The gasket is a key component in a flange assembly. As discussed in Mechanical Design of Heat Exchangers, flanges are composed of three sub-components. The complex interaction between these sub-components under bolting up and operating conditions determines the successful operation of the flange. However, it is through the gasket that any leakage occurs and for this reason it receives most attention.
A wide range of gasket materials and types are used, most of them to national standards which specify quality and dimensions. Gasket materials are described in the Heat Exchange Design Handbook, 1994, of which the following are the most important:
Rubber. Natural rubber was the first material used; synthetic rubbers now is widely available in many grades.
Compressed asbestos fiber jointing (CAF). This was the most common material used for low pressure service, although alternatives are now sought as manufacturers and operators try to minimize their use of asbestos. The gasket consists of asbestos fiber bound together with an elastomeric binder, sometimes with a steel mesh, and can be used up to 550°C.
Compressed synthetic fiber jointing. This new range of gasket materials utilize Kevlar,TM a registered trademark of Du Pont, and glass fibre with elastomeric binders and is an attempt to replace CAF.
Graphite foil. The material comprises almost 100% graphite laminated to a metal core to give it strength for handling and service. It has a wide temperature range.
Metal jacketed asbestos. A popular gasket for exchangers as it can be manufactured with integral pass partition bars for multipass heads. A section is shown in Figure 1 and shows the asbestos millboard core surrounded with a metal jacket which can be produced in a wide range of materials.
Spiral wound. These gaskets are made from stainless steel or high alloy metal wound in a spiral formation with a nonmetallic filler. These are suitable for high integrity gaskets and can be produced in a wide range of materials. Figure 2 shows a gasket with the addition of solid rings to provide a compression stop and a location stop against the bolts. The internal ring can be fitted with partition bars as e) above.
Solid metal joints. Used for high pressure applications, they are available in a range of sections, e.g., flat or octagonal, or as proprietary designs. The flange surface finish is critical in achieving a leak free gasket. The finish should be selected to suit the particular gasket used, and the gasket supplier involved if necessary. A "gramophone" finish is one of the most useful finishes.
The gasket is contained in a flange facing. A fully confined facing has machined steps inside, and outside the gasket which gives complete location and prevention of gasket blowout or extrusion. This facing has the highest integrity, partial or unconfined facings are also used for lower pressure/integrity.
Heat Exchange Design Handbook (1994) Begell House Inc., New York.