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Research Papers: Codes and Standards

J. Pressure Vessel Technol. 2018;140(2):021101-021101-10. doi:10.1115/1.4038903.

Fatigue design method for 2.25Cr-1Mo-V steel reactors in code case 2605 (CC 2605) is reviewed. Main factors such as the accelerating function of fatigue action, the cyclic frequency, the strain damage factor (β) related to the fatigue design curves are addressed, and the applicable stress level for pure creep rupture analysis in CC 2605 is also discussed. Results indicate that, for the high loading levels, the accelerating function of fatigue action and strain damage factor contribute relatively remarkably to the fatigue design curve. The increase of cyclic frequency leads to a remarkable increase of the allowable fatigue cycle number and hence reduces the conservativeness of fatigue design curve. It should be stipulated in CC 2605 that the applicable stress level is higher than a value of around 200 MPa (slightly dependent on temperature) for the adjusted uniaxial Omega damage parameter and 16 MPa for the creep strain rate when the Omega creep-damage method is employed.

Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2018;140(2):021201-021201-9. doi:10.1115/1.4038901.

Isobaric gas-tight hydrothermal samplers, with the ability to maintain pressure, can be used to keep in situ chemical and biological sample properties stable. The preloading pressure of the precharged gas is a major concern for isobaric gas-tight hydrothermal samplers, especially when the samplers are used at different sampling depths, where the in situ pressures and ambient temperatures vary greatly. The most commonly adopted solution is to set the preloading pressure for gas-tight samplers as 10% of the hydrostatic pressure at the sampling depth, which might emphasize too much on pressure retention; thereby, the sample volume may be unnecessarily reduced. The pressure transition of the precharged gas was analyzed theoretically and modeled at each sampling stage of the entire field application process. Additionally, theoretical models were built to represent the pressure and volume of hydrothermal fluid samples as a function of the preloading pressure of the precharged gas. Further, laboratory simulation and examination approaches were also adopted and compared, in order to obtain the volume change of the sample and accumulator chambers. By using theoretical models and the volume change of the two chambers, the optimized preloading pressure for the precharged gas was obtained. Under the optimized preloading pressure, the in situ pressure of the fluid samples could be maintained, and their volume was maximized. The optimized preloading pressure obtained in this study should also be applicable to other isobaric gas-tight hydrothermal samplers, by adopting a similar approach to pressure maintenance.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021202-021202-10. doi:10.1115/1.4038655.

Based on the unified analytical method of stress analysis for fixed tubesheet (TS) heat exchangers (HEX), floating head and U-tube HEX presented in Part I, numerical comparisons with ASME method are performed in this paper as Part II. Numerical comparison results indicate that predictions given by the unified method agree well with finite element analysis (FEA), while ASME results are not accurate or not correct. Therefore, it is concluded that the unified method deals with thin TS of different types of HEX in equal detail with confidence to predict design stresses.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021203-021203-5. doi:10.1115/1.4039072.

During the firing of guns, the barrel undergoes two major damaging processes: wear of its inner surface and internal cracking. Barrel's are condemned based on either the increase of their internal diameter due to wear or the severity of their internal cracking. The cost of replacing such a damaged gun barrel runs in the tenth of thousands of U.S.$. Therefore, cost effective methods are sought for restoring such gun barrels. In the present analysis, a new method is proposed for refurbishing vintage gun barrels by machining their inner damaged layer and replacing it by an intact, autofrettaged, shrink-fit liner that will restore the barrel to its original performance. The design of the shrink-fitted liner is based on two design principles. First, the von-Mises residual stress distribution through the thickness of the barrel at each of its cross sections along the inserted liner should be at least equal in magnitude to von Mises stress, which prevailed in the original barrel. Second, once the maximum pressure is applied to the compound barrel, the von-Mises stresses at the inner surfaces of the liner machined barrel should be equal to their respective yield stresses. The preliminary results demonstrate the ability of this process to mend such barrels and bringing them back to their initial safe maximum pressure (SMP) and their intact conditions, rather than condemn them. Furthermore, from the authors' experience, based on a preliminary rough estimate, such an alternative seems to be cost effective.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021204-021204-9. doi:10.1115/1.4039126.

In the present work, inelastic dynamic behavior of a pressurized stainless steel elbow is studied under harmonic base excitation with the emphasis on strain accumulation known as ratcheting. Initially, sine sweep test is carried out on the long radius stainless steel (SS 304L) elbow to evaluate the free vibration characteristics. Then, incremental harmonic base excitation with the first resonant frequency is applied to the elbow till failure and the resulting response is studied. The tested elbow is analyzed using a simplified method and the simulated ratcheting strain is compared with experimental results. The effect of thickness variation in the elbow on strain accumulation is also studied. Levels of base excitation corresponding to different failure criteria are evaluated and the details are provided in the paper.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2018;140(2):021401-021401-7. doi:10.1115/1.4038902.

For modern plate steels exhibiting high toughness and ductility, the conventional Charpy test is ostensibly stretched beyond its limits of applicability. Impact tests yield absorbed energy values in excess of 300–400 J, which are associated with limited material fracture and mostly derive from plastic deformation of the specimen (bending), friction, and vibrations of the swinging hammer. It would be therefore very desirable to measure the actual fracture toughness of very-high-toughness steels by means of an alternative specimen and/or methodology, entailing just a moderate increase of cost and test complexity with respect to Charpy testing. The investigation presented here was aimed at establishing a reasonable, yet cost-effective test procedure utilizing Charpy-type specimens for measuring the dynamic toughness of high-toughness steels, such as line pipe steels. Promising results have been obtained from notches cut by electrical-discharge machining (EDM) using a thin wire of 0.1 mm diameter, as compared to specimens where an actual crack was generated and propagated by fatigue at the root of the machined notch.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021402-021402-7. doi:10.1115/1.4038721.

The required thickness of welding tees is neither specified in ASME (2012, “Factory-Made Wrought Buttwelding Fittings,” American Society of Mechanical Engineers, New York, Standard No. B16.9-2012) nor is a clear calculation method provided in codes such as ASME (2016, “Process Piping,” American Society of Mechanical Engineers, New York, Standard No. B31.3-2016). This can lead to uncertainty regarding the pressure capacity of a tee fitting, particularly one that has suffered from erosion or corrosion. Code methods including area replacement (ASME, 2016, “Process Piping,” American Society of Mechanical Engineers, New York, Standard No. B31.3-2016) or pressure-area (ASME, 2015, “Boiler and Pressure Vessel Code Section VIII Division 2,” American Society of Mechanical Engineers, New York, Standard No. BPVC-VIII-2-2015; BSI, 2014, “Unfired Pressure Vessels Part 3: Design,” BSI, London, UK, Standard No. BS EN 13445-3) do not directly account for the effect which the curvature of the crotch region may have on the stress state in the tee. The approach adopted in this work is to liken the geometry of the tee crotch to the intrados of a torus or pipe bend. The shell theory applicable to the torus is adapted for the tee in order to derive a relationship for circumferential membrane stress. An equivalent tube radius is assigned by determining the local radius of shell curvature in the plane passing through the crotch center of the curvature. The actual stresses in the tee crotch are significantly reduced by the adjoining straight portions. This effect is difficult to quantify theoretically and has thus been investigated by means of finite element analysis (FEA)-based assessments. An empirical relationship was then established providing a conservative correlation between the theoretical stresses and the program calculated local stress intensities.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021403-021403-13. doi:10.1115/1.4038824.

Hydrogen has been proposed as a potential partial solution to the need for a clean-energy economy. In order to make this a reality, large-scale hydrogen transportation networks need to be engineered and installed. Steel pipelines are the most likely candidate for the required hydrogen transportation network. One historical barrier to the use of steel pipelines to transport hydrogen was a lack of experimental data and models pertaining to the fatigue response of steels in gaseous hydrogen. Extensive research at NIST has been performed in conjunction with the ASME B31.12 Hydrogen Piping and Pipeline committee to fill this need. After a large number of fatigue crack growth (FCG) tests were performed in gaseous hydrogen, a phenomenological model was created to correlate the applied loading conditions, geometry, and hydrogen pressure to the resultant hydrogen-assisted fatigue crack growth (HA-FCG) response of the steels. As a result of this extensive data set, and a simplification of the above-mentioned phenomenological model, the ASME B31.12 code was modified to enable the use of higher strength steels without penalty, thereby resulting in the potential for considerable installation cost savings. This paper details the modeling effort that led to the code change.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021404-021404-8. doi:10.1115/1.4039069.

Repair welding is a popular method to repair the leakage zone in tube-to-tubesheet joint of shell-tube heat exchangers. But the repaired residual stresses are generated inevitably and have a great effect on stress corrosion cracking (SCC). In this paper, the effects of repair welding on residual stress were studied by finite element method (FEM) and neutron diffraction measurement. The original weld residual stresses calculated by FEM showed good agreement with neutron diffraction measurement results. After repair welding, the transverse residual stresses change very little while the longitudinal residual stresses are increased in the repair zone. In the nonrepair zone, both the transverse and longitudinal stresses are decreased. The repair welding times have little effect on residual stress distribution. With the increase of welding length and heat input, the residual stresses increase. Repair opposite to the original welding direction is recommended because the opposite welding direction minimizes the residual stresses.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021405-021405-10. doi:10.1115/1.4039124.

A novel principal stress-based high cycle fatigue (HCF) model is proposed for preloaded threaded fasteners under cyclic tensile-shear loads. The model uses the fastener principal stress amplitude in order to construct a singular multi-axial S–N curve from the conventional uniaxial S–N curve with zero mean stress of bolt along with some experimental data. An material testing system (MTS) fatigue testing system is used first to generate the fastener preload by applying a direct tensile-shear load using a special fixture. Subsequently, the same system is used for applying combined cyclic tensile-shear loading of the fastener at various levels of mean stress. Results show that for the same level of axial stresses, the multi-axial loading would significantly reduce bolt fatigue life as compared to that of uniaxially loaded bolt. Moreover, only one S–N curve would be able to predict the multi-axial HCF of preloaded threaded fasteners, when the maximum principal stress amplitude is used. Detailed discussion of the proposed model results and test data are provided.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021406-021406-11. doi:10.1115/1.4039071.

Water jet peening (WJP) is a mechanical surface strengthening process, which can improve the residual stress (RS) of the peened surface and then improve the fatigue life of components. In this paper, erosion experiments are conducted to investigate the influence of peening parameters on erosion. On this basis, RSs induced by WJP are studied in relation to the peening parameters. In addition, the coupled Eulerian–Lagrangian (CEL) technique is used to model and simulate the dynamic impact process of WJP on Al6061-T6. The influence of peening parameters such as jet pressure p, jet traverse velocity vf, and the number of water jet pass n on the modification of residual stress field (RSF) is examined by simulation and experiment. The influence of incidence angle α and water jet diameter d on RSF is also investigated by simulation. Results show that compressive RS σcrs is a result of the action of water-hammer pressure alone. Furthermore, σcrs increases with an increase in p, n and α. The optimal peening parameters for Al6061-T6 are found to be p = 60 MPa, vf = 2000 mm/min, n = 4, α = 90 deg and d = 2.0 mm. Finally, the depth of compressive RS layer D0 increases greatly with an increase in water jet diameter d and can reach 984 μm.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021407-021407-7. doi:10.1115/1.4039127.

Creep strength ferritic/martensitic modified 9Cr-1Mo-V-Nb (P91) steel also designated as ASTM A335 and P92 steel are used for piping, cladding, ducts, wrappers, and the pressure vessel in Gen IV nuclear reactors. In the present investigation, a comparative study of the effect of autogenous tungsten inert gas welding (A-TIG) with double pass and multipass gas tungsten arc (GTA) welding with filler on microstructure evolution in the weld fusion zone and the mechanical properties of P91 and P92 steel welded joints was carried out. The microstructure evolution was studied in as-welded and postweld heat treatment (PWHT) condition. The study also focused on the evolution of δ-ferrite patches and their influence on the tensile properties of welded joints. PWHT was carried out at 760 °C with durations from 2 to 6 h. To study the effect of δ-ferrite evolution on mechanical properties, Charpy toughness, microhardness, and tensile tests were performed. The acceptable microstructure and mechanical properties were obtained after the 6 h of PWHT for A-TIG arc welding process while for GTA weld with filler wire, it was obtained after the 2 h of PWHT at 760 °C.

Commentary by Dr. Valentin Fuster

Research Papers: Operations, Applications and Components

J. Pressure Vessel Technol. 2018;140(2):021601-021601-9. doi:10.1115/1.4039122.

The compression-resilience performance of new metal-to-metal contact (MMC) gasket at different temperatures was studied by compression-resilience test, and the influences on sealing performance of gasket caused by internal pressure of medium and assembly stress were studied by sealing test. Then the influences on performance of gasket caused by internal pressure of medium, bolt pretightening force, and temperature were studied through finite element method of ANSYS. According to the results of test and numerical analysis, conclusions are as follows: The new MMC gasket has good high-temperature stability from results of tests at different temperatures. When the MMC occurs between flange and gasket, the contact stress of new MMC gasket is about 50 MPa. When applying loads, the limiting ring of gasket is able to bear parts of bolt loads, and when reducing loads, its resilience can compensate loss of gasket stress. In this case, the stress on sealing surface of gasket remains stable so that the leakage rate can keep in a stable range. Because of its unique structure in design, the inner contact stress of new MMC gasket increases with the rise of internal pressure of medium so that in a certain range, the leakage rate changes gently, which shows that the gasket has a certain self-sealing ability. Besides, the new MMC gasket is able to adapt to high temperature and fluctuation of internal pressure of medium.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021602-021602-8. doi:10.1115/1.4039003.

In this paper, a seal with triangular cross section was proposed and its performance behavior under compression and various hydraulic pressures was analyzed through experimental and numerical methods. The seal was designed to have a 90 deg corner located near the extrusion gap while hydraulic pressure was applied at an inclination. With this design, it was found that even at hydraulic pressures of up to 18 MPa, the seal offered good fluid tight sealing capabilities without indications of extrusion failures. Such high pressure offers new possibilities for successful application of the seal in aircraft and rocket propulsion equipment. Moreover, the resistance of the seal against leakages was assured because measured contact stresses were greater than applied pressures. A numerical simulation through finite element analysis (FEA) showed that tilting of the delta ring even at angles of ±5deg did not have any effect on the Von Mises stresses. The FEA results also demonstrated that the deformations and fringe patterns of delta ring were similar to the experimental results.

Topics: Pressure , Stress , Extruding
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021603-021603-7. doi:10.1115/1.4038726.

Many piping networks in processing plants, such as refineries, chemical plants, and electric power generation plants, are operated at elevated temperatures (≥250 °F or 121 °C). Failure of these insulated high temperature pipes can cause a major disruption of plant operation. In addition to inspection during the regular plant shutdowns, processing industries are looking for ways to inspect and monitor these pipelines on-line to ensure safe operation of the plants. Permanent monitoring of high temperature structures would require addressing the following technical problems: supporting the sensor functionality at high temperatures, ensuring the probe durability, and maintaining good coupling of the probe to the structure. In this work, a probe utilizing magnetostrictive transduction was tested on a mockup at 200 °C and produced a steady high amplitude signal over a period of 270 days. Probe performance parameters such as signal to noise ratio, data reproducibility, and sensitivity to anomalies are discussed.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):021604-021604-9. doi:10.1115/1.4039073.

In this study, the ratcheting behaviors of pressurized Z2CN18.10 austenitic stainless steel elbow pipe influenced by the thermal aging process were experimentally investigated in controlled constant internal pressure and reversed in-plane bending after different thermal aging periods (1000 h and 2000 h) at thermal aging temperature of 500 °C. It is shown that the ratcheting behavior of pressured elbow pipe is highly affected by the thermal aging process. The evaluation of ratcheting behavior of pressured elbow pipe was performed using Chen–Jiao–Kim (CJK) kinematic hardening model as a user subroutine of ANSYS. The relationships of yield stress σs and multiaxial parameter χ with thermal aging time were proposed. Ratcheting shakedown boundary of aged elbow pipe was evaluated by CJK model with thermal aging time.

Topics: Pipes
Commentary by Dr. Valentin Fuster

Technical Brief

J. Pressure Vessel Technol. 2018;140(2):024501-024501-4. doi:10.1115/1.4038900.

Flexibility is the most important requirement of the pipe system. A general approach is to include pipe bends in the system to provide flexibility. The design of the pipe routing requires either rigorous pipe stress analysis or hand calculation based on the beam theory and finite element method. In this paper, a simple methodology has been developed for pipe routing to provide flexibility to absorb thermal expansion and other secondary displacements. The method uses the basic theory of beam and based on the data fitting from the pipe stress analysis results. This method provides general and simple equations of the common bends in the pipeline industry including L, Z, and U bends, for determination of the minimum length requirement for enough flexibility.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(2):024502-024502-5. doi:10.1115/1.4039123.

In order to study the influence of cryogenic temperature on the mechanical properties, a series of uniaxial tensile experiments were performed at different temperatures (20 °C, 0 °C, −20 °C, −40 °C, −80 °C, −120 °C, −196 °C) for the austenitic stainless steel S30403 (both the base material and weld joint). Rp0.2 (0.2% proof strength), Rp1.0 (1% proof strength), Rm (tensile strength), A (elongation after fracture), Z (reduction of area), σcr (a critical threshold stress for onset of discontinuous yielding), and Rh (second hardening ratio, Rm/σcr) were taken into consideration. It was found that in GB150, ASME VIII-1, and EN13445, the maximum allowable stress for austenitic stainless steel at low temperature (≤20 °C) was dependent on the yielding strength at room temperature (20 °C). Compared with Rp0.2, Rp1.0 had a linear relationship with temperature. Synthetically considering the first hardening and the second hardening, both the base material and weld joint presented a better strength performance at low temperatures. The plasticity of base material dropped as the temperature decreased, and it was kept at an acceptable level. Nonetheless, the plasticity of weld joint was nonlinear because of the nonuniform structure components.

Commentary by Dr. Valentin Fuster

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