Plates end plates tube sheets anti vibration bars

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plates, end plates, tube sheets, anti-vibration bars, neigh- boring tubes,impingement plates, loose parts, and attach- ments clamped or welded to a tube. NOTE 4: Interference from ferromagnetic or conductive objects can be of practical use when RFT is used to confirm the position of an object installed on a tube or to detect where objects have become detached and have fallen against a tube. 527 8.3.2 Neighboring Tubes 8.3.2.1 In areas where there is nonconstant tube spacing (bowing) or where tubes cross close to each other, there are indications which may be mistaken for flaws. 8.3.2.2 Neighboring or adjacent tubes, according to their number and position, create an offset in the phase. This phenomenon is known as the bundle effect and is a minor source of inaccuracy when absolute readings in nominal tube are required. 8.3.2.3 In cases where multiple RFT probes are used simultaneously in the same heat exchanger, care should be taken to ensure adequate spacing between differ- ent probes. 8.3.3 Conductive or magnetic debris in or on a tube that may create false indications or obscure flaw indications should be removed. 8.4 Tube Geometry Effects 8.4.1 Due to geometrical effects (as well as to the effects of permeability variations described in 8.2.2), local- ized changes in tube diameter such as dents, bulges, expan- sions, and bends create indications which may obscure or distort flaw indications. 8.4.2 Reductions in the internal diameter may require a smaller diameter probe that is able to pass through the restriction. In the unrestricted sections, flaw sensitivity is likely to be limited by the smaller probe fill factor. 8.4.3 RFT End Effect-The field from the exciter is able to propagate around the end of a tube when there is no shielding from a tube sheet or vessel shell. A flaw indication may be obscured or distorted if the flaw or any active probe element is within approximately three tube diameters of the tube end. 8.5 Instrumentation 8.5.1 The operator should be aware of indicators of noise, saturation, or signal distortion particular to the instrument being used. Special consideration should be given to the following concerns: 8.5.1.1 In a given tube, an RFT system has a frequency where the flaw sensitivity is as high as practical without undue influence from noise. 8.5.1.2 Saturation of electronic components is a potential problem in RFT because signal amplitude increases rapidly with decreasing tube wall thickness. Data acquired under saturation conditions is not acceptable. 8.5.2 Instrument-induced Phase Offset. During the amplification and filtering processes, instruments may introduce a frequency-dependent time delay which appears as a constant phase offset. The instrument phase offset may be a source of error when phase values measured at different frequencies are compared.
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  • Fall '16
  • Heri
  • Nondestructive testing, RFt

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