Leakage occurs at the connection
1. Like a fluid flowing laterally through a single cylindrical object,
2. Turbulent buffeting at pitch diameter ratio
3. Acoustic vibration when steam or gas enters the shell side, it is in contact with the flow direction and;
4. Fluid elastic excitation is first caused by the movement of the pipe;
5. When the fluid flows through a single row of pipes with a pitch diameter ratio less than 1.5; As far as possible, the pipe can be damaged anywhere in the heat exchanger,
and the pipe can be damaged; At ordinary times, the vibration of the equipment shall be closely monitored.
Leakage occurs at the connection with tube sheet;
Strong noise occurs in the shell side; The differential pressure on the shell side increases.
Therefore, only by paying attention to the possibility of vibration during equipment operation in design and manufacturing and taking necessary measures can it be avoided.
The research of some institutions shows that the main causes of vibration induced by transverse flow are as follows.
Karman vortex
As the fluid flows laterally through a single cylindrical object, when it flows through the tube bundle, there is also a Karman vortex behind the tube, as shown in figure 1-111.
When the shedding frequency of the Karman vortex is equal to the natural vibration frequency of the pipe, the pipe will vibrate violently.
Turbulent buffeting
In the dense tube bundle with a pitch diameter < 1.5, there is not enough space, so it is difficult to fall off the Karman vortex. However, the extreme turbulence of the shell side fluid will also induce the vibration of the pipe. The turbulent vortex makes the pipe subject to random wave action. Moreover, turbulence has a fairly wide frequency band.
When a certain frequency in the frequency band is close to or equal to the natural frequency of any vibration mode of the pipe, it will lead to large amplitude pipe vibration.
Acoustic vibration
When the steam or gas enters the shell side, an acoustic standing wave will be formed in the direction perpendicular to the flow direction and the pipe axis.
If the frequency of the acoustic standing wave is consistent with the vortex shedding frequency or turbulence buffeting frequency,
the vibration of the acoustic standing wave will be excited, resulting in strong noise.
Hydroelastic excitation
First, it is caused by the movement of the pipe. If a tube in the tube bundle deviates from the original or stationary position and produces displacement,
it will change the flow field and destroy the balance of forces on the adjacent tubes.
These pipes are subjected to fluctuating pressure and are in a vibration state under their natural vibration frequency.
The excitation frequency is related not only to the flow rate but also to the resonance frequency of the surrounding pipes. That is, fluid-induced vibration is the result of the dynamic interaction between fluid flow and pipe motion.
The vibration of each pipe is closely related to the movement of the surrounding pipes. The fluid elastic vibration belongs to self-excited vibration. Once the vibration starts, the amplitude will increase sharply
Jet conversion
When the fluid flows through a single row of pipes with a pitch diameter ratio of less than 1.5, the emergence of jet pairs can be observed in the wake. If the single row of pipes has enough time to move upstream or downstream alternately, the jet direction will also change.
If the direction of the jet pair changes synchronously with the direction of the pipe movement, in this case, the energy absorbed by the pipe from the fluid is much greater than the energy consumed by the pipe due to damping, and the vibration of the pipe will be intensified.
Generally speaking, when the cross-flow velocity is low, it is easy to produce periodic Karman vortex or turbulent vortex. At this time, both pipe vibration and acoustic vibration may occur in the heat exchanger.
When the cross-flow velocity is high, the vibration of the pipe is generally excited by fluid elasticity, but there will be no acoustic vibration.
conversion
When the cross-flow velocity is very high, the jet conversion will occur and cause the pipe vibration.
As far as possible, the pipe can be damaged anywhere in the heat exchanger. The most likely section of pipe damage is the high-speed flow area, such as the middle span of the largest end support between the two baffle support plates in the tube bundle; Those tubes around the tube bundle in the notch area of the bow baffle;
U-shaped tube bundle and U-shaped elbow area; Pipe under inlet nozzle; Tubes located in the bypass area of the tube bundle and in the flow passage of the tube side split diaphragm;
In the section interface where the tube and the structural components of the heat exchanger have relative movements, such as the interface between the tube and the baffle and the interface between the tube and the tube sheet.
General external reasons, such as failure of spring supports and hangers of pipes conveying fluid, loosening of anchor bolts of the heat exchanger itself, and unstable supporting foundation of equipment, will cause equipment vibration.
The most dangerous thing is that during the process start-up, the pressure rise or load increase is fast, which is easy to cause the vibration of the heating pipe. Especially at the diaphragm,
the vibration frequency of the pipe is high, which is easy to cut off the pipe, resulting in broken pipe leakage. In this case, it must be shut down for disassembly, inspection, and maintenance.
At ordinary times, the vibration of the equipment shall be closely monitored. Strictly control the vibration value to no more than 250 μ m。 If this value is exceeded, it needs to be checked and handled immediately.
Anti-vibration measures
Change flow rate
The vibration can be eliminated by reducing the shell side flow or velocity. But this is often not allowed by production operations.
The common method is to reduce the flow rate by increasing the pipe gap,
especially when the pressure drop needs to be limited in the design, but the shell diameter should be increased.
Changing the arrangement angle of the tube bundle can also reduce the velocity of the fluid in the tube.
The setting of the guide cylinder is an effective measure to prevent fluid from scouring the tube bundle and reduce the flow of shell-side fluid into the tube bundle.
Change the natural vibration frequency of the pipe
The most effective way is to reduce the span of the pipe.
It can be seen from the calculation formula of the natural vibration frequency of the pipe that the span is shortened by one time and the natural vibration frequency is increased by about three times.
Appropriate materials can also be selected to increase the elastic modulus of the pipe.
Increasing the diameter of the pipe to increase the moment of inertia of the section can improve the natural vibration frequency of some pipes, but it has little practical significance.
Inserting slats or rods into the gap of the pipe to limit the movement of the pipe can increase the natural vibration frequency of the pipe. This method can effectively prevent vibration in the U-shaped tube area of the heat exchanger.
Without laying tubes at the gap of the baffle, each baffle can support all tubes. Compared with the tubes in the heat exchanger with baffle, the span of the tubes in the central part is shortened by twice, which greatly increases the natural vibration frequency.
Moreover, a support plate can be set between the baffles to further increase the rigidity of the pipe, which has no substantive impact on heat transfer and pressure drop.
Reducing the gap between the pipe and the baffle hole and thickening the baffle can not substantially change the natural vibration frequency of the pipe,
but it can reduce the sawing effect of the pipe by the baffle and increase the damping of the system. If the material of the baffle is softer than that of the pipe, the damage can sometimes be reduced.
Set silencing diaphragms,
and set longitudinal diaphragms parallel to the pipe axis on the shell side, which can effectively reduce the noise: the position of the diaphragms shall be away from the parking lot
The node is close to the antinode.
Restrain the influence of periodic vortex
Winding a metal wire around the outer surface of the pipe or setting a metal strip along the axis can suppress or weaken the influence of periodic vortexes and reduce the alternating force acting on the pipe. If the baffle rod is used to replace the traditional baffle plate,
it can not only prevent vibration but also strengthen the conversion and reduce dirt and shell side pressure drop (see figure 1-112).
Causes of vibration of shell and tube heat exchanger
1. The vibration frequency of the tube bundle is close to the natural frequency of the gas flow
2. The baffle spacing is not set properly
3. If the operating environment is improper, adjust the operating parameters and replace it with superheated water
4. The steam carries water, and the steam produces a water hammer in the heat exchanger.
5. The lack of support for the steam pipeline and unreasonable pipeline design leads to the vibration of heat exchangers caused by pipeline vibration.
6. Excessive steam temperature and quantity will cause partial vaporization of the tube side, resulting in a water hammer.
acoustic resonance
When the exciting frequency of the fluid is close to the natural frequency of the column vibration of the air in the heat exchanger, acoustic resonance will occur in the heat exchanger.
The reason is that under certain conditions, the vortex separation of the Karman vortex will arouse a certain order of standing waves between the chamber walls.
This standing wave reflects back and forth between the tubes and shells and continuously propagates energy, but the Karman vortex continuously inputs energy.
When the ratio of Karman vortex frequency FV to acoustic standing wave frequency FA is in the range of 0.8 ~ 1.2,
strong acoustic resonance and noise may occur in the air chamber.
When the shell side fluid is a liquid, this vibration will not occur due to the extremely high sound speed of the liquid.
prevention and effective utilization of vibration
The mechanism of fluid-induced vibration in heat exchangers is quite complex, and the complete design criterion that can effectively prevent vibration has not been established.
This requires different measures to prevent the vibration of the heat exchanger according to different operating conditions. Vibration is inevitable, but slight vibration will not bring damage,
but also strengthen heat transfer and reduce scaling.
However, in case of strong vibration, necessary anti-vibration measures should be taken to slow down the vibration and avoid vibration damage to the heat exchanger.
The fundamental way to resist vibration is to avoid the natural frequency of the pipe as far as possible. The following anti-vibration measures are often adopted in engineering practice:
(1) Formulate reasonable start-up and shutdown procedures, strengthen online monitoring, strictly control the operating conditions, and set buffer plate or guide cylinder in front of the fluid inlet,
which can not only avoid the direct impact of fluid on the tube bundle, and reduce the flow rate, but also reduce the fluid pulsation.
(2) Reducing the fluid velocity on the shell side of the shell and tube heat exchanger is the most direct method to prevent tube bundle vibration. Because when the natural frequency of the heat transfer element remains unchanged,
reducing the flow rate can reduce the frequency of fluid pulsation, so as to avoid resonance, but at the same time, the heat transfer efficiency will also be reduced.
(3) Improving the natural frequency of the heat transfer element is another key factor to prevent vibration.
Reducing the span and effective mass and increasing the elastic modulus and moment of inertia of the material can improve the natural frequency of the heat transfer element.
Appropriately increase the thickness of the pipe wall, the diameter of the circular pipe, and the thickness of the baffle plate.
The pipe hole on the baffle plate shall be closely matched with the pipe, the gap shall not be too large, and the structural design can be optimized.
(4) Change the tube bundle support form and adopt new longitudinal flow tubes bundle support, such as baffle rod type, hollow ring type, full circular special-shaped hole baffle plate,
and baffle belt or baffle rod can also be used to replace baffle plate. These methods can effectively prevent tube bundle vibration.
Conclusion
It is better to prevent vibration problems in advance, rather than correct them after vibration occurs. This requires us to fully consider various factors in the design process. Only in this way can we make the designed products more perfect and the operation more safe and reliable.
(1) Under the influence of transverse velocity in the tube bundle, the tube of shell and tube heat exchanger will vibrate.
If the amplitude is large, one or more hazards may occur (the pipe wall is thinned due to repeated mid-span vibration between baffle support plates)
(2) The pipe interface is worn due to collision with the baffle,
(3) Fatigue or corrosion fatigue due to high wear rate.
(4) Excessive shell side pressure drop, because the pipe vibration needs to absorb energy,
(5) serious pressure corrosion;
The most likely section of pipe failure is the middle span of the largest end support between the two support plates in the pipe bundle;
U-bend area in U-tube bundle; The pipe under the inlet connecting pipe is located in the side flow area of the tube bundle and the pipe side split diaphragm channel.
Turbulence induces vibration. The higher flow velocity in the tube bundle promotes heat transfer in the fluid, but the heat exchange tube responds to the vibration caused by turbulence in a random way.
In addition, flow turbulence will also promote and strengthen the formation of other vibration
mechanisms, such as vortex separation upstairs. The turbulent flow has greater randomness. When the central dominant frequency in the flow field is consistent with the lowest natural frequency of the tube bundle, Will vibrate, that is, resonance, causing damage.
Causes of vibration of shell and tube heat exchanger
1. The spacing between baffles is set improperly so that the vibration frequency of the tube bundle is close to the natural frequency of airflow
2. The steam carries water, and the steam produces a water hammer in the heat exchanger.
3. The lack of support for the steam pipeline and unreasonable pipeline design leads to the vibration of heat exchangers caused by pipeline vibration.
4. Excessive steam temperature and quantity will cause partial vaporization of the tube side, resulting in a water hammer.
Anti-vibration measures:
1. Change the flow rate: reduce the flow on the shell side, replace the single shell side with the split shell side, and replace the single conformal baffle with the double bow baffle, which can reduce the cross-flow velocity and prevent vibration. However, the heat transfer efficiency will change.
2. Change the natural frequency of the heat exchange tube:
① Reduce the span of the shell and tube heat exchange tube,
② No pipes are arranged in the notch area of the baffle;
③ Without affecting the cross-flow velocity, a support plate should be added between the baffles;
④ The elbow section of the U-line pipe is provided with a support plate or support strip.
3. Insert a longitudinal partition along the direction parallel to the airflow on the shell side to reduce the characteristic length, improve the audio frequency and prevent acoustic vibration.
4. Rod or strip support is used instead of a baffle.
When the elastomer is disturbed, it will vibrate. The tube bundle, diaphragm, pull rod, and shell of the shell and tube heat exchanger are elastomers, which tend to be disturbed and cause vibration. Among these elastomers,
the rigidity of the tube bundle in the structure is the smallest and is most likely to be excited by vibration. The vibration problem of the shell and tube heat exchanger we study refers to the vibration of the tube bundle.
The vibration of the tube bundle is caused by interference force or excitation force. These exciting forces can be divided into two categories: mechanical exciting forces and fluid-induced exciting forces.
The excitation frequency can generally be predicted accurately and corresponding measures can be taken to prevent it.
1. Mechanical exciting force: the vibration transmitted through the support and pipeline connected to the heat exchanger, as well as the pulsating exciting force brought by reciprocating fluid conveying machinery (such as air compressor). Mechanical excitation force
2. Exciting force caused by fluid flow: this exciting force can be divided into exciting force caused by fluid longitudinal flow and exciting force caused by fluid transverse flow.
In the actual analysis, it is known that the excitation force caused by the longitudinal flow of fluid is ignored because of its small amplitude and little harm.
Therefore, the emphasis is placed on the study of the exciting force caused by the transverse flow of fluid. The excitation mechanism of transverse flow is complex.
2.1. Mechanism of fluid excitation:
(1) When the fluid flows through the back of the cylinder alternately, the vortices form on the back of the cylinder
The reason for the vortex is that after the fluid is blocked, the kinetic energy and pressure energy are converted to each other,
and the pressure changes along the circumference of the cylinder and the thickness direction of the boundary layer.
The larger pressure of the fluid outside the boundary layer forces the particles with smaller pressure inside the boundary layer to flow in the opposite direction,
so as to thicken the boundary layer, form a vortex, and then separate from the surface of the cylinder,
The vortex is elongated and disappears with the increase of velocity.
When the vortex on one side grows and separates, the vortex on the other side is forming and grows. In this way, two rows of vortex wake are formed alternately, which looks like a “vortex street”.
(2) Turbulence buffeting)
Formation mechanism: when the fluid flows laterally through the tube bundle and flows in the curved channel under the action of the baffle,
it will produce a variety of random turbulence or turbulence, resulting in random turbulence pressure fluctuation.
This turbulence pressure fluctuation is a random excitation force in a wide frequency range. The dynamic response of the pipe has frequency selectivity.
It will absorb the part of the vibration energy corresponding to its own frequency in the broadband random excitation force caused by turbulent pressure fluctuation to produce vibration.
This phenomenon is called turbulent buffeting or turbulent buffeting.
(3) Elastic excitation
Formation mechanism: fluid elastic excitation is caused by the disturbance of the pipe. When a tube in the tube bundle deviates from its original position and displaces instantaneously,
it will change the condition of the flow field,
and destroy the force balance state on the adjacent tubes through the action of fluid elastic force,
so that these tubes are in the vibration state corresponding to their natural frequency.
When the transverse flow velocity of the fluid reaches a certain critical value,
the work done by the total elastic force of the flow on the tube bundle will be greater than the work consumed by the damping effect of the tube bundle,
so that the tube begins to vibrate with large amplitude. This vibration is called fluid elastic excitation. The fluid cross-flow velocity that causes the pipe to vibrate with large amplitude is called the critical cross-flow velocity.
(4) Acoustic resonance 1
Generation mechanism: when the airflow stably flows laterally through the tube bundle to form vortex separation,
a longitudinal wave with periodic pressure changes perpendicular to the airflow direction and the tube axis direction will be generated.
This longitudinal wave is reflected and propagated in the shell wall of the heat exchanger,
which may form a certain order of acoustic standing waves between the shell walls.
Acoustic resonance 2
This acoustic standing wave is reflected back and forth on the shell wall and continuously absorbs the energy of Karman vortex street and turbulent buffeting.
When its frequency is coupled with the frequency of Karman vortex street and the natural frequency of the pipe or shell,
it will produce strong acoustic resonance and noise. This phenomenon is called acoustic resonance.
Damage caused by tube bundle vibration
The vibration of tube bundles is a common problem.
Almost all heat exchanger tube bundles will produce large or small vibrations,
but the most harmful ones generally occur in some large heat exchangers, especially those with gas or steam shell side.
Because the baffle spacing of such a heat exchanger is large, the shell side velocity is high, and the damping of gas or steam is small.
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