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FSEC · RFT · MFT · IRIS

Tube Inspection Methods for Air-Cooled Heat Exchangers (ACHE)

Technical Reference Leaflet · Rev. 1 · API 510 / API 661 / ASTM E2884

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Overview

Air-cooled heat exchangers (ACHEs) use carbon steel or low-alloy steel tubes with external aluminium or steel fins. This geometry creates a unique inspection challenge: external methods requiring surface contact (MFT) are physically blocked by the fins, while internal methods must handle ferromagnetic material with variable permeability. This leaflet compares the three primary NDT methods — FSEC, RFT and MFT — and positions IRIS as the complementary verification tool for selected tubes.

Method Comparison

Parameter FSEC RFT MFT IRIS
Principle DC bias + AC eddy current Remote field (double wall transit) DC saturation, flux leakage (Hall) Rotating UT mirror, TOF wall measurement
Role Primary screening — 100 % coverage Alternative screening — limited applic. Not applicable (fins block magnet) Verification / FFS — selected tubes
Fin compatibility No influence — fully internal sensor Signal shift possible (Al fins alter phase) Not possible — magnet contact blocked No influence — fully internal sensor
Inspection speed 0.5 – 1.0 m/s (~30–60 s / 30 m) 0.05 – 0.3 m/s (~3–10 min / 30 m) N/A 0.02 – 0.05 m/s (~10–25 min / 30 m)
Wall thickness Relative (% loss), referenced to calibration std. Relative (% loss), phase vs. reference N/A Absolute (mm), ± 0.1 mm resolution
Ferromagnetic tube Excellent — DC bias eliminates µr noise Good — low freq. overcomes µr Requires saturation + external contact Excellent — material-independent TOF
Coupling / medium Dry — no water required Dry — no water required N/A Water-filled tube required; turbidity < 5 NTU
Optimum WT range 1.5 – 6 mm — ideal for typical ACHEs 2 – 8 mm — thin wall: sensitivity drop 3 – 12 mm — thin wall unsuitable Any wall thickness — frequency-independent
Key standards ASTM E2884, API 510, API RP 585, API 661 ASTM E2096, API 510 ASTM E570 ASTM E2929, API 510, API 579 (FFS)
Air cooler suitability Recommended — first choice for CS / LAS Restricted — speed & fin signal issues Not suitable — fin geometry incompatible Verification only — 5–15 % of tubes post-FSEC

Why FSEC Is the Preferred Method for ACHEs

1

Fin Geometry Is Not a Constraint

FSEC probes operate entirely inside the tube. Fins on the OD are invisible to the measurement process. Neither the DC bias coil nor the AC sensing coil has any interaction with external fin material, provided that fin-root corrosion on the OD is correctly represented in the calibration standard.

2

Ferromagnetic Material — Permeability Noise Eliminated

Carbon steel has a relative permeability µr of 100 – 1,000, depending on cold working, weld zones and thermal history. Conventional eddy current is swamped by permeability noise. FSEC applies a DC bias field sufficient to drive the tube into magnetic saturation (typically 0.5 – 2 T flux density). At saturation, the differential permeability µdiff → 1, so the superimposed AC field behaves as if the material were paramagnetic. Skin depth δ = √(2/ωσµ) becomes stable and predictable, enabling reliable phase-based signal analysis.

3

Inspection Speed Enables Full Bundle Coverage

At 0.5 – 1.0 m/s, a typical 400-tube bundle (30 m tubes) can be screened in 6 – 8 hours including set-up. RFT at 0.05 – 0.3 m/s requires 50 – 130 hours for the same bundle — economically incompatible with a 48 – 72 h turnaround window. MFT is physically excluded by the fin geometry.

4

Optimum Wall Thickness Range

Typical ACHE tube wall thicknesses are 1.8 – 3.2 mm (API 661 / TEMA R). FSEC frequency is tuned to set skin depth δ ≈ 1.2 – 1.5 × nominal wall, achieving the best balance of full-wall penetration and phase resolution between ID and OD defects. RFT suffers reduced phase discrimination at thin walls due to the exponential double-transit attenuation e−2d/δ.

IRIS — Quantitative Verification for Selected Tubes

IRIS (Internal Rotary Inspection System) is not a screening method. It is a metrological verification tool used on a small subset of tubes (typically 5 – 15 %) identified by FSEC screening. Its principal advantage over FSEC and RFT is that it measures wall thickness in absolute millimeters, independent of a reference calibration standard.

Physical principle: A focused ultrasonic beam is deflected 90° by a rotating mirror (3 – 15 rev/s). The time-of-flight between the ID echo and the OD echo is converted to wall thickness using the known longitudinal wave velocity in steel (c ≈ 5920 m/s). The helicoidal scan produces a C-Scan image with 1 – 2 mm trace spacing, providing both axial and circumferential defect geometry.

Wall thickness = t(ID → OD) / 2 × csteel

IRIS Selection Criteria

WT loss ≥ 20 – 25 % FSEC flag exceeds acceptance level per API 510; IRIS provides absolute value for RSF per API 579
Ambiguous phase signal Overlapping ID/OD defects at same axial position cannot be resolved by FSEC phase alone; IRIS C-Scan separates spatially
Pitting cluster Isolated deep pits require exact depth for pressure-retaining calculation
Weld / roll transition FSEC calibration less reliable at dissimilar material zones; IRIS measures TOF material-independently
Pre-plugging confirmation Irreversible action requires absolute WT datum; IRIS is the normative basis for plugging decisions

Recommended Inspection Workflow

  1. 1

    FSEC 100 % Screening

    All tubes scanned at 0.5 – 1.0 m/s → indicator list generated (amplitude + phase)

  2. 2

    Indicator Triage

    Apply acceptance threshold (typically ≥ 20 – 25 % WT loss) → flag ambiguous phase signals

  3. 3

    IRIS Verification

    Selected tubes filled and flushed → rotating UT scan, C-Scan recorded

  4. 4

    FFS Assessment

    Absolute WT from IRIS → API 579 RSF calculation → plugging / repair / next-inspection decision

IRIS Operational Requirements

  • Tubes must be fully flushed and water-filled — turbidity < 5 NTU for adequate UT coupling
  • Flushing, filling and subsequent draining / drying adds significant time — IRIS of all 400 tubes in the above example would require ~80 – 130 h of probe time alone
  • Sludge or scale deposits can block the rotating mirror or attenuate the UT signal
  • IRIS confirms the absolute WT datum required before an irreversible tube-plugging decision (API 510 § 6.5)

Key Takeaways

  • ▸FSEC is the only method combining ferromagnetic capability, fin-geometry compatibility and practical inspection speed for ACHE bundles.
  • ▸RFT is technically feasible but economically unviable for large bundles within typical turnaround windows.
  • ▸MFT is excluded by fin geometry — external magnet contact on finned OD is not achievable.
  • ▸IRIS provides the absolute wall-thickness datum required for API 579 FFS calculations and plugging decisions.
  • ▸Best-practice workflow: FSEC 100 % screening → IRIS verification of flagged tubes (5 – 15 %) → API 579 RSF → maintenance decision.

Normative References

ASTM E2884 Standard Guide for Eddy Current Examination of Tubing Using Partial Saturation
ASTM E2096 Standard Guide for In Situ Examination of Ferromagnetic Heat-Exchanger Tubes Using Remote Field Testing
ASTM E570 Standard Practice for Flux Leakage Examination of Ferromagnetic Steel Tubular Products
ASTM E2929 Standard Practice for Guided Wave Testing of Above Ground Steel Pipework and IRIS of Heat Exchanger Tubing
API 510 Pressure Vessel Inspection Code — § 6.5 Heat Exchanger Tube Inspection
API 661 Air-Cooled Heat Exchangers for General Refinery Service
API RP 585 Pressure Equipment Integrity Management — Inspection Technique Selection
API 579-1 / ASME FFS-1 Fitness-For-Service — § 4 / 5 Remaining Strength Factor (RSF) Calculation
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