Analysis of the influence of billet copper mould tube cooling water on copper tube scaling

Abstract: During the operation of the 150×150 billet copper mould tube with indirect open circulation cooling, it was found that when the quality of the copper mould tube cooling water was basically stable (total hardness <250 mg/L, calculated as CaCO3, the same below), the cooling water outlet temperature When the temperature is higher than 41°C, it will significantly affect the scaling of the copper tubes of the copper mould tube. When the cooling water outlet temperature is high, a large flow rate can effectively improve scaling. Taking into account the copper mould tube cooling water inlet and outlet temperatures, water volume, flow rate, and water quality is the key factor. Analysis shows that controlling the outlet temperature of the cooling water to <41°C, the temperature difference to <7°C, the flow rate to about 14 m/s, the flow rate to >140 m3/h, and the total hardness to <250 mg/L can ensure the safety, stability, and normal operation of the copper mould tube.

Keywords: billet; copper mould tube; scaling; cooling water temperature

Preface

The quality of the heat transfer state of the copper mould tube directly affects the production and quality of the slab. Studies have shown that the temperature of the copper mould tube cooling water significantly affects the cold surface temperature of the copper tube. If the water temperature exceeds 313K, the maximum temperature of the cold surface of the copper tube will exceed the boiling point of water. An increase in water velocity of 0.49 m/s can eliminate the water temperature rise. The adverse effects of high 4K. When the water velocity increases, the degree of turbulence of the fluid increases, that is, the shear force of the fluid on the dirt layer increases, and the power of the dirt layer shedding increases. At the same time, the increase in turbulence force can reduce heat transfer. The thickness of the stagnant bottom layer in the boundary layer reduces the convective heat transfer thermal resistance. These two reasons reduce the fouling thermal resistance and increase the total heat transfer coefficient.

copper mould tube fouling and microbial reproduction are key issues that need to be addressed. Set up a reasonable water supply system. The cooling water volume and flow rate are important factors affecting abnormal scaling of the copper mould tube. Providing qualified water supply quality and reasonable water quality stabilization treatment to prevent scaling of the copper mould tube is to ensure good thermal conductivity of the copper mould tube. It is also an important measure to ensure the safety, stability and normal operation of the copper mould tube.

1 Crystallizer process and problems in a steel plant

A steel plant produces two 150×150 billet machines for rebar production. The copper tube length is 1000 mm, the seam width is designed to be 3.8±0.2 mm, the copper tube life is designed to be 450 furnaces, and the crystallizer cooling water system is an indirect open circulation cooling system. , the water replenishment is industrial water ring pipe network + softened water, the cooling water pump is on and off twice, each pump has a nameplate flow rate of 790 m3/h, a lift of 90 m, the water system is equipped with an industrial water replenishment filter, and the billet was efficient in mid-April 2018 The chemical transformation is completed, and the maximum design pulling speed is 4 m/min.

From June to July 2018, the offline copper mould tube pulling speed was 3-3.5 m/min, and the total cooling water flow of the dual-machine copper mould tube was about 1560 m3/h (the total water volume of the 1# machine was about 4% greater than that of the 2# machine). The inlet water temperature is 28~36℃, the temperature difference of the 1# machine is 4.7~6℃, and the temperature difference of the 2# machine is 5.1~7.4℃. The scale control of the crystallizer copper tube is basically normal, and the copper tube life is about 500 furnaces. In August 2018, the billet workshop reported scaling in the copper tubes of the off-line copper mould tube. This article discusses the analysis of cooling water issues related to this phenomenon.

2 Overview of copper mould tube cooling water system

2.1 The process parameter specifications for copper mould tube operation are as follows:

Inlet water temperature: 35 ℃;

Temperature rise: 10℃;

Pulling speed: 3.0~4 m/min;

Middle package temperature: 1520~1550℃.

2.2 The copper mould tube operating water quality is shown in Table 1

Table 1 Continuous casting mold operating water quality

projectpHConductivity/μS/cmCl-/(mg/L)Turbidity/NTUAlkalinity/(mg/L)Total hardness/(mg/L)Total phosphorus (PO43-meter)/(mg/L)
Industrial hydration8.2500205125 ~ 150200 ~ 2500.25
Soft water hydration500<3
circulating cooling water8.35302541232202.1

2.3 Continuous casting mold online time point and cooling water parameters in August 2018

The cooling water flow, temperature, and temperature difference data in Figures 1 and 2 come from the on-site operation records of a steel plant. The cooling water flow rate of one machine only records one data per channel per shift or every day. The temperature of one machine per furnace and six channels only randomly records one data. The process computer has no historical record curve query. The operation curve is listed with the Continuous casting mold online operation time point (not a specific time and date) as the abscissa and the cooling water parameters recorded by the corresponding team as the ordinate. The following table is the same.

2.4 The online time points and cooling water parameters of the continuous casting mold from September to October 2018 are shown in Figure 3

Figure 1 copper mould tube online time point and cooling water parameters

Figure 2 Online time points and cooling water parameters of 15#, 25#, and 17# copper mould tube

Figure 3 Online time points and cooling water parameters of 7#, 11#, and 12# copper mould tube

3 Analysis of abnormal scaling of copper mould tube

3.1 Continuous casting mold cooling water quality

The cooling water carbon steel plate is 0.0067 mm/a. In addition to the seasonal changes in water quality with water replenishment, from the daily laboratory test data and stability analysis of the copper mould tube water quality, the water quality remains basically stable (pH=8~8.6, total hardness 200~250 mg/L ), but water quality sterilization treatment and bypass filtration need to be more optimized to reduce the impact of microorganisms and suspended solids.

3.2 Influence of copper mould tube cooling water outlet temperature, flow rate and flow rate

From the statistical analysis of the operation data of the two sets of crystallizers in Figure 1, it can be seen that the cooling water outlet temperature of the 15#, 25#, and 27# crystallizers all reached above 42°C, and the temperature differences were basically similar. The cooling water flow rate of the 25# crystallizer is basically stable at more than 142 m3/h, and only local scaling occurs at the slag liquid level. During the operation of the 15# crystallizer, the flow rate was too low and the outlet water temperature was high, and the scaling tendency became more serious. Among the 4#, 7#, and 11# crystallizers, the cooling water flow rate of the 4# crystallizer is basically stable below 140 m3/h, and the cooling water outlet temperature (>42℃) and temperature difference are relatively high, and the copper tubes are scaled on all sides. .

Tracking the on-site production situation, it usually takes about 10 to 15 days for the crystallizer to go online. In July and August, the inlet temperature of the copper mould tube cooling water is generally 35°C, reaching a maximum of 37°C, and the temperature difference is generally 7~8°C. The cooling capacity problem of the cooling tower in the circulating cooling system causes the crystallizer cooling water inlet temperature to be high.

From the statistical analysis of the copper mould tube operation data in Table 2 and Figure 2, it can be seen that the cooling water inlet temperature has dropped significantly since the end of September, the cooling water outlet temperature of the 7# and 11# copper mould tube is <41°C, and the surface of the copper tube is basically free of scale. (There is thin scale locally on the slag liquid surface, but there is no deformation, so surface sampling is difficult). The subsequent copper mould tube that came off the production line in October also showed that the surface quality of the copper tubes was better when the cooling water flow rate was relatively large. When the copper mould tube cooling water outlet temperature is < 40°C and the flow rate is appropriately reduced (12 m/s), scaling on the surface of the copper tube is basically normal. The temperature of the copper mould tube cooling water significantly affects the cold surface temperature of the copper tube. The surface conditions of the 25# and 4# copper mould tube indicate that the water temperature exceeding 41°C may cause the maximum temperature of the cold surface of the copper tube to exceed the boiling point of water. Increased water flow rate can eliminate the adverse effects of increased water temperature.

3.3 Influence of the distribution of cooling water in each flow channel of the copper mould tube

The distribution of cooling water relies on manual adjustment. It is difficult to accurately adjust the water volume, and the water volume of each flow channel may also change within a day. Tracking the on-site production situation, we found that sometimes the water volume in individual flow channels is obviously too small, but the record of the day does not reflect it. For example, at 10:30 on September 10, 2018, the three-channel computer showed a flow rate of 113 m3/h and an inlet water temperature of 33°C. The temperature difference is 10.1°C, but the flow rate recorded in the operation that day is 142 m3/h. This difference should be caused by the cooling water flow rate recorded only once per shift in the on-site operation. This special water volume has a greater impact on the copper mould tube, but the trend analysis is basic reflects the actual situation.

3.4 Effect of copper mould tube cooling water gap adjustment

The water gap between the copper mould tube and the water jacket is designed to be 3.8±0.2 mm, and the designed outer dimension of the copper tube is D-0.2 mm. The width of the water gap in each copper mould tube was continuously adjusted and checked. It was found that during the maintenance process, the water gap was actually offline due to the copper pipe. There will be a big difference in replacement. Some copper mould tube show significant differences in local scaling on each side. During the maintenance process, it is necessary to maintain the balance of water volume on each side.

4 Conclusion

During the operation of the 150×150 billet copper mould tube with indirect open circulation cooling, it was found that when the quality of the copper mould tube cooling water was basically stable (total hardness <250 mg/L), the cooling water outlet temperature was higher than 41°C, and the cooling water A flow rate lower than 140 m3/h will significantly affect the scaling of the copper mould tube. When the cooling water outlet temperature is high, a large flow rate can effectively improve scaling.

Comprehensive consideration of the copper mould tube cooling water inlet and outlet temperatures, water volume, flow rate, and water quality are key factors. Controlling the copper mould tube cooling water outlet temperature to <41°C, the temperature difference to <7°C, the flow rate to 14 m/s, the flow rate to >140 m3/h, and the total hardness to <250 mg/L can ensure the safety, stability, and normal operation of the copper mould tube.

The temperature of the cooling water after the open circulation cooling tower of this steel plant reaches 36~37℃ in the hot season, and the total cooling water flow rate of the two machines is about 1560 m3/h. The existing open net circulation process and water quality conditions will have an impact on the copper mould tube. The pipes cause scaling, and the cooling effect of the cooling tower of this system is obviously insufficient. It is necessary to modify it to meet the requirements of controlling the copper mould tube cooling water outlet temperature to <41°C and the temperature difference to <7°C. At the same time, the amount of softened water replenishment must be sufficient, the addition of acid must be stable and controlled, and the pH value and total hardness of the cooling water must be controlled within the required range.

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