What is IRIS inspection?
Internal Rotational Inspection System (IRIS) is an ultrasonic immersion pulse echo technique. Unlike the Eddy Current, Remote Field and Magnetic Flux Leakage techniques, which are based on electromagnetic principles, the IRIS inspection technique is based on ultrasonics.
How IRIS inspection works
The probe is centred in the tube to be inspected for wall quality. Ultrasonic pulses are transmitted along a path parallel to the tube axis. These pulses are then redirected radically to the tube wall by a 45° rotating mirror. The mirror rotates at a high speed and scans the ultrasonic beam around the tube circumference.
Successive pulses build a screen image of the tube cross section at any given point. This provides information regarding the integrity of the tube wall. IRIS is slower than Eddy Current and generally costlier, but provides more information on the tube’s condition.
If a single testing technique were to be selected, then the IRIS would be recommended since it may be used for almost any material. The method is very accurate for thickness measurement as well as detecting internal and external diameter pitting.
Benefits of the IRIS inspection technique
- It is very accurate – wall thickness measurements can be made, with the use of a 15MHz-focused transducer, to an accuracy of within 0.1mm.
- It is a sensitive technique – The sensitivity achieved will depend on tube dimensions and tube cleanliness. In general, it should be possible to detect a 1.5mm defect in tubing up to 1 inch which has been properly cleaned.
- Both ferromagnetic and non-ferromagnetic tubes can be inspected.
- Three dimensional pictures produced can show the profile and depth of volumetric defects.
- Compared to other techniques, assuming reasonable tube cleanliness, it is easier to interpret the results.
IRIS inspection disadvantages
- It is a slow technique – testing speed depends on several factors but will generally be in the order of 0.04m/sec to achieve 100% coverage. However it must be noted that the tube has to be filled with water to provide acoustic coupling between the transducer and the tube wall every time prior to the actual inspection. This reduces the typical production rates to the order of 70-100 tubes per shift, which again depends on the tube length, access and the cleanliness of the tubes and the water pressure supplied at the point of inspection.
- Tubes must be very clean – while the other techniques are able to tolerate some degree of scaling, the tubes must be cleaned virtually down to the bare metal for a successful IRIS inspection hence Tube Tech’s “Enhanced Tube Polishing System” which guarantees IRIS standard cleanliness is favoured.
- Water must be introduced into the tube to act as a signal couplant – At times this may pose as a problem as no suitable water outlet is available at the point of inspection. In other cases the source of water may not be clean enough or may not be at the ambient temperature required for a successful inspection. In some cases, the introduction of water into the tubes may give rise to corrosion problems.
- Only volumetric defects can be detected and IRIS is therefore insensitive to cracking.
- The probe must be centralized in the tube to avoid a loss of signal.
IRIS Inspection / Cleaning / Enhanced Tube Polishing
Our usual comments to clients regarding the cleanliness of tubes in preparation of IRIS inspection are as follows:
As this is an ultrasonic method, the internal tube surface should be cleaned of any product, rust or loose scale as near to bare metal as possible. Based on our usual experience, and provided fouling severity allows, high pressure water jetting should be carried out to provide a good/clean condition for IRIS inspection.
Below is an extract from the IRIS manual regarding cleaning with additional comments from Tube Tech’s 30 years of experience.
The customer will also usually be responsible for having the unit properly cleaned prior to inspection. No other aspect of the job will be as important as a thorough cleaning
Hydro blasting is the usual type of cleaning employed. There are several different types and techniques.
Direct, high-pressure water blasting will often be sufficient for most inspections, especially in non-carbon steel tubes. It is the easiest and cheapest method and companies doing this type of cleaning are readily available. Cleaning standards drop when insufficient pressure or volume or incorrect jets and nozzles are used or if the deposit is off an oily nature.
Sometimes grit such as sand, silica, aluminium oxide, or tiny glass beads will be used in conjunction with water blasting (or with high-pressure air only) to add an abrasive nature.
IRIS recommends cleaning by water blasting with abrasives. Be aware that this method can be detrimental to the tube surface and create a profile (pitted) surface which then leads to more rapid fouling, allowing the deposit to adhere more thoroughly to a keyed surface. This makes it more difficult to clean in future.
Mechanical cleaning by brushing, boring, or swabbing can be used. Swabbing involves running a fibre cleaning brush, similar to a giant Q-Tip, down the tube. The brush may contain solvent to dissolve residue in the tubes.
Brushing uses a rotating, steel-bristle brush to scrub away harder deposits. Boring employs long drill bit-like shafts to remove the hardest deposits.
The latest technology on the market is “Enhanced Tube Polishing Technology “ which creates a much cleaner tube surface in a fraction of the time compared to any other mechanical system.
Chemical cleaning with solvents, acids, or detergents may be used when all other types of cleaning have failed, in combination with a mechanical alternative. These methods are costly and time consuming and usually reserved for only the most critical application.
When hydro blasting, the primary consideration will always be the pressure at the nozzle, not the pump rated pressure, including the method in which the lance is found down the tube (i.e. manually or remote control).
It is important to achieve the correct feed rate and dwell time through each tube to ensure the nozzle has the best chance of removing scale, not least the combined pressure and volume combination as one directly impacts the other. It doesn’t matter if a pump is rated at 20,000 psi at 10 gallons a minute if it is only generating 15,000 psi at the nozzle or the hoses are leaking.
When water blasting, the maximum allowable pressure should always be used. This water pressure, volume and nozzle configuration is derived from the tube material and characteristics of the deposit being removed, such as tenacity, thickness, consistency, volume and location, not least the actual thickness of the tube wall.
For most carbon steels this figure will be in the 20,000-25,000 psi range, although pressures of up to 60,000 psi (4000 bar) can also be used. The use of insufficient pressure poor operator training and subsequent handling will only cause delays. This includes the appreciation of “how clean is clean”.
The type of cleaning needs to be considered alongside the type of material to be cleaned. Caution must be used when cleaning softer materials such as brass, copper, and mild steels. Make certain the customer discusses cleaning with both the inspector and the cleaning contractor in the same room so that the cleaning crews will know what is required.
IRIS inspection is also used to verify the RFET measurements
Basic Principles of Remote Field Electromagnetic Technique (RFET)
The Remote Field Electromagnetic Technique (RFET) is a non-destructive method which uses low frequency AC and through wall transmission to inspect pipes and tubes from the inside. The through wall nature of the technique allows external and internal defects to be detected with approximately equal sensitivity. However, due to the nature of the remote field signals, we are unable to distinguish whether the indication is an internal or external defect while the phase shift is directly proportional to wall loss.
Flaw sizing with RFET is done using the Voltage-Plane curves. These curves are used to size tube wall loss but not pits. The curves relate flaw depth, flaw length, and the flaw circumference to the phase of the remote field signal. Inaccuracies result because the geometry of the actual flaw is not defined as in the calibration defects. Ultrasonic IRIS is therefore used to verify the RFET measurements.
NB: The above inspection methods do not allow hairpin bends to be inspected. Nevertheless, the cleanliness of U bends is as important as the straight sections. The bends can then be inspected using a video endoscope to detect visual flaws in the tube.