Inspectioneering Journal

Breakthroughs in Predictive HTHA Life Assessment: Time Dependent Nelson Curves and Inspection Planning Enhancements

By Dave Dewees, P.E., Mechanical Engineering Division Manager at Becht, and Gerrit Buchheim, P.E., Corrosion & Materials Expert and Pono Division Manager at Becht. This article appears in the January/February 2022 issue of Inspectioneering Journal.


Nearly two years ago, in an Inspectioneering Journal article, Practical HTHA Experience and Time-Based Nelson Curves for Improved Equipment Life Management, we described how to incorporate well-established void growth modeling (see Figure 1) into a high temperature hydrogen attack (HTHA) assessment approach [1]. Since then, we have made the following advances:

  • Inclusion of pressure and weld residual stresses, and nucleation effects [2, 3]
  • Addition of C-0.5Mo material ([4, 5])
  • Direct relation of the damage and life fractions the model predicts to both microstructural damage and toughness loss
  • Definition of a simple but predictive through-wall failure criterion
  • Model informed HTHA flaw detection and sizing guidance for component life extension
  • Validation of through-wall damage predictions in almost 100 case histories

Taken together, these advances have enabled a breakthrough in predictive component life assessment for carbon and C-0.5Mo steels in high temperature hydrogen service.

The ability to relate model predictions to physical damage is exciting and is a major focus of our planned work over the next 1 to 2 years. In fact, a primary motivation for pursuing a more quantitative model was the repeated HTHA nondestructive examination (NDE) false calls and the strain it put on industry. HTHA NDE has evolved significantly in the last five years, and we believe we’re at an inflection point where modeling and inspection can be used together to make safe and informed decisions in a much shorter time frame and with much less disruption. A real-life example is given to illustrate how this can be accomplished with an approach that will be validated by leveraging extensive ex-service material.

With a focus on practicality, we end this article by presenting time-dependent Nelson-Curves that consolidate volumes of information into convenient charts, which we hope will make much of the progress this article describes readily available to those who need it.

Figure 1. Idealized Void Growth Models.
Figure 1. Idealized Void Growth Models.

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