Asbestos gaskets, ropes, packings and similar products: this article describes the properties, manufacture, history, & visual identification of asbestos packings, ropes and gaskets. We describe how asbestos rope, gaskets, & paper gasket material were manufactured and give the uses & properties of these products.
This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
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The following text is Adapted from Rosato (1959) p. 178-184  © 2013 InspectApedia.com
Introduction Asbestos packing and gasket seals have provided the best means of positively sealing hydraulic or pneumatic equipment.
No subject is of more pertinent interest to the mechanical engineer and plant executive than the proper operation of hydraulic or pneumatic equipment. Presses, valves, hoists, autoclaves, pumps and similarly controlled machinery can be a source of great expense when improperly sealed.
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Packing is a compressible and resilient material used as a seal on moving parts. Gaskets are used wherever two stationary pieces of metal are brought together in equipment handling liquids or gases. Gaskets are also termed stationary or static seals.
The most common use of gaskets is in the joining of flanges on pipe lines or valves. Seals and gaskets are identified basically by the method of application or type of construction; i.e., asbestos-"Teflon" seals, asbestos-fiber sheeting, floating seals, mechanical seals, asbestos packings, rubber-sheet packings, V-U- and 0-rings, plastic seals, tadpole, silicone-rubber sheeting, asbestos rope, compression packings, asbestos felt seals, asbestos braiding packing, flexible metallic packings, and oil-retainer seals.
While the use of packings and gaskets is not new, there are always manufacturing and engineering problems associated with the development of new seals for new equipment. These types of problems involve the application of different types of asbestos-base seals.
The continually growing field of hydraulics and pneumatics has created a need for specialty packings and gaskets of varying characteristics and design to meet the many variations in speeds and temperatures. Experience has shown that no one style or type of packing can be recommended as suitable for all different types of applications. The principal reason for this is due to the variations to which the parts are subjected—pressure, medium of operation, temperature, availability of lubrication, metal clearances, speeds of operation, motion of operation (rotary or reciprocating) and allowable space of operation.
Chrysotile asbestos fibers, as well as crocidolite, are used in gaskets and packings along with such binders as natural rubbers, synthetic rubbers, asphalt, "Teflon," "Kel-F," phenolic resin, plasticizers, and other materials. Crocidolite is used principally in packing that requires acid resistance.
Asbestos can be made to have the ability to expand and contract with heat and cold in exactly the same manner as the metal it contacts. Chrysotile fibers are desirable in the seals because of their softness, durability, resistance to charring, silkiness, and slipperiness.
What is needed in the packing is low friction, resistance to high pressure, resistance to heat, and resistance to chemical gases. Having a combination of these requirements, asbestos surpasses all other materials in these properties.
Types of Asbestos Packing, Gasket, & Rope Products
Asbestos packings and gaskets are made by different techniques. Yarn can be twisted or braided into different shapes conforming to packing. or gasket requirements. The braided forms or asbestos fabrics can be coated, or impregnated with different compounds. Cloth is one of the bases for sheet packing, folded, and wound packing. These packings in turn are formed into various shapes and sizes.
Sheet asbestos packing is considered one of the first applications of asbestos in the form of packing. This type of packing is available with such different combinations as wire reinforced asbestos fabric, graphite impregnated asbestos fabrics and others. Metallic products can be made from woven cloth which contains asbestos yarn—metal core. This type of construction, impregnated with the various binders, has been very popular and useful in the manufacture of gaskets and packings.
Numerous government specifications are available which pertain to asbestos gaskets and packings. The asbestos packing or sheet requirements generally specify type and amount of fiber in proportion to binders and other additives. Parts can be used at temperatures up to about 700°F. For example, for the higher temperature resistant rubber packings, the composition generally specified includes 70 to 75 per cent, by weight asbestos fiber, 9 to 16 per cent rubber, 1 1/2 to 1 per cent sulfur, and 10 to 14 per cent mineral fillers.
Braided asbestos packing, which is rubberized and graphited, is suitable for high pressure and high temperature applications. See Figure 10.1. Approximately 10 per cent rubber together with such suitable fillers as talcum and graphite is added in this type of product in order to thicken and build up the body of packings. Graphite is a very popular additive since it principally provides low frictional and binding properties.
Packing for high temperature must be firm and hard. The packing which is vulcanized in large coils is generally difficult to force into engineered cavities of equipment such as stuffing boxes. A common procedure is to wind these coils in spirals during the vulcanizing process so that they can be easily installed.
Figure 10.1. Asbestos braided and woven packings_froni top to bottom—interlocked braided, square-plaited braid, braid over braid, and twisted.
Although graphite is a popular ingredient in packings, it is not suitable in packings which will be in contact with stainless steel shafts, rods, and valve stems. In these applications pitting of the metal will occur when packings are lubricated with graphite.
Fabricated packing get its name from being made by molding asbestos cloth with rubber. Different types of synthetic rubber as well as natural rubber are used in order to withstand operating temperatures up to 700°F.
Special packings made to withstand high steam pressures, which in turn are subjected to both high pressures and temperatures, are similar to millboard and paper products.
See Figure 10.2 below. The special packings are made in different shapes and sizes including ring-type gaskets, V-grooves, and others. Medium length asbestos is generally used.
The mixture can be 66 per cent fiber; the remaining constituents are magnesia, iron oxide, clay, barite, cellulose and/or graphite.
Below we show Figure 10.2. the Fourdrinier paper machine adapted for manufacture of asbestos paper sheet gasketing material. - Courtesy Armstrong Cork Co.
Resins, gums, or rubber are sometimes added to provide for more bonding action. These type of binders are generally dissolved in a volatile solvent such as naphtha and toluene. The mixtures are pressed into any desired shape depending on specific applications. They can be reinforced with copper or lead foil. These mixtures can be directed through heated calenders and pressed into sheets. See Figure 10.3 below.
Figure 10.3. Compressed asbestos sheet packing. - Courtesy Raybestos Manhattan, Inc.
Automatic mechanisms are available with asbestos sheeters to control the opening between the front and back rolls as the machine forms a sheet of asbestos from which packings and gaskets are made.
In actual operation the stock is built up on a steam-heated roll to the required thickness, layer by layer, as the volatile liquids are evaporated.
Even pressure must be maintained by the smaller front roll which requires that the opening between the rolls be widened as the sheet becomes thicker. This adjustment of the opening between the rolls is automatically accomplished by means of a ratchet controlled worm adjustment driven by controls directed from the front roll.
Asbestos-rubber compound sheets are sometimes not completely vulcanized until after the material is installed and heat is applied in the packing or gasket area. This allows the packing to become a well fitted part in the machine, making it almost impossible for a leak or blowout to occur.
Asbestos fabric packings are also available constructed of an outer cover of asbestos cloth wrapped around an intercore of resilient material. These products are also referred to as "tadpole" gasketing tape.
The outer cloth can be made with or without metallic inserts. Synthetic rubber, silicone, or "Teflon" can be used as the binder in the cloth. The cores can be made with asbestos alone, synthetic rubber, and a metal such as stainless steel. The shape of the tadpole tape can vary from round to flat gaskets.
Asbestos wick packing is a soft twisted packing made from asbestos roving and is principally used in dry heat where there is relatively no steam pressure involved. The asbestos wick is also available in plied form to make the asbestos rope which can also be used as a packing. When it is made in the form of rope, it is usually supplied on reels so that any length can be cut. In the manufacture of rope packing, the rope is covered with one or two braids of asbestos yarns in order to prevent the rope from untwisting during handling.
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Web search 01/20/2011, original source: http://epa.gov/asbestos/pubs/verm_questions.html
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