Large Diameter Pressure Vessels Research Articles

Underwater Dynamic Collapse of Sandwich Composite Structures

An experimental investigation is conducted to study the mechanics of underwater implosion of cylindrical bonded sandwich composite shells. Sandwich structures comprised of concentric carbon-fiber/epoxy shells with PVC foam cores of different densities are imploded in a large-diameter pressure vessel. High-speed photography in conjunction with Digital Image Correlation (DIC) measurements are employed to obtain full-field displacements of the dynamic collapse process. Local dynamic pressure histories are also simultaneously recorded to investigate fluid structure interaction during implosion. Observations of collapse mode, radial displacement and velocity of collapse, interaction between the concentric shells and the foam core and post-buckling failure sequence are made. Increasing foam core shear modulus linearly increases the experimental buckling pressures. Weaker growth of incipient modal deformations are understood to play a pivotal role in obtaining higher critical buckling pressures from bonded sandwich shells than previously studied unbonded sandwich constructions. Three dynamic collapse behaviors as determined by the relative orientation of collapse between the inner and outer shell are observed. Core foam density and imperfections also strongly influence the impulse, energy and pressure pulses released from the implosions.

Hydrostatic and shock-initiated instabilities in double-hull composite cylinders

The dynamic collapse of hollow and foam-filled double hull composite cylinders is investigated experimentally. Concentric carbon-fiber/epoxy double cylinders with and without parametrically-graded PVC foam cores are collapsed in a large-diameter pressure vessel under critical hydrostatic pressure as well sub-critical hydrostatic pressure and shock loading. Dynamic pressure data is used in conjunction with underwater Digital Image Correlation (DIC) to determine the effect of the double hull structure on implosion mechanics. Buckling initiation and overall collapse behavior are studied, as well as the pressure pulses released during the dynamic event. Incidents of partial collapse are reported, in addition to cases where the entire structure collapses. The physical mechanisms responsible for this behavior are identified, and the time between inner and outer cylinders collapsing is related to buckling phase angle. For hydrostatically initiated implosions, results show heavier foam cores increase critical collapse pressure linearly with foam crushing strength. Pressure pulses emitted during collapse are shown to occur in distinct phases, with an additional under- and overpressure region present if the inner cylinder collapses. Impulse is demonstrated to be primarily a function of collapse pressure, with energy released increasing with core density. For shock initiated cases, specimens are shown to implode below their natural collapse pressure when subject to explosive loading, with the addition of a foam core substantially increasing structural stability and sometimes preventing collapse. Specimens with foam cores are shown to undergo prolonged vibrations before collapsing. Post-mortem specimens are used to elucidate fracture and failure mechanisms.

Design Consideration for the Erection of Heavy Wall and Large Diameter Pressure Vessels

Heavy wall reactors and large diameter pressure vessels usually face unique challenges in their erection onto their foundations at the jobsite. Today, 800 ton to 1,500 ton reactors and or large diameter vessels with diameters ranging up to 15 meters are common in the refining industry. These pressure vessels are usually fabricated in shops dispersed globally and require unique heavy duty equipment for their safe handling, transportation and erection.The challenge is to find a safe, efficient and cost effective solution based upon the limitations of the equipment, job site and the lift equipment available for erection. Equipment for the handling heavy reactors and large diameter pressure equipment is usually scarce, expensive. Planning well in advance of the lift date is required.Careful consideration must be given to the choice and design of attachment to the pressure vessels used for the lift to ensure that the vessel and lifting components are not overstressed during the lifting process. This paper identifies the types of evaluations that must be performed to determine the stresses in the vessel and lifting attachments during the lifting. The paper also discusses different types of lifting lugs that may be attached to each end of the vessel either by bolting or welding and discusses the pros and cons of each. The paper also provides an example of a finite element analysis (FEA) of a top nozzle, a FEA of a pair of lifting trunions and a FEA of welded on lifting lugs for buried pipe. The intent of the paper is to outline the effects on the vessel due to different types of lifting configurations in combination with the lifting parameters. This paper is not intended to provide guidance on how to design lifting lugs.

Fourier series analysis of a cylindrical pressure vessel subjected to axial end load and external pressure

This paper presents the comparison of a reliability technique that employs a Fourier series representation of random axisymmetric and asymmetric imperfections in a cylindrical pressure vessel subjected to an axial end load and external pressure, with evaluations prescribed by the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2 Rules. The ultimate goal of the reliability technique described herein is to predict the critical buckling load associated with the subject cylindrical pressure vessel. Initial geometric imperfections are shown to have a significant effect on the calculated load carrying capacity of the vessel. Fourier decomposition was employed to interpret imperfections as structural features that can be easily related to various other types of defined imperfections. The initial functional description of the imperfections consists of an axisymmetric portion and a deviant portion, which are availed in the form of a double Fourier series. Fifty simulated shells generated by the Monte Carlo technique are employed in the final prediction of the critical buckling load. The representation of initial geometrical imperfections in the cylindrical pressure vessel requires the determination of respective Fourier coefficients. Multi-mode analyses are expanded to evaluate a large number of potential buckling modes for both predefined geometries in combination with asymmetric imperfections as a function of position within the given cylindrical shell. The probability of the ultimate buckling stress exceeding a predefined threshold stress is also calculated. The method and results described herein are in stark contrast to the “knockdown factor” approach as applied to compressive stress evaluations currently utilized in industry. Further effort is needed to improve on the current design rules regarding column buckling of large diameter pressure vessels subjected to an axial end load and external pressure designed in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 2 and ASME STS-1.

Metallographic investigations of the heat-affected zone II/parent metal interface cracking in 18Ni maraging steel welded structures

During the fabrication of a large diameter pressure vessel out of 18 Ni maraging steel by manual TIG welding, microcracks were noticed at the heat-affected zone (HAZ)/parent metal interface. The location of these cracks was very different from those reported at the fusion zone/HAZ I interface due to “constitutional liquation”. Extensive optical metallography, scanning electron microscopy and energy dispersive X-ray analyses were carried out to identify the cause for the occurrence of these cracks. It is inferred from the experimental results that the microsegregation of titanium and nickel due to repeated thermal cycling during multipass welding led to the formation of TiC/Ti(CN) and stable austenite film on the grain boundaries. Under severe thermal stresses developed during welding, microvoids generated at the interface of TiC/Ti(CN) inclusions and austenite and further propagated intergranularly due to the premature failure of the austenite films.