Heating furnace_ Speed control system fault of walking beam heating furnace
Heating furnace_ Speed control system fault of walking beam heating furnace
(1) Overview
The original hydraulic system of walking beam reheating furnace in a steel mining plant is imported from abroad. The hydraulic system consists of power system, control system and circulation system. The power system is the core of the walking beam hydraulic system, which has four main pumps, three of which are working and one is standby.
The speed control of walking beam of heating furnace is completed by three main pumps of power system controlled by PLC. The system is a typical multi system parallel combination oil supply, variable pump cascade volume speed control system.
The schematic diagram of single pump set and four stage pressure regulation is shown in Figure v. The main pump is the external gear pump, the front pump is the external gear pump, the set pressure of the four stage pressure control valve is 1MPa, 2.2MPa, 2.7mpa, 3.5Mpa respectively, and the four stage pressure controls the displacement of the valve core of the mechanical hydraulic servo valve respectively, so as to control the inclined angle of the swash plate of the piston pump and achieve the purpose of controlling the flow of the piston pump.
Since most of the hydraulic components of the original hydraulic system were produced in the 1970s and 1980s, some of them tended to be eliminated at that time, which laid a serious hidden danger. After more than ten years of wear and tear in production, the faults of the hydraulic system components are gradually increasing, which seriously threaten the normal operation of the walking beam and the normal production of the hot rolling line. In order to maintain the normal operation of the system, the original hydraulic system must be transformed.
(2) Pump control transformation scheme of electro hydraulic proportional control two way variable
Due to heavy production task and large rolling amount of oriented silicon steel in steel plant, it is not allowed to shut down the furnace for a long time
① Advanced technology, mature and reliable, to ensure that the system should have the technical indicators, performance and function;
② The main system and control system shall not be changed greatly, and the original hydraulic system equipment shall be kept unchanged or less changed as far as possible;
③ In order to reduce the investment of transformation and the influence of transformation on production time, the components of electrical control system remain unchanged or change less;
④ Standard and general hydraulic components and related components are adopted for future replacement.
Combined with the basic principles of the transformation and the control mode of the original system, it can be seen that the local pump control transformation is better than the open system transformation. The characteristic of the pump control scheme is that the civil engineering does not move, and the electric control system can be changed slightly to ensure that the electric control system program remains unchanged. The pump control scheme of electro-hydraulic proportional control two-way variable simplifies the flow control circuit, makes the flow control circuit very simple, and makes the main pump interchangeable, which has nothing to do with the electric control system and process of the original system, so that the problem of spare parts of the main pump can be solved. The construction period of on-site reconstruction can be basically completed during the annual repair period. Considering comprehensively, the pump control scheme with electro-hydraulic proportional control of two-way variable is suitable.
The original main pump is replaced by coaxial make-up pump, and the four stage pressure control with complex structure and backward control mode is eliminated to realize the flow control loop of cascade variable. PLC is used to control the electro-hydraulic proportional valve to realize the step variable of the main pump. At the same time, a transition module is provided to adapt to the signal of the original four stage pressure regulation. The flow of the main pump is controlled by the electro-hydraulic proportional valve to meet the corresponding flow output of the original four stage pressure.
The variable control mode of hydraulic pump is that the electric controller gives the corresponding control signal, the proportional amplifier power amplifies, drives the proportional directional valve, controls the variable piston of hydraulic pump to move to the corresponding position, and the hydraulic pump discharges the required flow for the system to work. At the same time, the swash plate inclination of the main pump is measured by the displacement sensor, converted into electrical signal, and sent to the proportional amplifier as the feedback signal of closed-loop control, so the flow control accuracy is high. The control mode is flexible, the speed of the hydraulic cylinder is automatically controlled by the program, and the stepless speed regulation can be realized.
In this transformation scheme, the main pump, pump frame and its accessories are replaced, and the four stage pressure regulating circuit is cancelled; The electro-hydraulic proportional valve is used to control the main pump, and a transition module is set between the original electric control system and the electric controller of the proportional valve to convert the switching value of the process related control electromagnet output from the original electric control system into the corresponding analog value, so as to control the variables of the main pump, and the variable gradient is adjustable; The hydraulic servo control mode is changed to electric control mode, which simplifies the hydraulic system and control system and reduces the fault points; In this way, the control system is simplified, the control mode is advanced, and the flow regulation is more simple, practical and reliable.
(3) Improved hydraulic system
The reformed walking beam hydraulic system is composed of three subsystems as before. The main transformation is the main pump unit. Figure w shows the hydraulic schematic diagram of the main pump group.
In general closed system, the flow of make-up pump is only about 15% of that of main pump. Because the hydraulic cylinder in the system is asymmetric cylinder, the oil inlet of rod cavity and non rod cavity is different, and the phenomenon of air suction is easy to occur. Therefore, a large flow make-up pump is used.
If the makeup pump has no pressure oil output, the main pump will be damaged in dozens of seconds. In order to protect the main pump, a pressure relay is set at the pressure port of the make-up pump, which sets the alarm pressure as 1.0MPa. If the pressure of the make-up pump is lower than 1.0MPa, the system will stop automatically within 3S.
In order to ensure the oil supply pressure of the pump to the hydraulic cylinder, two relief valves are connected in parallel at the outlet of the pump to play the role of safety valve, so as to prevent the vibration and impact of the hydraulic cylinder caused by too large pressure change.
(4) Summary
The reconstruction of hydraulic system of walking beam heating furnace in hot rolling plant cancels the flow control loop of the original main pump which is controlled by four stages of pressure and realizes step variable; By controlling the main pump, the hydraulic servo control mode is changed to electric control mode, which simplifies the hydraulic system and control system and greatly reduces the fault points. Since the system was put into operation after transformation, it has been running normally without any fault affecting the normal production, which ensures the output and quality of silicon steel production.
