Tube Sheet Research Articles

Refined General Theory of Stress Analysis for Tubesheet

The stress analysis method for fixed tubesheet (TS) heat exchangers (HEX) in pressure vessel codes such as ASME VIII-1, EN13445, and GB151 is based on the classical theory of thin plate on elastic foundation. In addition, these codes all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS is needed to be considered. In this study, a refined general theory of stress analysis for TS is presented which also considers unequal thickness for two TSs, different edge conditions, pressure drop and deadweight on two TSs, the anisotropic behavior of the TS in thickness direction, and transverse shear deformation in TS. Analysis shows floating and U-tube heat exchangers are the two special cases of the refined theory. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the refined theory. Numerical comparison results indicate that predictions given by the refined theory agree well with finite element analysis (FEA) for both thin and thick TS heat exchangers, while ASME results are not accurate or not correct. Therefore, it is concluded that the presented refined general theory provides a single unified method, dealing with both thin and thick TSs for different type (U type, floating, and fixed) HEXs in equal detail, with confidence to predict design stresses.

Stress corrosion cracking of a circulation water heater tubesheet

Abstract After only three years' service of a circulation water heater, tubesheet showed sever leakage and led the plant to emergency shutdown. On-site investigation revealed extensive cracking initiated at weld area and through the tubesheet holes. Samples from failed area were removed and investigated. The chemical and micro structural properties were analyzed by optical and scanning electron microscopy. Micro structural observations have shown intergranular and transgranular crack growth. Sulfur was detected in corrosion products by EDS examinations. Sulfur compounds and chromium carbides also were detected by XRD analyses. Wet carbonaceous deposits were found on surfaces of the tube sheet. Analysis on the deposits depicted presence of caustic and chlorides. Considering all aspects of failures led to this conclusion that the material was in sensitized condition possibly due to overheating at the deposit sites. The sensitized material failed due to the acronym SCC induced by polythionic acid. Presence of chlorides and caustic aid the failures.

Self-Standing Carbon Nitride-Based Hydrogels with High Photocatalytic Activity.

Here we report a facile synthesis of carbon nitride-based hydrogels with adjustable shapes, ranging from cylinder to tube and thin sheet, by photopolymerization process in confined templates. The fabricated hydrogel shows enhanced mechanical properties compared to the reference gel without carbon nitride incorporation, good adsorption capacity, and promising photocatalytic activity toward hydrogen production. Meanwhile, the hydrogel also exhibits selective pollutants adsorption properties that could be attributed to the negative-charged carbon nitride as well as relatively high stability alongside enhanced light harvesting. The novel carbon nitride-based hydrogels offer a facile approach as photocatalyst and open up many opportunities for smart-catalyst design with adjustable reaction sites.

SERS-active metal-organic frameworks with embedded gold nanoparticles.

Surface-enhanced Raman scattering (SERS) has been widely used in the detection of targets and strongly depends on the interaction and the distance between the targets and nanoparticles. Herein, metal-organic frameworks (MOFs) were first easily synthesized on a large scale via a water bath method, especially Uio-66 and Uio-67. MOFs embedded with gold nanoparticles (AuNPs) for SERS enhancement were successfully fabricated via an impregnation strategy. The synthesized AuNPs/MOF-199, AuNPs/Uio-66, and AuNPs/Uio-67 composites, with LSPR properties and high adsorption capability of MOFs to preconcentrate the analytes close to the surface of the AuNPs, exhibited excellent SERS activity. The effects of the reducing concentrations of sodium citrate on the SERS activity, and the stability and reproducibility of the AuNP/MOFs have been discussed via the detection of acetamiprid. The SERS intensity enhanced by the composites was retained for more than 40 days under ambient conditions with the reducing concentrations of sodium citrate at 0.16%, 0.20%, and 0.16%. The limits of detection with the signal/noise ratio higher than 3 at the characteristic peak 632 cm-1 were 0.02 μM, 0.009 μM, and 0.02 μM for acetamiprid. Most interestingly, the AuNP/MOF-199 composites, whose morphology was long tube sheet, exhibited excellent SERS activity. These novel composites with high sensitivity, stability, and reproducibility provide a new route for the detection of pesticides via the SERS technology.

Mathematical Modelling for Thermal and Mechanical Design of Shell and Tube Type Gas Cooler Used in Transcritical CO<sub>2</sub> Refrigeration System

This paper is about Mathematical Modelling for Thermal and Mechanical Design of Shell and Tube type Gas Cooler used in Transcritical CO2 Refrigeration system. Transcritical refrigeration system refers to system whose condenser temperature is above critical temperature of refrigerant. To achieve it, the condenser in conventional refrigeration system is replaced by Gas Cooler where Refrigerant vapour is cooled sensibly without condensation. The gas cooler is used for cooling of refrigerant by using water as coolant. The temperature of refrigerant vapour coming out of compressor in transcritical system is more as compared to conventional refrigerant system. So gas cooler can be effectively used for heating of water. This paper describes a mathematical model that can be used in predicting the heat transfer performance of a shell and tube type Gas Cooler used in transcritical CO2 refrigeration system. The model uses Kern Method of Heat exchanger design. Given the fluid inlet and outlet temperatures flow rates of fluid & fluid properties the model determines (a) the necessary heat transfer surface area, (b) Outside and inside heat transfer coefficient, (c) overall heat transfer coefficient, (d) Pressure drop on shell and tube side, (e) It also determines mechanical design parameters such as shell O. D, Shell thickness, Tube sheet thickness, Flange thickness.

Finite Element Analysis of Opening Plate, Fixed Tube Sheet and Floating Sheet of Shell & Tube Heat Exchanger

Finite Element Analysis of Opening Plate, Fixed Tube Sheet and Floating Sheet of Shell & Tube Heat Exchanger

Numerical investigation of erosion of tube sheet and tubes of a shell and tube heat exchanger

Abstract The failure of shell and tube heat exchangers caused by solid particle erosion has been a major problem in the oil and gas and other industries. Predicting erosion is still a developing art, an accurate simulation method is then significant to analyze the erosion characteristics in such complex geometry and determine erosion rate of metal surface. In this work a physical model was proposed to simulate the erosion of two-pass shell and tube heat exchangers with computational fluid dynamics. The simulation was performed for different feed fluid rates and a range of sand particle sizes from 0.1 to 1000 μm. The erosion rates of tube sheet, tube ends in the inlet plenum and the inner wall of tubes were monitored and the influences of flow pattern, particle size and particle behaviors on erosion were studied. The predictions are compared with the earlier studies and a good agreement was found. The particles can be classified into three groups based on the dependence of erosion rates of tube sheet and tubes on the particle size. The large particles (>200 μm) exhibited a near-linear influence on the erosion rates. The small particles (about 50–200 μm) produced approximate size-independence facet-average erosion rate of tubes, but the maximum local erosion rates of the tubes and tube sheet sharply increased with the decrease of particle size. The fine particles (

Structural Analysis and Optimization of Transition Section of Convex Tube Sheet

The convex tube sheet which is used in heat recovery boiler consists of three parts: the high temperature tube sheet, the low temperature tube sheet and the transition section.Three-dimensional finite element model of convex tube sheet in new type of heat recovery boiler is established in this paper. Using the ANSYS Workbench software, thermal stress of the convex tube sheet is analyzed. The temperature fields and thermal stress distribution of convex tube sheet are obtained, and its structure strength is checked. The effects of the high temperature tube sheet thickness, low temperature tube sheet thickness and transition section thickness on the maximum equivalent stress of the convex tube sheet are analyzed. The results show that: temperature of most parts of convex tube sheet is close to the tube side fluid temperature, and the large temperature gradient only existed in the thinner regions of shell side of convex tube sheet; temperature distribution shows obvious skin effect. At the transition section, the temperature along the thickness direction is more evenly distributed, with little change in temperature gradient; larger thermal stress mainly concentrated at tube layout area which close to the shell side of the high temperature tube sheet and the connecting parts of transition section and low temperature tube sheet in the tube side. Through checking the strength intensity, convex tube sheet structural strength meets the requirements.The transition section thickness are optimized. The optimum thickness of the transition section analyzed in this paper is 31mm.

Stress Analysis for Clad Steel Tube Sheet with Oval Tube Hole

Models are established for oval tube hole clad steel tube sheet and circular tube hole clad steel tube sheet. Finite element analysis software ANSYS Workbench is used. Stress distribution characteristics of tube sheet with oval tube hole are investigated. Stress distribution characteristics under different ellipticity are analyzed and the general law of stress distribution of oval tube hole clad steel tube sheet is summarized. Results show: effects of tube hole shape on stress of clad steel tube sheet mainly focus in the tube zone, oval tube hole could effectively improve the phenomenon of stress concentration at the connection of basic layer and clad layer. When tube hole is oval, the maximum values of thermal stress and mechanical stress increase comparing to clad steel tube sheet with circular tube hole. With the increase of ellipticity, the total stress of tube sheet increases, stress increases on the minor axis of tube hole and decreases on long axis, and the deviation between the two stresses increases. The results provide reference for the design of the clad steel tube sheet.

Weld Residual Stress in Tube to Tube Sheet Joints of a Heat Exchanger by Experimental and Finite Element Simulation

Shell-and-tube heat exchangers are widely used in petrol and petro-chemical industries. But weld residual stresses are generated in the tube-to-tube sheet joint and have a great effect on stress corrosion cracking. Therefore, a good prediction and an efficient evaluation of the weld residual stress are necessary to assure the structure integrity. In this work, X-ray diffraction measurement and finite element method have been used to study the residual stress distribution. The results show that the large tensile residual stresses, which have exceeded the yield stress, are generated in the tube to tube sheet joints, leading that the joints become the weakest parts of the heat exchanger. In the central surface part, a wave shape stress distribution is generated along the centerline of tube 1 and tube 3.

Manufacturing and mechanical evaluation of cooled cooling air (CCA) heat exchanger for aero engine

Recent studies on improving the efficiency of gas turbine engines have focused on increasing the inlet temperature of gas flowing into the turbine. For the high inlet temperature, it requires the development of a super-alloy based material that can withstand harsh operating conditions. However, it is essential to utilize alternative solutions such as turbine blade cooling technology. Unfortunately, lightweight, high performance, and high mechanical reliable heat exchangers for aircraft have not yet been examined because of the difficulties associated with design optimization and reliability verification under high temperatures and pressures. The purpose of this study is to develop a manufacturing process for a heat exchanger that can be used in aircraft gas turbine engines. The manufacturing process involved preparing fine tubes through multi-step drawing and annealing processes, and joining these to a tube sheet through brazing. In this work, we reported on the total fabrication processes and mechanical integrity tests of a cooled cooling air (CCA) heat exchanger for the aircraft turbine engine. Through the work, a prototype model of a heat exchanger based on a gas turbine assembly was then developed using each of the individual processes. An X-ray CT test and an endoscopy test were performed to inspect the heat exchanger. The results indicated good manufacturing integrity; thus, the developed heat exchanger can be used for cooling turbine blades

A Proposed General Method of Stress Analysis for Tubesheet of Heat Exchanger

The stress analysis method for fixed tubesheet (TS) heat exchangers (HEX) in pressure vessel codes such as ASME VIII-1, EN13445, and GB151 all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS is need to be considered. In this study, the midplane symmetry assumption is discarded. More common situations are considered such as unequal TS thickness, different edge conditions, pressure drop, and dead weight on two TSs. Based on the classical thin plate and shell theoretical solution, an analytical method of stress analysis for TS is presented. The proposed method is suitable for different types of HEX due to fewer assumptions employed in this study. Analysis shows that floating and U-tube HEX are the two special cases of the proposed method. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the proposed method. Typical geometries and loading are considered, and the proposed method is used to check the adequacy of design. Predictions are compared with the results obtained from axisymmetric finite element analysis (FEA) and current ASME method. Comparison results indicate that predictions given by this paper agree well with FEA while ASME results are not correct or not accurate.

Sensitivity study of APR-1400 steam generator primary head stay cylinder and tube sheet thickness

The steam generator is a type of heat exchanger which produces steam to drive the turbine generator. For the APR-1400, Doosan Heavy Industries & Construction manufactures a vertical type of steam generator that has a horizontal tube sheet with a central support design, called a stay cylinder. In this design, the stay cylinder limits tube sheet deformation and effectively reduces thickness of tube sheet and increases structural integrity. A concentric hole in the stay cylinder facilitates welding to the primary head and to the tube sheet. In this study, the APR-1400 steam generator tube sheet thickness parameters were investigated within the constraint of maintaining the overall deformation of the tube sheet in comparison to the current design. A parametric evaluation to reduce the thickness of the steam generator tube sheet is performed by varying the inner diameter of the stay cylinder and the thickness of the tube sheet. The solutions are developed based on two-dimensional finite element analyses. The effect of a reduction of the tube sheet is beneficial in the manufacturing of steam generator by using less material and reducing manufacturing time and allowing easier assembly process of U-tube bundles to the tube sheet.

Buckling of hybrid nanocomposites with embedded graphene and carbon nanotubes

With the aid of atomistic multiscale modelling and analytical approaches, buckling strength has been determined for carbon nanofibres/epoxy composite systems. Various nanofibres configurations considered are single walled carbon nano tube (SWCNT) and single layer graphene sheet (SLGS) and SLGS/SWCNT hybrid systems. Computationally, both eigen-value and non-linear large deformation-based methods have been employed to calculate the buckling strength. The non-linear computational model generated here takes into account of complex features such as debonding between polymer and filler (delamination under compression), nonlinearity in the polymer, strain-based damage criteria for the matrix, contact between fillers and interlocking of distorted filler surfaces with polymer. The effect of bridging nanofibres with an interlinking compound on the buckling strength of nano-composites has also been presented here. Computed enhancement in buckling strength of the polymer system due to nano reinforcement is found to be in the range of experimental and molecular dynamics based results available in open literature. The findings of this work indicate that carbon based nanofillers enhance the buckling strength of host polymers through various local failure mechanisms.

A Mobile Robotic System for the Inspection and Repair of SG Tubes in NPPs

The reliability and performance of a steam generator (SG) is one of the serious concerns in the operation of pressurized water nuclear power plants. Because of high levels of radiation, robotic systems have been used to inspect and repair SG tubes. In this paper, we present a mobile robotic system that positions the inspection and repair tools while hanging down from the tube sheets where the tubes are fixed. All of the driving mechanisms of the mobile robot are actuated by electric motors to start its works, providing that the electric power is prepared without the additional need for an on-site air services. A special tube-holding mechanism with a high holding force has been developed to prevent falling from the tube sheets, even in the case of an electric power failure. We have also developed a quick installation guide device that guides the mobile robot to desired initial positions in the tube sheet exactly and quickly, which helps to reduce the radiation exposure of human workers during the installation work. This paper also provides on-site experimental results and lessons learned.

Application of Numerical Simulation on the Manufacturing Process Qualification of Nuclear Power Plant Steam Generator Tube-sheet

Nuclear power plant key parts-steam generator (SG) tube sheet forging operate in harsh environment, so it has high requirements for internal quality. According to RCC-M specification requirements (take CPR1000 reactor type for example), the manufacturer should carry out workshop qualification and part process qualification before delivery product manufacturing. It is commonly used that centre compaction process and conical plate upsetting process regard as two kinds of large forging upsetting ways. The numerical analogue simulation has been carried out on nuclear power large tube sheet forgings using France Forge simulation software. The results show that: when the same pressure size, 3-direction compressive stress in the core of the billet always exist in the process of upsetting using centre pressing process, and the strain create in the central of the billet, but stagnant zone exist on the bottom and end plane; tensile stress in the core of the billet always exist in the process of upsetting using conical plate upsetting process, it will force the billet that contact with conical plate to deform, so eliminate stagnant zone, but effective deformation don’t produce in the central of the billet. The conical plate continue press for conical plate upsetting process, tensile stress in the core of the billet disappear, and then gradually change into pressure stress, then tensile stress is generated at the bottom, finally tensile stress disappear and 3-direction compressive stress exist in the core of the billet.

Effect of the sequence of tube rolling in a tube bundle of a shell and tube heat exchanger on the stress–deformed state of the tube sheet

Rolling the tube sheet of a heat exchanger with U-shaped tubes, as exemplified by the vapor cooler GP-24, was simulated. The simulation was performed using the finite element method with account of elas- tic–plastic properties of the tube and tube sheet materials. The simulation consisted of two stages; at the first stage, maximum and residual contact stress in the conjunction of a separate tube and the tube sheet was determined using the “equivalent sleeve” model; at the second stage, the obtained contact stress was applied to the hole surface in the tube sheet. Thus, different tube rolling sequences were simulated: from the center to the periphery of the tube sheet and from the periphery to the center along a spiral line. The studies showed that the tube rolling sequence noticeably influences the value of the tube sheet residual deflection for the same rolling parameters of separate tubes. Residual deflection of the tube sheet in different planes was determined. It was established that the smallest residual deflection corresponds to the tube rolling sequence from the periphery to the center of the tube sheet. The following dependences were obtained for different rolling sequences: maximum deformation of the tube sheet as a function of the number of rolled tubes, residual deformation of the tube sheet along its surface, and residual deflection of the tube sheet as a function of the rotation angle at the periphery. The preferred sequence of tube rolling for minimizing the tube sheet deformation is indicated.

Computational and experimental studies of the causes of crack network formation in the area of the heat exchanger tube sheet in the BN'600 reactor

In order to examine the condition of intermediate sodiumtosodium heat exchangers (IHX) and to substantiate their operation life extension to 45 years at the BN600 reactor plant at Beloyarsk NPP Unit 3, one of the six heat exchangers was removed from the reactor in April 2006. Inspection revealed cracks with 7 maximum depth of mm on the outer surface of the upper tubesheet (UTS) and adjacent shell.To predict durability of substantially fatigued metal, verification of the existing relationship for the threshold stress intensity factor range for the 10Cr18Ni9 steel was needed. To this end, specimens of two structural elements in the IHX – upper tube sheet and protection block were tested.To identify failure mechanisms, fractographic studies were performed on the surface of cracks detected in the tube sheet and produced in the specimens. The studies led to the conclusion that the failure mechanism for cracks detected on the tube sheet was identical to the mechanism generated in the test specimens. In both cases the intergranular failure prevailed that is typical for the material stress level indicative of crack growth termination. This result makes it possible to speak about crack initiation and propagation in the IHX tube sheet in the high-cycle fatigue region at low-level strain ranges and stress intensity factor ranges.An analysis of causes of crack formation showed that the cracks could have formed as a result of the temperature pulsation effect produced by mixing of sodium flows having different temperatures – sodium entering the IHX inlet and sodium coming from the reactor vessel cooling system.Computational analysis results showed that for all thermal pulsation conditions and crack propagation cross-sections under consideration, the leak-tightness condition is met for the upper tube-sheet and IHX outlet chamber shell that divide the primary and secondary coolant circuits.The computational and experimental studies have proved that presence of these cracks does not limit the potential service life extension to 45 years for the IHX in the BN600 reactor plant.

Расчетно-экспериментальные исследования причин образования сетки трещин в районе верхней трубной доски теплообменника реактора БН-600

Расчетно-экспериментальные исследования причин образования сетки трещин в районе верхней трубной доски теплообменника реактора БН-600

Failure analysis of AISI 321 tubes of heat exchanger

The present study is about the failures of tubes from a tube shell heat exchange in one of the compression stages used to cool and purify the gas produced in an off-shore platform. Conventional hydrostatic tests did not detect leaking in the heat exchanger, but these tests were conducted at room temperature, i.e., very distant to the service temperature (163°C). However, tightness tests with helium gas have detected leaking. Some tubes were taken off for analysis and the liquid penetrating test detected cracks in crevices formed between the tube and the tube sheet. The cracks were found to be typical stress corrosion cracks with branched pattern. A non-destructive test with Eddy current probes was developed to give high percentage of detection. The work was conclusive about the main cause of failures. Due to the increase of chloride content of the water processed to concentrations higher than 1000mg/L over the years, the substitution of AISI 321 tubes by superduplex steel is recommended.