The boiler is an important power equipment in the whole plant. Its task is to supply qualified and stable steam to meet the needs of the load. To this end, all major parameters of the boiler production process must be strictly controlled. Boiler equipment is a complex control object. The main input variables are load, boiler feed water, fuel volume, desuperheating water, air supply and induced air volume. The main output variables include drum water level, superheated steam temperature and pressure, flue gas oxygen content and furnace negative pressure, etc. Therefore, the boiler is a complex control object with multiple inputs, multiple outputs, and interrelationships.
2. About the boiler computer control system
Boiler microcomputer control is a new technology developed in recent years. It is the product of a close combination of microcomputer software, hardware, automatic control, boiler energy saving and other technologies. There are more than 300,000 medium and small boilers in my country, which consume coal every year. The amount accounts for 1/3 of my country's raw coal production. At present, most industrial boilers are still in a state of serious production such as high energy consumption, large waste, and environmental pollution. Improving thermal efficiency, reducing coal consumption, and using microcomputer control is a task of far-reaching significance.
As a boiler control device, its main task is to ensure the safe, stable and economical operation of the boiler and reduce the labor intensity of the operator. Using microcomputer control, it can perform many functions such as automatic detection and automatic control of the boiler process.
The boiler microcomputer control system generally consists of the following parts, that is, the boiler body, primary instrument, microcomputer, manual automatic switching operation, actuator and valve, slip motor, etc. The primary instrument controls the boiler's temperature, pressure, flow, The amount of oxygen, rotation speed, etc. are converted into voltage, current, etc. and sent to the microcomputer. The operating part is automatically switched by hand. When it is manual, it is manually controlled by the operator. The operator controls the slip motor and valve, etc. When it is automatic, it sends a control signal to the microcomputer and executes it. Partially operated automatically. The microcomputer monitors, alarms, and controls the operation of the entire boiler to ensure the normal and reliable operation of the boiler. In addition, in order to ensure the safety of the boiler operation, when designing the microcomputer system, important parameters such as boiler water level and boiler drum pressure are Conventional instruments and alarm devices should be set up to ensure double or even triple alarm devices for water level and drum pressure. This is essential to avoid major accidents in the boiler.
3. Transformation requirements
1. Remove the original two low-configuration industrial computers and auxiliary boards in the boiler main control room, and use new industrial computers, PLCs and related software to form a PLC+IPC structured monitoring system to monitor the operating status of the two 35-ton boilers. Monitoring of various parameters and automatic control of drum water level.
2. The new system and the instrument system on the original control panel are independent of each other through the signal isolation distributor. The control priorities from high to high are: on-site manual → main control room control panel instrument system → microcomputer system automatic.
3. In terms of control, this transformation temporarily requires automatic control of the drum water level, but requires that a full set of procedures for automatic boiler combustion control be reserved in the PLC to facilitate automatic combustion control when conditions are mature.
4. The system mode is: PLC + IPC, which is the monitoring mode of programmable controller + industrial computer. This achieves bottom-level control; the best monitoring method for upper-level monitoring.
4. System configuration
1. Industrial computer software part
◆Microsoft WINDOWS 2000 operating system
◆Kunlun Tongtai MCGS 5.5 development version
◆Kunlun Tongzhou MCGS 5.5 running version
◆Siemens S7-300 programming software SETUP7 v5.2+sp1
2. Industrial computer part
Adopting the most stable Advantech original industrial computer in China, the configuration is as follows:
◆P4 1.8G CPU; 256M DDR memory; 32M graphics card; 40G hard disk; 1.44M floppy drive; 52XCDROM; 4 USB2.0 interfaces; dual 10/100M network cards;
◆17-inch Samsung CRT monitor, Lenovo Tiangong industrial keyboard and mouse set.
3. Programmable controller PLC
Adopts S7-300 series programmable controller from Siemens of Germany, built-in PID module, 32K memory, I/O can be expanded to 2048 points
5. Control scheme design
※PLC control design※
■Boiler drum water level control system
The drum water level is an important parameter that affects the safe operation of the boiler. If the water level is too high, it will destroy the normal operation of the steam-water separation device. In severe cases, it will cause an increase in water in the steam, increase scaling on the tube wall and affect the steam quality. If the water level is too low, it will destroy the water circulation and cause the water wall tube to rupture. In severe cases, it will cause the pot to dry out and damage the steam drum. Therefore, if its value is too high or too low, it may cause major accidents. Its adjusted quantity is the steam drum water level, and the adjustment quantity is the feed water flow. By adjusting the feed water flow, the materials inside the steam drum can reach a dynamic balance, and the changes are within the allowable range. Since the boiler drum water level has a negative impact on the steam flow The response to changes in feedwater flow rate is positive. However, when the load (steam flow) increases sharply, it exhibits "reverse response characteristics", which is the so-called "false water level". The reason for this is that when the load increases, the drum pressure decreases, causing the water in the drum to The boiling point temperature drops, and the boiling of water suddenly intensifies, forming a large number of bubbles, causing the water level to rise. The drum water level control system is essentially a system that maintains the balance of water inlet and outlet of the boiler. It uses the water level as a control indicator for whether the water volume is balanced or not. It adjusts the amount of incoming water to achieve inlet and outlet balance, and maintains the drum water level near the midline of the drum with the largest steam-water separation interface to improve the evaporation efficiency of the boiler and ensure Production safety. Since the boiler water level system is a controlled object with self-balancing capability, there is a false water level phenomenon during operation. In practical applications, the single impulse of water level, the double weight of water level and steam volume, and the three impulses of water level, steam volume, and water supply volume can be used according to the situation. Control System.
Control target value: (The uniform amount of drum water level is: ±220, the water level is controlled at the intermediate value, and the deviation is ≤±10
1. Use the mouse and keyboard to control the start and stop of the water pump on the operation interface.
2. Use the original DDZ-III operator on the instrument panel to perform manual/automatic adjustment and control of the water pump.
The automatic adjustment of system water supply is divided into three modes:
Single water level control mode: only control the water supply volume by detecting the drum water level
Dual-impulse water level control mode: monitors the drum water level and steam flow, uses the steam flow as a feedforward signal, and forms a feedforward-feedback control mode with the drum water level.
Three-impulse water level control mode: monitor the drum water level, steam flow, and feed water flow. The steam drum water level is used as the main control, the feed water flow is used as the auxiliary controlled variable, and the steam flow is used as the feed forward signal to form a cascade control system. —Cascade feedback control method,
Implementation method of impulse water level control:
1) Under abnormal circumstances, such as when the liquid level deviates greatly from the normal value, rule control can be used to quickly restore the water level and ensure the safe and stable operation of the boiler.
2) When there is a conflict between water level control and main steam temperature control, the two can be coordinated and controlled based on the main aspects of the conflict.
3) It contains two control loops: feed water flow control loop and drum water level control loop. It is essentially a composite control system composed of steam flow feedforward and water level-flow cascade system. When the steam flow rate changes, the feed water flow control loop in the boiler drum water level control system can quickly change the incoming water volume to complete coarse adjustment, and then the drum water level regulator completes the fine adjustment of the water level.
■Boiler combustion process control system
The boiler combustion process has three tasks: coal supply control, air supply control, and furnace negative pressure control. Maintaining the ratio of gas to air so that the air excess coefficient is around 1.08, the economy of the combustion process, and maintaining the furnace negative pressure, so the automatic adjustment of the boiler combustion process is a complex issue. For the 35t boiler to burn the released blast furnace gas, the requirement is to maximize the use of the released blast furnace gas, so it can be operated according to the maximum output of the boiler, and there are no strict requirements on the steam pressure; there are no higher requirements on the economy of combustion. In this way, the automatic adjustment of the boiler combustion process is simplified to the constant gas flow adjustment with the furnace negative pressure as the main parameter.
The size of the furnace negative pressure Pf is affected by the induced air volume, blast volume and gas volume (pressure). If the negative pressure in the furnace is too small, the furnace will spray fire and leak blast furnace gas, endangering the safety of equipment and operators. If the negative pressure is too large, the furnace air leakage will increase, the smoke exhaust loss will increase, and the power consumption of the induced draft fan will increase. Based on years of manual adjustment and exploration, the 35t boiler is designed with Pf=100Pa. The adjustment method is to manually adjust the ratio of air and gas in the initial state to achieve the ideal combustion state. When the induced draft fan is fully opened, the furnace negative pressure reaches 100Pa. After being put into automatic operation, only the gas butterfly valve is adjusted to make the blast furnace gas flow rate consistent with pressure fluctuations. The gas flow rate is at the initial state to maintain the optimal ratio of blast furnace gas and air during combustion. Therefore, the boiler combustion process automatic control system can be divided into three sub-control systems according to different control tasks, namely the steam pressure control system, the flue gas oxygen control system and the furnace negative pressure control system.
■Superheated steam temperature control system
The temperature of superheated steam is an important parameter in the boiler production process and is generally determined by the boiler and turbine production processes. From the perspective of production safety and economic and technical indicators, the temperature of superheated steam must be controlled within the allowable range. In the 35T boiler computer control system, the superheated steam temperature control system is designed as shown in the figure below. The adjustment method is to change the flow rate of desuperheating water. Structurally, this is a simple single-loop control system, but the actual system has the following problems: There are actually three regulating valves in the boiler's water inlet system, namely the boiler main water supply valve (V1), the desuperheating water valve (V2) and Steam drum feed water branch valve (V3). Because these three valves all control the water supply, they will have a correlation effect through the change in pressure P at the water inlet intersection.
■Deaerator control system
The deaerator control system includes two control subsystems: deaerator pressure and liquid level. In the 35T boiler computer control system, the deaerator pressure control system and deaerator liquid level control system are both designed as single-loop PI control methods. On the premise of meeting the actual requirements of boiler production, the single-loop PI control method has the advantages of simple structure, easy setting and implementation.
For the deaerator pressure system, when the deaerator pressure changes, the pressure control system adjusts the steam inlet valve of the deaerator and changes the steam inlet volume of the deaerator, thereby controlling the pressure of the deaerator at the target value. Above; similarly, for the deaerator liquid level system, when the deaerator liquid level changes, the liquid level control system adjusts the water inlet valve of the deaerator to change the water inlet volume of the deaerator, thereby reducing the liquid level of the deaerator. The bit is controlled at the target value.
※Software system※
The above control system is generally controlled by PLC or other hardware systems, and the following functions must be completed in the host computer
■System functions:
Real-time and accurate detection of boiler operating parameters: In order to fully grasp the operating conditions of the entire system, the monitoring system will monitor and collect boiler-related process parameters, electrical parameters, and equipment operating status in real time. The system has a rich graphics library. Through configuration, the boiler equipment graphics and related operating parameters can be displayed on the screen; in addition, the parameters can also be displayed in the form of lists or groups.
Comprehensive analysis and timely issuance of control instructions: Based on the monitored boiler operation data and the set control strategy, the monitoring system issues control instructions to adjust the operation of the boiler system equipment to ensure that the boiler is efficient and can operate normally.
Fault diagnosis and alarm management: The main control center can display, manage and transmit various alarm signals of boiler operation, thereby greatly improving the safety, explosion-proof and safe operation level of the boiler. At the same time, the file management of alarms can enable owners to have a clear understanding of various problems and weaknesses in boiler operation. In order to ensure the safety and normal operation of the boiler system, the monitoring system will perform fault diagnosis based on the monitored parameters. Once a fault occurs, the monitoring system will promptly display the alarm point on the operator's screen. Alarm-related display capabilities associate user-defined displays with each point so that when an alarm occurs, the operator can immediately access the details of the alarm point and follow recommended emergency measures.
Historical records of operating parameters: The real-time database of the monitoring system will maintain historical records of boiler operating parameters. In addition, the monitoring system also maintains historical records of boiler operating parameters. There is a special alarm event log to record alarm/event information and operator changes. According to the operator's requirements, the system can display historical data as instantaneous values or average values within a certain period of time. Historically recorded data can be displayed in a variety of ways, such as curves, specific graphics, reports, etc.; in addition, historically recorded data can also be applied by a variety of network-based application software.
Calculate operating parameters: Some operating parameters of boiler operation cannot be measured directly, such as annual operating load, steam consumption, water replenishment, condensate return, cumulative operating time of equipment, etc. The monitoring system provides a wealth of standard processing algorithms to calculate these derived quantities based on the measured operating parameters.
■Implementation
MCGS configuration software integrates control technology, human-machine interface technology, graphics technology, database, and communication technology. It includes dynamic display, historical trend records, alarms, control strategy components, etc. It also provides a friendly user interface, allowing users to generate the required application software without coding.
Establishing a database: The first step in monitoring configuration is to establish a real-time database. The premise is that after completing the design of the continuous control chart and ladder logic diagram, the relevant loop points, signal points, register points, alarm points, etc. are stored in the real-time database. .
Interface configuration: The most important one is the flow chart screen design, using the drawing tools and rich graphics and other elements provided by the ACTIVE library to form a specific human-computer interface. Each boiler is designed to have 9 operating screens, specifically: boiler body flow chart, adjustment system screen, parameter display screen, electrical operation screen, report display screen, comprehensive trend chart screen, loop tuning screen, alarm overview screen, setting screen parameter setting screen.
Alarm configuration: Define each alarm point according to process requirements, and set the high and low limits of the alarm point. When the limit is exceeded, it will be displayed on the alarm overview screen and send out audible and visual signals.
Report configuration: The shift report and monthly report are produced according to the process requirements, that is, using the DDE method to output the real-time database data to EXCEL to generate report data, and make relevant links and set the printing time. The shift report is automatically printed, and the monthly report is automatically printed. Reports are printed manually.
Historical trend configuration: Use software to define, collect, and display historical trend data of important parameters such as drum water level, drum pressure, steam flow, and furnace negative pressure. It can be printed when necessary.
Login and permission configuration:
Operator level: can only switch, operate and monitor the set screens, and does not have the right to change parameters, exit or minimize the system.
Engineer layer: Configure the visiting team system, set parameters, and exit the system.
Administrator level: The highest authority and can perform all operations.
Controller parameters can be set and modified on the monitoring station during operation. However, in order to ensure system security, when entering settings such as parameter setting, historical data query, monthly report production, etc., you must enter a password before entering each screen for operation.
Menu configuration: all face-changing switching buttons, printing function buttons, system login and exit buttons, help buttons, system clock, operator authority display and other function buttons.