TECH NOLOGY
Pressure Vessel Codes Under Study Refrigerated storage vessel and unfired pressure vessel design standards are due for revision by API and ASME The American Petroleum Institute has completed the first design and structure standard for refrigerated storage tanks. Just sent to the printer, Appendix R to API Standard 620 will likely be available by the end of April. It extends API's recommended rule for design of low-pressure tanks to include design metal temperatures down to - 6 0 ° F. Within the past five years, such tanks have virtually replaced pressurized vessels in new bulk-storage installations for anhydrous ammonia. Their use for storing liquefied petroleum gas (LPG) is also growing. Meanwhile, the American Society of Mechanical Engineers is moving toward an extensive upgrading of its standards for unfired pressure vessels. During meetings this week of the ASME Boiler and Pressure Vessel Code Committee and its subcommittees in San Francisco, Calif., a panel will discuss the proposed Division 2 for Code Section VIII, which covers unfired pressure vessels. Under development by the Special Committee to Review Code Stress Basis, Division 2 will apply to vessels that are custom-designed and fabricated. This takes in a large share of the vessels built for chemical and petroleum processing. The new rules likely will permit higher design stresses (and incorporate other more advanced design concepts) than does present Section VIII. Though preliminary drafts of parts of Division 2 are beginning to appear, a complete approved text is still some time away. John Rees, chairman of the special committee, estimates mid-1966 for completion of Division 2. Cold Tanks. Typical past practice for storing ammonia in bulk was to hold it at about 25° F. (about 50 p.s.i.g.) in a refrigerated pressure vessel—often a sphere. Rising ammonia production, though, has pushed storage requirements upward at plants and terminals. One engineering firm that routinely specified 15,000-bbl. 68
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ammonia vessels five years ago has recently designed refrigerated tanks to hold well over 100,000 bbl. Designers found that the economic balance between design pressure and refrigeration load shifts markedly as capacity goes up. A tank that holds ammonia at just under its boiling point (—29° F . ) , and thus at atmospheric pressure, need not be a pressure vessel. Even when the greater cooling load for a tank is figured in, savings over a pressure vessel are substantial—they can be more than 50% for very large tanks. Fabricators have had to rely partly on their own experience in low-temperature design when specifying and building refrigerated tanks. Since these tanks aren't pressure vessels, the ASME code doesn't apply. The most nearly applicable standard is API 620, which deals with tanks of up to 15 p.s.i.g. design pressure. Until now,
though, API 620 has specifically excluded refrigerated tanks. The American Insurance Association, an advisory group to capitalstock fire insurance companies, nevertheless has been using API 620 as the basis of its own Pamphlet 59. This standard covers refrigerated LPG storage tanks at utility gas plants. The Agricultural Ammonia Institute's Standard M-l is also based in part on API 620. M-l covers refrigerated storage Of agriculture-grade (inhibited) ammonia. The institute has just revised its standard to accept the new API Appendix. Bringing refrigerated tanks under a design standard probably will have little effect on insurance rates that tank owners pay. Fire and explosion insurance typically takes the form of blanket policies for a tank farm or an entire plant. The nature of the tanks' contents, layout of the tank farm, and
REVISED STANDARDS. The American Petroleum Institute has completed its recommended design and structure standards for refrigerated storage vessels, which now include design metal temperatures down to —60° F.
degree of fire protection available are the prime factors determining rates. Structural design is secondary. Boiler and machine underwriters, who often insure process equipment against rup ture, consider each tank or vessel in dividually for coverage. While they are prone to check closely the design calculations for a noncode vessel, standard rates apply if the design of the vessel is approved by the under writers. Appendix R will supplement Stand ard 620 by recommending materials of construction for several ranges of design metal temperatures from - 6 0 ° F. to 40° F. It details impact test requirements for primary tank components, describes additional welding rules, and discusses test pro cedures beyond those for nonrefrigerated tanks. It covers both single-wall and double-wall designs. AS Μ Ε Code. Though the new por tion being developed for the ASME code has been tagged Section VIII Di vision 2, it hasn't been decided whether it will in fact be part of Sec tion VIII or a new code section in it self. The special committee, which has been working on this addition for about two years, was created in 1955 to review the entire code. Its first task—by popular demand—was to in corporate the most advanced design concepts in current use into a stand ard for vessels in nuclear service. This was accomplished in 1962 with the issue of Code Section III. The special committee is now at tempting to extend this thinking to large industrial vessels. One member of the code committee characterizes the work as a completely new ap proach to pressure vessel design. No extensive rewriting of present Section VIII is involved. The new provisions won't cover production line vessels such as air receiver tanks and similar vessels. The committee may take a look at extending Section VIII coverage be yond 3000 p.s.i.g. design pressure—its present limit. This has been consid ered and rejected several times in re cent years, however, since so few in dustrial vessels are built for such pressure. There is also some reluc tance to freeze specifications while ultrahigh-pressure technology is in a pe riod of rapid growth. As one commit tee member puts it, it is important to gain some experience with the new materials and designs now available before ASME says "thou shalt."
New Chemicals Pattern for Kraft Pulpers Could cut cost of kraft pulping and bleaching by as much as a quarter to a third No need to buy chlorine, caustic soda, or salt cake. This is the prospect held out to kraft pulpers—and to sup pliers of the chemicals—by a new chemical supply system for pulping and bleaching. Only sodium chlo rate, sodium chloride, and sulfuric acid, in addition to the usual lime, are required. The system, which its developers believe could cut bleaching costs by as much as a quarter to a third, is now in mill trials at the Crofton, B.C., mill of British Columbia Forest Products. If it proves out, it could be a shot in the arm for the so-called R2 process developed several years ago by Dr. W. Howard Rapson, professor of chemical engineering at the Univer sity of Toronto, along with Electric Reduction Co. of Canada (ERCO) and Hooker Chemical. Originally developed to produce chlorine dioxide for bleaching, the R2 process involves reactions of sodium chlorate, sodium chloride, and sulfuric acid. Besides chlorine dioxide and chlorine, however, sodium sulfate is also produced. Success had hinged on a method for crystallizing sodium sulfate from solution as Glauber's salt (sodium sulfate decahydrate) and evaporating and recycling sulfuric acid. It's this R2 process that's now the basis for the new chemical supply con cept. The concept was described by Dr. Rapson at the 50th Annual Meet ing of the Technical Association of the Pulp and Paper Industry, in New York. Even as he was speaking, ERCO engineers were starting up a new pilot plant on a revised R2 proc ess—a simpler one, with lower capital cost. The revised process is designed to produce anhydrous sodium sulfate and a more concentrated chlorine. Integrates. The new concept inte grates and changes somewhat the chemical flow patterns in pulping and bleaching. In a typical kraft pulp plant, pulp is digested with white liq uor, the fresh cooking liquor made up primarily of sodium sulfide and so dium hydroxide. After cooking, spent (black) liquor washed from the pulp goes to chemical recovery.
Pulp to be bleached then goes to a chlorination stage in the bleaching op eration. Chlorinated alkali lignin, however, is almost insoluble in acid solution; so chlorination is followed by caustic extraction with sodium hy droxide. The pulp is then further bleached with sodium hypochlorite or chlorine dioxide. Another caustic ex traction follows. Back in the pulping operation, black liquor that has entered the chemical recovery system is evapo rated (and tall oil skimmed off) and sent to a combustion furnace. There it burns to a molten mixture of pri marily sodium carbonate and sodium sulfide. The melt discharges to a quench tank to form green liquor, which is then causticized with lime to convert the sodium carbonate to so dium hydroxide. The calcium carbon ate that forms in this step is filtered and burned to calcium oxide, availa ble again for causticizing. The re maining solution, white liquor, is ready for feeding to the digesters. To make up sodium sulfide losses in the pulping process, salt cake (crude sodium sulfate) is added to the solu tion entering the furnace. It is re duced in a furnace to sodium sulfide. Thus a kraft mill generally buys salt cake for makeup, chlorine for bleaching, and sodium hydroxide for caustic extraction and for making so dium hypochlorite. It also buys so dium chlorate, sulfuric acid, and ei ther sulfur dioxide, methanol, or sodium chloride for reducing sodium chlorate to chlorine dioxide. Added to these is calcium carbonate or lime for recausticizing. All told, Dr. Rapson says, chemicals used to make bleached kraft pulp range on an aver age from $13 to $17 per ton of pulp. Change Supply. Two factors ac count for the R2 process being able to change this supply picture. First, the process makes both chlorine and chlorine dioxide, and it can be ad justed to give these in whatever pro portion is needed. Second, labora tory experiments and mill operation have shown that it's perfectly feasible to use white liquor instead of sodium hydroxide for caustic extraction. MAR.
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