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A flaw 185 mm long was found in a 125 mm thick circumferential weld of a medium pressure vessel at a late stage of manufacture. The weld had passed all normal radiographic tests but was subject to an ultrasonic examination just prior to delivery in order to provide a record of all minor flaws as an aid to subsequent in service inspection. This ultrasonic examination revealed the presence of the flaw which was almost ten times longer than the standard allowed. The criticality of the defect for the service conditions of the vessel has been evaluated using a failure analysis diagram and found to be low. A safety factor of four was calculated even after making a number of pessimistic assumptions. The influence of defect length upon the stress intensifying effect of a defect is discussed. Attention is drawn to the variation in allowable length in several standards and the difficulties in applying standards based on radiography to the more sensitive ultrasonic measurements. It is concluded that an allowable defect length of at least 200 mm could be tolerated in some circumferential welds without loss of structural integrity. (Peters)