1、. MILITARY HANDBOOK i ! GASKET MATERIALS (NONMETALLIC) Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-_- - . “. .I . .- - i . .- . .1 . .- . -. _. . -e.-.- Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS
2、-,-,-. . _“ . . MIL-HDBK-212 53 W 7777770 000bBb7 T W MIL-HDBK-2 12 26 SEPTEMBER 1958 SUPERSEDING ORDP 20-307 DECEMBER 1956 DEPARTMENT OF DEFENSE OFFICE OF THE ASSISTANT SECRETARY OF DEFENSE (SUPPLY AND LOGISTICS) STANDARDIZATION DIVISION Washington 25, D. C. Gasket Materials (Nonmetallic) 26 Septem
3、ber 1958 MIL-HDBK-212 1. This handbook on Gasket Materials (Nonmetallic) has been de- veloped by the Ordnance Corps, under the direction of the Standardization Division, OASD (S this loss is instantaneous at 1300 deg. F. When the water of crystallization is driven from asbestos, slight rubbing will
4、reduce the material to a powder. In actual application, however, asbestos is rarely used alone or in its pure form. Therefore, the heat resistance of asbestos materials also depends upon the heat resistance of the constituents in the composition. Asbestos and asbestos-composition gas- kets have exce
5、llent resistance to the pres- sure of heavy loads and the cutting action of narrow, sharp-edged flanges, Asbestos materials are, for the most part, unaffected by changes in humidity. As a re- sult, they maintain their original size and 3 I 000b872 3 m MIL-HDBK-212 26 SEPTEMBER 1958 shape regardless
6、of variations in the amount of moisture present in the atmosphere. Pure asbestos products are porous and weak, and therefore, are almost always com- bined with other substances. Asbestos fibers may be combined with cellulose or metallic fibers for added strength ; they may be joined with binders and
7、 saturants for greater im- perviousness ; or they may be added as a filler to rubber and plastic compounds. 7, Principal Compositions. The two types of asbestos fibers from which asbestos gasket materials are made are chrysotile and cro- cidolite. Crocidolite is more commonly known as blue asbestos
8、and is a sodium-iron-silicate complex. It is used chiefly for the manufac- ture of acid resistant gaskets. Chrysotile is common white asbestos, a mineral consisting primarily of magnesium silicate combined with approximately 14 percent water of crystallization. It is resistant to weak acids and most
9、 alkali solutions, but is susceptible to the action of strong acids. Resistance to corrosive agents may be in- creased by combining these two fiber types with binders such as rubber. The character- istics of the particular binder and of the as- bestos will determine the properties of the resulting c
10、omposition, The most common type of asbestos combination is the asbestos- rubber composition. It is considered the principal asbestos material, although woven or braided asbestos and mill board or paper are also used. Each of these three groups of asbestos materials is analyzed in the fol- lowing pa
11、ragraphs. The rubber-asbestos compositions include “compressed asbestos” and several other forms of asbestos sheeting or composition with similar characteristics and applicabil- ity. Of the three groups of asbestos ma- terials, this group is produced in greatest quantity and has the widest range of
12、use. Rubber-asbestos sheets vary according to the formula and process of manufacture : and some forms are pigmented while others are given some external after-treatment. The type known as “compressed asbestos” is made by calendering a dough of asbestos Provided by IHSNot for ResaleNo reproduction or
13、 networking permitted without license from IHS-,-,-MIL-HDBK-2 12 26 SEPTEMBER 1958 TABLE 1. GENERAL PROPERTIES AND USES OF GAsKET MATERIALS Classification Asbestos, Compressed Cork-and- Rubber Cork Composition Rubber and Plastic Paper, Untreated Paper, Treated Leather Combination Constructions Data,
14、 courtesy Special Characteristics Tough and durable. Dimensionally stable. Relatively incompressible. Good steam and hot water resistance. Oil and solvent re- sistance determined by characteristics of rubber binder. Provides fluid barrier and resilience of rubber with compressibility of cork. Prop-
15、erties of flow versus compression subject to controlled variation according to pro- portions of cork and rubber. Some sacrifice of rubbers tensile and elongation proper- ties. Chemical properties about same as base polymers. Higher in cost than cork com- position or fiber types, but lower than strai
16、ght rubbers. Does not extrude from joint. Die cuts easily. High coefficient of friction. Versatile, dependable general-purpose ma- terial. Variable as to binder, texture and hardness (density). Truly compressible. High friction whether dry, wet or oily. Low cost. Excellent oil and solvent resist- an
17、ce. Poor resistance to alkalies and cor- rosive acids. Highly variable according to compound- ing, hardness, modulus, fabric reinforce- ment, etc. Generally impervious, but en- tirely incompressible. Low in cost; may induce corrosion. General-purpose material having better tensile strength than cork
18、 composition, but less compressibility. Good oil, gasoline and water resistance, but alternate wet and dry cycles may cause shrinkage and hard- ening of some types. Tough, porous, abrasion resistant, flexible at extreme low temperatures. Tanning and impregnation are important variables. Can be molde
19、d to simple shapes - cups, Vs, Us. Innumerable modifications available, de- pending on materials used and methods of combining. Engineekg. 4 . I 0- /“ 1.: General Uses For heavy duty bolted and threaded joints, as in water and steam pipe fit- tings, manifold connections. Tempera- tures to 500F. For
20、general-purpose gasketing, except steam lines, combustion chambers, etc. Enables design of metal-to-metal joints with gasket positioned in channel or counter-bore and with no allowance for flow. High friction keeps gasket positioned even where closing pressure is not per- pendicular to flange faces.
21、 For mating rough or irregular parts, as glass, light stampings, unfinished cast- ings. Oil sealing at lowest cost in normal range of temperatures and pressures. For installations involving sfxetching over projections, or where flow of gas- ket into threads or recesses is desired. For lowest compres
22、sion-set and maxi- mum resistance to fluids such as alka- lies, hot water and certain acids. Ability to be molded permits use for special de- sign and assembly conditions. Spacers, dust barriers, splash seals, where breathing and wicking not objec- tionable. For machined or r.easonably uniform flang
23、es where adequate bolt pressures can be applied. Relative firmness and high tensile strength permit use of thin gaskets to give good alignments of cov- ers and connected parts. Widely used in dynamic packing appli- cations and as back-up for rubber 0- Rings in high pressure joints. Avoid steam, acid
24、s, alkalies. Usually employed for extreme conditions and special purposes. I Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-212 53 m 7779970 0006874 7 W fibers and rubber cement. It has a pro- nounced grain and usually contains clumps of un
25、dispersed asbestos fibers. It is hard, tough, unyielding, and dimensionally stable. Composition of rubber and asbestos may be made by several other methods, including latex treatment of asbestos paper or im- pregnation of asbestos felt with a rubber solution. These forms are not as hard as the compr
26、essed asbestos, but they are more re- silient. The rubber latex, beater solution treatment is a recently developed process that produces a more homogeneous, non- directional material with greater flexibility and resilience. Natural rubber and synthetic rubbers are also used to make certain com- posi
27、tions. Asbestos materials made with natural or SBR rubber are not oil-resistant, but neoprene and nitrile compositions are, especially the nitrile compositions which are extremely resistant to oil and aromatic fuel. The rubber binder constitutes 10 to 25 per- cent of the total weight in typical comp
28、osi- tions. Woven and braided asbestos are softer than compressed sheets and are often satu- rated by special treatments. They may be processed into cloth, spun into yarn, or braided into various rope-like forms. The asbestos yarn may be reinforced with metal- lic wire or strands for added strength
29、with- out the sacrifice of high temperature re- sistance. Heat resistance is lowered, however, in forms reinforced with cotton. Braided forms are frequently impregnated with a lubricant and graphite treatment, while woven asbestos fabrics may be frictioned with rubber or saturated with latex or resi
30、n. Both the blue and white asbestos fibers are used for these forms, although the blue as- bestos is generally used for applications that require acid resistance. The millboard and paper materials are relatively pure forms of asbestos fiber, bonded perhaps with starch or sulphites. They do not have
31、a wide range of applica- bility and are not as important as the other forms of asbestos materials MILHDBK-212 26 SEPTEMBER 1958 8. Properties and Limitations. In comparison with other gasketing materials, asbestos products have properties and limitations that classify them as midway between the soft
32、 gasketing materials and the metallic or hard nonmetallic types. The asbestos products used .for gaskets are primarily compressed asbestos and the rubber-asbestos materials grouped with it. For purposes of a genera1 analysis, the properties and limitations of these asbestos materials can be evaluate
33、d by indicating how well these products fulfill the basic requirements of gasket design. Thus, the following discussion will briefly analyze the impermeability of asbestos, its ability to make complete contact with the flanges, and to maintain this contact despite various pressures, forces, and envi
34、ronmental factors. The impermeability of asbestos is limited. The characteristic clumps of undispersed fibers in compressed asbestos may offer some conduction to the sealed liquids unless bolt- ing pressures are high and there is sufficient compacting force to compress the material into impermeabili
35、ty. To achieve complete contact with the flange faces the asbestos gasket must be sub- jected to a heavy load. There is a threshold pressure that is necessary to achieve com- plete seating and ensure contact even under very light or atmospheric pressures. This threshold pressure averages about I500
36、psi, with variations according to the type of rub- ber-asbestos used, and the flange finish, rigid- ity and bolt spacing. Complete contact between gasket and flange can be maintained by proper applica- tion rather than as a result of the charac- teristics of the material. Compressed asbes- tos has l
37、ittle elasticity and does not follow flange movement. The material is relatively incompressible and forms a solid inert filler between the flanges. Moreover, the gasket becomes almost as unyielding as the metal assembly components after the flange bolts have been tightened to the point where the gas
38、ket is well seated and conforms to the irregularities of finish and alignment in the 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-2 12 26 SEPTEMBER 1958 joint. Continuing tightening of the bolts causes them to stretch ; it is the elasti
39、city of these stressed bolts that provides the force that maintains the tight contact. However, over a period of time, and especially in ap: plications where high temperatures are en- countered, creep will occur within the strc- ture of the gasket and the bolts. This creep will produce a reaction kn
40、own as stress re- laxation, cold flow, or stress decay ; the bolts become loose and must be tightened to main- tain the seal, If the joint is not separated during operation, it is best to permit the gasket to adhere to the flanges and thus pre- vent leakage in the seal. For applications where adhere
41、nce is undesirable, compressed asbestos can be treated with graphite as a preventive measure. (See paragraph 40.) Compatability between the gasket and its environment depends upon the type of as- bestos composition and the particular ap- plication. For example, chemical properties are determined by
42、the rubber used in the composition while heat resistance is gov- erned by the temperature range in the ap- plication. When certain rubbers are com- bined into rubber-asbestos, the resulting composition is resistant to the effects of petroleum products. In general, rubber-as- bestos is unaffected by
43、most common indus- trial chemicals such as alcohol, glycerine, alkalines, caustic solutions, and weak acids. It may also be used satisfactorily in high or low-pressure water joints. But, in chemical processing and similar uses, the application must be carefully studied to ensure that the asbestos do
44、es not come into contact with any substance capable of attacking it, such as nitric acid. A careful examination of the application is also necessary for require- ments that involve high temperatures. While asbestos may be used at temperatures of 500 deg. F., the design engineer is cautioned to consi
45、der the effects of continued exposure on the expected performance and service life of the gasket. Asbestos is not recommended for applications where temperatures con- tinually exceed 500 degs. F. .- 9. Application. From the discussion of properties and limitations it is evident that compressed rubbe
46、r-asbestos gaskets are best used in heavy, rugged construction employ- ing flat, rigid flanges, adequate bolting pres- sures, a- minimum of machining irregular- ities, and where temperatures do not exceed 500 deg. F. General pipe-flange applications such as found in oil refineries and chemical proce
47、ssing plants meet these requirements and, therefore, utilize rubber-asbestos gas- kets. Other applications that specify re- sistance to particular chemicals require the appropriate rubber-asbestos composition. For certain applications woven and braided as- bestos or millboard and paper may be pre- f
48、erred. Their uses are discussed below. The appropriate specification should be consulted for more precise data. Woven and braided materials are used for packing in stuffing boxes on reciprocating and rotating assemblies to prevent leakage of oil, water, steam, etc. For these purposes they may be in
49、the form of coils or split-wall rings. Woven fabrics may be die cut to va- rious sizes and shapes, or rubber-treated, then folded and pressed to make rings, ovals, squares, and other shapes required for ap- plications such as tube plates in boilers. In tape form it may be used for gasketing in steam cylinder heads, heater doors, and other power-plant applications. As continuous- length tubing, asbestos fabric gaskets are used for steam service, hand holes, man holes, heater
