ET16.pdf

Nondestructive test techniques for tubes include

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Nondestructive test techniques for tubes include conventional eddy current, full saturation eddy current, remote field eddy current, magnetic flux leakage, internal rotary ultrasonic testing and laser profilometry. No single nondestructive test technique can be applied to all of the tube materials. Each nondestructive test technique has advantages and limitations. For example, conventional eddy current testing is very sensitive to pits and cracks but its application is limited to nonferromagnetic materials. Internal rotary ultrasonic testing is accurate in measuring wall thickness but will miss small discontinuities such as pin holes and cracks. Optical techniques are limited to inside surface discontinuities. Proper selection of nondestructive test techniques is therefore a key to testing heat exchangers. Technique Selection Several nondestructive test techniques for testing heat exchanger tubing are discussed below. The selection of technique depends on the material and type of discontinuity. Table 2 lists tube materials and the recommended techniques. Eddy Current Testing The conventional eddy current technique is based on measuring the impedance of the test coil. 4 The impedance of the coil 405 Electric Power Applications of Electromagnetic Testing P ART 2. Electromagnetic Techniques for Heat Exchanger Tubing 3 T ABLE 2. Selection of nondestructive test techniques for tube testing. Discontinuity Type Technique Comments Nonferromagnetic Metals Pits, cracks, wall loss conventional eddy current reliable for discontinuity detection; accurate for pit sizing; maximum speed 1.8 m·s –1 (6 ft·s –1 ) Cracks in finned tubes multiple-coil pancake probe detects cracks greater than 50 percent deep Cracks in tubesheet rotating pancake probe slow but reliable expansion zone multichannel transmit/receive fast; special probe required Partial and Thin Ferromagnetic Metals a Pits, cracks, wall loss full saturation eddy current same as conventional eddy current; inside surface pit sizing inaccurate; maximum speed 1.8 m·s –1 (6 ft·s –1 ) Thick Ferromagnetic Metals (Carbon Steel) Wall loss remote field eddy current only for wall loss, limited pit sizing accuracy; maximum speed 0.25 m·s –1 (0.9 ft·s –1 ) partial saturation eddy current only for wall loss, limited accuracy for pits and pin holes Pits and wall loss magnetic flux leakage cannot size discontinuities; more sensitive to inside surface discontinuities; rust on inside surface affects sensitivity; maximum speed 0.9 m·s –1 (3 ft·s –1 ) All Materials Wall loss and large pits internal rotary ultrasonic testing accurate for thickness measurement; slow; requires clean tubes; cannot detect cracks; may miss pin holes and small pits; maximum speed 0.075 m·s –1 (0.25 ft·s –1 ); minimum thickness about 0.9 mm (0.035 in.) Inside surface laser profilometry limited to inside surface testing; requires clean tubes; maximum speed 0.075 m·s –1 (0.25 ft·s –1 ) a. Unified Numbering System S31803, S44627 and S44736 duplex stainless steels.
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  • Fall '19
  • Test, Magnetic Field, Nondestructive testing, electromagnetic testing

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