Heat exchanger vibration
As the scale of production increases, the size of the heat exchanger, the flow rate of the fluid and the span of the support all increase, even beyond the permissible limits, thus reducing the rigidity of the tube bundle and increasing the potential for vibration.
Vibration can cause tube leakage, wear, fatigue, fracture and even accompanying ear-splitting noise, which not only reduces the life of the equipment, but is also detrimental to people's health. Once an incident has occurred, vibration often takes a long time to analyse and repair. Due to the complexity of the factors affecting vibration, the difficulty in estimating the magnitude of the damping effect and the difficulty in determining the rate of tube wear and damage, which cannot yet be described by simple mathematical equations, it can be said that the theoretical calculation methods used so far cannot be used to analyse vibration accurately in engineering practice. The existing codes for
Heat Exchangers also lack clear regulations on vibration analysis methods and design guidelines for vibration prevention. However, it has been proven that if the existing research results can be used to make the necessary estimates and analyses of vibration during design, and some vibration prevention measures can be taken, then some destructive vibrations can mostly be avoided.
Causes of fluid-induced vibration
The tube bundle of the heat exchanger belongs to the elastomer and is disturbed by the fluid flowing through it, leaving its equilibrium position, the tube produces vibration, this vibration is called flow-induced vibration. In fact, each heat exchanger in the work of more or less vibration, the source of vibration may be the shell side or tube side of the fluid flow caused by vibration; fluid velocity fluctuations or pulsation caused by vibration; through the pipe or support propagation of power mechanical vibration and so on. Sometimes the sources of vibration may be more, and one or more of them may be the main source of vibration. There are sources of vibration that are relatively easy to predict, while fluid-induced vibration is more difficult to anticipate.
Some experimental and operational experience has shown that vibrations in heat exchangers are mainly caused by the flow of fluid on the shell side, with vibrations caused by fluid flow on the tube side often negligible. In general, in the shell-side fluid, the longitudinal flow parallel to the direction of the tube axis is excited by the small amplitude of the vibration, the vibration caused by the probability of structural damage, but also much smaller than the transverse flow. Therefore, it is the vibration caused by transverse flow that is of greater concern.
Three different causes of fluid-induced vibration have been recognised: vortex shedding, turbulent jittering and fluid elastic rotation (or fluid elastic instability).
(1) Vortex shedding
When the fluid flows transversely through a single cylinder, at a large Reynolds number, the tube after the tail flow in the formation of the Carmen (karman) vortex (or Carmen vortex street) so that the two columns of vortices in opposite directions alternate off periodically, resulting in a certain shedding frequency. When the fluid flows laterally through the bundle, the same karman vortex is created behind the bundle and for small pitch bundles this phenomenon only occurs in the first few rows at the periphery of the bundle, for large pitch bundles it can occur throughout the bundle. When vortex shedding, the fluid exerts an alternating positive and negative force on the circular tube, the frequency of this force is the same as the vortex shedding frequency, which causes the circular tube to vibrate perpendicular to the direction of flow at or near the vortex shedding frequency. If the vibration frequency of the circular tube is equal to a multiple or approximate of the vortex shedding frequency, the vortex is shed uniformly at the same time along the full span of the cylinder (circular tube) at the same frequency, with the shedding frequency and vibration frequency synchronised, which is known as resonance.
The vortex shedding itself can also produce a certain sound. This is due to the fact that under certain conditions it will excite a standing wave of some order between the two walls of the gas chamber, perpendicular to both the tube and the direction of flow, as shown in the diagram below. This standing wave is reflected back and forth between the walls where the tube bundle is located and does not spread energy outwards, while the vortex shedding is constantly inputting energy. When the standing wave frequency and the vortex shedding frequency are coupled, a strong acoustic standing wave vibration of the gas chamber is induced - the gas vibration - which generates a lot of noise.
(2) Fluid elastic rotation
When a gas flows laterally through a tube bundle, the fluid forces generated by the asymmetry of the fluid can cause a tube in the bundle to be instantaneously displaced from its original position, thus alternating the flow field and destroying the equilibrium to the adjacent tubes, causing them to also be displaced and in a state of vibration. If there is not enough damping to dissipate the energy, the amplitude will increase until the tubes hit each other and cause damage, such vibration is called fluid elastic vibration. Unlike the former, vortex shedding is an unstable phenomenon that occurs behind the tube and causes the tube to vibrate. It is a hydrodynamic phenomenon that does not depend on the tube motion at all, while fluid elastic rotation is not determined by any unstable phenomenon, but is generated by the interaction of the flow fields of adjacent tubes.
(3) Turbulent jitter
Turbulent flow of fluid in all directions in a wide range of frequencies have random fluctuation components, when the fluid downstream or lateral flow around the outside of the tube, these turbulent components to the tube transfer energy, resulting in random vibration of the tube, this kind of turbulence generated by the shell side of the fluid flow through the tube bundle caused by the vibration of the tube, is the most common form of vibration, when the main frequency of this turbulent fluctuations and the tube's intrinsic frequency coincide, then A typical resonance occurs. If the fluid on the shell side is a gas, the main frequency of the turbulent jitter vibration may also produce acoustic resonance at a certain velocity.
The above three studies show that the vibration of the tube bundle is closely related to both the tube's intrinsic frequency and the acoustic vibration frequency of the gas chamber.
Vibration prediction and prevention
The hazards caused by vibration are so great that consideration should be given at design time to minimise the potential for fluid induced vibration. Eliminating all possibilities of excitation of the heat exchanger tube bundle is the most fundamental way to prevent vibration, and therefore predicting or calibrating the vibration of shell and tube heat exchangers should be done as an important part of ensuring the safe operation of the heat exchanger.
However, vibration does not necessarily cause mechanical damage, and many heat exchangers vibrate but do not have accidents. This does not mean, of course, that vibration can be ignored. When the predicted results are likely to occur vibration, some of the following vibration prevention and damping measures can be taken.
(1) Reduce the flow rate on the shell side. If the flow rate on the shell side is constant, the pipe distance can be increased. This method is more feasible when there is a pressure drop limit in the design, but it will increase the shell diameter or increase the length of the tube.
If the original single inlet and outlet located at both ends of the shell (fluid flow through the shell at once around the folding plate) into the inlet in the middle, the outlet at both ends of the split-flow heat exchanger, the fluid will be divided into two halves from either end of the shell outflow, as shown in the figure below, can greatly reduce the cross-flow velocity.
(2) Increase the inherent frequency of the tube. The inherent frequency of the tube is inversely proportional to the square of the support span, so reducing the support span of the tube is the most effective way to increase the inherent frequency of the tube.
If the tube is not lined up at the notch of the bowed baffle, those spans that were originally supported only at each interval of the baffle can be shortened, increasing the inherent frequency. This method is claimed to be the most effective solution to the vibration problem and is shown in the diagram below. If required, an intermediate support plate (a support plate with both ends cut away) can be added between the two folded plates, which has no effect on the pressure drop but has a beneficial effect on heat transfer. The intrinsic frequency can also be increased by changing the tube or increasing the wall thickness, but the effect is not significant.
(3) Increase the sound vibration frequency. Insert vibration dampening plate in the shell, so that its width direction parallel to the direction of cross-flow and its length direction parallel to the axis of the tube, so that the sound vibration frequency can be increased, so that it is not consistent with the frequency of vortex shedding and turbulent jitter vibration. The position of the vibration dampening plate should be on the wave belly of the sound vibration standing waveform.
(4) from the structure, increase the thickness of the folded plate or intermediate support plate, when the hole gap is certain, can reduce the shear effect on the tube and increase the damping of the system. Machining chamfers on both sides of the baffle plate bore has a certain effect on reducing the damage of vibration.
In addition to structural attention to avoid vibration, attention must also be paid to the operating heat exchanger with regard to some of its influential factors, for example: not allowing the shell diameter flow rate to exceed the limits allowed by the vibration analysis, even for short periods of time, is detrimental to the service life of the heat exchanger.
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