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Eddy-Current Testing of Heat Exchanging Tubes of the NPS Steam Generators
P. Shkatov, V. Kolinchenko,
Moscow State Academy of Instruments Engineering and Computer Science (Moscow).
Steam-generator refers to the one of the most intensive and destruction exposed NPS elements. Nowadays the eddy-current testing of heat- exchanging tubes are recommended according to the current specifications based on American standards (ASME).
At present the eddy-current probes used for heat-exchanging testing react to the axis fixing elements as to continuity defect (crack). At the same time the cracks more likely appears at the areas of fixing elements. It happens because these areas have special places of strength mechanic effects and they favour corrosion. Thus to make the testing more precise the special eddy-current probes are in demand. These probes have to react only to the cracks but not to the axis fixing elements.
Moscow State Academy of Instrument Engineering and Computer Science (MSAIECS) created such a probe that meets these requirements. The probe consists of four deferentially connected sensitive elements which are symmetrically disposed about the testing tube axis.
Wraps of the sensitive elements have the form of parallelogram. They give an opportunity to depress the discontinuity. Such choice of the sensitive elements disposition and of their forms were made because of some reasons:
This probe is worked out for testing a set of tubes (bars) which has not only straight line section but crooked ones either (so-called transitional zones). It imposes limitations on the tube length and assumes the availability of such flexible elements which pave the way for success while the probe passes over a crooked part of the tube.
- the probability of the axis symmetric defects occurring is rather small
- cracks (continuity defects) set out symmetrically about two nearest sensitive elements have to be brought out at the moment when the eddy-current probe is moving down the tube axis without any rotation.
The experiments and practice show that if the inside steam-generator tube diameter (Di) is 13.8 mm the outside diameter of the probe (Dpr) mustn't be more than 10.5 mm and its length has to be not less than 11.5mm.
After making a lot of experiments we came to conclusion that all the cracks that had been precisely detected on the straight line parts of the tubes could be easily find out in the area of the fixing elements. In this case special signal complex mathematics calculation isn't needed, while using a traditional probe it is necessary to do.
Figures 1 and 2 show the diagrams received after using 2 types of probes: the usual differential internal one and the new probe. As it is seen, the traditional probe hardly detects the crack in the area of tubes set (bars) because the signal uses to change in such a way that the tubes set signal effect is much more than the crack signal effect. It is impossible to distinguish the defect effect from the tubes set one, for example, because of lift off changing between the bars and the tube.
Fig 1: Signal from old probe
Fig 2: Signal from probe by authors|
In the new probe the bars signal changing is less than 50% against the signal changing made under the defect effects. All above mentioned confirm the efficiency of the new probe applications.