A gasket, abbreviated GSKT, is a material that is used to seal the face of flanges, valves and equipment. Gaskets can be made from materials that are soft or hard, but will compress to make a tight seal. When compressed between two flange faces it will deform to match the surface grooves and irregularities. A gasket must be installed properly to prevent leakage. The surface of the flange must be clear of all foreign bodies such as dust, dirt or grease that could prevent a proper seal. To ensure a seal through out the life of the gasket, sufficient pressure or stress should be maintained to prevent leakage. It is very important to select the right gasket material to be used.
Nomenclature & Symbols
Gasket materials can be divided into three basic categories:
- Non-metallic gasket - or "soft gaskets" are made from materials that are easily compressed under a low bolt load. They can be used with both flat and raised face flanges in low pressure applications. Materials used are different types of rubber, PTEF, ceramic fiber, glass fiber and more. ASME B16.21 - Nonmetallic Flat Gaskets for Pipe Flanges.
- Semi-metallic gasket - Made from both non-metallic and metallic materials and can operate at both high temperature and pressure applications. They can be used with raised face, male and female, and tongue and groove flanges, and many sizes and styles. ASME B16.20 - Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral-Wound, and Jacketed
- Metallic gasket - Made from a combination of materials. These gaskets are used for special applications where a tight fit is needed. Ring joint flanges (RJF) often use these gaskets. ASME B16.20 - Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral-Wound, and Jacketed.
SPIRAL WOUND GASKET
Ring Gaskets and spiral wound gaskets are used with raised face flanges.
FULL FACE GASKET
Full Face Gaskets are used with flat face flanges.
RING JOINT GASKET
Gasket API STAMPING
- Manufactured to standard ASME B16.20 or applicable standard.
- Inner ring material stamp other than carbon steel.
- Outer ring material stamp other than carbon steel.
- Nominal pipe size and pressure class.
- Winding material and filler material.
- Stamping Location
- Metaillac fillers are color coded on full rim of outer ring.
- Non-metallic fillers are marked with color stripes on the outer ring.
- Inner ring material stamped on inner of outter ring.
- Nonmetallic gasket dimensions and tolerances in accordance with ASME B16.21 for ASME B16.5 and ASME B16.47 flanges.
- Standard thicknesses are 1/16" and 1/8", but others are available.
- Spiral wound gaskets for ASME B16.5 1/4" to 24" dimensions and tolerances in accordance with ASME B16.20.
- Spiral wound gaskets for ASME B16.47 22" to 60" Series A (MSS SP-44) dimensions and tolerances in accordance with ASME B16.20.
- Spiral wound gaskets for ASME B16.47 26" to 60" Series B (API Std 605) dimensions and tolerances in accordance with ASME B16.20.
- ASME B16.20 - Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral-Wound, and Jacketed
- ASME B16.21 - Nonmetallic Flat Gaskets for Pipe Flanges
- ASME B16.47 - Large Diameter Steel Flanges: NPS 26 Through NPS 60 Metric/Inch Standard
- ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through 24
- ISO 4709 - Composition cork -- Gasket material -- Classification system, requirements, sampling, packaging and marking
- ISO 7483 - Dimensions of gaskets for use with flanges to ISO 7005
- A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z
- Adhesion - Sticking of the gasket material to either or both flanges of a mechanical assembly.
- Adhesive - A compound that adheres or bonds two items togeather.
- Antistick - A coating or surface treatment that prevents the gasket material from adhearing to the flange.
- Barlow's formula - Relates to burst pressure, the point just before or at when a pipe or tubing fails under pressure.
- Binder - The elastomer or rubber used in gasket material.
- Blowout - Occures when internal pressure tends to push the gasket material out from between flanges.
- Bolt fracture - A type of fastener failure that occures when a fastener is over-tightened and causes the bolt to break.
- Bolt hole distortion - The flange is permanently distorted directly beneath the bolt. Caused by initial bolt torque.
- Bolt load - The load generated by a bolt when it is tightened.
- Bolt tightening procedure - When tightening, always use the correct sequential bolt order for the flange.
- Bolt torque - A rotational moment. It is a measure of how much twisting is applied to tighten (turn) the nut on a bolt.
- Cocking - When the flange of a mechanical assembly is not parallel to each other.
- Cold flow - The continued deformation under stress.
- Compressibility - The state of being compressed. For a gasket, it is the percent loss of thickness when subjected to a given load.
- Compression set - The residual deformation of a material after removal of the compressive stress.
- Cork - A gasket material manufactured from from the cork tree. Corke material is typically combined with rubbers to give it greater resistance to chemicals and solvents.
- Creep - Deformation with time when a part is subjected to constant stress. Metals creep can occur at elevated temperature however with gasket materials it can occur at normal ambient temperatures.
- Corrosion resistance - The ability of a material to resist chemical destruction from an environment.
- Creep relaxation - A transient stress strain condition in which the strain increases concurrently with the decay of stress.
- Crush resistance - A measure of a gasket's ability to resist initial bolt load and /or extreme stress generated by the expansion of a bolted assembly upon heating.
Deformation - Is measured by how much an object is deformed from its origional dimensions.
- Density - The ratio of the amount of matter in an object compared to its volume.
- Ductility - The ability of the material to deform before it fractures.
- Elastic limit - The limiting extent to which a object my be deformed and yet return to its initial shape after removal of the deforming force.
- Elongation - The increase in length expressed numerically as a fraction or percentage of the initial length.
- Expansion - The increase in any linear dimension or volume.
- Extrusion - Permanent displacement of part of a gasket into a gap, under the action of fluid or bolting pressure.
- Flange insulating kit - See isolation gasket
- Flat faced flange - A flange that is machined flat and does not have a ridge like a raised face or ring type joint flange.
- Gasket extrusion - The tendency of a gasket to flow from between flanges, particularly around bolt holes where stresses are concentrated.
- Graphite - A gasket material commonly used with steam.
- Initial flange pressure - The stress on a gasket attained when a joint is initially tightened.
- Isolation gasket - Used to stop the current flow across metallic pipelines by separating two flanges.
- Nominal diameter - The external size diameter designated of pipe or threads.
- Nominal pipe size - See nominal diameter
- Permeability - The quality or condition of allowing passage of media through a material.
- Pipe flange - A bolted connection where two pieces of pipe, equipment, fittings or valves are connected together to form a piping system.
- Preload - A load that sufficiently stretches the bolt / screw to give a tight joint and overcome any forces tending to pull the joint apart.
- Pressure - The force exerted perpendicular to the surface of an object and is expressed as force per unit area.
- Proof load - The amount of stress that the fastener can withstand without experiencing deformation.
- PTFE - A plastic used for making gaskets. PTFE isd extremely chemically inert, and as such is resistant to chemicals and corrosives. PTFE also has a low co-efficient friction.
- Raised face flange - The most common type of flange facing is the raised face flange. It is used in almost all applications in high and low pressures and temperatures.
- Rubber - A material that is capable of recovering from large deformations quickly and forcibaly.
- Seal - Any device designed to prevent or control the movement of fluid from one chamber to another to exclude contaminants.
- Sealing stress - The amount of flange pressure in a flanged joint assembly to compress and seat a gasket material in order to create an effective seal.
- Shear strain - Opposing forces acting parrallel to the cross-section of a body.
- Shear stress - Tends to deform the material by breaking rather than stretching without changing the volume by restraining the object.
- Strain - The deformation, stretched or compressed, of a material compared to its original length.
- Stress - The force per unit area of cross-section.
- Swelling - The increase volume or linear dimension of a specimen immersed in liquid or exposed to vapor.
- Tensile strength - The maximum stress a material can resist before it starts to elongate.
- Thermal conductivity - The ability to transfer heat within a material without any motion of the material.
- Thermal expansion - The increase in length, area or volume due to the increase (in some cased decrease) in temperature.
- Warp - The lengthwise yarns in a woven fabric.
- Weathering - The surface deformation of a rubber object during outdoor exposure, such as cracking, crazing or chalking.
- Wicking - Leakage through a gasket.
- Working pressure - The normal pressure that a system operates at.