Connecting the Proper Inspection Strategies to Damage Mechanisms

Introduction

To perform an effective inspection, one must develop a strategy that provides value. Otherwise we are wasting money, time, and creating a false sense of security or raising issues where there are none. The value of the inspection strategy should be tested in various ways, for example:

• Will the output improve the accuracy of my equipment reliability prediction?
• Will the output reduce the uncertainty/ies in my risk-based inspection analysis sufficiently?
• Will the output improve the accuracy of my fitness for service prediction, if applicable?
• Will the output achieve compliance with applicable codes and regulatory requirements?

For years the industry has performed inspections that yielded minimal value in answering the above questions. For example, perhaps the damage mechanisms of concern were crack-like. For many years we just used visual inspection and basic NDT, such as spot UT thickness readings, and thought that was OK for everything. The industry has also confused mechanisms, especially hydrogen-related ones like HIC/SOHIC with sulfide stress cracking and high temperature hydrogen attack. The industry has not always understood the connection between the limitations and benefits of NDT methods in their probability and accuracy of flaw detection, characterization, measurement or monitoring damage mechanisms of concern. This resulted in confusion about what types of NDT to select to detect, characterize and size the damage and often led to over-conservatism in fitness-for-service (FFS) calculations, perhaps. 

Thanks to the work of the American Petroleum Institute’s committees on inspection and corrosion and materials, we now have documents like API RP 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry that provide important information on damage mechanisms. This information helps inspection strategists understand what the various types of damage look like and when and where to expect them both within the process or system and in the component. In addition, the recommended practice addresses materials of construction, parameters causing and driving the damage, and damage mitigation practices. There is some guidance provided, although minimal, on suggested NDT methods for detecting the specific type/s of damage.

A new Recommended Practice entitled, API RP 586, NDT Methods for Equipment Damage Mechanisms is under construction. This document will provide more thorough guidance on NDT methods, sub-methods (techniques) for common inspection applications, and types of degradation on a component level. The document is being built in stages, starting with heat exchanger tubing inspection, followed by pressure vessel inspection, piping inspection, and tank inspection. Its expected publication date is year end of 2020.