1、 附录一 外文原稿： Anhydrous Ammonia Pressure Vessels In The Pulp And Paper Industry The purpose of this article is to ensure that pulp and paper operating companies, their engineering consultants, and inspection contractors are informed about stress corrosion cracking in anhydrous ammonia service. The info
2、rmation was written by a task group of the TAPPI Engineering Division Nondestructive Testing and Quality Control Subcommittee. Bacteria in some activated sludge effluent treatment systems require supplementary food. In some cases, this food is provided by ammonia and phosphoric acid which are stored
3、 on the mill site. Ammonia is commonly stored as anhydrous liquid ammonia in carbon steel vessels at ambient temperature and 16 bar (250 psig) pressure. These vessels can be subject to stress corrosion cracking (SCC).SCC could cause release of ammonia, which is a hazardous chemical. SCC of carbon st
4、eel vessels in anhydrous ammonia service is somewhat analogous to that experienced in continuous digesters. For example, the importances of stress relief during fabrication and of in-service inspection are common to both. This article concerns storage in horizontal pressure vessels at ambient temper
5、ature, as this type of vessel is used in pulp and paper applications. Large refrigerated storage tanks are used for atmospheric pressure storage in the chemical industry. History of Scc In Ammonia Storage Vessels The history of SCC in carbon steel ammonia storage vessels was reviewed by Loginow (1)
6、and is also briefly summarized in a NACE Technical Committee Report entitled “Integrity of Equipment in Anhydrous Ammonia Storage and Handling” (2). In the 1950s, liquefied ammonia began to be injected directly into soil for fertilization. Failure of carbon steel storage vessels by SCC began to occu
7、r. These failures were unexpected since 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 recommendations were made in 1962 that still form the ba
8、sis 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 0.2% water to inhibit SCC. Loginow reported that adoption of these recommendations practically eliminated SCC in carbon steel v
9、essels 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 magnetic particle testing (WFMT) (3). Despite the above measures SCC continued to occur in road transport tanks constructed from high streng
10、th steels, in refrigerated storage vessels and in vessels which had been weld repaired but not subsequently stress relieved. An additional recommendation to limit steel tensile or yield strength was embodied in the U.S. and British ammonia storage codes, respectively (4, 5). ANSI K61.1Nominal tensil
11、e no greater than 70,000 psi (580 MPa) U.K. CodeMinimum 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 pl
12、anning to fabricate such vessels. In view of the industrys experience with SCC in continuous digesters the governing objectives should be to control fabrication and inspection to prevent, or at least minimize, in-service problems including over-reaction to relatively minor crack indications. Guidanc
13、e 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
14、 for interpretation of subsequent in-service 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
15、welded joints plus post weld heat treatment. ANSI K-61.1-1989, “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. In
16、spection and testing under UG-90(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 (2
17、50 psig). Post weld heat treatment is mandatory and a furnace of sufficient size to accommodate the entire vessel is recommended. 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 shells c
18、ircumference. 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 compared with the base plate. 100% magnetic particle inspection of all internal welds in order to provide a record against which all future inspections of the vessel can be assessed.
