Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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38 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM CHEMICAL TREATMENT indicates that corrosive acid salts were formed (product of strong acids and weak bases) when chlorides reacted with NH 3 and various amines trafficking through the process vapor. The presence of MDEA was alarming, since it can decompose in a fur- nace environment to produce monoetha- nolamine, which is an aggressive salt for- mer. Large losses of MDEA from the amine treating facilities had been reported. How- ever, from the solids analysis it was not possible to determine which base made the salt first. Once a salt forms, it constitutes a new phase in the system, and all other ionic species in the surrounding environment establish an equilibrium with the salt. Since the deposit is acidic, it readily reacts with bases in the vapor phase, and these com- pounds become part of the deposit compo- sition. The only way to sort these reactions and determine original salt deposit con- tributors is IEM. IEM indicated that the unit operation was vastly improved over previous years, but still not within a safe operating enve- lope. Salt formation nomographs of NH 3 levels in the overhead vapor with increas- ing chloride contents were plotted for sys- tem temperatures (230 to 270 °F [110 to 132 °C]) over a period of one year (Figure 4). NH 4 Cl salts were found to be forming at temperatures greater than system temper- atures exiting the heat exchangers, thereby causing localized corrosion of the overhead line. This refinery had a history of corrosion failures resulting in overhead line replace- ment multiple times. Although a materials upgrade would have mitigated this corro- sion mechanism, it could not be imple- mented since construction of Ti piping is not common. Improved process control and monitoring were the only options. Hence, continued IEM and water analysis, wash water optimization, and corrosion monitoring were recommended. Insulation was applied to this line to prevent conden- sation that could cause corrosion. IEM revealed that corrosive salts were forming due to the NH 3 in the process. New simula- tions helped better adjust contaminant lev- els. Also, significant pH depression in the condensing water of the exchangers (pH <1) was noted, implying loss of neutralizer and an unreliable neutralizer injection sys- t em . S hor tcomings in th e wat er wash design were also found, and a new design w a s r e c o m m e n d e d u p s t r e a m o f t h e planned water wash location. Case History 2—Overhead Line Failure in Crude Unit at Refinery B Refiner y B suffered an overhead line failure on its crude unit atmospheric tower. A pipe section from the CS line was submit- ted for failure analysis. Corrosion was observed on the inner diameter (ID) only, with a strip of protected metal along the pipe bottom (Figure 5, blue arrows). The throu g h wal l fai lure s app eared to b e grouped around a weld line (Figure 6). The ID was coated with flaky scale throughout the area of attack. X-ray fluorescence indi- cated that the scale contained 45% chlo- rides and 35% oxides, consistent with salt deposits rather than aqueous acid corro- sion. Also, acid attack does not leave such a scale accumulation at the point of greatest activity. The uncorroded section along the pipe bottom also testifies to pH buffered water f lowing in this area and not acidic media. Although this unit had a water wash system installed with proper distribution equipment, its location was not at an opti- mal spot. The presence of the uncorroded surface along the pipe bottom was evi- dence that pH control was maintained by neutralizer application practice at the time. However, the water wash was inade- quate in reducing contaminant levels suf- ficiently to prevent salt formation down- stream. The location for water injection was several feet upstream of the pipe elbow at the bottom of the vertical run of pipe coming down the tower. This site left very little time for the distributed spray from the nozzle to interact with the vapor before the directional change collapsed the spray pat- tern into a stream along the bottom. Hence, FIGURE 4 NH 4 Cl salt formation nomographs. FIGURE 5 As-received photo of the overhead line. Blue arrows indicate corrosion. FIGURE 6 Outside view of through wall failure.

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