机械毕业设计外文翻译---纸浆和造纸行业中的无水氨压力容器(编辑修改稿)

机械毕业设计外文翻译---纸浆和造纸行业中的无水氨压力容器(编辑修改稿)内容摘要：

should be rated for ammonia service and should not contain copper or copper alloy ponents. In one case, a nickel rupture disc corroded to failure at its periphery due to formation of an ammonia solution at a gasketed joint exposed to the weather. Inservice inspection Vessel entry Liquid or gaseous ammonia is hazardous and in some jurisdictions release of ammonia vapor to the atmosphere is prohibited by law. Vessels must be properly purged by water and/or steam. Detailed procedures for vessel purging and entry are available from ammonia suppliers (6). Inspection procedures The ANSI standard does not address inservice inspection but does state weld repair or alteration must conform to the current edition of the National Board Inspection Code (NBIC). The 1992 edition of the NBIC includes nonmandatory guidelines for inspection of liquid ammonia vessels (7). These guidelines remend: • Power buffing or light sandblasting as surface preparation for inspection • All interior welds be examined by WFMT. • Cracks should be removed by grinding without encroaching on the minimum thickness required by ASME Section VIII and the original design. • Weld repairs, regardless of size, should be post weld heat treated wherever possible. Light grinding does increase the sensitivity of WFMT pared to sandblasting or power buffing (8). For example the NBIC mandates grinding as surface preparation for deaerator inspection. The omission of grinding in the guidelines for ammonia vessel inservice inspection may be due to concern that rough grinding may produce residual stress sufficient to initiate SCC in anhydrous ammonia service. If welds have been properly profiled for WFMT on initial fabrication, then grinding for inservice inspection should not be needed. The NBIC guidelines also state that other inspection methods such as acoustic emission or ultrasonics may be used and that fracture mechanics may be used to assess the integrity of vessels where plete removal of cracks is not practical. Normally the only corrosion that occurs in anhydrous ammonia vessels is due to water ingress during out of service periods. Shallow pitting, however, has been found in the bottom of some vessels beneath oily deposits. The source of oil is presumed to be from pressors used to handle the ammonia. In view of concerns over air contamination due to vessel entry and residual stress imparted by grinding nonintrusive inspection, techniques like acoustic emission and UT could be considered by vessel owners. The British Code does not mention nonintrusive inspection of ambient temperature pressure vessels but does state that, if acoustic emission is to be used for spherical storage vessels, a reference base should be taken during initial hydrotesting. Nonintrusive inspection is being used in other industries (9). Vessel refilling Safety procedures should be established for refilling a vessel that has been emptied for inspection. It is also very important to purge the vessel of air to prevent the occurrence of SCC. Detailed instructions are available from ammonia suppliers (10). If a vessel is not to be returned to service immediately after inspection, then care should be taken to dry it and possibly nitrogenpad it depending on the time it will remain out of service. Inspection frequency Neither the ANSI document nor the NBIC deals with inspection frequency. The British Code remends the following: • WFMT inspection of 100% of all internal butt welds within the first three years of service • WFMT reinspection within 2 years if significant defects are found • Subsequent to no significant defects being found, any subsequent inspection should include WFMT of all Tee junctions and 10% of the total length of butt welds • In no case should。