Stress intensification factor, sustained stress index and flexibility factor analysis of large D/T elbows

Paper presents new equations for calculation of stress intensification factors, sustained stress indices and flexibility factors for large diameter-to-thickness ratio elbows (i.e.,[Formula: see text]). The proposed equations overcome the need for pipe stress engineers to use numerical analyses in order to evaluate the piping which falls outside of the scope of typically used piping design codes. Regression analysis of the newly proposed equations is based on an extensive numerical database developed specifically for purposes of this paper, alongside with the data (both numerical and empirical) found in the open literature. Proposed equations prove to be a better fit to the database than the existing equations and have been shown to be applicable not only for large pipes, but also for normal piping (having[Formula: see text]) typically found in industrial plants. In case of the adoption of the proposed procedure, it is implied that the proposed equations should be applied to all the bends within a piping system subject to the analysis and not only to the selected bends.

Challenging the Industry Practices of Temperature Distribution, SIF and its Effects on Piping Design

The piping, welding and its fittings contributes to around 25–35% of the total material [1] of any process plant. The quantity of the piping and fittings required in critical pipes (lines) is largely influenced by the flexibility requirements to comply with applicable codes. Since the pressure and weight driven stresses are primary in nature and taken care by the thickness & supports respectively. The Secondary stresses are mainly caused due to temperature and determines the route of the pipe. Unfortunately, there is no sufficient awareness amongst the Process engineers, who dictate the temperature, about the effect of temperature (listed in line list) on the plant cost. To make the situation worse, very often the pipe stress engineers follow thumb rules, conservative design basis while applying temperature to pipes without questioning process engineers the basis and without having experimental evidences which in turn ends up adding to the cost, labor and time of the plant. Another factor influencing the ‘flexibility need’ is ‘Stress Intensification Factor’ which, as well, incorrectly multiplied by conservative numbers, especially in case of 45° branch fittings. Even ‘Flange leakage analysis’ needs to be performed to ensure safety of the plant. This paper challenges the mis-concepts in temperature distribution of a piping system by analyzing a real-life problem. Also, it provides the actual temperature measurements and compares it with theoretical values. At the end it provides a ‘Temperature de-rating factor, @(Taa)’ which can be used to derive actual expected temperature distribution. Further this paper provides a table to get the actual SIF of Tee and Fittings obtained by FEA. At the end it provides comparison between the results obtained by present industry practices against practical analysis and its effect on cost.

Analytical Determination of Stress Indices and Stress Intensification Factor for an Extruded Nozzle of Super Pipe

The super pipe nozzles in nuclear power plants are usually designed to be in compliance with the requirements of Class 2 piping of Section III of the ASME Boiler and Pressure Vessel Code. The stress indices B2 and stress intensification factor i are required for the stress evaluation. In the past two decades, the hot extrusion forming technology has been widely used to manufacture those nozzles, instead of traditional insert weldolets. However, previous extruded nozzle stress analyses have shown B2 that the calculated stresses may exceed the limits in some working conditions. The objective of present study is to determine the stress indices and stress intensification factor for an extruded nozzle of the supper pipe by the finite element method and to evaluate the conservatism of those factors from the ASME Code formulae. In this paper, a three-dimensional finite element model of an extruded nozzle is developed. Four load cases are considered, which are corresponding to an in-plane bending moment and an out-plane bending moment applied at the run pipe side and at the branch pipe side, respectively. The magnitude of bending moment is assumed to be 1000Nm. The stress indices B2r, B2b, C2r, C2b, K2r and K2b, where the subscript r and b refer to the run pipe and B2r the branch pipe, are calculated based on the finite element analysis results. The stress intensification factor ir and ib are determined by the empirical formula: ir = C2r*K2r/2 and ib = C2b*K2b/2. Further, the developed factors are compared with those calculated from the ASME code formulae. It is found that the stress indices B2r and B2b obtained from the linear elastic finite element analysis are conservative. Currently, the values of B2r and B2b gained from the ASME code formulae are more appropriate for the stress evolution. The stress intensification factors ir and ib obtained from the analytical determination are lower than those calculated from the ASME code formula. For the extrude nozzle studied, the factor ir decreases 30% and the factor ib decreases about 3.3%.

Finite Element Analysis Based Stress Intensification Factor Calculation and Comparison With Various Approaches for Stress Intensification Factor Calculation

Piping caters a major role in the process industries wherein stress intensification factor (SIF) express the Piping flexibility of the system. A typical Piping system consists of combination of pipes and various fittings with intersection geometries namely bend, tee, reducer, etc. A SIF is a multiplier on nominal bending stress so that the effect of geometry and welding can be considered in a flexibility analysis. An attempt has been made to compare the SIF values among ASME Piping B31.3, Welded Research Council (WRC) Bulletin 329, Paulin Research Group (PRG) empirical data and shell-based finite element analysis (FEA) for various tee sections based on in-plane and out-plane bending moments through this paper. The bending moment which causes tee to open/close in the plane formed by two limbs of tee is called in-plane bending moment. The bending moment which causes branch of tee to displace out of the plane retaining run pipe steady is called Out-plane bending moment. ASME B31.3 provide guidelines to evaluate SIF values through empirical formulation as per Appendix-D with few limitations listed below. 1. Valid for d/D < 0.5 only 2. Non-conservative for 0.5 < d/D < 1.0 3. Valid for D/T ≤ 100 4. SIF values calculated with respect to header pipe. There is no difference in SIF values for header and branch pipe and it is the average value. WRC 329 was published in 1987 and has not been updated taking ASME B31.3 latest edition into account. PRG carried out SIF for the various sizes and types of tee fittings and prepared correlation equations through detailed FEA using nonlinear regression and test data.

A New Stress Intensification Factor of Pipe-In-Pipes Based on Finite Element Analyses

For the design of a transmission piping system, a stress intensification factor (SIF) is generally used for the stress calculations of piping components due to external forces, and the solutions for the single-walled piping components can be found in the existing design codes. However, it is quite difficult to obtain the reliable estimations for pipe-in-pipes (PIPs) from the existing solutions, because the PIPs show significantly different behaviors compared to the single-walled piping components due to the restraint effect induced by the outer pipe of the PIP. In this paper, the estimation schemes for the stress behaviors of the PIPs were proposed based on the detailed finite element (FE) analyses. In order to quantify the restraint effect, the FE analyses were conducted by considering various geometric variables of the PIPs under an internal pressure and a global bending moment. Based on the FE results, the tabular and closed-form solutions of the SIFs of PIPs were newly proposed. Finally, the proposed SIF estimations were validated against numerical results.

Failure of Thin-Walled Pipes With D/T up to 140 and Conclusions for the Design Codes

The influence of imperfections on the instability bending moment of thin-walled straight pipes with D/t-ratios (D - outside diameter, t - wall thickness) up to 140 is determined using nonlinear Finite Element (FE) analyses. The analyses show that the type and size of the imperfection, the D/t ratio and the material properties have significant influences on the instability moment. The nominal bending stress of pipes (yield stress 500 MPa) with D/t > 70 and an ovality of 0.5% is smaller than the yield stress at the instability point. That means, the failure occurs by buckling in the elastic range of the nominal bending stress. In static analyses the moment decreases abruptly after reaching the instability moment. In the dynamic analyses the pipe jumps abruptly to the state with smaller moment. The obtained results are applied to calculate the B2 index for pipes with D/t ≤ 140. The B2 indices for thin-walled straight pipes with D/t > 40 are considerably higher than 1.0. In general, there is a good agreement between the calculated B2 values and the values of the ASME Code. A correction factor for higher temperatures is not necessary. The allowable moments calculated with the B2 index and the stress intensification factor i are compared. The bending moments from disabled thermal expansion and anchor movements have the same effect on the failure due to (plastic) buckling as the primary moments and must be taken into account.

Stress Intensification Factor for Large Diameter Welding Tee Connections

Stress Intensification Factors (SIFs) are factors relating to fatigue characteristics of piping components. SIF is fatigue correlation which compares fatigue life of a typical piping component such as a tee and elbow to the reference fatigue life, that is girth butt welds in straight pipe subjected to bending moments. In order to calculate localized stress of such piping component, above mentioned figured out SIF shall be multiplied by nominal stress. ASME B31 contains several formulas for stress intensification factors considering limitation that those formulas are valid only for D/T≤100 (diameter to thickness). Extending the valid range mentioned in ASME B31 this paper is dedicated to SIF calculation for D/T≤100 and also for D/T>100 by utilizing Finite Element Analyzing (FEA) for welding tee. The computed SIF for D/T>100 welding tee can now be placed to typical pipe stressing program which analyzes piping system using beam elements. In addition, this paper investigates the effect of Rx (the magnitude of corner radius of shoulder at branch) on SIF of Welding Tee Connections.

Creep-Fatigue Crack for a 2¼Cr1Mo and Inconel Transition Weld

Transition welds have been used in some Advanced Gas-Cooled Reactors to attach a 2¼Cr1Mo spool piece to a hot reheat header, which is fabricated from 316L material. These welds were made using Inconel-82 filler and were post-weld heat-treated (PWHT) at around 705°C. In this paper, a creep-fatigue crack growth assessment has been carried out on the 2¼Cr1Mo ferritic side of the weld using the R5 procedure. As the welds have been PWHT rather than solution heat treated, therefore there are residual welding stresses present. On the 2¼Cr1Mo ferritic side of the weld, a 60MPa through-wall bending stress has been applied in the axial direction and a 60MPa membrane stress in the hoop direction. These are consistent with the recommendations of R5 Volume 7. Due to the differing thermal expansion coefficients of Inconel and ferritic materials, thermal mismatch stresses induced from changes in temperatures have been considered. It has been assumed that at the PWHT temperature of 705°C there are no thermal mismatch stresses. Any reduction from this temperature generates thermal mismatch stresses which are accounted for by using secondary Stress Intensification Factor values in the calculation. On the 2¼Cr1Mo ferritic side of the weld, a 5mm deep by 50mm long circumferential defect (semi-elliptical surface-breaking surface defect) postulated to exist on both internal and external surfaces has been used. Limiting defect sizes have been calculated following the R6 procedure. Four cases have been investigated. After presenting the investigation results, conclusions are drawn.

Evaluation of the Stratification Loads With the EDF Fatigue Assessment Device

A fatigue assessment device, Fatiguemètre, has been developed by EDF especially for sensitive zones of components of French PWR NPP. The main objective of this device is the evaluation of a realistic usage factor (damage criterion) by using operating data. Two requirements are to use as less specific transducers as possible and simplified thermomechanical methods for the calculation of stress variation. Some monitoring devices had nevertheless been necessary to get correlations between in service data, for instance flow rate and thermal stratification distribution. This instrumentation, called a stratimeter, consists of a ring of thermocouples installed around a section of a pipe. The present studies were carried out on stabilized thermal stratification states and also on transients of different rates. These input data are always representative of the operating and design transient occurring on the FWL next to the connection with the steam generator. The first use of the Fatiguemètre device revealed the need of optimization on some thermomechanical methods. Investigating steady-state (stabilized) or slow enough transients of temperature stratifications, it was found that it is possible to calculate sufficiently accurate stresses without the use of any stress intensification factor, when applying temperatures from outer wall measurement as mean wall temperatures. The new discretization of Duhamel formula gives a satisfying evaluation of the local stratification stress value.

GB ASME & RCCM Analysis of Piping Design Camparison about Class 2 Components

Three primary nuclear technique of RCCM, ASME and the GB50267-97 code of China, have the similar classification of nuclear facility in nuclear power plant, including the code class and security class. The class 2 pressure pipeline design clauses in the 3 codes are similar, but are not identical. The earthquake input and the clauses about the class 2 pressure pipeline are compared. The results show that, when use the alternative damping values for response spectra analysis, GB and ASME are somewhat safer than RCCM in level B criteria, RCCM is somewhat safer than GB and ASME in level D criteria. In class 2 piping design code, RCCM stress assessment focuses more on the pressure, the GB and ASME is more focuses on considering the weight and occasional loads, in the lower pressure and the same stress intensification factor conditions, GB and ASME criteria are conservative than RCCM.