An automated ultrasonic inspection method for thickness checking of dryer cylinders for paper making machines

摘要

Paper machine dryer cylinders are steam heated cast iron cylinders typically 7.2 m long by 1.5 m diameter with a wall thickness of between 25 and 35 mm. In-service inspection requirements specify that dryer non-destructive testing is performed to the standard document TIS-0402-16: 'Guidelines for the Inspection and Non Destructive Examination of Paper Machine Dryers'. This Standard requires periodic thickness surveys to be conducted to assure that the shell thickness is consistent with the maximum allowable working pressure. Ultrasonic thickness readings can determine the worn or thin spots that threaten the integrity of the dryer, but a problem exists in using conventional pulse echo techniques. Accurate ultrasonic thickness measurement depends on consistency in the material properties between the calibration test-piece and the work-piece. On any given dryer cylinder the material properties of the cast iron and therefore the velocity of sound varies over the length and around the circumference. Therefore, reliable and accurate thickness measurements cannot be made from a single point calibration and it is physically impossible to obtain multi-calibration points on a dryer cylinder using a conventional, pulse echo technique. This paper describes the development and application of a self-calibrating ultrasonic imaging system using aspects of the time-of-flight diffraction (TOFD) method to provide a cross section image and accurate point readings along the whole length of a dryer cylinder. This enables accurate, rapid monitoring of the vessel thickness and a reliable ongoing hard-copy record of the test.