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Pipe testing method

Application for pipe testing

Condenser tubes

Condensers are used in power plants, chemical plants as well as various other industrial plants. The paramagnetic tubes of brass, cupronickel, stainless steel or titanium usually present are tested highly efficiently using eddy current (up to 700 tubes per shift/team). The accuracy of the test results is at ± 10 %. In addition, the erosion through droplet erosion present in power plants can be detected with a detection limit of approx. 0.2 mm defect size with highly sensitive probes.

Heat exchanger tubes

Heat exchangers can be found in almost all chemical plants. Where other test methods like IRIS or RFT fail at testing airfin cooler tubes, DELTA TEST can competently perform test tasks. The advantage of this inspection method is that the channel of the air cooler does not need to be dismounted for a test. Very precise test results are achieved during inspections of carbon steel tubes, as the cooling fins do not affect testing.

High-pressure tubes

Specifically for high-pressure tubes, i.e. stainless steel pipes with a wall thickness over 5 mm, DELTA TEST has realised a test system for complete control. With this, cracks and material homogeneities are detected in one pass on the internal and external surface of the tube and documented with precise location via C-scan. For this, an adjustable and mobile test device is available in our efficient laboratory.

Tube bends and U tube heat exchangers

The probe systems developed by DELTA TEST enable damage detection in tube sections that are difficult to access. For this, rotating as well as integral probes in a flexible guideway system are used. The probe system is able to pass through several tube bends and overcome changes of the inner tube diameter (swaging). This is difficult to realise with other NDT methods. Testing U tube heat exchangers is very difficult due to the tube sections bent to different degrees. Standard sensors cannot be used in these bends, but specifically adjusted, flexible sensors are required. Additionally, U tube coolers can remain in their casing, which means significantly reduced costs for dismounting.

Expanded tube sections

Testing tube ends in expanded or tube sheet areas is a big problem for integral standard eddy current probes. DELTA TEST has developed a rotating probe system specifically for this challenge. It enables us to test paramagnetic as well as ferromagnetic tubes in the tube sheet area for cracks, corrosion or other inhomogeneities. Testing tube ends is very efficient, whereby up to 2,500 tube ends can be inspected, analysed and documented per shift/team. This rotating tube system is of course also available for testing the entire tube length, and for bended tubes.

BOILER AND FURNACE TUBES

Due to a severe steam boiler detonation in October 1994 near Bonn, Germany, at which six people lost their lives, special tests of boiler tubes were called for by the responsible Federal and Federal State Authorities.
According to this, at least all three ascending tubes up to and including the first bend were to be subjected to a visual internal test, supplemented by endoscopy. In addition, a radiography test was to be performed at 20% of the bends.

Due to the usually present coatings (protective coating, furring etc.), the endoscopy could not always achieve the required detectability of the damage typical for the accident in Bonn: longitudinally oriented strain-induced crack corrosion. Radiography testing as well as ultrasound testing could also not always be sufficiently performed due to hampered local accessibility. One disadvantage of these test methods is that the insulation must be removed for the test

to be carried out properly, and that the resulting measured values do not achieve the required resolution.

Ultrasonic testing (e.g. with an IRIS system) for example presupposes that the IRIS system is precisely inserted in the centre of the tube and that the tube to be tested is com- pletely filled with water, to ensure precise connection. Particularly in bent sections, these conditions cannot be met. For the above reasons, together with TÜV Süddeutschland, DELTA TEST has developed an internal test system, based on the eddy current method, which has already been successfully used for more than 10,000 tubes.

This procedure includes an rotating eddy current probe technique, which scans the internal surface of the tubes seamlessly up to an inserting depth of approx. 20 m. This covers the first tube bend and possibly also the complete second one. Normal firmly adherent protective layers do not affect testing. Only crude inhomogeneous deposits may have to be removed with a hydrojet or chemical cleaning before testing. The eddy current probe was designed specifically for the defect type to be detected, so longitudinally oriented crack-like defects from a defect depth of approx. 0.5 mm can safely be detected.

Additionally, also small corrosion pits are detected. The tests results so far on more than 10,000 tubes confirm the excellent detection sensitivity and good interpretability of the eddy current rotating probe method. For this reason we regard the eddy current method as suited to significantly increase the evaluation safety on the integrity of the boiler tubes.

The probe system consists of a number of centering elements on a flexible hose and the rotating sensor itself. These centering elements have a system for automatic self positioning on the surface to be tested. During testing, the detected data are compared to the data of the calibration bodies.

The results of the analysis are stored in a table as well as in the DELTA TEST documentation software.

IRIS - INTERNAL ROTATING TEST SYSTEM

IRIS is a technique that can be applied on both ferrous and non- ferrous materials and even on non-conductive materials like plastics. With IRIS the remaining wall thickness of tubes can be accurately measured. IRIS is more accurate than other tube inspection techniques and has the advantages of presenting information about the geometry of defects. Local defects and wall-loss on both sides of the tube can be accurately measured. Defects under support plates can be measured without any limitations and IRIS allows detection of corrosion and thinning on a wide range of tube diameters (ID 14,5 – 76 mm … 0.57″ – 3.0″) and wall thicknesses (1.25 – 8.0 mm … 0.06“ – 0.31“) depending on the tube OD.

Fields of Application
The IRIS technology is mainly used as an analysing tool in combination with other techniques such as conventional eddy current or saturated eddy current. Due to its low inspection performance (6 – 12 tubes / h) only few selected tubes shall be IRIS tested in order to analyse or confirm results from other NDT techniques. Furthermore IRIS is the only suitable technique to detect defects within the tube sheet area of heat exchangers with C-steel tubes (air fin coolers).

Theory
The probe used in IRIS examination is made up of a centering device, an ultrasound transducer and a rotating mirror. An ultrasound pulse will be generated in the transducer that is mounted in axial direction, then A 45 degree rotating mirror in the probe will guide the sound bundle towards the tube wall. Next an ultrasound reflection (echo) will take place at the inner and at the outer wall of the tube. These echoes are reflected back and processed by the equipment. The time between these two echoes represents the wall thickness of the tube. Knowing the sound velocity in the material under test enables accompanying wall thickness to be calculated. Water is then used to rotate the probe mirror and is also needed as a couplant between the transducer and the tube wall. A calibration standard of the same material and dimensions as the tubes to be examined is used to check the IRIS system response.

After an inspection an “on-site” report detailing the condition of each tube will be presented to the client.

Limitations
IRIS is not suitable/reliable to detect small pittings and cracks. It is not or only hard to apply in confined space. Bended tubes (U- tubes) cannot be inspected within the bended area. Tubes to be tested have to be perfectly clean to the bare metal in order to acquire adequate data.

VIDEOSCOPE INSPECTION OF TUBES

The visual inspection or visual testing (VT) in terms of non-destructive testing methods is the optical quality control of a product (tube, shell, weld) through a technician qualified and certified according to ISO 9712. The type of visual inspection can be adapted and defined as per customer’s requirements. For instance an inspection can be done for acceptable or rejectable material-inhomogeneities such as corrosion, cracks or leakages or a tube cleaning quality control. The visual inspection is one of the most important tools for the root cause analysis of defects. It can be used to clearly determine defect geometry and orientation to get conclusion on the defect mechanism. Visual testing gives outstanding possibilities to confirm and underline defects or indications, previously found with other NDT methods like eddy current or ultrasonic by means of expressive images.

Because of the fact that integral NDT methods like eddy current or IRIS are much more suitable for a full coverage inspection of larger areas (tubes) we use the visual testing for the verification of defects, cleaning control or random spot check only.

The use of visual testing makes it possible to inspect tubes as well as other components which are difficult to access with conventional tools. By means of state-of-the-art video technology, combined with qualified personnel certified according to ISO 9712, DELTA TEST guarantees to all clients a competent visual inspection, defect analysis and documentation.

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