Abstract : This paper mainly introduces the use of horizontal screw discharge centrifugal separator in catalyst wastewater treatment device and the main faults that occur in production. The causes of these faults are found through monitoring analysis and production capacity calculation. Several fault solutions and modification suggestions are introduced from three aspects. Keywords : horizontal screw separator; vibration; material accumulation; production capacity 1. Overview of centrifugal separator Centrifugal separator is a machine that uses the centrifugal inertial force generated by the rotation of the drum to separate or concentrate suspension, emulsion and other materials. It is widely used in resource development, chemical production process and industrial production such as waste treatment[1]. There are many types of centrifugal separators. The following mainly discusses the horizontal screw discharge sedimentation centrifugal separator (hereinafter referred to as horizontal screw separator). The horizontal screw separator uses centrifugal sedimentation to separate suspension. It can run at full speed, continuously feed and separate, and discharge by screw conveyor. It is suitable for separating medium and fine particles with high viscosity, fine particles, and difficult filtration, while the solid phase density is greater than the liquid phase density, the solid phase concentration is between 2 and 50%, and the particle diameter varies greatly. The catalyst wastewater treatment unit is a wastewater treatment device that treats the suspension of bleaching clay particles, acids, alkalis, ammonia, etc., discharged from the catalyst production unit. Its main methods include neutralization, staged sedimentation, and centrifugal dewatering. The decanter is a key piece of equipment in the wastewater treatment unit; its normal operation, processing capacity, and separation effect directly affect the unit's processing capacity and whether the wastewater discharge meets standards. Since the unit started operation, the decanter has been malfunctioning, requiring multiple repairs due to excessive vibration, bearing damage, belt failure, and non-discharge. Because of its complex structure and the fact that most parts are made of stainless steel, repairs are difficult and maintenance cycles are long, putting significant pressure on normal production. Frequent repairs have also greatly increased maintenance costs, putting routine maintenance in a very passive situation. 2. Fault Cause Analysis 2.1 Main fault phenomena (1) High vibration intensity. The maximum vibration value at the bearing part can reach 200 mm/s, and the vibration can be clearly seen. The vertical vibration of the bearing box is 2 to 3 times greater than that in the horizontal direction. Using a CSI2115 vibration analyzer, the vibration is mainly caused by the imbalance between the drum and the screw; (2) Many auxiliary equipment are involved in the vibration. The discharge port, pulley guard, clear liquid outlet pipeline, feed pipe, etc. all experience strong vibration. The feed pipe has broken many times; (3) The screw bearing is generally damaged during disassembly and inspection. The bearing box skeleton seal ring fails, the liquid enters the bearing box, a large amount of material is blocked in it, the grease is basically gone, and the seal ring is worn into the shaft contact area; (4) Due to the large vibration, the main bearing and screw bearing are easy to burn out. The short shaft at the differential end of the drum has suddenly broken into two pieces during operation. From the above fault phenomena, it can be seen that vibration is the most common problem in centrifuge failure and the direct cause of damage to most parts. 2.2 Analysis of the cause of failure The structure and working principle of the LW horizontal screw discharge sedimentation centrifuge are described in reference [2]. In order to clarify the cause of vibration, the centrifuge was dynamically balanced multiple times, all bearings of the drum and screw were replaced, the differential (cycloidal pinwheel reducer) was repaired and dynamically balanced together with the outer drum. The advanced CSI2115 vibration analyzer was used for tracking and monitoring. The monitoring revealed that: (1) The machine was in good operating condition and the vibration was relatively normal when it was first fed. However, the vibration gradually increased over time, especially when the machine suddenly stopped. When it was restarted, the vibration suddenly increased. From the spectrum diagram of the CSI2115 vibration analyzer, it can be seen that the vibration value of the inner screw changed the most significantly during this process, while the vibration value of the outer drum did not change much. Therefore, it can be concluded that the increase in the vibration of the whole machine is the result of the rapid increase in the imbalance of the inner screw. (2) The machine was disassembled and inspected. It was found that the separation zone inside the spiral was severely blocked. It can be inferred that the increased vibration was caused by the solid material separated by centrifugal force accumulating inside the spiral and rotating with the spiral, which could not be discharged and destroyed the dynamic balance of the spiral, causing the vibration to increase. At this time, the separated material could not be pushed to the discharge port, and the machine almost lost its production capacity. The violent vibration directly caused the shaft and feed pipe to break. Therefore, a preliminary analysis of the separation process when the material was accumulated was carried out [3]: Since the material entered the machine body not far before entering the drum from the spiral feed port to start separation, the fluid moved a short distance inside the machine and had poor self-washing performance. The accumulated layer would gradually fill the entire separation zone. The frictional resistance would gradually reduce the distance between the spiral and the drum, eventually synchronizing, causing the differential load to increase sharply, heat up severely, and eventually be damaged; Due to the accumulation of material, the material flow cross-sectional area decreased, the overall pressure at the feed end increased, and the liquid mixed with a large number of solid particles accumulated near the spiral bearing (there should be no liquid accumulation under normal conditions). It would gradually enter the bearing box, and the violent vibration would quickly damage the bearing. The liquid would spray directly from the overflow hole [4]; According to the above analysis, the main factor causing the machine failure is the accumulation of materials, and other failures are caused by this. 3. Production capacity calculation In order to find the cause of material accumulation, the production capacity of the centrifuge was calculated for this material and working condition. 3.1 Determination of minimum separation particle size According to the requirements, the water content of the clear liquid should be greater than 99% during normal production, the solid content of the mud cake should be about 25%, and the measured average water content of the material at the feed port should be 96.46% (mass percentage). To achieve this result, it is calculated according to the material balance theory [5]. Let: the solid recovery rate be 1-x, and the liquid phase recovery rate be y. Solid content in mud cake: According to the above formula, 74.74% (mass percentage) of solid particles must be separated. According to the above particle size test results, the minimum separation particle size is about 4.5μm. 3.2 Material characteristics The maximum speed that the machine can reach is 3200r/min. 3.3 Production capacity process calculation [6] The production capacity is (1) The theoretical maximum production capacity. According to the above calculation process, the theoretical maximum production capacity for the existing medium is Q = 6.39 m3/h. The feed pump flow rate is 15 m3/h, which simultaneously provides feed for two centrifuges. The feed flow rate of each centrifuge is 7.5 m3/h, which obviously exceeds the production capacity of the machine. (2) Actual production capacity. In actual operation, the highest speed will not be used for separation because centrifuge separation requires a suitable separation factor Fr: to ensure the separation effect, while not making the centrifugal force too large, increasing the resistance of the screw conveyor, leading to a decrease in conveying efficiency, or even causing material blockage. According to experience, a separation factor of about 1500 is more suitable for separating this material, at which time the speed is 2600 r/min. The production capacity calculated according to the actual situation is 5.34 m3/h. 3.4 The impact of feed flow exceeding production capacity: As calculated above, the feed flow significantly exceeds the production capacity. If maintenance is required and one pump supplies feed to one centrifuge, the feed rate will exceed the capacity by nearly double. According to the production capacity theory of sedimentation centrifuges, the above production capacity calculation formula can be derived when the residence time t2 of particles in the drum is greater than or equal to the time t1 required for solid particles to settle from the liquid surface R0 to the drum wall R. In this case, according to the formula, when other conditions remain unchanged, an increase in flow rate corresponds to a corresponding increase in the maximum diameter of particles that can be separated. When the flow rate is 7.5 m³/h and the rotation speed remains constant at 2600 r/min, the maximum diameter of particles that can be separated is 5.33 μm. Based on particle size analysis, the solid recovery rate at this point is 70%. Material balance calculations are performed at this time. When the feed flow rate is greater than the production capacity and the actual feed flow rate (7.5 m3/h), the mass of dry solids separated is 190.04 kg/h; when the feed flow rate is equal to the production capacity (5.34 m3/h), the mass of dry solids separated is 144.97 kg/h. It can be seen that due to the increase in feed flow rate, relative to the production capacity at this speed, the mass of dry solids separated per hour is 45.1 kg more. When the drum rotates at a certain speed, the differential speed between the screw and the drum is fixed, and the screw pushing torque is completely transmitted by the belt pulley through the differential. Its conveying capacity cannot be improved, and the sludge cannot be conveyed and discharged in time, which causes the gradually serious accumulation phenomenon. As the accumulation process develops, the screw pushing torque increases, the belt begins to slip, the differential speed gradually decreases, and the screw conveying capacity further decreases, forming a vicious cycle until the differential speed is 0, the belt is severely worn or even broken, and the production capacity is completely lost [7]. 4 Solutions and Improvement Methods Based on the above analysis, the key to avoiding various malfunctions of the machine lies in solving the problem of material blockage, that is, solving the problem of matching production capacity with feed flow rate. This can be solved through several approaches [8,9]. 4.1 Improve production capacity (1) Increase the inlet medium temperature and reduce the liquid phase viscosity, which can increase the mass settling velocity vg of the particles, thereby increasing the production capacity; (2) Through calculation, it was found that appropriately increasing the overflow radius and reducing the liquid layer depth can slightly increase the production capacity. Moreover, it can reduce the thickness of the sludge layer and reduce the screw resistance. 4.2 Reduce inlet flow rate This is the fundamental way to solve this problem. By increasing the frequency conversion speed regulation or increasing the return bypass to adjust the inlet flow rate, the inlet flow rate can be equal to or less than the production capacity, thus completely solving this problem. 4.3 Improve screw capacity (1) Replace the pulley and increase the differential speed so that the excess sludge that settles down can be transported out in time, which is equivalent to increasing the production capacity; (2) By designing the transmission part, the screw torque can be increased, thereby improving the screw's anti-clogging ability and conveying capacity. Currently, centrifuge design pays great attention to the selection of this torque, with similar models selecting a torque of around 6000 Nm; (3) Adding axial ribs to the inner wall of the drum increases the circumferential friction through the material filling between the ribs, improving the screw conveying efficiency and protecting the inner wall of the drum from wear. Combining the above methods, if the screw differential speed can automatically increase according to the increase of torque, while the feed flow rate decreases or is cut off, and even the drum speed is appropriately reduced, the possibility of material blockage can be basically eliminated. Currently, new centrifuges all adopt this method of control, which can realize the differential speed changes with torque, controllable feed rate, and automatic unblocking and flushing programs, basically eliminating the phenomenon of vibration exceeding the standard and being unable to start due to material blockage or unexpected shutdown. 5 Conclusion After analyzing and making a preliminary judgment on the failure phenomenon of the catalyst centrifuge, the production capacity was calculated and the root cause of the failure was found. A practical solution was proposed. Practice has proved that after modifying the centrifuge through several solutions, the downtime and maintenance time were greatly reduced, and the effect was obvious. It has significant economic and social benefits for the continuous production process, especially for the sewage treatment device in the environmental protection aspect. References : [1] Zhang Hanzhu. Chemical Machinery [M]. Beijing: Chemical Industry Press, 2005. [2] Jin Dingwu. Chemical Engineering Handbook - Liquid-Solid Separation [M]. Beijing: Chemical Industry Press, 2000. [3] Chen Minheng, Cong Dezi, Fang Tunan. Chemical Engineering Principles Volume 1 [M]. Beijing: Chemical Industry Press, 1985. [4] Li Lifeng, Wang Jinze. Analysis of Vibration Causes of Sedimentation Centrifuge [J]. Refining and Chemical Industry, 2003, 14(4): 44. [5] Bi Qinling. Analysis and improvement of high torque in spiral sedimentation centrifuge [J]. Petrochemical Technology and Application, 2001, 19(4): 35-37. [6] Sun Qicai, Jin Dingwu. 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