B2 and C2 Stress Indices for Large-Angle Bends

Abstract

B2 and C2 stress indices are needed for evaluation of nuclear piping components. These indices are used in calculation of stress terms corresponding to moment loading. The formulas (B2=1.3/h2/3 and C2=1.95/h2/3) given in the ASME Boiler and Pressure Vessel Code for calculating these stress indices for elbows and bends are independent of bend angle. Author’s earlier work indicated that the values obtained by these formulas are conservative for bend angle ≤90 deg. The objective of the present study was to investigate the values of B2 and C2 stress indices for elbows with bend angles larger than 90 deg and to evaluate the limit of the applicability of the ASME Code formulas in terms of the bend angle. For this purpose, elbows with bend angles ranging from 90 to 180 deg have been investigated in detail. Elbow sizes studied ranged from 2.5 to 20 NPS. Finite element models have been used for detailed analysis. To cover the worst case of stresses in the elbows, several hundred load cases were analyzed by varying the direction of the moment loading. It was found that the three independent loading cases corresponding to three orthogonal axes do not bound the worst-case stress in the elbows. The results of the analysis show that the calculated value of B2 stress index increases with bend angle; but its value, even for 180-deg bend angle, remains below the ASME Code value with a margin of at least 15%. Considering all analyzed elbows and bend angles from 90 to 180 deg, the calculated maximum value of C2 stress index is found to be within 2 percent of the ASME Code value. The maximum increase in the value of B2 stress index is 12.5 percent as the bend angle is increased from 90 to 180 deg. The corresponding increase in the C2 stress index is less than 8%. Based on these results and authors’ earlier work, a modification to ASME Code formulas is proposed for calculation of more realistic values of the stress indices for bends up to 180 deg.