A. N. Shuaib

Evaluation of optimization methods for machining economics models

Abstract In machining operations it is desirable to operate under optimal machining conditions. The optimal cutting conditions are obtained by solving machining optimization models. The formulated machining models are non-convex non-linear programs of complex nature. This paper compares the performances and the utilities of six algorithms to identify the most suitable one(s) for solving the machining models. The algorithms are evaluated empirically with respect to their reliability, precision, convergence, sensitivity to input vector and their preparational effort. The Generalized Reduced Gradient method (GRG) implemented as GINO is found to be the most suitable for solving machining optimization models.

Experimental Investigation of Heat Exchanger Tubesheet Hole Enlargement

An experimental study has been conducted to evaluate the effect of roller expansion of heat exchanger tubes in enlarged tubesheet holes on joint strength, tube wall reduction (i.e., thinning), and strain hardening of the tubes and surrounding tubesheet ligaments. Different levels of tube-to-tubesheet hole clearance, or overtolerance (OT), were investigated to simulate tubesheet in-service, and during-maintenance, hole enlargement process. The experimental results have shown that, in spite of increasing overtolerance level by up to seven times the maximum value allowed by the Tubular Exchanger Manufacturer Association (TEMA) standard, the average strength of the test joints has not declined below the strength of joints in the allowed TEMA OT range. Percent wall reduction (WR) of the expanded tube was found to increase linearly with increasing overtolerance value; although its maximum value remained well below the wall reduction limit for which a joint is rejected in industry. An observed increase in hardness of the expanded tube material is evidence of the high degree of strain hardening to which the expanded tubes are subjected compared with the surrounding ligaments. Pull-out forces, calculated using existing analytical methods, were found to compare well with the experimental results.

Experimental and Finite Element Modeling of Friction Stir Seal Welding of Tube-Tubesheet Joint

Tube-tubesheet joints are critical in some applications, where contact between shell and tube side fluids is not tolerable. To ensure joint tightness, standards (ASME and TEMA) recommend performing a combination of rolling-or expansion of tube-tubesheet and seal welding. Available techniques for seal welding are based on fusion welding that sometimes results in a number of defects such as cracking and porosity formation, and such defects may take a newly fabricated heat exchanger out of service. In this work, friction stir welding (FSW) was used for tube-tubesheet seal joint and simulated using a 3D thermo-mechanical finite element model (FEM). The model was analyzed using a commercial finite element (FE) package. The model included the thermal effect of the tool workpiece interaction along with axial load, ignoring the metal flow around the tool. The material model took into account temperature dependency of thermal and mechanical properties. The model objectives were to evaluate the temperature distribution and residual stress in the workpiece resulting from the thermal cycle and axial load during welding for various process parameters, and to study how residual stresses in adjacent roller expanded tubes are affected during welding. The FE results show that the maximum temperature at the welding zone does not exceed the solidus temperature (except at high tool rotational speeds); the process can thus be classified as cold working. Moreover, adjacent tubes temperature does not exceed the annealing temperature. An experimental setup was designed and manufactured to show the feasibility of the process in this constrained size joints and to validate the numerical results. A test cell and a special FSW tool were designed and manufactured for this purpose. Many tests were performed with welding quality depending on process parameters.

Combined Effects of Tube Projection, Initial Tube-Tubesheet Clearance, and Tube Material Strain Hardening on Rolled Joint Strength

The tube-to-tubesheet joint strength is measured in terms of interfacial pressure between the tube’s outer surface and tubesheet bore. The strength of a rolled joint is influenced by several design parameters, including the type of tube and tubesheet materials, initial tube projection, and the initial radial clearance between the tube and tubesheet, among other factors. This paper uses finite element analysis (FEA) to evaluate the effect of several parameters on the strength of rolled joints having large overtolerances, a situation that applies to used equipment. An axisymmetric finite element model based on the sleeve diameter and rigid tube expanding roller concepts was used to analyze the effects of tube projection, initial tube-tubesheet clearance, and tube material strain-hardening property on the deformation behavior of the rolled tube and on the strength of the tube-tubesheet joint. The FEA results show that for zero tube projection (flush) the initial clearance effect is dependent on the strain-hardening capability of the tube material. For low strain-hardening tube material the interfacial pressure remains constant well above the Tubular Exchanger Manufacturer’s Association maximum overtolerance. A drastic reduction in joint strength is observed at high values of radial clearances. The cut-off clearance (clearance at which the interfacial pressure starts to drop) is found to vary linearly with the tube material hardening level, and the contact stress increases slightly for moderate strain-hardening tube materials but shows lower cut-off clearance levels. Furthermore, with flush tubes the maximum contact pressure occurs close to the secondary face (at the end of rolling) while for joints with initial tube projection the contact pressure shows two maxima occurring near the primary and the secondary faces. This is attributed to the presence of two elbows in tube deformation near the primary and secondary faces. The average interfacial pressure increased with increasing projection length for all clearances. Tube material strain hardening enhances the interfacial pressure in a similar fashion for all initial tube projection lengths considered in the analysis.

Integrity of Roller-Expanded Tube-to-Tubesheet Joints With Overenlarged Tubesheet Holes

This paper presents the results of an experimental investigation conducted to evaluate the effect of initial tube-tubesheet diametral clearance on the integrity of roller-expanded tube-to-tubesheet joints in overenlarged tubesheet holes. The criteria of evaluating joint integrity include the pullout force and hydraulic tightness of the joints in which tubes were expanded to 5% wall reduction after metal to metal contact of the tube with the tubesheet hole. The levels of clearance investigated were selected to cover a range that exceeded by ten times that prescribed by the Tubular Exchangers Manufacturers Association (TEMA) standards (Standards for the Tubular Exchanger Manufacturers Association, 1988, 7th ed., TEMA, Tarrytown, NY). The pullout test results showed that, within the investigated range of clearance, the joint strength has not declined below the strength of joints with clearance levels within the TEMA range. In fact, the pullout force exhibited a slight linear increasing trend with the increase in initial clearance, which is attributed to the effect of tube strain hardening. The hydrotest results indicated that all joints within the investigated clearance levels were hydraulically tight when they were tested at 1.5 times the design pressure of the heat exchanger for a duration of 30 min. A relationship between rolling specific energy and initial joint clearance has been developed.

Power quality indices: A Saudi steel mill case study

This paper presents results of field measurements of power quality indices at the terminals of an Electric Arc furnace connected at 34.5 kV to a 230 kV 60 Hz network in Saudi Arabia. The power quality indices that are measured are total harmonic distortion for voltage and current (THD V, THD I), individual harmonics, and flicker for short and long terms (Pst, Plt). In addition, other electrical parameters are measured as well such as voltage (Vrms), current (Irms), active power (P), reactive power (Q), power factor (PF), and frequency. All power quality parameters for both study cases are found to be within the IEEE and IEC standards. The installation of SVC and filters are mandatory. Long term Monitoring and country wide PQ study is recommended.

On process capability and product tolerancing as affected by measuring device

Statistical process control seeks to monitor the quality characteristics stipulated in the product design specifications to assure that these are achieved by the operation. Measuring devices are subject to variations but usually the possibility of obtaining fluctuating results is often neglected and the measurements provided by these devices are taken for granted as true values. Presents, briefly, process control and shows the interaction between process capability and measuring device error. Presents a model for determining process target limits which minimize cost of production taking into account measuring device variability. Provides a criterion for establishing optimal process capability as well.

Microstructure and Hardness of Friction Stir Weld Bead on Steel Plate Using W-25%Re Pin Tool

One of the challenges that impede the use of the relatively new friction stir welding (FSW) process in joining steels and high temperature alloys, as well as dissimilar materials, is the development of the right pin tool material that can stand the severe welding conditions of these alloys. Recent developments in FSW tool materials include tungsten rhenium (W-Re) alloys. The ductile to brittle transition temperature of pure tungsten is reduced by the addition of rhenium (Re).. The addition of Re also improve fracture toughness of the alloy. The major focus of this paper is studying the process of making a friction stir welding bead on mild steel using a proprietary W-25%Re alloy pin tool and investigating the effects of process parameters (i.e. tool rotational and welding speeds) on microstructure, microhardness as well as tool reaction loads. Grain refining of the steel microstructure was observed in all beads. Certain process conditions produced a bead with needle like microstructure with the highest values of hardness. Reaction forces were found to increase with the increase in the tool welding speed and to decrease with the increase of the tool rotational speed. Although the spectroscopic analysis of the beads confirmed the diffusion wear of the tool, the overall tool has shown excellent resistance to mechanical wear.Copyright

Experimental Investigation of Friction Stir Seal Welding of Tube-Tubesheet Joints

To ensure heat exchanger tube-tubesheet joints tightness, industrial standards may recommend performing a combination of roller expansion and seal welding, using conventional fusion welding processes. Solid state friction stir welding (FSW) has several advantages over the conventional fusion welding but has not yet proven its usefulness in seal and strength welding of heat exchanger tube-tubesheet joints where the available space is very limited and weld pass is of a relatively complex contour.In this work, a newly designed tool and procedure are applied to friction stir seal weld (FSSW) tube-tubesheet joints. The effects of process conditions such as welding speed and tool offset on dependent process parameters including welding loads and joint quality are investigated on a 600-series aluminum three-tube test cell.The results revealed that the quality of the seal weld of tube-tubesheet joints is affected by the preceding parameters at different levels.Copyright

3-D finite element analysis of roller-expanded heat exchanger tubes in over-enlarged tubesheet holes

Repeated de-stubbing and re-tubing of heat exchangers in petrochemical plants during their useful lifetime may result in over-enlarged tubesheet holes with overtolerances that exceed those prescribed by Tubular Exchanger Manufacturing Association (TEMA) standards (1988). Roller expansion of tubes in these over-enlarged holes may result in tube thinning and weakening of the joint due to a decrease of interfacial pressure between the tube and tubesheet. In the present work, a 3-D finite element (FE) model of a tube-tubesheet joint was used to determine displacement and stress distributions along the axial direction of roller expanded tube-tubesheet joint and to evaluate the combined effects of large initial clearance and strain hardening of tube material on interfacial pressure and tube deformation. The results obtained from the present model are compared to those of axisymmetric FE analysis and to the experimental results. Both axisymmetric and 3-D models are found to yield comparable trends showing that for elastic perfectly plastic tube material the residual contact pressure remains constant well above the prescribed TEMA maximum overtolerance values. In addition, both models show that for strain hardening tube materials the interfacial pressure increases with increasing clearance. An appreciable difference is observed at high overtolerances where the 3-D model predicts cut-off clearances (clearance at which the interfacial pressure starts to drastically drop) which are about 30% lower than those predicted by the axisymmetric models. The tube inner surface deformation and pull out forces estimated from 3-D results compares very well with those obtained from the experimental tests.