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.