The water supply requirements of the closed circulation cooling water system of desalted water in the crystallizer of the continuous casting machine must be constant pressure water supply, stable water quality, and water supply temperature that can meet the requirements of production changes. Taking the closed circulation cooling water system of desalted water in the crystallizer of a 3* slab continuous caster as an example, we will discuss how to ensure that the system’s water supply pressure is stable, the water supply quality is up to standard, the water supply temperature meets the production change requirements, and the closed cooling tower and copper pipes are prevented from freezing in winter. Several aspects of frost cracking were analyzed and studied. Corresponding technical transformation has been made to the process, and the above problems have been effectively solved.
Keywords: continuous caster crystallizer; desalted water closed circulation cooling system; closed cooling tower; pressure stability; water quality compliance; process improvement
The cooling water system of the crystallizer is often a closed system, and the water quality requirements are relatively strict, with the return water temperature being 45°C. Therefore, a soft water (or demineralized water) closed-circuit circulation system is generally used. It is not recommended to use a desalted water open circuit circulation system for crystallizer water, because desalted water removes all dissolved substances in the water and does not have buffering properties for the pH value. Therefore, water is in contact with air in an open circuit system, and carbon dioxide in the air will dissolve in the water, which is highly corrosive to metals, and it is difficult to control water quality stability. On the other hand, the evaporation loss of desalted water during the cooling process of the cooling tower is also serious, resulting in waste. .
The cooling water for the crystallizer of Shougang Qiangang’s 3* and 4* twin-flow slab continuous casters is supplied by a closed circulation system. This process has extremely strict requirements on the stability of the water supply pressure, which must be between 1.03 and 1.05 MPa to prevent large-scale fluctuations in pressure. The cooling water uses demineralized water, which requires total iron to be controlled below 1.0 mg/L and total phosphorus to be controlled between 2.0 and 5.0 mg/L. The water supply temperature is controlled below 33°C. In order to meet the user’s various index requirements and ensure stable production, we have made corresponding technical transformations and improved the operation methods of the relevant aspects of the circulating water system process based on the original design.
Introduction to circulating water system
The process flow is shown in Figure 1.
Figure 1 Process flow chart of closed circulation system for desalted water
3* The crystallizer cooling water supply system mainly includes a closed cooling tower, mechanical seal circulating water pump, spray water supply pump, frequency conversion water replenishment device, automatic dosing device, etc.
Closed cooling tower
The closed cooling tower is mainly used as the cooling equipment of the circulating cooling water system. 3* The cooling equipment of the crystallizer cooling water system adopts 4 CXW-U440ASW series surface evaporative air coolers, each with a water treatment capacity of 250 m²/h, and an inlet and outlet water temperature of 45~35°C. A closed cooling tower (also called an evaporative air cooler or a closed cooling tower) places a tubular heat exchanger inside the tower to ensure the cooling effect through heat exchange between circulating air, spray water and circulating water. The working principle of the closed cooling tower is to spray cooling water downwards above the horizontally placed brass light tube coils. A thin water film is formed on the outer surface of the copper tube to absorb the heat emitted by the circulating water in the copper tube. At the same time, the air sweeps across the copper tube from the side of the tower, and the fan draws out the hot air, thereby reducing the temperature of the spray water. That is the basic cooling principle of cross flow cooling tower. Because it is a closed cycle, it can ensure that the water quality is not polluted, well protect the efficient operation of the main equipment, and improve the service life. When the outside temperature is low, the sprinkler system can be stopped to save water. With the implementation of national energy conservation and emission reduction policies and the increasing scarcity of water resources, closed cooling towers have been widely used in steel metallurgy, power electronics, mechanical processing, air conditioning systems and other industries in recent years.
Mechanical seal circulating water pump
This system uses three 250SS150B mechanical seal horizontal centrifugal pumps. The pump unit operates in a dual-use and one-standby mode.
Cooling tower refrigerant water supply pump
This pump unit mainly provides spray water for the cooling tower. It uses three 500S-35 mechanical seal horizontal centrifugal pumps. The pump unit operates in a dual-use and one-standby mode.
Frequency conversion water replenishing pump
Frequency f=50Hz, power P=7.5kW, head H=53.1~70.3m, flow rate Q=30m²/h. The operation mode of the pump unit is dual use and one standby.
Automatic dosing device
Generally, the circulating water temperature of a closed desalted water system is 10~13°C higher than that of an open system. For every 10°C increase in water temperature, the corrosion rate of steel increases by approximately 30%. In order to reduce corrosion, an automatic dosing device is installed in the pumping station, and the corrosion rate is controlled by adding organic high-efficiency phosphorus corrosion and scale inhibitor ZY-268B into the system.
Automated testing instruments (temperature sensors, pressure sensors, etc.)
Problems encountered during operation practice and solutions
Problems of closed cooling tower freezing and copper pipes being cracked in winter
Causes of the problem
The winter temperature in the northern region is usually below zero, and the antifreeze problem in the operation of closed cooling towers is prominent. If not solved properly, the heat exchange tubes or other components of the cooling tower may freeze and damage. According to different process characteristics, some closed cooling towers operate all day in winter, some operate in divided time periods, and some are hardly used, but antifreeze issues need to be considered.
Qiangang is located in Qian’an City, Hebei Province. According to local meteorological data, the annual average temperature here is 10.10℃ and the extreme minimum temperature is -28.2℃. It is particularly important to consider the anti-freeze issue of closed towers. During the operation of the closed cooling tower of the 3* crystallizer water system, it was found that the spray water splashed out seriously. In winter, when the crystallizer is changed or the crystallizer is replaced, the temperature of the circulating water in the system drops. If the outside temperature is too low, the surface of the copper pipe will freeze and be cracked. The wall thickness of the copper pipe is only 0.8 mm. Moreover, ice will also appear on the closed tower louvers and the inlet and outlet valve operating platform, causing great inconvenience to daily operations. Based on the analysis of the actual operation situation for one year, the crux of this problem is that there are problems with the basic structure of the closed tower, such as the tower body is too small, the width of the louvers is unreasonable, there are debris in the copper pipes, and the water is not drained completely.
In view of the process characteristics of the 3* crystallizer, the cooling tower operates year-round. However, changes in production load and temperature will also cause changes in the number of operating cooling towers. Based on the above analysis results, we take the following antifreeze measures:
(1) Add bypass pipes and reasonably adjust the number of cooling towers operating in winter. All cooling towers do not need to be put into operation in winter, but the original designed 3* crystallization system circulating water volume is 1000 m³/h, so it is necessary to ensure that all four towers are put into operation. For this reason, we add bypass pipes to the cooling tower inlet and outlet pipes. In winter, the bypass valve can be appropriately adjusted to allow part of the flow to pass through the bypass pipe, thereby reducing the number of operating cooling towers. Operational practice has proven that for a 3* crystallizer water system, only two cooling towers can be operated in winter to meet the water supply temperature requirements and greatly reduce the antifreeze intensity in winter.
(2) Install a filter to solve the problem of copper pipe clogging. When replacing the frozen-cracked copper pipes, it was discovered that there were debris in the copper pipes. The analysis showed that when the cooling tower stopped operating, the water was not completely drained due to the presence of debris in the pipes. In addition, since each closed cooling tower is divided into two modules, each module has 10 layers of horizontal coils, if there is a problem with the installation slope of the copper pipes during construction, there will be residue when the cooling tower releases water, resulting in Copper pipes cracked. To this end, we install an online cleaning filter before the system returns water into the tower to intercept debris in the water. In addition, when the copper pipes are drained, compressed air is used to blow the copper pipes to ensure that there is no accumulated water in the copper pipes.
(3) Modify the louver configuration to solve the problem of spray water splashing. When the spray water falls on the blinds, the blinds are not wide enough to block the splashing water. Therefore, we refer to the principle of using blinds to widen the blinds appropriately and enable them to close freely. When the crystallizer stops pouring steel, the louvers are closed to maintain the temperature of the circulating water in the copper tube. When the crystallizer starts pouring steel, the louvers are opened and the cooling tower operates normally.
(4) Add a heat exchanger to the system return pipe to ensure that the temperature of the circulating water in the copper pipe does not drop too much when the crystallizer is stopped in winter. This can not only prevent freezing, but also meet the water temperature requirements before the continuous casting machine starts pouring steel, and solve the problem of corner cracks in the first two billets.
Ensure constant pressure water supply in the system
The condition for stable water supply in the water supply system is that the flow rate required by the user and the actual flow rate supplied are in balance. That is, if the water consumption is more, the system will supply more, and if the water consumption is small, the system will supply less. The necessary condition to achieve this is to always keep the system in a constant pressure water supply state.
The main factors causing pressure fluctuations during the operation of the desalted water closed circulation system are: system leakage, thermal expansion and contraction of the circulating water due to heat exchange, system filter discharge, etc. When a tiny water burst occurs in the system, pressure fluctuations will occur in the closed circulating water system. To this end, a variable frequency water replenishing pump is installed in the pumping station to ensure stable pressure in the closed circulation system.
The selection of constant voltage value can be based on the following situations depending on the location of the monitoring point.
(1) Pipe end pressure
Pipe end pressure refers to the end pressure from the water supply pipe to the user. When the water supply system design is completed, the pipe resistance characteristics of the pipeline are determined. When the system pressure fluctuates, the variable frequency water replenishing pump adjusts the water pressure by changing the rotation speed, and all operating points fall on the pipe resistance characteristic curve. The advantage of this control method is that it is more accurate, but the disadvantage is that the distance between the pressure sensor and the control cabinet is too long, which is difficult to implement, and the pressure change at the end of the pipeline lags behind, which can easily cause the frequency converter to oscillate.
(2) Return water pressure
For this closed circulating water system, the return water pressure refers to the pressure of the system return water main pipe, which is relatively stable. However, in the early stages of system operation and debugging, since there is no specific calculation result for the return water pressure, it is difficult to retrieve the water pressure at the pressure signal point, and the pressure preset value needs to be explored, which is not conducive to the debugging of the crystallizer.
(3)Water supply pressure
The water supply pressure here refers to the pressure of the circulating pump out of the jellyfish tube. The monitoring point is installed on the main pipe at the outlet of the pump, usually close to the control cabinet and easy to implement.
In view of the selection of the above pressure points, combined with the operation conditions of Qiangang’s 1* and 2* billet closed circulating water systems, the frequency conversion water supply pump signal of the 3* slab closed circulating water system selects the water supply pressure. When the system water supply pressure fluctuates, the pressure sensor transmits the pressure fluctuation signal to the frequency converter. According to the pressure preset value, the frequency converter transmits the frequency conversion command to the frequency conversion water replenishment pump motor, and then the frequency conversion water replenishment pump can work, making the system water supply pressure constant and realizing automatic water replenishment. Figure 2 is the control diagram of the 3* crystallizer frequency conversion water replenishment system.
Figure 2 Control diagram of frequency conversion water replenishment system of 3# crystallizer
3* The operation practice of the closed circulation cooling water system for slab continuous casting machine crystallizer desalted water found that when the entire system operates stably, the frequency of the frequency conversion water replenishing pump motor is almost zero. However, when the system needs to replenish water, the frequency converter will automatically increase from 0 Hz to 50 Hz in about 20 s. At this time, the system pressure will fluctuate up to about 0.2 MPa, seriously affecting the quality of the billet. In order to solve this problem, a DN20 return pipe is added to the system outlet pipe to enable the frequency converter to run at low load for a long time and shorten the frequency change time of the variable frequency water replenishment pump. After the transformation, the pressure of the entire system was stable and no large-scale fluctuations occurred, as shown in Figure 3.
Figure 3 Comparison of pressure fluctuations before and after transformation
Water supply quality stability issues
In any industrial circulating cooling water system, the stability of water supply quality is crucial, because it is related to the quality of products, equipment, pipeline service life, utilization rate of circulating water, etc. The main ways to ensure qualified and stable water quality in the desalted water system are: adding corrosion and scale inhibitors, water stabilizers, and bactericides according to the prescribed plan; when a certain indicator in the water exceeds the standard, replace it with new water. When the 3° crystallization circulating water system was put into operation, the water quality was extremely unstable, the total iron content exceeded the standard, and the color turned yellow. However, the closed circulating water system is designed without replacement pipes, and it is considered that even after a normal delivery, if a certain water quality index of the circulating water exceeds the standard, new water must be replaced to ensure stable water quality. Therefore, we decided to add a replacement pipe to add a reliable way to adjust the water quality of the closed circulation system, which has been proven to play an important role in daily production. As shown in Figure 4, the stability control of total iron and total phosphorus in the water was significantly improved after the transformation compared with before the transformation. (The data in Figure 4 are water quality record values during a certain period before and after the transformation)
Figure 4 Comparison of total iron and total phosphorus before and after transformation
3* After various technical transformations, the closed circulation cooling water system of the slab continuous caster crystallizer desalted water has solved the problems of constant system pressure, stable water quality and anti-freezing of the closed tower in winter. Operation practice shows that the water supply stability of this system has been significantly improved. We have also gained a deeper understanding and mastery of the closed circulating water system, accumulated experience in operation and management, and improved the level of operation and management.