Inspectioneering Journal

The Role of Process Unit Corrosion Reviews on Inspection Planning, RBI and IOWs

By Alphonse W. Hegger Jr., P.E., Staff Consultant at Stress Engineering Services. This article appears in the November/December 2014 issue of Inspectioneering Journal.


Today, more than ever, refinery operators are focused on maintaining safe operations for their employees and extending the life cycle of valuable operating equipment to optimize efficiencies and avoid expensive unforeseen shutdowns. When process unit corrosion assessments, risk-based inspection (RBI) methodologies, and integrity operating windows (IOWs) are properly integrated into refining operations, the result is improved safety and longer run times.


Corrosion assessments for refinery process units have been performed informally for many years to assist inspection planning for stationary pressure equipment. With the introduction of RBI (circa 1992), corrosion reviews became more formalized and the likelihood of the potential damage mechanism was integrated into the work process. In generic terms, the likelihood (or probability factor) is the chance that a specific damage mechanism will penetrate the pressure boundary over some defined interval. For general corrosion damage mechanisms, the probability factor is a function of the corrosion rate. For damage mechanisms such as environmental cracking which do not necessarily propagate through the wall thickness at a uniform or predictable rate, the probability factor is based on a process sensitivity assessment. Probability factors will generally follow the likelihood component of the owner-user risk matrix. API 581, Second Edition, Section 2 provides a basis for probability factors for many damage mechanisms, which can be adjusted to match the owner-user risk matrix. API RP 571 has captured and defined most of the corrosion damage mechanisms used in corrosion reviews. For the refining industry, this standard has frequently employed “process specific” damage mechanisms. Process specific damage mechanisms are more conducive to the identification of necessary process monitoring and the development of IOW’s, in comparison to general or generic damage mechanisms.

The first step in a proper corrosion assessment involves a survey of the existing materials of construction, a survey of the primary stream compositions, and a survey of the temperature profile. Equipment and piping are divided into common corrosion loops. Corrosion loops are characterized by similar metallurgy, process environments, anticipated damage mechanisms, and damage rates. The development of common corrosion loops improves the efficiency of the subsequent inspection planning process. It is common for corrosion loops to change when there is a change in the metallurgy, temperature, or process chemistry.

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