Operation methods and adjustment techniques for horizontal spiral centrifuges

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Section 1: The Key Role of the Spiral in the Operation of Centrifuges. In the operation of horizontal screw centrifuges, especially when it comes to separating materials, the movement of the spiral inside the centrifuge can be considered its \"soul\"; without proper operation of this spiral, the centrifuge cannot carry out its basic functions. The most basic function of a horizontal screw centrifuge is to continuously separate the materials fed into it. This requires the machine to remove the materials that have been separated inside it, so that it can continue to process new materials that enter it. Industrial production demands that this process of \"separation – discharge – continued separation – continued discharge\" be automated and continuous. The feed screw inside the centrifuge is used for continuous discharge, and this discharge function is achieved through the relative rotational movement between the screw and the centrifuge drum; this relative rotational movement is what is referred to as the \"centrifugal differential\" of the centrifuge. Since the feed to the centrifuge is continuous, in order for the centrifuge to be able to process materials continuously, the differential speed must also be continuous. To prevent failures caused by the accumulation of material inside the centrifuge, a differential speed must always be present, and this differential speed always serves as the driving force for pushing the material. The so-called “material pushing mechanism” refers to the “difference in speed” generated between the screw and the drum, which pushes the separated solid residue in the direction of the centrifuge’s discharge port. For the same spirogram, depending on the rotation direction of the drum, the differential can be designed as a positive differential or a negative differential; however, their pushing behavior is the same. The feed screw is able to “sense” the dryness of the solid residue during operation. This feeling is reflected through the load of the rotational motion, namely the so-called \"torque\" at that moment. The perception of torque in the SIMP gearbox differential mode is indirectly reflected through the load on its drive motor, while in the hydraulic differential drive mode, the perception of torque is indirectly reflected through the oil pressure of the hydraulic drive unit. When the speed of the drum remains constant, if we reduce the differential speed of the screw, we can obtain a relatively dry solid residue output. Since the differential speed is reduced, more solid residue is pushed out with each rotation of the screw; and because the solid residue is drier at lower differential speeds, the thrust torque required by the screw increases. If we increase the screw differential, the solid slag pushed out by the screw will be more moist, and at this point the thrust torque of the screw will decrease. Therefore, when the solid residue is too dry or the pushing torque is too high, we can increase the differential speed to accelerate residue discharge and thereby reduce the pushing torque; when the solid residue is too wet, we can decrease the differential speed to increase its dryness. During the operation of the centrifuge, we adjust the operating parameters continuously in order to achieve a relatively stable dryness level of the solid residue. In practice, we monitor the load or torque of the differential drive motor, or the oil pressure in the hydraulic system. If the load on the differential drive motor or the oil pressure in the hydraulic lines remains stable, we can conclude that the dryness of the solid residues discharged by the centrifuge is very stable. Therefore, one of the important operational requirements for a centrifuge is to obtain a stable thrust torque or thrust hydraulic pressure. Section 2: The dependence of centrifuge operation on the material. A well-designed centrifuge enhances the efficiency of material separation, but the performance of the centrifuge depends on the material being processed. Due to the high-speed rotation of its drum system, the centrifuge creates a centrifugal force field for the materials that enter it. The centrifugal force field accelerates the settling speed of materials with natural settling properties. The better the natural sedimentation property of a material, the faster its sedimentation rate in this accelerated centrifugal force field, resulting in a \"better and faster\" separation effect. To achieve the best and fastest separation results, we often use auxiliary methods to aggregate small material particles into larger ones. A common auxiliary method is to add flocculants to the material; when flocculants are added properly, the material is then separated using a centrifuge, allowing for more thorough separation and resulting in less content of small particles in the liquid after separation. The viscosity of the material is one of the key factors that hinder the settling rate of the solid particles within it. Excessively high viscosity will make centrifugal separation very difficult or even impossible; the centrifuge may achieve very poor separation results when handling such materials, as they do not have good natural settling properties. They require a very long residence time inside the centrifuge for separation, which leads to a sharp decrease in the processing capacity of the centrifuge (i.e., the amount of material it can handle). The most effective method is to directly increase the temperature of the material. This is quite common in the food industry. To obtain a drier solid residue for discharge, we want the settled solid residue to have good compactness, and such a compact structure should not be easily disrupted by the flow of liquid above it. If the settled solid residue can be easily broken apart by the flowing liquid above, then the solid content in the liquid discharged from the centrifuge will increase. Finally, the material fed into the centrifuge must have an appropriate solid volume concentration. Although weight-to-concentration is one of the very useful material properties, the separation of materials inside a centrifuge is based on the proportion of solid volume that forms the interface with the liquid. Therefore, for separation efficiency, volume concentration is more meaningful than weight concentration. If the proportion of solid volume is too large, the thickness of the clear liquid will be smaller, and sometimes it is even impossible to obtain a clear separated liquid. Section 3: One of the three key parameters for the operation of a centrifuge is the drum speed. The drum speed determines the magnitude of the centrifugal force acting on the materials inside the centrifuge; it also influences the settling speed of solids and the processing capacity of the device. An increase in drum speed enhances the separation speed of materials, resulting in a clearer liquid after separation. The solid residues that settle become more compact and firm, and the solids discharged from the centrifuge are drier. Additionally, due to the increased settling speed, the centrifuge’s capacity to handle materials increases, thereby raising its processing capacity. However, excessively high drum speeds can sometimes have some adverse effects. The most direct drawback is the increased power consumption of the drum motor, as not all materials need to be processed at the centrifuge’s highest speed in order to be separated. Excessively high drum speeds generate high centrifugal forces, which in turn result in a high degree of solid settlement density; this increases the load on the screw conveyor. Another drawback is that it raises the power consumption of the screw motor. For certain viscous or slippery materials, or those with extremely fine solid particles, excessive centrifugal force can make it difficult for the settled solids to reach the discharge outlet of the centrifuge through its cone section, resulting in difficulties in discharging the material. Improving the design of the spiral element and the drum cone can enhance the centrifuge’s ability to discharge such materials. Section 4: The second of the three key parameters for the operation of a centrifuge – the differential thrust of the screw. Under the action of specific centrifugal forces, this differential thrust plays a crucial role in assisting the centrifuge in separating materials. Without an appropriate differential thrust from the screw, it is not possible to achieve material balance inside the centrifuge, and thus good continuous separation cannot be accomplished. The function of the screw feeder is to push the solid sludge that has been separated and settled in the drum smoothly and continuously toward the discharge port of the centrifuge, so as to remove it from the machine. However, since both solid residues and liquid are present inside the centrifuge, the pushing motion of the screw will undoubtedly have a similar effect on the liquid as well, acting to enhance its flow. As a result, the differential speed of the screw is usually low; this lower differential speed not only helps to suppress the screw’s effect of accelerating the liquid’s flow but also prolongs the time during which the settled solid residues remain compressed inside the centrifuge. Only in this way can we obtain a drier solid residue for discharge. By reducing the differential speed of the screw feeder, the solids are pushed out of the centrifuge more slowly; at the same time, the discharge of the liquid creates a flow, which increases the tendency of the solids to penetrate in the direction of the liquid. Therefore, while we reduce the differential pressure and increase the dryness of the slag, we may end up with a relatively turbid separated liquid, meaning that the solid content in the liquid increases. If we increase the differential speed of the screw pusher, we can prevent the solid residue from penetrating toward the liquid phase, thereby obtaining a clearer separated liquid. However, this increases the effect of the screw in pushing the material forward, and at the same time reduces the compression time of the solids inside the centrifuge, causing them to be pushed out early by the screw; as a result, we may end up with a more moist solid residue. Under the effect of the differential speed of the feed screw, the dryness of the solid residue and the clarity of the liquid represent such a pair of opposing factors; the purpose of machine tuning is to determine the differential speed value that allows for a balance between these two aspects. Finding the optimal differential speed means identifying the best value for this difference that allows for a proper level of dryness in the solid residues while also ensuring that the liquid is clear. With this optimal differential speed in place, the dryness of the residues discharged remains relatively constant; and this constant dryness in turn requires a relatively constant pushing force (torque and hydraulic pressure) from the screw used for moving the residues. We refer to this pushing force as the characteristic pushing force. It can be used to automatically control the differential in the future. Different factories, different materials, different processing volumes, and various approaches to achieving the desired level of dryness in the solid residues and clarity in the liquid – all these factors result in different thrust torques. However, within the normal operating conditions of any given factory, this thrust torque remains relatively constant. We need to value the role it plays. Section 5: The third of the three key parameters for centrifuge operation – liquid pool depth. The liquid pool depth refers to the thickness of the solid sludge+liquid mixture ring that is formed on the inner wall of the drum inside the centrifuge, as a result of the centrifugal force acting on the material. Within this ring, the solid settles at the outermost edge of the ring because its density is greater than that of the liquid; the closer to the inner edge of the ring, the clearer the separated liquid. The greater the depth of the liquid pool and the thicker the ring, the further the liquid in the innermost circle is from the solid-liquid interface, and the clearer this liquid is. Therefore, the depth of the liquid pool is also one of the very critical auxiliary parameters for centrifuges. By increasing the depth of the liquid pool, we obtained a clearer separated liquid; however, as the liquid ring thickens, its inner edge gradually spreads toward the solid residue discharge port, and the length of the anhydrous zone (dry zone) near the discharge port decreases, which causes the discharged solid residue to become increasingly moist. On the contrary, if we reduce the depth of the liquid pool, the liquid ring inside the centrifuge becomes thinner; the inner edge of this ring gradually moves away from the slag discharge port, and the length of the water-free area near the discharge port increases gradually. This results in the solid slag discharged becoming progressively drier. However, since the inner ring of the liquid ring is close to the solid-liquid separation surface, the separated clear liquid obtained is rather turbid. The specific adjustment of the liquid pool depth is achieved through the centrifuge’s overflow baffle or a variable impeller. It is worth noting that, due to the exacerbating effect of the screw feeder differential, at a certain depth of the liquid pool, an excessively high differential can easily cause the slag discharge area to be affected by the spread of liquid; as a result, we must reduce the depth of the liquid pool again. If we choose a lower screw feed differential, we may be able to increase the allowable depth of the liquid pool. By utilizing Flowserve’s variable impeller technology, we can easily make corresponding dynamic adjustments to the liquid level depth while changing the differential speed. Once the centrifuge starts discharging sludge, the gap between the screw and the drum, along with the accumulation of separated solid sludge in the area near the discharge opening, allows us to increase the depth of the liquid pool by 1–2 mm. This can only be achieved using a machine equipped with VaneTech’s variable impeller. Section 6: One of the technical adjustments is that the startup sequence follows the principle that a differential speed must always be present during the operation of the centrifuge; whether in program-controlled operations or under any other conditions, the screw must be operated first. There are two reasons: First, after regular cleaning of the equipment, there is always a small amount of residue left inside the centrifuge. If the centrifuge is not used for an extended period of time, this remaining solid residue will settle at the bottom and become quite hard. Therefore, by utilizing the principle of starting with the spiral, we can ensure a smoother start-up of the drum in the future to a greater extent. Secondly, since there is relative rotational motion between the screw and the drum during the operation of the centrifuge, and this relative motion relies on their high-speed rotation, by using the method of starting the screw first, we can check and listen for any signs of collision or friction between the screw and the drum during this relative motion, thereby further improving the safety checks prior to startup. Therefore, we must adhere to the principle of starting the spiral first to ensure the safe operation of the centrifuge. Before starting each motor, we must first set its operating frequency in advance. To this day, quite a number of operators start the equipment blindly, without considering what value the motor’s rotation frequency should be set to. If there is significant vibration once the equipment starts running, and it is not noticed until the speed has become very high, then even an emergency stop will not be able to halt the operation of the centrifuge immediately, resulting in severe damage to the equipment. We have programmed the centrifuge, and even the entire system, to start and stop automatically. However, we strongly recommend that operators first fully master the sequence for starting and stopping the equipment manually, as well as fully understand the reasons behind the logical order of these steps. Otherwise, if an anomaly occurs while the equipment is in automatic mode, the operators may not know at which stage the equipment or system is currently operating, and they will be unable to take the appropriate emergency actions. Once we confirm that the screw operates perfectly on its own, we can start the drum drive motor. We recommend adding water to the inside of the centrifuge in moderation once the drum is running at 300–500 rpm. The purpose of injecting water is to further ensure a smooth start-up of the drum, eliminating rotational imbalance that may arise from residual amounts of material. Throughout the acceleration process of the entire drum, we must pay close attention to and listen to the sound of acceleration, comparing it with the normal sound under usual conditions in order to detect any minor abnormalities in advance. Section 7: Technical Adjustments II – Starting Feeding. We can initiate the feeding system once the centrifuge’s speed has reached the set value and it is operating properly. When it comes to feeding systems, people naturally think of feed pumps and dosing pumps. But the most critical aspect is that all the outlets of the centrifuge must be “unblocked”. The outlet of the centrifuge refers to the slag discharge system below the solid residue discharge port, as well as the outlet piping for the separated liquid and the corresponding valves on that outlet piping. If the outlet of the centrifuge is blocked, the material being discharged will quickly spread or get trapped within the drum system of the centrifuge. It will then come into intense friction with the rapidly rotating drum, causing the centrifuge to vibrate violently and produce noise until it shuts down due to overload. Severe blockages by solid residues may also occur inside the centrifuge. If the outlet of the centrifuge becomes frequently blocked or if the material spreads upward, solid residues will gradually accumulate in the centrifuge housing as well as in the gaps between the housing and the drum, eventually causing severe wear on the drum and the formation of grooves, which poses safety risks. Therefore, before starting the feed pump, it is essential to first activate the slag discharge system and clear the liquid outlet pipeline. We have implemented logical protection in the control system, but in some cases customers may have special requirements that lead to the disregard of this logical protection. As operators, it is essential to be fully aware of this logical sequence and to adhere to it strictly. The feed to the centrifuge should be started at a low flow rate; while adding the flocculant, we must pay close attention to the liquid that is discharged and wait for the solid residues to accumulate inside the centrifuge until they can be discharged. The discharged liquid must be clear in color; extreme turbidity is a sign of blockage. If the liquid from the liquid outlet flows out, and solid residue is discharged from the solid residue outlet (which may be quite moist), it indicates that material flow in and out of the centrifuge is unobstructed, and further parameter adjustments can be made. Section 8: Technical Adjustments – III, Parameter Adjustment. We have already discussed the basic principles of adjustment in the sections on screw feed differential and liquid level depth. After feeding material into the centrifuge, our task is to carry out the adjustment process in accordance with these basic principles. This process involves adjusting the drum speed, screw differential, position of the overflow weir or the position of the variable impeller, and taking into account the increase in feed volume as well as the addition of flocculants, in order to determine a set of operating parameters that allow us to achieve an optimal balance between the dryness of the solid residues and the clarity of the liquid. Once the operating parameters are set, one of the parameters that requires special attention is the screw differential speed. We already know that the screw differential speed, the thrust torque, and the dryness of the solid residues are a set of parameters that are closely related to each other, with the differential speed playing a dominant role among them. Once we have determined the characteristic thrust torque, we can use this torque value as a control point to automatically adjust the differential. When the measured torque value is higher than the control setpoint, the automatic control system increases the differential to reduce the torque ； When the measured torque value is below the control setpoint, the automatic control system reduces the differential speed to increase torque, thereby achieving a higher level of stability in the dryness of the slag. Section 9: Technical adjustments – 3. Shutdown procedure: Before shutting down, the feed must first be stopped, and then efforts should be made to remove the residual solid residues inside the centrifuge. Whether it is to execute the automatic relationship steps of the procedure or the manual key steps, the principle remains the same. Here are the steps for manual shutdown. If the program previously used differential automatic control, switch back to differential manual control, then increase the differential value significantly to facilitate slag discharge from the centrifuge. At the same time, reduce the speed of the drum considerably to help the solid residues settled inside the centrifuge loosen from the inner wall of the drum and be discharged via the screw. After the solid residues have been discharged, water can be poured into the centrifuge for rinsing; once clear water begins to flow out from the discharge port of the centrifuge, the cleaning process is essentially complete. At this point, the drum drive motor can be turned off; the screw motor and the slag discharge system can only be turned off after the drum’s rotation speed has dropped to zero. The specific details of the steps for automatically shutting down the execution program remain the same; the difference is that the operator must be clear about which step of the program is currently being executed, in order to insert steps such as water injection for cleaning at the right time. Section 10: Technical Adjustments V – Fault Diagnosis Ø Machine clogging Ø The first sign of a clogged centrifuge is the failure to discharge waste over an extended period of time. With other feeding conditions remaining unchanged, the discharged clear liquid gradually becomes turbid and darker, approaching the color and consistency of the feed over time. Along with an increase in the drum current and a significant decrease in the drum speed, the slag discharge torque gradually rises and stays at a high level. All these indicate a possible internal blockage in the centrifuge. Ø In such a situation, our first action should not be to press the emergency button; the centrifuge is running at high speed and has no braking mechanism, so the emergency button cannot provide any help at this time. First and foremost, we must immediately cut off the feed to the centrifuge. In most cases, the centrifuge can clear blockages on its own thanks to the pushing action of the screw. If it is unable to clear the blockages on its own, we should immediately reduce the speed of the drum or turn off the motor that drives the drum; however, the screw motor must not be turned off. Once the speed of the centrifuge drum is reduced, the blocked solid residues may gradually loosen up and can then be expelled from the machine by the screw. Ø For centrifuges that have indeed experienced blockages, even after the drum has completely stopped rotating, we should still pour water into the machine and do everything possible to restore the operation of the screw. As long as the screw can rotate, the centrifuge is not truly blocked, and it is still possible for us to return to normal operation. Ø Adding water to the machine and resuming spiral operation are the preferred methods for clearing blockages in a centrifuge. Ø Machine vibration Ø Machine vibration is divided into chronic vibration and sudden vibration. Chronic vibration is mostly indicative of wear in the centrifuge, wear or detachment of external components, and gradual damage to the bearing system. Ø Sudden vibrations usually indicate blockages of the material inside the centrifuge, as well as severe damage or detachment of its internal components. In most cases, we should first check whether there is a malfunction in the slag removal system that has led to an accumulation of solid residues, and whether there are blockages in the liquid flow channels that have caused the liquid to flow back into the centrifuge drum. Ø High bearing temperature. Ø If the bearing temperature is too high, we should first check whether too much grease has been applied to the bearing. Is there excessive vibration in the centrifuge, and are there any abnormal noises coming from the bearings? At the same time, we need to observe the color of the waste grease from the bearings. The normal color of such grease should be light gray (when Fulleva standard grease is used). If the waste grease is black, rust-yellow, or has any other abnormal color, it indicates that the bearings may be damaged, and they should be replaced in advance. Ø Regarding the noise in the bearing area, we must pay special attention to distinguishing it from the normal sharp noise produced by the oil baffle stacking rings. It is a grease-lubricated friction component inside the bearing housing; it emits a specific sharp noise when the centrifuge is operating normally, with the noise appearing intermittently. This noise disappears temporarily after applying grease 1–2 times, only to reappear later. Section 11: Technical Adjustments VI – Common Judgment Methods Ø Manual rotation of the drum Ø Manual rotation of the drum is one of the important methods used to determine, when the centrifuge is at rest, whether there is any accumulation of material inside, as well as whether the drum can start rotating smoothly. When we turn a normal freely rotating drum by hand, the drum spins very easily, and it can slow down freely until it stops; the angle at which it stops is random. If the rotation of the drum is very sluggish, and when it is turned by hand and then released, the drum reverses direction and returns to almost the same angle it was in before rotation, remaining at that angle after each attempt to turn it, this indicates that there is accumulated solid residue inside the drum. In such a case, increasing the speed of the drum will result in severe imbalance vibrations. Ø In such cases, please operate the screw feeder while injecting water, empty the contents inside the centrifuge, and then start it up. Ø Method of comparing interchangeable adjacent components Ø For some small components, especially control elements, we should make use of the method of comparing interchangeable components as much as possible in order to quickly identify the location of the fault. This is particularly true for speed probes, vibration probes, level sensors, PLC modules, frequency converters, and connection cables. Replacing the connection cables with identical components is also a very important technique for this purpose. We quickly determine the location of the fault by checking whether the fault or alarm points shift after replacing the components. Ø Pulse count estimation method Ø This method is mainly used to check whether the installation clearance of the speed probe and the detection point is appropriate. The speed probe requires a certain response time and pulse peak value; the fact that we can obtain speed detection pulse signals at low speeds does not mean that the speed probe will function properly when the centrifuge is operating at high speeds. Sometimes, when operating at high speeds, the probe is only able to detect two or three of the four speed pulses. When the centrifuge is running at full speed, if we find that the value displayed on the speed gauge is 1/3 or 1/2 of the rated speed, we should first suspect that the gap between the speed probe and the detection points is too large, which results in one or two of those detection points not generating pulse signals when the centrifuge is running at high speeds. Ø Gradual removal of pipes – Section 12: Technical adjustments, No. 7; Daily maintenance. Ø Proper cleaning and the concept of soaking. Ø If the machine needs to be left idle for an extended period, it is necessary to clean out any residual solids inside it. Ordinary cleaning methods often result in residual solids remaining, so a longer cleaning time is required. In such cases, soaking should be used to extend the cleaning time; this involves using a lower rotation speed of the drum, a very small spiral differential, and a low water flow rate, thereby significantly increasing the cleaning time and ensuring that all solids settled on the inner wall of the drum are completely removed. Ø Drive belt, pulley Ø The drive belt wears out and becomes loose after the centrifuge has been in operation for a certain period of time. It is necessary to regularly adjust the belt tension and check the degree of wear on the belt throughout its normal service life. The belt must be replaced in advance, just like the bearings, before it gets damaged; otherwise, it can cause the motor to vibrate and the machine to shake. If the belt breaks during operation, it often leads to damage to the speed sensor attached to it, resulting in unnecessary additional losses. Ø Before restarting a centrifuge after a long period of downtime, it is necessary to remove the rust from the pulleys, as rust is a major factor that accelerates belt wear. Ø Check for wear Ø The check for wear is carried out in three aspects. Ø First, regularly inspect the wear-resistant bushings at the solid residue discharge port of the centrifuge. A wear-resistant bushing should not be construed as a bushing that is immune to wear; it is merely a bushing that can be replaced easily, preventing the material from directly abrading the stainless steel surface. By checking the anti-wear bushings at the slag discharge port, we can gain an understanding of the abrasion characteristics of the material, thereby enabling us to predict the wear caused by this material on the spiral blades. Ø Second, regularly inspect the slag-scraping blades attached to the drum cone and the slag discharge outlet; if these blades fall off, the slag will directly abrade the stainless steel body of the drum, which in turn will cause an increase in vibration levels. Ø Third is to check the degree of wear of the bearings, which is primarily determined indirectly by observing the color of the old grease in the bearings, the temperature of the bearings, and their vibration levels.