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Causes and hazards of vibrations There are mainly two causes of vibrations in heat exchanger tubes: one is vibrations induced by external excitation sources, such as the pulsating airflow caused by reciprocating machinery (e.g., reciprocating compressors), or vibrations transmitted through support structures or connecting pipes. Another type is fluid flow excitation, which can be further divided into vibrations induced by the fluid on the tube side and those induced by the fluid on the shell side. Since, under normal conditions, the vibration amplitude induced by flow on the pipe side is small and not very harmful, it can often be ignored; excitation from the pipe side only needs to be considered when the flow velocity is much higher than normal. Vibrations in the heat exchanger are primarily induced by the fluid on the shell side; at normal flow rates, the flow on this side can generate large amplitudes of vibration, which pose the greatest threat to the heat exchange tubes. Research shows that only when the frequency of fluid-induced vibration is identical to or very close to that of the heat exchange element does it cause a sudden and significant increase in the element’s amplitude, leading to its failure. There are mainly two types of damage caused by vibration in heat exchange tubes: tube wear and fatigue fracture of the tube material. Tube wear can be divided into two scenarios: one is that under conditions of large-amplitude vibrations, tubes come into contact with each other and wear down (becoming smoothed and perforated) into a diamond shape; this situation occurs predominantly at the mid-span where the vibrational displacement is greatest. The thermal expansion of pipes increases the likelihood of vibration wear. This is because the thermal deformation of the tube increases the relative movement at the contact point, resulting in hexagonal wear on the tube. The other type is wear caused by relative motion between the pipe and the support plate due to vibration, which gradually thins the pipe wall until it eventually wears through. Furthermore, since the pipe holes in the baffle (support plate) are usually 0.8~1.2 mm larger than the outer diameter of the pipes, the wall of the vibrating pipe may be cut or broken by the baffle (or support plate), a problem that is particularly pronounced when the baffle is thin and its material is harder than that of the pipe. When relaxation and corrosion of the joint occur simultaneously, vibration wear increases, and this type of wear takes on a saddle shape. At the point where the tube passes through the tube sheet, vibration can also cause the sharp edges of the tube hole to cut into the tube. The fatigue fracture of pipes is caused by periodic cyclic excitation (including in cases of resonance or micro-vibration). Because when the tube vibrates, periodic alternating stresses resulting from repeated bending occur. If a pipe is subjected to high alternating stresses over a long period of time, fatigue cracking will occur at the areas of the pipe where the stress is highest. Vibration-induced damage usually occurs at the following locations: (1) in the middle of the support span of the heat transfer pipe fittings, where obvious wear marks appear due to collisions between the pipes ； (2) Right next to the gap in the baffle, the heat exchange tube collides with the baffle and gets worn out ； (3) Inside the tube holes of the baffle plates, when the heat transfer tubes vibrate, the edges of these tube holes can cut or collide with the heat transfer tubes; in severe cases, this can lead to the rupture of the tubes ； (4) Some existing minor cracks or defects in the heat transfer tubes gradually expand due to vibration, eventually leading to failure. Vibration-induced failure of tubes occurs most frequently in cases where the medium in the shell side is gas or steam, and this phenomenon is more pronounced when the operating pressure is above 0.8 MPa. When the medium in the shell side is liquid, tube vibration damage can also occur, but it generally affects only a few tubes in the areas where the fluid flows at high speeds. Vibration prevention measures must be taken from two aspects: reducing the flow velocity of the high-speed fluid in localized areas and altering the natural frequency of the heat exchange elements. The main methods adopted are the following three. (1) Reduce the flow velocity of the fluid in the shell side. When the natural frequency of the heat transfer tubes remains constant, reducing the flow rate in the shell side can prevent resonance from occurring. If the operating conditions cannot be changed, baffles, guide cylinders, or liquid outlet distributors can be installed at the inlet and outlet pipes of the heat exchanger to reduce the flow rates at these locations and minimize fluid pulsations. When vibration prevention measures have been taken for the tube bundle with little effect, it is necessary to consider adjusting the flow rate of the medium from a process perspective. (2) Increase the natural frequency of the tube. Increasing the natural frequency of the pipe can **reduce the likelihood of resonance. The most effective way to increase the frequency is to reduce the span. The natural frequency of a pipe is inversely proportional to the square of its span; if the span is reduced by 20%, the natural frequency can increase by 50%. For U-tube bundles, in order to increase the natural frequency, strips can be wound around the tubes or rods and plates can be inserted to prevent the tubes from moving. (3) Change the form of the baffles to alter the support condition of the heat exchange tubes. Options such as baffle rod-type tube bundles and spiral baffle-type tube bundles can all **improve the support conditions of the tubes.