FAILURE ANALYSIS OF BANK TUBE IN PACKAGE BOILER

摘要

Generally, it is well know that the bank tubes are one of the components of package boilers that fail frequently. In the investigated case the bank tube experienced a leak due to a circumferential through wall crack developed on the top half. This portion of the tube is swaged and connected to the steam drum. The failures occurred in tubes positioned in the first and second rows as a rule. The boiler has a design capacity of 140 tons per hour (140 Megagrams per hour) and is operated at a pressure of 42 barg (43 kg/cm~2) and temperature of 340°C (644°F). It was commissioned in 1993. Bank tubes have OD about 2.5-inches (63.5 mm) and wall thickness of 0.126-inches (3.2 mm). The results of metallographic and SEM/EDS examinations are presented and discussed. Numerous rounded pits about 0.8 mm in depth were found on the inner surface of swaged area. Intergranular cracks propagated from the bottom of these pits. The most probable failure mechanisms are also discussed. Most likely the bank tube failed due to caustic gouging followed by intergranular caustic cracking that developed from the bottom of the pits.