Global J. of Mech., Engg. & Comp. Sciences, 2012: 2 (1)

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1 Research Paper: Thombare et al., 2012: Pp FLOW INDUCED VIBRATION ANALYSIS OF TEMA J-TYPE U-TUBE SHELL AND TUBE HEAT EXCHANGER Thombare, T.R., Kapatkar, V.N*., Utge, C.G., Raut A.M and H.M. Durgawale Department of Mech. Engineering, Sinhgad College of Engineering, , Maharashtra, India *Walchand Technology Group-Walchandnagar Industries Ltd., , Maharashtra, India Corresponding Author: ABSTRACT Aim: Flow induced vibn (FIV) analysis of U tubes to design vibn free shell and tube heat exchanger (STHE), without affecting its thermal performance. Methodology: Effect of unsupported span on vortex shedding, crossflow amplitude and gap velocity involved in FIV were analyzed by using software Heat Transfer Research Incorporated (HTRI). Results: Unsupported tube length is seems to play a major role in contributing FIV. It is found that; for heat exchanger under considen, with addition of one support plate per baffle space having same thickness as baffle plate and using five support plates at U bend region vibn problems are unlikely. Conclusion: It is found that with reduction in unsupported span length due to addition of support plate s corrective action to FIV problem is taken, without changing thermal performance of heat exchanger at design stage. The detailed design, fabrication and analysis work was carried out at WTG-Walchandnagar industries Ltd., , Maharashtra, India. Keywords: Shell and Tube Heat Exchanger, Flow-Induced Vibn, TEMA and HTRI. INTRODUCTION Fluid flow, inter-related with heat exchanger geometry, can cause heat exchanger tubes to vibrate. This phenomenon is highly complex and the present state of the art is such that the solution to this problem is difficult to define. Most heat exchangers have multiple baffle supports and varied individual unsupported spans. In case of U- tube heat exchanger outer rows of U-bends have a lower natural frequency of vibn and therefore, are more susceptible to FIV failures than the inner rows. Damaging tube vibn can occur under certain conditions of shell side fluid flow relative to baffle configun and unsupported tube span. Mechanical failure of tubes resulting from FIV may occur in various forms such as collision damage, baffle damage, tubesheet clamping effect, material defect propagation and acoustic vibn. There are four basic FIV mechanisms that can occur in a tube bundle. These are the fluidelastic instability, vortex shedding, turbulent buffeting, and acoustic resonance. The first three mechanisms are accompanied by tube vibn amplitude while acoustic resonance causes a loud acoustic noise with virtually no increase in tube amplitude. Fluidelastic instability is the most damaging in that it results in extremely large amplitudes of vibn with ultimate damage patterns. The design approach in this case is to avoid the fluidelastic instability situation thereby avoiding the accompanying large amplitude of vibn. Vortex shedding may be a problem when there is a frequency match with the natural frequency of tube. This frequency match may result in vibn amplitude which can be damaging to tubes in the vicinity of the shell inlet and outlet connections. Turbulent buffeting mechanism is the fluctuating forces acting on the tubes due to extremely turbulent flow of shell side fluid. The turbulence has a wide spectrum of frequencies distributed around a central dominant frequency which increases as the cross flow velocity increases. This turbulence buffets the tubes which selectively extract energy from the spectrum of frequencies present. Acoustic resonance mechanism is due to gas column oscillation can be excited by phased vortex shedding or turbulent buffeting. Oscillation normally occurs perpendicular to both the tube axis and flow direction. When natural acoustic frequency of shell approaches the exciting frequency of the tubes, a coupling may occur and kinetic energy in the flow stream is converted into acoustic pressure waves (Thorngren, 1970). In this section various methods to predict FIV are described. Chen has shown that the vortex shedding frequency can be predicted by dimensionless Strouhal number (Chenoweth and Kistler, 1978). This Chen Strouhal number is a function of the tubefield layout and the tube pitch. This data is transformed into Chen experimental curves for layout arrangement (30 0, 60 0, 45 0 and 90 0 ) found in shell and tube heat exchangers. This curve is used for finding out vortex shedding frequency. Owen predicted the central dominant frequency and develops empirical equation to find out turbulent buffeting frequency (Owen, 1965 and Chen, 1968). Empirical equation given in TEMA standards is used to calculate turbulent buffeting amplitude (TEMA 2007). Connors postulated that the amount of energy input at these flow conditions exceeds that which can be dissipated by damping; therefore, the amplitude progressively builds up until the tubes collide. Developed damping correlations in the literature are used to predict fluidelastic instability by comparing average crossflow velocity to critical velocity. Method developed by Chen and Weber Global Journal of Mechanical Engineering and Computational Sciences ISSN Rising Research Journal Publication 10

2 (Chenoweth, 1988) is used to predict tube vibn induced by parallel flow to the tubes. In which the of parallel flow velocity in the window to critical parallel flow velocity is used to calculate the midspan amplitude. Brothman method (Brothman et al., 1974) is used to predict whether FIV is serious or not in STHE. The procedure described is based upon comparing the displacements due FIV mechanisms and worst case peak midspan deflection. SHELL AND TUBE HEAT EXCHANGER SPECIFICATION Problem definition & objective: At thermal design stage, geometry of heat exchanger is finalized by using given process parameters (Table 1). Literature survey shows that vibns in heat exchanger are caused by; shellside fluid flow, tubeside fluid flow, unsupported span etc. These vibns become dangerous when resonance occurs; which causes serious damage to tubes. In U- tube heat exchanger inlet, central and U- bend regions are most possible regions for FIV. Also outer rows of U-bends have a lower natural frequency of vibn therefore; these are more susceptible to FIV failures than the inner rows. Objective of this work is; to take corrective action on FIV with addition of support plate without changing thermal performance of heat exchanger (Table 2). Table 3 gives FIV analysis result of existing heat exchanger. Items with asterisk(*) such as unsupported span/tema maximum span, vortex shedding frequency and cross flow amplitude exceed a conservative lower limit for FIV (Table 4). METHODOLOGY FIV analysis is done by using HTRI software with considen of one support plate per baffle space having same thickness as baffle plate and five U- bend supports. Also FIV analysis of single tube in outer row of U-bend is carried out. RESULT AND DISCUSSION Table 5 shows output summary for FIV analysis of TEMA J type U tube STHE with one support plate per baffle space and five U-bend supports. It is observed that there is no any asterisk(*) value in output summary, which indicate that unsupported span/tema maximum span, vortex shedding frequency and cross flow amplitude within conservative limit for vibn free design. 6 shows output summary for vibn analysis of single tube. For first mode, maximum vortex shedding amplitude and maximum of gap velocity to critical gap velocity are less than conservative limit. Figure 2 shows that of gap velocity to critical gap velocity is increases with increase in unsupported span. For first mode of vibn maximum is which is less than conservative limit for FIV, observed at tubesheet region where unsupported span is maximum (Figure 1). Figure 3 shows vortex shedding amplitude increases with increase in unsupported span. For first mode of vibn maximum vortex shedding amplitude is mm which is less than conservative limit for FIV, observed at tubesheet region where unsupported span is maximum. CONCLUSION From the vibn analysis of STHE it is found that unsupported span has major impact on unsupported span/tema maximum span, cross flow amplitude, gap velocity, vortex shedding and vortex shedding amplitude. From Table 5, it is concluded that with reduction in unsupported span with addition of support plates; unsupported span/tema maximum span, vortex shedding and crossflow amplitude reduces to conservative limit for FIV [Table 4]. From Figure 2 and Figure 3 it is concluded that gap velocity/critical gap velocity and vortex shedding amplitude are maximum where unsupported span is maximum. We finally concluded that with reduction in unsupported span due to addition of support plate s corrective action to FIV problem is taken, without changing thermal performance of heat exchanger at design stage. Acknowledgements: The Authors thank the Walchand Technology Group-Walchandnagar Industries Ltd., , Maharashtra, India for their encouragement and infrastructure facilities to carry out this work. REFERENCES Brothman A., Devore A., Hollar G. B., Horowitz A. and H. T. Lee A tube vibn analysis method, AIChE Symposium Series, 138 (70): Chen Y. N Flow induced vibn and noise in tube-bank heat exchangers due to Von Karman streets, Trans. of the ASME, series B: J. of Eng. for Industry. 90: Chenoweth J. M Flow induced vibn in tube bundle with a pitch of 1.42, HTRI design report STV-03, Chenoweth J. M and R. S. Kistler Tube vibns in shell and tube heat exchangers, HTRI design report STV-01, Gupta J. P Heat Exchanger Design and Vibn Analysis, Heat Exchanger Design a Practical Look, 2 nd ed., C. S. Enterprises, V-46. Owen P. R Buffeting excitation of boiler tube vibn, J. Mech. Eng. Sci. 7: TEMA Standards of the Tubular Exchanger Manufacturers Association, 9 th ed., Tubular exchanger Manufacturers Association, Inc., New York. Thorngren J. T Predict exchanger tube damage, Hydrocarbon Processing, X, Global Journal of Mechanical Engineering and Computational Sciences ISSN Rising Research Journal Publication 11

3 Table 1: Process parameters Description Tube side Fluid flow rate 2175 m 3 / hr Shell Side Fluid flow rate 4600 m 3 / hr Density: Tube Side Fluid at Avg. Temp kg/m 3 Density: Shell Side Fluid at Avg. Temp kg/m 3 Dy. Viscosity: Tube Side Fluid at Temp. Dy. Viscosity: Shell Side Fluid at Temp mnsec/m mnsec/m 2 Global J. of Mech., Engg. & Comp. Sciences, 2012: 2 (1) Table 2: Geometric parameters Description Heat exchanger NTIW single segmental NJ12U type Size dia/length) ID x 6636 mm Mtl.-SA 213 TP 316L; Dia mm x 1mm thk.; Pitch- 19 mm Straight length 5850mm; U tubes- Tube 4136; Min. and max. bending radius mm and 760 mm respe. Baffle/support Mtl.-SA 516 Gr 70; Thk. 12 mm;hole plates dia.13 mm Table 3: Output summary of FIV analysis for existing STHE Vibn Analysis Xist Ver SP3 1/30/ :54 SN: Rating - Horizontal Multipass Flow TEMA NJ12U NTIW-Segmental Baffles Shellside (Level 2.3) Sens. Liquid condition Axial stress (MPa) 0 Added mass factor loading Beta Position In TheBundle Inlet Center U-Bend Length for natural requency (m) Length/TEMA maximum span (--) * Number of spans (--) Tube natural frequency (Hz) Flow Velocities Inlet Center U-Bend Window parallel velocity (m/s) Bundle crossflow velocity (m/s) Bundle/shell velocity (m/s) Fluidelastic Instability Check Inlet Center U-Bend Log decrement TEMA Critical velocity (m/s) Baffle tip cross velocity (--) Average crossflow velocity (--) Tube Vibn Check Inlet Center U-Bend Vortex shedding (--) Parallel flow amplitude (mm) Crossflow amplitude (mm) Tube gap (mm) Crossflow RHO-V-SQ (kg/m-s 2 ) Bundle Entrance/Exit (analysis at first tube row) Entrance Exit Fluidelastic instability (--) Vortex shedding (--) * Crossflow amplitude (mm) * Crossflow velocity (m/s) 1.18 Tubesheet to inlet/outlet support (mm) None None Shell Entrance/Exit s Entrance Exit Impingement plate No SI Units Flow area (m 2 ) Velocity (m/s) 1.18 RHO-V-SQ (kg/m-s 2 ) Shell type NJ12U Baffle type NTIW-Seg. Tube type Plain Baffle layout Perpend. Pitch Tube diameter, (mm) 12.6 Layout angle 60 Tube material 316 Stainless steel (17 Cr, 12 Ni) Number U-Bend supports Supports/baffle space Note: * Items with asterisk exceed a conservative lower limit for vibn-free design Global Journal of Mechanical Engineering and Computational Sciences ISSN Rising Research Journal Publication 12

4 Table 4: Conservative limits for vibn free design Figure 1: Drawing of TEMA J type U tube STHE Conservative limit for FIV Vortex shedding frequency [TEMA, 2007] 0.5 Vibn amplitude [TEMA,2007] Unsupported Span /TEMA max. span Gap velocity/critical gap velocity [Gupta, 1979] (0.02 x Tube OD) inch Table 5: Output summary FIV analysis of STHE with support plates and U-bend supports Vibn Analysis Page 1 Xist Ver /12/ :58 SN: SI Units Rating - Horizontal Multipass Flow TEMA NJ12U NTIW-Segmental Baffles Shellside condition Sens. Liquid (Level 2.3) Axial stress loading (MPa) 0 Added mass factor Beta Position In The Bundle Inlet Center U-Bend Length for natural frequency (m) Length/TEMA maximum span (--) Number of spans (--) Tube natural frequency (Hz) Flow Velocities Inlet Center U-Bend Window parallel velocity (m/s) Bundle crossflow velocity (m/s) Bundle/shell velocity (m/s) Fluidelastic Instability Check Inlet Center U-Bend Log decrement TEMA Critical velocity (m/s) Baffle tip cross velocity (--) Average crossflow velocity (--) Tube Vibn Check Inlet Center U-Bend Vortex shedding (--) Parallel flow amplitude (mm) Crossflow amplitude (mm) Tube gap (mm) Crossflow RHO-V-SQ (kg/m-s 2 ) Bundle Entrance/Exit (analysis at first tube row) Entrance Exit Fluidelastic instability (--) Vortex shedding (--) Crossflow amplitude (mm) Crossflow velocity (m/s) 1.18 Tubesheet to inlet/outlet support (mm) None None Shell Entrance/Exit s Entrance Exit Impingement plate No Flow area (m 2 ) Velocity (m/s) 1.18 RHO-V-SQ (kg/m-s 2 ) Shell type NJ12U Baffle type NTIW-Seg. Tube type Plain Baffle layout Perpend. Pitch Tube diameter, (mm) 12.6 Layout angle 60 Tube material 316 Stainless steel (17 Cr, 12 Ni) Number U-Bend supports 5 Supports/baffle space 1 Global Journal of Mechanical Engineering and Computational Sciences ISSN Rising Research Journal Publication 13

5 Figure 2: Figure 3: Tube length Vs vortex shedding amplitude Table 6: Output summary FIV analysis of single tube Output Summary Page 1 Xvib Ver SP3 1/24/ :52 SN: Tube Pass=2, Row=75, Tube=1 No Data Check Messages. No Runtime Messages. Vibn Data Number of spans 46 Average span length (m) Tube pitch (mm) 19 Fluid density (kg/m 3 ) Fluidelastic instability constant Lift coefficient 0.1 Added mass factor Log decrement Tube Material Information Tube material 316 Stainless steel (17 Cr, 12 Ni) Density (kg/m 3 ) Elastic modulus (MPa) Effective weight (kg/m) 0.55 Area moment of inertia (mm 4 ) Analysis Results Mode Frequency Gap Velocity / Max Vortex Shedding Span SI Units Critical Gap Velocity Amplitude Number (--) (Hz) (--) (mm) (--) Note: + Frequency s are based upon lowest natural or acoustic frequency *************** Global Journal of Mechanical Engineering and Computational Sciences ISSN Rising Research Journal Publication 14