Tubesheet Heat Exchangers Research Articles

Research on heat exchanger tube sheet to shell fillet weld testing and engineering application using total focusing phased array technique

Abstract According to structure of the heat exchanger tube sheet to shell fillet weld, the total focusing phased array technique was adopted to settle down this testing difficulty. Firstly, the inspection background and detection difficulties of the fillet weld was introduced in this paper. Secondly, the sound field simulation and typical flaw response simulation was carried out using CIVA software, and the special blocks were manufactured on the base of the simulation result. Thirdly, the total focusing phased array testing study was carried out in laboratory, and the drilled holes, grooves, as well as the typical flaws of incomplete fusion, incomplete penetration and crack in blocks were successfully detected. Finally, the total focusing technique engineering application of one heat exchanger in chemical device was put into effect, the incomplete fusion and incomplete penetration flaws were successfully detected. The total focusing phased array technique was successfully applied to the heat exchanger inspection project, the research result of this paper have high practical value for ensuring the safe and reliable operation of heat exchanger, and can be popularized and applied in the engineering project.

Lightweight Reseach of Tube Sheet for Spiral Wound Heat Exchanger Based on ANSYS

Abstract Tube sheet is the key pressure-bearing part in the spiral wound heat exchanger, and the high-strength and light-weight tube sheet structure is of great engineering significance for the development of a large-scale heat exchanger. In order to further realize the lightweight of the spiral-wound heat exchanger, this paper analyzes the influence of the thickness of the heat exchanger tube sheet on its strength based on the Ansys software and proposes the optimization design method of the tube sheet based on the strength safety factor F, which solves the problem that the software cannot take the stress linearization result as the state variable in the optimization calculation. The optimization results show that the thickness of the tube sheet is reduced by 40%. At this time, the stress strength at the connection between the tube sheet and the head and the center of the tube sheet first exceeds the design bearing capacity, indicating that the strength conditions of the two places are the controlling factors of the thickness. The results are important for the lightweight design of heat exchanger tube plate and material saving.

Application of Finite Element Simulation Technology in the Teaching of Process Equipment and Control Engineering

Improving the teaching level of the professional courses of Process Equipment and Control Engineering has an important role in promoting the cultivation of compound and applied equipment talents. In this paper, finite element simulation technology is introduced into the teaching of Process Equipment and Control Engineering. Taking the finite element analyses of truss beam in the filled tower and fixed tube-sheet heat exchanger as examples, the complementary role of finite element simulation technology for the theoretical and practical teaching of professional courses was introduced. Teaching practice shows that the visualization of finite element analysis results can effectively stimulate the interest of students in learning, deepen their understanding and mastery of theoretical knowledge, improve the ability of students to use finite element simulation technology to solve practical engineering problems, and achieve the unity of teaching and learning in professional courses.

Combined Gas Tungsten Arc Welding and Shielded Metal Arc Welding Processes for Joining Tube to Tubesheet in Shell and Tube Heat Exchangers

Abstract Flawless tube and tubesheet joints are necessary for shell and tube heat exchanger with high leakproof ability and durability. The consequence of mixing of transfer fluids due to the defective welded or expanded tube-to-tubesheet joint results in complete malfunctioning of the heat exchange process. In this case, there is a high demand for quality assessment of the fabricated tube-to-tubesheet joints (TTS) based on the international standards. Additionally, the literature on assessing the mechanical and metallurgical characteristics of tube-to-tubesheet joints using any one welding technique is available; however, articles dealing with the use of multiple welding techniques are scanty. In the current work, the quality assessment of strength welded and light expanded (3%) tube-to-tubesheet joints followed by light expansion was performed whereas for welding, combined gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) as root pass and cap pass were adopted in the fabrication stage. Quality and sound tube-to-tubesheet joints were produced using welding and tube expansion process. Linear and circular indications representing the cracks were absent on the developer in the liquid penetration test. The minimum leak path (MLP) was greater than the two-third of tube wall thickness. The results strongly qualify the use of combined gas tungsten inert gas and shielded metal arc welding process for manufacturing of tube-to-tubesheet joints. The methodology and results in the current article are beneficial for researchers and industrialists working close to shell and tube heat exchangers and boilers.

Analysis of the ductility dip cracking in the nickel-base alloy welding overlay on heat exchanger tube sheet

In this paper, the cracking failure of the weld overlay of the heat exchanger tube sheet was analyzed. The weld overlay, welded by ErNiCrMo-3, cracked several times within a short period after the heat exchanger was put into operation, resulting in internal leakage from the heat exchanger. The failure causes are investigated by means of macroscopic observation, metallographic analysis, scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) analysis, and hardness test. The synthetical analysis shows that a sudden drop in ductility of the weld overlay material occurs at the time of microcracking, and a phenomenon similar to grain boundary sliding is observed in the fracture morphology image at high magnification. Therefore, the microcrack on the weld overlay is a kind of ductility dip crack at elevated temperatures. Improper welding strategy and inappropriate choice of welding material is the root cause of ductility-dip microcrack during the welding process. The microcracks on the weld overlay propagate during the operation of the heat exchanger, causing it to leak several times in a short period.

Analysis of Tube-to-Tubesheet Welding in Carbon Steel Heat Exchangers of a Double Plate Header Box.

The rear wall of the header box serves as a tubesheet in heat exchangers of double plate header box. Tube-to-tubesheet welding must be performed using orbital Gas Tungsten Arc Welding (GTAW) with a head extension, which is passed through the corresponding hole in the front wall (plugsheet) of the header box, where the welding machine is supported. In this project, the effect of parallelism deviations between the plugsheet and the tubesheet of carbon steel header box is analyzed to evaluate its influence on the quality of the tube-to-tubesheet welding. Welded tube (SA-210 Gr. A1) to tubesheet (SA-516 Gr. 70) coupons are manufactured simulating the parallelism deviations previously analyzed in two double plate header boxes of air-cooled heat exchangers using two different preheating temperatures. Macrographic analysis is performed in order to evaluate the weld penetration (minimum leak path) and length of the weld leg in tube-to-tubesheet joints. The results obtained show important variations in those parameters when the parallelism deviations are equal to or greater than −1 mm over the theoretical distance as well as when the distance approaches +1 mm or more. Finally, the incorporation of dimensional controls prior to the welding process is discussed and the implementation of improvements in orbital GTAW equipment is recommended as an optimal solution for this kind of heat exchangers.

Numerical research on thermal stress of steam cooler tube sheet mechanical structure

Steam cooler is one of the most important mechanical equipment in thermal power generation system and nuclear power generation system. The steam cooler bears a huge temperature gradient load when the working conditions are switched. In order to analyze the thermal stress of steam cooler tube sheet with high temperature load under harsh working conditions, the thermal-structural coupling analysis model of steam cooler tube sheet is constructed with finite element method. The results show that the stress concentration exists at the position connection between heat exchanger tube and cylinder. The maximum stress is located at the outermost heat exchanger tube with the peak stress of 320MPa. The heat exchanger tube layout alone cause higher stress. This situation should be avoided in the design of heat exchanger tube sheet. Furthermore, the strength and safety of the steam cooler tube sheet are evaluated with the stress linearization method. The steam cooler tube sheet design meets the safety requirements of structural strength under high temperature load.

The Unified Theory of Tubesheet Design—Part III: Comparison With ASME Method and Case Study

Abstract Based on the unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEXs), floating head and U-tube HEXs presented in Part I and Part II, theoretical and numerical comparisons with ASME method are performed in this paper as Part III. Theoretical comparisons show that ASME method can be obtained from the special case of the simplified mechanical model of the unified theory. Numerical Comparison results indicate that predictions given by the unified theory agree well with finite element analysis (FEA), while ASME results are not accurate or not correct. Numerical comparisons also indicate that ASME results of TS stresses are not always conservative and may lead to unsafety design.

The Unified Theory of Tubesheet Design—Part I: Theoretical Foundation

Abstract The unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEXs), floating head and U-tube HEXs is developed by removing the midplane symmetry (MPS) assumption, which assumes a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the HEX or one TS need be considered. The unified theory can be successfully extended to all common types of HEXs, with arbitrary combinations of TS configurations for two TSs, and with arbitrary material properties and temperature for each component due to less assumptions. The effects of unperforated annual plate, TS flange, gravitational (e.g., dead-weight of tubes, catalyst inside tubes, shell side fluid and tube side fluid) and fluid flow pressure loss, bending stiffness of the tubes, TS in-plane stretch, pressure in TS perforations as well as temperature gradient across TS are also evaluated by the unified theory. Theoretical analysis shows that the existing theories of TS design can be derived from the unified theory as special deductions. The limitations and problems faced in the existing theories and codes are systematically reviewed and solutions to these limitations and problems are also provided by the unified theory. Numerical comparisons indicate that predictions given by the unified theory agree well with finite element analysis (FEA), while ASME results are not accurate or not correct and may lead to unsafety design.

The Unified Theory of Tubesheet Design—Part II: Supplement to the Theoretical Analysis

Abstract The theoretical foundation of the unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEXs), floating head, and U-tube HEXs are presented in Part I, in which only the most important equations are addressed to show the main points of the theory. The equations, conclusions, and other aspects referenced in Part I are presented or proved in this paper as Part II to supplement the theoretical analysis in Part I.

Manufacturing of molten salt reactor heat exchangers using inner hole welding

ABSTRACT Inner hole welding was applied to weld UNS N10003 nickel-based alloy tubes (φ13.72*1.65 mm) to heat exchanger tubesheet in the Thorium Molten Salt Reactor for the first time. Welding defects were eliminated after the welding technology optimization after a great deal of trials and errors. And proper inner hole welding procedure and good butt joints were obtained. Welding procedure qualification shows that the strength of the welded joints at room temperature, 650°C and 700°C was higher than the qualified values of the base metal. After the confirmation of exact welding conditions on full-size mockup, the first batch of two heat exchangers of Thorium Molten Salt Reactor was fabricated by using inner hole welding technics in the world. The tube bundles containing 58 tubes (116 joints) were welded to tubesheet successfully. Penetration and radiation inspection revealed all the welds had no defects and were 100% qualified. Finally the completed unit passed 0.85 MPa hydrostatic testing. The successful development and application of inner hole welding process also establish a good foundation for the manufacture of high-temperature heat exchangers of other reactors besides Molten Salt Reactor.

含温度载荷的管板变形的解析解

A mechanical model for calculating tubesheet deformation was established according to the structure of the heat exchanger. The effective elastic constant of the tubesheet was calculated with reference to code ASME Ⅷ-1, the outer cylinder and the heat exchange tube bundle were respectively equalized to springs and elastic foundations axially deforming due to temperature and pressure loads. An analytical solution of tubesheet deflection was obtained with the Ritz method, and compared with the numerical simulation results of 3 heat exchangers of different scales. The results show that, the analytical solution is in good agreement with the numerical simulation results, which verifies the correctness of the derived analytical solution of tubesheet deformation. The research work has guiding significance for the design of fixed tubesheet heat exchangers.

Failure analysis on abnormal leakage between tubes and tubesheet of spiral-wound heat exchanger for nuclear power plant

In order to ensure the safety and reliability before actual operation in the nuclear power plant, the reliability test, i. e. bench test was parallelly carried out on two spiral-wound heat exchangers. During the test, both of the two heat exchangers suddenly failed. In our previous paper, the root causes analysis and mechanisms discussion were carried out on one of the spiral-wound heat exchangers, and concluded that the stress corrosion cracking induced by the interaction between inappropriate fabrication and aggressive environment upon the susceptible material was the root cause; while in the current paper, the abnormal leakage near the junction between the heat transfer tubes and the tubesheet of the other spiral-wound heat exchanger was investigated. The obtained results indicated that the interactive effect of crevice corrosion and stress corrosion cracking was the root cause of the failure, which was resulted from multiple factors including design, fabrication, surface treatment and operation. Then the failure mechanism was discussed in detail and the relevant countermeasures were put forward to prevent the similar failure cases occurring again. The achievement of this paper was expected to provide useful experience and suggestion for safe management of spiral-wound heat exchangers in the future.

Optimization of the multi-hole drilling path sequence for concentric circular patterns

Determination of the optimal path sequence in a multi-hole drilling operation is a challenging task in a manufacturing industry as it facilitates substantial reduction in tool travel distance (path length), machining time and machining cost. It is quite analogous to the travelling salesman problem, which is one of the most fundamental NP-hard optimization problems. In this paper, six well-known metaheuristics, i.e. ant colony optimization, artificial bee colony algorithm, particle swarm optimization, firefly algorithm, differential evolution and teaching learning-based optimization algorithm are applied to determine the optimal path sequences in computer numerically controlled multi-hole drilling operations. Two layouts consisting of four and five concentric circular patterns, and a heat exchanger tube sheet with 2600 holes are considered here as three different test problems. The minimum drill path lengths as estimated using these algorithms are observed to be better than that as determined by the spiral path method. Amongst them, teaching learning-based optimization algorithm performs best with respect to the derived optimal path length, consistency of the solution, convergence speed and computational time. Its distinctiveness over the others is also validated using the paired t-test.

Ratcheting assessment in the tubesheets of heat exchangers using the nonlinear isotropic/kinematic hardening model

In this paper, the ratcheting phenomena in the tubesheet of a heat exchanger, which made of modified 9Cr–1Mo steel, has been investigated. The nonlinear isotropic/kinematic hardening model (combined model) is used to investigate the progressive strains due to the cyclic pressure and temperature loading. Therefore, three series of tests were carried out at different temperatures (room temperature, 300 C∘, 520 C∘) to gain stable hysteresis cycles and to obtain the hardening parameters of the desired material. Via least-squares nonlinear regression, the constitutive coefficients are obtained. In addition the effect of tube number on the progressive strains has been investigated. Therefore, three models of tubesheets with different tube numbers are considered. The results show that the progressive strains initially are significant at each cycle and gradually decrease from their initial amount, they reach a stable state, as the number of the tubes increases, and the more progressive strain accumulates.

Study of Various International Codes and Standards for Flanged and Flued Type Expansion Bellows Design

AbstractDifferential longitudinal thermal expansion between the shell and the tube bundle is a well-known problem in fixed tubesheet heat exchanger design. An expansion bellows provide flexibility for thermal expansion and also function as a pressure retaining part. In this paper, guidelines for design of flanged and flued type (thick wall) expansion bellows available in international codes and standards including ASME VIII-1 and 2, EN-13445, and TEMA and EJMA codes are presented. These codes and standards are compared in terms of information available for thick wall expansion bellows design with regard to condition of applicability of design formula, spring rate determination, parameter to define the initial geometry, stress determination, and fatigue evaluation. Inherent limitations of these codes with respect to expansion bellows design, research gape, and recommendations for effective design are also presented in this paper. Brief history and information provided in various codes and standards related to unreinforced thin wall expansion bellows (bellows expansion joints) are also presented to understand evaluation of expansion bellows design.

Research on Heat Treatment Process Optimization of Welding Workstation under New Engineering Background

With the development of new engineering, more research began to transform to energy-saving technology in equipment manufacturing industry. Studies about measurement and analysis of residual stress after surfacing of Heat exchanger tube sheet & the causes of PWHT (post weld heat treatment). Results revealed that the residual stress distribution presented certain trends. PWHT have a limited impact on general trend. Strongest point of stress appears in the ends of test piece or at the surface center of test piece. To some extent, annealing time of 300 minutes is more conducive to removing the residual stress than 90minutes. Annealing temperature of 610°C has more significantly effect than 580°C. Stress of measuring point reduced on average by 40%. At 580°C to 610°C and when annealing time is from 90 minutes to 300 minutes, main cause of removing the residual stress is annealing temperature.

Tube bundle buckling analysis with support effects on tubesheet in heat exchangers

Tubesheet strength calculation is important work in design of heat exchangers. The thickness of the tubesheet depends on the two major factors: loads and structure. The former includes fluid pressure and metal thermal gradient. The later involves tubes bundle located at inner perforated area of tubesheet and constraints applied by flanges or shells at the outer ring of tubesheet. As an elastic foundation, tubes bundle is one of critical factors in calculation of tubesheet system. The effect and current problems of tubes bundle support is reviewed in this paper. Analysis and experiments show that current single tube buckling criterion used in the strength design of the tubesheet may lead to an inaccurate calculation of the tubesheet thickness since this criterion is conservative for a long tube or aggressive for a short tube. A more reasonable and safe criterion for considering the tube bundle support is proposed in this paper which could make the tubesheet thinner and thus, decrease the cost of the heat exchangers. In addition, the idea proposed in this paper may be helpful to develop a new standard for the strength design of the tubesheet or even the tubesheet system.

Numerical simulation of shell-and-tube heat exchanger and study of tube bundle support

In this paper, the method for numerical simulation of fixed tubesheet heat exchangers was analyzed and the support of the tube bundle was studied. It is found that for low or medium pressurized shell-and-tube heat exchangers, no matter which type of solid elements, solid-shell elements or shell elements is utilized for meshing the cylinder and tubes, the axial stresses at the cylinder and tubes as well as the tubesheet deflection are almost the same. The number of tubes has no effects on the hoop stress at the shell, but has significant effects on the axial stress. The less the number of the tubes, the larger the axial stress at the cylinder is. The tubesheet deflection and the action of the pressure on the outside surface of the tubes and inside surface of the cylinder affect the axial stress at the cylinder remarkably. Accurate stress calculation for the combined tubesheet, cylinder and tubes system should be performed using finite element method. The error would be large if the axial stress were simply calculated with the axial force divided by the axial cross section area bearing the load.

Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop

A 400 l capacity nitric acid loop (NAL) test facility simulating inactive plant operating conditions was designed, constructed and evaluation of candidate materials was carried out for nuclear spent fuel reprocessing applications. Austenitic stainless steel coupons were exposed to flowing nitric acid medium at various temperatures in NAL. Assessment of critical components of NAL after 10,000 h operation revealed failure of titanium heater tubes and stainless steel heat exchanger tube sheet welds. Failure analysis and detailed characterisation of corrosion products on the failed tubes were carried out using SEM, EDX and XRD techniques. The brown colour corrosion product on titanium heater tubes comprised iron and small amounts of chromium indicating that failure of titanium tubes was due to deposition of corrosion products of stainless steel, leading to galvanic corrosion. Cracks observed on titanium heater tubes were attributed to hydrogen-induced cracking, resulting in severe degradation. Surface morphology and EDX analysis of the corrosion product on stainless steel heat exchanger tubes revealed dislodged grains from 304LSS and XRD analysis confirmed the deposit as stainless steel. Grain dropping occurred due to intergranular corrosion of 304L SS in nitric acid and deposition of the dislodged grains at the seal weld joints resulted in localised corrosion and failure. Based on the studies carried out, surface heaters instead of immersion heaters and periodic inspection and frequent removal of corrosion products from the nitric acid loop were suggested to mitigate such failures. Based on the results of the investigation, repair and refurbishment of the nitric acid loop was undertaken.