Improved Methods for Paper Dryer Shell Inspection, 1996 Engineering Conference Proceedings

Alan Dorsey

Longview Inspection, Inc.

Paper machine dryer rolls must be operated continuously to obtain maximum productivity. These rolls must also operate at the highest allowable pressure rating to maximize throughput. Determining the integrity of pressure vessels such as dryer rolls is of prime importance for both safety and reliability. Many of these components have been in service for many years, which is also an important consideration. Various nondestructive testing methods are used to determine the condition of areas on dryer rolls corresponding to locations in which previous failures have initiated on other rolls. The ultrasonic pulse-echo method is used to determine remaining wall thickness on dryer shells. Data analysis in recent years has proven that the accuracy of ultrasonic thickness measurement is poor when used on cast iron materials, as opposed to use on carbon steel. This is unfortunate since it is very important to have accurate data to remain on the thin line between maximizing output and minim&g safety risks. Measurement accuracy on each roll is important since only one marginal roll left in service can derate machine operation. The intent of this paper is to discuss the sources of inaccuracies in ultrasonic thickness measurements and to offer suggestions for achieving higher accuracy.