In 1987 and 1989 the Ammonia Plant Secondary Reformer Exit line experienced severe cracking which developed in the fusion line between the CrMo low alloy and the high alloy CrNi (Incoloy 800H) materials. The major failure mechanism was identified as “hydrogen disbonding,” due to combinations of high material temperatures, high weld stresses and too high cooling rates during plant shutdown. An undetected development of defects could not be tolerated, and after a review of applicable monitoring techniques, it was decided to install a four-channel PC based AE monitoring system. The objective was on-line detection of developing defects in the weld area. According to in-house evaluation criteria the acoustic activity was verified by on-line (high temperature) ultrasonic examination.

The safety philosophy behind the project, the AE monitoring system and the analysis programs, and the summarized experience from nearly two years of operation is presented here.