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Design modifications of pressure boilers with flat endplates with stress relief groove

Szybiński B., Wygoda M.

Journal of KONES

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2017

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Vol. 24, No. 3

309--316

EN

Stress concentration is still present in the vicinities of structural notches in engineering structures. Zones with higher level of stress are particularly dangerous for structures subjected to fatigue or dynamic loads. The vessels with flat endplates with stress relief grooves are the common examples of such structures. Unfortunately, no clear evidence is given in existing codes how optimally to choose the circular groove radius and the optimal value of that radius depends on the dimensions of the boiler, operating conditions and the material used in manufacturing. Additionally, commonly used grooves with circular shapes are not optimal. Series of experimental tests, numerical or analytical studies presented in numerous articles confirmed these facts. In the presented article, the Authors proposed two-stage modification of the investigated vessel. The first step relies on change of the shape of the groove in the endplate, which provides certain reduction of stress concentration but still plastic deformations in the groove vicinity are not eliminated. The second step of the design modification is proposed for the boiler with the optimal elliptic groove configuration. In this step, some material is added around the top of the outer edge of the endplate. Two simple shapes are proposed for these parts - the short cylindrical ring or alternatively the circular ring welded on the top the endplate is used. For both concepts, the search of optimal dimension/parameters is performed. The numerical results of that study clearly show that the full elimination of plastic deformations near the groove is possible. The numerical analysis and optimizations were made with the well- established finite element software ANSYS, which is accepted by commonly used codes for designing of pressure vessels.

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Influence of crack presence on operating conditions of pressure vessels with flat endplates

Szybiński B., Romanowicz P.

Journal of KONES

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2016

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Vol. 23, No. 1

377---384

EN

The paper discusses influence of crack presence on operating conditions of boilers and pipes. In the study particular attention is paid to the vessels with flat endplates with circular stress relieve grooves. The endplates with such grooves are common alternative for widely used dished ends, particularly for smaller boiler diameters and also for vessels with non-circular cross-sections. The existing codes set the rules how to calculate the circular groove parameters – the groove radius and the location of the arc centre. These design parameters are limited by set of inequalities, which indicate the ranges for admissible radii values and the groove centre positions. The choice of the optimal values for the endplate design parameters has been worked out in recent Authors’ investigations. Such approach provides the minimum stress concentration in the vicinity of the cutout, but this notch area still can be the source of crack nucleation and its propagation leading to the premature fracture of the considered vessel. In standard calculations of pressure boilers, the fracture parameters are not taken into account. The current study concentrates on the assessment of the influence of the crack existence on the stable vessel operation. The cracks existing in inner wall of the vessel are of the main interest, because these are difficult to identify and monitor. In these studies the influence of different parameters like the crack length and orientation, and the fracture toughness were taken into account. The detailed numerical results were obtained by means of the finite element code ANSYS [1] for several crack sizes and locations. The cracks in the weld joining shell and end plate and cracks in the groove area, where the maximum stress concentration appeared were subject of the numerical study. The obtained results show the risk of possible premature failure of boilers with cracks.

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Designing of stress relieve groove parameters for flat endplates of pressure boilers

Szybiński B., Barski M., Wygoda M.

Journal of KONES

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2016

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Vol. 23, No. 4

513--520

EN

The application of flat ends as closures in pressure boilers is a certain alternative for commonly used dished ends. However, it is inevitably joined with the stress concentration presence in the vicinity of the junction between the cylindrical pipe and the ending flat plate. The notch effect, which is observed in the connection area, is usually caused by the abrupt change of the inner wall shape and the lack of the curvature continuity in this region. The problem of the stress reduction in the transition zone between the cylindrical pipe and the endplate has been studied by researchers from many years and several proposals how to modify the shape of the respective elements have been established. On the base of these results, several designs for flat end closures are proposed in the codes applied in pressure boilers calculations. This paper shows the effectiveness of one of the design proposals, admitted by code EN 13445-3. In the proposed analysis the special attention are paid to flat ending plates with the circular stress relieve grooves. The parameters of the groove radii and location of its centre are set by a system of inequalities, which usually defines the region of admissible values for radii and position of the circular groove centre. The choice of these values strongly influences the stress concentration and in the considered code, no clear suggestions or justification is given how to choose the best groove parameters, providing the minimum stress concentration. In this report, the Authors show how to choose the best combination of the design parameters providing the minimum stress concentration.