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 beof practical use when RFT is used to confirm the position of an objectinstalled on a tube or to detect where objects have become detached andhave fallen against a tube.5278.3.2Neighboring Tubes126.96.36.199In areas where there is nonconstant tubespacing (bowing) or where tubes cross close to each other,there are indications which may be mistaken for flaws.188.8.131.52Neighboring or adjacent tubes, accordingto their number and position, create an offset in the phase.This phenomenon is known as the bundle effect and isa minor source of inaccuracy when absolute readings innominal tube are required.184.108.40.206In cases where multiple RFT probes areused simultaneously in the same heat exchanger, careshould be taken to ensure adequate spacing between differ-ent probes.8.3.3Conductive or magnetic debris in or on a tubethat may create false indications or obscure flaw indicationsshould be removed.8.4Tube Geometry Effects8.4.1Due to geometrical effects (as well as to theeffects 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 ordistort flaw indications.8.4.2Reductions in the internal diameter may requirea smaller diameter probe that is able to pass through therestriction. In the unrestricted sections, flaw sensitivity islikely to be limited by the smaller probe fill factor.8.4.3RFT End Effect-The field from the exciter isable to propagate around the end of a tube when there isno shielding from a tube sheet or vessel shell. A flawindication may be obscured or distorted if the flaw or anyactive probe element is within approximately three tubediameters of the tube end.8.5Instrumentation8.5.1The operator should be aware of indicatorsof noise, saturation, or signal distortion particular to theinstrument being used. Special consideration should begiven to the following concerns:220.127.116.11In a given tube, an RFT system has afrequency where the flaw sensitivity is as high as practicalwithout undue influence from noise.18.104.22.168Saturation of electronic components is apotential problem in RFT because signal amplitudeincreases rapidly with decreasing tube wall thickness. Dataacquired under saturation conditions is not acceptable.8.5.2Instrument-induced Phase Offset.During theamplification and filtering processes, instruments mayintroduce a frequency-dependent time delay which appearsas a constant phase offset. The instrument phase offsetmay be a source of error when phase values measured atdifferent frequencies are compared.
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