GB547714A - Improvements in or relating to the detection of variation in wall thickness of tubes of magnetic material - Google Patents

Abstract

547,714. Magnetic tests. ARMOUR, A. M., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. May 12, 1941, No. 6115. [Class 37] In an arrangement for detecting variations in wall-thickness of magnetic tubes, and specially for detecting pitting of the outer surfaces of the tubes of multi-tubular boilers, a magnetic flux is passed through a length of the wall and magnetic particles are introduced into the tube-bore and their distribution on the internal surface of the tube, or on a membrane contiguous thereto; is observed. Thus in Fig. 1, the pitted surface 3 of tube 1 causes a bunching of the particles on the interior surface, as shown, due to the diminished reluctance of the wall at that point, Preferably a 2 per cent. by volume concentration of iron powder in water-free paraffin oil is employed for introducing the magnetic material into the bore, and a flux density of 17,000 gauss is employed. The electromagnet comprises a core 5 containing a central rod 8 with a truncated end 9 engaging in correspondingly internally-tapered segments 14 which are surrounded by a garter spring 17, the arrangement being that, in order to grip the tube 1 and hold the core 5 centrally therein, the segments, three in number, Fig. 2 (not shown), are expanded by movement of the rod 8 to the right on rotation of a tommy bar 19 pivoted at 20 to a threaded sleeve 18. The winding 23 is carried by a sleeve 22 closely surrounding and slidable on the core 5 and carried by a magnetic plug 21 which can be pressed tightly into the tubemouth. The carrier liquid 33 is contained in a flexible bag 32 connected by tubing and ducts 29 to non- magnetic nozzles 28. In use the bag 32 is shaken to distribute the particles and is then squeezed so as to force some of the contents in a fine spray into the bore and cause a piling up of particles against a flaw. The unit 21, 23 is then withdrawn, the segments 14 are slackened back, the tommy bar straightened and the whole pushed into the tube so as to enable an observation to be made. A period of magnetization of two to three seconds and 15 ccs. of liquid yield a satisfactory record, and reductions in thickness from a normal of three to two mms. can be easily detected. An arrangement using a liquid-containing cell as described in Specification 439,324 is also set out, Fig. 3 (not shown). The Specification refers also to an inspection torchlight fitted with a periscope mirror system, to coloured magnetic particles for ease of observations and to an A.C, solenoid, and states that sensitivity may be adjusted by varying the winding voltage.