Short answer
RFT (Remote Field Testing) and ECT (eddy current) are fast electromagnetic techniques, ideal for scanning many tubes and locating defects and wall loss. IRIS (Internal Rotary Inspection System) is rotating ultrasound: slower, but it measures remaining wall thickness quantitatively. In practice they complement each other: electromagnetic screening of the whole bundle and IRIS on the tubes that need an exact measurement.
How RFT works
RFT uses an exciter coil and receiver coils spaced about two to three tube diameters apart. The field crosses the wall twice, making the technique sensitive to internal and external wall loss in ferromagnetic tubes such as carbon steel.
The probe travels through the tube at high speed and needs no couplant, which is why it is widely used on heat exchangers, boilers, feedwater heaters and air coolers with steel tubes.
Eddy current (ECT) in non-ferromagnetic tubes
For brass, copper-nickel, austenitic stainless steel or titanium tubes, conventional bobbin eddy current is the right choice. It is fast and detects pitting, cracking, baffle wear and general thinning.
How IRIS works
IRIS spins a mirror at high speed inside a water-filled tube, directing an ultrasonic beam at the wall. The result is a 360° thickness map along the tube with direct measurement of the remaining wall.
It requires a clean tube, water as couplant and a slow pull speed, so it is much slower per tube than RFT or ECT.
RFT/ECT vs IRIS compared
| Criterion | RFT / ECT | IRIS |
|---|---|---|
| Principle | Electromagnetic | Rotating ultrasound |
| Speed | High (full bundle) | Low (tube by tube) |
| Wall result | Electromagnetic assessment of wall loss | Quantitative thickness measurement |
| Tube cleaning | Moderate | Strict |
| Couplant | None | Water |
| Typical use | 100% tube screening | Confirmation and measurement |
When to use each
Use RFT/ECT when the goal is to inspect the whole bundle in a short shutdown, identify tubes with indications and prioritise actions. Use IRIS when you need a remaining-wall value for life assessment, sample verification or confirmation of an indication.
The most efficient strategy in most shutdowns is a combination: electromagnetic scanning of 100% of the tubes and IRIS on a sample and on flagged tubes.
Reference standards
ISO 15549 (eddy current — general principles), ASTM E690 (eddy current testing of non-magnetic heat exchanger tubes), ASTM E2096/E2096M (remote field testing of ferromagnetic tubes) and ASME BPVC Section V, Article 8. Compliance depends on the complete procedure, calibration, qualified personnel and applicable acceptance criteria.
Frequently asked questions
Does RFT replace IRIS?
No. RFT assesses wall loss electromagnetically and is ideal for fast screening; IRIS measures thickness by ultrasound. RFT/ECT assessment is not equivalent to IRIS quantitative measurement.
Which technique is faster?
RFT and ECT are much faster per tube, allowing 100% of the bundle to be inspected in a short shutdown.
Which technique for carbon steel tubes?
RFT for screening; IRIS for remaining thickness measurement.
And for stainless, brass or titanium tubes?
Conventional bobbin eddy current, since these materials are non-ferromagnetic.
Equipment and technical support
Synvor supplies non-destructive testing equipment and provides technical support in selecting the right method and configuration for your application. The inspection itself is performed by the asset owner or a qualified inspection company.