A 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.
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. Flat ring 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.
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 diameters are 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.Spiral wound gaskets are designed to suit ASME B16.5 and DIN flanges.
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 Fig. A7.4, DIN 2697, Profile for Camprofile Gasket. 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. Camprofile gaskets are used on all pressure classes from Class 150 to Class 2500 in a wide variety of service fluids and operating temperatures.
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.
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.
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. #Little_PEng