Risk-based inspection (RBI) methods have been around the refining and petrochemical industry for nearly 3 decades. The concept of focusing inspection programs on areas of highest risk has proven to be an effective approach for managing fixed equipment integrity and optimizing inspection programs. For RBI, risk is defined as the product of probability of failure and consequence of failure. Related to fixed equipment, the focus is on losses of primary containment (LOPC) and the negative consequences associated with those losses.
All RBI programs rely assignment of damage mechanisms to properly assess the probability of failure. Understanding the credible damage mechanisms and their levels of severity is critical to proper categorization of risk and planning future inspections. Regardless of the type of RBI assessment, qualitative or quantitative, the severity of the expected damage mechanisms directly affects the determination of probability of failure. Severity is this sense means either the corrosion rate determination or susceptibility to an environmental cracking mechanism, or in some cases both.
The problem with this type of assessment is assumptions are made that the damage mechanisms and levels of severity assigned today will continue to be the same in the future. That’s where IOWs come into play. Integrity Operating Windows (IOWs) are defined as established limits for process parameters that affect the integrity of fixed equipment and piping. When operating parameters deviate from these limits, degradation is more likely to occur. IOWs should be considered a subset of a larger group of unit operating parameters. If operations are kept within these limits, degradation should be predictable.
IOWs provide limits to ensure that the original, static damage assessments are still valid. For example, a corrosion rate would be assigned to the equipment and piping in a fractionator tower overhead system as part of an RBI analysis. This system would also normally have IOWs around chemical injections, water wash, containments, and/or pH. If the process is kept within the IOW limits, then it is likely the original corrosion rate assessments would be valid.
The example above also illustrates another key interaction between IOWs and RBI programs. If the IOW limits associated with the Fractionator overhead are exceeded on some basis, it indicates that the original RBI assessment should be reevaluated based on the new information. Corrosion rates or cracking potentials could be significantly worse than originally expected causing a higher risk for the system.
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For more information on Integrity Operating Windows and Risk-based Inspection, please contact Becht.