknesses of 2mm (3/32”) or 3mm (1/8”) nominal. The thickness of jacketed gaskets cannot be held to as precise dimensions as non-metallic (soft) gaskets, due to accumulated tolerances of the metal, filler and the metal spring back when it is formed. Because of limited resilience, they should not be used in joints requiring close maintenance of the compressed thickness. They can be made with or without pass bars for use in heat exchangers. If pass bars are utilized, the pass bars can be integral (not welded but formed from the same piece of material as the outer ring) or welded (pass bars are separate pieces and welded into the ID of the outer ring). Materials which must be considered are the metal jacket, material used and the filler material.
Grooved Metal Gaskets with Covering Layers (Kammprofile/Camprofile)
Kammprofile gaskets are a solid metal ring with grooved faces and a soft facing material is usually present on the grooved faces to improve sealability. Typical facing materials are flexible graphite, phyllosilicates (mica and vermiculite), or PTFE.
A=Metal Core; B=Soft Facing Material; C=Metal Outer Ring
When the gasket is compressed the serrated faces create concentric rings of high stress, enhancing the sealing capabilities of the gasket. Typical configurations include a grooved sealing section or core with soft facing material, a serrated sealing section or core with an independent outer ring (outer ring made with separate piece of material) and a serrated sealing section or core with an integral outer ring. The function of the outer ring is to locate or center the sealing core onto the sealing face (i.e. flange raised face) utilizing the bolts in much the same way an outer ring on a spiral wound gasket would on a raised face flange. However, the outer ring on a kammprofile is not typically used as a torque stop and is thinner than the serrated sealing section of the gasket to ensure the bolt load is concentrated on the sealing section or core and soft facing material. Materials which must be considered are the metal core and soft facing materials used.
Spiral Wound Gaskets
These gaskets are comprised of a preformed “V” or chevron shaped metal strips alternately wound with a conformable filler material. The metal windings provide strength and resilience, while the non-metallic filler portion conforms to the irregularities of the flanges aiding in the joint seal. These gaskets can be constructed in a variety of densities accommodating available bolting and pressure conditions.
Sealing is achieved through a combination of yielding and flowing of the “V” shaped metal material and conformable fillers during the compression phase. They can be made in several configurations, to accommodate various flange facing shapes. This gasket can be made with windings to include a solid metal outer ring, solid metal inner ring or both.
A=Outer Ring; B=Filler Material; C=Metal “V” Windings; D=Inner Ring
The solid metal inner and outer rings serve various functions. The outer ring serves to center the windings onto the sealing face, utilizing the inner edge of the bolts. The inner ring acts to help support the ID of the windings/ to help prevent inward buckling of the windings. Both inner and outer rings are typically made of a thickness that is within the optimal compression thickness range of the windings. Materials which need to be considered for the manufacture of such gaskets includes the inner ring metal, metal winding strip, filler material and outer ring. If an inner ring is present, it should be compatible with the fluid being contained and be able to withstand the temperatures encountered in the application. The metal winding strip has the same considerations as the inner ring, and is often made utilizing the same material type. The filler material utilized should be compatible with the fluid and again, be suitable for the temperatures to be encountered in the application. The outer ring should also be suitable for the temperatures encountered, but its material selection is less critical than the winding strip and inner ring material, as it is not exposed to the process fluid.
An envelope type gasket is a composite consisting of two parts; envelope (shield) and insert (filler). These gaskets are primarily used in conjunction with corrosive resistant equipment constructed of stoneware, glass, glass-lined metal, etc. The envelope serves as the corrosion resistant part of the gasket and is usually PTFE.
A=Slit Type; B=Folded Type; C= Machined/Square Type
There are basically three (3) designs of envelopes which are “Slit”, “Machined” and the “Square” cut (inside diameter, flat) or “Folded” type (inside diameter, round). The “Slit” type is most commonly used with inside diameters of 24″ and under. The “Machined” type is used where close tolerances, narrow flange widths or a reduction in the dead space is desirable. The “Folded” shield gasket is used on larger than 24″ I.D. gaskets. “Slit” and “Machined” gaskets are lathe cut from billets or sleeves; the “Folded” shield is made from tape, which produces a continuous jacket or shield. The insert material may be selected for a particular environment or designed to cover a wide variety of conditions. It provides the gasket with good compressibility and recovery to allow the minimum seating stress and proper conformability that is required for this type of service. The insert may be made of any recognized gasket material, with or without metal reinforcing, taking into consideration temperature, pressure and corrosive conditions. The most popular insert materials are compressed and beater add products. Elastomeric materials have a tendency to flow and cause envelope splitting; whereas extremely hard materials require excessive bolt loading.
2. Filler and Facing Materials
The following is a listing of typical materials that can be used in semi-metallic gaskets, either as facing (jacketed gaskets, kammprofile gaskets, corrugated gaskets) or filler materials (jacketed gaskets, spiral wound gaskets). A listing of metals commonly used in the construction of semi-metallic gaskets is covered at the end of the discussion concerning metallic gaskets, as they are typically available for both metallic and semi- metallic gaskets.
This material has excellent chemical resistance and very low creep relaxation. The recommended operating temperature is from cryogenic to a maximum in oxidizing conditions of 454°C (850°F) for facing material and spiral wound gaskets. These limits are dependent on the application and grade of flexible graphite used. In some cases, the maximum service temperature may exceed these limits. Avoid use with strong oxidizing fluids such as concentrated sulfuric acid. Contact the manufacturer for specific applications.
A phyllosilicate (specifically, chlorite), graphite and cellulose-based paper with a rubber latex binder, has been used as an asbestos substitute. When subjected to temperatures over 230°C (450°F), this product starts to lose volume, which has an adverse effect on performance.
PTFE, Filled PTFE and Expanded PTFE
Typically used in tape form, these materials are used for high chemical resistance. The temperature limits are cryogenics to 260° (500°F). Maximum temperature shown does not account for all operating conditions.
Aluminum oxide materials, commonly referred to as ceramic, are used in some corrosive environments. There is a wide variety of compositions that can be classified as ceramic materials.
Phyllosilicates are a group of minerals based on the mica family which can be used to make non-oxidizing, high temperature gasket materials. Sheet, spiral wound and kammprofile gaskets can be made from phyllosilicates. The two main phyllosilicates used are mica and vermiculite.
C. Metallic Gaskets
Metallic gaskets can be fabricated from a single metal or a combination of metallic materials, in a variety of shapes and sizes. Metallic gaskets are suitable for high temperature and pressure applications. Higher loads are required to seat the gaskets. Types include flat, grooved, round cross-section solid metal, lens rings, ring type joints (RTJ’s) and welded gaskets. A brief description of the more popular styles are described below.
1. Metallic Gasket Styles
Flat Metal Gaskets
These are defined as gaskets that are relatively thin compared to their width. They are cut from sheet metal and typically have a reduced area to increase unit load and improve sealability. Plain metal, washer shaped gaskets are relatively inexpensive to produce and can perform satisfactorily in simple applications. Surface finish on the gasket and flange facing is critical.
Serrated or Grooved Flat Metal Gaskets
This is a flat metal gasket with concentric serrations or grooves, which reduce surface contact area between the gasket and flange face. Thus creating concentric rings of high stress when loaded Round Cross-Section Solid Metal Gaskets.
These gaskets are generally made from round wire of the desired diameter cut to the length of the gasket mean circumference, then formed into a circle and welded. They provide positive, gas tight seals at relatively low flange pressures. Since only line contact occurs, they have high local seating stress at low bolt loads. The contact faces increase in width as the gasket is compressed, effectively flowing into the flange faces. Round solid gaskets are used on equipment designed specifically for them. Flanges are usually grooved or otherwise faced to accurately locate the gasket during assembly. However, there are some applications in which they are used between flat faces.
Corrugated Metal (No Filler of Facing)
Corrugated metal gaskets are plain metal with concentric corrugations. For low pressure
34.5 bar (500 psi) applications such as valve bonnets, gas turbines and combustion lines.
Solid Metal-Heavy Cross Section Gaskets
These gaskets are machined from solid metal and are designed for high pressure, high temperature service where conditions require a special joint design. Solid metal-heavy cross-section gaskets may seal by initial line contact or wedging and coining action, causing high unit stresses and the metal surface to flow at this line contact. Surface finish and dimensional accuracy is critical on both elements. Another type of solid metal-heavy cross-section gaskets includes welded gaskets (i.e. weld rings and weld membrane gaskets). The more common solid metal-heavy cross-section gaskets are described below:
Ring Type Joints (RTJ)
• Style R, Oval and Octagonal
The oval cross-section is the original ring joint design.
A=Octagonal Style; B=Oval Style
The octagonal cross-section is an evolution of the oval design. Both oval and octagonal rings can be used with flanges having the standard ring joint flat bottom groove.
The former round bottom groove is no longer shown in the flange specifications and can only be used with an oval gasket. Standard sizes of these gaskets are manufactured to ASME B26.20 and API 6A specifications.
The RX style ring joint has a unique self-sealing action. The outside bevels of the ring make the initial contact with the groove as the flanges are brought together with the flange bolting.
This provides initial sealing of the joint with the gasket seating against the groove surfaces. During pressurization the gasket loading increases against the groove. Style RX ring joint gaskets as specified in ASME B16.20 and API 6A are completely interchangeable with the oval and octagonal series of identical reference numbers and are used in the same flange grooves.
The BX style ring is designed to specifications shown in ASME B126.20 or API 6A, for use with grooved flanges on special applications involving high pressures from 344 bar (5,000 psi) to 1,034 bar (15,000 psi).
The pitch diameter of the ring is slightly larger than the pitch diameter of the groove, thus initial contact is made on the outside of the ring, pre-loading the gasket and creating a pressure-energized seal. Connections utilizing Style BX have a limited amount of positive interference, ensuring that the gasket will be “coined” in the flange grooves. Style BX ring joint gaskets can only be used with API BX flanges and are not interchangeable with the Style RX series.
• Style SRX/SBX for Subsea Type
Style SRX and SBX ring joint gaskets are made per API 17D for subsea wellhead and tree equipment. These ring joint gaskets with “S” suffix designations, indicate that these gaskets have cross-drilled holes which connects fluid volume located between the flange joint groove, the ring joint gasket and the bore or ID. This hole prevents fluid located between the joint groove and the ring joint gasket from interfering with proper seating of the gasket. During installation, the gasket is compressed into the flange groove and fluid is allowed to vent into the bore or ID.
A=SRX Style; B=SBX Style
The SRX and SBX gaskets have identical overall measurements to the RX and
BX ring joint gaskets with the same number designation. These additional vent holes are typically installed in one of two different patterns.
NOTE: The use of vent holes can also reduce the possibility of trapping pressure between one side of the ring joint gasket and the groove, creating a potentially dangerous situation during disassembly.
These are for high temperature, high pressure applications on pipework, valves and pressure vessels.
Lens rings have two (2) spherical faces and are used between flanges with straight tapered twenty degree (20°) faces. Providing a line contact seal approximately one-third across the gasket face, the specially designed cross-section affects a pressure-energized seal.
This pressure-activated design is used for pressure vessel and valve bonnet gaskets, at pressures 103 bar (1500 psi) and higher.
This design has also been adapted to pipe joints which are subject to extreme thermal shock conditions.
The pressure-activated Delta cross-section is a pressure vessel or valve bonnet gasket, useful for pressure ranges of 344 bar (5000 psi) and higher.
Welded Gaskets have originally come from the German industrial market. There are two
(2) typical variations; Weld Membrane Gaskets, in accordance with DIN 2695 and Weld Ring Gaskets. Welded gaskets are typically used where a welded joint is required and limited disassembly will occur. Typically, Welded Gaskets can be reused/rewelded up to approximately five (5) times. It is important to note that when utilizing Welded Gaskets in bolted flange joints, the bolting is still relied upon to carry the hydrostatic end loads on the joint. Also, Welded Gaskets are sometimes utilized in conjunction with machined face profiles to accept installation of other auxiliary gasket types, such as spiral wound or kammprofile gaskets. In these cases, the Welded Gasket may be utilized and welded on the OD in the event of failure of the auxiliary gasket. Or the auxiliary gasket may be utilized to perform hydro tests or other functions, thereby not requiring welding of the OD of the Welded Gasket set until final assembly
Weld Membrane Gaskets
Weld Membrane Gaskets are used in pairs, one welded to each flange.
Weld Membrane Gaskets made to DIN 2695 are 4mm thick. They are typically made of material the same or similar to the flange material/ in terms of weld compatibility, thermal characteristics and corrosion characteristics. Each ring is seal welded to its mating flange on the ID. Then upon flange assembly, the two Weld Membrane Gaskets are seal welded to each other at their outer diameter, providing a fully welded joint. Special flanges made to DIN 2526 Type M are utilized in conjunction with Weld Membrane Gaskets, to ensure that there is adequate room to perform the OD seal weld.
Weld Ring Gaskets
Similar to Weld Membrane Gaskets in that Weld Ring Gaskets are also used in pairs.
Materials utilized should be similar to the flange material as noted above for Weld Membrane Gaskets.
Each ring is seal welded to its mating flange on ID or OD (depending upon style utilized).
Then upon flange assembly the two Weld Ring Gaskets are seal welded to each other at their outer diameter.
2. Common Metallic Gasket Materials
The following is a listing of the more common metals used in manufacturing both semi- metallic and metallic gaskets. It is not intended to be a list of all available materials, only the typical materials. For a more complete listing, contact your gasket supplier or manufacturer to discuss all the materials which they may have available. Refer to EN 10027-1 and EN 10027-2 Designations Systems for Steel Part 1: Steel Names and Part 2: Steel Numbers and The Unified Numbering System (UNS) for Metals and Alloys for complete list of metal standards.
Has excellent corrosion resistance to organic acids except nitric acids.
Copper alloys are generally used with non-oxidizing acids, alkaline and neutral salt solutions.
The most commonly used material for manufacturing double jacketed gaskets. It has poor resistance to corrosion and should be used with caution when in contact with water or diluted acids.
Used successfully in acetic acids, nitrates and many organic chemicals
This is a corrosion resistant alloy resists corrosion of hydrochloric acid under most conditions, as well as, other halogen acids. It is also resistant to phosphoric acid and reducing conditions.
Offers exceptional resistance to severe oxidizing conditions encountered with nitric acid, free chlorine as well as strong aqueous and acid solutions.
Withstands high temperatures and has excellent resistance to corrosion by halogen gases and compounds.
Good resistance to sulfuric, chromic and phosphoric acids. It is soft and malleable.
Excellent resistance to most acids and alkalis except extremely oxidant acids.
Excellent resistance to caustic substances. Has a high degree of corrosion resistance to neutral and distilled water.
Has a good resistance to wet chlorine and chlorine dioxide.
Type 304 Stainless Steel (1.4362)
This material is widely used in the manufacture of industrial gasketing, due to its low cost and excellent resistance to corrosion.
Type 316 (1.4401) /316L (1.4404) Stainless Steel
This material generally offers a higher resistance to corrosion than type 304SS.
Type 321 Stainless Steel (1.4541)
This alloy is similar to 304SS but titanium is added. It is widely used in high temperature corrosive applications.
Type 347 Stainless Steel (1.4550)
This alloy is similar to 304SS but columbium and titanium are added. It has good performance in high temperature corrosive applications.
Type 410 Stainless Steel (1.4006)
This stainless steel is a heat treatable 12% chromium steel, which combines good general corrosion resistance with high strength.