DETECTION OF CIRCUMFERENTIAL CRACKS IN HEAT EXCHANGER TUBES

DETECTION OF CIRCUMFERENTIAL CRACKS IN HEAT EXCHANGER TUBES
The detection of circumferentially oriented cracks in heat exchanger tubing is difficult to detect with the usual bobbin probes for two reasons.
1. The bobbin coil creates eddy currents in the tube wall which flow in the circumferential direction; therefore, they pass a circumferential crack without having to cross its crack face. This gives a low sensitivity to this defect type.
2. Most circumferential cracks are located adjacent to support structure or an expansion transition, both of which create a very large signal, masking the small signal from the crack. Circumferential cracks occur in several heat exchanger applications as follows:
1. At the tube sheet expansion transition zone in nuclear steam generators.
2. On bends that occur in nuclear steam generators at the top of the sludge pile.
3. In nuclear steam generators at the top edge of the top support plate.
4. In titanium condenser tubes at the tube sheet expansion transition zone when the tube was overrolled in the tube sheet.
5. In finned copper tubing (integral fin) used in air conditioners. The crack initiates from the root of the fin and may or may not be in an expansion transition zone, but even away from expansion transition zones, the tube has high background noise, masking the signal from the crack.
6. Fatigue cracks mid-span between support plates on the peripheral tubes of titanium condensers.
In 1983, Eddy Current Technology Incorporated designed the first probe in the world that was capable of detecting circumferential cracks at a tube sheet expansion transition zone. The probe design canceled signals from expansions, support plates, and tube sheets, while increasing sensitivity to circumferential cracks by having current flow in the longitudinal direction in the tube, so that the current flow would pass over the crack face. For further information on this subject, please request the report DETECTION OF CIRCUMFERENTIAL CRACKS IN HEAT EXCHANGER TUBES.