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UT of Furnace Tubes

Failure of radiant tubes in a hydrosulfurisation furnace promoted development of an ultrasonic technique for detecting the presense of polythionic acid stress corrosion cracks in the austentic stainless steel tubes in these furnaces. The initial hand-scanning ultrasonic method was too slow for complete furnace inspection in the time available. In order to reduce the time required for inspection, a semi-automated UT tool was designed and fabricated.

The UT tube inspection tool, shown in the figures, consists of a split hinged frame containing 12 piezoelectric transducers ultrasonically coupled to the tube wall by means of a water bath. Two groups of three transducers are positioned to search longitudinally down the length of the tube while the second two groups of three transducers are directed to search circumferentially around the tube. Each of the four groups of three transducers is electrically paralleled and connected to one of a total of four pulse echo flaw detectors. Individually, a transducer is aimed like a search light and will detect energy reflected from a flaw if it is in the path of the ultrasonic beam. To completely inspect the tube wall with a single transducer it would be necessary to move the transducer in a spiral path around the tube. To minimize the extent of transducer movement for 100% inspection, 12 transducers were used; 6 for longitudinal scanning and 6 for circumferential scanning. Four groups of three transducers were thus operated from four pulse echo flaw detectors. By means of this configuration, a fixture containing 12 transducers requires an oscillatory movement of only 60 degrees to achieve complete coverage of the tube wall. The movement of the fixture traces out a zig-zag path down the tube. After calibration, the inspection operation involves monitoring of CRT screens of four pulse echo units for flaw echoes. For instruments equipped with a gate/alarm, this operation can be simplified by adjusting the gate to trigger the audible alarm if a flaw echo appears. If a flaw indication is detected, its position on the tube can be located by sequentially passing a finger over the face of each transducer until the flaw echo is made to disappear. The indication will be found near the transducer which has been blocked.