WHAT IS A FLANGE GASKET

Flange gaskets are used to create a static seal between two flanges faces, at various operating conditions, with varied pressure and temperature ratings.
A gaskets fills the microscopic spaces and irregularities of the flange faces, and then it forms a seal that is designed to keep liquids and gases.
Correct installation of damage-free gaskets and demage-free flange faces is a requirement for a leak-free flange connection.
If it would be technically possible, in order to manufacture flanges perfectly flat and smooth, and perfectly compatible with one another under all operating conditions, a gasket would not be necessary.
But in normal practice it is not possible, because flange connections under ANY circumstances should be made. Small impurities and a small bit of dirt, is in practice not be avoided and therefore it is necessary to use a gasket.

TYPES OF GASKETS

Materials for gaskets can be divided into three main categories:

Non-metallic types
Semi-metallic types
Metallic types

Non-metallic gaskets are usually composite sheet materials are used with flat-face and raised-face flanges in low Pressure Class applications. Non-metallic gaskets are manufactured from arimid fiber, glass fiber, elastomer, Teflon (PTFE), graphite et cetera. Full-face gasket types are suitable for use with flat-face flanges. Flat-ring gasket types are suitable for use with raised face flanges.
ASME B16.21 covers types, sizes, materials, dimensions, dimensional tolerances, and markings for non-metallic flat gaskets.
Image of both Non-metallic gaskets types.

Semi-metallic gaskets are composites of metal and non-metallic materials. The metal is intended to offer strength and resiliency, while the non-metallic portion provides conformability and sealability. Often used semi-metallic gaskets are spiral wound and camprofile, and a variety of metal-reinforced graphite gaskets. Semi-metallic are designed for almost all operating conditions and high-temperature and pressure applications, and are used on raised face, male-and-female, and tongue-and-groove flanges.
ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and semi-metallic gaskets.
Image of a typical Spiral Wound gasket.

Metallic gaskets are fabricated from one or a combination of metals to the desired shape and size. Often used metallic gaskets are ring-type-joint gaskets (RTJ). They are always applied to special, accompanying flanges which ensure good, reliable sealing with the correct choice of profiles and material.

Ring Type Joint gaskets are designed to seal by "initial line contact" or wedging action between the mating flange and the gasket. By applying pressure on the seal interface through bolt force, the "softer" metal of the gasket flows into the microfine structure of the harder flange material, and creating a very tight and efficient seal.
ASME B16.20 covers materials, dimensions, dimensional tolerances, and markings for metallic and semi-metallic gaskets.
Image of two types of RTJ gaskets.

OFTEN USED SEMI-METALLIC GASKETS

Here below you will find a short description of a number of semi-metallic gaskets, which are largely used. For information on other types, I refer to the World Wide Web. There you can find a lot of reliable information about gaskets.

Spiral wound gaskets
The spiral wound gasket meets the most exacting conditions of both temperature and pressure in flanged joints and similar assemblies and against every known corrosive and toxic media.

The spiral wound gasket depends upon the mechanical characteristics of a formed metal spiral strip, rather than the compressive virtues of more traditional gasket materials. This makes it particularly suitable for low or fluctuating bolt loads. The sealing strips, or fillers, are usually graphite, although other materials such as Teflon (PTFE) may be used, the windings are always stainless steel. For this type of gasket to work the spiral must not be over compressed, hence one of two types of compression control is usually used.
The completed gasket is fitted into a steel ring of specific thickness. When the gasket is fitted into a flange and the bolt load is applied, flange closure is governed by the outer steel ring of the gasket. To further improve the pressure rating of the spiral wound gasket, a steel ring may be added to the inside. This gives an additional compression limiting stop and provides a heat and corrosion barrier protecting gasket windings and preventing flange erosion. It is customary to select inner ring material to be the same as the metal winding.
ASME B16.20 which covers spiral wound gaskets requires the use of solid metal inner rings in: Pressure Class 900, nominal pipe sizes 24 and larger, Pressure Class 1500 from nominal pipe sizes 12 and larger, Pressure Class 2500 from nominal pipe sizes 4 and larger and all PTFE filled gaskets. In the same standard is also described how a spiral wound gasket should be characterized, below you will find a image on it.
Image of marking Spiral Wound Gaskets.

Camprofile gaskets
Camprofile or "Grooved" gaskets have proven themselves in all industrial applications. Camprofile gaskets are found in industrial power plants and in the primary circuits in nuclear installations. Used either between flanges or in heat exchanger units in nuclear applications. The Petro and chemical industry benefit too, as the gaskets are used in applications where high pressures and temperatures are maintained and consequently high bolt loads need to be controlled.

Camprofile gaskets consist of a metal core (generally Stainless Steel) with concentric grooves on either side with sealing materials. The sealing layers (depending on the service duty) can be Graphite, PTFE (Teflon), CAF or Metal (e.g. Aluminium or Silver). Camprofile's can be used without sealing layers to provide an excellent seal but there is a risk of flange surface damage - especially at high seating loads. The sealing layers protect the flange surfaces from damage in addition to providing an effective seal. In the main menu "Others" you will find a link to the dimensions of that kind of grooved gasket.

Metal jacketed gaskets
Metal Jacketed gaskets, as the name suggests, are comprised of a metallic outer shell with either a metallic or non-metallic filler. The filler material gives the gasket resilience, while the metal jacket protects the filler and resists pressures, temperatures and corrosion.

They are traditionally used for heat exchanger applications, pumps and valves, however the resilience and recovery properties of these gaskets are limited. Metal Jacketed gaskets require smooth flange surface finishes, high bolt loads and flange flatness in order to seal effectively.
There are many different styles of jacketed gaskets available. In the main menu "Others" you will find a link to the dimensions of double jacketed flange gaskets. In that type the filler material is completely enclosed by a two piece metal jacket, which covers both the inside and outside diameters and both contact surfaces.

from : http://www.wermag.org/