In a petrochemical plant, a steam leak incident happened due to a through-wall crack, which was found around the branch connection between the superheated steam pipe header and the branch pipe for the safety valve. Similar cracks had been observed in the Normally No Flow (NNF) line pipes sometimes. We decided to investigate the cause of these cracks. The crack was found along the circumferential welding line of the branch connection. According to the microfractography, the origin of the crack was internal surface of the pipe and it propagated through the thickness of the wall. Striations were observed on the fracture surface, and it showed the fatigue effect. All the NNF line pipes where the leak incidents occurred were installed upward and then connected to the horizontal pipe toward the safety valve. Infrared thermographic testing revealed that the steam drain pooled in the horizontal pipes was flowing intermittently toward the superheated steam pipe header. Cyclic thermal stress was sure to occur in the damaged area with calculation by FEM, and the cause was assumed to be thermal fatigue cracking due to intermittent dripping of drain water condensed in the NNF pipe. We decided to correct the slope of the horizontal pipe so that the drain was not pooled. Also, we checked all other NNF line pipes upwards and confirmed that there was no possibility of a similar incident due to drain in the horizontal pipe.