The use of blue, brown and some uses of white asbestos were prohibited in the UK by the Asbestos [Prohibition] Regulations of 1992. There is now a proposal for these Regulations to be amended to prohibit the remaining uses of white asbestos (chrysotile).
The four common areas of remaining use of white asbestos are asbestos cement, friction materials, seals and gaskets and textiles and composites. As far as seals and gaskets are concerned, there are alternatives available.
When the Victorian sealing industry developed sealing materials based upon asbestos fibre, it was the only material available which could survive high temperature and pressure.
More recently, the aim has been to replace these with non-asbestos products. But these seals often have a different physical form from the asbestos based original. Thus, where previously a BS 1832 Grade A compressed asbestos fibre cut gasket had been used, the best solution might now be a spiral wound gasket using exfoliated graphite as the filler in a gasket construction. This only requires a low seating stress and is within the capability of a low pressure class flange and its bolts.
Today, a range of types of flange sealing solutions are available which either use a fibre-based sealing material on its own or in combination with metal.
With the former, the fibres used in sheet sealing material is generally aramid fibre, often in combination with other fibrous materials such as man made mineral fibre, glass fibre or carbon fibre, or it is cellulose fibre.
Whilst there is a debate about the health and safety aspects of some of the alternative fibrous materials, there is no concern about the use of cellulose. With the latter, the sheet sealing material is based upon polytetrafluoroethylene [PTFE], exfoliated graphite or phyllosilicate sheet.
Often asbestos sealing materials were overspecified as the service conditions were very modest – the sealing of water services for example. Here the asbestos based product can be replaced very economically by a cellulose fibre based sheet sealing material made by the calendering process. Sheet sealing materials based upon aramid fibre, in combination with other fibrous materials and structural filler systems, produced by the calendering process also give good service in a range of applications.
Both these types of sheet material are less forgiving to installation malpractices than the original asbestos materials. For that reason, the European Sealing Association [ESA], has produced a comprehensive booklet on the correct methods of assembly for flanged connections. Entitled `Guidelines for Safe Seal Usage’, this can obtained from Flexitallic, or direct from the ESA (UK phone & fax 01524 844 222 ].
For sealing applications where chemical resistance is the prime concern, then PTFE is without peer. In the past, PTFE gaskets have had a poor reputation as a flange sealing material because of their inherently poor creep characteristics. However, development work has resulted in more sophisticated forms of PTFE gaskets now being available. These are based upon either biaxial orientation or expansion of the PTFE. These materials have greatly improved creep characteristics and are suitable for service at up to about 320 degrees C, outperforming in terms of creep resistance and chemical resistance, the specialist asbestos based materials formerly used.
Perhaps the most significant new sealing material introduced has been exfoliated graphite. This has extremely good sealing functionality because it is inherently highly impermeable due to its plate structure. It is also soft and conformable and boasts excellent stress retention. When used in combination with a reinforcing metal core, it can very satisfactorily replace asbestos sheet material in most applications. However,it is not suitable where oxidising media are being sealed or the service temperature/time combination is such that the stress loss due to thermal oxidation is likely to be significant. For sheet gasket applications this combination temperature/time service limitation means, for instance, that for a service life of 5 years, the temperature limit is of the order of 320 degrees C to 350 degrees C. For chemical resistance, modern PTFE materials in sheet form will generally be suitable and where thermal oxidation is likely, phyllosilicate materials are now available.
A phyllosilicate is a silicate mineral similar to graphite. Being a silicate it has extremely good chemical resistance, at least as good as white asbestos, and better thermal resistance. The latest form of phyllosilicate sheet sealing material is based upon exfoliated vermiculite, and produces a material which has the same structure as exfoliated graphite sheet but which has peerless thermal resistance and excellent chemical resistance. It is currently available commercially as `Thermiculite’. Its combination of attributes means that it can take over the chemical resistance role from PTFE where the service temperature is too high. When intended for sheet gasket use this material, like exfoliated graphite, is supplied on a tanged 0.1mm 316 stainless steel core.
All of these sealing materials can be also combined with metal in various forms to provide semi-metallic gaskets giving additional security. The additional metal can be in a number of forms, one of these being a flexible corrugated metal core with a facing of the sealing material on both sides. The corrugations provide both compressibility to the gasket and act as local stress intensifiers so that such gaskets are useful where the pressure rating of the flange is low. A particularly useful form of this gasket uses graphite sheet as the sealing layer.
Another form of semi-metallic gasket, one which is noted for the security of the seal that it provides, is the spiral wound gasket. This gasket depends upon a helix of sealing material coaxial with a helix of metal. When the former is graphite or PTFE, both the seating stress required and level of sealing achieved by this type of gasket are very significantly better than for the original asbestos forms. Both of these types of sealing material allow spirals with very low assembly load requirements to be produced, so that even in Class 150 conditions good sealing performance can be achieved .
A more recent form of semi-metallic gasket is the `Kammprofile’. This is basically a gasket with a 3mm or more thick core of appropriate metal that has had concentric grooves machined into each face and to which sealing layers are adhered. This design not only provides the security of other semi-metallic forms provide but also has extremely good sealing capability in combination with a low seating stress requirement. Such gasket types are proving to be very successful, especially for heat exchanger applications and others where large diameter robust gaskets are required.
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