Reversing control system of regenerative heating f

The reversing control system of regenerative heating furnace in Laiwu Iron and Steel Co., Ltd. (hereinafter referred to as Laiwu Iron and steel rolling mill) the original strip steel heating furnace is a stepping type, which has the disadvantage of using heat exchanger for air preheating, low preheating temperature and high exhaust gas temperature, which inhibits the improvement of heating capacity of the heating furnace. With the continuous acceleration of the pace of strip steel production, the heating furnace is increasingly unable to adapt to strip steel production. Therefore, in November 2002, the original walking beam heating furnace was changed into a regenerative walking beam heating furnace. The biggest feature of regenerative walking beam heating furnace is to preheat the air by using the regenerator. During the heating process, the two regenerators are in a state of continuous alternation of heat storage and heat release, so as to improve the air preheating temperature and control the exhaust gas temperature at 100 ~ 150 ℃. The key to the work of the regenerative heating furnace is to control the exchange between the two regenerators in the heat storage and exothermic state. If the two regenerators cannot be exchanged in time, the temperature of the regenerator in the heat storage state will be too high and the function of absorbing heat from the flue gas will be lost. At the same time, the temperature of the regenerator in the exothermic state will be too low and the function of preheating the air will be lost. Therefore, the key technology of regenerative elimination method: check whether there is a curvilinear coordinate heating furnace steel burning control is the automatic reversing system

1 control principle

the reversing control of regenerative walking beam furnace reversing control system in Laigang rolling mill is time-based control. As shown in Figure 1, for example, when the fire burns to the oil tank, in the combustion state, the normal temperature combustion supporting air from the blower first enters the left channel through the change-over valve, and is heated when passing through the heat accumulator. It reaches the temperature close to the furnace temperature in a very short time (generally 80% - 90% of the furnace temperature). The coal gas is injected near the thin high-temperature air by the on-off valve, and the fuel is burned in the oxygen poor state; At the same time, the hot flue gas after combustion in the furnace foam granulator will store the heat in the heat storage body when it passes through the heat storage body on the other side, and then the low-temperature flue gas below 150 ℃ will be led out by the induced draft fan through the reversing valve. After the specified time, the change-over valve automatically reverses, the normal temperature combustion supporting air is changed into the right channel through the heat accumulator, and the hot flue gas is discharged from the left channel. The two heat accumulators automatically switch the state of heat storage and heat release, so as to achieve the purpose of energy conservation and environmental protection

Figure 1 working principle of regenerative walking beam heating furnace

2 reversing control system

2.1 system configuration

the reversing control system of regenerative walking beam heating furnace in Laigang rolling mill adopts programmable controller s; The upper computer adopts industrial computer Pentium IV 1.7g, memory 128M, and software platform Microsoft Windows; The powerful step-7 software package is used for software development, so as to realize various functions of the computer control system, which can reduce the growth rate by 2.5 percentage points; Use WinCC (Windows Control Center) configuration software to establish man-machine interface

2.2 control function

the control functions of the reversing system of regenerative walking beam heating furnace include: (1) the sequence control of air and flue gas reversing valves, and the setting of reversing cycle and sequence interval cycle. (2) The gas on-off valve reverses in sequence, and the reversing cycle, sequence interval and cycle setting time correspond to the air/flue gas reversing valve. (3) Smoke exhaust temperature is detected and displayed in real time, and it participates in burner reversing control. (4) Smoke exhaust temperature over temperature alarm, forced commutation, alarm temperature manually set. (5) Valve position display and valve position fault alarm of air/flue gas reversing valve and gas on-off valve

2.2.1 startup initialization in the initial startup state, the system automatically detects the status of each change-over valve and on-off valve. The next operation is allowed only when it is confirmed that 24 sets of air/flue gas change-over valves are in the air on position and 24 sets of gas on-off valves are in the gas off position

2.2.2 manual state (furnace temperature lower than 800 ℃) the system has three control modes: manual, automatic and remote control. Manual control is adopted in the furnace temperature lower than 800 ℃. Manual control is to set the valves to be operated through the channel selector on the panel of the control cabinet, and then operate the "air inlet (gas supply)" and "flue gas exhaust (gas cut-off)" buttons in sequence to control each reversing valve or on-off valve. In order to ensure the safe operation of the heating furnace, manual control is preferred under accident conditions

2.2.3 automatic state (furnace temperature higher than 800 ℃). When the furnace temperature is higher than 800 ℃, start the automatic control program. The automatic reversing combustion control adopts the timing principle, and the reversing cycle and reversing interval are set to 90s and 4S respectively according to the process requirements. The whole heating process of regenerative walking beam heating furnace is divided into three parts: soaking section, heating section I and heating section II. Each section has 8 air/flue gas reversing valves, and their reversing control principles are the same. In order to facilitate operation, set two opposite directional valves on both sides of the furnace as a group, and set eight air/flue gas directional valves and gas on-off valves in each section as a section

generally, there are 4 gas on-off valves and 4 air/flue gas reversing valves in each section. After the specified cycle, the reversing valve will automatically switch to complete the switch between the heat storage state and the heat release state. Under abnormal circumstances, such as when the exhaust gas temperature exceeds the set value, the commutation can be forced

the system can realize local and remote control. Local operation refers to the commutation operation on the upper computer of the commutation system, and remote control refers to the commutation operation on the upper computer of the combustion system. No matter what kind of operation, it can realize the control of group on, group off, section on, section off, etc

2.2.4 safety protection function this system is equipped with safety protection functions such as reversing interlocking, numerical detection, alarm, fault display, emergency stop, etc

the conditions of reversing interlock are: at the beginning, air is supplied first, and then gas is turned on; When reversing, turn off the gas first and discharge the flue gas. During the operation of the system, if the gas on-off valve cannot be opened in place or the air reversing valve cannot be opened in place, the system will automatically turn off the gas on-off valve. At the same time, the buzzer will alarm, and the corresponding valve position "not in place" or "not in place" will be displayed in the state diagram of each heating section on the upper computer screen. The operator will find the fault valve in time through the fault indication and take corresponding treatment measures, It can avoid all kinds of safety problems caused by inadequate valve position in the process of reversing

in the process of automatic reversing, if one side of a group of burners fails, the burner on that side will be automatically cut off, and the burner on the other side will be burned as a conventional burner. In case of abnormal conditions, you can click the "emergency shutdown" button on the upper computer to turn on and off all air/flue gas reversing valves and gas of the heating furnace