Abstract: There are 20 bends of 90°, 406 mm×8 mm in the circulate system of some experiment units. Each bend is made by welding two plates of 316 stainless steel, which are rolled to semicircular-arc shape. During in the construction, three correlative unqualified occurrences were found about these bends. The cause of occurrences was analyzed and the dealing course modifying the shape for the third occurrence was described.