Abstract
Atmospheric distillation is the primary unit of the oil refining industry. Any major upset or unplanned shutdown in this unit greatly affects the profitability of the refinery. As an industry's usual practice, depending on crude quality, plant configuration and fouling conditions, a time based inspection interval of four years is the optimum run length between two major overhauls.
Ever-growing demand for petroleum products and utilization of existing plants beyond the design capacity is not uncommon across the industry. Any process changes, usually up to 10 to 15% over the design capacity, may not affect plant reliability and integrity, but subsequently any changes and deviations in the process conditions, will significantly affect the overall safety and integrity of plant. In-service monitoring program may not often be effective to detect degradation such as localized corrosion without detailed internal inspection of critical equipment.
For first time, after approximately 25 years of service, during a recent four year run length of the distillation tower shell has experienced severe localized thinning and erosion-corrosion in the overhead section (interface of Monel† clad plate). This paper describes in detail the cause of failure including design deficiency, process and material changes made over a period of time, synergistic corrosion phenomena, corrosion monitoring and mitigation measures undertaken to overcome the problem.
