The collapse of heat exchanger tubes with ovality and simulated defects

An experimental program has been carried out on typical PWR heat exchanger tubing with two objectives; firstly to investigate the collapse pressure of the tubes with varying amounts of initial tube ovality, secondly to simulate tube defects which might be expected to occur during operation. The defects were artificially induced to simulate: longitudinal tube cracking; fretting at tube support; erosion; and were produced by means of a 60 deg V groove machined longitudinally partially through the wall of the tube, spark erosion, and chemical thinning, respectively. The collapse of the oval tubing is compared with the theoretical limit using plastic limit analysis theory and a geometric idealisation of an oval tube, taking into account the effect of elastic changes in ovality prior to collapse. The collapse of the tubes containing deliberate defects are compared with the theoretical strength of the original tube, and also with the limiting theoretical local membrane stress at the reduced wall section. The effect of varying the length of defect along the tube is also considered, and it is found that with a limited length of defect, considerable support is obtained from surrounding material in the unimpaired part of the tube. Collapse is of a ductile nature, and in no instance did the defect propagate through the wall of the tube, although some partial propagation and slight extrusion into the bore of the tube occurred in one instance as shown on a micrograph of the section. The findings are also of interest in situations where any ductile tube is subjected to external pressure in its normal operational state