Plugging criteria based on stress analysis for high-pressure feedwater heater tubes with external circumferential wall-thinning flaws

Tubes in high-pressure feedwater heaters often experience localized wall thinning due to long-term mechanical and environmental degradation. To prevent catastrophic rupture at such weakened locations, damaged tubes are typically withdrawn from service through plugging. This study integrates theoretical formulations with finite element simulations to examine the stress characteristics in feedwater heater tubes containing external circumferential thinning under simultaneous internal pressure and thermal gradients. Using regression analysis of numerical data, new analytical relationships are proposed to predict hoop and axial stresses at the inner surface of tubes with localized defects. Based on the von Mises stress criterion, plugging thresholds are established to define allowable stress limits. The results indicate that stress concentration is most severe at the inner wall, while radial stress remains almost constant regardless of the ratios of defect depth to wall thickness or axial length to radius of tube. The defect depth (c) primarily governs the overall stress values which increase significantly as the thinning deepens. The influence of defect length (b) becomes notable only for deeper defects. Moreover, as the circumferential angle of the defect (θ) extent enlarges, the stress concentration intensifies, particularly at higher c/t ratios. • Plugging criteria established for feedwater heater tubes with external circumferential wall-thinning flaws. • The defect depth (c) primarily governs the overall stress values which increase significantly as the thinning deepens. • The influence of defect length (b) becomes notable only for deeper defects. • The circumferential angle (θ) of the tube defect also has effect on the stresses. • Proposed method is independent of material properties and operating conditions.