Special Emphasis Programs and TML Placement as Entry Points for RBI Implementation

Introduction: Finding the Entry Point into RBI

The application of risk-based inspection (RBI) typically begins with improving the management of assets such as pressure relief devices or aboveground storage tanks, or mitigating damage mechanisms such as corrosion under insulation (CUI). As the return on RBI investment produces results in the context of inspection deferment and optimum inspection effectiveness selection, the commitment can expand into other equipment types, plants, and business units for one’s organization.

Two common challenges aided by the guiding principles of API RP 580, both of which are addressed in this article, include:

• Implementing a special emphasis program (SEP) for CUI.
• Managing the efficiency of TML/CML placement for the purpose of returning an accurate representative corrosion rate for the identified damage mechanism.

Corrosion Under Insulation Special Emphasis Programs

The progression of CUI depends on multiple parameters including, but not limited to, environmental conditions, insulation material, material of construction, operating temperatures, and type of installation (e.g., coating on equipment surface). A sample life-cycle progression of failure by CUI is provided as follows:

• New equipment; coating and insulation in “as-new” condition
• Insulation becomes wet; water is trapped in low-lying areas
• Degradation of coating system begins
• Localized failure of coating system; CUI initiates on base metal
• CUI corrosion rates vary based on the parameters summarized previously
• Equipment/piping through-wall leak

The corrosion rate generated by CUI is directly related to:

• The surface temperature of the piping/equipment.
• Amount of water contained with the insulation and in contact with the equipment/piping surface. In order of increasing effectiveness against CUI, insulation types such as halide controlled mineral wool, fiberglass, perlite, and foam glass are used.
• Percentage of time operating in a dry external service versus percentage of time operating in a wet external service.

Applying a SEP RBI process that generates an inspection plan for CUI will require follow-up assessments for the lifetime of the asset and follow a workflow such as the one illustrated in Figure 1 and can be summarized as follows: