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Inspectioneering Journal

When Bad Things Happen to Good Pressure Vessels: A Story of Localized Metal Loss

By Michael Turnquist at Quest Integrity Group. This article appears in the November/December 2013 issue of Inspectioneering Journal

Introduction

While there are many types of damage mechanisms that can occur in a piece of equipment, localized metal loss is one of the most common. If an inspection reveals that metal loss has occurred, many questions are raised: Can I continue operating? If so, for how long? Is a repair needed right away? Finding a solution to these questions can be difficult and costly. However, a solution can be reached through the application of finite element analysis (FEA) as outlined in API 579/ASME FFS-1 Fitness for Service1.

This article describes the process of performing a fitness-for-service (FFS) assessment of a CO2 absorber tower located at a refinery, where internal corrosion was discovered on the vessel shell. Ultrasonic thickness (UT) inspection uncovered four localized regions of metal loss, referred to as local thinned areas (LTAs). Using the methodology identified in Part 5 of API 579, the localized metal loss was evaluated to determine if the damaged equipment could be considered fit for continued operation.

Inspection Data

Thickness data for each LTA was provided in the form of a 1 inch by 1 inch equally-spaced, rectangular thickness grid, and two sets of thickness readings were provided from separate inspection dates. This allowed for the estimation of a corrosion rate. When estimating the remaining life of a component subject to local metal loss, it is important to obtain a good estimate for the corrosion rate so that the equipment can be evaluated with respect to a future date, such as a planned shutdown. This helps determine whether the equipment can be operated up to the next scheduled shutdown without the need for repair.

Level 2. Vs. Level 3 FFS Assessment

Generally, as outlined in API 579, a Level 2 FFS assessment is performed before a Level 3 assessment, since the Level 2 assessment is simpler and performed more quickly than a Level 3. Level 2 calculations can generally be performed by hand, in a spreadsheet, or with the aid of a software package such as SignalTM Fitness-For-Service, while a Level 3 assessment requires numerical evaluation of the component, typically through the execution of FEA. However, Level 2 assessments present certain limitations: they can only be applied in instances of simple geometry and loading conditions. For example, when performing a local metal loss assessment, a Level 2 assessment cannot be performed if the LTA is too close to a structural discontinuity, such as a nozzle or manway. The presence of a structural discontinuity invalidates the Level 2 assessment procedure.

In the assessment of the absorber tower, only one of the four LTA’s was sufficiently far from a structural discontinuity for a Level 2 FFS assessment to be applicable. The absorber tower passed the Level 2 assessment at this location. This assessment considered loads due to gravity and internal pressure, as well as supplemental loading due to wind. The wind loads were calculated using ASCE-72, a standard that is commonly used for the evaluation of wind loading on a structure.

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