Materials Performance

AUG 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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47 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 the process environment and the severity of historical H 2 S cracking in that equip- ment. If the material is likely to suffer, or has already suffered high corrosion rates due to the presence of H 2 S in the process stream, a coating system may be the best solution to protect the asset. While this is not a long-term solution to HIC, it has helped to prolong the life of the affected vessels and saved replacement costs. The decision to apply a coating system is based on many factors, including the time remaining until the next turnaround, shutdown, or other optimal maintenance points. In other words, coatings may not be used as the permanent solution. Realisti- cally, however, this may mean several years of relying on the coating for protection, since asset replacement is expensive and often requires process downtime. The age of the equipment is also a factor. A new piece of equipment would not be installed with a plan to coat it, but a previously exist- ing asset may be coated as part of preven- tive maintenance. The practicality and logistics of apply- ing the potential coating are also consid- e re d b e f o re u si n g a c o a t i n g sy st e m . Although their usage is widespread in some facilities, coatings are stopgap arrange- ments with limitations. Most coatings do not perform well with hot streams, and can flake off and fail; the surfaces being worked on require safe human access; and some asset geometries do not facilitate easy application, such as towers or exchanger sh el l s w ith many att a chm ent s. Stitch welds, which are also present on towers, cannot be properly coated due to crevices between adjacent stitch welds that cannot b e p en etrat ed by th e b arri er c o atin g. Attempting to coat an exchanger with a bundle or baffle plates is also difficult and not usually done, because the bundle or p l a t e s , w h e n re i n s e r t e d , m a y c a u s e mechanical damage to the newly installed coating. Drums tend to work well for coat- ing application because they are open and have accessible weld seams. There are also surface temperature limitations to ensure the coating cures properly. These tempera- ture limitations have a wide range due to the multitude of coating types. Some coat- ings are engineered to withstand hot envi- ronments up to 500 °F (260 °C). FIGURE 4 A photo taken during the subsequent turnaround of approximately the same area shown in Figure 3. FIGURE 5 A close-up photo shows the same area depicted in Figure 4 during the subsequent turnaround. A saturated light ends unit at the same refiner y, w hich includes C 1 through C 6 hydrocarbons, is in use for less than six months ever y six years. The inspector stated that coating systems have been used nearly everywhere in the unit. These coat- ings have held up satisfactorily and pro- vided adequate corrosion protection. The coating adhered well to the metal surface and prevented corrosion and/or cracking due to hydrogen damage. Equipment included in the coated areas are the caustic knockout drum, debutanizer overhead drum, and diethanolamine (DEA) wash tower. For coatings that were applied correctly, no further corrosion was found, which indicated the coating created an effective barrier. At regions where coating defects were present, hydrogen permeation was noticed. Examples include manway areas. These breaches were attributed to improper application of the coating system. In each area that the coating failed, the refinery plans to remove the coating and coat the surfaces afresh. Some failure is expected, but this does not deter the com- pany from using the same coating in the future. The refinery is also experimenting with spray coating application instead of the traditional coating methods using brushes, rollers, and mitts; and anticipates faster coating times and better coating adhesion. Coating Application to Minimize Hydrogen Cracking in Gas Dehydration Units

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