Procedure for Eddy Current Testing of Non-Ferromagnetic Tubes

Eddy Current Testing (ECT) is a non-destructive testing method widely used for the inspection of non-ferromagnetic heat exchanger and condenser tubes. Because these materials, such as copper-nickel alloys, stainless steel, and titanium, do not exhibit magnetic properties, they are ideally suited for ECT, which relies on the electromagnetic induction principle.

1. Purpose and Scope

The primary objective of this procedure is to detect, locate, and size discontinuities such as pitting, cracking, wall thinning, and erosion in non-ferromagnetic tubing. This ensures the structural integrity and operational safety of critical industrial components.

2. Equipment Requirements

To ensure accurate results, the following equipment is necessary:

• Eddy Current Instrument: A multi-frequency digital flaw detector capable of phase analysis and real-time signal processing.
• Probes: Bobbin probes are typically used for routine inspections, while array probes or rotating probes (RFT) may be used for specific defect characterization.
• Calibration Standard: A tube of identical material, diameter, and wall thickness as the component under test, featuring artificial defects such as EDM notches or flat-bottom holes.
• Data Acquisition Software: For recording signals and performing post-test analysis.

3. Pre-Inspection Preparation

Before testing begins, the tubes must be thoroughly cleaned. Any debris, sludge, or scale inside the tubes can cause background noise or interfere with the probe's sensitivity, leading to false calls. The tube sheet should also be inspected for accessibility.

4. Calibration Procedure

Calibration is the most critical step in the ECT process. It involves setting the instrument to recognize known discontinuities:

1. Insert the probe into the calibration standard.
2. Adjust the gain and phase angle to ensure that the signals from the artificial defects (e.g., 20%, 40%, and 60% through-wall holes) are clearly distinguishable and oriented on the impedance plane.
3. Perform a "balance" operation to ensure the system is at the null point for a defect-free section of the tube.

5. Inspection Execution

Once calibrated, the inspection proceeds as follows:

• Probe Insertion: The probe is inserted into the tube. Most modern systems utilize a motor-driven pusher-puller unit to ensure a constant, steady speed of travel, which is essential for signal repeatability.
• Signal Monitoring: The operator monitors the screen for deviations from the baseline signal. Signals appearing at specific phase angles are flagged for further investigation.
• Data Recording: All data, including the tube identification and signal characteristics, must be captured digitally for final reporting.

6. Data Analysis and Evaluation

The evaluation phase involves interpreting the signals captured during the inspection. Technicians analyze the amplitude and phase shift of the signal to estimate the depth and volume of the identified defect. Discontinuities are categorized based on their severity relative to the rejection criteria established by the site specification or relevant industrial codes.

7. Reporting and Documentation

A final report must be generated, which includes:

• Identification of the heat exchanger or system tested.
• Calibration records and equipment settings.
• Summary of findings, including the location and estimated size of all detected flaws.
• Recommendations for further action, such as plugging, sleeving, or replacing affected tubes.

8. Safety Considerations

All personnel must adhere to site-specific safety protocols. This includes the use of personal protective equipment (PPE), ensuring proper electrical grounding of the test equipment to prevent shock, and practicing caution regarding the hazardous environments often found near heat exchangers and boiler systems.