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The distinctive material features of expanded graphite rings include:
Low friction coefficient, estimated at a level of 0.1 for wet friction and 0.16 for dry friction
Remarkable chemical resistance against most process fluids, including all fluids used in energy generation
High resistance to nuclear radiation making them eminently suitable for applications in nuclear engineering
Resistance to high temperatures exceeding the resistance of the most common sealing materials
Relatively low cost graphite ring
High resistance to compression
On the negative side, low resistance to tensile and shear forces
It is also significant that in terms of the applications where gland packings are used, our knowledge of this material and our experience is the most extensive. Initially in the seventies, the rings were made directly from expanded graphite particles by filling them into a tool or even by forming directly in the device (pump or valve). In this case, a problem occurred related to the low density of the expanded graphite particles and the high compression set. Because of this graphite foils with a higher density of 0.7 – 1 g/cm³ were used to form rings because they are easier to handle.
Graphite foil is thus currently the basic intermediate product used in the production of graphite gland packings and graphite seals in general. Such foil is supplied by manufacturers of graphite products in different densities (usually 0.7-1.0 g/cm³), carbon contents (in process seals carbon contents of 95- 99.9% of carbon are applied), in different thicknesses (0.3-1.0 mm) and with various modifying additives. Graphite foil materials can also differ in terms of the content of soluble chlorides, fluorides and sulphur, which have an impact on the development of corrosion, and in the amount of traces of rare earth metals which have an impact on the rate of oxidation.
These properties are not always specified by the supplier. While the thickness of the foil or its density may be treated as a purely technological quality, the remaining material characteristics have a fundamental impact on the functionality, durability and operational reliability of a packing.