Effect of Blend Radius on Stress Concentration Factor of Crossbored Holes in Thick Walled Pressure Vessels

Many studies have been performed on the effect of stress concentration factor in thick walled cylinders caused by holes drilled to the wall perpendicular to the vessel ID, commonly called crossbores. Recent developments in FEA analysis and computer technology have allowed detailed analysis in their effect on the stresses in pressure vessels. This allows the reevaluation of many theories developed in the past. The following is a study of how applying a blend radius to the inside intersection of a vessel bore and a crossbore affects the stresses in vicinity of the hole and the stress concentrations developed near the hole.

A New Approach to Reducing the Stress Concentration Factors of Cross-Bores in Blocks Under High Pressure

The fundamental design of high pressure joints such as crosses and tees has remained the same for many years. However, the introduction of commercially available high pressure equipment operating at 600 MPa and higher has demanded improved designs for these classic connections. This study presents a new design concept for reducing the stress concentration at intersecting crossbores. Both the finite element analysis and the fatigue test results from the standard high pressure design and the new design are compared. The new approach realizes a 17–25% reduction in the stress concentration factors and a 40% improvement in fatigue life test results when compared to the standard design.

Advanced Design Methods for Packing Cups of Hypercompressor Cylinders

Reciprocating Compressors for LDPE applications operating at pressures over 3000 bars require advanced methods for designing cylinders components. Packing cups undergoing very high-pressure fluctuations and severe operating conditions are designed by FEM to determine the stress level in a more accurate way. Considering boundary conditions, complex geometry and the consequent stress risers, the procedure allows to optimize the cups profile, giving a strong contribution to machine performance and plant availability. Challenging performance and capacity change requirements are faced by using innovative solutions related to simulation methods, technologies and diagnostic systems. FEM is used to make a comparison of the stress state during normal operating conditions, between the original solution and a modified one. The modified application requires a higher capacity and consequently a bigger plunger diameter and a different geometry of packing cups. In addition, site feedback and Service engineering are essential to improve safety, reliability, availability, and maintainability, as well as general performance of the machines, particularly for high-pressure cylinder components.

Stress Intensity Factors for a Curved-Front Internal Crack in an Autofrettaged Tube With Bauschinger Effect

Pressure vessel steels exhibit the Bauschinger effect that significantly reduces post-autofrettage residual compressive hoop stresses in the near-bore region in comparison with ‘ideal’ (elastic-perfectly plastic) behavior. These reduced hoop stress profiles were calculated using Von Mises’ criterion via a non-linear analysis for the case of open-end (engineering plane strain) autofrettage. These profiles were then used to obtain stress intensity factor solutions via the Boundary Integral Equation (BIE) method, commonly known as the Boundary Element Method (BEM). Results are presented for tubes of diameter ratio 2 and 2.5 with an internal semi-elliptical surface crack having a maximum depth/surface length ratio of 0.4 (i.e. an eccentricity of 0.8). Crack depths range from 20% to 80% of wall thickness and results are presented for seven locations on the crack front from maximum depth to free surface. For crack depths up to 20% of wall thickness there is a significant reduction in magnitude of autofrettage stress intensity factor due to Bauschinger effect. For typical overstrain levels this reduction is approximately 30% of ‘ideal’ values. Such a reduction may, in turn, cause an order of magnitude reduction in the fatigue lifetime of the vessel.

ASME Code Ductile Failure Criteria for Impulsively Loaded Pressure Vessels

Ductile failure criteria suitable for application to impulsively loaded high pressure vessels that are designed to the rules of the ASME Code Section VIII Division 3 are described and justified. The criteria are based upon prevention of load instability and the associated global failure mechanisms, and on protection against progressive distortion for multiple-use vessels. The criteria are demonstrated by the design and analysis of vessels that contain high explosive charges.

Stress Intensity Correction Factors for Radial-Longitudinal Surface Cracks in Thick-Walled Cylinders

Section XI and Appendix D of Section VIII, Div. 3 of the ASME Code include an influence coefficients method for calculating Mode I crack tip stress intensity factors, K1. When outlining the technical basis for this method, Cippola mentions that the fiee surface correction factors G0, G1, G2 and G3, that are tabulated for uniform, linear, quadratic and cubic stress variations, respectively, were derived using Shen and Glinka’s weight functions for surface cracks in plates. In the interim, the authors and colleagues have extended the validity range of their weight functions for cracks in plates and developed solutions for internal or external cracks in thin- or thick-walled vessels. This paper is a comprehensive comparison of correction factors, Gi, obtained from weight functions for an edge or surface semi-elliptic crack in a plate with those obtained for a radial-longitudinal edge or surface semi-elliptic crack in a cylinder. The differences, which in some cases are large, and the well know uncertainties when calculating K1 for the surface points of cracks are discussed.

Utilizing Finite Element Analysis to Accurately Predict Fatigue Life of Autofrettaged Ultra High Pressure Components

Computer Aided Design and Finite Element Analysis packages that have been developed are capable of providing a relatively accurate fatigue life prediction. These software packages have made nonlinear analysis more reliable in forecasting a component’s fatigue life. A safe-life (in which the components are safe from failure during the estimated service life) can be predicted during the design process. The autofrettage technique has long been applied in high-pressure industries in order to extend the components’ life. The critical parameters that must be understood during a fatigue life analysis are material properties, including cyclic loading properties and stress excursion during the service cycle. In this paper, a three-dimensional finite element analysis of an autofrettaged manifold is presented. This assessment investigated an ANSI 316 stainless steel Tee-fitting, which was exposed to different cyclical loading conditions (two autofrettage conditions at a normal operation level). This was done in order to compare finite element analysis results to actual laboratory experimental results.

Power Actuated Pressure Relief Valve for High Pressure Vessels

The pressure required for treatment of food ranges from 300 to 600 MPa, depending on the food being processed and the desired results, higher pressure giving better and more economical results. However, there are currently no predictable, reliable, and repeatable safety devices available for very high pressures like 600 MPa. Rupture disks have a short life at these pressures, and pressure relief valves for very high pressure are not repeatable. Thus the possibility of using a system design is an attractive alternative that will make the overpressure protection more reliable and controllable. In current applications, high-pressure vessels normally operate from a few MPa to 200 MPa, for example when extracting substances, compacting powder materials, or healing defects in materials. The pressure medium is typically a pure gas or a liquid. Here existing devises serve their purpose. The request for mild treatment of food to enhance safety and quality has created a niche for very high pressure. With this new technique the food is treated at low temperature and high pressure for a short time. The treatment inactivates micro organisms but maintains the nutritional and organoleptic values of the food, achieving a food with high quality and increased safety throughout its commercial shelf life.

Lessons Learned From a Vessel Explosion Caused by Human Error

The object of this paper is to describe the details of a vessel explosion caused by human error, the incident investigation including engineering analysis of the explosion observations, and the recommended changes that were implemented to avoid a similar incident in the future. Fortunately, this incident did not cause any personal injuries. Stress analysis of the Breech Nut threads was performed to determine if the Breech Nut could be safely used after the incident. Released energy and missile analyses were performed to verify the assumptions made on the source of the explosion.

The Favorable Effect of Autofrettage on Uniform Arrays of 3-D Unequal-Depth Cracks in a Thick-Walled Cylindrical Pressure Vessel

The favorable effect of autofrettage on the mode I stress intensity factor (SIF) distributions along the fronts of radial, semi-elliptical surface cracks pertaining to large uniform arrays of unequal-depth cracks emanating at the bore of a pressurized thick-walled cylinder is studied. The analysis is based on the, previously proposed, “two-crack-depth level model”. SIF values are evaluated by the finite element method (FE) using the ANSYS 6.1 code. In the FE model singular elements are employed along the crack front and an equivalent temperature load simulates the autofrettage residual stress field. The distribution of KIN = KIP + KIA, the combined stress intensity factor due to pressurization and full autofrettage, for numerous uneven array configurations bearing n = n1 + n2 = 8 to 128 cracks, a wide range of crack depth to wall thickness ratios, a1/t = 0.01 to 0.4, and various crack ellipticities, a1/c1 = 0.3 to 1.5, are evaluated for a cylinder of radii ratio Ro/Ri = 2. The accuracy of the evaluated SIFs is increased using an improved displacement extrapolation. The results clearly indicate the favorable effect of the residual stress field on the fracture endurance and the fatigue life of autofrettaged cylindrical pressure vessel bearing uniform arrays of 3-D unequal-depth cracks emanating from its inner bore. This favorable effect is governed by Ψ = σo/p — the ratio of the vessel’s material yield stress to its internal pressure. The higher ψ is the more effective autofrettage becomes. The “interaction range” for the various configurations of uneven crack arrays is evaluated. The range of influence between adjacent cracks on the maximal combined SIF, KNmax, is found to be dependent on the density of the array, as reflected in the inter-crack aspect-ratio, as well as on the cracks’ ellipticity.