Ultrasonic Examination – Calibration

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Ultrasonic Examination – Calibration

Calibration of longitudinal wave UT consists of developing a “distance amplitude curve (DAC)” for the instrument and transducer, using test blocks manufactured from a material similar to the workpiece (i.e., with the same velocity of sound). Calibration for determining the remaining thickness of a corroded component can be accomplished by placing the transducer sequentially on […]

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Ultrasonic Examination – Longitudinal Wave UT

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Longitudinal wave UT is used primarily to determine the remaining thickness of corroded shell components, including the depth of pits. It can also be used to detect internal flaws that have developed during service and that have a reflecting surface essentially parallel to the surface of the workpiece (i.e., perpendicular to the longitudinal ultrasonic wave), […]

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Ultrasonic Examination – Couplants

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Attenuation of ultrasonic waves propagating through air is very high, and normal surface roughness (especially of the workpiece) will always create an air gap between an ultrasonic transducer and the workpiece. It is not practical to improve the surface of the workpiece to the extent necessary to effectively eliminate the air gap. Therefore, a “couplant” […]

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Ultrasonic Examination – Ultrasonic Transducers

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Ultrasonic Examination – Ultrasonic Transducers

Ultrasonic waves for UT are created in a metal workpiece with a piezoelectric transducer that is coupled to the workpiece. The piezoelectric transducer converts high frequency electrical impulses into correspondingly high frequency mechanical vibrations that induce ultrasonic waves in the workpiece. For detection of reflected waves, the piezoelectric transducer operates opposite to the manner in […]

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Ultrasonic Examination Instruments

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Ultrasonic Examination Instruments

Most UT systems used for the in-service inspection of a pressure vessel employ a “pulse-echo” method of operation, in which the same transducer (described below) that creates the ultrasonic wave (pulse) is also used to detect any ultrasonic waves (echos) reflected by flaws. A typical ultrasonic instrument for the pulse-echo UT is illustrated in Figure […]

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Cylindrical Shells Subjected to External Pressure

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Cylindrical Shells Subjected to External Pressure

The critical collapse pressure (Pc) of a cylindrical shell under external pressure depends on two characteristic geometric ratios: t/Do and L/Do, where t is the shell thickness, L is the unstiffened length, and Do is the outside diameter. If L is short enough, the cylinder can fail by plastic yielding in compression at a stress […]

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Stresses in Pressure Vessel Shells Due to External Pressure

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External pressure on a vessel most commonly occurs when a vacuum or partial vacuum is created inside of the vessel by (1) design, (2) discharge of its contents, (3) steam-out cleaning (condensation of steam), or (4) mechanical action, such as on a compressor suction, during off-design events. In these circumstances, the atmospheric pressure surrounding the […]

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Pressure Vessels Thermal Stresses

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Thermal-expansion problems can occur whenever there is: (1) a considerable difference between the vessel operating temperature and the temperature of the environment surrounding the vessel; (2) restricted expansion or contraction; or (3) a temperature gradient within a vessel component that creates a differential expansion. Problems due to external constraint are solved differently than those due […]

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