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Comparison of NDT Methods for Heat Exchanger Tube Inspection

ECT · PSEC · FSEC · RFT · NFT · MFT · IRIS

Comparative Reference Card · Rev. 2 · DELTA TEST ME LLC

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ECT

Conventional Eddy Current

Applicable for: All non-magnetic materials (SS, Cu-Ni, Ti, Inconel, brass)

Advantages

  • +Suitable for all paramagnetic / non-magnetic metals
  • +Very good defect depth & size estimation (phase)
  • +Internal + external defects (ID/OD)
  • +Suitable for WT up to ~5 mm
  • +High speed — up to 100 tubes / h
  • +Tube-sheet area with rotating probes
  • +Defects under baffle plates detectable
  • +Cracks (circumferential + longitudinal)
  • +No coupling medium (water) needed
  • +Cleaning less stringent than IRIS
  • +Defect type need not be known in advance

Disadvantages / Limitations

  • −Not applicable for ferromagnetic tubes (CS, LAS)
  • −Magnetic deposits → false signals
  • −No absolute wall thickness
  • −Skilled personnel required (calibration)
  • −Reference standard must match production tube
PSEC

Partial Saturation EC

Applicable for: Slightly / partially magnetic (Duplex, super-duplex, CW SS, thin CS)

Advantages

  • +Bridges the µr gap between ECT and FSEC
  • +Internal + external corrosion
  • +Cracks (circ. + long.) — phase preserved
  • +ID/OD discrimination via phase
  • +Defects under baffle plates
  • +High speed — up to 70 tubes / h
  • +U-bend capable (compact magnet)
  • +Tube-sheet area with rotating probes
  • +No coupling medium needed
  • +Same instrument as ECT and FSEC
  • +Covered by ASTM E2884

Disadvantages / Limitations

  • −Mixed defect types hard to interpret
  • −Depth via amplitude in saturated zones
  • −No absolute WT measurement
  • −Highly skilled personnel required
  • −Accuracy ± 10 % to ± 20 %
  • −Sensitive to material variations
FSEC

Full Saturation EC

Applicable for: Strongly ferromagnetic (CS, LAS, T11, T22, P91, ferritic SS)

Advantages

  • +DC bias → permeability noise eliminated
  • +Internal + external corrosion
  • +Stable, predictable skin depth
  • +ID/OD discrimination via phase
  • +Defects under baffle plates
  • +General wall loss reliably detected
  • +High speed — up to 70 tubes / h
  • +Tube-sheet area with rotating probes
  • +No coupling medium needed
  • +Same instrument as ECT and PSEC
  • +Covered by ASTM E2884

Disadvantages / Limitations

  • −Heavy magnet → straight sections only
  • −Crack signature flattened by saturation
  • −Mixed defect types hard to interpret
  • −No absolute WT measurement
  • −Highly skilled personnel required
  • −Accuracy ± 10 % to ± 20 %
  • −Larger probe diameter than ECT/PSEC
RFT

Remote Field Testing

Applicable for: Ferromagnetic CS / LAS (heavy wall > 4 mm)

Advantages

  • +Effective for heavy-wall ferromagnetic tubes
  • +Inspection speed considerably > IRIS
  • +No coupling medium needed
  • +Tolerant to scaling / less cleaning effort
  • +Cracks (circumferential) detectable
  • +Equal ID/OD wall-loss sensitivity

Disadvantages / Limitations

  • −Not applicable for finned ACHE tubes
  • −Not sensitive to small local defects
  • −No reliable ID/OD discrimination
  • −Cannot detect defects under baffle plates
  • −Mainly limited to general wall loss
  • −10 – 20× slower than EC family
  • −Compressed phase-vs-amplitude dynamic range
  • −Highly skilled personnel required
NFT

Near Field Testing

Applicable for: Ferromagnetic finned ACHE tubes (OD wall loss + fin-root corrosion)

Advantages

  • +Optimized for ACHE finned tubes
  • +Detects fin-root corrosion (bobbin EC may miss)
  • +Internal probe — no external access required
  • +Faster than RFT (~ 0.3 – 0.6 m/s)
  • +No coupling medium needed
  • +Tolerant to fin geometry

Disadvantages / Limitations

  • −Niche method — finned ACHE tubes only
  • −Qualitative output (amplitude-based)
  • −Limited crack sensitivity
  • −WT range limited to 1 – 3 mm
  • −Manufacturer-specific calibration
  • −Cannot replace bobbin EC for ID-side defects
MFT

Magnetic Flux Leakage

Applicable for: Ferromagnetic CS only (requires external magnet contact)

Advantages

  • +General + localized corrosion in CS tubes
  • +Cracks (circumferential) detectable
  • +Fin-fan tubes with external geometry access
  • +No coupling medium needed
  • +Tolerant to internal scale / deposits

Disadvantages / Limitations

  • −Not for general gradual erosion
  • −Sizing of pits / defects not reliable
  • −Defects under baffle plates difficult
  • −Irregular pulling speed → misinterpretation
  • −Bad data on rough surfaces / scaling
  • −High set-up effort, error-prone
  • −Highly skilled personnel required
  • −No near-/far-surface discrimination
  • −Longitudinal cracks not detected
  • −Slower than FSEC
  • −Excluded from ACHE, U-bends, insulated tubes
IRIS

Internal Rotary Inspection System

Applicable for: All materials (incl. non-metals) — verification tool, not for screening

Advantages

  • +Absolute wall thickness in mm (high accuracy)
  • +Excellent defect geometry quantification
  • +Excellent defect size quantification
  • +Internal + external defects equally well
  • +Defects under baffle plates
  • +Suitable for general wall loss
  • +All material types (material-independent TOF)
  • +Provides datum for API 579 FFS

Disadvantages / Limitations

  • −Misses small pin holes (e.g. 1 mm dia. in SS)
  • −Very low speed (max. 6 – 12 tubes / h)
  • −High cost per tube due to low throughput
  • −Bare-metal cleaning essential
  • −Water coupling mandatory
  • −"Dirt reflection" causes false signals
  • −Pulling speed critical for 100 % coverage
  • −Pulling speed normally not closed-loop
  • −Water turbidity < 5 NTU required
  • −Skilled personnel for false-signal rejection
  • −Hard to apply in confined-space entry
  • −Tube must be filled with water
  • −U-bends cannot be inspected
  • −Sensor centring critical
Author: Stefan Köllner, DELTA TEST ME LLC · Comparative Reference Card, Rev. 2
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