ABSTRACT

Corrosion of overhead system in the distillation unit has been a chronic and unresolved issue even with the injection of neutralizer and corrosion inhibitors. Recently, a light naphtha discharge pump (made of UNS J91150 casing and impeller) malfunctioned, which caused severe damage to the impeller. It was subsequently realized that, prior to the malfunction of the impeller, the pH of the water at the second overhead accumulator unexpectedly dropped to 2.6 with simultaneous increase of Fe concentration. Preliminary analysis of the impeller revealed that it had failed from a combination of corrosion and cavitation. To understand the origin of corrosion, extensive water chemistry analysis was performed, which revealed the presence of corrosive species such as sulfate and nitrate ions, presumably from the flue gas, which would have caused pH to drop. Autoclave corrosion tests also revealed that these species would cause significant corrosion of UNS S41000 which is similar to UNS J91150 used in the pump. In-depth electron microscope study (i.e., SEM, EBSD) on corrosion scale and damaged impeller surface was performed to confirm that the damage mechanism was corrosion-induced cavitation. Details of the analysis will be presented and discussed.

INTRODUCTION

Corrosion in the crude unit overhead is a complex phenomenon that impacts refinery reliability and profitability. It is well known in the refining industry that hydrochloric acid (HCl) is the primary corrosive substance to cause overhead system corrosion. Neutralizer, film-forming inhibitors, and wash water systems are some of the typical industry practices to mitigate acidic attack by condensed water containing HCl. The pH and concentration of chloride ion of the water at overhead accumulator is monitored for corrosion control.1-3 Although most of overhead corrosion is well controlled as expected based on properly designed metallurgy and chemical programs, there are still unresolved corrosion issues.

Recently, the pH of the water at the second overhead accumulator of condensate splitting unit unexpectedly dropped to 2.6, with a simultaneous increase of Fe concentration of up to more than 50 ppm. Several days later, a light naphtha (LSR) discharge pump (made of UNS J91150) malfunctioned. Neutralizer injection into the second stage was immediately applied which recovered the pH and Fe concentrations. Early investigations revealed that the pump was out of order from the severe damage of the impeller, which might be corrosion-related. Field monitoring data showed that the chloride ion concentration of water in second stage accumulator was less than 10 ppm, which is still under the maximum limit. Extensive water chemistry analysis and laboratory autoclave corrosion test was performed to determine unknown substances causing the pH drop of the water and to measure the corrosion rate of various alloys in low pH water. In-depth electron microscope study including scanning electron microscope (SEM) and Electron backscattered diffraction (EBSD) of corrosion scale and impeller metal surface was performed as well to investigate the fundamental damage mechanism of pump impeller.

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