Tube Bundle Vibration

The vibration induced by the fluid flowing over the tube bundle is caused principally by vortex shedding and turbulent buffeting. As fluid flows over a tube vortices are shed from the down-stream side which cause disturbances in the flow pattern and pressure distribution round the tube. Turbulent buffeting of tubes occurs at high flow-rates due to the intense turbulence at high Reynolds numbers. 

ESDU 87019 (1987) Flow induced vibration in tube bundles with particular reference to shell and tube heat... 

Baffles. Longitudinal baffles are used in the shell to control the overall flow direction of the shell fluid as in F, G, and H shells. The transverse baffles may be classified as plate baffles and axial-flow baffles (rod, NEST, etc.). The plate baffles (see Fig. 17.4) are used to support the tubes, to direct the fluid in the tube bundle at about 90° to the tubes, and to increase the turbulence of the shell fluid. The rod (and other axial-flow) baffles (see Fig. 17.5) are used to support the tubes, to have the fluid flowing axially over the tubes, and to increase the turbulence of the shell fluid. Flow-induced vibration is virtually eliminated in rod and other axial-flow baffles having axial shellside flows. 

Tube vibrations in a tube bundle are caused by oscillatory phenomena induced by fluid (gas or liquid) flow. The dominant mechanism involved in tube vibrations is the fluidelastic instability or fluidelastic whirling when the structure elements (i.e., tubes) are shifted elastically from their equilibrium positions due to the interaction with the fluid flow. The less dominant mechanisms are vortex shedding and turbulent buffeting. 

Flow-induced vibration problems are mostly found in tube bundles used in shell-and-tube, duct-mounted tubular and other tubular exchangers in nuclear, process, and power industries. Less than 1 percent of such exchangers may have potential flow-induced vibration problems. However, if it results in a failure of the exchanger, it may have a significant impact on the operating cost and safety of the plant. 

Now we will briefly describe two additional mechanisms vortex shedding and turbulent buffeting. It should be noted that these mechanisms could cause tube vibrations, but their influence on a tube bundle is less critical compared to the fluidelastic instabilities described earlier. 

Tube Bundle Natural Frequency. Elastic structures vibrate at different natural frequencies. The lowest (fundamental) natural frequency is the most important. If the vortex shedding or turbulent buffeting frequency is lower than the tube fundamental natural frequency, it will not create the resonant condition and the tube vibration problem. Hence, the knowledge of the fundamental natural frequency is sufficient in most situations if / is found to be higher than / or fb. Higher than the third harmonic is generally not important for flow-... 

According to Chenoweth [130], acoustic vibration is found more often in tube bundles with a staggered rather than inline layout. It is most common in bundles with the rotated square (45°) layout. 

Heat exchanger vibrations can be reduced either by increasing the tube bundle natural frequency or reducing excitation mechanisms. Methods to accomplish that and eventually prevent vibration and potential damage can be summarized as follows [130] ... 

The IHX is a vertical, counter current flow, shell and tube heat exchanger (fig. 6). Each IHX has 3000 straight tubes (19 mm OD x 0.8 mm WT) with primary sodium on shell side and secondary sodium on the tube side. The tubes are arranged in circumferential pitch. A variable flow distribution is provided inside the IHX tubes with a higher flow on the outer rows to improve the thermohydraulic behaviour of the tube bundle. A mixing device is also provided at the secondary outlet to reduce the temperature differences between inner and outer shell of the secondary outlet header. Absence of flow induced vibration of tube bundle and the drain pipe in the downcomer have been verified by theoretical analysis. 

The RBM bands correspond to the coherent vibration of the carbon atoms where all the tube atoms vibrate radially in phase. These features are unique in CNTs and occur with frequencies between 120 and 350 cm for SWCNTs with diameters in the range 0.7 nm < dt<2 nm. The RBM frequency (corbm) varies as Hd through the relation corbm = dt + B, where the parameters A and B are determined experimentally. Different values of the constants A and B have been reported in the literature, whereas the variations in the A and B parameters are often attributed to environmental effects, namely whether the SWCNTs are present as isolated, supported or in the form of bundles. Therefore, from the corbm measurement of an individual SWCNT, it is possible to obtain its diameter value. Also, by recording Raman spectra using... 

Leaks erosion/corrosion/vibration/improper tube finishing/cavitation/lack of support for tube bundle/tube end fatigue. 

Hydraulic tests to assess the flow distribution and measure vibration of tube bundle are carried out on a 60 degree sector model of SG (fig 13a). The velocity distribution measured in the inlet plenum was found matching with prediction of the 3D hydraulic calculations (fig 13b). Vibration measurements are carried out in the straight spans and expansion bend regions. Although the vibration level is found higher in the inlet span where cross flow takes place across the tube bundle, it is within the acceptable values based on structural mechanics analysis (fig 13c). 

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