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Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
14 Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead Glass Lining Most engineers in the chemical process industry may have no familiarity with glass lining but are interested in learning about its basic properties and what makes glass-lined equipment so much better compared to standard stainless steel and alloy vessels in corrosion protection. Glass-lined equipment has served as a workhorse for some of the most challenging applications for over 50 years [1]. Its nearly universal chemical compatibility and time-tested designs result in equipment lasting for decades. Even so, there are a lot of misconceptions and common mistakes that users make that can lead to premature failure. However, if one adheres to the best practices that follow, one can ensure to increase the life of the glass-lined equipment. 1. Do not use any glass or metallic instruments during operation or maintenance of the glasslined vessels. 2. Make sure to inspect new equipment and check accessories upon arrival. Upon arrival, a full inspection of the interior and exterior of the equipment should be performed to confirm its condition. 3. Do not operate the glass-lined equipment outside of the design conditions. Pressure and temperature ranges are assigned to each vessel manufactured and are to be clearly labeled on the vessel nameplate. These design limits are based solely on the steel pressure vessel in accordance with the ASME code. 4. Make sure to wear clean, rubber-soled shoes when entering a glass vessel. Also, when it comes to footwear, it is a good idea to have special shoes that have not been worn for normal day-to-day activities. Small pebbles or other debris can find themselves stuck in the treads of the shoes and scratch or otherwise damage the lining in the vessel.
5. Special care must be used in handling the equipment during removal and installation. The standard operational temperature range for most glass-lined reactors is −200°F (−28.90°C)–5000°F (2600°C). One major cause for vessel failure is thermal shock to the glass lining. Exceeding the recommended “safe temperature differential” will cause thermal shock. In general, the higher the operating temperature, the lower the safe temperature differential. Thermal stress caused by improper piping connections may be a reason for failure occurring below the safe temperature differential. The glass lining will withstand a wide variety of chemical reactants. In addition, all accessories used must be compatible with the process. The term “glass lining” is used instead of “enameling” to distinguish products from various paints and lacquers that are sometimes called enamels. The material used for glass lining vessels is truly glass, which is fused silicate. The chief constituent of all commercial glasses and glass enamels is silica. The other ingredients are mainly oxides of alkaline and alkaline earth metals and fluorides of sodium and calcium. The raw materials used for glass manufacture are quartz, borax, soda, nitrates, feldspar, fluorspar, carbonates, cryolite, and various coloring oxides to produce the desired color. Glass-lined equipment has served the process industries for several years combating most challenging environments. Its nearly universal chemical compatibility and time-tested designs result in equipment lasting decades.
Historical Exact knowledge of the discovery of glass is lacking, but it is thought that it was discovered in Persia many centuries before the Christian era. Probably, it was discovered when some ancient tribe built fires on a sandy beach where there was
Anticorrosive Rubber Lining. http://dx.doi.org/10.1016/B978-0-323-44371-5.00014-1 Copyright © 2017 Elsevier Inc. All rights reserved.
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sufficient concentration of sodium and calcium salts present, which, when heated with silica, formed crude glass. Certainly, it is safe to assume that covering metals with glass was a much later development [2]. However, there is a biblical reference and evidence (Exodus 38:8, which reads “And he made the laver of brass, and the foot of it of brass, of the looking glasses of the women assembling, which assembled at the door of the tabernacle of the congregation”) that suggest that long before the time of Christ metals were covered with glass to obtain artistic effects. The actual art of enameling is probably of western Asiatic origin. The earliest work was done in various colors, applied to gold and used for jewelry. As the art developed, the highest order of artists and craftsmen did the work, and many of the objects finished by them were of a religious nature. Previous to the 18th century practically all enameling had been applied to gold, silver, and copper. At about that time the enameling of ferrous metals began. Cast iron had already reached a fairly high state of development and first attempts at enameling or glass lining were made with that metal.
Development of Industrial Glass Lining With the advent of the Bessemer and Siemens processes for steel making [3], rapid strides were made with cast iron, and it was first successfully enameled in the 19th century. With the enameling of ferrous metals began the manufacture of enameled articles for mostly utilitarian purposes. Kitchenware was first made of cast iron, but later on, when steel was developed, it was found that those articles could be much more economically fabricated from steel pressings. One of the most recent developments in the enameled steel industry is the application of enameled steel sheets to the outside walls of dwellings and store fronts. Enameled steel tile for roofs and inside finish have been produced for a number of years. The chief advantages of this material are its ease of cleaning, long life, and its ability to retain its color and finish. Late in the 19th century a German American by the name of Casper Pfaudler [4], after observing that the employment of vacuum hastened the fermentation of beer, invented and patented a method of
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vacuum fermentation. At that time there was no suitable material from which large fermenting vessels could be made to maintain reduced pressure. Many experiments were made with various existing materials, including stone, cast iron, and plate glass. Later experiments led to trials with enameled steel, which it might be said was the actual beginning of the glasslined tank. At that time, glass lining of steel was not nearly as common as today, and no glass had been applied to the heavy gauge material required for beer fermenters working under vacuum. A great amount of experimental and developmental work was done before the process was finally brought to commercial stage. Although originally intended for fermentation, brewers soon adopted this material for holding beer at nearly all subsequent stages of the brewing process. Where originally developed for maintaining diminished pressure, the vessels are now almost universally operated under pressures varying from 10 to 50 lb/sq inch. As the market for these vessels was originally in existing brewery cellars, the problem arose of transporting and installing tanks of sufficient size to meet the capacities demanded. A method of bolting together flanged rings and ends was developed to give almost any desired capacity. These tanks were suitable for vacuum and low pressures but possessed the disadvantage of joints at each section. There has been a steady increase in pressures used by brewers, and it was soon discovered that the ring construction was unsuitable in most cases. For that reason, large one-piece tanks became necessary, which meant the development of larger furnaces and improved glass lining. One-piece tanks not only withstood higher pressures better and successfully, but they were also more easily cleaned, and because of the elimination of joints there were no places for bacterial growth. Today, tanks of one-piece construction of 250 barrels capacity can be processed in the United Kingdom. Although developed for the brewing industry, it soon became apparent that glass-lined steel vessels were advantageous in other industries. Their chief advantage is ease of cleaning and resistance to acids and corrosion. The food industries were quick to realize this, and many glass-lined tanks were later used in service for processing, storing, and transporting various food products. Also there has been steady development in the use of glass-lined steel in the chemical and pharmaceutical industries where metallic contamination must be avoided.
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Manufacturing Process
Manufacture of Glass/Enamel
The primary step in the manufacturing sequence is to create the enamel or glass that is used to construct the glass lining. Each batch of enamel is comprised of carefully selected and rigidly controlled raw materials, which are melted in a rotary furnace at about 2500°F (1400°C). The melted glass is then poured into water. This sudden tempering breaks the enamel into small particles. This is then dried, milled, and screened into a powder. The enamel is available in two colors (blue and white), each having exactly the same chemical and mechanical properties. The preference for one color versus the other is typically determined by which enables better visibility for cleaning purposes based on the chemicals involved in the end user’s application. For convenience the manufacturing process can be divided into five steps, namely:
The raw materials (quartz, borax, soda, nitrates, feldspar, fluorspar, carbonates, cryolite, and various coloring oxides to produce the color) are mixed in proper proportions and charged into a reverberatory furnace. After the glass is completely melted, it is quenched into cold water. This sudden cooling at reduced temperature breaks up the glass into small particles called frit. After being separated from the water, the frit or broken glass is charged into pebble mills and ground wet with a definite charge of water. After grinding to proper fineness, the wet enamel has the consistency of thick cream.
1. Fabrication of vessels 2. Manufacture of glass/enamel 3. Application of the enamel
4. Firing or curing of glass 5. Fitting
Fabrication of Vessels The steel for glass lining must be of superior quality. Its chemical and physical characteristics must be practically perfect. Basic open hearth steel of highest quality obtainable is used for vessels that are to be glass lined. The raw steel sheets must be reasonably free from surface defects that might later influence the perfection of the finished lining. Flanged and dished ends are pressed from steel plate of proper thickness to meet the pressure involved. The ends are then welded to the body and the tank marked for the various openings, such as manway, inlet, outlet, thermometer, gauge glasses, etc. After welding, all of the inside welds must be ground down to the level of the virgin metal to take a smooth glass coating without breaks or joints. After grinding the welds, the fabricated tank is blasted with sand under high pressure to produce an absolutely clean surface before applying the glass lining. It is after this blasting that the steel is most carefully inspected for flaws in the surface. Should any blemishes be discovered, they are ground out and reblasted.
Application of the Enamel In the glass lining of steel or enameling, two kinds of enamel are used. First is the ground or bonding coat, and second is the cover or gloss coat. The first coat has the quality of firmly bonding with the steel, generally lacks gloss, and is fused into and onto the steel surface at higher temperature than the succeeding coats. The cover or gloss coat is smoother and glossier than the ground coat, and is less subject to corrosion. All vessels are given a ground coat and one or more cover coats. The pebble-milled enamel is sprayed from pistols by compressed air onto the inside sandblasted surface of the steel.
Firing or Curing of Glass After drying with warm air, which on larger vessels may require several hours, the vessels are charged into the furnace. They are slowly heated to a temperature that fuses the glass onto the steel. The heating period depends upon the size, weight, and thickness of the vessel. A three-barrel vessel would require as little as 15 min, and a 250-barrel vessel as much as an hour or more. After the first heating, the vessels are allowed to cool down to room temperature before the cover coat is applied. This cooling period also varies with the size, weight, and thickness of the vessel, but for large, heavy tanks, as much as 5 h are necessary before the tank can be entered for inspection and further application. The cover coat is then sprayed, dried, and fired similar to the ground coat. After one or more cover coats the tank is passed as satisfactory, or, if some condition exists that cannot be corrected by another firing, the tank is returned to the sandblast, and after blasting, the defective spot rewelded
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and reground. The whole enameling process is then repeated until the tank is satisfactory for the intended purpose. The firing operation is the most important in the entire process, and it is here that the most careful control is necessary. Two methods of temperature control are used: one to measure the temperature of the furnace and the other to gauge the actual temperature of the vessel being fired. It is highly important that the entire vessel be brought to the required temperature as uniformly as possible. If this is not done the finished vessel would have nonuniform appearance, and probably the glass lining would be of poor quality in those areas where it was too hot or too cold. It is to obviate this possibility that the vessels are revolved during heating, and the uniformity of temperature controlled.
Fitting After the tank is finished the manway doors are individually fitted, and after fitting they are enameled. This is done to ensure each door fits properly and because slight deformations are apt to occur in the tanks during the heating process. After the manway is fitted, other operations are necessary to complete the tank. For some services, such as heating or cooling, jackets must be welded on after the tank is finished in the glass lining process. In nearly all tanks, inlet and outlet cocks, thermometers, observation glasses, pressure gauges, safety valves, etc., must be properly fitted before the tank can be tested and dispatched. After all fitting is finished the vessel is then painted, placed on the necessary wooden supports, and dispatched.
Furnace Designs Since the early days of sectional tanks, there has been steady development in the designs of furnaces and furnace-charging equipment. The first furnaces were very low compared to their floor area. The largest single section processed was 10 ft in diameter. With the advent of large one-piece tanks, the furnaces have been increased correspondingly in height; currently it is possible to process tanks more than 20 ft in length. The first furnaces were of the rectangular box type charged from the front. This design was retained in the furnace developed for large one-piece tanks, notwithstanding the fact that nearly all vessels fired were circular in shape. The
Anticorrosive Rubber Lining
heat losses from such furnaces are terrific. The same heat loss occurs again when the vessel is removed. To offset these losses, a furnace in Scotland (which incidentally is believed to be the largest enameling furnace in the United Kingdom) was designed circular in shape with the bottom of the furnace serving as the door. This means that when the door is opened there is no great outrush of heated gases because of the tendency of heated gases to rise. Large one-piece tanks have also necessitated much heavier handling equipment.
Precautions to be Taken With Glass-Lined Equipment After being sprayed onto the metal surface and dried the enamel is very fragile, and it is essential that the vessel should not be subjected to mechanical shock at this stage of the process. Therefore the charging machinery must maneuver the vessels very gently into position in the furnace. The largest vessels weigh several tons, and therefore it can be realized how stable the charging machine must be. The charging crane used in conjunction with the furnace in Scotland is believed to be capable of lifting an 8-ton tank from a horizontal position, tipping it into the vertical position, and charging into the furnace. On removing the tanks, the operations are repeated in reverse order. All of these machine operations are accomplished with practically no sudden shock to the vessels.
Industrial Applications of GlassLined Equipment Glass-lined steel process equipment is used in virtually all of the world’s pharmaceutical manufacturing facilities and is also widely employed by the chemical, petrochemical, pesticide, metallurgical, and food industries. There are several advantages as described next [5]. These unique characteristics of glass lining make this material of construction a top selection for design engineers.
Corrosion Resistance Glass-lined steel provides superior corrosion resistance to acids, alkalis, water, and other chemical solutions (with the exception for hydrofluoric acid
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
and hot concentrated phosphoric acid). As a result of this chemical resistance, glass lining can serve for many years in environments that would quickly render most metal vessels unserviceable.
Flexibility The chemical, mechanical, and thermal properties of glass are proof that this material can handle a diverse range of operating conditions. Users of glasslined equipment are therefore able to make drastic changes to their processes with no added investment for new equipment needed for their various processes. This versatility makes glass-lined steel the equipment of choice for research and development projects, and batches of corrosive chemicals that require frequent change out, and other multifaceted applications.
Purity Aggressive reaction environments tend to dissolve metals from unlined mild steel or alloy reactors. Extractable metals, such as chromium, nickel, molybdenum, and copper, can leach into and contaminate the product, producing undesirable catalytic effects that can cause harmful fluctuations in the process reactions. These metals can compromise product quality, negatively affect product yield, and in some cases even cause unwanted reactions. Glasslined steel is inert so it is impervious to contamination. Additionally, it does not adversely affect flavor or color, which is of extreme importance to food and drug applications where purity is essential.
Ease of Cleaning Especially in the case of pharmaceutical processes, cleanability is critical. Between batches, each reactor and its associated process equipment must be thoroughly cleaned to ensure product quality and minimize heat transfer resistance caused by products or the buildup of their reactants. Its high degree of surface smoothness makes it easy to clean using noncorrosive, low-pressure cleaning systems. The smooth surface of glass-lined steel also resists the buildup of viscous or sticky products, which means less frequent cleaning.
Economy When properly handled and maintained, glasslined steel reactors can be a cost-efficient solution
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compared to steel and alloy vessels, whose service life can be drastically shortened because of their inability to resist corrosion the way glass lining can. The combination of glass and steel provides the best of both materials of construction; fusing glass to steel produces a composite material with an inside that offers product protection and an outside that provides structural strength and durability. Whether this technology is new or not, glass-lined steel is not new; it has existed for over 300 years. Its usage has changed over time and manufacturing practices have certainly improved. Today, companies choose to use glass-lined equipment for the same reasons they did centuries ago.
Absence of Catalytic Effect Glass lining eliminates the possibility of catalytic effect that can occur in vessels made with various exotic metals.
Fiberglass Reinforced Plastic Lining Fiber-reinforced plastic (FRP), also called fiberreinforced polymer, is a composite material made of a polymer matrix reinforced with fibers. It is also known as glassfiber-reinforced plastic (GRP). The fibers are usually glass, carbon, aramid, or basalt. Rarely, other fibers such as paper, wood, or asbestos have been used. The polymer is usually epoxy, vinyl ester, or polyester thermosetting plastics, and phenol formaldehyde resins are still in use. FRPs are commonly used in the aerospace, automotive, marine, and construction industries and in ballistic armor. They belong to a category of composite plastics that specifically use fiber materials to mechanically enhance the strength and elasticity of plastics. The original plastic material without fiber reinforcement is known as the matrix. The matrix is a tough but relatively weak plastic that is reinforced by stronger, stiffer reinforcing filaments or fibers. The extent that strength and elasticity are enhanced in an FRP depends on the mechanical properties of the fiber and matrix, their volume relative to one another, and the fiber length and orientation within the matrix. Reinforcement of the matrix occurs when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.
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Historical Bakelite was the first FRP discovered by Dr. Baekeland. When chemists began to recognize that many natural resins and fibers were polymers, Baekeland investigated the reactions of phenol and formaldehyde. He first produced a soluble phenolformaldehyde resin called “Novolak,” which never became a market success. He then turned to developing a binder for asbestos, which, at that time, was molded with rubber. By controlling the pressure and temperature applied to phenol and formaldehyde, he found in 1905 that he could produce a hard moldable material, the world’s first synthetic plastic, namely, Bakelite. The development of FRP for commercial use was being extensively researched in the 1930s. In the United Kingdom, considerable research was undertaken by pioneers such as Norman de Bruyne. It was of particular interest to the aviation industry [6]. Mass production of glass strands was discovered in 1932 when Games Slayter, a researcher at Owens-Illinois, accidentally directed a jet of compressed air at a stream of molten glass and produced fibers. Originally, fiberglass was a glass wool with fibers entrapping a great deal of gas, making it useful as an insulator, especially at high temperatures. A suitable resin for combining the “fiberglass” with a plastic to produce a composite material was developed in 1936 by DuPont. Peroxide curing systems for the resins were used by then. With the combination of fiberglass and resin, the gas content of the material was replaced by plastic. This reduced the insulation properties to values typical of the plastic, but now for the first time the composite showed great strength and promise as a structural and building material. Many glass fiber composites continued to be called “fiberglass” (as a generic name) and the name was also used for the low-density glass wool product containing gas instead of plastic.
FRP—A Potentially Advantageous Material Glass fibers are the most commonly used fibers in all industries, although carbon-fiber and carbonfiber-aramid composites are widely found in aerospace, automotive, and sporting goods applications. These three (glass, carbon, and aramid) continue to be the important categories of fibers used in FRP composites.
Anticorrosive Rubber Lining
Global polymer production on the scale present today began in the mid-20th century, when low material and production costs, new production technologies, and new product categories combined to make polymer production economical. The industry finally matured in the late 1970s when world polymer production surpassed that of steel, making polymers the universal material that it is today. FRPs have been a significant aspect of this industry from the beginning. Often, a major advantage of FRP is its lower cost. When comparing materials for corrosion service, rubber lining, titanium, Monel, Hastelloy, and the exotic stainless materials are very frequent alternatives to FRP. In these cases, FRP may offer both a satisfactory solution to corrosion problems and the lowest cost. There is no rule of thumb for comparing costs of FRP with other materials. These costs depend upon the application, the design considerations, the pressures (or vacuums) involved, the product configurations, and raw material cost and availability [7]. The prime reason for using FRPs is because of their inherent corrosion resistance. In many cases, they are the only materials that will handle a given service environment with a few exceptions, such as in mercury cells where rubber lining alone is suitable for the in situ chlorine environment, and in other cases where their corrosion resistance is combined with their economy to make them the most economically acceptable solution. Corrosion resistance of FRP is a function of both the resin content and the specific resin used in the laminate. Generally speaking, the higher the resin content, the more corrosion resistant the FRP laminate. Another very distinct advantage of FRP is its low weight-to-strength ratio. As a rule of thumb, for the same strength, FRP will weigh approximately oneseventh as much as steel and half as much as aluminum. Lightweight properties are important when considering the cost and ease of installation, especially for pipe and tanks. FRP’s inherent lightweight is an advantage when equipment must be mounted on existing structures, such as scrubbers on mezzanine floors or rooftops, and for specialty applications such as FRP tank trailers. Many people overlook the versatility of FRP. It is best for many applications because things can be done with it that cannot be done economically with other materials. Almost any configuration can be molded, or a temporary or permanent mold can be built for a piece of equipment such as a large road tanker (Fig. 14.1). For ductwork, for example, all
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
Figure 14.1 A custom-fabricated FRP tank.
types of elbows, rectangular to circular transitions, tee inlets, and flanges in a wide proliferation of round and rectangular sizes and shapes at minimal tooling cost can be made. It is also possible to use FRP to line existing structures. The major industrial sectors that use FRP are: • Oil and gas
• Desalinization
• Coal
• Water purification
• Nuclear • Mining and minerals
• Wastewater management
• Chemical processing
• Pulp and paper • Architectural
Resins Used in the Manufacturing Process The manufacture of FRP involves two distinct processes: the first is the process whereby the fibrous material is manufactured and formed; the second is the process whereby fibrous materials are resin bonded with the matrix during molding. The most commonly used thermosetting resin families used are vinyl ester, bisphenol-A fumarate polyester, teraphthalic polyester, and isophthalic polyester. Similarly, each family of resin has its own unique usefulness depending on the application and operating conditions like temperature, pressure, etc.
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The following provides basic information that pertains to the common resins used when designing FRP products for a wide range of industrial and commercial applications: Polyester resins are thermoset polyesters. They are versatile, offer good dimensional stability, and have good mechanical, chemical resistance and electrical properties. Vinyl ester resins are flexible (double-bonded vinyl group) in nature and are useful when creating products that are designed to withstand flexing, impacts, or compression. Epoxy resins have an extended range of properties when compared to polyester and vinyl ester resins. They demonstrate extremely low shrinkage, good dimensional stability, high-temperature resistance, as well as good fatigue and adherence to reinforcements. In addition, they have excellent resistance to basic (alkali) environments/solutions. Generally speaking, epoxies require heat curing to develop higher heat distortion temperatures. Polyurethane resins are known throughout the fiberglass industry for their durability and robustness. They are flexible in nature. Phenolic resins possess many desirable attributes in the fiberglass world. They offer formability to complex contours, as well as flexibility. They are heat and chemical resistant and demonstrate flame retardance. They are ideal for high-temperature applications where parts/components must meet fire safety, smoke emission, and combustion and toxicity requirements. In addition, they also have electrical nonconductivity characteristics. Hybrid resins are unique in that they are a customized blend of various resins and fillers to create superior properties that allow the design and product to be optimized. For fiberglass products to perform right in the field, it takes more than just quality manufacturing and resin selection. It takes a high level of engineering and design skills with project-related expertise— the kind that comes from years of experience.
Application Techniques There is a wide variety of techniques by which FRP composites can be fabricated, although there are differences between the techniques available for thermosetting and thermoplastic because of their intrinsically different properties. The different application techniques are described next.
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1. Pultrusion
Pultrusion is a process for manufacturing reinforced plastic products in which a bundle of glass fibers is pulled through a resin bath and then through a heated die to shape the resin. Tightly packed rows of fibers, impregnated with polymer, are pulled through a heated, shaped die to form aligned, continuous sections. Solid and hollow profile sections may be produced with a high fiber content and high degree of fiber alignment. Off-axis fibers may also be introduced if required, such as pultruded shapes and concrete reinforcing bars and tendons. Suitable for I-beams and other sections. 2. Filament winding This process involves winding fibers over a mandrel that rotates while a moving carriage lays down the reinforcement in the desired pattern. The orientation of the fibers can also be carefully controlled so that successive layers are plies or oriented differently from the previous layer. Suitable for cylindrically symmetric structures such as hollows and vessels. Wrapping in retrofit strengthening is an adaptation of the process. 3. Compression and transfer molding Compression molding of thermosetting molding compounds in dough with chopped glass fibers (dough-molding compound) or sheets with longer fibers (sheet-molding compound). Suitable for simple or complex decorative panels. 4. Matched-die molding and autoclave Large panels and relatively complex open structural shapes are constructed by hot-pressing sheets of preimpregnated fibers or cloths between flat or shaped platens or by pressure autoclaving to consolidate a stack of impregnated sheets against a heated, shaped die. Composite reinforced with chopped-strand mat or continuousfilament mat reinforcements may also be press laminated. Suitable for laminates and retrofit strengthening sheets. 5. Continuous sheet production Chopped strand mat or chopped strands are impregnated with resin and sandwiched between two layers of film on a moving belt. The sandwich passes through guides that form the corrugated or other desired profile. Suitable for manufacturing corrugated plates.
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6. Resin transfer molding and vacuum-assisted resin transfer molding Precatalyzed resin is pumped under low pressure into a fiber preform, which is contained in a closed and often heated die. The preform may be made of any kind of reinforcement, but usually consists of woven cloths or continuousfiber mats. Suitable for structural components with varying shapes and degrees of anisotropy/ orthotropy, e.g., cladding and roofing panels, shell structures, and bridge decks. 7. Contact molding by hand lay-up or spray-up These are open mold methods, where fiber continuous strand mat and/or other fabrics such as woven roving are placed manually in the mold and each ply is impregnated with brushes and rollers. The product must also be built by spraying through a gun that simultaneously delivers short fiber and precatalyzed resin. Suitable for fabrication of one-off structures and small numbers of large components.
Testing of FRP Lining There are many important properties of FRP that are determined by international testing methods. These measurements of properties are particularly useful for quality control and specifications purposes. Flexural modulus is an engineering measurement that determines how much a sample will bend when a given load is applied, as compared to tensile modulus that determines how much a sample will stretch when a given load is applied, and compressive modulus that determines how much a sample will compress when a given load is applied. Because composites are nonisotropic (as opposed to metals, for example) these additional material properties are required to predict the behavior under load, which helps to solve design problems. ASTM D-790 is one such standard testing method that is used to determine flexural properties of FRP. According to ASTM D-790, flexural properties may vary with specimen depth, temperature, atmospheric conditions, and the difference in rate of straining. For example, because the physical properties of many materials (especially thermoplastics) can vary depending on ambient temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end user environment.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
According to ASTM D-790, these test methods cover the determination of flexural properties of unreinforced and reinforced plastics, including highmodulus composites and electrical insulating materials in the form of rectangular bars molded directly or cut from sheets, plates, or molded shapes. These test methods are generally applicable to both rigid and semirigid materials. However, flexural strength cannot be determined for those materials that do not break or that do not fail in the outer surface of the test specimen within the 5.0% strain limit of these test methods. These test methods utilize a three-point loading system applied to a simply supported beam. A four-point loading system method can be found in Test Method D6272. Another common standard for testing flexural behavior is ISO 178. Similarly, this standard specifies a method for determining the flexural properties of rigid and semirigid plastics under defined conditions. A standard test specimen is defined, but parameters are included for alternative specimen sizes for use where appropriate. It is important to note the differences between ASTM D-790 and ISO 178 standards. As per a well-established procedure of plastics testing, most commonly the specimen lies on a support span and the load is applied to the center by the loading nose producing three-point bending at a specified rate. These parameters are based on the test specimen thickness and are defined differently by ASTM and ISO. For ASTM D790, the test is stopped when the specimen reaches 5% deflection or the specimen breaks before 5%. For ISO 178, the test is stopped when the specimen breaks. If the specimen does not break, the test is continued as far as possible and the stress at 3.5% (conventional deflection) is reported. Note: ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized leader in the development and delivery of international voluntary consensus standards.
Lead Lining Lead lining tanks is a very highly skilled process and is used for process and storage of chemicals and mixtures of chemicals that are not suitable for rubber and PVC linings or need to be used at
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higher temperatures. Although very dense lead is malleable and can be formed into shapes, the majority of lead is chemically pure grades A and B for industry and grade C for construction. When alloyed with antimony or tin either the strength or electrical conductivity can be improved to suit requirements. The excellent chemical resistance of lead lining is mainly because of the insoluble film that is formed on the surface of the lead in many environments, which prevents further attack. Thus it is an excellent material for pickling sulfuric, chromic, and phosphoric acid and also solutions of these salts because of the formation of an insoluble film that forms over all surfaces when it is first used, which helps to prevent further attack as a protective layer. Its resistance to hydrochloric acid is limited. Similarly, lead lining should not be used for handling nitric acid or ammonia solutions. Lead lining is used in the following industries. • Electroplating and metal finishing • Chromic acid tanks
• Effluent treatment
• Chrome solutions • Various plating solutions
• Aerospace industry
• Anodizing • Chemical and pharmaceuticals industry Lead sheets from 0.5 to 50 mm thickness, lengths up to 30 ft, and widths up to 6 ft are the commonly available products on the market. There are three composition options in the lead sheet, namely, pure or chemical lead, antimonial lead, and calcium lead. Custom alloys that meet specific applications are also developed by certain suppliers. Lead sheet’s uniform density, high atomic number, level of stability, ease of fabrication, high degree of flexibility in application, and its availability at reasonable cost are principal factors in lead lining technology. Rolled lead sheet is formed by passing a slab of refined lead back and forth on a rolling mill between closing rollers to a predetermined thickness. The sheet is then slit to width and cut to length for final packing and distribution. Such is the consistency of the process that lead sheet will not vary in thickness by more than ±5% at any given point.
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Lead sheet is a construction material of major importance in chemical and related industries because lead resists attack by a wide range of chemicals. Lead sheet is also used in building construction for roofing and flashing, shower pans, flooring, X-ray and gamma-ray protection, and vibration damping and soundproofing. Lead sheet used in chemical industries and building construction is made from either pure lead or lead-antimony alloy normally up to 6%. Calcium-lead and calciumlead-tin alloys are also suitable for many of these applications.
Lead for Radiation Protection Lead has long been recognized as a highly effective material in providing protection from various sources of radiation, and as such has become a standard in the design of radiation protection systems [8]. Lead for radiation protection is very familiar to architects, designers, users, and engineers who focus on the radiation shielding properties, design, and fabrication information of lead and lead-based products. No shielding systems are undertaken without consultation with a qualified radiation consultant or certified radiation physicist. Radiation is energy propagated through space, and may encompass two kinds of phenomena: (1) electromagnetic waves, e.g., X-rays, gamma-rays, and (2) particle emulsions, e.g., alpha- and beta-particles from a radioactive substance or neutrons from a nuclear reactor. The universe is flooded with radiation of various energy levels, but the earth’s atmosphere shields us from most of the harmful radiation. Without such shielding, human life would not be possible. Shielding is used to dissipate excessive heat from high absorption of radiation energy and is usually termed thermal shielding. Theoretically, all materials could be used for radiation shielding if employed in a thickness sufficient to attenuate or weaken the radiation to safe limits. Lead and concrete are among the most commonly used materials to shield radiation. The choice of the shield material is dependent upon many varied factors such as final desired attenuated radiation levels, ease of heat dissipation, resistance to radiation damage, required thickness and weight, multiple use considerations (e.g., shield and/or structural), uniformity of shielding capability, permanence of shielding, and availability.
Anticorrosive Rubber Lining
Properties of Lead for Radiation Shielding The properties of lead that make it an excellent shielding material are its density, high atomic number, high level of stability, ease of fabrication, high degree of flexibility in application, and its availability. The following is a description of these properties as related to the criteria of selecting a shield material.
Attenuation of Neutron Particles In shielding against neutron particles it is necessary to provide a protective shield that will attenuate both the neutron particles and the secondary gamma radiation. When applied as part of a neutron particle shielding system, lead has an extremely low level of neutron absorption and hence practically no secondary gamma radiation. If the shield material has a high rate of neutron capture, it will in time become radioactive, sharply reducing its effectiveness as a shield material. Lead itself cannot become radioactive under bombardment by neutrons. Therefore lead shielding, even after long periods of neutron exposure, emits only insignificant amounts of radiation caused by activation. In the lining design of the protective shielding system, one of the key factors is preventing the penetration of the rays. The property of the shield material of most significance in preventing this penetration is its density. Lead enjoys the advantage of being the densest of any commonly available material. It is recognized that lead is not the most dense element (i.e., tantalum, tungsten, and thorium are higher on the density scale), but it is readily available, easily fabricated, and has the lowest cost of the higher density materials.
Other Factors Being a metal, lead has an advantage over various aggregate materials such as concrete, being more uniform in density throughout. In addition, because commonly used forms of lead exhibit smooth surfaces, lead is less likely to become contaminated with dirt or other material, which, in turn, may become radioactive. Regarding its reuse, lead contains only small quantities of other elements that can be adversely effected by exposure to radiation, and therefore it is immediately available for reuse, adaptation, or for sale as scrap. For example, the price of scrap lead may be as high as 80% of the prevailing price of virgin lead.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
Lead Lining Application Procedure Exhaustive details on the application procedures of lead lining are available in lead sheet suppliers’ manuals or from the codes of practice of lead lining of vessels and tanks for chemical process industries stipulated in many national/international standard specifications like ASTM International, BS, ISI, ISO, etc. Although presenting a detailed description of the application procedures is beyond the scope of this book, a brief description thereon as per IS 4682 (Part III)—1969 Indian Standard “Code of practice for lining of vessels and equipment for chemical processes: Part III Lead lining” [9] is given next.
Design of Vessels and Equipment For designing vessels for lead lining, the following precautions should be observed, as their non-observance may lead to early failure of the lining: 1. Branches and openings should be so positioned and proportioned that local stresses in the lining are avoided; 2. Supports should be evenly spaced and should be adequate in number; 3. As far as possible, care should be taken that side branches and other openings do not interfere with the uniform spacing of the supports; and 4. The design should be such as to minimize the possibilities of vibration, and atmospheric and external corrosion.
Cladding Lead-clad mild steel sheets can be preformed to make cylindrical or flat-sided tanks. Access should be provided for sealing the lead lining after fabrication. Where a vessel is to be homogeneously lined, ample working space should be provided for the lead welded over the whole area to be covered. The vessel should be designed to give the operator access to all parts of the surface and adequate ventilation should be provided during all lining operations. The structural design of the vessel should be such that the application of heat in the lining process will not cause buckling of the shell or weakening of the welds. Where the lining of branches or connections has to be welded to the lining of the vessel, they should be located as far as possible to permit welding in the flat or downhand position. In proportioning a vessel, thought should be
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given to the effect of the location of branches and connections on the maneuverability of the vessel during the lining operation. All sharp corners and angles should be avoided, unavoidable corners formed by lap welds being filled by fillet welds finished smooth to as large a radius as practicable.
Sheet Linings Sheet linings are the cheaper form of lining and should be used whenever circumstances permit. This is the only form of lining suitable for nonmetallic vessels. Sheet linings should never be used for vessels and equipment operating at pressures less than atmospheric pressure.
Homogeneous Linings Homogeneous linings are produced by fusing the lead to the base metal either directly or by using an intermediate layer of an alloying metal. This is usually tin or tin-lead alloy. Tin and tin alloys are not recommended where the highest corrosion-resistant properties are required, or were operating temperatures in excess of 130°C are involved. These can be applied only to metal vessels. Being more expensive, they are commonly employed only where conditions preclude the use of sheet linings. Instances of these are: 1. Where the surface to be covered is large and where it would not be practicable to provide adequate support for a sheet lining, 2. Where the vessel operates at pressures below atmospheric pressure, 3. Where good heat transfer properties are important, as in steam-jacketed vessels, heating coils, and thermometer pockets, 4. For the prevention of creep, particularly where the vessel operates at an elevated temperature (above 150°C), 5. Processes where the temperature or pressure or both fluctuate rapidly, and 6. Where agitators are used or where the vessel is subjected to vibration.
Thickness of Linings The minimum thickness of lead lining/cladding to be used is 3.2 mm for sheet linings and 5 mm for
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Anticorrosive Rubber Lining
Table 14.1 Thickness and Weight of Lead Lining in General Applications Thickness (mm)
Weight Per Unit Area (g/m2)
0.5
5.7
0.8
9.0
1.0
11.3
1.6
18.1
2.0
22.7
2.5
28.3
3.2
31.2
3.5
39.7
4.0
45.4
4.5
51.0
5.0
56.7
5.5
62.0
6.0
68.0
7.0
79.1
Courtesy: IS 4682 (Part III)—1969.
homogeneous linings. For some processes thicker lead should be used, the appropriate thickness being agreed between user and contractor. The weight and thickness of sheet lead in general use are given in Table 14.1.
Factors Affecting Design of Lining The choice of type of lead and the design of a lining will be influenced by the chemical and mechanical conditions in the plant. The following aspects are considered important: Corrosion—Care should be taken to select a lead with the desired corrosion resistance to avoid or minimize erosion. Unalloyed lead is generally most corrosion resistant but does not have such good mechanical properties as alloyed lead. Fatigue—Fatigue failures may be caused by highfrequency vibrational stresses or by low-frequency stresses such as those arising from alternate heating and cooling. These stresses may result in intercrystalline cracking and this is more likely with lead of coarse grain size. Such failures are likely when using alloyed lead that has a finer grain structure and
higher fatigue resistance at elevated temperatures than has unalloyed lead. Fine grain size is particularly important in welded joints, where unalloyed lead is liable to have a very coarse structure. The designer should endeavor to avoid conditions that give rise to vibration and also avoid rapid temperature fluctuations. Creep—Creep failures are associated with the action of a continuously applied static load. Creep has been defined as the slow and progressive plastic deformation that occurs during the prolonged application of stress, which, if applied for a short period, would not cause permanent deformation of the material. The coarser-grained unalloyed lead may have a somewhat higher resistance to distortion under static loads, but alloyed lead has a much higher ductility and therefore can sustain more distortion before failure occurs, thus giving warning of impending breakdown. Unalloyed lead has low ductility under creep stress and shows little distortion before failure. Alloyed lead containing copper and tellurium has the property of work hardening, which is of value where the lining may be subject to severe local stress. Unalloyed lead does not possess this property. It should be noted that lead can creep under its own weight at normal temperatures.
Lead Burning Lead burning is also known as lead welding and may be performed during the installation or maintenance of lead linings and coatings. The process involves using a lead burning bar that matches the same chemical compound of the lead being fused together. The material being joined along with the burning bar is heated with a torch; the two are then melted and fused together.
Adhesion Test After completion of the lead lining, vessels should be assembled complete with any leadcovered coils, agitators, dip pipes, etc. The vessel is then filled with steam at 0.7 kgf/cm2 gauge pressure and maintained at this pressure for 1 h. The steam pressure is then released and the vessel subjected to a vacuum of 635 mm Hg for 1 h. The lining is then examined visually and any part of the lining that has lifted up from the surface of the metal is cut out and renewed.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
References [1] http://www.ddpsinc.com/blog-0/bid/95889/12. [2] h t t p : / / w w w. w i l e y. c o m / d o i / 1 0 . 1 0 0 2 / j.2050-0416.1935.tb05612.x/pdf, Onlinelibraryfree access, Journal of the Institute of Brewing, 41 (5), first published online: 9 APR 2013. [3] https://www.britannica.com/technology/ Bessemer-process. [4] The history, development and manufacture of glass lined steel equipment for the brewing and kindred industries by S.W. McCann, Paper presented at the meeting of the Midland counties section held at the white horse hotel, Congreve Street, Birmingham, on Thursday 2nd May, 1935.
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[5] http://www.ddpsinc.com/blog-0/bid/95229/5Reasons-your-Process-Could-Benefit-fromGlass-Lined-Steel-Equipment. [6] Norman de Bruyne. http://www.iom3.org/society-adhesion-adhesives/awards-adhesion-andadhesives. [7] Blog from: http://beetleplastics.com/what-arethe-advantages-of-fiberglass-reinforced-plastic/. [8] http://www.canadametal.com/wp-content/ uploads/2016/08/radiation-shielding.pdf, A Guide to the Use of Lead for Radiation Shielding. [9] IS: 4682 (Part III), 1969.
5. Special care must be used in handling the equipment during removal and installation. The standard operational temperature range for most glass-lined reactors is −200°F (−28.90°C)–5000°F (2600°C). One major cause for vessel failure is thermal shock to the glass lining. Exceeding the recommended “safe temperature differential” will cause thermal shock. In general, the higher the operating temperature, the lower the safe temperature differential. Thermal stress caused by improper piping connections may be a reason for failure occurring below the safe temperature differential. The glass lining will withstand a wide variety of chemical reactants. In addition, all accessories used must be compatible with the process. The term “glass lining” is used instead of “enameling” to distinguish products from various paints and lacquers that are sometimes called enamels. The material used for glass lining vessels is truly glass, which is fused silicate. The chief constituent of all commercial glasses and glass enamels is silica. The other ingredients are mainly oxides of alkaline and alkaline earth metals and fluorides of sodium and calcium. The raw materials used for glass manufacture are quartz, borax, soda, nitrates, feldspar, fluorspar, carbonates, cryolite, and various coloring oxides to produce the desired color. Glass-lined equipment has served the process industries for several years combating most challenging environments. Its nearly universal chemical compatibility and time-tested designs result in equipment lasting decades.
Historical Exact knowledge of the discovery of glass is lacking, but it is thought that it was discovered in Persia many centuries before the Christian era. Probably, it was discovered when some ancient tribe built fires on a sandy beach where there was
Anticorrosive Rubber Lining. http://dx.doi.org/10.1016/B978-0-323-44371-5.00014-1 Copyright © 2017 Elsevier Inc. All rights reserved.
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sufficient concentration of sodium and calcium salts present, which, when heated with silica, formed crude glass. Certainly, it is safe to assume that covering metals with glass was a much later development [2]. However, there is a biblical reference and evidence (Exodus 38:8, which reads “And he made the laver of brass, and the foot of it of brass, of the looking glasses of the women assembling, which assembled at the door of the tabernacle of the congregation”) that suggest that long before the time of Christ metals were covered with glass to obtain artistic effects. The actual art of enameling is probably of western Asiatic origin. The earliest work was done in various colors, applied to gold and used for jewelry. As the art developed, the highest order of artists and craftsmen did the work, and many of the objects finished by them were of a religious nature. Previous to the 18th century practically all enameling had been applied to gold, silver, and copper. At about that time the enameling of ferrous metals began. Cast iron had already reached a fairly high state of development and first attempts at enameling or glass lining were made with that metal.
Development of Industrial Glass Lining With the advent of the Bessemer and Siemens processes for steel making [3], rapid strides were made with cast iron, and it was first successfully enameled in the 19th century. With the enameling of ferrous metals began the manufacture of enameled articles for mostly utilitarian purposes. Kitchenware was first made of cast iron, but later on, when steel was developed, it was found that those articles could be much more economically fabricated from steel pressings. One of the most recent developments in the enameled steel industry is the application of enameled steel sheets to the outside walls of dwellings and store fronts. Enameled steel tile for roofs and inside finish have been produced for a number of years. The chief advantages of this material are its ease of cleaning, long life, and its ability to retain its color and finish. Late in the 19th century a German American by the name of Casper Pfaudler [4], after observing that the employment of vacuum hastened the fermentation of beer, invented and patented a method of
Anticorrosive Rubber Lining
vacuum fermentation. At that time there was no suitable material from which large fermenting vessels could be made to maintain reduced pressure. Many experiments were made with various existing materials, including stone, cast iron, and plate glass. Later experiments led to trials with enameled steel, which it might be said was the actual beginning of the glasslined tank. At that time, glass lining of steel was not nearly as common as today, and no glass had been applied to the heavy gauge material required for beer fermenters working under vacuum. A great amount of experimental and developmental work was done before the process was finally brought to commercial stage. Although originally intended for fermentation, brewers soon adopted this material for holding beer at nearly all subsequent stages of the brewing process. Where originally developed for maintaining diminished pressure, the vessels are now almost universally operated under pressures varying from 10 to 50 lb/sq inch. As the market for these vessels was originally in existing brewery cellars, the problem arose of transporting and installing tanks of sufficient size to meet the capacities demanded. A method of bolting together flanged rings and ends was developed to give almost any desired capacity. These tanks were suitable for vacuum and low pressures but possessed the disadvantage of joints at each section. There has been a steady increase in pressures used by brewers, and it was soon discovered that the ring construction was unsuitable in most cases. For that reason, large one-piece tanks became necessary, which meant the development of larger furnaces and improved glass lining. One-piece tanks not only withstood higher pressures better and successfully, but they were also more easily cleaned, and because of the elimination of joints there were no places for bacterial growth. Today, tanks of one-piece construction of 250 barrels capacity can be processed in the United Kingdom. Although developed for the brewing industry, it soon became apparent that glass-lined steel vessels were advantageous in other industries. Their chief advantage is ease of cleaning and resistance to acids and corrosion. The food industries were quick to realize this, and many glass-lined tanks were later used in service for processing, storing, and transporting various food products. Also there has been steady development in the use of glass-lined steel in the chemical and pharmaceutical industries where metallic contamination must be avoided.
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Manufacturing Process
Manufacture of Glass/Enamel
The primary step in the manufacturing sequence is to create the enamel or glass that is used to construct the glass lining. Each batch of enamel is comprised of carefully selected and rigidly controlled raw materials, which are melted in a rotary furnace at about 2500°F (1400°C). The melted glass is then poured into water. This sudden tempering breaks the enamel into small particles. This is then dried, milled, and screened into a powder. The enamel is available in two colors (blue and white), each having exactly the same chemical and mechanical properties. The preference for one color versus the other is typically determined by which enables better visibility for cleaning purposes based on the chemicals involved in the end user’s application. For convenience the manufacturing process can be divided into five steps, namely:
The raw materials (quartz, borax, soda, nitrates, feldspar, fluorspar, carbonates, cryolite, and various coloring oxides to produce the color) are mixed in proper proportions and charged into a reverberatory furnace. After the glass is completely melted, it is quenched into cold water. This sudden cooling at reduced temperature breaks up the glass into small particles called frit. After being separated from the water, the frit or broken glass is charged into pebble mills and ground wet with a definite charge of water. After grinding to proper fineness, the wet enamel has the consistency of thick cream.
1. Fabrication of vessels 2. Manufacture of glass/enamel 3. Application of the enamel
4. Firing or curing of glass 5. Fitting
Fabrication of Vessels The steel for glass lining must be of superior quality. Its chemical and physical characteristics must be practically perfect. Basic open hearth steel of highest quality obtainable is used for vessels that are to be glass lined. The raw steel sheets must be reasonably free from surface defects that might later influence the perfection of the finished lining. Flanged and dished ends are pressed from steel plate of proper thickness to meet the pressure involved. The ends are then welded to the body and the tank marked for the various openings, such as manway, inlet, outlet, thermometer, gauge glasses, etc. After welding, all of the inside welds must be ground down to the level of the virgin metal to take a smooth glass coating without breaks or joints. After grinding the welds, the fabricated tank is blasted with sand under high pressure to produce an absolutely clean surface before applying the glass lining. It is after this blasting that the steel is most carefully inspected for flaws in the surface. Should any blemishes be discovered, they are ground out and reblasted.
Application of the Enamel In the glass lining of steel or enameling, two kinds of enamel are used. First is the ground or bonding coat, and second is the cover or gloss coat. The first coat has the quality of firmly bonding with the steel, generally lacks gloss, and is fused into and onto the steel surface at higher temperature than the succeeding coats. The cover or gloss coat is smoother and glossier than the ground coat, and is less subject to corrosion. All vessels are given a ground coat and one or more cover coats. The pebble-milled enamel is sprayed from pistols by compressed air onto the inside sandblasted surface of the steel.
Firing or Curing of Glass After drying with warm air, which on larger vessels may require several hours, the vessels are charged into the furnace. They are slowly heated to a temperature that fuses the glass onto the steel. The heating period depends upon the size, weight, and thickness of the vessel. A three-barrel vessel would require as little as 15 min, and a 250-barrel vessel as much as an hour or more. After the first heating, the vessels are allowed to cool down to room temperature before the cover coat is applied. This cooling period also varies with the size, weight, and thickness of the vessel, but for large, heavy tanks, as much as 5 h are necessary before the tank can be entered for inspection and further application. The cover coat is then sprayed, dried, and fired similar to the ground coat. After one or more cover coats the tank is passed as satisfactory, or, if some condition exists that cannot be corrected by another firing, the tank is returned to the sandblast, and after blasting, the defective spot rewelded
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and reground. The whole enameling process is then repeated until the tank is satisfactory for the intended purpose. The firing operation is the most important in the entire process, and it is here that the most careful control is necessary. Two methods of temperature control are used: one to measure the temperature of the furnace and the other to gauge the actual temperature of the vessel being fired. It is highly important that the entire vessel be brought to the required temperature as uniformly as possible. If this is not done the finished vessel would have nonuniform appearance, and probably the glass lining would be of poor quality in those areas where it was too hot or too cold. It is to obviate this possibility that the vessels are revolved during heating, and the uniformity of temperature controlled.
Fitting After the tank is finished the manway doors are individually fitted, and after fitting they are enameled. This is done to ensure each door fits properly and because slight deformations are apt to occur in the tanks during the heating process. After the manway is fitted, other operations are necessary to complete the tank. For some services, such as heating or cooling, jackets must be welded on after the tank is finished in the glass lining process. In nearly all tanks, inlet and outlet cocks, thermometers, observation glasses, pressure gauges, safety valves, etc., must be properly fitted before the tank can be tested and dispatched. After all fitting is finished the vessel is then painted, placed on the necessary wooden supports, and dispatched.
Furnace Designs Since the early days of sectional tanks, there has been steady development in the designs of furnaces and furnace-charging equipment. The first furnaces were very low compared to their floor area. The largest single section processed was 10 ft in diameter. With the advent of large one-piece tanks, the furnaces have been increased correspondingly in height; currently it is possible to process tanks more than 20 ft in length. The first furnaces were of the rectangular box type charged from the front. This design was retained in the furnace developed for large one-piece tanks, notwithstanding the fact that nearly all vessels fired were circular in shape. The
Anticorrosive Rubber Lining
heat losses from such furnaces are terrific. The same heat loss occurs again when the vessel is removed. To offset these losses, a furnace in Scotland (which incidentally is believed to be the largest enameling furnace in the United Kingdom) was designed circular in shape with the bottom of the furnace serving as the door. This means that when the door is opened there is no great outrush of heated gases because of the tendency of heated gases to rise. Large one-piece tanks have also necessitated much heavier handling equipment.
Precautions to be Taken With Glass-Lined Equipment After being sprayed onto the metal surface and dried the enamel is very fragile, and it is essential that the vessel should not be subjected to mechanical shock at this stage of the process. Therefore the charging machinery must maneuver the vessels very gently into position in the furnace. The largest vessels weigh several tons, and therefore it can be realized how stable the charging machine must be. The charging crane used in conjunction with the furnace in Scotland is believed to be capable of lifting an 8-ton tank from a horizontal position, tipping it into the vertical position, and charging into the furnace. On removing the tanks, the operations are repeated in reverse order. All of these machine operations are accomplished with practically no sudden shock to the vessels.
Industrial Applications of GlassLined Equipment Glass-lined steel process equipment is used in virtually all of the world’s pharmaceutical manufacturing facilities and is also widely employed by the chemical, petrochemical, pesticide, metallurgical, and food industries. There are several advantages as described next [5]. These unique characteristics of glass lining make this material of construction a top selection for design engineers.
Corrosion Resistance Glass-lined steel provides superior corrosion resistance to acids, alkalis, water, and other chemical solutions (with the exception for hydrofluoric acid
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
and hot concentrated phosphoric acid). As a result of this chemical resistance, glass lining can serve for many years in environments that would quickly render most metal vessels unserviceable.
Flexibility The chemical, mechanical, and thermal properties of glass are proof that this material can handle a diverse range of operating conditions. Users of glasslined equipment are therefore able to make drastic changes to their processes with no added investment for new equipment needed for their various processes. This versatility makes glass-lined steel the equipment of choice for research and development projects, and batches of corrosive chemicals that require frequent change out, and other multifaceted applications.
Purity Aggressive reaction environments tend to dissolve metals from unlined mild steel or alloy reactors. Extractable metals, such as chromium, nickel, molybdenum, and copper, can leach into and contaminate the product, producing undesirable catalytic effects that can cause harmful fluctuations in the process reactions. These metals can compromise product quality, negatively affect product yield, and in some cases even cause unwanted reactions. Glasslined steel is inert so it is impervious to contamination. Additionally, it does not adversely affect flavor or color, which is of extreme importance to food and drug applications where purity is essential.
Ease of Cleaning Especially in the case of pharmaceutical processes, cleanability is critical. Between batches, each reactor and its associated process equipment must be thoroughly cleaned to ensure product quality and minimize heat transfer resistance caused by products or the buildup of their reactants. Its high degree of surface smoothness makes it easy to clean using noncorrosive, low-pressure cleaning systems. The smooth surface of glass-lined steel also resists the buildup of viscous or sticky products, which means less frequent cleaning.
Economy When properly handled and maintained, glasslined steel reactors can be a cost-efficient solution
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compared to steel and alloy vessels, whose service life can be drastically shortened because of their inability to resist corrosion the way glass lining can. The combination of glass and steel provides the best of both materials of construction; fusing glass to steel produces a composite material with an inside that offers product protection and an outside that provides structural strength and durability. Whether this technology is new or not, glass-lined steel is not new; it has existed for over 300 years. Its usage has changed over time and manufacturing practices have certainly improved. Today, companies choose to use glass-lined equipment for the same reasons they did centuries ago.
Absence of Catalytic Effect Glass lining eliminates the possibility of catalytic effect that can occur in vessels made with various exotic metals.
Fiberglass Reinforced Plastic Lining Fiber-reinforced plastic (FRP), also called fiberreinforced polymer, is a composite material made of a polymer matrix reinforced with fibers. It is also known as glassfiber-reinforced plastic (GRP). The fibers are usually glass, carbon, aramid, or basalt. Rarely, other fibers such as paper, wood, or asbestos have been used. The polymer is usually epoxy, vinyl ester, or polyester thermosetting plastics, and phenol formaldehyde resins are still in use. FRPs are commonly used in the aerospace, automotive, marine, and construction industries and in ballistic armor. They belong to a category of composite plastics that specifically use fiber materials to mechanically enhance the strength and elasticity of plastics. The original plastic material without fiber reinforcement is known as the matrix. The matrix is a tough but relatively weak plastic that is reinforced by stronger, stiffer reinforcing filaments or fibers. The extent that strength and elasticity are enhanced in an FRP depends on the mechanical properties of the fiber and matrix, their volume relative to one another, and the fiber length and orientation within the matrix. Reinforcement of the matrix occurs when the FRP material exhibits increased strength or elasticity relative to the strength and elasticity of the matrix alone.
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Historical Bakelite was the first FRP discovered by Dr. Baekeland. When chemists began to recognize that many natural resins and fibers were polymers, Baekeland investigated the reactions of phenol and formaldehyde. He first produced a soluble phenolformaldehyde resin called “Novolak,” which never became a market success. He then turned to developing a binder for asbestos, which, at that time, was molded with rubber. By controlling the pressure and temperature applied to phenol and formaldehyde, he found in 1905 that he could produce a hard moldable material, the world’s first synthetic plastic, namely, Bakelite. The development of FRP for commercial use was being extensively researched in the 1930s. In the United Kingdom, considerable research was undertaken by pioneers such as Norman de Bruyne. It was of particular interest to the aviation industry [6]. Mass production of glass strands was discovered in 1932 when Games Slayter, a researcher at Owens-Illinois, accidentally directed a jet of compressed air at a stream of molten glass and produced fibers. Originally, fiberglass was a glass wool with fibers entrapping a great deal of gas, making it useful as an insulator, especially at high temperatures. A suitable resin for combining the “fiberglass” with a plastic to produce a composite material was developed in 1936 by DuPont. Peroxide curing systems for the resins were used by then. With the combination of fiberglass and resin, the gas content of the material was replaced by plastic. This reduced the insulation properties to values typical of the plastic, but now for the first time the composite showed great strength and promise as a structural and building material. Many glass fiber composites continued to be called “fiberglass” (as a generic name) and the name was also used for the low-density glass wool product containing gas instead of plastic.
FRP—A Potentially Advantageous Material Glass fibers are the most commonly used fibers in all industries, although carbon-fiber and carbonfiber-aramid composites are widely found in aerospace, automotive, and sporting goods applications. These three (glass, carbon, and aramid) continue to be the important categories of fibers used in FRP composites.
Anticorrosive Rubber Lining
Global polymer production on the scale present today began in the mid-20th century, when low material and production costs, new production technologies, and new product categories combined to make polymer production economical. The industry finally matured in the late 1970s when world polymer production surpassed that of steel, making polymers the universal material that it is today. FRPs have been a significant aspect of this industry from the beginning. Often, a major advantage of FRP is its lower cost. When comparing materials for corrosion service, rubber lining, titanium, Monel, Hastelloy, and the exotic stainless materials are very frequent alternatives to FRP. In these cases, FRP may offer both a satisfactory solution to corrosion problems and the lowest cost. There is no rule of thumb for comparing costs of FRP with other materials. These costs depend upon the application, the design considerations, the pressures (or vacuums) involved, the product configurations, and raw material cost and availability [7]. The prime reason for using FRPs is because of their inherent corrosion resistance. In many cases, they are the only materials that will handle a given service environment with a few exceptions, such as in mercury cells where rubber lining alone is suitable for the in situ chlorine environment, and in other cases where their corrosion resistance is combined with their economy to make them the most economically acceptable solution. Corrosion resistance of FRP is a function of both the resin content and the specific resin used in the laminate. Generally speaking, the higher the resin content, the more corrosion resistant the FRP laminate. Another very distinct advantage of FRP is its low weight-to-strength ratio. As a rule of thumb, for the same strength, FRP will weigh approximately oneseventh as much as steel and half as much as aluminum. Lightweight properties are important when considering the cost and ease of installation, especially for pipe and tanks. FRP’s inherent lightweight is an advantage when equipment must be mounted on existing structures, such as scrubbers on mezzanine floors or rooftops, and for specialty applications such as FRP tank trailers. Many people overlook the versatility of FRP. It is best for many applications because things can be done with it that cannot be done economically with other materials. Almost any configuration can be molded, or a temporary or permanent mold can be built for a piece of equipment such as a large road tanker (Fig. 14.1). For ductwork, for example, all
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
Figure 14.1 A custom-fabricated FRP tank.
types of elbows, rectangular to circular transitions, tee inlets, and flanges in a wide proliferation of round and rectangular sizes and shapes at minimal tooling cost can be made. It is also possible to use FRP to line existing structures. The major industrial sectors that use FRP are: • Oil and gas
• Desalinization
• Coal
• Water purification
• Nuclear • Mining and minerals
• Wastewater management
• Chemical processing
• Pulp and paper • Architectural
Resins Used in the Manufacturing Process The manufacture of FRP involves two distinct processes: the first is the process whereby the fibrous material is manufactured and formed; the second is the process whereby fibrous materials are resin bonded with the matrix during molding. The most commonly used thermosetting resin families used are vinyl ester, bisphenol-A fumarate polyester, teraphthalic polyester, and isophthalic polyester. Similarly, each family of resin has its own unique usefulness depending on the application and operating conditions like temperature, pressure, etc.
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The following provides basic information that pertains to the common resins used when designing FRP products for a wide range of industrial and commercial applications: Polyester resins are thermoset polyesters. They are versatile, offer good dimensional stability, and have good mechanical, chemical resistance and electrical properties. Vinyl ester resins are flexible (double-bonded vinyl group) in nature and are useful when creating products that are designed to withstand flexing, impacts, or compression. Epoxy resins have an extended range of properties when compared to polyester and vinyl ester resins. They demonstrate extremely low shrinkage, good dimensional stability, high-temperature resistance, as well as good fatigue and adherence to reinforcements. In addition, they have excellent resistance to basic (alkali) environments/solutions. Generally speaking, epoxies require heat curing to develop higher heat distortion temperatures. Polyurethane resins are known throughout the fiberglass industry for their durability and robustness. They are flexible in nature. Phenolic resins possess many desirable attributes in the fiberglass world. They offer formability to complex contours, as well as flexibility. They are heat and chemical resistant and demonstrate flame retardance. They are ideal for high-temperature applications where parts/components must meet fire safety, smoke emission, and combustion and toxicity requirements. In addition, they also have electrical nonconductivity characteristics. Hybrid resins are unique in that they are a customized blend of various resins and fillers to create superior properties that allow the design and product to be optimized. For fiberglass products to perform right in the field, it takes more than just quality manufacturing and resin selection. It takes a high level of engineering and design skills with project-related expertise— the kind that comes from years of experience.
Application Techniques There is a wide variety of techniques by which FRP composites can be fabricated, although there are differences between the techniques available for thermosetting and thermoplastic because of their intrinsically different properties. The different application techniques are described next.
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1. Pultrusion
Pultrusion is a process for manufacturing reinforced plastic products in which a bundle of glass fibers is pulled through a resin bath and then through a heated die to shape the resin. Tightly packed rows of fibers, impregnated with polymer, are pulled through a heated, shaped die to form aligned, continuous sections. Solid and hollow profile sections may be produced with a high fiber content and high degree of fiber alignment. Off-axis fibers may also be introduced if required, such as pultruded shapes and concrete reinforcing bars and tendons. Suitable for I-beams and other sections. 2. Filament winding This process involves winding fibers over a mandrel that rotates while a moving carriage lays down the reinforcement in the desired pattern. The orientation of the fibers can also be carefully controlled so that successive layers are plies or oriented differently from the previous layer. Suitable for cylindrically symmetric structures such as hollows and vessels. Wrapping in retrofit strengthening is an adaptation of the process. 3. Compression and transfer molding Compression molding of thermosetting molding compounds in dough with chopped glass fibers (dough-molding compound) or sheets with longer fibers (sheet-molding compound). Suitable for simple or complex decorative panels. 4. Matched-die molding and autoclave Large panels and relatively complex open structural shapes are constructed by hot-pressing sheets of preimpregnated fibers or cloths between flat or shaped platens or by pressure autoclaving to consolidate a stack of impregnated sheets against a heated, shaped die. Composite reinforced with chopped-strand mat or continuousfilament mat reinforcements may also be press laminated. Suitable for laminates and retrofit strengthening sheets. 5. Continuous sheet production Chopped strand mat or chopped strands are impregnated with resin and sandwiched between two layers of film on a moving belt. The sandwich passes through guides that form the corrugated or other desired profile. Suitable for manufacturing corrugated plates.
Anticorrosive Rubber Lining
6. Resin transfer molding and vacuum-assisted resin transfer molding Precatalyzed resin is pumped under low pressure into a fiber preform, which is contained in a closed and often heated die. The preform may be made of any kind of reinforcement, but usually consists of woven cloths or continuousfiber mats. Suitable for structural components with varying shapes and degrees of anisotropy/ orthotropy, e.g., cladding and roofing panels, shell structures, and bridge decks. 7. Contact molding by hand lay-up or spray-up These are open mold methods, where fiber continuous strand mat and/or other fabrics such as woven roving are placed manually in the mold and each ply is impregnated with brushes and rollers. The product must also be built by spraying through a gun that simultaneously delivers short fiber and precatalyzed resin. Suitable for fabrication of one-off structures and small numbers of large components.
Testing of FRP Lining There are many important properties of FRP that are determined by international testing methods. These measurements of properties are particularly useful for quality control and specifications purposes. Flexural modulus is an engineering measurement that determines how much a sample will bend when a given load is applied, as compared to tensile modulus that determines how much a sample will stretch when a given load is applied, and compressive modulus that determines how much a sample will compress when a given load is applied. Because composites are nonisotropic (as opposed to metals, for example) these additional material properties are required to predict the behavior under load, which helps to solve design problems. ASTM D-790 is one such standard testing method that is used to determine flexural properties of FRP. According to ASTM D-790, flexural properties may vary with specimen depth, temperature, atmospheric conditions, and the difference in rate of straining. For example, because the physical properties of many materials (especially thermoplastics) can vary depending on ambient temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end user environment.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
According to ASTM D-790, these test methods cover the determination of flexural properties of unreinforced and reinforced plastics, including highmodulus composites and electrical insulating materials in the form of rectangular bars molded directly or cut from sheets, plates, or molded shapes. These test methods are generally applicable to both rigid and semirigid materials. However, flexural strength cannot be determined for those materials that do not break or that do not fail in the outer surface of the test specimen within the 5.0% strain limit of these test methods. These test methods utilize a three-point loading system applied to a simply supported beam. A four-point loading system method can be found in Test Method D6272. Another common standard for testing flexural behavior is ISO 178. Similarly, this standard specifies a method for determining the flexural properties of rigid and semirigid plastics under defined conditions. A standard test specimen is defined, but parameters are included for alternative specimen sizes for use where appropriate. It is important to note the differences between ASTM D-790 and ISO 178 standards. As per a well-established procedure of plastics testing, most commonly the specimen lies on a support span and the load is applied to the center by the loading nose producing three-point bending at a specified rate. These parameters are based on the test specimen thickness and are defined differently by ASTM and ISO. For ASTM D790, the test is stopped when the specimen reaches 5% deflection or the specimen breaks before 5%. For ISO 178, the test is stopped when the specimen breaks. If the specimen does not break, the test is continued as far as possible and the stress at 3.5% (conventional deflection) is reported. Note: ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized leader in the development and delivery of international voluntary consensus standards.
Lead Lining Lead lining tanks is a very highly skilled process and is used for process and storage of chemicals and mixtures of chemicals that are not suitable for rubber and PVC linings or need to be used at
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higher temperatures. Although very dense lead is malleable and can be formed into shapes, the majority of lead is chemically pure grades A and B for industry and grade C for construction. When alloyed with antimony or tin either the strength or electrical conductivity can be improved to suit requirements. The excellent chemical resistance of lead lining is mainly because of the insoluble film that is formed on the surface of the lead in many environments, which prevents further attack. Thus it is an excellent material for pickling sulfuric, chromic, and phosphoric acid and also solutions of these salts because of the formation of an insoluble film that forms over all surfaces when it is first used, which helps to prevent further attack as a protective layer. Its resistance to hydrochloric acid is limited. Similarly, lead lining should not be used for handling nitric acid or ammonia solutions. Lead lining is used in the following industries. • Electroplating and metal finishing • Chromic acid tanks
• Effluent treatment
• Chrome solutions • Various plating solutions
• Aerospace industry
• Anodizing • Chemical and pharmaceuticals industry Lead sheets from 0.5 to 50 mm thickness, lengths up to 30 ft, and widths up to 6 ft are the commonly available products on the market. There are three composition options in the lead sheet, namely, pure or chemical lead, antimonial lead, and calcium lead. Custom alloys that meet specific applications are also developed by certain suppliers. Lead sheet’s uniform density, high atomic number, level of stability, ease of fabrication, high degree of flexibility in application, and its availability at reasonable cost are principal factors in lead lining technology. Rolled lead sheet is formed by passing a slab of refined lead back and forth on a rolling mill between closing rollers to a predetermined thickness. The sheet is then slit to width and cut to length for final packing and distribution. Such is the consistency of the process that lead sheet will not vary in thickness by more than ±5% at any given point.
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Lead sheet is a construction material of major importance in chemical and related industries because lead resists attack by a wide range of chemicals. Lead sheet is also used in building construction for roofing and flashing, shower pans, flooring, X-ray and gamma-ray protection, and vibration damping and soundproofing. Lead sheet used in chemical industries and building construction is made from either pure lead or lead-antimony alloy normally up to 6%. Calcium-lead and calciumlead-tin alloys are also suitable for many of these applications.
Lead for Radiation Protection Lead has long been recognized as a highly effective material in providing protection from various sources of radiation, and as such has become a standard in the design of radiation protection systems [8]. Lead for radiation protection is very familiar to architects, designers, users, and engineers who focus on the radiation shielding properties, design, and fabrication information of lead and lead-based products. No shielding systems are undertaken without consultation with a qualified radiation consultant or certified radiation physicist. Radiation is energy propagated through space, and may encompass two kinds of phenomena: (1) electromagnetic waves, e.g., X-rays, gamma-rays, and (2) particle emulsions, e.g., alpha- and beta-particles from a radioactive substance or neutrons from a nuclear reactor. The universe is flooded with radiation of various energy levels, but the earth’s atmosphere shields us from most of the harmful radiation. Without such shielding, human life would not be possible. Shielding is used to dissipate excessive heat from high absorption of radiation energy and is usually termed thermal shielding. Theoretically, all materials could be used for radiation shielding if employed in a thickness sufficient to attenuate or weaken the radiation to safe limits. Lead and concrete are among the most commonly used materials to shield radiation. The choice of the shield material is dependent upon many varied factors such as final desired attenuated radiation levels, ease of heat dissipation, resistance to radiation damage, required thickness and weight, multiple use considerations (e.g., shield and/or structural), uniformity of shielding capability, permanence of shielding, and availability.
Anticorrosive Rubber Lining
Properties of Lead for Radiation Shielding The properties of lead that make it an excellent shielding material are its density, high atomic number, high level of stability, ease of fabrication, high degree of flexibility in application, and its availability. The following is a description of these properties as related to the criteria of selecting a shield material.
Attenuation of Neutron Particles In shielding against neutron particles it is necessary to provide a protective shield that will attenuate both the neutron particles and the secondary gamma radiation. When applied as part of a neutron particle shielding system, lead has an extremely low level of neutron absorption and hence practically no secondary gamma radiation. If the shield material has a high rate of neutron capture, it will in time become radioactive, sharply reducing its effectiveness as a shield material. Lead itself cannot become radioactive under bombardment by neutrons. Therefore lead shielding, even after long periods of neutron exposure, emits only insignificant amounts of radiation caused by activation. In the lining design of the protective shielding system, one of the key factors is preventing the penetration of the rays. The property of the shield material of most significance in preventing this penetration is its density. Lead enjoys the advantage of being the densest of any commonly available material. It is recognized that lead is not the most dense element (i.e., tantalum, tungsten, and thorium are higher on the density scale), but it is readily available, easily fabricated, and has the lowest cost of the higher density materials.
Other Factors Being a metal, lead has an advantage over various aggregate materials such as concrete, being more uniform in density throughout. In addition, because commonly used forms of lead exhibit smooth surfaces, lead is less likely to become contaminated with dirt or other material, which, in turn, may become radioactive. Regarding its reuse, lead contains only small quantities of other elements that can be adversely effected by exposure to radiation, and therefore it is immediately available for reuse, adaptation, or for sale as scrap. For example, the price of scrap lead may be as high as 80% of the prevailing price of virgin lead.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
Lead Lining Application Procedure Exhaustive details on the application procedures of lead lining are available in lead sheet suppliers’ manuals or from the codes of practice of lead lining of vessels and tanks for chemical process industries stipulated in many national/international standard specifications like ASTM International, BS, ISI, ISO, etc. Although presenting a detailed description of the application procedures is beyond the scope of this book, a brief description thereon as per IS 4682 (Part III)—1969 Indian Standard “Code of practice for lining of vessels and equipment for chemical processes: Part III Lead lining” [9] is given next.
Design of Vessels and Equipment For designing vessels for lead lining, the following precautions should be observed, as their non-observance may lead to early failure of the lining: 1. Branches and openings should be so positioned and proportioned that local stresses in the lining are avoided; 2. Supports should be evenly spaced and should be adequate in number; 3. As far as possible, care should be taken that side branches and other openings do not interfere with the uniform spacing of the supports; and 4. The design should be such as to minimize the possibilities of vibration, and atmospheric and external corrosion.
Cladding Lead-clad mild steel sheets can be preformed to make cylindrical or flat-sided tanks. Access should be provided for sealing the lead lining after fabrication. Where a vessel is to be homogeneously lined, ample working space should be provided for the lead welded over the whole area to be covered. The vessel should be designed to give the operator access to all parts of the surface and adequate ventilation should be provided during all lining operations. The structural design of the vessel should be such that the application of heat in the lining process will not cause buckling of the shell or weakening of the welds. Where the lining of branches or connections has to be welded to the lining of the vessel, they should be located as far as possible to permit welding in the flat or downhand position. In proportioning a vessel, thought should be
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given to the effect of the location of branches and connections on the maneuverability of the vessel during the lining operation. All sharp corners and angles should be avoided, unavoidable corners formed by lap welds being filled by fillet welds finished smooth to as large a radius as practicable.
Sheet Linings Sheet linings are the cheaper form of lining and should be used whenever circumstances permit. This is the only form of lining suitable for nonmetallic vessels. Sheet linings should never be used for vessels and equipment operating at pressures less than atmospheric pressure.
Homogeneous Linings Homogeneous linings are produced by fusing the lead to the base metal either directly or by using an intermediate layer of an alloying metal. This is usually tin or tin-lead alloy. Tin and tin alloys are not recommended where the highest corrosion-resistant properties are required, or were operating temperatures in excess of 130°C are involved. These can be applied only to metal vessels. Being more expensive, they are commonly employed only where conditions preclude the use of sheet linings. Instances of these are: 1. Where the surface to be covered is large and where it would not be practicable to provide adequate support for a sheet lining, 2. Where the vessel operates at pressures below atmospheric pressure, 3. Where good heat transfer properties are important, as in steam-jacketed vessels, heating coils, and thermometer pockets, 4. For the prevention of creep, particularly where the vessel operates at an elevated temperature (above 150°C), 5. Processes where the temperature or pressure or both fluctuate rapidly, and 6. Where agitators are used or where the vessel is subjected to vibration.
Thickness of Linings The minimum thickness of lead lining/cladding to be used is 3.2 mm for sheet linings and 5 mm for
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Anticorrosive Rubber Lining
Table 14.1 Thickness and Weight of Lead Lining in General Applications Thickness (mm)
Weight Per Unit Area (g/m2)
0.5
5.7
0.8
9.0
1.0
11.3
1.6
18.1
2.0
22.7
2.5
28.3
3.2
31.2
3.5
39.7
4.0
45.4
4.5
51.0
5.0
56.7
5.5
62.0
6.0
68.0
7.0
79.1
Courtesy: IS 4682 (Part III)—1969.
homogeneous linings. For some processes thicker lead should be used, the appropriate thickness being agreed between user and contractor. The weight and thickness of sheet lead in general use are given in Table 14.1.
Factors Affecting Design of Lining The choice of type of lead and the design of a lining will be influenced by the chemical and mechanical conditions in the plant. The following aspects are considered important: Corrosion—Care should be taken to select a lead with the desired corrosion resistance to avoid or minimize erosion. Unalloyed lead is generally most corrosion resistant but does not have such good mechanical properties as alloyed lead. Fatigue—Fatigue failures may be caused by highfrequency vibrational stresses or by low-frequency stresses such as those arising from alternate heating and cooling. These stresses may result in intercrystalline cracking and this is more likely with lead of coarse grain size. Such failures are likely when using alloyed lead that has a finer grain structure and
higher fatigue resistance at elevated temperatures than has unalloyed lead. Fine grain size is particularly important in welded joints, where unalloyed lead is liable to have a very coarse structure. The designer should endeavor to avoid conditions that give rise to vibration and also avoid rapid temperature fluctuations. Creep—Creep failures are associated with the action of a continuously applied static load. Creep has been defined as the slow and progressive plastic deformation that occurs during the prolonged application of stress, which, if applied for a short period, would not cause permanent deformation of the material. The coarser-grained unalloyed lead may have a somewhat higher resistance to distortion under static loads, but alloyed lead has a much higher ductility and therefore can sustain more distortion before failure occurs, thus giving warning of impending breakdown. Unalloyed lead has low ductility under creep stress and shows little distortion before failure. Alloyed lead containing copper and tellurium has the property of work hardening, which is of value where the lining may be subject to severe local stress. Unalloyed lead does not possess this property. It should be noted that lead can creep under its own weight at normal temperatures.
Lead Burning Lead burning is also known as lead welding and may be performed during the installation or maintenance of lead linings and coatings. The process involves using a lead burning bar that matches the same chemical compound of the lead being fused together. The material being joined along with the burning bar is heated with a torch; the two are then melted and fused together.
Adhesion Test After completion of the lead lining, vessels should be assembled complete with any leadcovered coils, agitators, dip pipes, etc. The vessel is then filled with steam at 0.7 kgf/cm2 gauge pressure and maintained at this pressure for 1 h. The steam pressure is then released and the vessel subjected to a vacuum of 635 mm Hg for 1 h. The lining is then examined visually and any part of the lining that has lifted up from the surface of the metal is cut out and renewed.
14: Technoeconomic Aspects of Nonrubber Linings—Glass, FRP, and Lead
References [1] http://www.ddpsinc.com/blog-0/bid/95889/12. [2] h t t p : / / w w w. w i l e y. c o m / d o i / 1 0 . 1 0 0 2 / j.2050-0416.1935.tb05612.x/pdf, Onlinelibraryfree access, Journal of the Institute of Brewing, 41 (5), first published online: 9 APR 2013. [3] https://www.britannica.com/technology/ Bessemer-process. [4] The history, development and manufacture of glass lined steel equipment for the brewing and kindred industries by S.W. McCann, Paper presented at the meeting of the Midland counties section held at the white horse hotel, Congreve Street, Birmingham, on Thursday 2nd May, 1935.
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[5] http://www.ddpsinc.com/blog-0/bid/95229/5Reasons-your-Process-Could-Benefit-fromGlass-Lined-Steel-Equipment. [6] Norman de Bruyne. http://www.iom3.org/society-adhesion-adhesives/awards-adhesion-andadhesives. [7] Blog from: http://beetleplastics.com/what-arethe-advantages-of-fiberglass-reinforced-plastic/. [8] http://www.canadametal.com/wp-content/ uploads/2016/08/radiation-shielding.pdf, A Guide to the Use of Lead for Radiation Shielding. [9] IS: 4682 (Part III), 1969.