Causes and prevention of tube bundle vibration of heat exchanger
Vibration of heat exchanger
With the expansion of production scale, the size of heat exchanger, fluid flow rate and support span increase, even exceeding the allowable limit, thus reducing the rigidity of tube bundle and increasing the possibility of vibration.
Vibration can make the pipe leak, wear, fatigue, fracture, and even accompanied by harsh noise, which not only reduces the service life of the equipment, but also damages people’s health. Once an accident is caused by vibration, it often takes a long time to analyze and repair.
Because the factors affecting vibration are complex, the magnitude of damping effect is difficult to accurately estimate, and the speed of pipe wear and failure is difficult to determine. They can not be described by simple mathematical formulas. It can be said that the theoretical calculation methods so far can not be used to accurately analyze vibration in engineering practice.
In the existing specifications for heat exchangers, there are no clear provisions on vibration analysis methods and anti vibration design criteria. However, practice has proved that if the existing research results can be used to estimate and analyze the vibration, and some anti vibration measures can be taken, some destructive vibration can be avoided.
Causes of fluid induced vibration
The tube bundle of the heat exchanger belongs to elastomer, which is disturbed by the flowing fluid and leaves its equilibrium position. The tube vibrates. This vibration is called flow induced vibration. In fact, each heat exchanger has more or less vibration during operation, and its vibration source may be the vibration caused by fluid flow on the shell side or tube side; Vibration caused by fluctuation or pulsation of fluid velocity; Power transmitted through pipes or supports, mechanical vibration, etc. Sometimes there may be many vibration sources, and one or several of them may be the main source of vibration. Some vibration sources are relatively easy to predict, while fluid induced vibration is difficult to predict.
Some experiments and operation experience show that the vibration of heat exchanger is mainly caused by the fluid flow on the shell side, and the vibration caused by the fluid flow on the tube side can often be ignored. In general, in the shell side fluid, the vibration amplitude excited by the longitudinal flow flowing parallel to the tube axis is small, and the probability of structural damage caused by vibration is much smaller than that of transverse flow. Therefore, people are more concerned about the vibration caused by transverse flow.
At present, it has been recognized that three different causes of fluid induced vibration are vortex shedding, turbulent buffeting and fluid elastic rotation (or fluid elastic instability).
(1) Vortex shedding
When the fluid flows laterally through a single cylinder, at a large Reynolds number, the Karman vortex (or Karman vortex street) formed in the wake behind the pipe makes the two rows of vortices with opposite directions fall off periodically and alternately, resulting in a certain shedding frequency. When the fluid flows laterally through the tube bundle, Carmen vortex will also be generated behind the tube bundle. For small spacing tube bundles, this phenomenon only occurs in the first few rows around the tube bundle, and for large spacing tube bundles, it can occur in the whole tube bundle. During vortex shedding, the fluid applies a positive and negative alternating force to the circular pipe, and the frequency of this force is the same as the vortex shedding frequency, which will make the circular pipe vibrate perpendicular to the flow direction with the vortex shedding frequency or its similar frequency. If the vibration frequency of the circular tube is equal to the multiple or divisor of the vortex shedding frequency, the vortex falls off evenly at the same frequency at the same time along the full span of the cylinder (circular tube), and the shedding frequency is synchronized with the vibration frequency, which is the so-called resonance.
Vortex shedding itself can also produce a certain sound. This is because under certain conditions, it will excite the air chamber
There is a standing wave between the two walls, which is perpendicular to both the pipe and the flow direction, as shown in the figure below. This standing wave is reflected back and forth between the walls where the tube bundle is located, and does not propagate energy outward, while the vortex shedding continuously inputs energy. When the standing wave frequency is coupled with the vortex shedding frequency, it will induce a strong acoustic standing wave vibration of the air chamber – air vibration, resulting in great noise.
(2) Fluid elastic rotation
When the gas flows laterally through the tube bundle, the fluid force generated by the asymmetry of the fluid can make a tube in the tube bundle move instantaneously from its original position, so the flow field changes alternately, which destroys the balance of adjacent tubes and makes them also move and vibrate. If there is not enough damping to consume its energy, the amplitude will continue to increase until the pipes collide with each other and cause damage. Such vibration is called fluid elastic vibration. Different from the former, vortex shedding is an unstable phenomenon that causes pipe vibration behind the pipe. It is a hydrodynamic phenomenon that does not depend on the motion of the pipe. The elastic rotation of the fluid is not determined by any unstable phenomenon, but due to the interaction of the flow fields of adjacent pipes.
(3) Turbulent buffeting
The turbulent fluid has random wave components in a wide frequency range in all directions. When the fluid flows downstream or laterally around the tube, these turbulent components transmit energy to the tube, resulting in the random vibration of the tube. The vibration of the tube caused by the turbulence of the fluid on the shell side flowing through the tube bundle is the most common form of vibration, When the dominant frequency of the turbulent wave coincides with the natural frequency of the tube, a typical resonance occurs. If the fluid on the shell side is gas, the dominant frequency of turbulent buffeting may also produce acoustic resonance at a certain speed.
The above three studies show that the vibration of the tube bundle is closely related to the natural frequency of the tube and the acoustic vibration frequency of the air chamber.
Prediction and prevention of vibration
The harm caused by vibration is great, so the possibility of fluid induced vibration should be considered to minimize in the design. Eliminating all the possibility of excitation of heat exchanger tube bundle is the most fundamental way to prevent vibration. Therefore, the prediction or verification of vibration of shell and tube heat exchanger should be done as an important link to ensure the safe operation of heat exchanger.
However, vibration does not necessarily cause mechanical damage. Although many heat exchangers have vibration, there is no accident. Of course, this does not mean that you can turn a blind eye to vibration. When the predicted results may cause vibration, the following anti vibration and vibration reduction measures can be taken.
(1) Reduce the flow rate on the shell side.
If the shell side flow remains unchanged, the pipe distance can be increased. When there is pressure drop limit in the design, this method is feasible, but it will increase the shell diameter or increase the pipe length.
If the single inlet and outlet at both ends of the shell (the fluid bypasses the baffle and flows through the shell at one time) is changed into a split flow heat exchanger with the inlet in the middle and the outlet at both ends, the fluid is divided into two parts and flows out from either end of the shell, as shown in the figure below, the cross flow speed can be greatly reduced.
(2) Increase the natural frequency of the pipe.
The natural frequency of the pipe is inversely proportional to the square of the support span, so reducing the support span of the pipe is the most effective way to increase the natural frequency of the pipe.
If the tubes are not arranged at the notch of the bow baffle, the spans that are supported by each baffle can be shortened and the natural frequency can be improved. It is said that this method can most effectively solve the vibration problem, and its structure is shown in the figure below. If necessary, an intermediate support plate (the support plate cut off at both ends, as shown in the elevation) can be added between the two baffles, which has no effect on the pressure drop and is beneficial to heat transfer. The natural frequency can also be increased by changing the pipe or increasing the pipe wall thickness, but the effect is not great.
(3) Increase the sound and vibration frequency.
Insert a damping plate into the shell so that its width direction is parallel to the cross flow direction and its length direction is parallel to the pipe axis, which can improve the acoustic vibration frequency and make it inconsistent with the frequency of vortex shedding and turbulent buffeting. The position of the damping plate shall be on the antinode of the acoustic vibration standing wave waveform.
(4) Structurally, increase the thickness of baffle or intermediate support plate
When the clearance of the hole is constant, the shear effect on the pipe can be reduced and the damping of the system can be increased. Chamfering on both sides of the baffle tube hole plays a certain role in reducing the damage of vibration.
In addition to avoiding vibration from the structure, attention must also be paid to some factors affecting the heat exchanger in operation, such as not allowing the shell diameter flow rate to exceed the allowable limit of vibration analysis. Even short-term overspeed is unfavorable to the service life of the heat exchanger.
Source: China Heat Exchanger Tubes Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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