A Damage Assessment of Autofrettaged Tubes Exposed to Decompositions in an LDPE Facility

The manufacture of low density polyethylene (LDPE) by radical polymerization regularly subjects components to extreme pressures exceeding 20 ksi and, possibly, to runaway decomposition reactions with temperatures exceeding 1500 °F and pressures above 30 ksi. Components subject to such extreme conditions are often autofrettaged to induce a beneficial residual stress distribution that retards crack growth and extends fatigue life. Three samples of autofrettaged tubes extracted from these components are examined here. Only one of these samples is known to have been exposed to multiple decompositions while in service. Measurements of the remaining residual stress were taken for each of these tube samples, and a number of other metallurgical tests were performed. The results show that the tube experiencing decompositions lost almost all of the beneficial residual stress induced by autofrettage and actually has a large, detrimental tensile stress at the inner surface. Corresponding to this is a band of embrittled material with a significantly altered microstructure that was most likely caused by thermal excursions. The tubes that experienced no decompositions showed no such alterations, and their residual stress distributions were relatively intact. An FFS assessment of crack-like flaws was performed on these tubes in accordance with API 579-1/ASME FFS-1 in order to determine the effect of this loss of residual stress on remaining life and quantify this loss in terms of a damage parameter.

Optimization of Hyper Compressors Configuration and Related Plant Arrangement

A good design of hyper compressors is heavily dependent by the good understanding of all critical aspects of the LDPE plant and on the implementation of appropriate technical solutions, during the design phase, to identify and resolve problems that may arise during the operation of the plant. The various aspects have to be investigated keeping in mind to reach an optimum performance in agreement with the customer’s requirements. The real effort is starting at the first design stages where all different departments are involved for dynamic loads acting on the foundation, the torsional analysis involving the motor supplier and the pulsation/vibration considerations interfacing the engineering company, the suppliers of high pressure equipment and the end users. All the stress levels, the pulsation and vibrations have to be minimized. Such results are influenced by the layout of the plant and the arrangement of the cranks of the compressor’s crank-shaft. This latter decision can improve the pulsation level, but affect the loads on the foundations. Here is the optimization of some technical aspects to be faced with the approval of all actors involved.

Metal Magnetic Memory Testing Technique for Typical Defects in Pressure Vessels and Pipelines

Metal magnetic memory technique has been extensively applied in different fields due to its unique advantages of time-saving, low cost, and high efficiency. However, very limited research has been carried out on studying the characteristics of metal magnetic memory signals of different defects except crack, and also the effect of orientation angle between testing direction and defect on magnetic memory signals. To promote study in this area, the magnetic memory signals of typical defects (such as crack, slag inclusion) are investigated as well as hydrogen-induced cracking. In addition, the characteristics of magnetic memory signals when measured with different angle between testing direction and defects were obtained. The results indicate that the metal magnetic memory technique is a promising method to detect typical defects of welding and also hydrogen-induced cracking. Moreover, the technique has high sensitivity on defects no matter the angle between testing direction and defect. However, further research is needed because it can only find the possible location of defects but cannot quantitatively describe the defect.

Ultrasonic Guided Wave Testing of Finned Tubing

In the past few decades, ultrasonic guided waves have been utilized more frequently Non-Destructive Testing (NDT); most notably, in the qualitative screening of buried piping. However, only a fraction of their potential applications in NDT have been fully realized. This is due, in part, to their complex nature, as well as the high level of expertise required to understand and utilize their propagation characteristics. The mode/frequency combinations that can be generated in a particular structure depend on geometry and material properties and are represented by the so-called dispersion curves. Although extensive research has been done in ultrasonic guided wave propagation in various geometries and materials, the treatment of ultrasonic guided wave propagation in periodic structures has received little attention. In this paper, academic aspects of ultrasonic guided wave propagation in structures with periodicity in the wave vector direction are investigated, with the practical purpose of developing an ultrasonic guided wave based inspection technique for finned tubing. Theoretical, numerical, and experimental methods are employed. The results of this investigation show excellent agreement between theory, numerical modeling, and experimentation; all of which indicate that ultrasonic guided waves will propagate coherently in finned tube only if the proper wave modes and frequencies are selected. It is shown that the frequencies at which propagating wave modes exist can be predicted theoretically and numerically, and depend strongly on the fin geometry. Furthermore, the results show that these propagating wave modes are capable of screening for and identifying the axial location of damage in the tube wall, as well as separation of the fins from the tube wall. The conclusion drawn from these results is that Guided Wave Testing (GWT) is a viable inspection method for screening finned tubing.

Improving Prediction of Limit Bending Load for Wall-Thinned Pipes by Reconsidering the Effect of Biaxiality

In this paper, a failure criterion applicable to large-strain finite element analysis (FEA) results was studied to predict the limit bending load Mc of the groove shaped wall-thinned pipes, under combined internal pressure and bending load, that experienced cracking. In our previous studies, Meshii and Ito [1] considered cracking of pipes with groove shaped flaw (small axial length δz in Fig. 1) was due to the plastic instability at the wall-thinned section and proposed the Domain Collapse Criterion (DCC). The DCC predicted Mc of cracking for small δz by comparing the von Mises stress σMises with the true tensile strength σB. However, it was indicated that the predictability of Mc was not necessarily sufficient. Thus, in this work, attempts were made to improve the accuracy of Mc prediction with a perspective that multi-axial stress state might affect this plastic instability. As a result of examination of the various failure criteria based on multi-axial stress, it was confirmed that the limit bending load of the groove flawed pipe that experienced cracking could be predicted within 5 % accuracy by applying Hill’s plastic instability onset criterion [2] to the outer surface of the crack penetration section. The accuracy of the predicted limit bending load was improved from DCC’s error of 15% to 5%.

Finite Element Analysis for the Evaluation of Cracks at a Pipe-Flange Welded Part by the Direct-Current Potential Difference Method

The use of a flange joint is a popular method to close the end of pipes or connect pipes in manufacturing industries. As the pipes are often subjected to vibrations and cyclic bending, fatigue cracking may occur at the welded part between the pipe and flange. It is therefore important to detect and monitor the cracking in this part to ensure safety of the whole piping system. The direct-current potential difference method (DC-PDM) is known as a suitable non-destructive technique to monitor the initiation and growth of cracks and it has been applied to cracks and wall thinning on the inner surface of pipes. In this study, finite element analyses were carried out to clarify the relationship between the size and location of cracks at the pipe-flange welded part and the potential difference. An evaluation method of circumferential crack length angle by DC-PDM was proposed.

The Effects of Specimen Size on the Very High Cycle Fatigue Properties of FV520B-I

The authors researched the effects of specimen size on the very high cycle fatigue properties of FV520B-I through ultrasonic fatigue testing. The test results showed that the very high cycle fatigue mechanism was not changed and the fatigue properties declined as the specimen size increased. The S-N curve moved downward and the fatigue life decreased under the same stress level maybe due to the heat effects of large specimens in tests. The fatigue strength and the fatigue life were predicted by relevant models. The prediction of fatigue strength was close to test result, and the prediction of fatigue life was less effective compared with the previous prediction of small size specimen test results.

ASME Sec VIII Div 3 Fatigue Life Predictions Using Critical Plane Approach

ASME VIII Div 3 fatigue evaluation is based on the theory that cracks tend to nucleate along the slip lines oriented in the maximum shear stress planes. This code provides methods to calculate the fatigue stresses when the principal stress direction does not change (proportional loading) and axes change (nonproportional loading). When principal stress direction does not change within a fatigue cycle, shear stress amplitude is calculated only on the three maximum shear stress planes. But when the principal stress directions do change within a loading cycle, the plane carrying the maximum shear stress amplitude (also known as critical plane) cannot be easily identified and all planes at a point needs to be searched for the maximum shear stress amplitude. This paper describes the development of an ANSYS-APDL macro to predict the critical plane at each surface node of an FE model using the FEA stress results. This macro searches through 325 planes (at 10° increments along two angles) at each surface node and for each load step to identify the maximum shear stress and the corresponding normal stress for each surface node. The fatigue life is calculated for each surface node and is plotted as a color contour on the FEA model. This macro can be extended to calculate the fatigue life using other critical plane approaches such as the Findley and Brown-Miller models.

Overview of Revisions to the ASME Boiler and Pressure Vessel Code Section VIII Division 3 for the 2015 Edition and Near Future

This paper is to give an overview of the major revisions pending in the upcoming 2015 edition of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section VIII Division 3, Alternative Rules for Construction of High Pressure Vessels, and potential changes being considered by the Subgroup on High Pressure Vessels (SG-HPV) for future editions. This will include an overview of significant actions which will be included in the upcoming edition. This includes action relative to test locations in large and complex forgings, in response to a report from the U.S. Chemical Safety and Hazard Investigation Board (CSB) report of a failed vessel in Illinois. This will also include discussion of a long term issue recently completed on certification of rupture disk devices. Also included will be a discussion of a slight shift in philosophy which has resulted in the linear-elastic stress analysis section being moved to a Non-Mandatory Appendix and discussion of potential future of linear-elastic stress analysis in high pressure vessel design.