API RP 945-2003 pdf download.Avoiding Environmental Cracking in Amine Units.
1 Scope This recommended practice discusses environmental cracking problems of carbon steel equipment in amine units. Stress corrosion cracking of stainless steels in amine units is beyond the scope of this document although there have been isolated reports of such problems. This practice does provide guidelines for carbon steel construction materials including their fabrication, inspection, and repair to help assure safe and reliable operation. The steels referred to in this document are deÞned by the ASTM designation system, or are equivalent materials contained in other recognized codes or standards. Welded construction is considered the primary method of fab- ricating and joining amine unit equipment. See 3.1 and 3.2 for the deÞnitions of weld and weldment. This document is based on current engineering practices and insights from recent industry experience. Older amine units may not conform exactly to the information contained in this recommended practice, but this does not imply that such units are operating in an unsafe or unreliable manner. No two amine units are alike, and the need to modify a speciÞc facility depends on its operating, inspection, and maintenance history. Each user company is responsible for safe and reli- able unit operation. 2 References 2.1 REFERENCED PUBLICATIONS The following publications are referenced by number in this recommended practice. 1. H. W. Schmidt et al., ÒStress Corrosion Cracking in Alkaline Solutions,Ó Corrosion, 1951, Volume 7, No. 9, p. 295. 2. G. L. Garwood, ÒWhat to Do About Amine Stress Cor- rosion,Ó Oil and Gas Journal, July 27, 1953, Volume 52, p. 334. 3. P. G. Hughes, ÒStress Corrosion Cracking in an MEA Unit,Ó Proceedings of the 1982 U.K. National Corrosion Conference, Institute of Corrosion Science and Technol- ogy, Birmingham, England, 1982, p. 87. 4. H. I. McHenry et al., ÒFailure Analysis of an Amine Absorber Pressure Vessel,Ó Materials Performance, 1987. Volume 26, No. 8, p. 18. 5. J. Gutzeit and J. M. Johnson4.2.2 Environmental Cracking Problems with environmental cracking occur when carbon steels are in regions of high hardness, high residual stress, or both. In particular, areas of high hardness in and adjacent to welds have been problematic. Cracks have also been reported in areas where high hardness levels were not detectable with standard Þeld hardnessÐmeasurement equipment. The crack- ing of weld-repaired areas has also caused serious problems when excessively hard zones or regions of high residual stresses have not been eliminated by the repair procedure. In some instances, cracking has occurred in base metal at sites of internal arc strikes, or opposite external welds for vessel attachments, such as ladders. Four different cracking mechanisms have been identiÞed in carbon steel components in amine units: a. SulÞde stress cracking (SSC). b. Hydrogen-induced cracking (HIC) associated with hydro- gen blistering. c. Stress-oriented hydrogen-induced cracking (SOHIC). d. Alkaline stress corrosion cracking (ASCC).
Several serious cracking problems have been reported over the past 50 years. ASCC of carbon steel by amine solutions was Þrst mentioned in a report published in 1951 by the NACE Technical Practices Committee 5C on Sub-Surface Corrosion by Alkaline Solutions . The report noted that piping, regen- erators (strippers), absorbers, and heat exchanger shells and heads made from carbon steel had cracked after 6 months to 10 years of exposure to 15-percent monoethanolamine in water (containing unspeciÞed amounts of both hydrogen sulÞde and carbon dioxide) at temperatures up to 149¡C (300¡F). Com- plete stress relieving was recommended as a solution to the problem. In 1953, ASCC was reported in MEA solutions in gas treat- ment plants . Requirements for cracking included the pres- ence of both a high stress and a particular corrosive amine solution. The elimination of either factor was found to prevent cracking. Recommended preventive measures included main- taining the reboiler temperature and the regenerator pressure at the lowest practical levels, using reclaimers, and preventing air contact to minimize the corrosiveness of the amine solutions. Frequently, such process changes cannot be readily imple- mented, so stress relieving was recommended as an effective alternative to the recommended practices.