Probabilistic Characterization of B₁ for Nuclear Pipe Bends Subjected to Internal Pressure with In-Plane Closing Moment

Abstract eng:
For a nuclear pipe bend or elbow, the primary stress index B₁ is calculated in terms of the pipe's bend factor (h) as per ASME B&PV Code, Section III (NB-, NC-, ND-3600): B₁=0.4h-0.1 where, h = tR/r²_m, t = nominal thickness, R = nominal bend radius, and r_m = mean pipe radius. The plastic collapse behaviour of pipe bends and elbows under internal pressure and moment is characterized by geometric nonlinearity, and it changes depending on the orientation of flexure. The nonlinear behaviour is affected by the pipe geometry, defined by its nominal outer diameter D_o, t, and R. These effects can be included, only in an approximate sense through the stress indices B₁ and B₂, while using a simplified analysis method. Eq.(1) cannot represent all pipe bends with varying D_o, t, and R and different flexure conditions. For the reliability-based design, it is necessary to obtain probabilistic models of all relevent parameters ('random varaibles'). The primary objective of this paper is the probabilistic characterization of B₁ for pipe bends subjected to internal pressure with in-plane closing moment. The scope of this study is limited to Class 2 and 3, stainless steel (Type 304), thin shell pipes with a 90° long radius bend. The probabilistic characterization is obtained using a 'crude' Monte Carlo simulation with repeated random samplings of the basic random varaiables: D_o, t, h. The stress index B₁ is calculated as B₁=2t/PD_o(S_y-B₂M_c/Z_p) where, Z_P = plastic section modulus and P = design internal pressure for considering Service Level B and target reliability 3 at room temperature. The collapse moment (M_c) is evalutated from the moment versus end rotation plot for each pipe specimen by nonlinear finite element analysis. From the linear regression analyses the best fit line is obtained for large number of samplings as B₁=-0.03380 + 0.1054h + e where, e is a random error with mean zero and standard deviation 0.07232. The values of B₁ are very low, which indicates that the effect of internal pressure in the collapse behavior of pipe bends is negligible. This new expression for B₁ can be used for reliability-based code calibration of nuclear pipe bends and elbows.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket