ABNORMAL CRACKS LED TO PREMATURE DECOMMISSIONING OF BOILER FEED WATER EXCHANGER - WHAT HAPPENED AND WHY?

摘要

A small leak from top tubesheet-to-tube welds prompted further inspection of the 11/4Cr- 1/2Mo Ammonia Converter Boiler Feed Water (BFW) Exchanger during a planned shutdown. Further cracks were identified in the top channel to tubesheet butt weld that operated at 700° F. After grinding halfway through thickness with these cracks still being present, excavations of the 2 1/8" thick shell were stopped. As well, attempts to repair the top tubesheet-to-tube welds resulted in further cracks developing. The BFW exchanger was scrapped after being in service for 7 years. In contrast, previous exchangers in this service had operated without known cracks for more than 15 years. The findings as to the causes and potential measures to obtain a better service life are presented below.