機(jī)械畢業(yè)設(shè)計(jì)外文翻譯---紙漿和造紙行業(yè)中的無水氨壓力容器-文庫吧在線文庫
【正文】 ce liquefied ammonia had been used for many years in the refrigeration, chemical, and metal heat treating industries without reported problems. Investigation confirmed SCC to be the cause of cracking. Three remendations were made in 1962 that still form the basis of modern codes: ? Pressure vessels should be fully stress relieved. ? Extreme care should be used to eliminate oxygen from ammonia systems. ? Ammonia should contain at least % water to inhibit SCC. Loginow reported that adoption of these remendations practically eliminated SCC in carbon steel vessels in the agriculture industry. However, in a recent Western Canadian survey SCC was found in 100 of 117 field storage vessels inspected by wet fluorescent magic particle testing (WFMT) (3). Despite the above measures SCC continued to occur in road transport tanks constructed from high strength steels, in refrigerated storage vessels and in vessels which had been weld repaired but not subsequently stress relieved. An additional remendation to limit steel tensile or yield strength was embodied in the . and British ammonia storage codes, respectively (4, 5). ? ANSI —Nominal tensile no greater than 70,000 psi (580 MPa) ? . Code—Minimum specified yield strength shall not exceed 350 MPa (51,000 psi). PRACTICAL CONSIDERATIONS This article is concerned mainly with practical considerations important to pulp and paper mills already possessing anhydrous ammonia storage vessels or planning to fabricate such vessels. In view of the industry’s experience with SCC in continuous digesters the governing objectives should be to control fabrication and inspection to prevent, or at least minimize, inservice problems including overreaction to relatively minor crack indications. Guidance is available in the published codes and detailed information is available from some ammonia suppliers. Fabrication The two main objectives in fabrication should be to provide the most crack resistant vessel possible at reasonable cost and to ensure that an adequate inspection baseline is available for interpretation of subsequent inservice inspections. ASME Section VIII Division 1 does not require stress relief for anhydrous ammonia storage pressure vessels unless the owner specifies a lethal service designation. The lethal service designation requires radiographic testing (RT) of all butt welded joints plus post weld heat treatment. ANSI , “American National Standard Safety Requirements for the Storage and Handling of Anhydrous Ammonia,” adds several requirements: ? Fabrication to ASME Section VIII Division 1 Table UW 12 at a joint efficiency less than 80% is not allowed. ? Inspection and testing under UG90(c) (2) (multiple, duplicate pressure vessel fabrication) is not allowed. ? Steel used for pressure containing parts shall have a nominal tensile strength no greater than 580MPa (70,000 psi). ? The minimum design pressure for ambient temperature storage shall be 16 bar (250 psig). ? Post weld heat treatment is mandatory and a furnace of sufficient size to acmodate the entire vessel is remended. Welded attachments may be made to pads after post weld heat treatment. ? Horizontal vessels shall be mounted on saddles which extend over at least one third of the shell’s circumference. Thermal expansion and contraction shall be allowed for and means provided to prevent corrosion between the shell and the saddles. The 1986 British Code “Storage of Anhydrous Ammonia under Pressure in the United Kingdom” requires: ? Steel must have specified minimum yield strength less than 350 MPa (51,000 psi). ? Weld filler must have minimal strength overmatch pared with the base plate. ? 100% magic particle inspection of all internal welds in order to provide a record ag
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