Analysis on the explosion accident of the drum of

2022-08-07
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Analysis of the centrifuge drum burst accident

1 basic information

the 2methyl sugar separator (model xz1200b top hung automatic discharge centrifuge. Hereinafter referred to as the centrifuge) in the boiling workshop of a sugar factory suddenly burst during operation, hitting the centrifuge drum protective shell, causing some shells to fly off and hit the four people on the operating table, Causing major casualty accident: according to the introduction of Zhuguang, the centrifuge has been running for 18 years. Before the safety operation accident occurred in the pressing season, the first screening separation of methylose was completed. Then, the first screening was carried out. It operated normally during charging. However, during the acceleration process of Danli operation, the drum of the centrifuge suddenly broke, causing an accident

after the accident, after on-site investigation, we cut the broken samples and carried out various shuttle tests, strength calibration and edge analysis. We investigated, analyzed and studied the causes of the rupture of the centrifuge drum. The specific analysis is as follows: 2. Various test results after the rupture of the centrifuge drum

(1) mechanical properties of materials

first, we need to understand the material changes and the impact on the safe operation after the machine has operated for I8 years. Therefore, Three samples are respectively cut from the screen drum and top cover of the drum, and processed into tensile samples according to the requirements of the national standard. See Table 1 for the comparison between the tensile test results and the standard values

the comparison results in Table 1 show that: after 18 years of use, it meets the national standard for experimental motor gas, and the yield point of the drum material is significantly increased (by 50% and 49% respectively) and close to the tensile strength, which indicates that the plasticity of the drum material is significantly reduced due to stress fatigue, resulting in material hardening, which is a hidden danger for the sudden rupture of the drum. It is also found that there is no obvious yield point during the tensile test, It also shows that the material has fatigue hardening phenomenon, which is easy to induce brittle fracture

(2) drum wall thickness

in order to understand the change of drum wall thickness after 18 years of service, the thickness of the top cover, screen drum and screen hole edge of the drum was measured at multiple points with ultrasonic thickness measurement. The test results are compared with the original wall thickness as shown in table 2

from the results listed in Table 2, it can be seen that the wall thickness of drum material has been seriously reduced. It can be seen from the strength calculation results of drum. The calculated wall thickness of screen drum is 9mm, However, the existing small wall thickness (there are many parts of the whole screen drum) is 5.5mm, which is far from meeting the minimum requirements of strength design. Therefore, there is a very unsafe hidden danger in the continuous operation of the drum. The main reason for the thinning of the drum wall thickness is the wear and erosion of the drum by the grains in the syrup during long-term operation, especially in the middle and lower parts of the screen drum. The thinning of the screen hole edge is the most serious, However, the drum near the cover plate (the upper part of the drum) has a small wall thickness. At the same time, the stress changes repeatedly during operation and the stress corrosion caused by the action of the working medium may also reduce the wall thickness

(3) the mechanical strength calibration

is not provided in the original data of the accident centrifuge. The design calculation sheet of the centrifuge is not provided. In order to understand the rationality and reliability of the drum design and provide a basis for safety and reliability analysis for the thin drum, We checked the mechanical strength of the drum. The check results show that:

① turn 10 Experimental rotation direction: the design of the two-way drum is reasonable and reliable. 20g material with an original wall thickness of 12mm has been taken, and a large safety factor has been left (the design wall thickness is 9mm). Based on this wall thickness, the theoretical total stress of the drum is 77.2'mpa, which is far lower than the allowable rejection stress of the material (10omp port)

② when the wall thickness is reduced to the design wall thickness, the total stress is equal to the allowable stress; When the wall thickness is reduced to 7mm, the total stress increases to Ili 9Mpa, which has exceeded the allowable stress by l1.9%; When the wall thickness is reduced to 5mm, the total stress increases to 145mpa, which has exceeded the allowable stress by 45%

⑧ when the added material in the drum increases from 500kg (normal) to 600kg, the stress of the drum increases. If the wall thickness is reduced to 7mm, the total stress increases to 136.9mpa, which has exceeded the allowable stress by 36.9%; If the wall thickness is reduced to 5mm, the total stress increases to 180MPa, which has exceeded the allowable stress by 80%

to sum up, the obvious thinning of the drum wall thickness and the increase of the drum loading will greatly increase the total stress borne by the drum, which greatly exceeds the allowable stress of the material, and become one of the main reasons for the drum fracture. However, according to the yield stress, the minimum safety factor ~=245/180=1.36 is selected based on the integrated, intelligent and networking technology. Although this value is lower than the specified value =2 ~ 2.5, the material is still far from yield. It is difficult to explain the sudden burst of the drum only from the viewpoint of the membrane stress of the drum, which is very similar to the burst accident of a 800mm diameter tripod centrifuge drum reported in the literature. The drum material reported in the literature is 1Crl8Ni9Ti, and the yield stress =200mpa. In allowable stress [when Ding 1=67.5mpa, the drum wall thickness should be 6.2mm. When the actual wall thickness is reduced to 3mm due to corrosion, the centrifuge drum bursts. When the drum wall thickness is 3mm, the actual stress is calculated to be 1 pa.

the safety factor n selected according to the yield stress, =200/140=-1.43. Although this value is lower than the normal value =2 ~ 2.5, the material is still a long way from yielding, and it is difficult to judge the root cause of the accident only from the viewpoint of membrane stress of the drum From the point of view of drum buckling (out of round), it is estimated that the minimum wall thickness of ssn800 centrifuge drum should be 6mm to maintain its stiffness at the China International Building Systems and materials exhibition held in Shanghai. It is pointed out that the root cause of the accident lies not only in the poor quality of the weld, but also in the insufficient rigidity of the drum

according to the literature, the proportion of drum burst accidents caused by thin drum wall and weld quality of tripod centrifuge is very high. It is reported in the literature that the four centrifugal machines were used for too long and corroded seriously, which made the drum thinner and led to the drum burst accident during operation. In one case, "the centrifuge was severely corroded by sodium alum, the wall thickness of the drum was only 3mm, the shell was made of cast iron, the thickness was uneven, and the feeding amount was too large, resulting in the centrifuge suddenly broken into 126 pieces". Mechanical Engineering Manual

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