Replacing Internal Visual Inspection with Non-Intrusive Techniques in Refineries: Practical Recommendations

The original promise of Risk-Based Inspection (RBI) was that better risk understanding (probability X consequence) would allow operators to reduce intrusive inspections, especially vessel entries, exchanger pull-and-clean, and widespread stripping, by substituting credible non-intrusive inspections (NII) and smarter targeting. In practice, that promise has largely not been fulfilled. Here are my reasons for this not happening:

• Despite marketing claims, most field-applied NII methods cannot reliably replace intrusive inspection for many refinery damage mechanisms.
• Corporate risk tolerance is extremely conservative. This is organizational risk aversion, not technical failure, but it defeats the RBI promise.
• Despite the RBI language in API 580/581, the standards API 510/570/653 still strongly anchor to direct internal condition assessment. Many inspectors and insurers explicitly distrust purely analytical justifications and unvalidated Probability of Detection (POD) claims.
• RBI has not solved the "Unknown Damage Mechanism" problem. NII, being specifically targeted, cannot reliably discover unknown mechanisms.

To fulfill the promise, I believe RBI would need to be paired with:

• Damage-mechanism-specific inspection strategies
• Validated POD curves for NII methods for each strategy
• Field verification of each NII's probability of detection
• Explicit risk acceptance criteria
• As a minimum industry consensus or at best regulatory alignment on equivalency

In modern refineries, internal visual inspection (IVI) has long been the standard for evaluating the integrity of pressure vessels, piping, heat exchangers, and tanks. While visual inspection is straightforward and well understood, it comes with limitations:

• Human factors such as fatigue and experience
• Poor detection of small pits (< 2-5 mm)
• Inability to detect subsurface defects
• Safety and access constraints

With advances in Non-Intrusive Inspection (NII) technologies, many refineries are now supplementing or even replacing IVI to increase Probability of Detection (POD) while reducing risk and downtime.

Why Move to NII?

Non-Intrusive Inspection methods are capable of detecting:

• General and localized corrosion
• Pitting and under-deposit corrosion
• Cracks, hydrogen-induced cracking (HIC), and sulfide stress cracking (SSC)
• Wall thinning under insulation

Moreover, these methods often provide quantitative measurements, enabling better trend analysis and risk-based maintenance decisions.

While consensus is still evolving, it isn't only about documents; it's also about shared understanding and competence across practitioners. Roadblocks still exist:

• Code equivalency language that would allow NII to replace intrusive inspections is still largely absent in major codes like API 510/570.
• Unified damage-specific POD thresholds for NII techniques are not yet universally agreed upon or codified.
• Regulatory acceptance frameworks remain "case-by-case" rather than universally accepted.

Therefore, in recognition of the fact that most company MI programs are not maintained by inspection experts, I want to start a conversation on what NII techniques are/could be equivalent to internal visual inspection. Below is a table that I have assembled to serve as a starting point. What are your thougnts? Comment below, join the conversation on LinkedIn, or contact me directly. I am excited to hear your thoughts.

REFERENCES

1. "Results of the Program for the Inspection of Nickel Alloy Components", August, 2010, S.E. Cumblidge, et al, United States Nuclear Regulatory Commission, https://www.govinfo.gov/content/pkg/GOVPUB-Y3_N88-PURL-gpo5392/pdf/GOVPUB-Y3_N88-PURL-gpo5392.pdf
2. "Non-Intrusive Inspection (NII) - Holistic inspection approach using advanced ultrasonic systems", 2024, Eddyfi Technologies, https://content.eddyfi.com/m/2ce94fe1b3d8da86/original/Non-Intrusive-Inspection-NII-Applications-Brochure.pdf
3. "Focused Computed Tomography", 15th World Conference on Non-Destructive Testing, October, 2000, Gede B. Suparta, https://www.ndt.net/article/wcndt00/papers/idn143/idn143.htm
4. "Publication of HOIS REcommended Practice for Non-Intrusive Inspection and Guidance Notes", 11/3/2020, British Institute of Non-Destructive Testing, https://www.bindt.org/News/March-2020/publication-of-hois-recommended-practice-for-non-intrusive-inspection-and-guidance-notes/

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