TYPES OF GASKET

 GASKET:

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.

A flange leak results in loss of product and energy, and sometimes with disastrous consequences. No plant operator wants a leak of toxic or hazardous material that can harm humans and the environment. The gasket helps to achieve a reliable seal to prevent the leak from the flange joints.

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.
• Inexpensive.

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

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 micro inches 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.

• 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.

Materials of Construction

Gaskets are made of resilient or pliable materials. Metal or nonmetal composites may be incorporated as reinforcement or filler material. Non-metallic gaskets are manufactured non-asbestos material or Compressed Asbestos Fibre (CAF). Non-asbestos types include :

1. Aramid Fibre,
2. Glass Fibre,
3. Elastomer,
4. Teflon (PTFE)
5. Flexible Graphite.

PTFE or RPTFE are used in high corrosive applications.

Elastomer Gaskets

SBR : Styrene Butadiene Gasket

SBR is a synthetic rubber that has excellent abrasion resistance and has good resistance to weak organic acids, alcohols, moderate chemicals and ketones. It is not good in ozone, strong acids, fats, oils, greases and most hydrocarbons. Its temperature range would be from approximately -65° F to 250° F.

CR (CHLOROPRENE) (NEOPRENE)

Chloroprene is a synthetic rubber that is suitable for use against moderate acids, alkalies and salt solutions. It has good resistance to commercial oils and fuels. It is very poor against strong oxidizing acids, aromatic and chlorinated hydrocarbons. Its temperature range would be from approximately -60° F to 250° F.

NBR : Nitrile Rubber

This is used as binder for flat gaskets for general services like Cooling Water, Fire Water, Raw Water, Tempered Water, Plant Air, Nitrogen, Pumped oily Water, Sanitary Waste Water.

This synthetic rubber is unusual in being generally resistant to oil, fuel, and other chemicals (the more nitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material).

EPDM (ETHYLENE PROPYLENE)

This synthetic material has good resistance to strong acids, alkalies, salts and chlorine solutions. It is not suitable for use in oils, solvents or aromatic hydrocarbons. Its temperature range would be between -70° F to 350° F.

Compressed Asbestos Fibre Gaskets

Early efforts to replace asbestos resulted in the introduction and testing of compressed non-asbestos products in the 1970s. Many of these products have seen extensive use since that period however there have been enough problems to warrant careful consideration in choosing a replacement material for compressed asbestos. Most manufacturers of non-asbestos sheet materials use synthetic fibers, like Kevlar, in conjunction with an elastomeric binder. The elastomeric binder makes up a larger percentage of this sheet and thereby becomes a more important consideration when determining applications.

Flexible Graphite Gaskets

This is an all graphite material containing no resins or inorganic fillers. It is available with or without a metal insertion, and in adhesive-back tape form. Flexible Graphite has outstanding resistance to corrosion against a wide variety of acids, alkalies and salt solutions, organic compounds, and heat transfer fluids, even at high temperatures.

MICA Gaskets

Mica sheet is a readily-processible form comprised of a high percentage of mineral held together with a small amount of silicon binder. Its lamellar and non-fibrous structure, together with the low ratio of binder allows for a significant reduction of weight loss at elevated temperatures, and especially when compared to other high temperature compositions. It resists a wide array of chemicals and is unaffected by water, acids, bases, solvents and mineral oils.

PTFE Gaskets

Of all the plastics, PTFE (Polytetrafluoroethylene) have emerged as the most common plastic gasket material. PTFE’s outstanding properties include resistance to temperature extremes from -140ºF to 450ºF (for virgin material). PTFE is highly resistant to chemicals, solvents, caustics and acids except free fluorine and alkali metals. It has a very low surface energy and does not adhere to the flanges. PTFE gaskets can be supplied in a variety of forms; either as virgin or reprocessed material, and also with a variety of filler material. The principal advantage in adding fillers to PTFE is to inhibit cold flow or creep relaxation.

Codes & Standards for Gasket Design

The following standards are normally adopted for specifying gaskets.

• ASME B16.21 Non-metallic flat gaskets for pipe flanges.
• ASME B16.20 Metallic Gaskets for steel pipe flanges, Ring Joint, Spiral Wound and Jacketed
• IS2712 Specification for compressed Asbestos fiber jointing.
• BS 3381 Spiral Wound Gaskets to suit BS 1560 Flanges

How to Select a Gasket?

The gasket material selected should be one which is not adversely affected physically or chemically by the service conditions.

The following factors dictate the proper selection of gaskets.

• The gasket material should be compatible with the fluid service.
• Ability to withstand the pressure-temperature of the system.
• The service life of the gasket
• Gasket material must be corrosion resistant against the service fluid.
• The gasket shall be readily available.
• Economy- Clients always prefer cheap and reliable gaskets.