HOMOGENEOUS LEAD-COVERED COPPERHEATINGCOILSFOR UBEWITH 150 POUNDSSTEAM PRESSURE(GAGE)
LEAD-LINEDEQUIPMENT HIS article does not intend to recommend the use of lead for any particular apparatus or to recommend the selection of a sDecific grade of lead but assumes that lead has already been chosen as the proper metal for the particular purpose. Successful lead construction requires not only the proper design of the equipment but also the selection of the correct grade of lead.
E. MANTIUS AND H. F. FREIHERR National Lead Company, New York, N. Y .
HANDLING.An essential part of lead construction is the handling of sheet lead and pipe while it is being moved around the plant and then erected. Lead (even hard lead) is relatively soft and may be easily damaged by nails, cinders, and sharp tools. Such damage may pass unnoticed or seem unimportant during inspection of the completed apparatus, but early failure of the lead sheets or pipe can be the result of careless handling. For hoisting lead material, ropes are used instead of cables or chains unless the latter are suitably covered with cloth. Rolls of sheet lead should be unrolled with wooden prys instead of iron bars. The lead sheets and pipes are dressed into the desired shape by the use of lead beaters, wooden dressers, or hard-rubber mallets, or by placing soft wood on the lead and beating the wood with a heavy hammer. Lead material should be laid preferably on smooth wooden planking or clean steel plate, but never on the ground or on a dirty floor where sharp pieces of dirt or metal may become imbedded in the lead. BURNING.An important feature of lead construction is the welding or ‘(burning” of lead. Lead sheets and/or pipe are joined together by fusing them a t the well-cleaned joint by means of a torch, preferably an oxygen-hydrogen flame. Acetylene has been used to some extent for this purpose but is not generally recommended because the regular acetylene flame is very hot and may cause oxidation of the lead, whereas a milder flame often deposits carbon which may become occluded in the weld. It pays to have lead burning done by skilled mechanics. It is almost impossible to write a description of the correct
Sheet Lead Equipment The type of construction to be described in this section involves the use of lead sheets and pipe, principally for apparatus in the chemical industry. These sheets and pipe are made of straight lead (any one of a number of different brands containing 99 per cent or more lead) or hard lead (straight lead mixed with antimony in percentages up t o 8 per cent). Sheet and pipe made from different varieties of SELECTION. lead vary greatly in physical properties, mechanical strength, creep, and resistance to corrosion. The technical man must determine by means of laboratory or small-scale apparatus the grade of lead that will best serve the purpose. If lead has been successfully used previously in similar construction, it is necessary only to be certain that the same grade of lead is used for the new construction. It is not unusual to hear of cases where two similar types of apparatus handling the same material but constructed with different grades of lead have not given equally satisfactory service. Such discrepancies are often baffling because the designers of the equipment have not been aware of the variations in properties of lead. Chemical and physical analyses of lead are not necessarily conclusive in determining its suitability. The best method is t o purchase, for testing purposes, small quantities of different grades from reliable manufacturers who can give reasonable assurance of continued uniformity of their ores and methods of refining and fabrication so that the purchase a t a later time of larger quantities of the selected grade will come up to expectations. 373
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Couilesv, Slrillheia- Wails Cornpony
EVAPORATOR OF HEAYY CASTANTIM~NIALLEAD method of burning lead or to give the qualifications of a good lead burner. However, the worth of a lead burner can be reanlily judged by his reputation and especially by the lasting quality of previous work. The foregoing paragraphs have not touched upon the actual details of lead construction, but they should be kept in mind because the best designed equipment can be successful and serviceable only if the precautions described have been observed.
Sheet Lead Construction The following descriptions refer chiefly to the design of equipment such as tanks, stills, evaporators, and piping generally used in the chemical industry. Sxcept for heavy-walled hard-lead castings or small equipment, lead does not have sufficient mechanical strength to be used without adequate means of support. A skeleton structure may be used as support but is not recommended. The usual procedure is to make a solid structure to which a lining of sheet lead, either soft or hard may be fastened. The structure may be of wood in the case of moderate-sise storage tanks or vats. Digesters, stills, evaporators, mixing tanks, etc., are preferably of steel. The use of concrete tanks is also acceptable but not so common. These supporting vessels may be of any desired shape but are preferably round with flat, dished, or cone-shaped tops and bottoms, or ends in the case of horizontal tanks. These vessels must be strong enough not only to give rigid support for a lead lining but also to take care of stresses occurring in the completed apparatus during operation, since the lead lining has such a relativefy low mechanical strength.
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The construction of the supporting vessels is important. They should be rigid, and the interior surfaces must .be smooth and free from raised welds, rivets, nails or any protrusions that might damage the lead lining. The lead sheets to be used as a lining for these vessels should be designed so as to reduce the number of joints to a minimum without making the sheets too heavy to handle (about 3000 pounds is a maximum). Furthermore, the joints should be located where they will be accessible in case of repairs, preferably at points where they will be subjected to the least stress during the use of t.he vessel. Joints, directly a t the corners, should be avoided if possible. In the actual lining of the vessels, the lead sheets are placed solidly against the surface of the supporting vessel. This point is important because hydrostatic or gas pressure or the expansion of a brick lining may rupture an unsupported lead sheet. Allsloping, vertical, or overhead portions of a lead liing must be securely fastened a t frequent intervals to the supporting structure because the weight of the l e d itself, especially with an increase in temperature, will cause the lining to stretch and sag. The usual manner of fastening is to place %inch quarter-round steel flats against the lead and bolt the flats with countersunk bolts, through the lead lining, to the supporting vessel. These flats, in turn, are covered with lead straps burned to the lining. The spacing for the vertical Bats is about 24 to 30 inches, and even closer for cone or dished tops. If a vessel is more than 10 feet high, the lining must also be supported by horizontal steel flats placed not more than 10 feet apart. After the lead sheets have been placed and fastened to the vessel, the joints between the sheets are burned. The “flat seam,” which can be burned only in a horizontal position, is the strongest and most satisfactory type of joint. Small tanks may be turned by a crane so that all seams are burned flat. Even quite largehorisoutaltanksmay be rolled, if space permits, so that flat seams can be used. Large cylindrical vessels and oddihaped apparatus cannot be moved after the lead lining has been placed, and therefore the joints are burned by some form of flat or vertical seam. Frequently a lead liniug is, in turn, protected by bricks or tile laid in suitable cement. For this type of construction the supporting vessel is essentially the same but is made somewhat stronger to take care of a possible stress due to thermal expansion of the bricks. Bricks or tile are a suitable support for a lead lining, and therefore the steel flats are usually omitted. The bricks must be placed tightly against the lead, with or without an intermediate protective coating of cement, ashestas paper, or plastic. The bricb or tile must be properly cut and fitted at those points where they come over a burned joint to prevent cutting or shearing of the lead. Brick liuings for the support and protection of lead are generally used at high temperatures (above 200” F.) and a t points where corrosion and mechanical wear might unduly shorten the length of service of the lead. Brick l i n g s must always be used in lead-lined apparatus operated under vacuum (excepting homogeneous lead-lined equipment). For severe conditions of temperature, agitation, and corrosion, a special type of lead-lined steel apparatus has been designed which cau be readily lined with brick or tile. There are many advantages in this type of construction. The tank is made of heavy steel plate in the form of numerous curved sections. The plates are stsened by placing extra heavy angles a t all four edges in such amanner that the edge of the angles is flush with the edge of the plate. The angles are properly drilled so that the curved plates cau be assembled and firmly bolted together to form a rigid, round structure. Before being assembled, the steel sections are covered with one solid sheet of lead, flanged over the edges of the steel at
all four sides. These sections can be readily moved and turned, permitting the use of the flat seam where it is necessary to burn flanges, pipe connections, etc. When the steel sections have been covered with lead, they are assembled and bolted together. The inside of the tank now presents a smooth and practically flush lead surface suitable for brick lining. The joints between the lead sheets on the steel sections are burned together on the outside of the tank where they are always visible and accessible. This type of construction relieves the joints from strain and removes them from the inside of the tank where they might be subject to shearing by the bricks, to cracking due to vibration, or to the effects of corrosion which are usually more severe a t the joints than on the surface of the rolled sheet.
Lead Pipe and Coils Lead pipe, either straight or antimonial lead, must be suitably supported along its entire length in steel or wooden troughs in the horizontal position or by clamping the pipe a t short intervals (about 18 inches) to a rigid support in the vertical position. Long lines, carrying hot solutions, must be provided with expansion bends since such a line will continually stretch without equal contraction upon cooling. A lead pipe line is made up of short sections of pipe joined together by means of cup joints or, preferably, by butt or rolled joints. Flanged joints are used for connections to tanks, pumps, and valves. Such flanged joints may be of the Van Stone type where the lead pipe is flanged over against a loose steel ring drilled for bolts. A more suitable joint is made by burning a drilled lead flange to the pipe, the flange being backed by a loose steel flange. A full-face gasket of rubber or, for high temperatures and corrosive liquids, of hard-pressed composition can be readily used with this type of joint.
to purchase the pipe coiled to the approxim a t e finished diameter since there is l e s s d a n g e r of obtaining uneven w a l l thickness in the pipe when t h e latter is bent while still hot as it comes from the press. Uneven wall thickness is often the cause of undue twisting and rupture in coils.
Cast Lead Many types of apparatus can be readily made of cast lead throughout. Cast lead is straight lead with 6 to 10 per cent antimony, and the design and fabrication are similar to those for cast iron except that the wall thickness must be relatively great because the mechanical strength of cast lead is low and in some cases it is subject to cracking. Castlead construction is not usually recommended for temperatures over 220’ F. or when the conditions of corrosion or erosion are sufficiently severe to wear off the lead to a point where the strength of the cast-lead structure is insufficient.
Lead Equipment for Handling Acids
One of the many useful and necessary applications of lead is in the manufacture and handling of various acid solutions. The class of lead used has an important bearing as to the service and life of this metal in contact with acids. “Chemical lead,” in the form of sheet lead and pipe, is generally satisfactory according t o chemical tests and research. This is a designation which has been used in the trade for many years to describe the undesilverized lead produced from southeastern Missouri ores. Chemical lead is produced from the sulfide ore, galena, in a blast furnace after roasting or in a Scotch hearth furnace, and is refined without desilveri~ation. The presence of a small amount of copper is considered advantageous; other impurities are practically negligible. The composition of this lead is remarkably constant year after year. For some uses in the chemical and allied industries, lead itself lacks sufficient mechanical strength, and this defect has been taken care of in two ways. First, there is the use of lead as a lining for pieces of equipment where the acid-resisting qualities of lead are combined with the strength of iron or steel. Secondly, there is the use of hard lead. Cast hardlead products are not suitable for as high working pressures as lead-lined products, but they offer greater strength than straight lead equipment and can be used a t normal temperatures under pressures up to 60 pounds per square inch. Wherever equipment is necessary for the handling of sulfuric and allied acids, lead or lead-lined products are the HARDLEAD,HORIZONTAL TYPEACID PUMPWITH CAST-IRON logical solution because they have the acid-resisting qualities BASEFOR DIRECTOR BELTDRIVE and the strength necessary for maintaining high pressure, which is essential a t present when the demand is for economiLead pipe coils used for cooling, but especially for heating, cal and continuous production. require some care in fabrication. The wall thickness of the The first requisites for the conveying of acid are piping pipe varies with the steam or water pressure to be used. and valves, or some other means of controlling the flow of Steam pressures up to 45 pounds gage are common practice solutions. It is not sufficient that a pipe line be installed; with coils made of 1 1 / 2 X 2I/z inch lead pipe coiled in the kind of pipe, its connections or fittings, its size, and the diameters of about 30 inches but preferably not smaller. The methods of support and protection utilized are all important coil must be adequately stiffened by five or six rows of verfactors which must be considered if satisfactory and efficient tical spacer blocks about 21/2 X 6 inches in size and burned distribution is to be secured. Breakdowns must not occur, together to form a continuous vertical support. The spacer for they disable the units they feed and sometimes interrupt blocks are also burned to the pipe to minimize the twisting the entire operation of a plant. Such difficulties may conof the coil and the natural tendency to unwind when subsume many times the original cost of proper piping in the jected to pressure from within. idleness which it enforces. The use of storage or supply If pipe coils are to be fabricated in the field, it is advisable tanks is also entailed, and, since it is not possible to have 375
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gravity flow at all times, pumps or air lifts are necessary to convey solutions to the succeeding stages of operation. The lead-lined piping can consist of steel, which is the most commonly used or, if necessary, copper or brass. I n order to simplify the description of the various processes, reference to metals other than steel or lead will be eliminated. It is possible to secure other metals lined or covered with lead, and the same general process of manufacturing is used as for the steel product. Steel pipe is thoroughly cleansed and then fluxed with a bonding medium. The required lead pipe is manufactured in the usual manner by extrusion and, in the process, receives a coating of a bonding medium on the outside only. The lead pipe is then carefully inserted in the steel pipe and is tightly expanded against the wall of the steel pipe. The second step is to fuse the lead and the steel together by steam pressure at a temperature sufficient to melt the flux and bring about an inseparable bond between the lead and the steel pipes. This class of pipe is furnished in the same random lengths as standard steel pipe, which varies from 16 to 22 feet, depending upon the diameter. The ends of pipe are furnished threaded, and the installation can be made either with threaded or flanged connections. However, the most practical method of joining the pipe is with flanges which assure a continuous lead contact through the entire length of the piping. Flanges used on the pipe are first lined or faced with lead; this lining is flush with the face of the steel flange and extends to slightly within the bolt circle. The flange is then screwed onto the pipe, and the lead is made continuous by burning or welding the lining of the flange to the lining of the pipe. For some installations, burning the flange lead lining to the lead lining of the pipe may be objectionable, since it is desirable to continue the lining of the pipe over the recessed face of the flange. This would require lining each piece of pipe separately. For such a process the lead lining is allowed t o extend beyond the length of the steel pipe and is turned over into t h e r e c e s s e d face of the flange similar t o a Van Stone joint. Flanged fittings are usually manufactured of cast iron and f u r n i s h e d from a standard 125-pound pressure p a t t e r n For lining purposes, however, since fittings usually increase to a considerable extent, the amount of friction necessary to move a certain quantity of s o l u t i o n , the iron fittings should be cast with a larger i n t e r n a 1 diameter than is normally specified. The lead lining, approximately 1/4 inch thick, is i n s e r t e d , and the full internal diameter of the fittings is retained, thereby reducing friction to a minimum. The faces of the flanged fittings are lead-covered in the same manner as the flanges which are used for the pipe; therefore, when bolting the two together, a positive lead-to-lead contact is obtained. If a vacuum is present in the system, a lead-bonded fitting should be specified. This would necessitate the use of
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steel fittings because it is not powible to secure a satisfactory bond with cast iron. For the same reason the pipe should be equipped with steel flanges. Flanges and flanged fittings are all standard, and therefore the dimensions are obtainable from the American Standard 125 Pound Pressure Table of Iron Pipe Flanges and Flanged Fittings. Acid valves of various types are obtainable in both leadlined and hard-lead patterns. The type of valve usually specified is the Y-pattern or what is known as a “free-flow” valve. The reason for this is that many acids contain sediment or grit, and, if a type of valve is used which permits accumulation of foreign matter at the seating surface, scoring of the lead plug normally results and causes the valve t o leak. For cases where strong acids are not being handled or the temperature does not exceed 180” F., valves are equipped with a removable rubber plug which can be easily renewed and which possesses some self-seating qualities. Where conditions require, other patterns of valves can be furnished, such as globe, gate, and angle patterns, as well as check and foot valves for pumping installations. Acid valves should be designed to give as clear and unobstructed flow as possible to permit the maximum passage of solution with minimum resistance. Stuffing boxes should be generously designed to aid in preventing leakage at such a point. Some acid valves include in their design a leadcovered bonnet, which gives a decided advantage. In many acid valves bonnet replacements prove a costly item which can be eliminated when the lead-covered bonnet is used. When gravity flow is not possible, acid pumps become LL necessity. The two types of hard-lead acid pumps are the open-pattern centrifugal type and the vertical pump; the latter does not require a stuffing box. The open-type centrifugal pump, however, is most commonly used and is similar to a standard open-impeller cast-iron pump, except that the casing is cast entirely of antimonial lead, heavily ribbed for additional strength. All parts with which the solution would come in contact are of a similar metal. The impeller is also of antimonial lead cast over a bronze spider. Shafts are usually made of an acid-resisting alloy, such as Monel metal or chromium steel alloys, etc., depending on the strength, temperature, and type of acid. The hard-lead vertical acid pump has the advantage of not requiring a stuffing box. This eliminates the necessity of a drip sump which is resorted to in many installations where the standard type of centrifugal acid pump is used. Packingless centrifugal pumps are built to meet the specific needs of the pumping specifications. Tank connections for wood- or lead-lined tanks are obtainable in a type which eliminates the use of bolts and easily permits the inlet and outlet pipe lines to be continued without resorting to lead burning. This connection consists of a bonded lead-lined nipple, which is threaded on the outside. One end of the nipple is equipped with a lead-covered flange. The flanged nipple is inserted from the inside of the tank and is securely held in place by a lock nut and washer on the outside of the tank. The lead lining of the nipple continues beyond the outside end, and a companion flange is supplied for bolting the connection to the continuation of the line. The hard lead designated in the manufacture of certain types of valves and all acid pumps is a cast metal composed of chemical lead with the addition of approximately 6 to 8 per cent of antimony.
Homogeneous Linings One of the most interesting developments in lead-lined equipment where strength is of prime importance, is the manufacture of homogeneous lined apparatus, such as storage
applying the covering by burning on the required t h i c k n e s s of lead with a lead burner's torch. This p r o c e d u r e i s no longer necessary because a more practical and satisfactory method of applying a homogeneous lead covering has been developed. For the manufacture of homogeneous lead-covered coils, it is both advisable and desirable to use copper tubing. Copper tubing lends itself more readily t o a homogeneous covering with lead and also facilitates bending. Lead-covered copper coils can be successfully bent to fairly small diameters, and, where lead-covered material is involved, all bending must be performed cold. Copper tubing in straight lengths, approximately 20 feet long, is homogeneously lead-covered by casting on the lead. The casting operation is performed by covering'a number of tubes at the same time, the covering being applied in a mold. The lead covering is applied several times heavier than the final specifications. The suggested minimum thickness of covering should be the same as for steel sheets-not less than "18 inch. After the casting process is completed, the tubes are run through a draw bench where the lead thickness is decreased to the final requirement. Then the lead-covered tubes are fonned to whatever diameter and type of coil may be required. The joints between the sections of lead-covered tubing are made by the cup method, and the copper is brazed together with silver solder. The continuity of the lead covering is continued over such joints by molding on the required lead. The specified pitch of the coils is maintained by casting into place lead spacer blocks. These blocks are continued beyond the bottom of the coil to whatever length may be necessary in order to form a support or legs so that the coil will not rest upon the bottom of the equipment. Coils of the foregoing description are manufactured to withstand 150 pounds working pressure, and with this type of heating unit maximum heat transmission is secured. It is stated that homogeneous coils eliminate many difficulties experienced in the paet with lead coils under their maximum operating pressure, such as weaving out of shape and bursting, with the resulting expense for repairs and loss in production. The most generally accepted design of homogeneous lead-covered coil is the helical type, although in many cases it is necessary to use the pancake or beehive type. The homogeneous process is also applied advantageously to the covering of agitators, propellers, and mixing devices of all descriptions. I n many cases a lead contact is necessary and, without the homogeneous principle of applying the lead, rigidity and durability, which result in economical operation, are not possible. Equipment for chemical and allied plants, petroleum refineries, and all industries where the use of corrosive solutions are necessary varies according to the type of operation for which it is intended. It is therefore necessary to fabricate homogeneous lead-lined apparatus in most cases to meet individual requirements. To secure best results, it is advisable to furnish the manufacturer with as complete details as possible. Because of the method of constructing homogeneous lead-lined equipment, designs can be, in many instances, advantageously revised to meet the fabricating process. This permits the user to benefit by the manufacturer's experience and obtain the most economical and efficient type of plant equipment.
tanks, autoclaves, and jacketed pressure tanks of all descriptions. Over a period of years, the industry struggled along with loose lead-lined equipment or, in some cases, with partially bonded apparatus, but the life of such material was very short and required continuous maintenance. Later developments brought about the use of tin for bonding purposes, but the temperature range of this material was limited to about 300" to 350" F. Even though the apparatus withstood the maximum temperature as given, continuous heating and cooling operations soon developed weaknesses in the bond, and blisters and breaks occurred in the lining. This process, however, has developed to the extent that homogeneous equipment now can be secured in which the lead linings are bonded to the steel or other metals with a nonmetallic flux. Such linings are therefore not subjected to the same faults as earlier equipment but can be used under various conditions of operation, such as high steam pressures, vacuum, etc., and the lead linings will not separate or come off unless heat is applied close to the melting point of lead (621' F.). The most successful way to manufacture homogeneous equipment, if the apparatus permits, is to treat the steel in order to cleanse it thoroughly, after which the required thickness of lining is applied to the flat sheets. The application of the lead to the steel is a casting process, and the lead is applied several times heavier than the final requirements. This is advisable because surface porosity often prevails in a cast-lead process, and by a method of removing the excess lead such a possibility is eliminated. A minimum final lead thickness of inch is a t all times recommended. When the bonding process of the lead to the steel is completed, the lead-covered sheets are machined in order to secure a smooth surface. I n machining off the excess lead, a knife or cutter is used and is followed by a pressure roller in order to increase the density of the lead. The sheets are then rolled in a manner similar to that utilized in the fabrication of plain steel tanks, t o whatever shapes or sizes may be necessary. The procedure generally recommended for making joints is to weld the steel electrically and then to line with lead the small area a t the section of weld which has been left bare. The lining of this joint is similar to the rest of the process, so that when the apparatus is completed, except for the outside seam in the weld, there are no indications in the lead surface as to where a joint may exist. Homogeneous lead-lined equipment has been perfected to the extent that specifications covering such products can be written along the following lines: The two metals, lead and steel, shall be so attached together that they are inseparable until they approach the melting point of lead, and shall be capable of withstanding shock, vibration, vacuum, and rapid changes in temperature. Homogeneous lead-lined jacketed vessels offer the opportunity to operate a t 125 to 150 pounds per square inch steam pressure. Although in many cases these operating pressures are a valuable asset from the standpoint of time, it is not always economical and practical to make an installation of this type. I n such cases heating coils are required. Lead coils are recommended only up t o maximum steam pressures of 50 pounds; therefore for the higher pressures the lead must be reinforced.
Homogeneous Lead-Covered Coils Lead-covered steel coils have been used a t various times but with unsatisfactory results. The faults which developed were mainly due to porosity, probably brought about by the method of covering the steel pipe; this method consisted in
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