Document 373w2wKN9oMxyD4Vmne4x7JjD

Hows and Whys of Packing and William J. Bowler, Paper Mill Specialist, Garlock, Inc. Palmyra, N.Y. A working knowledge of their design and Materials, instal lation and maintenance can extend their life, forestall emergency shutdowns and reduce maintenance costs. The pulp and paper industry, pre sents some of the most rugged and demanding applications for packing and gaskets. A better understanding of the "how to" and "why's" of pack ings--their design, materials used in their construction, installation and maintenance procedures--can enable puip and paper engineering and maintenance personnel to extend packing and gasketing life by 50 per cent or more, forestall emergency shut-downs, increase overall process efficiency, and reduce operating and maintenance costs. What follows are the "ABC's" of dynamic sealing (packing) and static sealing (gaskets) as they relate to the pulp and paper industry. Dynamic Packing Materials Dynamic packings seal moving parts: reciprocating, rotary, or heli cal. They are used from vacuum through high pressures, over a broad range of temperatures to retain all types of fluids and to exclude foreign materials. The five basic materials used in packing construction are: fibers, metals, plastics, rubber and leather, which are combined with various impregnants and lubricants depending on the application. The function of packing is to throttle leakage between a moving and stationary part. Packing does not stop leaks; rather it controls leaks by throttling. If packings were permitted to run dry, the re sult would be heat, wear, destruc tion of the packing and scoring of the shaft. There must, in other words, be some leakage--to a point where it merely lubricates and per mits cool operation of the packing but does not result in a serious loss of the fluid being sealed. Asbestos is the most widely used of fiber materials, because of it3 strength, temperature resistance and versatility. White asbestos, mixed with an organic fiber to fa cilitate weaving, is normally used against alkaline, neutral or weak acid solutions. Blue asbestos, usu ally without any other fiber, is used against strong mineral acids. Graphite fibers also have excellent chemical and temperature resist ance. Vegetable fibers--flax, ramie, jute and cotton--lose their heat resist ance above 200F.. and are primar ily cold water packings. An advan tage of flax and ramie is that their long, strong fibers get stronger as they get wetter. Synthetic fibers such as rayon and nylon are used in water service with temperatures below 250*F, Tef lon (TFE) fiber, with its low fric tion co-efficient and near-complete chemical resistance, is good to tem peratures up to 500F., has a life span many times that of other ma terials in use against highly corro sive fluids. In compression packings, metal is usually in foil form, although it is sometimes used in braided and shredded forms. Lead, copper and aluminum are the commonly used metals. These packings serve as end rings, and as pressure or thermal barriers in high pressure or high temperature service. Most common used plastics are Kel-F (Minnesota Mining & Manu facturing Trademark) and TFE (Teflon), both highly resistant to chemical attack. Kel-F is used with in the temperature range of minus 400F. to 390F. in unplasticized form, up to 300F. when plasticized. TFE. good within the range from minus 350F. to 500F., has the added virtue of its absolutely non stick properties. . In addition to natural rubber, a few of the more popular synthetics are: synthesized natural and sub stitute natural elastomers with good rubber-like qualities: Buna-N and neoprene with excellent heatand oil-resistant qualities; butyl with it3 impermeability to gases; and silicone which withstands tem perature extremes from minus Leather packings have high ten sile strength and can handle pres sures up to 100,000 p.s.i. All of these packing materials-- plastics, rubber, leather--are -most commonly found in molded or ma chined "V"s, "U"s and "O" rings. Impregnants and lubricants are vital to packing service, and consti tute a good percentage by weight of some types of braided packing. Impregnants include: tallow, lard oil, fish oil, soap, castor oil, paraffin, lubricating oil. silicones, fluorocar bons, graphite and mica, their se lection again depending on the packing application. Fibrous Packing Construction Packing constructed of fibrous materials is conventionally in twisted or braided form. Twisted packing does not have the strength of braided packing, and is primar ily suitable for valve stems. It can be easily untwisted to obtain the requisite size. The three most commonly used types of braided packing are: 1. Square braid. Its loosely locked braids are produced by one pass through the braiding machine. Its advantage--assuming thorough in ternal lubrication--is that its strands can adjust to uneven rod motion. 2. Braid-over-braid. Construction is a series of braided tubes, one over the other. A dense packing, it is used where stem or rod is in good condition for higher pressure job3 on valve stems, or where there is slow relative motion. Since it re sists extrusion, it is particularly useful in reciprocating service. 3.. Lattice Braid (Garlock Trade mark). Every braiding strand passes diagonally through the body of the packing. This type of braid combines flexibility and firmness in a completely unified structure, with each strand contributing to the Reprinted from PAPER TRADE JOURNAL, October I !, 1965 dii AA FIGURE 1. ILLUSTRATING THE HAZARDS OF SUCCESSIVE OVERTIGHTENING OF COMPRESSION PACKINGS. packing's overall strength. Unlike other types of braid, wear will not loosen the strands of lattice braid packing. Cloth woven from basic asbestos fibers may be reinforced with various kinds of wire such as brass. Monel, etc., or impregnated with rubber or other compounds to act as a binder and reduce porosity. The cloth is either wrapped around itself, or ac cordion-folded. or laminated with rubber and cemented with a binder. These types of construction render such packings highly suitable for reciprocating motion applications in volving higher pressures, slow speeds, or heavy duty service. They are used infrequently for very slow rotary service, as end rings in com bination with another type of pack ing for sealing. Metal packings are of spiralwrapped. folded, crumpled or braided construction. They are lubricated between layers to form lubrication reservoirs, thus simulating a lubri cated metallic bearing while serving as a seal. Generally used for higher speeds, they often prove to be the only feasible packing for high-tem perature applications. They may aiso be used as bull rings with other types of packing to prevent extrusion. Shredded fibers (asbestos), plas tics (TFE) and metals are combined with graphite and a binder to mak* shredded packing. Also cailed `'plas tic'' parking because of its softn's and ease of compression, it can b*' obtained in bulk, coils or die-formed rings. Excellent for sealing gases with little leakage, it requires end rings to prevent extrusion. There are four broad functional classes of dynamic seals: compres sion. automatic, floating and me chanical. Compression Packing Compression or jam-type packings are "soft-packings" that are installed in the stuffing box and adjusted pe riodically by taking up a gland. Compression types include twisted, braided, laminated cloth, shredded and metal foils. In a typical com pression packing installation, the fluid is "throttled" to an average leakage rate of 10 or 12 drops per minute, flowing through the slight FIGURE 2. COR RECT METHOO OF INSTALLING DIEFORMED PACKING RINGS. clearance between tne moving .--a:: and the packing to act as a lubricant (sketch "A." Figure 1). The lubri cant in the packing itself takes over during possible periods of dry opera tion. and acts as a safety valve in the event the packing is over-tight ened and the fluid flow is cut off. The hazards of successive over tightening of the gland are sketched in Figure 1. Over-tightening will eliminate the leakage, but the pack ing in turn will run dry on the shaft, building up friction heat. This heat will meit the lubricant in the pack ing, and temporarily prevent the packing from burning up and the shaft or rod from scoring (sketch "B." Figure D. This lubricant loss reduces the packing's volume, ana as leakage from the casing again resumes 'sketch "C", Figure I1 further tightening may be necessary. If the gland is again tightened to eliminate aii fluid leakage, the cycie is repeated until (as shown in sketch "D", Figure 1) there is no saturant left in the packing and its volume cannot be further reduced. The ulti mate resuit: burned up packing, .scored shaft. ` The "moral": avoid over-tighten ing. IF ALL PERSONNEL WITH WRENCHES WOULD ADJUST PACKING PROPERLY ITS LIFE COULD BE MORE THAN DOU BLED. Impregnated packings cannot pro vide sufficient lubricant where high operating temperatures are involved or where conditions create high fric tional heat. In such cases, externa! lubrication is supplied through the use of a lantern ring, usually located near the middle of the stuffing box. Lantern rings, by which a "sealing liquid" is injected into the packing at a pressure higher than that of the fluid being contained, are also re quired for all packings against gases which, because of low viscosity are not adequate lubricants and also leak I <i 4 TOP ADAPTER RING CHEYRON RINGS BOTTOM ADAPTER RING excessively; for liquids of low vis cosity or high volatility under con ditions of high speed or high pres sure; for liquids containing large amounts of dissolved or suspended matter: and for fluids of any kind that are poisonous, corrosive, or too valuable to lose. Maintenance pointers for compres sion packing include: Cut rings when packing is wrapped around shaft to insure par allel ends. When installing foil wrapped packings, inside edges should face in direction of shaft rotation. When die-formed rings are in stalled. they should be opened side ways (Figure 2) to prevent break age at opposite side from gap. Use split-bushing to seat each ring individually. Stagger the ring joints so that fluids will not have a straight path to leak past the packing. Keep spare coils of packing in stock for all key equipment. Clean stuffing box thoroughly after removing old packaging. Avoid scratching shaft. OVER TIGHTENING IS THE GREATEST SINGLE CAUSE OF PACKING FAILURE. * Automatic Packing Automatic packing is so called because it responds automatically to fluid pressure to establish a The flexible packing lips fluctuate with changing pressures . . . tighten ing with increasing pressures, yield FIGURE 3. HOW DIFFERENT SEC* TIONS OF V RINGS SHOULD SE ASSEM BLED. ing with diminishing pressures. They are ideally suited for recipro cating rods, pistons and rams. In cluded in this classification are 4#V" rings, flange cup and "U" packings and "0" rings, generally supplied in molded or machined forms. An excellent example of automatic packing is Chevron (Garlock Trade mark) packing, an improved form of "V" ring named from its functional shape. The hinge in the design is the factor which gives these rings their greater pressure-compensating and friction-relieving characteristics. As with compression types, the stuffing box should be thoroughly cleaned before replacing automatic type packing. The use of multiple nested rings--Chevron rings, for example--makes cut-open sets prac tical and thereby minimizes down time during servicing. The open side of the <rV" rings always points toward the pressure (Figure 3), so the male adapter is put into the stuffing box first with the flat surface seated on the bottom of the box. If split rather than end less rings are used, joints should be staggered just as in the case of soft compression packing. Here again, excessive gland pressure will drasti cally--and uneconoraicaily--curtail the operating life of the packing. Floating Packing Lik-' automatic packings, floating nvva! tvp*s of packing also depend on internal pressure to achieve and maintain a seal. For use as rod racking wherever gas or air is com pressed or expanded, they are desig nated floating type because their segmented rings are free-floating in their case, can move laterally or "float" with a shifting rod. The segmented rings are held together by garter springs so they bear on the rod until fluid pressure of the gas provides the seal. Annular surface of these rings is a ten micro-inches or less, and they are flat within light band readings. They are housed to gether in grooves in a metal packing case of either split-case or annular solid-cup design. The sealing theory of floating type packings i3 that on the compression r.roke. the gas or air is forced along ~e rod to the packing groove. This presses the packing ring3 tightly to gether and against the sealing face of the groove (3B). Rod packing sections consist of two single seg mented rings: a radial cut ring on the pressure side, doweiled to a tan gentially cut sealing ring. The pressure passes along the radial cut, then envelops the outside of the rings, forcing them* in tighter to the rod. On the discharge stroke of the compressor, the pressure in the cylinder becomes less than the pressure in the packing case, thus allowing the air or gas to escape back into the cylinder. The ring seg ments are so constructed as to auto matically close-in uniformly to the rod to compensate for wear. Meehanieai Seals Mechanical seals are designed for precise rotary shaft sealing, at pres sures up to 1200 p.s.i., temperatures of minus 60F. to 500*F., and face velocities up to 5000 f.p.m., depend ing on style and application. Normal mechanical seal construction consists of a smooth rotating face sealing against a smooth stationary face at right angles to the shaft. Sealing faces are precision lapped; there is no need for initial break-in. periodic repacking or adjustment. The spring loaded and flexibly mounted desigD affords continual and automatic com pensation for shaft run-out, axial and play, vibration and wear. They are recommended where leak age requirements are extremely low as in the case where fluids are of a hazardous, corrosive or costly na ture: and are problem solvers for high-speed, high-temperature or high-pressure applications. They are a virtually ieakiess =eai and consequently, if a mechanics; seal drips steadily, maintenance per sonnel should: Check compression of gland for water or gases. TABLE 1--TYPICAL TEMPERATURE AND PRESSURE LIMITATIONS Maximum Maximum Gasketing Material Temperature X Pressure Temperature *F. Kabber ........................................................ Vegetable Fiber ....................................... Goth-Inserted Rubber ........................... All TFE .................................................... Compressed Asbestos ............................. Spiral-Wound: Stainless Steel/TFE ............... Stainless Steei/Asbestos ... . Stainless Sieei/Maznesiumlithium ceramic 15.000 40.000 12S.000 150.000 250.000 over 250.000 over 250,000 over 250.000 225 225 225 500 750 500 850-1200 varies with alloys 1200-1900 ' varies with alloys Homogeneous TFE offers the ulti mate in chemical resistance, yielding only to molten alkali metals and certain fluorine compounds at ele vated temperatures. However, it coldflows rather readily, and TFE gas kets should, therefore, be kept as thin as possible or confined. TFE envelope gaskets provide a thin ma terial with a variable core density and thickness to adjust for flange conditions and pressure require ments. They are ideal for glass or glass-lined piping and other require ments where a high degree of re siliency is required because of flange waviness and low bolt loads. A pat gasket. Look for deflected faces caused by improper gland bolting. Check for installation damage `o shaft packing. Check for excessive shaft vibra tion caused by misalignment, impeiler imbalance, cavitation or de fective bearings. area to internal pressure is referred to a3 the "m" factor. This "m" factor is actually a safety factor: he factor by which the residual com pressive force in p.s.i. on the gasket at operating conditions is greater than the pressure to be contained. Garlock design engineers, for exam ple, recommend a minimum "m" factor of three for spiral wound ented etched TFE envelope gasket has been developed to create mere friction between the gasket and flange faces. Envelope gasket styles will handle pressures up to 300 p.s.i. Of the semi-metallic gaskets, the most versatile and widely used is the spiral-wound gasket. Gariock Guar dian (Garlock trademark) spiraiwound gaskets, for example, are made from a continuous strip of Statie Sealing Static seals--gaskets--perform a simple function: that of making a pressure-tight joint between two rigid elements, usually flanges. There are three principal forces involved in the make-up of a joint that affect the functioning of a gasket: X. Compressive Load. The com pressive load is the force available for initial gasket compression and is usually applied by bolting. The force must be sufficient to squeeze the gasket into all voids in the flange face--that is. the gasket material must "flow" into hills and valleys and surface imperfections of joint faces to mate satisfactorily and achieve a tight seal with no leakage pathways. The compressive load must also com gaskets. Typical temperature and pressure limitations for various gasket ma terials are given in Table I. Rubber, both natural and syn thetic, is used most widely against cool aqueous liquids and gases at relatively low pressures. Pressure range can be extended through the use of a reinforcing fabric or wire insert, the resultant gasketing ma terial being known as cloth-inserted sheet. In extreme temperature con ditions. but at low pressures, some of the specialized synthetics such as Viton (DuPont Trademark) or sili cone may be used. Vegetable fiber is a light-service, economical gasket material more suitable for fluids such as oils than preformed metal, wound spirally from the inside to the outside with a soft filler such as asbestos or TFE between each ply. Variations in the number of metal plies, and changes in filler, in filler thickness and in wind-up tension permit spiral-wound gaskets to be used over a broad range of bolting conditions and sealing re quirements. The success or failure of any gas ket depends in large measure on the condition of the flange itself. Seat ing surfaces should be flat--free from waviness or warpage. A surface finish in the range of 125 RMS is recommended for hazardous service. For general service, satisfactory per formance can be achieved with al most any commercial flange surface. pensate for the hydrostatic end force to be encountered and maintain a residual stress on the gasket suffi cient to prevent leakage. 2. Hydrostatic End Force. When internal pressure is applied to the assembly, the hydrostatic end force tends to force the flanges apart and thus reduces the load on the gasket. The difference between the initially applied force and the hydrostatic end force is the residual gasket load. 3. Pressure at the joint tends to bypass or blow-out the gasket ma terial. The strength of the gasket material and the residual gasket load must be sufficient to contain in ternal pressure. The ratio of residual gasket load ''compressive or bolt load minus hydrostatic end load) on the gasket FIGURE 4. CORRECT BOLT TIGHTENING SEQUENCE OF GASKETS. 25to 50 fcs&i&feS^ STEAM rCOKMECTIOftd TO SEAL.HOUSING 'tS^fs-wassSS^" ,--~-- --* * r>-<-*r.' x=^--- - - -- rr*r- \ : !**/.-- ?f' -* - ' .r . .':T:.\ "'"77 FIGURE 5. HEATING ARRANGEMENTS FOR 9LACX LIQUOR PUMP MECHANICAL SEAL up to the range of 250 RMS. Concentric or even phonographic grooves are acceptable if proper seat ing loads are provided. A surface made by a planer, however, should never be used because lines machined into the surface by the planer pro vide straight-line leakage paths. Additional pointers on proper gas keting are: Tighten flange bolts in proper sequence, going back and forth be tween opposite sides of the flange (Figure 4). Since hydrostatic end pressure is exerted against exposed portions of flange faces as well as the internal diameter of the pipe, gasket inside diameter should conform closely with ID of flange * Bolt holes in gasket should be larger than bolts If flanges are wide apart, metal spacer should be used, with gaskets on each side. and chip- from working into the bearings. Gaskets in Chemical Pulping Of the various pulping operations, both the sulphite and kraft processes involve strong hot chemical solutions for the digestion of wood chips. Cir culating systems for acid towers, digesters, recovery towers, coolers, and their equipment and accessories require chemical and heat-resistant packings, as do the hot and corrosive white, black and green liquors in volved in the alkaline system. These demands are best met by either a pure aii-TFE braid, or biue asbestos with TFE impregnation, or mechanical seals. Both the pure TFE braid and the TFE-impregnated as bestos braid, because of TFE's high coefficient of thermal expansion, re quire extreme care in adjusting at start-up to avoid the possibility of almost immediate seizure. In black liquor pumps, the tem perature of the liquor should be normally kept at above 170'F. Below this temperature the liquor becomes extremely viscous and tends to tear packing apart or to destroy a me chanical seal. If packing is used, a steam tracer coil around the box will keep the liquor temperature from falling below the prescribed level. With a mechanical seai on black liquor pumps i Figure o' a steam flush is ideal. A one-quarter inch line with a needle valve is brought from any convenient 50 p.s.i. steam line to the flush connection on the mechani cal seal housing. The needle vaive is opened slightly to allow a trickle of steam to the stuffing box. thus main taining sufficient temperature to keep the black liquor around the seal in a liquid state. A steam tracer around the outside of the box can also be used. Upon shutdown, the drain-cock on the volute is opened, the needle valve is opened wide and the stuffing box is blown clear of fluid, to prevent solidification around seal and result ant damage at start-up. Two special gasketing applications should be noted: 1. TFE envelope gaskets are rec ommended for goose-neck connec tions over solid TFE or blue asbestos. BRAIDED LEAD FOIL PACKING "SUPPORT RINGS' Woedroom Applications This general background informa tion of packings and gaskets can now be related specifically to the pulp and paper industry, which involves rug ged and demanding applications. Barking drums, chippers and.other equipment in the wood room are subject to heavy thrust loads. While this has a greater effect on bearings than on seals, the bearings can be better protected by using a doub! opposed lip seal rather than a sing1*-* seai. This recommendation--well worth the extra cost--L-eeps sprnv wn'M- ...... . . `OVERSIZE" LATTICE BRAID ASBESTOS PACKING FOLLOWS SHAFT CONTOURS FIGURE < RACKING WORN SHAFT ON AGITATOR OR ROBERTS GRINDER. T1 ' "T TTT tightening in such cases causes ex trusion of and actual loss of packing into the media. Bull rings offer an effective and relatively simple solu tion to this problem (Figure 8). One of the best of the variety of firm packing materials from which they can be made is white hydraulic duck and rubber packing. The ring must be made 1/16 inch less cross-section than the packing. For example, with Vs inch round soft braided packing, bull rings should be made from 13/16 inch hydraulic packing. Reason for this size differential is that at high surface speeds, the bull ring should not be in tight contact with shaft, or it will burn and weaken. The sole function of the buil ring is to reduce With proper care, they can be re TFE packings, such as square ring3, '.he clearance at the throat and giand. used. wedge ring3 and "V" rings, have In many such installations, the use 2. Plain woven asbestos gaskets been found partially satisfactory. A of these bull rings has increased pack 1 --the same as ordinary boiler gas packing consisting of stacked TFE ing life three- or four-fold. Nor i3 the kets--are recommended for digester washers cut from inch sheet, how use of bull rings limited to jordans head covers. More efficient than puip ever. has proven much more univer and modem refiners. They art or rubber gaskets, they can be re sally successful (Figure 7). equally applicable to any equipment used many times. In stock refining equipment a pri having large throat clearances. Older mary requirement is to keep the equipment such as fan pumps, par Mechanical Pulp Service pulp out of the packing. This is ticularly if the packing life is snort, done most effectively by means of a should be checked, since -bull rings In groundwood pulping, wood par seal ring or lantern fed with clean can often result in much improved ticles and water, rather than corro water at a slightly higher pressure service. sive fluids, must be contended with. than pumpage. IN FACT. EXCESSIVE CLEAR In pocket type grinders, Chevron ANCE AT THROAT AND GLAND packings on the rod and leather cups on the pistons have proven univer sally satisfactory. With its large shafts expensive to replace, the Roberts grinder is rep resentative of the tough packing de mands encountered in pulp and paper making. The stuffing box is subjected Problem of Packing Adjustment Proper packing adjustment is of extreme importance and, particularly in the pulp and paper industry, is often made difficult by the rather high pressures and large throat clearances encountered. Excessive AND WORN SHAFTS ARE THE PRINCIPAL CAUSES OF PACK ING. FAILURE THROUGHOUT ALL AREAS OF PULP AND PA PER MAKING. Whip or deflection can drastically reduce the efficiency and life of pack ings. This can be caused by uneven to an abrasive condition, and conse quently requires packing which will compensate for considerable wear of the shaft. Here, the answer is a com bination of a very soft packing that can be made to conform to the worn shaft, supported by a firm bearing- type packing, such as braided lead foil (Figure 6). This arrangement performs satisfactorily until shaft wear exceeds Vi inch. Washing and Bleaehing In bleaching and washing opera tions, pump applications generally pose no serious packing problems. The outlet seal on the deckers and washers is best provided by multiplenested packing rings of the automa tic type, such as Chevron. Lubrica tion of rings with vaseline assists the installation and helps insure that although the rings are narrow in re lation to diameter, they are not twisted. Rings should be seated (rue and square. For chlorine and hypochlorite valve packing, several versions of FIGURE 8. USE OF BULL RINGS SOLVES PROBLEM OF PACKING LOSS INTO MEDIA. stilled and adjusted, 13 me best solu tion. Super calenders and embossing rolls in converting operations re quire high-temperature ml seals of Viton rubber. loading, which may be corrected by replacing worn bushings or bear ings; or by bent shafts, which should be replaced. Pumping Abrasive Materials In pumps handling abrasive slur ries such as day, titanium dioxide or lime slurries in the recovery oper ation. a water seal should be used for FIGURE f. USE OF SFOOl-TYFE RU** 8ft EXPANSION JOINT PROVIOIS PIFE-UNE INSUR ANCE. maximum life and efficiency. A dou ble mechanical seal is often the most suitable in such applications. Rosin pumps may require a steam tracer to keep the material fluid and prevent tearing the packing apart. If excessive leakage is encountered with agitator shafts of stock chests, it is often due to over-tightening which causes shaft wear and a re sultant unsatisfactory seal. Here again, a water seal, properly in Rubber Expansion Joints One final item which has univer sal application throughout the mill and power plant is the spool type rubber expansion joint < Figure 9). They represent "pipe-line insurance." especially on transite pipe. Where pumps must be chanced frequently, expansion joints readily pay for themselves by just the lime saved in removing one pump and replacing i: with a spare. They reduce vibra tion. relieve torsionai strum and com pensate for misalignment. In one in stance, for example, torsional strain in a long river intake line cracked the body of a 36 inch gate valve. A 36 inch rubber expansion joint-- many times less the cost of the gate valve--corrected this situation. To sum up, the savings realized through proper selection, installation and maintenance of packing and gas kets can more than offset the cost of a paper plant's entire annual pack ing purchases. APPLICATIONS AND GENERAL recommendations Application Sulphite Acid tad Waste Liquors Kraft Green Liquor White Liquor Black Liquor Chlorine. SO (Valrea) Packing R 11 omnndolien Preferred Special TFE fiber yarn, lattice braid Acceptable Mechanical seal or African blue asbestos impregnated with TFE. lattice braid Special TFE fiber yam. lattice braid Mechanical seal or African blue asbestos impregnated with TFE. lattice braid Ga>k t Preferred TFE envelope TFE envelope Reeommendotlon Acceptable African blue asbestos compressed sheet, '.rich SBR* rubber binder African blue asbestos compressed sheet, with SBR* rubber binder TFE Washers African blue asbestos impreg nated with TFE. lattice braid as end rings with plastallic centers rings of TFE powder, synthetic binder, ungraphited TFE White a-bestos compressed sheet, with SBR* rubber bioder Fresh Water White Water Long fiber white asbestos yam. white lubrication, surface treated with TFE. regular square braid Coodensate 4 Steam Long fiber white asbestos < Power House) yam lubricated and graphited. lattice braid Wet Strength Solution Dyes and Alum Long fiber white asbestos yarn, white lubrication, surface treated with TFE. regular square braid Clay Pumps Long fiber white asbestos yarn, white lubrication, surface treated with TFE. regular square braid Long fiber white asbestos yam. lubricated but un graphited White asbestos compressed sheet, with SBR* rubber or4* binder Long fiber white asbestos yarn .lubricated and graphited. regular square braid Long fiber white asbestos yarn, lubricated but un graphited White asbestos compressed sheet, with SBR* rubber or binder White asbestos compressed sheet, with SBR rubber binder Double mechanical seal or long fiber white asbestos yam. lubricated but uneraphited White asbestos compressed sheet, with SBR rubber binder SBR* rubber sheet SBR* rubber sheet SBR* rubber `beet *St*rene Butadiene ``Depends on Flanges