BS ISO 20890-5:2020 pdf download – Guidelines for in-service inspections for primary coolant circuit components of light water reactors Part 5: Eddy current testing of steam generator heating tubes
During the eddy current testing of steam generator heating tubes, it is the task of the evaluation todifferentiate the recorded signals according to
– operational indications (location, expansion and possibly their type, e.g.cracks, abrasions, dents).
geometric indications (e.g.of structural parts such as spacers, tube bottom, rolled-in slugs), andother indications(e.g. of local fluctuations in the electrical conductivity and permeability, electricalinterference, wobble effect of the probes impurities on the inside or outside of the tube, productionindications as well as any external parts present).
To attain a sufficiently accurate reproduction of the signal sequence, the spatial resolution shall be setso that the required spatial resolution is ensured by the selected sampling rate and the probe speed.The probe speed shall be dependent on the base frequency and sample rate and shall be no faster thanthe speed required to obtain a clear signal from the reference standard through-wall hole, within thecalibration limitations described in 220.127.116.11.
The following inspection technique can be used for the eddy current testing of steam generator heatingtubes as part of the standard technique:
—Multiple frequency technique with internal bobbin probe (differential arrangement,see 4.3.1;
absolute arrangement,see 4.3.3);
– Linking of the signals of multiple frequencies with internal bobbin probe in differentialarrangement
lf the standard technique is inadequate for evaluation of the eddy current signals，eddy currenttechniques for more extensive evaluation (see 4.4 for additional techniques,e.g. rotating scanningprobes, matrix probes with T/R technique) or other non-destructive testing methods (e.g. ultrasonictesting, visual inspection) shall be applied.
Supplementary to the standard technique,eddy current techniques for more extensive analyses canalso be used as localisation systems in case of specific requirements (e.g. testing in the area of tubebottom/roll-in slugs).
4.3.1 Multiple frequency technique with internal bobbin probe in differential arrangement
18.104.22.168 Basic frequency
The total tube circumference is recorded integrally when testing with an internal coaxial probe. Theextension of indication can therefore only be determined in the axial direction. ln the undisturbedarea, evaluation of the signals of a frequency for determining location, depth and axial extension of anoperational change is sufficient.
lf indications overlap (e.g.operational indications and form indications), a statement is generally onlypossible with restrictions. The multifrequency technique shall then be used in the mix (linking ofmultiple frequencies) (see 4.3.2].
When testing operationally indications, the test frequencies used are generally in the range from10kHz to 1 MHz.The basic frequency shall be selected so that the phase offset angle between a 20 %flat-bottom hole applied on the outside of a reference specimen and a through-wall drilled hole is in therange from 90° to 120°.
22.214.171.124 Comparison between the signals of several individual frequencies with internal bobbin probe If indications occur, further frequencies can be used for the differentiation of interference signals and operationally indications in addition to the basic frequency. The individual frequencies shall be selected so that a significant phase offset angle is reached between indications and interference signals.
4.3.2 Linking the signals of multiple frequencies with internal bobbin probe in differential arrangement (mixed technique)
126.96.36.199 General Two to four frequencies are used generally for this inspection technique. Linking the signals of multiple frequencies (mix) allows a quantitative analysis of operationally indications even when indications with interference signals overlap. However, the test sensitivity (fault detectability) generally declines for areas in which the evaluation is conducted in the mix. The following two mixes shall be used as a minimum:
— 2‑frequency mix for the area of the spacers and the area of the tube bottom;
— 3‑frequency mix for the area of tube bottom with tube expansion (roll‑in).
188.8.131.52 Two-frequency mix The frequencies shall be selected so that the interference signals of all structural parts (e.g. spacers, corrugated iron, and oscillation limiters) are suppressed. The phase offset angle between a wall thickness weakening of 20 % applied on the outside of a reference specimen and a wall penetrating reference fault shall be in the range from 80° to 100°.BS ISO 20890-5 pdf download.