Document JNDGQbg9aqepZQaDeR7m2ojy6

GASK2T5 are a small -- even insignificant -- part of the first cost of process equipment. As a resuit, their true importance is frequently overlooked. Yet, leaking joints from improper gasketing can cost petroleum processors millions of dollars every year. Yester day, moderate process conditions often made gaskets a secondary consideration. Normal materials of that day were ade quate. Today, however, new proc esses and more severe operating conditions have created needs for new materials and techniques. This special 16-page report will tell you . . . 3 S;1-*"'" u -4 ht CASKET ii n< <1 vtm..................p. M li tp #f GASKETE5 imti............................. p. tl IMta twin tinnj a CASKETS........................... p. M ) ftotn in <fmct tl CASKET moiimi..........p. M nl ntitntli sr< CIIPI lor CASAETS........p. 100 t CASKET nit<ha hit............................... p. 101 nt ,* it Pnt it mttu in< nqw CASKET ............................................................... P-'t how imki CASKETS n ctniuvcite....................104 CASKET tftii*": 2 ltta, 2 nipt.....................p. I0 iptcitJ CASKET tfttijns far iptatl itnid---- p. TCI h it yit iht moil out tf s CASKET...............p. 110 a special report by RON CANNON Soutnwtittrn Editor Petroleum Processing Copyright, 1957. McGraw-Hill Publishing Co. Marcn. * A -- 80 microinch & -- 200 microinch what a C --425 microinch D -- 600 microinch E -- 800 microinch F -- Coar# pirol, or Phonographic G -- Concentric Mrrated 23 MM scale Fig. 1---Only a gasket eon seoi imper fections left in flange finish by tools A GASKET is a packing designed for inclusion between rigid pans of a fluid container in essentially sta tionary relationship. Gasketing (col lective) is material in sheet, strip, or bulk form, from which gaskets may be prepared. (Definitions from the American Society for Testing Materials.) Stated another wav. a gasket is a static seal, used where there is no relative motion between jointed pans. This definition differentiates be tween gaskets and other types ot packings that seal between moving pans. The most common use of gaskets in refineries is in pipe flanges, manway openings, and similar circular mating parts. This will be taken as a representative ex ample. although the same principles apply to all gasketed joints. The gasket creates and maintains a tight seal between separable and rigid members of a mechanical as sembly. Gasketed joints, the most commonly used, are easily assem bled and dismantled. There are three components of the gasketed assembly: 1 ) flange. 2) gasket, and 3) bolting. The flange does not require a gasket for sealing if the mating flange faces are poiished perfectly smooth and can be expected to re main in that condition throughout their lives. This precision, how-ever. is usually impractical and alwavs expensive. The inexpensive and easily reciaceable gasket substitutes for costly machining and exacting assembly. . Machine finishes of standard pipe flanges are usually concentrically serrated or spirally grooved, i Fig. I) Commercial serrations are ! 6-- in. deep, with 32 serrations/ inch. Commercial "smooth'' finishes have $0-1 00 serrations/inch, about 0.0005-in. deep. No amount of bolt loading will make a metal-to-metal seal with these imperfections in the mating pans. Gaskets provide the "give" that is needed to seal these joints. The gasket is squeezed by bolt pres- f-- leokage pressure, psi Fig. 3---There is no universal gasket moteriol (or oil types of flange finishes. A flot copper gasket (chart ot fight) it totelly unsuited far all ezeept the rela tively rough concentric finish 6, for eiomple. Corrugated steel jacketed as bestos on the other bond (chart below) worts best with fine finish A. Another finish (chart below ot right) hat more versatility. See chart above for finish designations /--leokoge pressure, psi A BOG C Ftg. 2--How flonga finish af fects gosket portormonee. At o given bolt load, gasket motet good teol on smooth flonge. has poor contact on rough flange 96 0 2000 4000 6000 Gosket stress, psi Corrugated Steel Joaceted Asbestos Spirally Wound Metal-Asbestos Gcuten Petroleum Processing. March. ;f>5~ and how it works sure on the flanges into imperfec tions in flange facts. {Fig. Z) This flow of gasket material fills the voids between mating parts and seals the joint. Complete filling of these voids is not necessary, de pending on the pressure and sur face tension of the fluyi contained. Gas. however, has no surface ten sion. so complete filling of voids is a necessity for a gas-tight joint. In addition, the pressure exerted by the gasket on the flange faces must be greater than the fluid pressure trying to leak past it. Flange finishes are important to gasket selection and greatiy influ ence how well the gasket performs its sealing functions. Gasket mate rial and flange finish must be matched to insure "wedging" ac tion necessary to squeeze gasketing injo flange irregularities. (Fig. 3) For deep serrations, or for flanges with extremely rough or pitted faces, a soft gasket might be re quired. The same purpose can be accomplished with a firmer gasket and higher bolt loading, however. The spiral, or phonographic, finishes (Fig. 4) are tough to seal because of the continuous leakage path from the fluid side of the joint. Likewise, the planed surface with parallel machine-tool marks is hard to seal because of relatively short, straight leakage paths. (Fig. 5) Some surface roughness, how ever, is desirable on flange faces to keep the gasket from "walking out" of the joint, particularly when the gasket material is a smooth, flat metal, or when flanges are like ly to become oily. Bolting provides the pressure necessary to squeeze the gasket into these imperfections. This pressure must do two things: 1) provide sufficient total pressure to seal the joint, and 2) distribute this pres sure evenly over the entire gasket contact area. (Fig. 6) Two large bolts might be sufficient total load ing for a given gasket and pressure, but the joint will probably fail be cause the gasket will not seal over its entire area of contact. A num ber of smaller bolts, properly spaced, will be needed. Pressure on the gasket is always greatest close to the bolts, and least midway be tween. There are two other methods of flowjng a gasket into imperfections of mating pans: heat and attrition. These methods do not properlv ap ply to flanged joints, but thev are wormy of mention. Heat flow of gaskets is typified by the leaded joint of bell-anaspigot cast iron piping. Further caulking of the poured joint creates additional flow by compression. At trition is a combination of com pression and a dragging action of the gasket as it rotates on sealing surfaces, filling voids by squeezing and dragging the gasket material into imperfections. Typical of this type flow is the sealing gaskets of a spark plug. Rough flange faces on this type flow are hard on the gas ket. and will tear the material if too rough. Rq. 4--A phonographic finish permits least through continuous path from fluid lid* of joint Fiq. TouqhesTfinish^o <oai it planed surface, with iti straight, short leakage paths Pig. fr-~Why bait loading is important. In four-bolt flonge most of load concentrates at bolts, there it little teoting between bolts and leakage can occur. The eight-bolt flange has better load dutribufion, een seating, and will hold design pressures - Petroleum Processing. March. 1957 97 five types of 4 -- S*if-<on{in*d S -- S*lf-*nrgi2*d 1-- Confined joint: In this type, the gasket cannot flow except for slight clearances between flanges, and it cannot blow out. Typical are the tonque-and*groove joints. A large tongue-and-groove joint is useful where a non-metallic gasket is used for a wide contact area and low bolt load. The small tongueand-groove is most often used with a metallic gasket and high bolt load. 2-- Unconfined joint: Gaskets are free to flow and can be blown out if loading is too light or pressures excessive. Probably the most widely used todav. this joint is best for moderate pressures and services. 3-- Partially confined joint: Gas kets can move in one direction only, and there is some measure of protection against blowout. Small partially confined, seldom used to day. aie primarily for metal gas kets and high bolt loads. The more common large size is used most often with soft gaskets and on frequentiy-opened closures. 4-- Self-confining joint: Most me tallic gaskets are of this type, ex cept the softer metals, which be have much like non-metals under high pressures and temperatures. Typical is the ring joint, with gas kets of various metals. 5--Self-energizing joint: Tr.is type dors not depend on boit pres sure to make the joint, except for initial seating. Pressure on the gas ket is caused by pressure ot the fluid contained. Important factor in this joint: while internal pres sure is being raised, the bolt load gradually reduces, sometimes to zero. Therefore, bolts must be .tightened as fluid pressure rises. 9 two factors in choice Resistance to mechanical forces is largely a function of the physical properties of the gasket. Each material has inherent limita tions. but these characteristics can be improved by reinforcing inserts, combinations with other materials, variations in construction, or by the type of joint it seals. Mechanical forces are important to design of the complete joint, but the primary selection of the gasketing material will be governed by two factors: 1) Temperature of fluid or gas contained. 2) Nature of fluid or gas con tained. Temperature Operating temperature will limit the choice of materials that may be used for-the gasket. Non-metallic gaskets and those incorporating rubber, or the low-melting point metals, in their construction, arc with few exceptions, limited to temperatures below 250* F. The semi-metallic group, w-hich comprises gaskets made of asbestos, with or without metal jackets, may be used at temperatures up to 850* F. Over 850* F. only gaskets of the all-metal type should be used. With very few exceptions, a material should not be used at tem peratures higher than those recom mended for its group. Fluid Contained Resistance to corrosion attack by the confined fluid or gas may limit the selection of the gasket material to the all-metal group, regardless of temperatures. For a full study of the effects of various chemicals on a gasket mate rial. the following factors must be given careful consideration. 1-- Concentration of corrosive agents may have a decided effect upon the selection of a resistant gasket material. Dilute solutions are not necessarily less corrosive than those full strength; the re verse is often true. Example: sul furic acid in concentrations over 909 may be handled with iron, yet below this concentration, rate of attack increases rapidly. 2-- Purity of corrosive agent, or the absence of contaminating com pounds. Dissolved oxygen in other wise chemically pure water can cause rapid oxidation of high tem perature steam generating equip ment. 3-- -Temperature of the corrosive agent will influence the rate of attack. This is in addition to the temperature effect on the mecr.anical properties of the gasket. 4--Location of the gasket some- 98 Petroleum Processing. Marcr.. i?57 # three forces T 0 FULLY understand how a gasket works, it is necessary to know something of the three main, definable, mechanical forces acting on the gasket, plus a host of lesser, indefinite forces. 1-- Bolt load: Caused by the ini tial pressure of the bolts, this is the force that squeezes the gasket into the face voids to form the seal. The initial seating stress in duced by this load varies with different gasket materials, and is the stress that experience has shown to be necessary to seat the gasket for any subsequent pressure. Until the pipe or vessel is subjected to internal pressure, this is the only force acting on the gasket. 2-- Hydrostatic end force: When internal pressure is applied to the vessel or pipe, this force tries to force the flanges apart. This, of course, reduces the load on the gasket. The difference between the initial load and the hydrostatic end force is the residual force on the gasket. As has already been stated, this residual force must be sufficient to exert a pressure on the flange faces greater than the pressure of the fluid contained within the pipe or vessel. 3--Internal pressure acts in one other direction on the gasket. Pres sure acting on that portion of the gasket exposed to the pressure side of the joint will tend to blow the gasket out of the joint. Resistance to this force is a function of the gasket material itself, and to a lesser degree, of the residual stress on the gasket. If the material is not suited to the pressure it contains, the gasket will rupture and blow out. # material times affects its chemical resistance. Gaskets at or above the solution level, or in partially filled lines, may be more subject to attack than those below the surface. 5--Construction of the gasket may have a decided effect on its chemical resistance. A gasket jacketed with a thin sheet of metal might fail in a service where a solid design of the same metal would have reasonable life. Mate rials abnormally subject to stresscorrosion should not be used in a construction that relies upon highly localized stressing to form the seal. The cost of materials for gaskets is not generally a factor in selec tion. However, where a closure will normally be opened qufte fre quently. a less expensive, though less resistant material mav be sat isfactory. % It is generally poor economy to use an inferior material w'here ex tra labor charges will be involved in replacing a failed gasket. Where closures are opened infrequently, the most resistant material available si. >uld be used. This may lead to a choice of a gasket made of a rare metal, such as platinum. Although the cost of this material is such that no one would seriously con sider its use except where no other material is suitable, the end result of a platinum installation might be the cheapest in the long run. This is particularly true for critical serv ices. and if the high salvage value of the gasket is taken into account. Reaction between the gasket and the fluid mnv help seal a joint if it results m a slight swelling of the gasket. Otherwise it can lead to gasket failure and fluid contamina tion. A material that shrinks or dries out can also cause failure. Other considerations may in fluence material selection, but to a lesser degree than the temperature and nature of the fluid. Operating pressure does not gen erally have a direct influence on the selection of gaskets reinforced wuh metal. By denning the bolting, the internal pressure indirectly governs the choice of gasket con struction. The strength of the gas keting material must be considered only when using a non-metallic gasket. Operating cycle, vibration, and the frequency of dis-assemoly will also affect the selection of the material. Sudden variations in op erating conditions may require a verv resilient material and design. The number of times a closure must be opened, may call for a casket that can be reused manv times. In some installations, bolt re laxation caused by thermal expan sion and creep should be consid ered. Expansion and contraction or the line or gasket and excessive bending moments on the flanges mav prevent the use of an other wise satisfactory gasket materia! .ind construction. Consideration ot all factors will usually result in narrowing possible choices down to one or two mate rials. The final selection can then he made on the basis of relates cost, jvuiiability. nr personal ex perience. On touch problems, connuA vour gasket manutacturer. Petroleum Processing. March. 1957 >9 what materials are available lor GaSKETS are made from an astonishing vanety of materials. There are countless types, classes, styles, grades, and combinations of gasket materials. This discussion, together with the table on the next two pages, will be a rundown on important materials, their uses and limita tions. It must be emphasized that this table is a compilation of general recommendations, based on in formation concerning common ma terials. Any generalization is dan gerous if applied to a specific problem without a thorough con sideration of all the variables in volved. For example, almost any material listed may be specially compounded for resistance to al most any condition. Rubber, both natural and syn thetic. is very important, (Table 1). An ideal gasket material in many ways, rubber is elastic and squeezes into joint imperfections under light boh loading. Its resili ence maintains complete face con tact over a wide range of loading, and variations in compounding makes the material suitable over a wide range of conditions. It is generally available in sheet form in thicknesses from 1/32-in. to 1/4-in. Its suitability f<jr gaskets is usually indicated in terms of its Durometer hardness. For gas kets. Duro numbers run from 35 to 95. Softer materials with a low Duro number are best for light bolt loads and rough flanges; high er Duro numbers with heavier bolt loads and smoother flange faces. Rubber is not usually considered as gasketing for oil or solvent serv ice. The synthetics, however, have greatly widened the range of rub ber gaskets; they can be com pounded for almost any service. 1'V 1 *f ,)OfaaOO;a^'JOu.o-Oa O'e--oO When using rubber gaskets in oil service, however, always consider the aniline point of the oil con tained (Fig. 7, opposite page), an index of its solvent action. Asbestos is the workhorse of the gasket w'orid. particularly where heat is involved. It is readily avail able in pressed sheets, woven cloth, reinforced sheets, and compounded with a variety of binders for specific services. White asbestos, known techni cally as chrysotile. is mostly mag nesium silicate with about I4T0 water of crystallization, which makes it a hydrated magnesium silicate. Up to 750-900 F. as bestos is relatively unaffected by heat. Above this range, water of crystallization is driven out. caus ing rapid loss of strength. At 1300 F, strength is just about gone. With little or no water as bestos is reduced to a powder. Blue asbestos, or crocidolite, is about equal or inferior to white asbestos in most respects, but is more expensive. Heat limits are about the same. Compressed sheets of asbestos fibers, compounded with one ot a variety of binders, are made by vulcanizing the compound into a homogeneous structure, under high pressure. Sheet is usually 65-759$ fiber, remainder binder. Natural and synthetic rubbers, common binders, will determine the chemi cal resistance of the gasket. Many users like a flexible gasket, one that's tough and not too brittle. The "feel" is not always a good indication of a gasket's qualities, however. Toughness and flexibility might be caused by a high per centage of binder, not enough fiber. Asbestos is provided in woven cloth, also as gasketing materia!. r i i+ iIi Because asbestos has little strength of its own. it is always reinforced, either with cotton duck or fine brass wire. Reinforcing will deter mine heat resistance. Plastics are the glamour girls among gaskets. Comparatively new. they are already stepping into the spotlight and stealing the show from some of the older performers. Complete range of usefulness is not >e: known, bu: the performances of some wnjely used plastics have given some idea of what they'll do and where they are best used. Generally speaking, the oiastics have widened the range of the nonmetaiiic gaskets. Where high tem peratures and strong cnemicais are impossible to hold with other nonmetallics. the plastics have taken over. Information on some of the .now-familiar plastics show* that there are still some limitations, but most plastics can be specifically compounded for resistance to a wide variety of chemicals and serv ices. Other non-metallics include cork, vegetable fiber, and leather. These are important gasketing materials for specific services. They are gen erally not suited to heavy-duty plant use. but are widely used in light services throughout the re finery. Combinations, such as corkand-rubber combine desirable prop erties of both componeois for special services (Fig. 8 below). Metallic gaskets are the "heavy weights". At high temperatures and pressures, metals--alone, plated, or in combination with other materials --are used almost universally. Material, as usual, will be deter mined by temperature contained (Table 21 and corrosion resistance needed, and the construction will be governed, among other things, by the oressure racing of the joint. !1 i 1 ' Q% s.'AQ Cork -- truly compressible, no lateral flow Rubber -- non-compretsible. flows laterally Fig. Compressibility properties of thr#* types of qoilet moterioit Cork-and-Rubbor -- controlled compressibility 100 Petroleum Processing. March. 1/5" Table 1--How various rubbers rate against important operating conditions General Classification Notvral Buaa S Buaa N Low Hiqh Swell Swell Neoprene Butyl Type Non-Oil ON Resistant Thiokol Type Type PR-1 ST Silicone Rubbers I Specific gravity...................................... 0.93 0.94 1.00 1.00 1.25 0.91 1.35 1.35 1.2 to 16 Tensile strength. psi: Pure gum............................................ Reinforced .......................................... Tear resistance ...................................... Abrasion resistance ............................. Aging: 3000 4500 Excellent Excellent 400 3000 Poor-fair Good 600 4000 Fair Good 600 4000 Fair Good 3500 3500 Good Excellent 3000 3000 Good Fair 300 1500 Fair Poor 300 1500 Fair Poor 200 to 50 __ Poor-fair Poor Sunlight .............................................. Oxidation ............................................ Heat, max temp *F........................... Static (in storage)............................. Flex craciting resistance: Poor Good 300 Good Poor Fair 250 Good Poor Fair 300 Good Poor Fair 300 Good Excellent Good 300 Very good Excellent Good 300 Good Good Good 160 Fair Good Good 160 Fair Good Very good 450 Good Slow rate ............................................ Fast rate ............................................ Compression set resistance.................. Solvent resistance: Excellent Excellent Good Good Poor Good Good Good Excellent Poor Poor Excellent Very good Very good Poor Excellent Excellent Fair Fair Poor Poor Fair Poor Poor Fair Poor Good Aliphatic hydrocarbon .................. Aromatic hydrocarbon .................. Oxygenated solvent ......................... Halogenated solvent......................... Oil resistance: Very poor Very Poor Good Very poor Very poor Very poor Good Very poor Excellent Good Good Fair Poor Poor Very poor Very poor Fair Poor Fair Very poor Poor Very poor Good Poor Excellent Good Fair Poor Excellent Good Fair Poor Poor Very poor Poor Very poor Low aniline........................................ Very poor Very poor Excellent High aniline ...................................... Very poor Very poor Fair Gasoline resistance: Fair Excellent Fair Good Very poor Excellent Excellent Poor Very poor Excellent Excellent Good Aromatic ............................................ Very poor Very poor Poor Non-aromauc .................................... Very poor Very poor Fair Acid resistance: Good Poor Excellent Good Very poor Excellent Excellent Poor Very poor Excellent Excellent Good Dilute (under 10%)......................... Good Cone, (except nitric A sulf).......... Fair Good Poor Good Poor Good Poor Fair Fair Good Fair Poor Poor Fair Very poor Very poor Poor Low temp resistance, max *F............. --65 --70 --65 --65 --50 --65 --SO --65 --120 Permeability to gases............................. Fair Fair Fair Fair Very good - Very good Good Good ' '` Fair Water resistance.................................... Good Very good Very good Very good Poor Very good Fair Fair Fair Alkali resistance: Dilute (under 10%)......................... Good Concentrated...................................... Fair Good Fair Good Fair Good Fair Good Good Very good Poor Very good Poor Poor Poor Fair Poor Resilience ............................................... Very good Fair Fair Fair Very good Very poor Poor Poor Good Elongauon, max %............................... 700 500 500 500 500 700 . 400 400 300 Swelling of blackloaded neoprene in oils of various aniline points. Oil Ne. 6 9 4 11 7 8 Type of Oil SAE 30 motor SAE 30 motor SAE 30 motor SAE 30 motor SAE 30 motor SAE 30 motor Aniline p. *C. 77.7 107.0 109.0 109.5 110.8 119.8 Table 2 -- Maximum service temperatures of gasket mate rials in oxidizing atmosphere (Temperature shown may be raised or lowered by operating conditions. These values are for general reference only) Material Tin........................................... Lead . Zinc .... Magnesium . Admiralty brass . *F. ... 212 . 212 . . 400 . . 500 High brass.................................. Copper......................................... Everdur . Aluminum Stainless steel. 304 . ... .. . 500 600 600 800 800 Stainless steel. 316 Rems iron .... Armco iron . . . Low carbon steel Silver . . ... . . . . 800 ... . .. 1000 . 1000 . 1000 1200 Gold................. Chrome moly steel. 5Q2 Chrome steel. 410 . Nickel . Monel . 1200 1200 1300 . U00 1500 Stainless steel, 347 Inconel Hastelloy. A or B Platinum Tantalum . . 1700 2000 2000 2300 3000 '*14-; point Check List You'll need this information in choosing a gasket: Fluid or gas to be sealed. If a chemical or corrosive, concentra tion must be known. Q Pressure to be sealed, both operat ing and test 0 Operating temperature range and cycle time. Type of flange. 0 Nomina! flange size and material. 0 Actual flange dimension---ID. OD, and thickness. 0 Type of flange facing. .0 Contact surface dimensions. 0 Condition of contact surface. 0 Contact surface finish. 0 Number and size of bolts, and bolt circle diameter. 0 Bolt material. 0 Limitations on gasket thickness. 0 Number of times joint must be opeaed. Petroleum Processing. March. 1957 101 waai you neeu 10 Know M'i/ j:uhh ELASTOMERS Natural Tough, resilient. Duro hardness up to 80. Imper vious. Not compressible. Buna-S Buna-N Neoprene Butyl Thiokol Silicone Better water resistance than natural. Duro up to 90. Good shear strength. Abra sion resistance. Not com pressible. Duro. 40-90. depends oncure. Resistance to aorasion. heat, compression set. High tensile strength. Nonndhesive. Duro 40-80. Resistant to abrasion ft tear. Incompressioie. Cannot vulcanize with sulfur. Impervious. Good aging qualities. Good resilience at high temperatures, poor at room temperatures. Duro 50-90. Best solvent resistance any rubber. Good low temp properties. Low swell, shrinkage. Cannot be vulcanized with sulfur. Resilient at high and low temp. Oxidation and weath ering resistant. Good heat stability. For hot or cold water and water sol'ns. low pressure steam and gas. some dilute acids and alkalis. General purpose for moist type joints: water, dilute acids and alkalis. Hot and cold water, steam, dilute acids and aixahs. gasoline, oil. aromatic and aliphatic solvents. Hot or cold water, gas. non - aromatics, oils itn aniline pt over 225. some solvents, where sulf. cor rosion likely. Water, steam, gas. alkalis, dilute acids. Excellent re sistance to gas diffusion. Hydrocarbons, esters, ethers, ketones, solvents, water. & where solvent re sistance is second to phys. properties. Good for ta rings. For joints where other rub bers are used, but at higher temp. Also for oils with high aniline point. -- 65 to -250 -- 70 to -250 Low pressure only, except in con fined joints. Not for strong acics. Never for oil or petroleum deriva tives. L'nsuiced for gasoline, cii, solvents, concentrated acids. -- 65 to -- 300 -- 50 to -250 Net tor oxygenated or -.a-ozenateo soi'-ents. Poor resistance to .tg.-.L Not for aromatic gasolines. Low ani line pt Otis cause sweumg. -- 65 to -- 250 Very poor against oiis. solvents, gaso line. -- 65 to -212 Not for joints with high bolt loads. Not good against halogenaied sol vents. -100 to -450 Attacked by low aniline pt oils. Not for such solvents as gasoline, ben zene. Relatively low tensile strength. Not tor steam at high pressures. ?1AS71CS Teflon Kel-F Polyethylene Phenolic resins Good heat resistance. Very inert to chemicals. Incom pressible. Non-resilient. Non-adhesive. Resistant to chemicals, sol vents. weatnering. Low cold flow. Resilient. Stays flex ible at low temperatures. Is non-flammable. Duro *5-95. depends on compounding. Resilience comparand to nail ruboer. Stays flexible at low temp. Strong and solid. Hard and impervious. Best used in combination with other materials that give resilience to gasket. For resistance to chemical attack. Unplasticized form has extreme cnemical in ertness. For chemical resistance and low temperature service. Combination gasket and insulation (electric) to re strict flow of stray currents. -- 90 to -500 -- 320 to -390 funpiast. form > -- R0 to -250 Some cold flow when first loaded. Decomposes to gas at 750 F. Plasticized form show* some shrink age i*uh some nydrocarocns. solvents, ana hot water. Not for glacial acetic acid, aniline (100^). formaidenvoe. rtnrocenzene. or HC1 at hign temperatures. Generally restricted to use m insuU!me rtanees tor control of eiectroiy^c corrosion. CORKS Cork compositions Soft, resilient, and com pressible. High friction properties. Cork-ondrubber Chemical resistance vanes with rubber. Duro 25-90. High friction. Controlled compressibility. For light duty at low bolt loads. Resists oil and aromane solvents. In confined mints for metal contact of flanges and with low bolt loads. In uncon fined joints tor light duty only. :2 to 160 0 to 250 Not for sustained contact wuh waier. Not for alkalis and corrosive acids. Moderate pressure onlv. Snrinxs and hnroens m some services. Oise polymer governs resistance. Not lor sieam or huh iemp. Hard and unyielding below u F. 102 Petroleum Processing. Marcn. 1957 White asbestos Blue asbestos Soft A pliable in woven cloth. Tougn A duraole in compressed sheets. Rela tively non-compresstble. About equal or inferior to white asbestos in physical properties. Good all-purpose for rela tively high temp. For weak acids, alkalis, oil. solvents, steam, and hot water. Best for- use against acids. to "50 to 750 Binder or saturant governs resistance and temp range. Generally not icr strong mineral acids. More costly than white asoestos. Temp ranee depends on btneer or reiniorcement. Vegetable fiber Firm A homogeneous. High tensile strength. For seai and lube otls. water, non-corrosives. For It or hvy bolt loads and smooth uniform flanges. to 2: I Generally for low press and moderate temp. Saturant governs soiven: A o:i resistance. Some tvpes tend to harder. A shrink with alternate wet a drying. leather Tough, porous, resistant to aorasion. Flexioie at low temperatures. 'AE7ALS Lead Excellent corrosion resist ance. Tin Aluminum Copper, brass High corrosion resistance. Oxide film forms protec tive coating. Excellent corrosion resist ance. Monel High resistance to corro sion at all temperatures. Nickel Iron, steel Chrome steel Somewhat lower all-around corrosion resistance than Monel. SAE 1010 to 1020. Type 502 (4-6 Cr. 5 Mo) Type 410 (ll-U Cr) Stainless steel Type 504 (1S-8) Type 347 (l8-8.Cb stabilized) Type 516 MS-3. witn Moi. For static seal in piasnfaced flanges at low press. onlv. In confined joint for higher press. Best for dy namic seai. -- 70s -::o PermtaOilitv depends on tannine and imoreenation. Never use *nn steam. acids, or alkalis. Quahtv is r.izniv variable. Widely used for sulfuric acid and other corrosives at temperatures under 212F. For pure neutral solutions. Used for corrosion resist ance. Good against hot. sulfur-bearing gases. Good general-purpose gas ket for corrosion resistance at moderate temperatures. Versatile material, for most corrosives, acids, alkalis, and for steam and other high temperature applica tions. Good against chlorine up to 950 F. to 212 (400 in a confined jt) Excessive creep at higher tempera tures. to 200 Strongly attacked by acids and alkalis. to 800 to 600 to 1500 to UOO Slightly attacked by strong acids and alkalis, but this is not generally objectionable. Attacked by oxidizing acids, wet NH-. chlorine, sulfur. Embrittlement a: high temp when exposed to H-. CO. unless metal is deoxidized. Electroly sis sometimes oojectionaole. Attacked by strongly oxidising acids A strong hydrochloric. Embrittlement in sulfur-bearing gases aooie 500F. Stress corr. likeiv min steam aoo'e 800 F. Subiect to embrittlement tv steam over 800 F. For joints with low gasket stress. For strong acics and most alkalis. For resistance to oxidation: Good against steam. Wide ly used m ring type joints. Special use for resistance to oxidation at nigh temperature. Most widelv used gasket for corrosive service. For extremely high temp. For cases where additional corrosion resistance is re quired. to 1000 to 1200 to 1300 Strong attack bv sulfur-bearing gases over n00 F. Not for crude ;c:cs high gasxet stress causes corrosion. Generally more corrosion resistant than iron. Special purpose material only. to 800 Attacked by sulfuric acid A wet halo gens. Subject to intergranular corro sion alter long service H- 800-15CG F. to 1700 :o 300 Corrosion resistance is not raised ov suoilization. 1 Sunect to intercranutar corrosion j Srtu-tjOOF. Petroleum Processing. March. 1957 how metal are constructed PLAIN ALL-METAL TYPE PLAIN SOLID; nigh mechanical strength. good heat conductivity. Thickness must be uniform and sur face smooth. Good for flanges with concentric serrations. Pay close at tention to available load in flange bolts. Hardness ot material must suit flanges, too. PROFILE: macntned with concentric serrations to reduce bolt load neeaed. Concentric contact lines give many sealing surfaces. By varying profile pitch, gasket loading can match bolt design. Used mostly tor high pres sures A temperatures in narrow-tace closures. PROFILED-CLAD design has the profile jacketed with a separate metal cover. Jacket is usually a different metal to provide corrosion resistance for the inner gasket. Seating stress is the same for ail profile gaskets, jacketed or not. See also next design shown below. PROFILED-CLAD: a variation in the design above. Here, a heavy, solid metal with concentric V-shaped grooves is partially enclosed by a single metal shell covering one face, both edges, and a portion of the other face. In the preceding design, jacket covers all but the outer edge. SERRATED type: heavy solid metal machined with spaced concentric ruos. Use where wide gasket wuh re duced contact area is needed, espe cially high pressure A temperature with standard raised face flanges (smooth finish!. When made of lead or lead alloys, gasket is usually cast. ALL-METAL RING TYPE ROUND OR OVAL cross-sectional design for ring-type gaskets are very popular. They are normally made of steel. Monel, chrome-moty, or stain less steels, depending on conditions. BELLOWSEAL: This design is mace up of two metal plates, each provided with spaced concentric nos on me:r outer faces, and welded together around me outer peripnery. a mm. compressed asoestos sheet is oiacsd over botn serrated faces cur.-.g in stallation. S _AAAf MULTISEAL: Essentially same oasic design as that of bellowseai. but having raised cross ribs as wil. These gaskets are ftexioie enough to ailow bellows action during sudden temperature changes. Inside pressure forces gasket halves against flange faces. CORRUGATED gaskets follow sim ple design shown, are widely usea on machined flanges for seating steam, water, gas. 01L chemicals, under 1000 psi. Corrugations can be increased or decreased as needed. This design is good for narrow-faced joints, and for tongue-and-groove joiner S CORRUGATED gasket variation in which the deeply corrugated metaj core has been partly enclosed with a metal shell that covers one face, both edges, and a portion of the other face. In another variation, the duplex type, two gaskets are joined at the inner lap. CORRUGATED METAL CORE de sign is actually three gaskets, won me bottom one lapped over the outer two at both edges. This provides a more resilient construction mat is useful for high temperatures ana for temperature cycling problems. TRANSITION RING >s a turner variation for ring-type joints wnere there axe Jifferent ring groove diam eters in maung flanges. OCTAGONAL cross-sectional design is another variation for ring-type gas kets. Depending on material of con struction. these rings are good for pressures up 10 If.000 psi and tem peratures up to 1600F. CONVEX cross-sectional deign is still another variation from me oasic idea m rtng-iype gaskets. When cor rosive operaung conditions require it. rings can be plated for additional protection. RING CASKET PROTECTOR. In stead of plating the ring tor corrosion protection, it can be used nr, a seoaraie proiecior ring of a s-utaoie material. The design shown r.as a roiled inner edge. RrNG CASKET PROTECTOR vari ation using a U-shaped inner edie instead of a rolled inner edge. These protective devices also are designed to reduce turbulence. 104 Petroleum Processing. Marcn. :95 The 32 DRAWINGS and description* on these pages show most of the many forms of construction for metai gaskets. Three classes are shown--all-metal, all-metal ring-type, and combinations. AU-metal gasket design is both efficient and eco nomical. particularly on flanges with concentric serra tions. However, it is the most difficult to seal. Atten tion must be paid to flange bolt loading. All-metal ring types are the tough guys tor the toucn jobs. They seal under relatively light bolt loads, com pensate for minor pipe misalignment, and hold tight under varying conditions. Combinations extend the useful range of metal and non-mctallic materials, frequently permitting the use of a soft filler at higher temperatures and pressures, with added corrosion protection. COMBINATION TYPE SPIRAL WOUND gaskets are the most familiar and suecessiul example of combination gaskets. They are made of a continuous strip of pre formed metal. wound spirally from inside to outside, with a filler cushion between each ply. The cross section of the metal strip is sues that com pressing the gasket results in a spring loading effect on the metal Sketch A is basic design. B is the basic de sign plus a patented centering guide for accurately positioning on raisedface flanges. C shows tne basic de sign provided with a combination compression limiting gauge and cen tering guide. In tnese gaskets---beside the primary seal by the metal---the filler compresses under bolt load, filling minute imperfections in the flange face. Spring action permits low seating stress and light bolt loads. If not over-stressed, the gasket has a built-in resilience permitting re-use many times. Filler is usually asbestos, but Teflon is commonly used for corrosive services. FULLY ENCLOSED type of combi nation gaskets come m several shapes. They are most often used wnere tne !Oint ;$ to be oroken freauentty ana the casket ts to os re-used. The cushion ing effect of the soft filler permits proper sealing wmle the full raetaj' Jacket stands up under higher pres sures and corrosive chemicals. With asbestos as the filler, this construction can be used at temperatures us to *50'F. The sketch at G is the doublejacketed design in which the filler is enclosed by a metal shell and a top wasner. At H the filler is completely enclosed by a single metal shell wmch overlaps at one face. At I is a varia tion in the design shown at H in which the bottom face and thc meul washer are corrugated. CORRUGATED METAL - ASBES TOS combination gaskets have twist ed asbestos cemented into corruga tions on both sides of the gasket. When these corrugations are flattened under pressure of bolts, the asbestos is held firmly against the flanges. This thick gasket seals well on poorly aligned work and rough flange faces, with light bolt loads. Like the spiral-wound design, these gaskets can aiso be obtained with various types of centering devices. In addition to centering, however, the outside centering ring prevents lateral expansion of the gasket during com pression. Some types act as a com pression gage: when flanges touch the ring, gasket is stressea proper amount for good seal. Sketch D shows deeply corrugated core covered with woven asbestos cioth. E uses twisted and treated asbestos cord cemented into pUce. F is about the same but design includes a reinforc ing inner iac. H PARTLY ENCLOSED or singlejacketed combination gaskets have a soft filler and are partly enclosed-- one face, both edges, and a portion of the other face. Used mostlv for cylinder heads, and for narrow-taced. small diameter iovnts. or where easy compressions is a requirement. EDGE-REINFORCED type of com bination gasket uses a soft material reinforced by metal nms on both edges: sides are open. In mis and the preceding design, choice of metal will determine corrosion resistance of gasket, and filler will govern tempera ture limns, FRENCH type combination gascets also come tn a great variety of de signs. Basically, the soft filler is pro tected by a metai tacket over all but the outer edge, as mown in sketch J. This metai jacket can ee one-piece as .it J. two-piece as at K. or three-piece as illustrated at L. A further varia tion in the French tvpe is shown a: '1. Here, a imck woven asoestos niter has a one-piece metal jacket covering me inner edge ana an equal portion of. but not all. of both contact faces. This design is for glass-lined or otner lightly bolted flanges. M 53 INSIDE-OPEN TYPE is nothin? more than a switch m the French type. The soft niter as a single metai edge open. Petroleum Processing. March. 1957 mu iUULUio iii and two basic STARTING from scratch, the de signer can adapt flange construction to any desirable gasket design. Selection of a gasket for existing equipment, however, limits gasket design because the joint features are already fixed and cannot be changed as easily as the gasket. The approach the designer uses is differ ent in each case; the principles are the same. Gasket Yield Stress A tight seal is obtained by the flow ot the gasket material into the imperfections of the contact faces, filling all voids between mating flanges and creating an unbroken barrier to leakage from the joint. Unless this flow takes place, a tight seal is not possible without using sealing pastes or compounds. The amount of force that must be applied to the contact area of the gasket to cause this flow is called the "yield" force. The yield stress y, is the unit stress, in psi. which is required to cause flow. The term "yield" is misleading and has no relation to the "yield strength" as ordinarily used for materials tested under tension or compression. The yield stress is independent of the internal pres sure--it is the minimum stress that must be applied if the gasket is to seal at all. even very low pressures. Certain gasket materials--such as soft rubber--and a few gasket designs require so little stress to effect a seal that they are con sidered to be self-yielding. The majority of gasket materials and designs, however, do require a rel atively high initial stress if the gasket is to seat properly. Usually, after the application of this high yield stress, the load on the gasket may be reduced considerably with out loss in the efficiency of the joint. In practice, this reduction of toad is often the result of bolt expansion', creep, and gasket set.- The influence of yield stress upon efficiency of gasket perform ance is'emphasized by the follow ing example from actual test data: A corrugated iron-jackeied-as- bestos gasket was installed in an assembly and loaded with a very low gasket stress of 500 psi. This gasket was not stressed to its yield point and it leaked at a hydrostatic test pressure of less than 10 psi. A duplicate gasket was then placed in the assembly and yielded first at a gasket stress of 1500 psi. This stress was held for a short time and then reduced to 500 psi. same pressure as the initial load in the first test. Hydrostatic pressure was applied and the gasket remained perifectly tight until a pressure of -20 psi was reached. The first gas ket. installed with insufficient ini tial loading would not hold 10 psi; the secood gasket, properly seated and reduced to a bolt load of 500 psi. was 4000% more effective. ] j 1 ] ' j ! Gasket Factor ' When internal pressure is applied to the gasketed assembly, the hydrostatic end force tends to sep arate the flanges. This lowers the total load on the gasket, hence decreases the unit stresses induced by initial loading. If the resulting gasket stress falls below a certain value, leakage will occur. The abili ty of a gasket to maintain a seal at reduced stresses is a function of the resilience of the design and is directly affected by the internal pressure it contains. The ratio of the residual unit stress on the gasket (in psi) to the internal pressure of the system (in psi) is called the "gasket factor" >n. and is constant throughout a wide range of pressure for a given gasket material and construction. Gasket factors for all common ma terials and constructions are avail able and are values that have been proved satisfactory in actual service. A gasket factor of 2.0. for example, indicates that the residual stress of the gasket must be a min imum of twice the internal pressure of the system if a seal is to be main tained. 1 j 1 1 i i j j j j ] j T HE PHYSICAL characteristics of the gasket selected--yield stress and the gasket factor--determines the amount of force that will be required for a tight seal. Step 1--For an existing flange, the designer must first determine the total force available: 1--to yield the gasket, and 2--to main tain a tight seal under service con ditions. The total tores, usuaiiv surptitc ny boits. w;i) depend on tne size and numoer of bolts in the flange, and by the maximum allowable bolt stress. The maximum bolt stress uii[. in turn, depend on the bolt material and the operating temperature. Mild carbon steel bolts are usually limited to services below 450 F and to a stress of 10.000 psi. Alloy bolts may be stressed somewhat higher, depend ing on the composition and operat ing temperature. Generally accepted allowable stress for alloy bolts is 20.000 psi. for purposes of gasket selection. The total available bolt force can be determined by a simple calculation of the total bolt area times the allowable stress. Step 2--should be an approxima tion of the gasket dimensions. For some confined-joint facings, gasket dimensions are limited by the re cessed face dimensions. With other facings, considerable leeway is available. A full-face joint may be provided with a full-face gasket to reduce flange distortion. Often, particularly when metallic or sem.;metallic gaskets must be used, the bolt torce available may not be enough to allow tne use of full-face gaskets. In these installations tne width or the gaskets must be re duced to term a ring gasket, where available bolt force will give the proper seating stress. Before metal gaskets came into general use. a wide contact flange was considered necessary to pro vide suriace friction and reinforce ment to (he son gasket materials, and thus prevent blow-outs at h:gh pressures. With most metallic gas- 106 pEntOLELM Processing. Nla.-cr.. steps kets. however, the required gasket stresses will be obtained only by using a gasket considerably nar rower than the contact faces of the flange. Knowing the total bolt force available, and having worked out a rough approximation of the gasket dimensions, it is possible to find y and m. the seating stress and the gasket factor at the service pres sure. By pairing these findings with a previous selection of materials based on operating conditions, the proper gasket may be selected from a table of gasket factors and yield stresses (Tables 3 and 4). From the "ASME Code for Un fired Pressure Vessels", the re quired initial bolt load shall be at least sufficient to seat the gasket so as to assure a tight joint: Ww., --3.14 bGy. And, the required operating bolt load shall be suffi cient. under maximum operating conditions, to resist the hydrostatic eod force and, in addition, to main tain a compression load on the gasket that experience has shown to be sufficient to assure a tight joint: 'm|0.785 G-P 4- (2b X 3.14 Gmp) Nomenclature for the two formu las is: required bolt load for max imum operating conditions, lbs. Wmt-- required initial bolt load without internal pressure, lbs. G = diameter at gasket load re action. P -- maximum allowable working pressure, psi. b = effective gasket seating width, inches. 2i = effective gasket pressure width, inches. m = gasket factor, y = gasket seating load. psi. There are several variations and simplifications of these formulas. Important point is that all pub lished casket constants are not specifically adaptable to these form ulas.. Although the same principles appiy. the other constants are ac curate. they are tailored to partic ular formulas. Petroleum Processing. March. 1957 Material m Rubber without fabric or a high %age of asbestos fiber: Below 75 Shore Ourometer 0.50 75 or higher Shore Ourometer 1.00 Asbestos with a suitable binder for operating conditions: 1 /8 in. thick . 2.00 1/16 in. thick ................... 2.75 1/32 in. thick. . 3.50 Rubber with cotton fabric insen 1.25 Rubber with asbestos fabric insertion, with or without wire reinforcement: 3-ply.......................................................... 2.25 2-ply................................... 2.50 1-ply . . . Vegetable fiber .... 2.75 1.75 Spiral-wound metal, asb. filled: carbon sicel . . .. 2.50 stainless steel . 3.00 Serrated steel, asbestos filled 2.75 Corrugated metal, asb. inserted or jacketed asbestos filled: Soft aluminum . . ... 2.50 Soft copper or brass 2.75 Iron or soft steel. 3.00 Monel or 4-6 chrome . . . . 3.25 Stainless steels.................... 3.50 Corrugated metai: Soft aluminum . 2.75 Soft copper or brass . 3.00 Iron or soft steel 3.25 Monel or 4-6 chrome 3,50 Stainless steels 3.75 Flat metal jacketed, asb. filled: Soft aluminum : ............... 3.25 Soft copper or brass..............................3.50 Iron or soft steel.................................. 3.75 Monel ....................................................... 3.50 4-6 Chrome......................................... 3.75 Stainless steels..................................... 3.75 Grooved iron or soft steel with or without metal jacket: Soft aluminum . ............................. 3.25 Soft copper or brass............................. 3.50 Iron or soft steel ............................... 3.75 Monel or 4-6 chrome.......................... 4.00 Stainless steels....................................... 4.25 Solid fiat metal: Soft aluminum ..................................... 4.00 Soft copper or brass.......................... 4.75 Iron or soft steel ............................. 5.50 Monel or 4-6 chrome...........................6.00 Stainless steels.. ...................... 6.50 Ring joint: Iron or soft steel................................. 5.50 Monel or 4-6 chrome ........................... 6.00 Stainless steels................................... 6.50 y 0 200 1600 3700 6500 400 In Table 4. use: Faciag Limits 1,4.6 only 1, 4. 6 only I. 4. 6 only 1. 4, 6 only l. 4. 6 oniy none 1! II II II II II 2200 2900 3700 1100 2900 4500 3700 2900 3700 4500 5500 6500 3700 4500 5500 6500 7600 5500 6500 7600 8000 9000 9000 none none none I. 4. 6 la only la only la only It II II II II n n la only la oniy la only la oniy la only ii n n ii H la only. la only la only la oniy la only n ii ii ii n la. 2* only la. 2* only ia. 2* only la. 2* only la. 2* only la. 2* only ii ii ii ii 11 II 5500 6500 7600 8800 10100 8800 13000 18000 21800 26000 18000 21800 26000 1,2. 3 only 1,2.3 only 1.2. 3 oniy 1.2. 3. only 1.2, 3 only none none none none ' nose 8 only 8 only 8 only II 11 II II II I I I I I I I I Iwlef Unlit* (ea*ff ewt*e) CD 1o 1 , at lb "N-* _4-w Table 4--Effective Gasket Width b Uw GaUiet SMting WISMi fc. Celvmn 1 Celwmn II EHecne Gauei w^tfi 'b' bs*,. wk n. i 'U" NN :T ** b. V locenon el GeUti teoti teoebon W --T / W --N \ *2 ' \ m / fW 2--T \ W4-N \) W --N *4 W --JN 8 2 r^N^ T`(t-) 4 5 -N-* ~ET ~E=T rczrfzz- --p w A 8W 3n 8 H 4 w 7n id DN 8 NOTE: The qoilbi tadOA utlee omt aop'r io i'oqee po.nn w*<n ik* qouei a teroi<ie eniire'r w.ia.a ih iAAr eeget el <r, oe<> heie (ilreciM I 956 *4.ne* $<:.. VIII. Uelite* ft > Ve,,el,. *>.< Seoeiv * *oc*eAi<oi 29 Wei 19ih Sei. New leu IS, N. T. el ie e*i*>o. Radial Sai_us* whtrt parts to b* s*al*d might n**d adjustment, lik* turning pip* in 0>Ring Fig. 10--0*rings eon bt either radial or axial tools Axial Seal -- Ceunt*rbor* (loft) far internal protturos only: groove (right) for vacuum and pressure The development of various syn thetic rubbers and resilient plastics that withstand oil and chemical attack have made possible the wide application of the O-ring seal for both static and dynamic seals. An O-ring assembly consists es sentially of a molded doughnut of rubber (or other resilient material) placed in a groove, so constructed that the ring cross-section is slightly deformed when the mating parts are brought into contact. Sealing action is caused by internal pres sure that deforms the ring into leakage paths, assuring a tight seal (Fig. 9). The higher the pressure, the tighter the seal. * O-rings can be either radial seals or axial seals (Fig. 10). The radial type seal is best used where adjust ment or positioning of metal parts independent of the seal is required. Axial type can be installed in a groove cut into the flange face or in a counterbore. For an axial type seal, bolts should be stressed only enough to give metal-to-metal contact of mat ing parts, and to prevent flanges from parting under pressure. Metalto-metal contact allows the use of O-rings at high pressures with low bolt loads, even when subjected to vibrations and temperature changes. The Durometer hardness of the ring has a definite relationship to extrusion under pressure. At higher pressures and greater clearances between mating pans, the harder materials should be used to resist extrusion. (Table 5) A rectangular-section (Fig. 11) groove is usually best for most static seals, although the dove-tail (Fig. 12) has the advantage of holding the ring in position while the joint is being made up. What- Table 5--Maximum recom mended radial clearances, Inches ps) 0-250 250-500 500-1000 1000-1500 1500-2000 2000-3000 3300-5000 70 D*r* 0.010 0.008 0.005 0.003 80 Our* 0 010 0.010 0.008 0.005 0.004 0.003 SO Dura 0.010 0.010 0.010 0.008 0.005 0.004 0.003 ever the groove design, be sure to provide smooth surfaces, chamfered edges, and to remove all burrs or tool marks that might cut or tear the ring. Superpressures, say above 3000 psi. demand gaskets of special con struction and there are several such designs in common use today. They 108 Petroleum Processing. March. 1957 lor special services Rg. 11---Recttffigular-sectioa groov* it utualiy bait, bgt . . . Mean Diom. of O-Ring Fig 13--'ridqmaii joint, o s!f*n*r> giiing typ of tool Brook Corner* to Within limit* Specified by "A" Dimensions----- Fig. 12--Dove-toil hold* ring in position during joining Fig. 14---Leas-ring joint is o relatively simpio *eoi Rg. IS--`Delta joint hot *ide adopta bility os 0 tool are generally considered to be self* sealing. All of the several types are based on the same principle. While pressure governs gasket con struction. it influences the choice 0/ gasket material only to the extent of defining the bolting, which is primarily designed to withstand the pressure of the joint. The key re quirements of temperature and fluid still determine the choice of gasket material. Bridgman joint (Fig 13) uses a self-energizing type of seal for large openings in heavy-wall vessels. The gasket is seated initially when the floating head is pulled up by the small loading bolts, then, internal pressure takes over. Any ductile metal can be used for (he gasket, depending on the fluid contained and its temperature. This type joint requires careful machining and careful assembly, but is an excellent joint for very high pres sure operating conditions. Lens type joint (Fig. 14) is a simple seal, using basically a lens shaped cross-section seated in a conical flange surface. Initial seat ing should be a small surface as close to the inner edge of the joint as possible: the small seating area allowing high unit stress with low bolt loads. Casket can be harder or softer than flanges--some users prefer a gasket with a Brinneii hardness 30-40 numbers lower than flanges, others choose a harder material. Delta Joint (Fig. 15) has a gas ket with a triangular cross-section. It is based on the unsupported-area principle, and sealing action resuits from deformation of the gasket by internal pressure into flange voids. It is in wide commercial use at pressures up to 30.000 psi and its economical design makes it easily adaptable to any other existing joint. Materials are generally soft iron, aluminum, copper, or stain less steel. There are other equally success ful designs for high pressures, and many variations of construction, but the principles used are basically the same. All high-pressure joints require careful machining, smooth surfaces, clean seating surfaces, and proper application. Careful installa tion. important to any gasket, is even more imperative at extremely high pressures. A common mis conception is that some of the special designs compensate for mis alignment of flange faces. In prac tice. cocked flanges result in uaeven bolting pressures and sub sequent leakage. Further tightening usually results in full face contact of the gasket and the benefit of the unsupported area effect is lost. There is no substitute for good alignment and proper assembly. Petooleum Processing. March. 1957 109 how to get the most out of a ONE HANDY GUIDE to the sealing characteristics of a gasket is its compressibility, which varies with material, construction, thick ness. and temperature. For good sealing, you have to know what load to place on the gasket. Gas kets must be properly loaded, but if bolting won't take increased load, the answer to the problem may lie in another gasket material. Gasket Pastes A paste of graphite and water is often used to prevent the gasket from sticking to flange faces. Oil and graphite is not recommended because the oil in the paste will attack any rubber in the gasket material. Sealing pastes stir up arguments. Some engineers won't allow them-- others won't have a joint made without them. While pastes often help to create the intital seal, their use throws out all the factors enter ing into careful gasket design. Proper selection of gasket material and construction will, however, make sealing pastes unnecessary; permanent remedies for poor mate rials and assemblies are impossible with any paste. Pastes may be classified roughly as 1) soft setting and 2) hard set ting. First group includes com pounds based on linseed or castor oil. soaps, asphalt, rosin, or shellac. They may be of some aid in seal ing a joint with a non-rubber gas ket. The hard-setting types usually expand when set. bringing more pressure on gaskets and helping to build up pitted surfaces, but they reprints of this Specioi Report are qvailable at $0.50 each. Either circle R43 on the Reader's Service Card in this issue or send order to: Readers' Service Department PETROLEUM PROCESSING 330 West 42nd Street, New York 36, New York 110 have a tendency to crumble as the joint contracts and expands under pressure and temperature, ruining the seal. Weight of informed opinion says that gasket sealing pastes are not needed if the gasket is right lor the job. Right sequence Fig. Ifr--Always tighten bolts properly Nine Practical Tips For Making the Joint 1-- Make flanges as parallel as possible. Cocked flanges can some times be made parallel with heat, or by slightly bending the pipe. When truing flanges, make sure that no stresses are left in the joint, particularly in machinery piping. Flanges should be practically mat ing without bolts. 2-- Warped or badly corroded flanges should be replaced or re faced. Flanges with coarse tool marks or scratches that form leak age paths should be refaced or machined with concentric grooves to interrupt the leakage path. 3-- Where flanges are badly out of parallel or widely separated and cannot be bolted up. a spacer can be improvised to fill the space. Use a gasket on both sides of the spacer. -1--Remove all burrs, rust, and dirt from flange faces. This is probably the most overlooked step in proper gasket application. Flange scrapers and wire brushes do the best job. 5-- Use the thinnest gasket pos sible that will do the job. 6-- -Gasket must be wide enough to take bolt loads without crushing, narrow enough to realize the re quired unit stress. 7-- There is one correct wav to cut a gasket--with a good cutter. Hammering out a gasket against a flange is a practice of long stand ing. but produces gaskets that mav be lumped. Any variation in gasxe: thickness reduces its effectiveness. 8-- Tighten the bolts properly. (Fig. 16) To pull up one bolt full while the others in a flange are loose wiU cock the flange out of parallel. Tightening the rest of the bolts will not bring the flanges back into alignment. Proper sequence is to first tighten all bolts band tight. Then partially tighten two -opposite bolts, and move to two others as near 90 from the first two bolts as possible. Each pair of bolts tight ened should then be 90* from the preceding pair, until all bolts are tight. Bolts are good for just so much stress, and further tightening will do one of two things: bolts will stretch, or nuts will upset. But no more pressure will be exerted on the gasket. Hammering on flange bolting to stop a leak is common, but does not stop leakage usually, and the joint has to be broken and remade after cleaning the flange faces and replacing the gasket. 9-- Retighten bolts at operating conditions, if possible. Frequent leaks that can be stopped by tight ening bolts, however, indicate that creep stresses are too high. A change in boiting material or more resilient gasket may correct the fault. Bibliography 11 Duakle and Fetter: "Chemical and Heat Resistance of Gasket Materials". Chemical Engineering, Vol. 53. No. II, November, 1946. 2) Elonka, Steve: "Gaskets". Power: Vol. 98. No. 3, pp 105-124. March. 1954. 3) Fraser, E. C.: "Design Manual on Nonmetailic Gaskets". Machine Design. Vol. 26. No. H, pp 157-188. Novemcer. 1954. 4) Anon.: "The Gasket". The JoansManvdie Corp. Petroleum Processing. March. -957 in U S A