Ultrasonic Refracted Longitudinal Wave / TOFD - INSPECTION OF REFORMER TUBES

Creep cracking of cast tubing used in steam/methane reforming and pyrolysis heaters usually starts near the mid-wall of the heater tube and is normally longitudinal, resulting from hoop and thermal stresses in the tube.

Ultrasonic equipment that implements through transmission (pitch catch) has been used to inspect tubes. With this attenuation method, a grading of percent transmission is made to draw some conclusions about the degree of fissuring, which attenuates transmission of the ultrasound. Since tubes vary in the amount of equiaxed and columnar grains, the calibration standard used should reflect the tubes being inspected. Without an adequate standard, the judgment of percent transmission may be in error. In any case, subsequent inspections with this method may show a high degree of overlap or scatter in damage states when compared with previous inspections.

Ultrasonic time-of-flight diffraction (TOFD) has also been used to compliment through transmission tube inspection. TOFD is a method that detects diffracted waves coming from the tips of flaws and is best used to detect severe flaws, such as fissures in steam/methane reformer tubes.

Assessment of degradation using these ultrasonic techniques may be accomplished by performing a baseline examination and recording trends over a period of time. A “snapshot” inspection may not adequately assess damage because of the many variables involved.

Evaluations of tubes have indicated that the initiation of internal fissuring will eventually cause the tube to fail. Major fissuring, which is easily detected, indicates that failure may occur in up to 10,000 hours. Since such a wide range of tube life is available for evaluation, a risk analysis should be made. Tubes that are expected to fail in less than 1 year should be replaced. Tubes that may be good for several years may be allowed to remain in service until the next scheduled shutdown, when they can be re-inspected or replaced. Replacement tubes can be ordered and would be on hand when needed. All these evaluations must be based on the assumptions that the original design and casting quality are adequate and that operation, especially with respect to tube metal temperature, is within the design limits.