Application of double adjustment technology of crystallizer cooling water flow and pressure

This article introduces the implementation method of dual-adjustment technology of crystallizer cooling water flow and pressure in Caigang’s 800mm continuous casting machine, which achieves independent control of the inlet pressure of the crystallizer of each casting strand and the pressure stability of the closed water-cooling pipe network.

Keywords: cooling water; PID control; dual regulation

Preface

The Laigang Special Steel 800mm bloom five-strand continuous casting machine can not only produce φ800mm blooms, but also φ500mm and φ650mm cross-section round billets. Different specifications of crystallizers have different requirements for water supply pressure. The original design water supply pressures of φ500mm, φ650mm and φ8000mm cross-section crystallizers are 0.9MPa, 1.0MPa and 1.1MPa. The crystallizer cooling water is provided by the joint water pump room, and the water supply pressure can be changed at any time according to production needs. The crystallizer cooling water system is shown in Figure 1.

Figure 1 Crystallizer cooling water system structure

After being put into production, although the casting strand PLC achieved stable flow control by adjusting the opening of the crystallizer outlet regulating valve, the water supply pressure from the water pump room was very unstable. In addition, the water supply pressure of each crystallizer stream of the continuous casting machine is the same, and individual pressure control cannot be achieved. In order to arrange the production plan more rationally, it is necessary to cast billets with different cross-sections at the same time. As a result, the small-sized crystallizer has been subjected to excessive water pressure, which has adverse effects on it and threatens its safety and the normal production of the continuous casting machine.

To address the above shortcomings, we carry out technical transformation, install a regulating valve at the entrance of the crystallizer of each stream of the continuous casting machine, and use automation technology to effectively adjust the water supply pressure and water outlet flow of each stream of the crystallizer to control it within the range of process requirements.

Problems that exist

(1) Pressure fluctuation. The main reasons causing large fluctuations in crystallizer cooling water supply pressure are:. The crystallizer cooling water supply pressure is controlled separately by the combined water pump room. The combined water pump room and the continuous casting machine have their own automation systems and there is no communication between them. The water pump room operator sets the target pressure of the water pump based on the pressure notified by the continuous casting master control phone. Obviously, this manual adjustment method sacrifices the degree of automation of the system and has a pressure regulation lag. The combined water pump room that supplies cooling water is very far from the continuous caster, and the pipe network has a certain impact on the water supply pressure. Usually the water outlet pressure of the water pump room is 0.1MPa or even more than the water supply pressure on the continuous caster side. It is definitely not appropriate for the water pump room personnel to adjust the pressure according to the requirements of the continuous casting master control. It may be necessary to make multiple adjustments, causing pressure fluctuations in the pipe network.

(2) Each stream of crystallizer cannot be controlled individually.

The simultaneous production of multiple sections was not considered in the early design of this continuous casting machine. The crystallizer water supply pipes of each stream are directly connected to the water supply main pipe, so the water supply pressure of the crystallizers of each stream is the same. With the optimization of production planning methods and changes in the slab management system, sometimes two specifications of slabs are produced in the same pour. At this time, the pipe network can only use the required higher pressure to supply water, which brings pressure shock to the small-sized crystallizer, which not only affects its service life, but also reduces the quality of the slab.

Pressure and flow dual adjustment technical solution

This technology mainly takes measures to solve the problem of unstable water supply pressure and the inability to independently control the pressure of each stream of crystallizer. The joint water pump room performs automatic control based on the pressure at the continuous casting machine end of the water supply pipe network to stabilize the water supply pressure from the source. In addition, a regulating valve is added before the inlet of each stream of crystallizer. Through automatic program control, on the premise of ensuring the flow rate of the crystallizer, the opening of the regulating valve is adjusted to ensure that the crystallizer operates at a suitable pressure.

Implementation of automatic control technology to stabilize main water supply pressure

The first step in stabilizing the automatic control of main water supply pressure is to realize the networking of the continuous casting PLC system network and the joint water pump room PLC system network. Since the two places are relatively far apart, it is necessary to use optical cable communication to merge the joint pump room network into the continuous casting system network so that both parties can PING each other.

After the networking is completed, use the downtime to modify the hardware network configuration of the continuous casting public PLC, establish the TCP/IP network connection between the continuous casting public PLC and the water pump room PLC, and modify the continuous casting public PLC program. Send the actual pressure and target pressure of the main water on the continuous casting side of the water supply pipe network to the DB address specified by the PLC in the water pump room. The target pressure value is equal to the target pressure required for the largest size crystallizer in production.

Then, modify the water supply pressure control program of the continuous casting water pump room. The water supply control of the original water pump room adopts PID adjustment method, and the outlet pressure is adjusted by changing the power frequency of the water pump. The control mode is manual control; the target pressure is the pressure value input by the operator from the HMI; the actual pressure is the water pump outlet pressure.

Modify the program, add automatic control function on the basis of the original program, and set the PID control process parameters during automatic control:. The target pressure is the target pressure communicated by the continuous casting public PLC; the actual pressure is the continuous casting side water supply pressure communicated by the continuous casting public PLC.

In addition, a fault judgment alarm program is added. When the outlet pressure of the water pump room is greater than 1.4MPa or the outlet pressure of the water pump room is greater than the water supply pressure of the continuous casting side greater than 0.3MPa, an alarm signal is generated.

Implementation of automatic control technology for pressure and flow adjustment of each stream

Hardware installation

(1) Install a regulating valve before the inlet of each stream of crystallizer. The original crystallizer inlet pipe is directly welded to the main water supply pipe network, so the pressure on the crystallizer is the same as the main water supply pipeline, and each flow cannot control the pressure of the cooling water entering the crystallizer. In order to solve this problem, a regulating valve is installed at the inlet of each crystallizer to control the cooling water pressure at the inlet of the crystallizer. The original regulating valve at the outlet of the crystallizer is still used to control the water flow. The installation location of the new regulating valve is selected on the pipe between the crystallizer and the crystallizer water main pipe close to the crystallizer. Taking into account the importance of crystallizer cooling water and the on-site environmental conditions, select a pneumatic regulating valve with better performance.

(2) Install a pressure transmitter after the newly installed regulating valve to detect the actual pressure value at the inlet of the crystallizer for automatic pressure control.

Software design

Combined with the program control flow chart, the mold cooling water control program in the casting strand PLC is modified, and an automatic pressure adjustment program is added to the original flow adjustment only. The automatic control and adjustment of pressure does not operate independently, but is affected by flow control.

The crystallizer water flow adopts PID closed-loop control. By comparing the actual flow rate with the set value, the opening of the outlet control valve is automatically adjusted to maintain the actual flow rate consistent with the set value. Under automatic control, the target flow rate is provided by the second level. During manual control, the target flow rate can also be input from the HMI. Generally, manual control is only used for on-site equipment testing and can only be used when the continuous casting machine is in maintenance mode. The crystallizer water pressure also adopts PID closed-loop control. By comparing the actual pressure with the set value, the opening of the inlet regulating valve is automatically adjusted to maintain the actual pressure consistent with the set value. Under automatic control, the target pressure is provided by the secondary. During manual control, the target pressure can also be input from the HMI. Generally, manual control is only used for on-site equipment testing and can only be used when the continuous casting machine is in maintenance mode.

Due to process requirements and safety production needs, it is necessary to ensure that the water flow rate of the crystallizer reaches the process requirements at all times. Only when the cooling water flow rate of the crystallizer is met, the pressure PID control will be enabled. Under automatic conditions, the specifications of the crystallizer selected for each stream may be different, and the water pump room will supply water according to the highest pressure required by the crystallizer. When water is supplied to the system, the inlet regulating valves of each flow crystallizer will all be opened. Then the program controls the opening of the outlet regulating valve through flow PID adjustment to make the actual flow reach the target flow. When the actual flow reaches the target value and is relatively stable, the pressure control PID of the inlet regulating valve is enabled. If the crystallizer inlet pressure value is greater than the target value, the program will reduce the opening of the crystallizer inlet regulating valve through pressure PID control to adjust the crystallizer cooling water pressure. When the inlet pressure is adjusted, the opening of the inlet valve becomes smaller and the cooling water flow rate decreases. At this time, the flow control will increase the opening of the outlet valve to adjust the flow rate. It is set in the program that when the opening of the flow control valve is greater than or equal to 95%, the pressure PID adjustment will lose its initial ability and the inlet pressure will no longer be adjusted. After all, the crystallizer flow is the most important parameter for process and safety production.

The program control flow is shown in Figure 2.

Figure 2 Program control flow

Application effects

The control system well meets the requirements of the production process and achieves high stability, safety and reliability. Application effect:

(1) Each stream realizes double adjustment of crystallizer water supply pressure and flow rate to meet the water supply pressure requirements, avoids the impact of high-pressure water on the crystallizer, and extends the service life of the crystallizer.

(2) The flow and pressure control are more precise, which not only enables simultaneous casting of slabs of different specifications, optimizing the preparation of production plans, but also greatly reduces defects such as shrinkage cavities and central segregation of the slab, and improves the yield rate of the slab.

(3) The communication between the joint water pump room PLC and the continuous casting PLC is realized, which reduces the operator’s workload, eliminates the impact of different human operating abilities on the system, and improves the system’s automated operation level.

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