ASME Code Research Articles

Combined stability of cone-cylinder transition subjected to axial compression and external pressure

This paper examines the buckling behaviour of unstiffened mild steel cone-cylinder assembly under (i) axial compression and (ii) combined loading (i.e., axial compression and external pressure). Experimental results on ten (10) laboratory scaled axially compressed cone-cylinder models and their accompanying numerical predictions of collapse load are provided. The tested models are assumed to have the following range of geometric parameter: rcone/rcyl = 0.565–0.721, rcyl/t = 72.17–72.73, β = 8.536° – 16.69° and a constant wall thickness, t = 1.0 mm. Result confirms the repeatability of the experimental data. Besides that, there is a good agreement between experimental and numerically predicted collapse load with discrepancy calculated to be within 10%. Furthermore, numerical analysis of stability domains for cone-cylinder assembly subjected to simultaneous load actions of axial compression and external pressure was calculated for a range of geometrical parameters of: (i) 50 < rcyl/t < 400 and (ii) 10° < β < 30°. For comparison purpose, the equivalent cylinder approach was also deployed to complete calculation based on ASME code case 2286-2. For the case of dimensionless radius-to-thickness ratio, rcyl/t, the result confirms that the ASME code case 2286-2 is unsafe in designing the cone-cylinder shell with rcyl/t > 400. Whereas, for the case of different cone angle, β, it can be said that the ASME code case 2286-2 may perhaps be safe to use in designing the cone-cylinder shell with β < 20°. In addition, the current analysis confirms that increasing the (a) dimensionless radius-to-thickness ratio, rcyl/t and (b) increasing the cone angle, β, may further shrink the combined stability plot. Finally, imperfect cone-cylinder shell under combined loading confirms that (i) the shell is remarkably sensitive at the pressure dominant region compared to the force dominant region and (ii) the location of dimple/dent imperfection at cylinder mid-section proves to be the worst-case scenario.

Pressure Vessel Design by Design by Analysis Route

Pressure vessels are conventionally designed based on the Design by Rule (DBR) approach. Standard vessel geometries are designed using simple formulae and charts, formulated as standards and codes based on the rules proposed in the ASME Boiler and Pressure Vessel Code and the European Standard prEN13445-3. The stress categorization problem faced employing DBR methodology was overcome by employing Limit Load Analysis using the DBA approach. A simple unfired vertical cylindrical pressure vessel with torispherical pressure head and Y-forged skirt support was analyzed for its structural integrity using ANSYS software. The results obtained for two different materials, high strength steel P500-QT and the conventional low strength steel P355 employing DBA approach was compared with the calculated results using the DBR approach. The maximum allowable pressure (PMAX) for P500-QT material was found to be 2.5X the maximum pressure calculated using the DBR approach as per ASME codes and 3X times as per EN13445-3 standard. For P355 material the maximum allowable pressure (PMAX) was found to be 2.32X the maximum pressure calculated using the DBR approach as per ASME codes and 2.6X times as per EN13445-3. The DBR methodology proves to be highly conservative whereas the use of DBA leads to less conservative design parameters.

Microstructure-based prediction of thermal aging strength reduction factors for grade 91 ferritic-martensitic steel

This study aimed to develop a microstructure-based mechanistic approach to address the long-term thermal aging effect on yield stress and ultimate tensile strength and to provide a physical basis for developing thermal aging factors for G91 for a design life of 60 years. Several heats of G91 steel were examined. Controlled aging experiments were conducted on two heats at temperatures of 550, 600, and 650 °C for times up to ~64,000 h. Specimens were also taken from archived creep-tested specimens of G91 and from a tube removed from the Kingston coal-fired power plant to obtain data that cover a wide range of temperatures and for aging times up to 155,000 h. Thermal aging caused significant subgrain recovery, coarsening of M23C6 carbides at subgrain boundaries and MX carbonitrides within subgrains. The intermetallic Laves phase forms during aging and grows rapidly. The growth rate of the subgrain width and MX mean size during thermal aging were described by kinetic models. Thermal aging results in the reductions in the yield stress and the ultimate tensile strength of G91. The effects of thermal aging on the reductions in yield stress and ultimate tensile strength were well described by the microstructure-strength model that considers three superimposed strengthening mechanisms, namely, sub-boundary strengthening, MX precipitation hardening, and Mo solid solution strengthening. The model was independently validated by the ASME Code values and is being used by the ASME to develop the aging-induced strength reduction factors for G91 steel for a design life of 60 years.

Electromagnetic and mechanical analyses of the ITER electron cyclotron Upper launcher steering M4 mirrors for the vertical displacement event

Abstract The mm-wave power exiting the eight In-Vessel waveguides inside the ITER Electron Cyclotron Upper Launcher is reflected by three fixed mirror sets and finally aimed by two independent steering mirrors to specific plasma locations. A design solution for both Upper and Lower Steering M4 Mirrors capable to withstand the loads taking place during normal operation was already proposed. This design is now assessed against the Vertical Displacement Event Category III (VDEIII) linear decay (36 ms) scenario, which was identified as one of the most stringent load combinations for the steering M4 mirrors. This paper reports the analyses performed to quantify the mechanical loads induced in the steering M4 mirrors during the VDEIII scenario as well as the structural integrity assessment to validate the design against these loads. The transient electromagnetic simulations show that the highest forces are induced in the reflecting surfaces, which produce a bending moment that tends to rotate upwards the mirrors. The comparison between the categorized stresses obtained from the transient mechanical analyses and the allowable design limits according to the ASME code shows that the design of the steering M4 mirrors is widely capable of withstanding the expected loads taking place during the VDEIII scenario.

Preliminary structure design and stress analysis of preload structure of DC magnet for Super-X test facility

Abstract Super-X test facility can simulate the electromagnetic performances of a superconducting conductor under fusion reactor operating conditions, in which DC magnets are a core assemble. The DC test magnet provides 15 T central magnetic field. The field homogeneity of test area is more than or equal to 98 %. The magnet comprises three pairs of Nb3Sn coils (i.e., high-field, mid-field and low-field.). Included in the magnet assembly are a preload structure, joints, cooling pipe, etc. The preload structure includes preloaded rods, pre-tightening screws, end plates, an intermediate connection flange, buffer layers and shims between coils. The main function of the preload structure is to maintain registration of the coils under magnetic loads, and, secondarily, to support gravity so that they will not separate. It also provides axial preload compression on the coils. Orthotropic equivalent properties of the coil regions are calculated based on generalized Hook’s law and ANSYS software, the superconducting coils are modelled with equivalent orthotropic properties of winding to simplify the finite element model (FEM). A quarter FEM of the simplified DC magnet is established with the coupling solver, and the finite element analysis has verified the design satisfies appropriate design limits base on the ASME Code.

Failure bending stresses for pressurized pipes with circumferentially part-through cracks

Stress corrosion cracks detected in austenitic stainless pipes were circumferential cracks at inside of the pipes. Failure stresses for the cracked pipes were developed by net-section stress concept. Limit Load Criterion provided by the ASME Code based on the net-section stress concept can estimate fully plastic collapse stress, which is called as failure stress. Local Approach of Limit Load Criteria for estimating stress at crack penetration had developed for a part-through cracked pipe based on the Limit Load Criteria. Advanced Local Approach of Limit Load Criteria for estimating stress at crack penetration is newly developed based on the Local Approach in this paper. In comparing with experimental stresses of pipe crack penetrations, the Limit Load Criteria give un-conservative estimation, and the Local and the Advanced Local Approaches are close to the experimental stresses. In addition, the Advanced Local Approach always gives higher stresses than the Local Approach for internal cracks, and it is shown that the Advanced Local Approach is beneficial for thick wall pipes with large angle cracks.

Structural Integrity Assessment of ITER Torus Cryo-Pump Housing

In this article, a structural integrity assessment of ITER Torus Cryo-Pump Housing (TCPH) is presented. TCPH is a penetration structure located on the cryostat cylinder whose main function is to accommodate and support the Torus Cryo-Pump (TCP), connect it to the vacuum vessel (VV), and provide tritium confinement and primary vacuum boundary. The design of the TCPH structure was finalized during the final design review (FDR). After the FDR, based on the manufacturing feasibility study, changes in the design were proposed to facilitate the nondestructive examination of weld joints. To qualify the proposed changes, structural integrity assessment was performed as per Section VIII Div. 2 Part 5 of ASME code by finite-element analysis using ANSYS software. The structural and thermal assessment was done as per TCPH load specification for protection against plastic collapse, local failure, ratcheting, buckling, and fatigue to perform the structural integrity of the TCPH. The results obtained from the integrity assessment qualified the design criteria. Margins available over allowable stress values for plastic collapse, local failure, and ratcheting are 20%, 19%, and 30%, respectively. A buckling factor of 79 was achieved with a minimum allowable value of 13.42. For fatigue, a usage fraction of 0.516 was obtained against allowable maximum value of 1. Thus, the design with proposed changes was qualified and incorporated.

High-temperature mechanical behaviors of diffusion-welded Alloy 617

The development of a compact-type heat exchanger is one of the critical issues for realizing an efficient gas-to-gas heat exchanging system in a very high-temperature gas-cooled reactor (VHTR). The nickel-based superalloy Alloy 617 (Ni-Cr-Co-Mo, UNS N06617) is considered as the leading material for this application owing to the excellent thermal stability and strength in high temperature ranges. Sixty sheets having dimensions of 200 × 200 mm2 were diffusion-welded under uniaxial compressive pressure of 14.7 MPa at 1423 K (1150 °C) in a high vacuum condition. After 2 h of hot pressing, a post-weld heat treatment (PWHT) was applied to enhance the constituent atomic diffusion across the interface. To evaluate the high-temperature mechanical properties, tensile and stress-rupture tests were employed in this study. In tensile tests, the yield strength of the diffusion weldment is 318 ± 4 MPa at room temperature, and superior yield strengths over the ASME code requirement are maintained up to 1223 K (950 °C). Similarly, the tensile strength reaches 730 ± 15 MPa with ductility of 37.1 ± 1.4% at room temperature. However, both the tensile strength and ductility drop rapidly at elevated temperatures (≥973 K (700 °C)). During stress-rupture testing at a fixed temperature of 1173 K (900 °C) in an ambient air environment, the diffusion weldment shows lower creep properties. A plot of log time-to-rupture vs. log minimum creep rate is illustrated on the basis of the Monkman-Grant (M–G) relationship.

Fatigue Crack Growth for Ferritic Steel Under Negative Stress Ratio

Abstract The phenomenon of crack closure is important in the prediction of fatigue crack growth behavior. Many experimental data indicate crack closures during fatigue crack growths both under tensile–tensile loads and tensile–compressive loads at constant amplitude loading cycles, depending on the magnitude of applied load amplitudes and stress ratios. Appendix A-4300 of the ASME Code Section XI provides two equations of fatigue crack growth rates for ferritic steels expressed by stress intensity factor ranges at negative stress ratios. The boundary of the two equations is classified with the magnitude of applied stress intensity factor ranges, in consideration of the crack closures. However, the boundary value provided by the ASME Code Section XI is not technically well known. The objective of this paper is to investigate the influence of the magnitudes of the applied stress intensity factor ranges on the crack closures. Fatigue crack growth tests using ferritic steel specimens were performed in air environment at room and high temperatures. From the crack closures obtained by the tests, it was found a new boundary which is smaller than the definition given by the Appendix A-4300.

Mechanical Integrity Analysis of a Printed Circuit Heat Exchanger with Channel Misalignment

Printed circuit heat exchangers (PCHEs), which are used for thermal heat storage and power generation, are often subject to severe pressure and temperature differences between primary and secondary channels, which causes mechanical integrity problems. PCHE operation may result in discontinuity, such as channel misalignment, due to non-uniform thermal fields in the diffusion bonding process. The present paper analyzes the mechanical integrity, including the utilization factors of stress and deformation under various channel misalignment conditions. The pressure difference of the target PCHE is 19.5 MPa due to the high pressure (19.7 MPa) of the steam channel in the Rankine cycle and the low pressure (0.5 MPa) of molten salt or liquid metal in the primary channel. Additionally, the temperature difference between channels is around 25 °C, however the average temperature is around 500 °C. The PCHE has a relatively large primary channel measuring approximately 3 x 3 mm, and a steam channel measuring 2 x 1.5 mm. The finite element method (FEM) is applied to determine the stress by changing the misalignment to below 30% of the primary channel width. It was found that the current PCHE is operable up to 700 °C in terms of the ASME code under these design conditions. Additionally, the change of utilization factor due to the misalignment increases, but is still under the ASME acceptance criteria of 700 °C; however, it violates the criteria at 725 °C, which is the allowable temperature condition. Therefore, the mechanical integrity of the PCHE with low-pressure molten salt or liquid metal and a high-pressure steam channel is acceptable in terms of utilization factor.

Application Scope of Limit Load Criterion for Ductile Material Pipes With Circumferentially External Cracks

Abstract Bending stress at plastic collapse for a circumferentially cracked pipe is predicted by limit load criterion provided by the Appendix C of the ASME Code Section XI. The equation of the Appendix C is applicable for pipes with both external and internal surface cracks. On the other hand, the authors have developed a more precise equation taking into account the pipe mean radii at noncracked area and at cracked ligament area. From the comparison of Appendix C equation and the new equation, the plastic collapse stress estimated by the Appendix C equation gives about 20% less conservative bending capacity prediction for external cracked pipes with large crack angle and small Rm/t, where Rm is the pipe mean radius and t is the pipe wall thickness. This paper discusses the limitation scope to use the limit load criterion of the Appendix C equation.

Safety assessment of pressure vessel components for Reaktor Daya Eksperimental

The 10 MW Indonesia’s Reaktor Daya Eksperimental (RDE) which is to be constructed in Serpong Nuclear Zone, Puspiptek and was firstly developed in 2014 is expected to be operational in 2022/2023. The RDE is expected to be a landmark to Indonesia for a nuclear power technology provider in the imminent futurity. Till the end of 2018, the RDE is focused on the basic design including its reactor pressure vessel (RPV). This research paper presents mechanical stresses assessment on basic design of RPV structure components for RDE in ensuring the safety design targets of RDE which are no dangerous radioactive release to the workers, populations, and environments even in the worst accident. Results are presented from the structural stresses assessment under following conditions such as thermal loading, operating pressure, and mechanical loads. The geometric models and dimensions of pressure vessel components within the basic design calculations had been performed by using computer code RPV_RDE.exe in the previous work. These components were designed in accordance with ASME code specification for SA516-70 carbon steel plate. In this work, finite element analysis is applied to assess the occurred structure mechanical stresses upon those RPV components. This work recommends that the RPV components of RDE are safe in the basic design calculations by analysis approach as described in model simulations.

Technical Basis for Flaw Acceptance Criteria for Cast Austenitic Stainless Steel Piping

Abstract According to the current ASME Code Section XI, IWB-3640 and Appendix C flaw evaluation procedure, cast austenitic stainless steel (CASS) piping with ferrite content (FC) less than 20% is treated as wrought stainless steel. For CASS piping with FC equal or greater than 20%, there was no flaw evaluation procedure in the ASME Code prior to the 2019 Edition. In this paper, the technical basis for the recently approved Code change containing flaw acceptance criteria for CASS piping is presented. The procedure utilizes the current rules in ASME Code Section XI, IWB 3640/Appendix C and the existing elastic-plastic correction factors (i.e., Z-factors) for other materials in the Code. The appropriate Z-factor to use for the CASS piping is determined based on the FC (using Hull's equivalent factor). Experimentally measured fully saturated fracture toughness and tensile data of the three most common grades of CASS material in the U.S. (CF3, CF8, and CF8M) were used to determine the flaw acceptance criteria in the Appendix C Code method. As described here, the method is conservative since it utilizes the fully saturated condition of CASS materials. In addition, it is simple and consistent with the current regulatory guidance on aging management of CASS piping.

The effect of crack length on SIF and elastic COD for elbow with circumferential through wall crack

Many damages due to flow-accelerated corrosion and cracking have been observed during recent in-service inspections of nuclear power plants. To determine the operability or repair for damaged pipes, an integrity evaluation related to the damaged piping system should be performed by using already proven code and standards. One of them, the ASME Code Case is most popularly used to integrity assessment in nuclear power plants. However, the recent version of CC N-513 still recommends the simplified method which means a damaged elbow is assumed as an equivalent straight pipe. In addition, to enhance the accuracy integrity assessment in elbow, several previous studies recommend that the SIF and elastic COD values for an elbow with relatively large crack could be predicted by an interpolation technique. However, those estimates for elbow with relatively large crack might be derived to inaccurate results for crack growth analysis, such as for the allowable crack size and life estimation. Therefore, in this paper, the effect of crack length (0.3≤θ1/π ≤ 0.5) on SIF and elastic COD for elbow is systematically investigated. Then, for large crack in elbow, accurate estimates for SIF and elastic COD, which are widely used to assess the integrity of elbows, are proposed. Those proposed solutions are expected to be the technical basis for revisions of CC N-513-4 through the validation.

Design Study of Vacuum Vessel Concepts for COMPASS-U Tokamak

COMPASS upgrade (COMPASS-U) is a high-magnetic field, medium-sized tokamak with high-temperature (< 500 °C) operation. The scientific program is aimed to address the topics of plasma exhaust, liquid metals, enhanced confinement modes, and edge plasma physics. The plasma current is up to 2 MA and the toroidal magnetic field is up to 5 T. Therefore, plasma disruptions can produce large electromagnetic forces on the vacuum vessel (VV) and other conducting structures. This article presents the study of different VV design concepts, which were considered during the COMPASS-U conceptual design phase. This article describes the electromagnetic forces exerted during plasma disruption, high-temperature operation requirements, and other design constraints. INCONEL 625 is selected as a reference material for VVs. FE simulations of the 45° sector of the vacuum vessel were carried out for various load combinations. This article summarizes the wall thickness optimization that limits the vessel deformation and minimizes the stress in the shell. The results are broken down into different categories of stress according to the ASME code and compared with material limits.

Improvement of aseismic performance of a PGSFR PHTS pump

A design study was performed to improve the limit aseismic performance (LSP) of a primary heat transport system (PHTS) pump. This pump is part of the primary equipment of a prototype generation IV sodium-cooled fast reactor (PGSFR). The LSP is the maximum allowable seismic load that still ensures structural integrity. To calculate the LSP of the PHTS pump, a structural analysis model of the pump was developed and its dynamic characteristics were obtained by modal analysis. The floor response spectrum (FRS) initiated from a safety shutdown earthquake (SSE), 0.3 g, was applied to the support points of the PHTS pump, and then the seismic induced stresses were calculated. The structural integrity was evaluated according to the ASME code, and the LSP of the PHTS pump was calculated from the evaluation results. Based on the results of the modal analysis and LSP of the PHTS pump, design parameters affecting the LSP were selected. Then, ways to improve the LSP were proposed from sensitivity analysis of the selected design variables.

Design and thermal fluid structure interaction analysis of liquid nitrogen cryostat of cryogenic molecular sieve bed adsorber for hydrogen isotopes removal system

Efficient design of Tritium Extraction System (TES) for the fuel cycle of any fusion reactor is very important to maintain the tritium breeding ratio and hence sustain the fusion reaction. Hydrogen Isotopes Removal System (HIRS) for Indian Tritium Breeder Blanket removes Q2 (Q = H, D or T) and impurities using Cryogenic Molecular Sieve Bed (CMSB) adsorber at 77 K. The CMSB is maintained at liquid nitrogen temperature using a double walled cryostat made up of SS304. The paper describes the design and thermal Fluid Structure Interaction (FSI) analysis of cryostat assembly for CMSB of HIRS. The coupled analysis performed in this work involves solving for the fluid domain and transferring the results to ANSYS Thermal-Static structural set up to determine the stresses and displacement due to combined effects in the system. The mechanical design of the cryostat components is analytically performed using ASME codes. The velocity, pressure drop and time taken to cool the CMSB are determined by solving the fluid and energy equations in ANSYS Fluent analysis system. The solutions are imported into ANSYS Thermal-Static structural analysis system and the thermal-structural stresses and deformations are determined considering the temperature, pressure and acceleration loads. The space between inner and outer vessel is maintained at vacuum, which might lead to buckling. So, the critical buckling load multiplier factor is determined. These results are used in fabricating the complete cryostat system for CMSB of HIRS.

Comparative study of design methods for bolted flanges subjected to external loading

Bolted flange leak tightness evaluation has been an involved topic and design calculations are further complicated in the presence of external loads. The present work compares three design methods used for incorporating effect of external loads in flange design: ASME BPVC, ASME Code Case, and equivalent pressure method. Total 35 flanges of different standard sizes and rating classes are analyzed in this study. For each case, maximum allowable external loads are computed by each of the three methods. Based on the results obtained from this study, practical guidelines are developed which will help practicing engineers to select an appropriate method to meet a specific design requirement.

Effect of hold time on high temperature creep-fatigue behavior of Fe–25Ni–20Cr (wt.%) austenitic stainless steel (Alloy 709)

To understand high temperature creep-fatigue interaction of the Alloy 709, strain-controlled low-cycle fatigue (LCF) tests were performed at strain ranges varying from 0.3% to 1.2% with fully reversible cycle of triangular waveform at 750 °C. In addition, different hold times of 60, 600, 1800 and 3600 s were introduced at the maximum tensile strain to investigate the effect of creep damage on the fatigue-life at strain range of 1% at 750 °C. The creep-fatigue life and the number of cycles to macro-crack initiation and failure are found to decrease with increasing hold time indicating higher crack initiation and growth rates. Creep-fatigue life is evaluated by a linear summation of fractions of cyclic and creep damages according to ASME code. The fractographs of the samples deformed at 1% strain range indicated that fatigue might have been the dominant mode of deformation whereas, for the samples deformed at the same strain range with different hold times, both fatigue and creep have contributed to the overall deformation and fracture of the alloy.

Development of pressure-temperature operation limits for a PWR vessel considering beltline shell and extended beltline nozzles

The pressure-temperature limits (P-T limits) of embrittled reactor pressure vessels (RPVs) should be determined for normal heat-up and cool-down operations based on fracture mechanics to ensure the structural integrity. In general, the P-T limits are mainly determined by the fracture toughness of beltline region material which suffers the highest level of neutron embrittlement. However, some other components such as nozzles which suffer insignificant neutron embrittlement may restrict the P-T limits as a result of higher stress concentration from their structural discontinuities. Therefore, beltline region material with the highest reference temperature as well as nozzles that have structural discontinuities need to be considered when developing the P-T limits of RPV. In this paper, the P-T limit calculations for a Taiwan domestic pressurized water reactor (PWR) vessel were performed considering both beltline shell and extended beltline nozzles according to the nonmandatory Appendix G to ASME Code Section XI. The three-dimensional finite element analyses were conducted to obtain the pressure and thermal stress distributions of extended beltline nozzles for P-T limits calculation. It is found that the beltline region still dominates the cool-down P-T limit, but the extended beltline region will partially control the heat-up limit. Further, the reference temperature ranges of extended beltline nozzles that may influence the P-T limits of RPV were also investigated. Present study is helpful for safe operation and also provides a reference for regulation of PWR plants in Taiwan.