Automatic control system for cooling water circula

2022-07-31
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Automatic control system for cooling water circulation of continuous caster introduction the intense competition in the production market of industrial products has made people more and more strict about the requirements of equipment and control quality. With the continuous development of science and technology, some new technologies and processes are continuously applied to the process control of industrial production, so that the previously unsolvable problems can be solved. Based on the perfect combination of frequency converter and PLC, it is applied to the water level control system of the recovery pool in iron and steel enterprises. There are zigzag attachments and shear attachments, which equivalently solve the water level control problem that has been difficult to solve, and bring huge management and economic benefits

2 brief introduction to water level control process of raw water return pond

more than 60% of China's iron and steel plants use low consumption and high efficiency continuous casting billet process. The process relies on water cooling in the drawing process. Due to the global shortage of water resources, in order to reduce costs and save energy, the cooling water is recycled to the reflux pool and recycled after treatment. The continuous caster of No. 3 steelmaking plant of Lianyuan Iron and Steel Co., Ltd. also adopts this cooling water recycling mode

the cooling water reflux pool of No.3 Steelmaking Plant has a depth of 12m and an area of 55m2. Two 70kw water pump equipment and regulating valves are installed at the platform 8m away from the pool bottom. The water inlet suction head of the water pump is 1m away from the pool bottom. The water level of the original cooling water reflux pool is controlled manually. The personnel on duty observe the water level in the pool with their eyes. According to the change of the water level in the pool, they manually operate the start and stop of the water pump and manually adjust the flow of the control valve to transport the water from the reflux pool to the wastewater treatment place for recycling after treatment. 2. It is to promote the progress and development of the plastic blending technology, new plastic additives and utilization technology into the water cooling process of the continuous casting slab

the problem of this water treatment process is that when the water level is too high, the water pump is not fully put into operation, flooding the equipment on the 8m platform. When the water level is very low, the water pump runs too fast, resulting in cavitation, which leads to empty suction oscillation of the water pump and serious damage to the water pump blades. Therefore, the two water pumps have to be overhauled several times a month. Either the equipment is flooded or the water pump is damaged. In addition, the power distribution cabinet is often burned out and has to be repaired many times a month. Therefore, it not only causes a huge waste of human and material resources, but also affects the normal production

3 problem analysis of the original system

3.1 objective causes of time-varying randomness included in the production process

(1) during the production process, the number of working units of the combined casting machine changes greatly at random. If the number of working units is large, more water will be used, and the reflux will be large, and vice versa

(2) in case of emergencies, summer rainstorm or other emergencies, water will also flow into the reflux pool, which may cause the water level of the reflux pool to rise suddenly

(3) the reflux pool is deep and the light is not good, so it is difficult to observe the accurate water level

(4) the operation is complicated, and it is inconvenient to operate the upper and lower pools

3.2 reservoir volume and equipment configuration of the original system and working mode of equipment control

(1) reservoir volume

55 × () =385 (M3) (1)

(2) the pumping system consists of two water pumps, two regulating valves, two water pump motors, starting cabinet and operation console. The technical indicators of the water pump are as follows:

water pump model: y350m-4

power: 70kw

rated voltage: 380V

rated current: 140A,

rated speed: 1480r/min

drainage: 1100m3/h

(3) control mode

the operator shall observe the water level of the reservoir with his eyes, put the water pump into operation according to the deviation of the water level, and operate the corresponding regulating valve. Sometimes one or two are put into operation. The water pump motor is directly started and cannot be adjusted

3.3 analysis of problems existing in the original system

(1) analysis of pump flow

the flow of each pump is 18.3m3/min, and the maximum volume of the reservoir is 385m3. To empty the reservoir, even if only one pump is put into operation (the return water flow returning to the reservoir is not recorded temporarily), then:

385 ÷ 18.3=21 (min) (2)

if two pumps are put into operation at the same time, it will be 11min, However, in actual production, the water level of the pool is required to be maintained below the water level of half of the pool, so it can be seen that the pumping time is shorter, which will bring some problems analyzed below

(2) analysis of pump working mode

since there are only two working modes of the pump, either full load operation or shutdown, the pump pumps water too quickly and can not adjust the speed, and it is difficult for the operator to adjust the regulating valve in time and control the pumping flow at a suitable amount of water, so the pump can only start and stop frequently

(3) control mode analysis

since the control mode of the water pump is manual, the operator must first observe the water level of the reservoir, and then control the battery valve and start/stop water pump. The pumping flow of the water pump is large. To maintain a certain water level, the workers must frequently observe the water level and frequently operate the valve and pump motor. Due to excessive frequency, especially in the trend of consumption upgrading, the labor intensity is high, especially in the night shift. In order to avoid flooding the equipment due to high water level, most workers prefer to keep the water pump running without stopping. The long-term idling of the water pump causes cavitation erosion and damage to the pump blade pump

(4) the pump motor control cabinet is often burned. Analyze the working mode of direct starting of the pump motor, so that the starting current is 4 ~ 7 times the rated current, and the current impact is as high as hundreds of amperes to kiloamperes. As the pumping flow of the pump is too large and too fast, in order to ensure that the water level is at a reasonable level, the motor must be started frequently, causing continuous large current impact, thus burning the motor control cabinet

4 technical transformation scheme

4.1 new process technical requirements

(1) grasp the accurate water level signal in real time

(2) the water level in the pool remains basically unchanged regardless of the amount of water flowing into the pool

under normal conditions, the flow into the return pond is 80m3/h, ~320m3/h, which shows that the drainage capacity of the original pump is sufficient. From the operating mechanism of the water pump, it is concluded that the relationship among the shaft power P, flow Q and pressure h of the motor is:

p ∝ Q × H (3)

Q2=Q1 × (N2/N1) (4)

H2=H1 × (N2/N1)2 (5)

P2=P1 × (N2/N1) 3 (6)

4.2 new process technical scheme

it can be seen from the formula () that in order to ensure the stability of the water level of the reflux pool, the water discharge of the pump is required to follow the change of the inflow volume, which must be realized through the adjustment of the pump speed. Therefore, the MASTER-K120S series programmable controller and Mitsubishi FR series variable frequency governor are used in the solution to realize the automatic control of closed-loop variable frequency speed regulation of the water level through software programming, This perfect combination of frequency converter and PLC is applied to the water level control system of the recovery pool, which can effectively solve the control problem of continuous casting cooling circulating water that has been plagued by frequent accidents on site

5 structure and working principle of technical transformation system

in order to realize these functions, closed-loop automatic control of reflux pool water level must be adopted. When the water level changes, the flow of the pump will automatically follow the change, so as to ensure the dynamic and constant water level of the reflux pool. See Figure 1 for the working principle block diagram of the automatic water level control system of the reflux pool

Figure 1 working principle block diagram of water level automatic control system

the water level automatic control system is composed of water level sensor, water level regulator, PLC controller, frequency converter and water pump. The controlled object is the water pump motor

the water level sensor detects the water level parameters of the reflux pool. The water level sensor adopts diffused silicon isolated sensitive components, and is equipped with dynamic pressure compensation and temperature compensation circuits to eliminate the errors caused by liquid level fluctuation and water temperature, and finally convert the liquid level signal into standard electrical signal

plc controller receives the water level signal from the water level sensor to detect the water level of the reflux pool, and outputs the constant current signal after PID adjustment operation to determine the working state of the variable frequency governor. At the same time, the upper limit and lower limit of water level set according to the actual situation of the site are compared and calculated, the logic operation is carried out, and the upper limit and lower limit alarm signals are output

the start and stop of the variable frequency governor are controlled by the output of the PLC, while the frequency output of the variable frequency governor depends on the output signal of the "PLC water level regulator", and the output signal of the "PLC water level regulator" is determined by the water level of the water level sensor

plc main control program flow chart is shown in Figure 2

Fig. 2 flow chart of PLC main control program

6 conclusion

effect after transformation

6.1 failure rate decreased and productivity increased

(1) after transformation, the system uses variable frequency governor to supply power, which fundamentally solves the problem of starting current impact, from the original 700a to below 70A, and solves the problem of power distribution panel damage due to current overshoot

(2) the reformed system has realized the automatic control of water level, and solved the problem of shutdown due to the damage of water pump caused by flooding equipment or evacuation. Therefore, the shutdown accidents caused by flooded equipment have been reduced from 6 times/year to 0, and the equipment maintenance has been reduced from 24 times/year to once-a-year normal maintenance. It can be seen that the system failure rate has decreased and the productivity has increased after the transformation

6.2 energy saving effect let's take a look at how to maintain and protect the experimental machine and change the use benefit of the experimental machine.

from the original control valve and frequent starting and stopping of the motor to the variable-frequency speed regulation of the motor speed to control the displacement, which reduces the energy loss of the motor starting and full load operation, and also eliminates the unnecessary friction loss of the control valve

the comparison of input current (primary side) line current values before and after transformation is shown in the attached table

comparison of line current in the attached table

it can be seen from the attached table that the starting current of the motor after the transformation is 100 times lower than that before the transformation. As the water pump is a soft load, it is started by the frequency converter after the transformation, which fundamentally eliminates the starting impact of the motor, so the energy consumption is greatly reduced. In the "work" and "maintenance" stages, the energy consumption is also reduced exponentially

(1) energy consumption during motor startup before transformation

the motor startup time is 1 minute, with an average of 5 starts per shift and 15 starts per day. In a year, except for two months of overhaul, 300 days are in operation. Therefore, the power consumption for a year is:

700 × three hundred and eighty ×× 1/60 × fifteen × 300=19950 (kw.h)

after the transformation, the motor has been in the "working" or "maintaining" state after starting, so there is no starting power consumption

(2) power consumption in "working" state

the automatic water level control system of the cold reflux pool of No. 3 steelmaking plant is in "working" state for 40% of the time, and the power saved in one year is:

() × three hundred and eighty ×× twenty-four × three hundred × 40%=98496 (kw.h)

(3) power consumption in "maintained" state

the automatic water level control system of cold reflux pool of No. 3 steelmaking works in "maintained" state most of the time. Therefore, the power saved in one year is:

() × three hundred and eighty ×× twenty-four × three hundred × 60%=164160 (kw.h)

calculated at 0.5 yuan per kilowatt hour, the savings of one-year common electricity are:

(19950+98496+164160) × 0.5=1413 (ten thousand yuan)

it has been running for two years since the automatic control of water level has been realized. The operation is in good condition and the effect is very significant. (end)

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