Phase Analysis Discrepancy on a Stainless Steel Tubing String
A power plant condenser has 304 stainless steel tubes (19.05 mm OD, 1.24 mm wall, conductivity ~1.4 %IACS, relative permeability 1.0). Your Level II technician ran a 300 kHz absolute bobbin coil and flagged 14 tubes with indications in the support-plate regions, calling them 'OD pitting at 30-40 % wall loss.' The client is preparing to plug those tubes. You review the data and notice: (1) the flagged indications are all within ±15 mm of the support plates, (2) the phase angles cluster tightly at about 200 degrees, while your reference tube's OD EDM notch reads about 160 degrees and the ID notch about 100 degrees, (3) the amplitude of the flagged signals is only 1.5-2x the noise floor, and (4) the reference tube was made of the same 304 alloy. The support plates are 304 stainless, not carbon steel.
1. Assess the technician's call of OD pitting at 30-40 % wall loss. What is the most likely explanation, and what evidence supports it?
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The call is not defensible. Several red flags: the indications are localized to support-plate regions; the phase angle (~200 degrees) does not match either the OD notch (~160 degrees) or the ID notch (~100 degrees), so the phase does not support an OD-wall-loss interpretation; and the amplitude is near the noise floor, which is inconsistent with 30-40 % wall loss on a 1.24 mm wall at 300 kHz.
The most likely explanation is support-plate interaction: 304 stainless support plates are conductive and, although non-ferromagnetic, they perturb the eddy current field and produce signals that can mimic wall loss when the coil passes the plate. The tight phase clustering at a non-defect phase is characteristic of a systematic artifact, not random pitting. The correct action is to re-examine with a support-plate suppression technique (dual-frequency mix or a rotating coil at the support regions) and to re-establish the phase reference on a tube with known ID and OD notches at the same alloy and wall.
2. What frequency and reference-standard changes would you require before re-inspecting, and why?
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At 300 kHz on 1.24 mm 304, the standard depth of penetration is roughly 0.6-0.9 mm, so the wall is thicker than the effective depth and OD signals are weak and phase-compressed. Lower the frequency to roughly 100-150 kHz to push penetration through the wall and improve OD phase separation. Rebuild the reference standard with EDM notches at 10 %, 20 %, and 40 % of wall on both ID and OD surfaces in the same 304 alloy, and verify phase separation ≥40 degrees between ID and OD. Also include a support-plate simulation (a 304 plate of the same thickness as the plant plates) so the mix can be tuned to suppress the plate signal while retaining defect sensitivity.
3. The client wants to plug the 14 tubes immediately. What is your recommendation as the Level III?
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Do not plug on the basis of the current data. The indications are unverified artifacts until re-inspected with a technique that suppresses the support-plate signal and with a properly built reference standard. Recommend: (1) hold the plugging decision; (2) re-inspect the 14 tubes plus a sample of adjacent clean tubes with the revised frequency and mix; (3) if a rotating coil confirms wall loss, plug; if the signals disappear under support-plate suppression, return the tubes to service and document the finding. This protects the client from unnecessary plugging and from a false-clear if real wall loss is present.