IRIS internal inspection of heat exchanger tubes

IRIS (Internal Rotary Inspection System) is a nondestructive examination method that travels inside heat exchanger and condenser tubes and measures the absolute tube wall thickness with a rotating ultrasonic probe. Electromagnetic methods such as eddy current (ET) and remote field (RFT) scan a tube quickly and return a comparative, relative result. IRIS produces a direct tube wall thickness value in millimeters. For that reason IRIS is usually not the primary screening tool in tube bundle examination. It is the referee method that verifies and quantifies what the electromagnetic scan found.

1. How it works

IRIS is fundamentally a pulse-echo ultrasonic measurement. What sets it apart is how the sound is steered into the tube wall. A fixed ultrasonic transducer in the probe body sends the beam forward, parallel to the tube axis. Inside the water-filled tube the beam strikes a rotating mirror inclined at 45° (the turbine mirror) and is deflected by 90°, so it arrives normal to the tube wall. The mechanism here is reflection: the sound changes direction at the mirror, it is not refracted.

Sound entering the wall at normal incidence reflects first from the inside surface (ID) and then from the outside surface (OD). Multiply the transit time between the two echoes by the material sound velocity and divide by two (d = v · Δt / 2), and you have the tube wall thickness directly. A small water turbine spins the mirror at high speed, so the beam sweeps the full 360° circumference. As the probe is pulled along the tube at constant speed, that circular sweep becomes a helical scan pattern and the entire wall surface is mapped.

The quantity actually measured is elapsed time: the interval between the ID echo and the OD echo, converted to distance through a known sound velocity. In that sense IRIS is conventional UT thickness gauging carried into the tube — rotating and automated.

2. Setup and components

An IRIS setup consists of several separate parts, and each one has a direct effect on how trustworthy the result is:

Ultrasonic probe and transducer: Typically a focused immersion transducer in the 10–20 MHz range; on thin-walled tubes, work at the upper end of that range. High frequency gives the resolution a thin tube wall demands and keeps the ID and OD echoes cleanly separated.
Turbine and rotating mirror: Water drives the turbine, which spins the mirror. Mirror angle, bearing condition and rotational balance set the measurement quality; a worn or unbalanced mirror produces noise and data dropout in the image.
Water supply: In IRIS the couplant is water. Feed the tube continuously and bubble-free; an air bubble breaks the sound path and causes signal loss.
Probe centralizer: Keeps the probe on the tube axis. Off-axis, the beam no longer strikes the wall at normal incidence and the thickness reading is wrong.
Encoder and data system: Records the axial position of the probe, so every indication is located along the tube and B-scan / C-scan images can be produced.

3. Absolute tube wall thickness measurement

The most distinctive feature of IRIS is that it reports tube wall thickness in absolute, quantitative terms. In ET and RFT the signal is compared against artificial discontinuities in a calibration tube and interpreted as a "percentage of tube wall thickness loss"; the result is relative and depends on the calibration. IRIS measures the remaining tube wall thickness directly in millimeters — much like a thickness gauge carried inside the tube.

That absolute measurement is the basis of remaining life and corrosion rate calculations. Compare the nominal tube wall thickness with the current thickness measured by IRIS and you have the wall loss directly; repeat the measurement periodically and you have the corrosion rate. This is why operators re-measure with IRIS the tubes an ET scan has flagged as suspect, then decide "replace / plug / leave in service" from the real remaining tube wall thickness.

Correct measurement depends on a correct sound velocity: enter the wrong velocity for the material and every thickness reading shifts proportionally. Verify the instrument before the examination on a reference tube of known thickness or on a step block. The inspector who performs this calibration settles the reliability of the whole examination at the very first step. A reading taken with the wrong velocity is wrong from the start, no matter how clean the signal looks.

4. What it sees: ID/OD corrosion, erosion and pitting

Because IRIS measures tube wall thickness around the full circumference, it can distinguish where and in what form the loss occurs:

General corrosion/erosion: Uniform thinning of the wall over a broad area. IRIS shows this clearly as a drop in thickness; where ET struggles with gradual loss, IRIS is strong.
Pitting corrosion: Local, deep and narrow cavities. The C-scan maps them with their position and depth.
ID or OD? A major advantage of IRIS is that it can tell whether the loss is on the inside or the outside surface of the tube. Since the sound produces the ID and OD echoes separately, you can see which surface the thinning started from. That is critical information for understanding the corrosion mechanism — process side or shell side.
Baffle / support plate wear and fretting: Mechanical wear where the tube rubs against baffles and support plates also shows up on the thickness map.

The weak point of IRIS is cracking. As a thickness method that measures normal to the wall, it cannot reliably find tight planar cracks; when cracks are the target, ET array probes or other techniques take over. IRIS also has a practical lower thickness limit: on very thin walls (roughly below 1 mm) the ID and OD echoes may overlap in time and fail to separate, and in very thick or highly attenuating materials the signal weakens. That constraint is the real counterweight to the method's material independence.

5. Material independence of IRIS

The biggest limitation of electromagnetic methods is their dependence on the material: conventional ET works on non-ferromagnetic tubes, carbon steel calls for RFT/MFL/NFT, and on non-conductive materials none of them work. IRIS is an ultrasonic method, so it is independent of the magnetic and electrical properties of the material.

This gives IRIS real flexibility: one method covers stainless steel, copper alloys, titanium and ferromagnetic tubes such as carbon steel alike. IRIS works as long as sound propagates in the material and clean echoes come back from both surfaces. The only conditions are a known sound velocity for the material and a beam arriving normal to the wall. That material independence makes IRIS especially valuable on mixed-material tube bundles and for the quantitative verification of ferromagnetic tubes.

6. Compared with ET/RFT: speed or accuracy?

IRIS and the electromagnetic methods are not competitors but complements, and a correct examination plan depends on understanding the difference:

Criterion	ET / RFT	IRIS
Speed	Very fast (many tubes per minute)	Slow (minutes per tube)
Result type	Relative / calibration-dependent (% tube wall thickness loss)	Absolute / quantitative (mm)
Material	ET on non-ferromagnetic material; RFT/NFT on ferromagnetic	Material-independent
ID/OD discrimination	Limited	Clear separation
Cracking	Possible with ET array	Weak
Preparation	Cleaning is enough	Thorough cleaning + water + probe passage

In practice the right strategy is usually this: scan the whole bundle quickly with ET/RFT first and flag the suspect or above-threshold tubes; then measure those tubes with IRIS to quantify the real remaining tube wall thickness. You keep the speed of the electromagnetic scan and the certainty of an absolute measurement, and you assess the whole bundle economically. Measuring thousands of tubes end to end with IRIS is neither economical nor necessary on most projects.

7. Prerequisite: tube cleanliness and probe passage

IRIS demands more preparation in the field than any other tube examination method, and its success depends largely on the quality of that preparation. Two conditions are absolute:

Cleanliness: Clear the tube ID of deposits, scale, oil and loose corrosion product that would disturb the sound path and the ID echo. IRIS takes that echo from the inside surface, so heavy deposits both weaken the signal and throw a false echo off the deposit surface. Clean the tubes by high-pressure water jetting (hydro-blasting) and, where needed, by brushing or mechanical means.
Probe passage: The IRIS probe has to travel physically through the tube. It will not pass a blocked, ovalized or dented tube, or one still holding deposit; and even when it does pass, it cannot stay centered and the measurement degrades. Confirm before the examination that the tubes are open and passable, running a go/no-go gauge rod where necessary.

Skip these prerequisites and the result will mislead you: a tube believed to be clean but still scaled inside can read thinner or thicker than it really is. In the field the most common cause of IRIS failure is not the equipment — it is inadequate cleaning.

8. Slow by design: IRIS as a verification tool

The most obvious disadvantage of IRIS is speed — or rather the lack of it. The water supply, probe positioning, the helical scan and the separate handling of every tube make the method many times slower than an electromagnetic scan. Measuring a bundle of several thousand tubes entirely with IRIS is impractical on most sites; it costs both crew time and shutdown days.

That slowness also defines the role of IRIS in the field. IRIS is generally not a screening tool but a verification and quantification tool:

to measure the real remaining tube wall thickness where ET/RFT exceeded the threshold,
to resolve quantitatively the regions where the electromagnetic signal stays ambiguous (at the tubesheet line, for example, or under a baffle / support plate),
to feed engineering decisions that need ID/OD discrimination and absolute thickness.

In other words, IRIS acts as the referee for the fast method that scans the whole bundle: a definite, absolute answer on a small number of critical tubes. This division of labor — ET/RFT for speed, IRIS for certainty — is the standard approach in modern tube bundle examination.

9. Related standards

ASME BPVC Section V, Article 5 / Article 23: Article 5, "Ultrasonic Examination Methods for Materials," covers thickness determination; weld examination sits in Article 4. For IRIS — a tube wall thickness method — Article 5 is the correct reference. Article 23 (Ultrasonic Standards) holds the adopted ultrasonic standards such as SE-213 and SE-797.
ISO 16810:2024: Non-destructive testing — ultrasonic testing — general principles (2nd edition; also adopted as EN ISO 16810).
ASTM E213: Standard practice for ultrasonic testing of metal pipe and tubing — aimed mainly at detecting discontinuities (cracks, pitting, inclusions) during manufacture. It is not a standard for absolute or remaining tube wall thickness measurement. (Also adopted in Article 23 as SE-213.)
ASTM E797: Standard practice for measuring thickness by manual ultrasonic pulse-echo contact method (current edition E797/E797M-21). IRIS couples by immersion, but the sound velocity and calibration principles are shared, and E797 is the reference for those principles. (Also adopted in Article 23 as SE-797.)
In-service assessment: Heat exchanger tube bundles are assessed against the construction or in-service code they belong to (for example the API 510 framework for pressure equipment).

The IRIS method itself defines no acceptance criteria; those come from the construction or in-service code governing the equipment. The general framework for personnel qualification is ISO 9712 (UT method, Level I/II/III). Have the evaluation performed by an inspector certified to the appropriate level and experienced in tube examination.

From the field

The most common mistake with IRIS is reaching for it when what you actually want is speed. IRIS is not fast and nobody should expect it to be; its value is an absolute, millimetric measurement that separates ID from OD. The second classic mistake is taking cleaning lightly. A "clean" signal off a scaled tube returns a tube wall thickness that is not the real one, and that is the most dangerous kind of error — because the result looks right. The third is neglecting probe centering: an off-axis probe sends sound that is not normal to the wall, and the thickness reads high. The best IRIS examinations are the ones where the tubes were genuinely cleaned, probe passage was confirmed, sound velocity was set against a reference block, and the method was framed for what it is — the referee of the ET/RFT scan. An experienced inspector never tries to run IRIS on a whole bundle; the craft lies in applying it to the right tubes at the right time.

Take it to the field: To keep these NDT methods, standard references and field steps in your pocket — completely offline and free — take a look at the Doawise NDT Guide app.

At DoaWise we carry out nondestructive examination services, including IRIS (rotary ultrasonic), to international standards. On heat exchanger and condenser tube bundles we cover the whole chain — from ET/RFT screening to absolute tube wall thickness verification with IRIS — producing recordable, auditable results that support safe, planned maintenance decisions.