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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

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

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

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