Pressure Vessel Mechanical Integrity Program: Integrating Eddy Current Testing, IRIS, RFET, and MFL Inspections

A pressure vessel mechanical integrity (MI) program is an important element of process safety and asset reliability across refineries, chemical plants, power generation facilities, and other industrial operations. The objective of an MI program is to ensure that pressure vessels remain fit for service throughout their lifecycle by identifying degradation mechanisms, applying appropriate inspection methods, and implementing corrective actions.

Example eddy current tube inspection field inspection process.

An effective MI program begins with damage mechanism identification and risk-based planning. Industry standards such as API 510 and API 581 lay the groundwork for understanding how process conditions, materials of construction, operating temperatures, and environmental exposure influence degradation. Common pressure vessel threats include general corrosion, localized pitting, corrosion under insulation (CUI), erosion, fatigue, and various forms of stress corrosion cracking. Because many of these mechanisms initiate below surfaces or beneath insulation systems, inspection methods must extend beyond visual examination alone.

Non-destructive testing (NDT) plays a critical role in detecting and characterizing pressure vessel degradation. Conventional ultrasonic thickness testing remains a primary tool for quantifying remaining wall thickness; however, advanced methods such as eddy current testing (ECT) provide additional value when access is limited or when early-stage damage must be identified. ECT techniques are particularly effective for examining thin-wall components, heat exchanger tubing, and localized surface or near-surface degradation. Modern ECT systems analyze probe response using impedance-plane displays and multiple inspection frequencies, allowing inspectors to differentiate true metal loss from geometric or support-related signals.

Technical Capabilities of IRIS, ECT, RFET, and MFL Tube Inspection Techniques

1. Introduction

Tube inspection is a critical component of asset integrity management for heat exchangers, boilers, air coolers, and similar tubular systems. The degradation mechanisms affecting tubes—such as corrosion, erosion, pitting, fretting, and flow-accelerated wear—can lead to loss of pressure boundary, efficiency reduction, or catastrophic failure if not detected early. To address these risks, a range of nondestructive examination (NDE) technologies has been developed, each with unique physical principles and performance characteristics.

Among the most widely applied techniques are Internal Rotary Inspection System (IRIS) ultrasonic testing, Eddy Current Testing (ECT), Remote Field Eddy Current Testing (RFET), and Magnetic Flux Leakage (MFL) inspection. Understanding the technical capabilities and limitations of each method is essential for selecting the appropriate inspection strategy.

IRIS (Internal Rotary Inspection System)

IRIS is an ultrasonic technique capable of producing absolute wall thickness measurements for both ferrous and non-ferrous tubes. A transducer mounted within a turbine-driven probe emits an ultrasonic pulse along the axis of the tube. The pulse is redirected by a 45° rotating mirror, sweeping circumferentially around the tube wall as the probe is withdrawn at a controlled speed. The ultrasonic signal reflects from the internal diameter (ID) surface and then from the external diameter (OD) surface. By measuring the time-of-flight difference between these echoes, IRIS calculates true wall thickness and provides a detailed cross-sectional map of the tube.

Figure 1: Diagram illustrating the IRIS transducer, rotating mirror, turbine, and ultrasonic signal reflecting from the ID and OD surfaces of the tube.

IRIS is particularly effective for detecting and sizing:

• General corrosion and erosion

• Localized OD pitting

• Baffle wear and baffle cuts

• External corrosion near tube supports

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Despite its high accuracy, IRIS requires thorough tube cleaning and water coupling. Fouling, debris, or scale can significantly attenuate the signal and compromise data quality. Additionally, IRIS inspection speeds are relatively slow, making it best suited for baseline inspections or confirmation of screening results.

Figure 2: IRIS C-scan and thickness profile showing deep tube erosion.IRIS Thickness

3. Eddy Current Testing (ECT)

ECT is an electromagnetic technique used primarily on non-ferromagnetic materials, including cop per alloys, aluminum, and austenitic stainless steels. An energized coil induces eddy currents in the tube wall. Variations in wall thickness, conductivity, or geometry disturb these currents, producing measurable changes in signal amplitude and phase.

Figure 3: Eddy current absolute and differential probe setup for tube inspection.

ECT excels at detecting:

• ID and OD pitting

• Fretting and wear at tube supports

• Axial and circumferential cracking

• Small localized defects

  
  

Modern multi-frequency ECT systems allow analysts to suppress noise from tube supports and enhance sensitivity to specific defect types. However, ECT provides relative measurements, not absolute wall thickness, and requires careful calibration and expert interpretation.

4. Remote Field Eddy Current Testing (RFET)

   
   

RFET is specifically designed for ferromagnetic tubes, such as carbon steel. Unlike conventional ECT, RFET relies on the remote field effect, where the electromagnetic signal passes through the tube wall, travels along the outside of the tube, and re-enters the wall before reaching the receiver coil.

Figure 4: RFET Signal Propagation Path

Because the signal traverses the wall twice, RFET is sensitive to both internal and external metal loss, making it well suited for identifying corrosion mechanisms that may be inaccessible to direct inspection.

RFET is commonly used for:

• OD corrosion under deposits

• General wall thinning

• ID and OD pitting in carbon steel tubes

  
  

Resolution is lower than IRIS, and inspection speeds are slower than conventional ECT, but RFET remains one of the most effective screening tools for ferromagnetic tubing.

5. Magnetic Flux Leakage (MFL)

   
   

MFL inspection magnetizes ferromagnetic tubes to near saturation. Where metal loss is present, magnetic flux leaks from the tube wall and is detected by sensor arrays positioned along the probe.   Magnetic Flux Leakage (MFL) tube inspection relies on controlled magnetization of ferromagnetic tubing and the detection of magnetic flux that escapes the material at locations of wall loss or geometric discontinuity. Modern MFL probes employ multiple sensor types, most commonly lead sensors, trail sensors, and absolute sensors, arranged circumferentially around the probe. Each sensor type responds differently to flux leakage patterns, providing complementary information that improves defect detection, discrimination, and characterization.

MFL is particularly effective for identifying:

• General corrosion

• Large-area metal loss

• Severe localized thinning

MFL inspections can be performed at relatively high speeds and are less sensitive to electrical conductivity variations. However, MFL data is typically qualitative or semi-quantitative, and precise defect sizing usually requires follow-up inspection using IRIS or ultrasonic techniques.

Tables 1 through 3 provide a comparative summary of the inspection techniques discussed in this article, highlighting their applicability by tube material, defect detection capability, and key inspection characteristics. Together, these tables are intended to support informed selection of inspection methods by illustrating the relative strengths and limitations of IRIS, ECT, RFET, and MFL when applied to common degradation mechanisms and service conditions. While no single technique offers complete coverage for all damage types, the comparisons demonstrate how these methods are often used in a complementary manner to achieve reliable, code-aligned inspection outcomes.

6. Conclusion

   
   

IRIS, ECT, RFET, and MFL each serve distinct roles within a comprehensive tube inspection program. Understanding their physical principles and technical capabilities enables asset owners to select the most effective inspection strategy for their specific equipment and degradation risks. When applied in a complementary manner, these technologies provide the data necessary to support safe operation, informed maintenance planning, and long-term asset integrity.

REFERENCES

1. ASME Boiler and Pressure Vessel Code (BPVC), Section V – Nondestructive Examination, The American Society of Mechanical Engineers, New York, NY, latest edition.

2. ASME Boiler and Pressure Vessel Code (BPVC), Section I – Rules for Construction of Power Boilers, The American Society of Mechanical Engineers, New York, NY, latest edition.

3. ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1 – Rules for Construction of Pressure Vessels, The American Society of Mechanical Engineers, New York, NY, latest edition.

4. ASME PCC-2 – Repair of Pressure Equipment and Piping, The American Society of Mechanical Engineers, New York, NY, latest edition.

5. ASNT SNT-TC-1A – Personnel Qualification and Certification in Nondestructive Testing, American Society for Nondestructive Testing, Columbus, OH, latest edition.

6. ISO 15548-1 / ISO 15548-2 – Non-destructive Testing — Equipment for Eddy Current Examination, International Organization for Standardization, Geneva, Switzerland.

7. ASTM E243 – Standard Practice for Electromagnetic (Eddy Current) Examination of Copper and Copper-Alloy Tubular Products, ASTM International, West Conshohocken, PA.

8. ASTM E2096 – Standard Guide for Eddy Current Examination of Carbon Steel Tubes Using Remote Field Testing, ASTM International, West Conshohocken, PA.

9. ASTM E1444/E1444M – Standard Practice for Magnetic Particle Testing, ASTM International, West Conshohocken, PA.

10. ASTM E317 – Standard Practice for Evaluating Performance Characteristics of Ultrasonic Transducers, ASTM International, West Conshohocken, PA.

11. ASME Section XI – Rules for Inservice Inspection of Nuclear Power Plant Components (referenced for ultrasonic and electromagnetic NDE methodology and qualification principles).

12. API RP 571 – Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, American Petroleum Institute, Washington, DC.

13. API RP 579-1/ASME FFS-1 – Fitness-For-Service, American Petroleum Institute / ASME, Washington, DC / New York, NY.

14. ASNT NDT Handbook, Volume 4 – Electromagnetic Testing, American Society for Nondestructive Testing, Columbus, OH.

15. ASNT NDT Handbook, Volume 7 – Ultrasonic Testing, American Society for Nondestructive Testing, Columbus, OH.

16. TechKnowServ Corp., IRIS, ECT, RFET, and MFL Inspection Training Materials and Case Studies, internal technical documentation and field data.