Analysis of the Causes of the Explosion of the Air Compressor Impeller in the Experience Exchange Hou Feng, Xu Hong, Guan Kaishu, Ye Guoqing, East China University of Science and Technology, Shanghai 2∞237 Impeller cracking causes fatigue cracking, impeller welding defects produce welding craters, causing surface stress concentration, Airflow excitation accelerates crack propagation 1 Introduction A petrochemical company power plant air separation air compressor station and chemical fiber plant air compressor refrigeration station each line 4 sets 29.50.98 centrifugal compressor 240 type centrifugal compressor is a single intake double shaft stage The gear-increasing isothermal compressor, that is, a cooler between the stages, makes the compression process as close as possible to isothermal compression. The structure is roughly divided into a 1 main body air cooling system including a gear speed increasing device and a phase running system.
The Type 240 compressor has a total of four impellers, each stage is a section, a total section compression. The impeller is welded by chrome-nickel medium-gold steel. Class 1.2 is a meta-impeller, and Class 3 and 4 are elemental impellers.
The quality air enters the 1 volume flow 124, the inlet pressure is, the 9 core inlet temperature 321 discharge temperature is about 40, but the rear axle power 4001 tear speed gear is the receiving date 250303 revision date, 20,05 defense is 190337, 1 critical speed stage rotor is 2344., 1 s and turn ten is 29742 heart 240 type compressor stage impeller structure 1.
Since the eight sets of 40-type centrifugal compressors have been put into use, they have occurred several times in two years, which seriously affects normal production. among them.
The most serious fault is that the impeller has cracked and dropped blocks, especially the second-stage impeller has many failures. Due to the cracking of the impeller, the rotor dynamic balance is unbalanced, causing the bearing bush to damage the shaft head pump and damage the tooth surface damage. In this paper, the impeller cracking is analyzed, and the failure mechanism and cracking reason of the impeller are found out.
2 The macroscopic morphology of the crack is analyzed for a certain stage of the impeller, and the outer diameter of the impeller is not cracked by the cracking part 2. From a macroscopic point of view, the impeller is cracked at many places and there is a partial block phenomenon. From the cracking situation, most of the cracking is located at the outer edge of the impeller wheel. The crack originates from the welding heat affected zone between the wheel cover and the blade, and the blade of some parts of the impeller also cracks. The sampling of the crack line saw that the crack on the inner surface of the wheel cover is longer than the crack on the outer surface, indicating that the crack is from the inside of the wheel, and the expansion of the crack extends from the radial direction to the circumferential extension. .
A circumferential crack is seen outside the wheel cover away from the edge, which is a crack that expands in the radial direction to a constant depth after the wheel is cracked and spreads to the outside of the wheel cover.
The chemical composition analysis result of the failed impeller is as follows.
It can be seen from 1 that the chemical composition of the fan impeller wheel cover disc meets the standard composition requirements of 340, and the chemical composition of the impeller disc and the wheel cover material is not significantly related to 3435 steel.
The project wheel cover wheel 3401 view, the standard component 3 fracture analysis has a radial pattern on the fracture surface. The crack extends from the inside of the impeller wheel cover to the core. The fracture surface is smooth, the wall thickness is not thinned, and there is no obvious plastic deformation except for the tear at the last fracture. Look at the above characteristics. Cracking is a brittle fracture feature but not a weld cold crack. The weld cold crack propagation has no radial pattern. That is, brittle fracture occurs during use and is a species with low stress and brittle fracture.
From the outer edge of the impeller, the crack surface of the non-site is analyzed by scanning electron microscopy. ; 3 is the crack shape. 341 is to enlarge the crack initiation point morphology of different multiples. The cracking point is on the inner side near the outer edge of the impeller. It can be seen that there is a large gap in the section. The defect is the core of the radiation stripes. Indicate that this is the cracking point. From the direction of the radiation pattern, the crack of the dish is extended from the inner farmer to the outside and expands to the heart at the same time. Radiation of the wooden end, wide fatigue shell line. 7 is brittle fracture 3, 3 is the crack at the point where the radiation pattern is enlarged and the inner surface is cracked, and then expanded to the outside. It also extends toward the core of the impeller along the heat affected zone at the junction with the blade. 3 The crack at the cracking site is fine, the fatigue expansion rate is slow, and the crack occurs. The impeller lost its balance. The vibration is intensified. The crack spread rapidly from the 14-bit chat, and the fractures obtained were scanned by electron microscopy.
The fractures are all fatigue cracking, and there are pit defects at the cracking position. Since the cracks are all in the heat affected zone of the weld, these pit defects are weld craters.
According to the analysis results. The formation and expansion of the crack can be analyzed as follows: The weld crater on the inner side of the impeller wheel cover is affected by the welding heat. The village material organization has changed. The performance is degraded and a large stress concentration is caused by the crack, under the action of alternating load. The welding crater gradually forms a source of fatigue cracks. The source of the crack can be generated at the same time or at the same time in the welding crater of the multiple welds.
Once the crack is formed, the impeller loses its dynamic balance after the centripetal expansion crack is formed under the action of the alternating load and the centrifugal force. The vibration-increasing crack accelerates and expands and produces instantaneous fracture. At the same time, the vibration distribution is uneven, but the pressure distribution is relatively uniform. Therefore the pressure in the boundary layer is equal to the pressure of the main gas stream. As the cross section increases. The pressure on the boundary layer also increases. The airflow in the boundary layer is driven forward by the flow of the main airflow. When the main gas flow can overcome the frictional resistance and differential pressure resistance in the boundary layer, the airflow in the boundary layer will be stagnant. There is even a reverse flow that separates the boundary layer from the wall. The airflow continuously stagnates or even flows back along the surface of the diffuser, so that more and more blocked airflow will accumulate between the wall and the main airflow in a short time, so that the backflow gradually expands, and the main airflow is squeezed toward the middle of the channel. The vortex zone is formed close to the wall, and the vortex zone also extends into the stream outside the channel. The generation of the run-up zone will cause a lot of energy loss. This energy loss is called boundary 1 detachment loss.
A large number of experiments have shown that the boundary layer detachment is related to the coarse sugar content of the shape wall of the channel and the nature of the gas flow, especially the shape of the channel has a great influence on the separation of the gas flow. If the channel is not well designed, such as a large degree of channel expansion, the cross-section changes suddenly, causing a sudden increase in local pressure in the channel. In this case, the eddy current zone is easily deformed.
The impact of the airflow on the blades can also cause the airflow to whirl. The compressor is a constant flow pressure temperature equal to that designed according to the specified operating conditions. Therefore, it is only when the compressor is in the design condition that the gas flows into the impeller diffuser and other components. That is, the airflow flows into the blade into the flow channel. However, the compressor was turned into a field of view. Influenced by the ten-1 art process. Other parameters such as the amount of gas required and the speed of the compressor often change. For example, when the flow rate is greater than or less than the design flow I; the fatigue pattern 4 alternating load 4.1 the air flow excitation causes the impeller to vibrate the airflow and expands. 1 Hannah 1. When the airflow enters the impeller and the diffuser vane, the speed direction It is no longer in phase with the inlet angle of the blade and there is a phenomenon in which the airflow impinges on the working and non-working surfaces of the blade.
4.2 Airflow causes the stress caused by centrifugal force to be mainly considered in the impeller setting before the calculation of the vibration stress of the impeller, without considering the bending stress generated by the airflow. Xu Zili proposed a new impeller stress calculation model.
Although the static bending stress and the like centrifugal stress are large, the alternating stress caused by the airflow impinging on the impeller caused by the diffusing blade and the variable working condition of the compressor may cause fatigue damage of the impeller, resulting in fatigue cracking or even destruction.
The geometry of the meta-impeller currently designed is extremely complex, and it is still difficult to accurately calculate the shear stress of the shear stress. The mountain can be seen from the cracked impeller. The crack starts with ten impeller wheels. The outer edge of the cover is covered with overflow, Na, to the 蔌, the outer edge of the wheel cover of the city is the weakest link, so the calculation of shear stress and bending on the impeller should mainly consider the outer edge of the wheel cover.
According to the calculation formula 2 derived from the model, the impeller wheel cover size is substituted into this formula, and the maximum shear stress and the clever stress at the wheel cover can be obtained as 8.65, 224.51, which is the pressure difference of the first-stage impeller wheel cover,=0.357 The maximum shear stress and bending stress of the impeller wheel cover of the factory are 3.03 and 80 respectively. Still 1. The static 5 stress at the impeller wheel cover is better than the human body, reflecting that the airflow effect is greatly changed. stress. The observation of the fracture also shows that it is this alternating bending stress that causes the fatigue expansion of the crack. This is the main reason why the impeller can crack, and the airflow excitation causes the bending stress to be large.
5 Improvement scheme Considering that the main cause of impeller cracking is the impeller vibration caused by the excitation of the airflow, the impeller has a narrow working range, and there is also an increase in the vibration caused by the impeller operating frequency and the frequency multiplication of the gear.
The impeller must be improved. Improve airflow. To avoid resonance of the shafting.
The two proposed improvements are 1 to reduce the outer diameter of the impeller; to optimize the flow path structure.
According to the improvement plan, the compressor impeller redesigned by the manufacturer has been put into production for more than 2 years, and the operation is normal, indicating that the improvement scheme and design are reasonable.
6 Conclusion Crack metallographic analysis and fracture analysis. Type 1240 centrifugal compressor impeller cracking is a sub-fatigue cracking. There is an alternating load when the impeller is working. The crack is first produced from the weld crater on the inside of the wheel cover. And gradually expand to the outer surface of the wheel cover and the direction of the heart. Once the macro crack is formed, the impeller will lose its balance, the vibration will be intensified, and the crack will accelerate and expand and instantaneously break.
The impeller material has high strength, low plasticity and poor weldability. The quality of the impeller welding has the stress concentration caused by the welding crater, which is easy to become a fatigue crack source.
The source of the impeller alternating load is the vibration of the impeller itself caused by the air flow.
An improved scheme of the impeller was proposed and commissioned by the manufacturer. The impeller of the modified 5 is ideal for use.
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Corrosion and failure study, communication address 200237 Institute of Mechanical Engineering, School of Mechanical Engineering, East China University of Science and Technology, Shanghai.
According to the test, the system is reliable and meets the requirements of central air conditioning in small villas. The application of the device can play a role of shifting peaks and valleys for power, saving the operating cost for the user.
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