Tube Vibration

Flow-induced tube vibration occurs at critical fluid velocities through tube banks, and is to be avoided because of the severe damage that can result. Methods to predict and correct vibration problems may be found in Eisinger Trans. ASME J. Pressure Vessel Tech., 102, 138-145 [May 1980]) and Chen J. Sound Vibration, 93, 439-455 [1984]). 

Direct Mass Measurement One type of densitometer measures the natural vibration frequency and relates the amplitude to changes in density. The density sensor is a U-shaped tube held stationaiy at its node points and allowed to vibrate at its natural frequency. At the curved end of the U is an electrochemical device that periodically strikes the tube. At the other end of the U, the fluid is continuously passed through the tube. Between strikes, the tube vibrates at its natural frequency. The frequency changes directly in proportion to changes in density. A pickup device at the cui ved end of the U measures the frequency and electronically determines the fluid density. This technique is usefiil because it is not affec ted by the optical properties of the fluid. However, particulate matter in the process fluid can affect the accuracy. 

Pressure Fluctuation Turbulent pressure fluctuations which develop in the wake of a cylinder or are carried to the cylinder from upstream may provide a potential mechanism for tube vibration. The tubes respond to the portion of the energy spectrum that is close to their natural frequency. 

On initial start-up and shut-down the heat exchanger can be subjected to damaging thermal shock, overpressure or hydraulic hammer. This can lead to leaky tube-to-tubesheet joints, damaged expansion joints or packing glands because of excessive axial thermal, expansion of the tubes or shell. Excessive shell side flowrates during the "shake down can cause tube vibrations and catastrophic failure. 

The arrangement of baffle plates and nozzles. Figure 10-96C, are important to prevent (a) tube vibration, (b) maldistribution of the process boiling fluid, and (c) poor heat transfer coefficients due to uneven and stratified flow resulting in uneven and dry spot heat transfer from nonuniform tube wetting, and others. ... 

Chenoweth, J. W. and J. Taborek, Flow-Induced Tube Vibration Data Banks for Shell-and-Tube Heat Exchangers, Heat Transfer Eng,Y. 2, Oct.-Dec. (1980) p. 28. 

Similar considerations apply to the selection of pressure drops where there is freedom of choice, although a full economic analysis is justified only in the case of very expensive units. For liquids, typical values in optimised units are 35 kN/m2 where the viscosity is less than 1 mN s/m2 and 50-70 kN/m2 where the viscosity is 1-10 mN /m2 for gases, 0.4-0.8 kN/m2 for high vacuum operation, 50 per cent of the system pressure at 100- 200 kN/m2, and 0 per cent of the system pressure above 1000 kN/m2. Whatever pressure drop is used, it is important that erosion and flow-induced tube vibration caused by high velocity fluids are avoided. 

Much work has been done on tube vibration over the past 20 years, due to an increase in the failure of exchangers as larger sizes and higher flow-rates have been used. Discussion of this work is beyond the scope of this book for review of the methods used see Saunders (1988) and Singh and Soler (1992). 

See also, the Engineering Science Data Unit Design Guide ESDU 87019, which gives a clear explanation of mechanisms causing tube vibration in shell and tube heat exchangers, and their prediction and prevention. 

When a high-pressure drop is utilised, care must be taken to ensure that the resulting high fluid velocity does not cause erosion or flow-induced tube vibration. 

Figure 8.12. Arrangements of cross baffles and tube-side passes, (a) Types of cross baffles, (b) Rod baffles for minimizing tube vibrations each tube is supported by four rods, (c) Tube-side multipass arrangements.

In transitional flow, the flow switches between laminar and turbulent randomly (Fig. 8-5). It should be kept in mind that laminar flow can be maintained at much higher Reynolds numbers in very smooth pipes by avoiding flow disturbances and tube vibrations. In such carefully controlled experiments, laminar flow has been maintained at Reynolds numbers of up to 100,000. 

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