IRIS

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 technique is based on ultrasonics.

The probe is centred in the tube to be inspected and 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 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.

If a single testing technique were to be selected, then the IRIS would be recommended since UT may be used for almost any material. The method is very accurate for thickness measurement as well as detecting ID and OD pits.

The advantages of the IRIS inspection technique include the following:

1. 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.

2. It is a fairly sensitive technique. The sensitivity achieved will depend on tube dimensions and tube cleanliness. In general, it can be stated that, it should be possible to detect a 1.5mm defect in tubing up to 1 inch which has been properly cleaned.

3. Both ferromagnetic and non-ferromagnetic tubes can be inspected

4. A three dimensional picture of the defect is obtained, thus the defect profile and its depth is provided.

5. Interpretation of results is easier than in the other techniques assuming that reasonable tube cleanliness has been achieved

On the other hand, the IRIS inspection technique poses the following disadvantages:

1. It is a slow technique. The actual testing speed will depend on a number of factors but will generally be in the order of 0.04m/sec in order to achieve 100% coverage. However it must be noted that the tube has to be filled with water (couplant) 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.

2. Tubes must be very clean. While all 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.

3. Water must be introduced into the tube to act as a 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.

4. Only volumetric defects can be detected and is therefore insensitive to cracking.

5. Probe must be centralized in the tube to avoid a loss of signal.

IRIS Inspection/Cleaning

Our usual comment to clients regarding the cleanliness of tubes in preparation of IRIS inspection is as follows;

As this is an ultrasonic method, the internal tube surface should be cleaned of any product, rust or loose scale. Based on our usual experience, 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.

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.

1. 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.  Some problems are seen when insufficient pressure is used or the material 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.
2. Mechanical cleaning by brushing, boring, or swabbing can be used.  Swabbing is 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.
3. Chemical cleaning with solvents, acids, or detergents may be used when all other types of cleaning have failed.  These methods are costly and time consuming and usually reserved for only the most critical application.
4. When hydro blasting, the primary consideration will always be the pressure at the NOZZLE not the pump rated pressure.  It matters little if a pump is rated at 20,000 psi if it is only generating 15,000 psi at the nozzle.  When water blasting, the maximum allowable pressure should always be used.  This number is derived from the strength of the material being cleaned and the thickness of the tube wall.  (For most carbon steels this figure will be in the 20,000 – 25,000 psi range.)  The use of insufficient pressure will only cause delays.
5. When cleaning, the type of cleaning needs to considered alongside the type of material to be cleaned.  Caution must used when cleaning softer materials such as brass, copper, and mild steels.  Make certain the customer discusses cleaning with the vendor so that the cleaning crews will know what is required.

Remote Field Electromagnetic Technique (RFET):

Basic Principles

The Remote Field Electromagnetic Technique (RFET) inspection technique 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.