Introduction to Gaskets
A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression. Gaskets allow for “less-than-perfect” mating surfaces on machine parts where they can fill irregularities, while at the same time be sufficiently resilient to resist extrusion and creep under operating conditions. The exact function of a gasket in this respect is to prevent the escape or ingress of fluids (liquids or gases) even at extreme pressure and temperature. It is usually desirable that the gasket be made from a material that is to some degree yielding such that it is able to deform and tightly fill the space it is designed for, including any slight irregularities. A few gaskets require an application of sealant directly to the gasket surface to function properly.
Material and Properties
Gaskets are normally made from a flat material, a sheet such as paper, rubber, silicone, metal, cork, felt, neoprene, nitrile rubber, fiberglass, polytetrafluoroethylene (otherwise known as PTFE or Teflon) or a plastic polymer (such as polychlorotrifluoroethylene). Gaskets for specific applications, such as high pressure steam systems, may contain asbestos. However, due to health hazards associated with asbestos exposure, non-asbestos gasket materials are used when practical.
The optimum gasket material needs to have the following characteristics.
- Chemical resistance of PTFE.
- Temperature or heat resistance of graphite.
- Strength or mechanical properties of steel.
- Zero seating stress of soft rubber.
Obviously there is no known gasket material that has all these characteristics and each material has certain limitations that restrict its use. It is possible to overcome limitations partially by several methods such as including the use of reinforcing inserts, combining it with other materials, varying the construction or density, or by designing the joint itself to overcome some of the limitations.
One of the more desirable properties of an effective gasket in industrial applications is the ability to withstand high compressive loads. Most industrial gasket applications involve bolts exerting compression well into the 14 MPa (2000 psi) range or higher. The more compressive load exerted on the gasket, the longer it will last. There are several ways to measure a gasket material’s ability to withstand compressive loading. The “hot compression test” is probably the most accepted of these tests.
Many factors are to be considered when selecting a gasket to ensure its suitability for the intended application. Primary selection of a gasket type is based on the following.
- Temperature of the media to be contained
- Pressure of the media to be contained
- Corrosive nature of the application
- Criticality of the application
- Flange configuration
Hot Compression Test
An important characteristic for industrial gaskets is their capacity for tolerating compressive loads. Evaluations, such as the hot compression test, can be used to gauge a specific gasket’s ability to withstand various weights and temperatures. Typically, a gasket is placed between the exertion bolts of a hydraulic press. Temperature is increased, often up to nearly 600 degrees Fahrenheit, at an incremental rate over a given period of time while the press exerts constant load pressure on the gasket. Any decreases in material thickness are measured and used to assess the gasket’s effectiveness. Tests such as this can be helpful in selecting a gasket or deciding upon a given material or design configuration.
Is a Gasket Necessary?
While gaskets serve an important function as sealing joints, there are a handful of similar devices that may be better-suited to certain tasks. An application requiring a seal that forms a barrier between external and internal elements, such as a unit to prevent water leakage, usually needs a gasket. However, to fill small assembly gaps between components, manufacturers would be better served by a spacer, or “shim”, which is a narrow wedge used for packing or leveling purposes.
Likewise, o-rings, though similar to gaskets, have a subtly different designation. Unlike gaskets, o-rings are made almost exclusively of synthetic rubber or plastic polymers with elastomeric properties, and are produced solely in ring form. They are durable and reliable in sealing matched components by creating a barrier around an area with leakage potential. In addition, o-rings are distinct for their round or square cross-sectional configurations, as well as their high pressure resistance, making them valuable in some applications where a standard gasket’s resistance would not suffice.
Types of gaskets
As per material of construction, gaskets can be divided into three main categories:
- Non-Metallic – Non Metallic gaskets are are usually composite sheet materials which are used in low to medium pressure services. With careful selection these gaskets are not only suitable for general service but also for extreme chemical services and temperatures. Examples are elastomers, cork, compressed fibre sheets, poly tetra fluoro ethylene (PTFE), Bi-axially orientated reinforced PTFE, graphite, thermiculite, and insulating gaskets etc. ASME B16.21 covers types, sizes, materials, dimensions, dimensional tolerances, and markings for non-metallic flat gaskets.
- Semi-Metallic – Semi Metallic gaskets are composite of both metallic and non-metallic materials. The metal provides the strength and the resilience of the gasket and the non-metallic component provides the conformable sealing material. These gaskets are suitable for low and high pressure and temperature applications. A wide range of materials is available. Examples are spiral wound gaskets, gaskets with covered serrated metal core, metal jacketed gaskets, and metal reinforced gaskets etc. ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and semi-metallic gaskets.
- Metallic – Metallic gaskets can be fabricated in a variety of shapes and sizes recommended for use in high pressure/temperature applications. These gaskets require a much higher quality of the sealing surface than non-metallic gaskets. Except for weld ring gaskets, high loads are required to seat metallic gaskets, as they rely on the deformation or coining of the material into the flange surfaces. Examples are ring type joints, lens rings, weld rings, and solid metal gaskets etc. ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and semi-metallic gaskets.
As per type of construction, gasket can be divided into following types:
- Jacketed Gaskets – Jacketed gasket merges the efficiency and flexibility of soft gaskets (made of rubber or plastic) with the resistance and durability of an external metal coating. A single-jacket has soft filler with metal coverage along one face of the gasket, while a double-jacketed version has a fully coated metal facing, providing improved temperature, pressure, and corrosion resistance. Other variations include corrugated jacketed gaskets, and French Style jackets, which provide coating on either the inside or outside of the gasket.
- Solid Gaskets – Solid gaskets are typically formed of metal and are a relatively inexpensive alternative to jacketed gaskets. They have high thermal and pressure resistance, though they require higher compression force to form a seal and are usually effective only against surfaces that are harder than the metal itself. “Ring Joint Gasket” is a type of solid gasket. Ring joint gaskets come in two basic types, an oval cross section and an octagonal cross section. The octagonal cross section has a higher sealing efficiency than the oval and would be the preferred gasket. The sealing surface on the ring joint grooves must be smoothly finished to 63 microinches and would be free of objectionable ridges, tool or machining marks. They seal by an initial line contact or a wedging action as the compressive forces are applied. The hardness of the ring should always be less than the hardness of the flanges to ensure the RTJ is deformed and not the flanges when assembled.
- Spiral Wound Gaskets – Spiral wound gasket is formed by combining metal with softer plastics or synthetic rubber in a winding shape, often reinforced with additional layers of metal without filler. Its unique design yields high thermal and physical stress resistance, coupled with flexibility and resilient sealing. Spiral wound gaskets are often used in piping, pumping, and heat exchange systems.
- Kamm / Cam profile Gaskets – The Kammprofile or Camprofile design contains a corrugated metal core (generally Stainless Steel) covered with a malleable sealing material attached to both of its sides. This structure focuses physical stress onto the surface sealant, creating tight seals along the gasket’s edges while retaining the device’s flexibility and strong tensile core. Kammprofile gaskets provide reliable support in heat exchange systems and have improved cost-effectiveness due to their capacity for repair.
The flange connection is the widest spread gasket application. The flange connection is a sealing system, which consists of;
Only the right choice and combination of these three individual parts results in a leakage free operation with long service life.
For more on Flanged Connection check out:
Gasket is a device for sealing two surfaces, by storing energy between them. The gasket reacts to the forces generated by the bolts, and therefore the work and energy imparted to the bolted joint becomes ‘stored’ within the gasket itself. On seating, the gasket needs to be capable of overcoming the macro and micro imperfections.
In order to ensure the maintenance of the seal throughout the life expectancy of the assembly, sufficient stress must remain on the gasket surface to prevent leakage. The residual bolt load on the gasket at all times must be greater than the hydro-static end force acting against it. The hydro-static end force is the force produced by the internal pressure which acts to separate the flanges. Basically there are the following four different methods which are used either singly or in combination to achieve this unbroken barrier.
- Compression – This is by far the most common method of making a seal on a flange joint and the compression force is normally applied by bolting.
- Attrition – This is a combination of a dragging action combined with compression such as in a spark plug gasket where the spark plug is turned down on a gasket that is both compressed and screwed into the flange.
- By heat – Example is the case of sealing a bell and spigot joint on cast iron pipe by means of molten lead. However, in such case after the molten lead is poured, it is tamped into place using a tamping tool and a hammer.
- Gasket lip expansion – This is a phenomenon which occurs due to edge swelling when the gasket is affected by confined fluid. This causes the gasket material to swell and increase the interaction of the gasket against the flange faces.
Forces Acting on a Gasket Joint
- Internal pressure – These are the forces continually trying to unseal a gasketed joint by exerting pressure against the gasket (blowout pressure) and against the flanges holding the gasket in place (hydrostatic end force).
- Flange load – it is the total force compressing the gasket to create a seal. It is the effective pressure resulting from the bolt loading.
- Temperature – Temperature creates thermo-mechanical effects, expanding or contracting the metals, affecting the gasket material by promoting ‘creep relaxation’ which is a permanent strain or relaxation quality of many soft materials under stress. The effect of certain confined fluids may become increasingly degrading as temperature rises and attack upon organic gasket materials is substantially greater than at the ambient temperatures (about 25 deg C). As a rule, the higher the temperature, the more critical becomes the selection of the proper gasket. .
- Medium – It is the fluid (liquid or gas) against which the gasket is to seal.
- General conditions – These include type of flange, flange surfaces, type of bolt material, spacing, and tightness of the bolts etc.
There are other shock forces that may be created due to sudden changes in temperature and pressure. Creep relaxation is another factor that may come into the picture.
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