Introduction
Cement storage is the most widely used powder storage facility in the cement industry. With the increasing scale of cement production, the number of cement storage facilities continues to increase. There are various forms of cement construction, steel plate construction, with pressure reducing cones, without pressure reducing cones, multi-point discharge, and so on. Moreover, the diameter is getting larger and larger, and the storage capacity is also constantly increasing. There are more and more problems in daily production and operation. In the process of promoting the transformation and application of cement silo powder flow control technology, we have found that there are some deviations in people's understanding of cement inventory storage and unloading technology. Therefore, we propose a series of common problems to discuss and analyze with everyone.
1. Why do cement depots adopt a large number of pneumatic pressure reducing cone design and operation methods?
Answer: Modern cement industry technology in China is imported from abroad, and there is a lack of in-depth and detailed systematic research on the dynamics of large storage powder. The known powder engineering theories lack static and dynamic dynamics research on large storage, only vague and ambiguous concepts are accumulated, but they cannot explain the problem or guide its solution. Design institutes or related universities have not conducted specialized and in-depth research. The results can only be based on simple and direct engineering practices that promote powder flowability through pneumatic means. Both design institutes, cement enterprises, and silo design units are exploring and constantly trying under limited experience conditions. Although some improvements have been made, the results are not ideal and the problem has not been completely solved.
In practical applications, it has been found that there are common problems such as biased storage, dead material, core pulling, and funnel flow, and the larger the storage, the more problems there are. However, the real dynamic reasons behind them have never been carefully examined, especially the systematic neglect of the most critical multiple powder mechanical arch effects. As a result, the problem can only be simply attributed to the installation and processing quality of the gas tank and the operation and maintenance management.
2、Why do cement depots often require periodic cleaning operations?
Answer: It is precisely because of the incorrect design of the pneumatic unloading process at the bottom of the silo that the gasification tank array at the bottom of the isobaric gas source always naturally forms a local center convergence effect that weakens rather than hardens, resulting in the operation of the cement silo only forming a non-equilibrium single or multiple funnel flow for unloading. The hard material at the bottom wall of the annular silo often flows slowly and does not flow, eventually forming a large amount of dead material accumulation. The filter cloth of the air tank at the bottom of the reservoir will naturally experience wear and hydration blockage after continuous long-term operation. In addition, cement depots often pause for an indefinite period of time to balance production and sales needs, so there is always a large amount of static deposition of cement that promotes the escape and precipitation of gas content in the cement warehouse, gradually expanding and developing the hardening degree of the surrounding warehouse wall, and also promoting the growth of dead materials. If dead material accumulates and grows over a long period of time, it can cause a significant squeeze on storage capacity, or even block the material path completely without discharging even after blowing air, ultimately leading to a complete clearance of the warehouse.
The fundamental reason lies in the fact that the pneumatic unloading mode at the bottom of the cement silo is destined to not achieve overall flow equilibrium unloading; The necessary scheduling and storage pauses in cement production cannot guarantee continuous dynamic operation, and static sedimentation during shutdown can cause the hardening of bottom wall cement to intensify, ultimately leading to the formation of dead materials and comprehensive dead material blockage. Regardless of the management and operation, longer or shorter cycles will eventually occur.
3、Why do cement depots adopt a simple open-loop control method?
Answer: Due to the fact that cement silos are only used for storage and unloading, it seems that precise quantitative control of discharged materials is not necessary, which greatly simplifies the control design of outbound operations. Often, only the
simple control mode of adjusting the gate valve opening is used, without considering the real-time measurement, monitoring, and feedback control of the outbound flow rate. The screen in the central control room only provides the display of the start stop status, and the actual discharge flow rate can only be roughly judged by the subsequent increase of the electromechanical flow rate.
This extensive and simple open-loop control method can save a certain amount of equipment investment, but it also brings huge production risks. Due to the significant fluctuations in the flowability of powder in different storage conditions, the discharge amount at a fixed opening or speed has significant random variability. Abnormal situations such as material shortage, breakage, and flushing are inevitable. Even if someone is watching and adjusting the on-site environment, it is very difficult to sacrifice the efficiency of intermediate conveying equipment such as chutes and elevators, which ultimately seriously affects the efficient and stable operation of the cement silo.
4、Why do many technicians believe that the top feeding of cement silos is carried out by scattering and drifting?
Answer: During the process of powder flow control renovation and installation in the cement warehouse, we found that many technical management personnel of cement customers have very similar misconceptions about the impact of feeding materials at the top of the cement warehouse, and they all mistakenly believe that cement will fall to the bottom of the warehouse like a wildflower. As a result, the significant impact of high drop feed on the bottom of the reservoir was seriously ignored.
The reason for many misconceptions is that people believe that cement powder is very fine, soft and easy to float, and it is difficult to directly observe and verify the closed warehouse. But as long as we carefully deduce based on the principles of Newtonian dynamics, the large drop free fall material will not automatically scatter in the warehouse, but will form gravity acceleration and fall quickly. The following are the material velocities calculated based on the free fall formula for different height differences:
At a 10 meter drop, v=14 meters per second, equivalent to 50 kilometers per hour;
At a 20 meter drop, v=19.8 meters per second, equivalent to 71 kilometers per hour;
At a 30 meter drop, v=24 meters per second, equivalent to 87 kilometers per hour;
At a 40 meter drop, v=28 meters per second, equivalent to 100 kilometers per hour;
Generally, the height of cement silos is above 30 meters. When the equipment at the bottom of the silo is exposed, it will be subjected to continuous high-speed jet random impact from the feeding flow. Especially in cement silos with Tai Chi cones installed, if a variable direction diffuser is not installed at the feeding port, the direct high kinetic energy impact during low silo feeding will cause support mechanical resonance and serious impact friction damage to the cone top. Some cement depots may experience pressure relief cone damage and hole breaking, which is actually the same reason.
5、Why do many people believe that cement silos are uniformly distributed with equal density?
Answer: It is precisely because there is a lack of clear research on the dynamics of large storage powder, and it is difficult to observe and verify the enclosed environment inside the storage. People often subjectively believe that the powder is uniformly distributed in the large storage, and even use fluid dynamics methods to incorrectly calculate the pressure inside the storage. In fact, powder has unique dynamic characteristics, and it is difficult to discover its inherent laws solely through general observation. Kunming Aike Industrial Automation Company has found through a large number of targeted experiments that due to its incompressibility and non permeability, flowing powder under pressure will form a transient layered weak gravity arch to transfer material pressure to the storage wall for support, and form a vertical superposition on the storage wall. Therefore, the pressure and density inside the storage are unevenly distributed. The bottom wall of the storage is under maximum pressure, causing its gas content to be squeezed out and form a ring cup shaped hardness distribution, as shown in Figure 1; The higher the material level in the cement silo, the longer the sedimentation time, and the greater the density change. Materials with high gas content have lower soft density, while materials with low gas content have higher hard density. Overall, the middle is soft and the surrounding areas are hard.
The distribution pattern of natural hard and soft density of such powder is a fundamental disaster for pneumatic unloading, because the isobaric air blown into the bottom hard material area will inevitably automatically converge to the middle soft material area, causing the soft material in the middle and upper parts to preferentially form a central funnel flow, and the movement of the edge wall hard material area will become increasingly slow and stagnant. Generally, the design of a pressure reducing cone or Yibao reservoir is to avoid the central funnel flow, rather than to reduce pressure, but the result is essentially impossible to achieve overall flow except for becoming multiple funnel flows.
6、Why do some people worry about the enormous pressure on the flow of Tai Chi cones inside the reservoir?
Answer: Due to the natural enclosed structure inside the cement silo, the Tai Chi cone for powder flow control is installed above the outlet at the bottom of the silo. When the cement storage location is high and the storage capacity reaches 5000-10000 tons, some people believe that the huge material pressure will act on the Tai Chi cone. If the anti impact strength of the Tai Chi cone is insufficient, it will cause damage and collapse of the key structure, ultimately losing the overall flow and arch breaking effect.
In fact, the Tai Chi cone is designed as a high-strength ring-shaped steel multi-level grid permeable vertical cage structure, with extremely strong external anti extrusion pressure. The powder is also designed as a vertically graded multi-point circumferential balanced arch breaking entry mode, and the dispersed inflow in each feeding direction is also small. Even under full storage conditions, the pressure difference between the inner and outer cones of the cone is very small, resulting in very small actual stress on the steel grid and no significant pressure effect. The vast majority of thousands of tons of material pressure is actually transmitted naturally and evenly to the bottom wall of the reservoir through the gravity arch of the powder, with only a very small amount of material weight being transmitted to the Tai Chi cone. In practical applications, we added a hollow steel pipe to the top cavity of the Tai Chi cone and led it out of the warehouse. We tested that the air pressure at the top of the cone during unloading operation mostly fluctuated between the lower positive and negative pressures of about 10 kPa, indicating that the material pressure around the cone top in the warehouse was only around one atmosphere, far from the dozens of atmospheres imagined for liquid materials. This thoroughly proved the intermittent pressure arch isolation and collapse dynamic effects of periodic weak gravity arches and contraction arches outside the Tai Chi cone in the warehouse. Due to the fact that the Tai Chi cone operates with multi-level automatic adaptation and balanced feeding of the ring cone, it can fundamentally generate natural overall flow movement of the ring reservoir wall, without the occurrence of deviation flow hanging on the wall or corresponding large deviation reservoir arch collapse impact damage. Moreover, during the unloading and operation, the cement powder will form a layered sliding film flow around the metal contact surface, which will not have a direct friction effect on the steel structure, and there will be no corresponding wear. After using it for six months, some users found that the paint on the outer ring of the Tai Chi cone is still visible, which is enough to prove everything.
In fact, some users, in order to test the unloading situation, empty the storage material and feed it when it is exposed to the top of the Tai Chi cone. At this time, the high drop and large kinetic energy randomly impact the Tai Chi cone and its support, which has a serious destructive effect. Therefore, the powder flow control requires that the feeding port at the top of the storage must be equipped with a feeding and dispersing device to avoid direct impact on the Tai Chi cone components when the low altitude storage material is exposed to the cone.
7、How to prevent the phenomenon of discharge clumping in cement silos?
Answer: There are various reasons for cement agglomeration in the warehouse. Firstly, during grinding, the crystallization water brought in with the mixture and gypsum is released through high-temperature condensation; The second is the natural condensation of water vapor during long-term storage cooling process; Thirdly, the leakage of rainwater outside the reservoir causes the self absorption reaction of cement; Fourthly, during pneumatic unloading, a large amount of low-temperature moisture containing air is blown into the outside and evaporates and permeates when exposed to heat.
So it is recommended not to directly feed the kiln clinker into the mill at high temperatures, and to use cooling clinker as much as possible to reduce the cement grinding temperature. Strictly control the absolute moisture content of the mixed materials and gypsum at the source, completely eliminate rainwater leakage, and adopt advanced powder flow control and non pneumatic natural discharge technology.
The cement silo using powder flow control technology can completely avoid the entry of external air and moisture. At the same time, the overall flow discharge movement generated by the outer wall of the silo during discharge can greatly accelerate the continuous dynamic automatic discharge of accumulated blocks on the inner wall of the silo, which can greatly delay and improve the accumulation and growth of cement clumps. In addition, the grid structure size design of the steel Tai Chi cone in the warehouse can effectively and automatically shear and crack various large and low strength cement clumps without causing blockage and material breakage, which is very beneficial for users to concentrate their efforts to control and eliminate clumping from the source.
8、How to understand the natural emptying tailings in powder flow controlled cement storage?
Answer: The powder flow control technology is completely based on the natural overall flow unloading operation principle that uses the weight of powder material as the driving force. In common large warehouses with a diameter to height ratio of about 2.0, when the cement is emptied to the top of the Tai Chi cone, it basically reaches the limit of natural unloading. The remaining tailings will form a natural accumulation center funnel shape, and the height of the tailings depends on the comprehensive fluidity of the cement in the warehouse. If it is a hot storage operation, its fluidity is naturally high and the stacking angle is low. If it is a long-term static cold storage discharge, its fluidity is naturally low and the stacking angle is high; Regardless of the height, all tailings will be automatically pushed out of the warehouse after new feeding, rather than becoming permanent dead material like during pneumatic unloading. Cold storage high tailings only need to find ways to add tens of tons of new materials, and the use of high drop differential energy impact effect can greatly improve the fluidity inside the warehouse, thereby reducing tailings.
In actual normal operation, the powder flow control should be operated at a medium to high storage position of 8 meters or more to form a uniform overall flow discharge of the horizontal material surface. The cement discharge within this working point range has the best effect of high mixing uniformity and stable flow rate.
9、How to introduce powder flow control technology in cement depots?
Answer: The powder flow control technology is completely based on a brand new theory of powder dynamics and overall flow. The entire process design is very simple, only requiring the bottom of the large cement silo to have a low taper and approximate flat bottom, and the middle discharge outlet to be directly installed with a Tai Chi cone. Therefore, the renovation of old cement silos with a pressure reducing cone must completely remove the original pressure reducing cone to ensure the natural dynamic working environment of the Tai Chi cone powder. It has been proven that the removal of the original pressure reducing cone has no effect on the mechanical strength of the cement storage. It is most suitable to carry out powder flow control renovation for cement storage that has been arranged for cleaning during the off-season of sales each year.
A new cement silo, whether it is a steel plate silo or a concrete silo, can be easily designed and constructed according to a flat bottomed silo. It is generally recommended to have a diameter to height ratio of 2.0 or above, and can be designed for single or multiple discharge without the need for labor to design and construct pressure reducing cones. The design strength of the silo body does not need to add anything to the original design, so the construction period and cost of the new silo can be significantly reduced.
10、How to lay the foundation for unmanned and intelligent cement storage as soon as possible?
Answer: In order to operate efficiently, the cement industry is now pursuing full plant intelligence and unmanned operation. However, the actual technical situation of a large number of cement depots is clearly seriously disconnected, and a large number of pneumatic open-loop discharge control methods are still used for production. There is not only a continuous problem of biased storage and dead materials, but also a lack of basic closed-loop real-time feedback monitoring for abnormal working conditions such as shortage, interruption, and flushing. In fact, not only can it not be separated from a large amount of manual intervention and operation, and even the basic automation adjustment operation effect cannot be achieved. Answer: In order to operate efficiently, the cement industry is now pursuing full plant intelligence and unmanned operation. However, the actual technical situation of a large number of cement depots is clearly seriously disconnected, and a large number of pneumatic open-loop discharge control methods are still used for production. There is not only a continuous problem of biased storage and dead materials, but also a lack of basic closed-loop real-time feedback monitoring for abnormal working conditions such as shortage, interruption, and flushing. In fact, not only can it not be separated from a large amount of manual intervention and operation, and even the basic automation adjustment operation effect cannot be achieved.
Only by implementing comprehensive automation technology upgrades and renovations on the cement warehouse can we achieve seamless connection with the cement process of two grinding and one kiln, and truly lay the foundation for unmanned and intelligent production.
The powder flow controlled cement storage technology is the best choice for upgrading the cement storage technology that has emerged.
11、How to determine the value of identifying the technological transformation of cement storage?
Answer: The problem often lies in the value. The value of cement storage technology transformation is to achieve the solution of various inherent problems and defects. During the off-season each year, many enterprises are accustomed to periodic inventory cleaning, but have not further considered whether to choose to continuously deal with the problem of periodic dead materials in the warehouse, or to fundamentally improve the control technology of cement warehouse unloading and completely eradicate the problem of dead materials in the warehouse. If a new powder flow control technology is adopted to transform the cement warehouse, it can achieve unpowered overall flow discharge, significantly release the effective storage capacity of the cement warehouse by eliminating the occurrence of dead materials in the biased warehouse, and quantitatively control the discharge amount to achieve multi warehouse matching and stable process matching, achieving safety, science, energy conservation, and unmanned operation in one go, achieving the advanced level of optimal operation of the cement warehouse.
The operation of cement depots with defects has become a common situation in the industry. Many cement plants clean up every 2-3 years, and the problem of cement depots has become a major minefield for cement plants. Whether or not to use the cleaning and repair opportunity to gradually carry out radical technological transformation often tests the value strategic decision-making level and courage of enterprise leaders.
In the process of promoting the use of fan flow control, many people are asking what will happen in the next three years after the change? How will the five-year meeting be? What will happen in ten years? Often, people selectively forget the fact that they clear their inventory every 2-3 years, and forget the simple principle that extending the clearance period by an additional year will earn an extra year. Moreover, the powder flow control technology uses a Roots blower (commonly 37KW) without air or power for unloading, reducing dust, extending the service life of the dust collector, reducing personnel, improving the efficiency of conveying equipment, elevators and other equipment, and reducing the cost of shipping one ton of cement.
Each cement enterprise has multiple cement warehouses for dynamic buffering, storage, balance, and scheduling between production and sales. If the storage capacity of each warehouse does not have sufficient adjustment space and the operating efficiency is low, it not only increases the operating cost itself, such as dead materials, blockages, electricity consumption, dust, and internal consumption such as clearing the warehouse, but also seriously restricts the potential of cement production capacity. During the peak sales season, missing opportunities for market revenue from shipment volume will indirectly significantly increase production costs. On the contrary, it can greatly reduce operating costs.
So, it is important to increase the technological content of each cement warehouse as much as possible in order to significantly enhance the profitability of cement enterprises at a macro level.
12、Why is it said that the cement silo powder flow control is a software system equipment?
Answer: Cement production is mostly mechanized and continuous process production, so cement technicians are very familiar with various operating mechanical technologies. However, they often tend to form hardware technical thinking, focusing the technical focus of technical equipment on matching flow and quality. Sometimes, they may overlook the software flexibility adaptation caused by fluctuations in production technology conditions, which can be likened to focusing on the caliber and quantity of artillery fire, while neglecting the stability of shooting accuracy.
Although the front-end of the powder flow control presents people with the appearance of a mechanical system, it is actually a complex of large-scale electromechanical information technology. The Taiji cone device inside the warehouse is actually an automatic operation program code customized for the natural overall flow rate of the powder large warehouse. It is organically connected with devices such as the unloading device and impulse meter outside the warehouse to form a hard soft combination of closed-loop control system for the process, including supporting pneumatic switch gate valves and pneumatic blockage removal solenoid valves. All of them are unified into an essential working condition adaptive software system through a specially matched electronic control computer system at the backend. Each hardware link is actually the data generation and operation processing link of the powder flow control software system, so it can be fully integrated with the accurate automatic calculation and adjustment control of the cement silo working conditions, ultimately achieving the goal of high efficiency and stability. The mechanical knockoff combination ideas that are pieced together cannot achieve the comprehensive effect of powder flow control.
If the original cement storage technology is a traditional rigid direct fire inertial weapon system, the powder flow control technology is a modern precision curved missile weapon system for cement storage. Compared to the bulky and heavy firepower coverage of mechanical thinking, software thinking emphasizes precision, short size, and one hit. Cement production is essentially a war of input-output ratio, and advanced technology and equipment will definitely win with a better battle loss ratio.
