Heat Exchanger Tube Inspection Services: Eddy Current, RFT and IRIS

IRIS vs eddy current for heat exchanger tubes: which method to use

Short answer: eddy current methods are the fast screening tools for whole tube bundles: conventional eddy current (ECT) for non-ferromagnetic tubes such as stainless, brass, copper-nickel and titanium, and remote field testing (RFT) or related techniques for carbon steel and other ferromagnetic tubes. IRIS is an ultrasonic rotary technique that measures actual wall thickness in any tube material. It is slower and needs clean, water-filled tubes, so it is usually applied to a sample or to tubes flagged by screening.

The choice is rarely "IRIS or eddy current". It is which eddy current variant fits the tube material, and how much IRIS to add for sizing and verification. The sections below explain how each method works, what each misses, which standards apply, and how to write a tube inspection scope that a plant engineer or API 510 inspector can rely on when deciding which tubes to plug, sleeve or retube.

How each technique works

Conventional eddy current (bobbin, rotating and array probes)

A probe inside the tube carries coils driven with alternating current. The coils induce eddy currents in the tube wall; wall loss, pits, cracks and changes in material alter the coil impedance, and the instrument displays those changes as signals that an analyst interprets against a calibration standard with machined reference defects. Bobbin probes are fast and suit volumetric wall loss and pitting screening; rotating and array probes add resolution for cracks and characterisation. ASTM E690 covers in situ eddy current examination of installed non-magnetic heat exchanger tubes with internal probes. Its own scope notes two limits worth remembering: eddy current density falls rapidly with distance from the inner surface, so sensitivity to outer-surface damage is lower, and indications at tube ends cannot be separated from nearby discontinuities. The standard does not set acceptance criteria; those come from the owner.

Remote field testing (RFT)

In ferromagnetic tubes, magnetic permeability swamps conventional eddy current signals. RFT uses an exciter coil and a detector placed some distance away, so the detected field has passed through the tube wall twice. The result is roughly equal sensitivity to inner and outer wall loss in carbon steel tubes. ASTM E2096 covers RFT of installed ferromagnetic heat exchanger tubes; its published scope covers tubes from 0.500 to 2.000 inch outside diameter with wall thickness from 0.028 to 0.134 inch. RFT is good at general wall loss and larger defects but has lower resolution than conventional ECT on small pits, and it is slower than bobbin ECT. Near-field testing and tube magnetic flux leakage are related techniques used on ferromagnetic tubes for specific damage, such as near-ID pitting on finned air-cooler tubes.

IRIS (internal rotary inspection system)

IRIS is ultrasonic. A probe centred in the tube sends an ultrasonic beam onto a rotating mirror, driven by the water flow, which sweeps the beam around the circumference while the probe is pulled out. The time difference between the inner and outer wall echoes gives a direct wall thickness measurement. Because it is ultrasonic, it works on ferromagnetic, non-ferromagnetic and non-metallic tubes, and the result is a thickness map rather than an impedance-derived estimate. That is why IRIS is widely used to verify and size indications flagged by RFT or ECT, and as a reference when calibrating screening results.

Side-by-side comparison

Exact performance depends on probe, frequency, tube size and analyst. Ask for the procedure's demonstrated capability on a reference tube that matches your material and size.

Choosing by tube material, damage and bundle condition

Start with three questions: what is the tube material, what damage do you expect, and how clean can the bundle get?

• Stainless, copper alloy or titanium tubes: conventional ECT is the primary screening method. Use rotating or array probes where cracking (for example chloride stress corrosion cracking in stainless) is a concern. Add IRIS on selected tubes to size wall loss accurately where plugging decisions are close.
• Carbon steel tubes in water service or refinery exchangers: RFT is the usual screening method. Because RFT resolution is limited, use IRIS on a sample of tubes, including the worst RFT calls, to confirm depth and calibrate the screening data.
• Air-cooled exchangers with finned carbon steel tubes: near-field testing or tube MFL may suit inner-surface pitting near the tube ends, with IRIS for sizing.
• Bundles that cannot be cleaned to bare metal: IRIS data quality will suffer; rely more on eddy current screening and plan cleaning before any IRIS sample.
• U-tube bundles: IRIS typically covers the straight lengths; U-bend regions often need other approaches. Plan coverage explicitly.

A common and defensible pattern is screen-plus-sample: eddy current or RFT on all accessible tubes (or a statistically chosen large sample), then IRIS on a smaller set made up of the worst screening calls plus a random selection. The IRIS results confirm depth where decisions are made and reveal whether the screening method is under- or over-calling in that bundle.

A worked example, described qualitatively

A refinery has a carbon-steel shell-and-tube exchanger in cooling water service with a history of under-deposit pitting on the tube inner surfaces. It is pulled for cleaning during a turnaround.

1. Scope. The owner's engineer asks for RFT on all accessible tubes and IRIS on the tubes with the deepest RFT calls plus a random sample from each pass.
2. Cleaning. The bundle is hydro-blasted. The NDE crew checks a set of tubes for cleanliness before IRIS starts, because residual deposits would block the ultrasonic beam.
3. Screening. RFT shows general wall loss in the inlet pass and a group of tubes with localized indications near the baffles.
4. Sizing. IRIS on the flagged tubes shows the baffle-area indications are fretting wear, sized accurately, and that RFT under-called a few pits in the inlet pass.
5. Decision. The owner applies its own plugging criterion to the sized results and plugs a number of tubes. Because IRIS showed RFT under-calling pitting in this bundle, the owner's engineer increases the IRIS sample for the next outage.

The plugging criterion and the next inspection interval were the owner's decisions; the NDE provided the data.

Acceptance, plugging criteria and where codes fit

Neither ASTM E690 nor ASTM E2096 sets acceptance criteria, and no single code tells a refinery when to plug an exchanger tube. Plugging and retubing criteria are owner decisions, typically based on remaining wall relative to the minimum required for pressure and corrosion allowance, the damage mechanism, the corrosion rate, and the consequence of a tube leak. Inspection intervals for exchangers in refineries are usually managed within the owner's pressure vessel programme under API 510, supported by API RP 572 guidance on inspection practices for pressure vessels including exchangers; check the current editions for detail. Some owners also use risk-based inspection to set exchanger intervals; that is an engineering programme the owner runs, not part of the NDE scope.

What the NDE report must provide is the data those decisions need: tube identification by row and column, method and probe used, calibration standard, indication location (from tube end and relative to baffles and tubesheets), estimated depth or measured wall, tubes not inspected and why, and a tube-sheet map.

What to send your NDE provider before the outage

Most tube inspection delays come from missing information at mobilisation. Before the bundle is pulled, give the NDE provider:

• the exchanger data sheet: tube material and specification, outside diameter, wall gauge, length, number of tubes and passes, U-tube or straight, finned or bare;
• tubesheet layout drawings with row and column numbering, so every indication can be mapped to a tube the maintenance crew can find;
• previous inspection reports and tube maps, including tubes already plugged, so trending is possible and plugged tubes are not counted as uninspected;
• the damage mechanisms the owner expects (pitting, erosion at the inlet, baffle fretting, cracking) so probe and technique can be matched;
• the cleaning method and who verifies cleanliness, especially when IRIS is planned;
• the reporting thresholds and the format the owner's engineer wants for plugging decisions.

With that information the provider can prepare reference standards that match the tubes, select probes, and plan crew time realistically. Without it, the first shift is often spent finding out what the bundle is made of.

Common mistakes in tube inspection scopes

• Using conventional ECT on carbon steel tubes without saturation or a suitable technique; permeability noise makes results unreliable. Use RFT or related methods.
• Treating RFT depth calls as exact. Verify the worst calls with IRIS before plugging or leaving tubes in service.
• Scheduling IRIS on poorly cleaned tubes. The data will be incomplete; build cleaning verification into the plan.
• Ignoring U-bends and tube ends. State coverage limits in the scope and the report.
• No reference standard matching the tubes. Calibration tubes must match material, size and wall.
• Sample too small or not random. A sample of only the easiest tubes says little about the bundle.
• Not trending. Keep tube maps from every outage so the owner can see which tubes and zones are degrading.

Personnel, procedures and regulatory context

Eddy current, RFT and IRIS data quality depends heavily on the analyst. Personnel should be qualified in the relevant method (electromagnetic testing for ECT and RFT, ultrasonic testing for IRIS) under the employer's written practice to ASNT SNT-TC-1A or CP-189, with technique-specific training and documented analysis experience. Written procedures should state probe types, frequencies, calibration standards, pull speeds and reporting thresholds.

In US refineries and chemical plants, exchangers in covered processes fall under OSHA PSM, 29 CFR 1910.119(j), which requires inspection and testing to recognized and generally accepted good engineering practices. In Canada, provincial regulators such as ABSA in Alberta and TSSA in Ontario oversee pressure equipment integrity programmes. In power plants, condenser and feedwater heater tube inspection is generally driven by the owner's reliability programme and industry guidance. Confirm requirements with your jurisdiction.

Frequently asked questions about IRIS and eddy current

What is the difference between IRIS and eddy current?

Eddy current is electromagnetic and fast, interpreting signal changes against a reference standard. IRIS is ultrasonic and slower, measuring actual wall thickness. Eddy current screens bundles; IRIS sizes and verifies.

Is IRIS more accurate than eddy current?

For wall thickness sizing, IRIS is generally regarded as more accurate because it measures thickness directly. Eddy current with rotating or array probes can be better for cracks, and is far faster for full bundle coverage.

Can eddy current be used on carbon steel tubes?

Conventional ECT struggles with ferromagnetic tubes. Remote field testing (ASTM E2096), near-field testing or tube MFL are used instead, with IRIS for sizing.

Remote field testing vs IRIS: which should I use?

Use RFT to screen carbon steel bundles quickly, and IRIS to size the worst RFT calls and a random sample. Together they balance speed and accuracy.

How clean do tubes need to be for IRIS?

Clean to bare metal. Scale and deposits block or scatter the ultrasonic beam, so cleaning is typically more demanding than for eddy current.

Does IRIS detect cracks?

IRIS is designed for wall loss and thickness; it is not the preferred method for cracking. Rotating or array eddy current probes are better suited where cracking is expected.

What standards cover heat exchanger tube eddy current testing?

ASTM E690 for non-magnetic tubes, ASTM E2096 for RFT of ferromagnetic tubes, and ASME Section V Articles 8 and 17 where a code or contract invokes Section V. None of them sets plugging criteria.

Who decides when a tube is plugged?

The owner, using its own criteria for remaining wall, damage mechanism and consequence. The NDE provider reports measured or estimated wall loss.

How Atlantis supports heat exchanger tube inspection

Atlantis NDT performs eddy current tube examinations, conventional ECT and remote field, with ASNT-certified technicians and ASNT Level III-approved procedures, and reports results tube by tube on a tubesheet map. If you want IRIS sizing in the same campaign, include it in your request and we will confirm scope. Results go to your engineer and API 510 inspector, who set plugging criteria and intervals. Tell us the tube material, size, count and service and we will quote: request a tube inspection quote. Related: eddy current testing for heat exchanger tubes, IRIS definition, pressure vessel inspection services, NDE method selection by damage mechanism, and chloride stress corrosion cracking. Planning a turnaround? Send us your exchanger list, or ask about remote field testing for carbon steel bundles.