Protection against heat exchanger vibration

Heat exchanger tube vibration causes are mainly two kinds: one is the vibration caused by external excitation sources, such as reciprocating machinery (such as reciprocating compressors) pulsating airflow caused by the excitation, or through the support components or connecting the vibration transmitted by the pipeline.

The other is the fluid flow excitation, and can be divided into the tube side and shell side of the fluid excitation vibration.

As a result of the general situation of the tube side of the flow-induced vibration amplitude is small, not harmful, often can be ignored, unless the flow rate is much higher than the normal flow rate, the tube side of the vibration to be considered.

The vibration in the heat exchanger is mainly shell-side media excited by the shell-side flow at normal flow rate may trigger a large amplitude, the greatest harm to the heat exchanger tube.

Research shows that only when the fluid-induced vibration frequency and the frequency of the heat exchanger element consistent or quite close to the component will make a sudden and substantial increase in amplitude, which leads to its destruction.

There are two main cases of heat exchanger tube vibration damage, namely tube wear and fatigue fracture of the tube material.

Tube wear and tear is divided into two cases: one is in the case of vibration of great amplitude, the tube and tube contact with each other and wear (grinding flat through the leakage) into a diamond shape, the vast majority of this situation arises in the middle span of the largest vibration displacement. The thermal expansion of the tube increases the possibility of vibration wear.

This is due to the thermal deformation of the tube expanding the relative motion that causes contact, resulting in hexagonal wear of the tube.

Another type of wear occurs between the tube and the support plate as a result of relative motion due to vibration, leading to gradual thinning of the tube wall and eventual wear through.

In addition, because the hole on the folding plate (support plate) is usually 0.8~1.2mm larger than the outer diameter of the tube, the wall of the vibrating tube may be cut and fractured by the folding plate (or support plate), and it is especially prominent when the folding plate is thin and the material is harder than the tube.

In the case of joint relaxation and corrosion at the same time, the vibration wear increases, and this wear has a saddle shape. The sharp edges of the tube holes also cut into the tube at the point where the tube penetrates the plate due to vibration.

Fatigue fracture of the tube is then caused by cyclic excitation of the cycle (including in the case of resonance or micro-vibration).

This is because when the tube vibrates, periodic alternating stresses of repeated bending action occur. If the tube is subjected to very strong alternating stresses for a long time, fatigue rupture will occur in some of the highest stress areas of the tube.

Vibration damage generally occurs at the following locations.

(1) At the middle of the support span of the heat transfer tubing, where the appearance shows obvious abrasion due to the mutual collision between the tubes.

(2) Immediately adjacent to the folded plate gap, the heat transfer tube and folded plate collision and wear.

(3) inside the bending plate hole, the heat transfer tube vibration bending plate hole edge of the heat transfer tube sawing, collision, serious will lead to tube fracture;.

(4) some small cracks or defects in the original heat transfer tube, which gradually expand due to vibration and eventually lead to damage.

Tube vibration damage mostly occurs in the shell process media is gas or steam occasions, operating pressure higher than 0. 8MPa is more obvious. Shell process when the medium is liquid, will also occur when the tube vibration damage, but generally limited to a few tubes in the fluid local high-speed area.

Anti-vibration measures

Anti-vibration measures must start from two aspects, namely, to reduce the local high-speed fluid flow rate and change the inherent frequency of the heat transfer element. The main methods taken are the following three.

(1) reduce the flow rate of fluid in the shell process.

When the inherent frequency of the heat transfer tube is certain, reduce the shell process flow rate, you can avoid excitation resonance, if the operating conditions can not be changed, the heat exchanger can be designed at the import and export of the tube anti-punch plate, guide cylinder or liquid export distributor, etc., to reduce the shell process at the import and export of the flow rate, so that the value of fluid pulsation is reduced to a minimum. When the tube bundle has been taken to prevent vibration measures, and the effect is not obvious, the process should consider adjusting the flow rate of the medium.

(2) Improve the inherent frequency of the tube.

Improving the inherent frequency of the tube can greatly reduce the chance of resonance. The most effective way to improve the frequency is to reduce the span. The inherent frequency of the tube and the square of the span is inversely proportional, if the span is reduced by 20%, the inherent frequency can be increased by 50%. For U-shaped tube bundle, in order to improve the inherent frequency, can be wound between the tubes with strips or inserted into the rod, plate to stop the tube movement.

(3) Change the form of the folding plate to change the support condition of the heat exchanger tube.

Such as bending rod type bundle, spiral bending plate type bundle, etc., can greatly improve the support conditions of the tube