Flow Coefficient Evaluation of Poppet Valves Used in Reciprocating Compressors for LDPE

Poppet valves have been used for a long time for very high pressure reciprocating compressors, as for example in the case of Low Density Polyethylene. These applications are very critical because the final pressure can reach 350 MPa and the evaluation of the performance of the machines is strongly connected to the proper operation and performance of the valve itself. The arrangement of cylinders requires generally a certain compactness of valve to withstand high fatigue stresses, but at the same time pressure drop and operating life are very important. In recent years the reliability of the machines has been improving over and over and the customers’ needs are very stringent. Therefore the use of poppet valves has been extended to other cases. In general the mentioned applications are heavy duty services and the simulation of the valves require some coefficients to be used in the differential equations, able to describe the movement of plate/disk or poppet and the flow and related pressure drop through the valves. Such coefficients are often determined in an experimental way in order to have a simulation closer to the real operating conditions. For the flow coefficients it is also possible today to use theoretical programs capable of determining the needed values in a quick and economical way. Some investigations have been carried out to determine the values for certain geometries of poppet valves. The results of the theory have been compared with some experimental tests. The good agreement between the various methods indicates the most suitable procedure to be applied in order to have reliable data. The advantage is evident as the time necessary for the theoretical procedure is faster and less expensive. This is of significant importance at the time of the design and also in case of a need to provide timely technical support for the operating behavior of the valves. Particularly for LDPE, the optimization of all the parameters is strongly necessary. The fatigue stresses of cylinder heads and valve bodies have to match in fact with gas passage turbulence and pressure drop, added to the mechanical behavior of the poppet valve components.

Theoretical and Experimental Investigation of Pipe Wall Thinning Detection Using Guided Waves

A method to investigate pipe wall thinning using guided waves has been developed for pipes in thermal power generation facilities. In this paper, the reflection coefficient and the transmission coefficient are derived for the torsional waves which propagate along a pipe and a simplified method to predict the waveform is proposed. The predictions of the waveforms by the FEM and a simplified method based on the reflection of torsional waves are also examined by comparing with experimental data.

Modeling and Optimization of Ultra High Pressure Vessel With Self-Protective Flat Steel Ribbons Wound and Tooth-Locked Quick-Actuating End Closure

Ultra-High Pressure Vessel (UHPV) with self-protective Flat Steel Ribbons (FSR) wound and Tooth-Locked Quick-Actuating (TLQA) end closure is a new type of vessel developed in recent years. When the structural parameters of its TLQA and Buttress Thread (BT) end closure are determined using the ordinary engineering design method, Design by Analysis (DBA) shows that the requirement on fatigue life of this unique UHPV could hardly be satisfied. To solve the above problem, an integrated FE modeling method has been proposed in this paper. To investigate the fatigue life of TLQA and BT end closures of a full-scale unique UHPV, a three-dimensional (3-D) Finite Element (FE) solid model and a two-dimensional (2-D) FE axisymmetric model are built in FE software ANSYS, respectively., Nonlinear FE analysis and orthogonal testing are both conducted to obtain the optimum structure strength, in which the peak stress in the TLQA or BT end closure of the unique UHPV is taken as an optimal target. The important parameters, such as root structure of teeth, contact pressure between the pre-stressed collar and the cylinder end, the knuckle radius, the buttress thread profile and the local structure of the cylinder, are optimized. As a result, both the stress distribution at the root of teeth and the axial load carried by each thread are improved. Therefore, the load-carrying capacity of the end closure has been reinforced and the fatigue life of unique UHPV has been extended.

Effect of Ultrasonic Impact Treatment on the Stress Corrosion Cracking of 304 Stainless Steel Welded Joints

High tensile weld residual stress is an important factor contributing to stress corrosion cracking (SCC). Ultrasonic impact treatment (UIT) can produce compressive stresses on the surface of welded joints that negate the tensile stresses to enhance the SCC resistance of welded joints. In the present work, X-ray diffraction method was used to obtain the distribution of residual stress induced by UIT. The results showed that UIT could cause a large compressive residual stress up to 325.9MPa on the surface of the material. A 3D finite element model was established to simulate the UIT process by using a finite element software ABAQUS. The residual stress distribution of the AISI 304 stainless steel induced by UIT was predicted by finite element analysis. In order to demonstrate the improvement of the SCC resistance of the welded joints, the specimens were immersed in boiling 42% magnesium chloride solution during SCC testing, and untreated specimen cracked after immersion for 23 hours. In contrast, treated specimens with different coverage were tested for 1000 hours without visible stress corrosion cracks. The microstructure observation results revealed that a hardened layer was formed on the surface and the initial coarse-grained structure in the surface was refined into ultrafine grains. The above results indicate that UIT is an effective approach for protecting weldments against SCC.

Comparison Between API 579-1/ASME FFS-1 and ASME VIII Division 3 Stress Intensity Factor Solutions Used for Fracture Mechanics and Fatigue Analysis of Thick Walled Cylinders

The paper discusses the differences between API 579-1/ASME FFS-1-1/ASME FFS-1 [1] and ASME Section VIII Division 3 [2] stress intensity factor solutions. In addition to this, the use of the Failure Assessment Diagram (FAD) in leak before burst analysis is compared to the present Division 3 approach. The paper contains the background of both approaches and a worked example demonstrating the effect of both methods. Finally, a simplified fatigue crack growth based life time study is presented.

Evaluation of the Corrosion Rate for Acidic Bottom Plates of an Aboveground Oil Tank by the AE Method

An AE method is an effective technique that can inspect corrosion damage of tank bottom plates to prevent leakage accidents of oil storage tanks. However, a correlation between AE signals and the corrosion behavior for bottom plates is not fully clarified. In this study, the authors considered that the corrosion regions in bottom plates become a strong acid environment by chloride ions as shown our previous work. The correlation between the AE signals and the corrosion behavior with a potentiostat for test pieces was examined in the environment. The polarization resistance was measured with an AC impedance method using a frequency response analyzer. It was clear that the polarization resistance indicated the corrosion rate for a test pieces in the experiments. While measuring the AE signals, the corrosion rate was monitored with a test piece. As a result, the AE signal showed the good correlation with the corrosion rates of the test pieces.

A Real-Time DSPI System for Measuring Surface Deformation in Clamped Bolted Joints

A Digital Speckle Pattern Interferometry (DSPI) system is developed for the real-time measuring and monitoring the out-of-plane surface deformation around tightened threaded fasteners that are used to clamp bolted assemblies. Spatial phase shifting is employed to quantitatively determine the distribution of phase data by introducing a spatial carrier fringe pattern to the speckle interferogram. This is achieved by leading the object and reference beams to two separate apertures. The configuration is also suitable for collecting the real-time deformation during bolt tightening. The experimental DSPI system is set-up with optical components on a vibration-isolation table. A Matlab software is developed for the image acquisition and phase data calculation, which yields the out-of-plane surface deformation caused by the bolt preload. An aluminum joint is used with an M12 steel fastener. For miniature screw application, however, a plastic joint is used for collecting data.

Probabilistic Multiscale Models for Fracture Analysis of Functionally Graded Composites

This paper reports three multiscale models, including sequential, invasive, and concurrent models, for fracture analysis of a crack in a two-phase, functionally graded composite. The models involve stochastic description of the particle volume fractions, particle locations, and constituent material properties; a two-scale algorithm including microscale and macroscale analyses for determining crack-driving forces; and two stochastic methods for fracture reliability analysis. Numerical results indicate that the sequential and invasive multiscale models are the most computationally inexpensive models available, but they may not produce acceptable probabilistic characteristics of stress-intensity factors or accurate probability of fracture initiation. The concurrent multiscale model is sufficiently accurate, gives probabilistic solutions very close to those generated from the microscale model, and can reduce the computational effort of the latter model by more than a factor of two. In addition, the concurrent multiscale model predicts crack trajectory as accurately as the microscale model.

Radial Expansion of T5-6063 Aluminum Tubes: An Experimental Investigation and Numerical Simulation

This paper presents the design of an experimental set-up to mechanically expand T5-6063 aluminum tubes using conical hardened steel mandrels. The effect of the expansion ratio and mandrel angle on the tube wall thickness and drawing force required to expand the tubes as well as failure modes near embedded circular holes are investigated. The experimental study considered four radial expansion ratios of 1.5, 5, 7 and 10% and three mandrel angles of 10, 22.5, and 30 degrees. It was shown experimentally that most of the tubes failed at the 90 degrees position of the hole indicating that the maximum stress concentration takes place at that position as compared to the zero degree location. The experimental study was supplemented by finite element models reflecting the lab tests to determine the drawing force and study the stress concentration around the hole at zero and 90 degrees locations. The finite element analysis took into consideration various parameters such as expansion ratio and mandrel angle. The simulation results, which were calibrated through generated experimental data, confirmed the experimental observations when studying the stress concentration around the hole for various mandrel angles and expansion ratios at 90 degrees angle. The finite element results also showed that the stress concentration is much higher at 90 degrees position where failure occurred in all tested samples as compared to the zero degree angle.

3-D Stress Intensity Factors for Arrays of Inner Radial Lunular or Crescentic Cracks in Thin and Thick-Walled Spherical Pressure Vessels

Some spherical pressure vessels are manufactured by methods such as the Integrated Hydro-Bulge Forming (IHBF) method, where the sphere is composed of a series of double curved petals welded along their meridional lines. Such vessels are susceptible to multiple radial cracking along the welds. For fatigue life assessment and fracture endurance of such vessels one needs to evaluate the Stress Intensity Factors (SIF) distribution along the fronts of these cracks. However, to date, only one 3-D solution for the SIF for a circumferential crack in a thick sphere is available, as well as 2-D SIFs for one through the thickness crack in thin spherical shells. In the present paper, mode I SIF distributions for a wide range of lunular and crescentic cracks are evaluated. The 3-D analysis is performed, via the FE method employing singular elements along the crack front, for five geometries representing thin, moderately thick, and thick spherical pressure vessels with outer to inner radius ratios of η = Ro/Ri = 1.01, 1.05, 1.1, 1.7, and 2.0. SIFs are evaluated for arrays containing n = 1–20 cracks; for a wide range of crack depth to wall thickness ratio, a/t, from 0.025 to 0.95; and for various ellipticities of the crack, i.e., the ratio of crack depth to semi crack length, a/c, from 0.2 to 1.5. The obtained results clearly indicate that the SIFs are considerably affected by the three-dimensionality of the problem and by the following parameters: the geometry of the sphere-η, the number of cracks in the array-n, the depth of the cracks-a/t, and their ellipticity-a/c.