Protection against vibrations in heat exchangers

Thread Content

There are mainly two reasons for vibration in heat exchanger tubes: one is vibration caused by external excitation sources, such as the pulsating airflow from reciprocating machinery (like reciprocating compressors), or vibrations transmitted through support components 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. In general, the vibration amplitudes induced by flow along the pipe wall are small and pose little hazard, so they can often be neglected. Only when the flow velocity is significantly higher than the normal value does one need to consider such flow-induced vibrations. 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 cause large amplitudes of vibration, which poses the greatest threat to the heat exchange tubes. Research shows that only when the frequency of the fluid-induced vibration is identical to or quite close to that of the heat exchange element does it cause a sudden and significant increase in the element’s amplitude, leading to its destruction. 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 types: one is wear that occurs when the tubes come into contact with each other due to high-amplitude vibrations, resulting in a diamond-shaped pattern of wear and leakage; this type of wear mostly occurs at the middle span where the vibration 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. Another scenario is wear occurring due to the relative movement of the tube and the support plate as a result of vibration, causing the tube wall to gradually thin out 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 pipes. When relaxation and corrosion of the joint occur simultaneously, vibration wear increases, and this type of wear takes on a saddle shape. At the points where the tubes emerge from the tube sheet, the sharp edges of the tube holes can also cut into the tubes due to vibrations. 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 fracture will occur at the areas of the pipe where the stresses are highest. The locations where vibration-induced damage occurs are generally as follows: (1) At the midpoint of the support span of heat transfer tubes; due to mutual collisions between the tubes, obvious wear marks can be observed on their surfaces ； (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 inside the shell side is a liquid, tube vibration-induced damage can also occur; however, it generally affects only a few tubes in the locally high-velocity regions of the fluid. 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 velocity 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 velocity at those locations and minimize fluid pulsations. When anti-vibration measures have been taken for the tube bundle but with little effect, it is advisable to adjust the flow velocity of the medium from a process standpoint. (2) Increase the natural frequency of the pipe. 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 between the tubes, or rods and plates can be inserted to prevent tube movement. (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.