Fitness for Service Assessment of Circumferential Cracks at Pipe Penetrations

An engineering study of the main steam line pressure tap cracking was performed. The failure mechanism was thermal fatigue and the circumferential cracks initiate at the pipe penetration on the inside surface of the pipe. Based on the results, a parametric fitness for service study was performed to develop a flaw tolerance graph to be used for the evaluation and disposition of detected defects. The parameters are pipe diameter, penetration diameter, and inspection interval. The objective is to determine the value of the current crack size when weld repair is needed. The fatigue crack growth calculations used the NASGRO computer program. The crack geometries were a single corner crack from a hole in a plate and a corner crack from a hole in a plate with two cracks. The analysis used the properties for Grade 22 plate material given in the program and applied a correction factor (69 cycles for Grade 22 at room temperature equals 1 fatigue cycle with hold time of 12 hours for P11) to account for the pipe material of P11, 1-1/4Cr-1/2Mo and the nominal operating temperature of 482°C. There were approximately 40 cold starts and 210 hot starts per year for the unit. The magnitude of the remote stress depended on the temperature range for the hot and cold starts. The applied stress consisted of tension and bending stresses and was estimated by the observed failure life of approximately 20 years for the 3/4 penetration in the pipe size 18 pipe.