[Abstract] This paper introduces the configuration and control functions of the control system for Laiwu Steel's No. 10 oxygen generator unit. The control scheme combining fieldbus, DCS, and PLC technologies adopted in the design is very advanced and has great reference value for the design and application of automation control.
1 Overview
The No. 10 oxygen generator, commissioned by Laiwu Steel Group Co., Ltd. in December 2003, is a 22,000 m³/h internal compression process oxygen generator unit. The main equipment of this unit was provided by Hangzhou Air Separation Equipment Factory. It adopts advanced processes such as molecular sieve adsorption purification, turboexpander pressurization, structured packed tower, and internal oxygen compression. The complete unit includes: an air precooling system, a molecular sieve purification system, a turboexpander pressurization system, a fractionation tower system, a nitrogen compression system, and a liquid storage system. After evaluation, the Automation Department of Laiwu Steel Group Co., Ltd. adopted a control method combining advanced technologies such as fieldbus, DCS, and PLC to achieve automatic control, data communication, and host computer management of the unit.
2. Control System Hardware Configuration and Technical Characteristics
2.1 Control System Hardware Configuration
The AC800F DCS is a versatile integrated control system launched by ABB, combining the advantages of DCS and PLC. It supports multiple international fieldbus standards, allowing connection to both conventional I/O and remote I/O as well as fieldbus devices such as PROFIBUS, FF, CAN, and Modbus. The system is divided into two levels: the operation and management level (OS, ES, gateway GS) and the process control level (process station PS, field controller AC800F).
The DCS for Laiwu Steel's No. 10 oxygen generator uses an AC800F and Profibus fieldbus, and includes two operator workstations (OS) and one engineer workstation (ES), providing redundancy in power supply, operator stations, and controllers. Through a standard TCP/IP Ethernet protocol, it achieves automatic control, data communication, and upper-level computer management of subsystems such as air compression, air precooling, air purification, booster compression, expansion turbine, oxygen and nitrogen distillation, argon distillation, and nitrogen compression throughout the entire process, allowing for comprehensive monitoring of the entire system's production status. A hardware configuration diagram is shown in Figure 1 below.
Figure 1 Hardware configuration diagram
The ES and OS systems utilize the currently popular P4 microcomputers. One EPSON LQ-1600K dot matrix printer prints alarms and operations in real time; one color laser printer prints reports, programs, and screens.
The AC800F fieldbus controller consists of a basic unit, power supply, Ethernet card, and fieldbus interface card. The baseboard is the PM802F basic unit, which performs calculations and control according to the user-configured control application. There are seven slots: slot P1 houses the SA801F power supply module, providing 5VDC/5A operating power and 3.3VDC/5A auxiliary power to the card; slots E1 and E2 house redundant Ethernet cards EI803F; and slots F1-F4 house fieldbus interface cards. This system uses the Profibus FI830F fieldbus interface card.
Controller redundancy can be achieved using two fieldbus controllers, AC800F, allowing for seamless switching between the master and slave AC800Fs. Each AC800F has two Ethernet cards (EI803F) providing 10BaseT interfaces. The first card connects to the system network (DigiNetS), while the second cards of the two AC800Fs are interconnected, forming a dedicated redundant communication link (DigiNetR) to ensure information synchronization between the master and slave AC800Fs. In the event of a failure of the master AC800F, the slave AC800F can quickly and seamlessly take over.
Configuration results and real-time process data are stored in RAM. The Ethernet card and power card are equipped with RAM backup batteries to retain configuration content and data in the event of a power failure.
The AC800F controller connects to the S800 I/O via the Profibus fieldbus interface card FI830F, acquiring field signals in real time and sending the control calculation results to the corresponding signal output module.
Based on the actual needs of the system, a total of 3 S800 I/O stations are configured. Each S800 I/O station has 12 slots (0~11), of which slot 0 is used to install CI830 modules for connecting to the Profibus-DP master station and setting the Profibus-DP slave address; slots 1~11 can be used to install various types of I/O modules.
The air compressor is controlled by a SIEMENS S7-400 PLC, and the nitrogen compressor is controlled by an AB SLC5 PLC. Data communication with the main DCS system is achieved through standard TCP/IP protocol Ethernet.
2.2 New technologies and features adopted in system configuration
2.2.1 Variable Cycle Task Operation: The ABB AC800F controller uses a RISC processor chip and employs a "task-based" programming mode. Each task can have its operating cycle and priority set. Therefore, this controller can meet the control requirements of complex regulation loops as well as rapid electrical switching control. Thus, one controller can cover the control requirements of all aspects of the factory, and the system programming language conforms to the IEC61131-3 standard.
2.2.2 Open TCP/IP Protocol Ethernet Technology The ABB AC800F controller system communication module is a standard TCP/IP protocol Ethernet module, which allows the system to connect to the factory LAN without the need for additional equipment; since the system supports various OPC data exchange standards, it makes data exchange with various third-party databases or software easier.
2.2.3 Overall Redundancy Switching Technology: Two fieldbus controllers, AC800F, can achieve 1:1 controller redundancy. Each AC800F has two Ethernet cards (EI803F) providing 10BaseT interfaces. The first card is used to connect to the system network (DigiNetS), and the second cards of the two AC800F fieldbus controllers are interconnected to form a dedicated redundant communication link (DigiNetR) to ensure information synchronization between the master and slave AC800Fs. In the event of a failure of the master AC800F, the slave AC800F can quickly and seamlessly take over, achieving a smooth overall switchover.
2.2.4 PROFIBUS-DP Fieldbus Technology PROFIBUS-DP is used for high-speed data transmission at the field level. The master station periodically reads input information from the slave stations and periodically sends output information to the slave stations. The bus cycle time must be shorter than the master station (PLC) program cycle time. In addition to periodic user data transmission, PROFIBUS-DP also provides non-periodic communication required by intelligent field devices for configuration, diagnostics, and alarm handling.
3. Main Control Functions and Technical Features <br />This control system can realize automatic control of subsystems such as air compression, air precooling, air purification, booster compression, expansion turbine, oxygen and nitrogen distillation, argon distillation, and nitrogen compression in the entire air separation process, as well as remote manual operation and host computer management, and can completely monitor the production status of the entire air separation system.
3.1 In manual mode, all controlled valves, water pumps, and liquid product pumps can be controlled via computer keyboard and mouse, and the molecular sieve purifier can also be operated step by step.
3.2 In automatic mode, the main DCS primarily implements the following functions:
(1) Automatic switching of adsorption/regeneration cycles between two molecular sieve purifiers;
(2) Automatic adjustment of more than a dozen liquid level control loops, including air-cooled tower liquid level, water-cooled tower liquid level, lower tower liquid air level, main condenser liquid oxygen level, oxygen heat exchanger liquid oxygen level, crude argon tower liquid argon level, crude argon tower condenser liquid air level, upper pure argon tower liquid nitrogen level, and lower pure argon tower liquid argon level.
(3) Automatic control of more than a dozen flow and pressure loops, including the flow rate of waste nitrogen into the purification system, the inlet flow rate of the booster, the pressure of waste nitrogen out of the cold box, the pressure of the lower tower, the outlet pressure of the liquid oxygen pump, the flow rate of product oxygen to users and venting, the venting flow rate of product nitrogen, the outlet pressure of the liquid argon pump, and the pressure at the top of the pure argon tower.
(4) Automatic control of the resistance of the self-cleaning filter and automatic control of the temperature of the molecular sieve heating furnace;
(5) Air-cooled tower resistance and outlet pressure, expander and booster compressor sealing gas pressure alarm interlock control, oil pump start/stop interlock control, etc.
3.3 Air Compressor PLC Control
The system employs a SIEMENS S7-400 PLC, with an eView touchscreen as the human-machine interface (HMI). Serial communication is used between the controller and the HMI; the MPI interface of the CPU414-2 module is converted to an RS232 port via a SIEMENS PC adapter and connected to the HMI's RS232 serial port. Data communication with the ABB AC800F DCS is achieved through an Ethernet module. This enables centralized monitoring, recording, archiving, and alarm functions for various process parameters of the air compressor, including temperature, pressure, flow rate, displacement, and vibration. It also provides sequential interlocking control for compressor start-up and emergency shutdown, as well as crucial "three-line anti-surge" control.
Surge typically manifests as rapid flow and pressure oscillations, making the compressor's flow and pressure extremely unstable. Because it is usually accompanied by reverse axial thrust and reverse flow, it causes changes in clearance, reducing compressor efficiency, shortening its lifespan, and causing serious damage. The characteristic curve and surge curve of the Laiwu Steel No. 10 oxygen generator air compressor are shown in Figure 2 below.
(1) Fast-opening and slow-closing function: When any unit experiences surge, it is desirable for the anti-surge controller to respond quickly and open the anti-surge valve rapidly to prevent danger; however, it is also desirable for the valve to close slowly during the closing process to prevent surge oscillation. Upon receiving a surge signal, the fast-opening and slow-closing function can quickly and promptly open the anti-surge valve, and when closing the valve, the control signal will also slowly close the valve exponentially.
(2) Manual/Automatic Switching and Protection Function: The system has a seamless manual/automatic switching function. In addition, in manual mode, operators may inevitably make mistakes. The system's unique manual/automatic protection function can overcome this shortcoming. It can automatically detect the signals sent by the operator. If the signal is incorrect, it will not accept it and will still set it to the default value to ensure the safe operation of the unit.
(3) Closed-loop control near surge control line A: Establish anti-surge control line A in the software. It is to the right of surge line C. When the compressor is running to the right of anti-surge control line A, the output is 20 mA and the anti-surge valve is completely closed. When the compressor operating point is about to reach anti-surge control line A, the anti-surge algorithm is designed to open the anti-surge valve. When the operating point moves slowly, the anti-surge control will use a PID loop to control the anti-surge valve so that the operating point is kept at the control point.
(4) Open-circuit control to surge control fast-opening line B: Surge control fast-opening line B is established in the software. It is to the left of surge control line A and to the right of surge line C. This is effective for larger disturbances. If the anti-surge control cannot keep the operating point at the control point, the operating point will still move to the left. Once it reaches the fast-opening line B, the anti-surge control will output a step signal from the controller to open the anti-surge valve to the predetermined opening degree. If the operating point stops moving, the controller signal will slowly increase exponentially (slowly closing the valve) until it enters PID control.
3.4 The nitrogen compressor PLC control adopts the American AB SLC5 PLC, which mainly completes the monitoring and alarm of various process parameters such as compressor temperature, pressure, flow rate, displacement, and vibration; sequential interlock control of compressor start-up and emergency shutdown; and automatic adjustment of inlet guide vanes and anti-surge valve.
3.5 Sequential and Logic Control of Molecular Sieves Based on the process requirements of molecular sieves, the control function characteristics of the Freelance2000 control system, and user needs, the following control functions are mainly performed: sequential and logic control of molecular sieve valves; position feedback and monitoring of each valve; temperature interlock control of electric heaters; running time modification; abnormal alarm printing, etc., making the control more complete and the operation simpler and more convenient.
3.5.1 Sequential and Logic Control: Based on the technical requirements for adsorbent adsorption and regeneration, determine the time for each stage of molecular sieve adsorption and regeneration (including pressure equalization, pressure relief, heating, and cold blowing). Edit the control program using a ladder diagram to control the opening and closing states of valves at each stage.
3.5.2 Position Feedback and Monitoring To ensure safe production, position feedback signals were added to each valve. These signals are provided by local proximity switches and the valve status is displayed on the monitoring screen. To easily distinguish between the operating and regeneration states of the two molecular sieves, the images of the two molecular sieves and pipelines are dynamically displayed. The status of the four sets of electric heaters is also dynamically displayed, facilitating operation.
3.5.3 Electric Heater Temperature Control The electric heater consists of two sets of thyristors: a main heater and an auxiliary heater. The electric heater has an internal protective switch and two temperature measuring points at the outlet to measure the temperature of the regeneration gas at the heater outlet. The operation of the heater is mainly controlled by these two temperatures and the operating stage of the molecular sieve.
3.5.4 Pressure Interlock Control Because the internal pressures of the two molecular sieves used for adsorption and regeneration differ significantly, direct switching would damage the structure of each adsorbent layer. Therefore, pressure equalization is required before switching, and the molecular sieve to be regenerated is depressurized after switching. This part of the control is designed as a pressure interlock control. During pressure equalization, the pressure must be higher than the set value for confirmation; otherwise, the program will stop running and generate an abnormal alarm. During depressurization, the pressure must be lower than the set value for confirmation; otherwise, the program will also stop running and generate an abnormal alarm. Only after pressure confirmation can the next stage of the program run.
3.5.5 The incoming call confirmation system uses a time-delay relay to monitor the controller's own power supply and interlock the start/stop of the molecular sieve switching program. The operating characteristics of the time-delay relay used are shown in Figure 2:
Figure 3. Operating characteristics of the time delay relay
In Figure 3, t represents the relay delay time. If the delay time is long enough, the controller will start up within a time less than t when the power supply is normal. At this time, the controller detects that the contacts of the delay relay are open, and uses this signal in the software to set a holding relay C, causing the molecular sieve program to stop running. Only when the operator presses a reset button AN on the screen and the delay relay contacts close will the relay be reset, and the program will restart. The ladder diagram is shown in Figure 4.
Figure 4. Molecular sieve program run/stop ladder diagram
In addition, a program was added to control whether the electric heater can be put into operation based on the molecular sieve operating status. The ladder diagram is not described in detail here.
In this way, when the system suddenly loses power, the controller also loses power and cannot detect the power failure signal. However, when the power is restored, the controller will start up normally within time t and can detect the power failure signal. Only when the operator manually presses the "Confirm Power" button on the monitoring screen to reset can the program continue running from where it stopped.
In practice, after numerous experiments, we found that setting the relay delay time to 90 seconds ensures the safe stopping of the program.
3.6 Host computer monitoring function
(1) Screen display: Displays the system main menu, process flow parameters and operating conditions, operating mode, equipment status, fault status, real-time and historical trends, etc.
(2) System operation: automatic switching or single-step execution of the molecular sieve purifier adsorption/regeneration cycle, and start/stop control of large single equipment (such as air compressor, nitrogen compressor, expander, water pump, etc.).
(3) Report management: Display and add system record reports, print system event reports in real time, and print system operation reports, daily reports, monthly reports and other reports on a schedule.
(4) Alarm function: Displays a flowchart of the fault area, changes the color or flashes the graphic of the faulty equipment, displays the time and nature of the fault in Chinese characters at the top of the screen, and the printer automatically prints the fault content and the time of occurrence.
4. Conclusion <br />The comprehensive adoption of advanced control technologies such as fieldbus, DCS, SIEMENS, and AB PLC, along with the realization of "variable cycle task operation," "three-line joint prevention" anti-surge, and advanced control functions, has ensured that the unit operates stably, with high safety and reliability, low failure downtime rate, and stable product quality and output since its construction and commissioning, thus guaranteeing smooth production and creating considerable economic benefits.
References:
1. Programmable Logic Controller Network Communication and Applications, Tsinghua University Press, 2000.3
2. Liu Encang, Computer Control System Analysis and Design [M], Wuhan: Huazhong University of Science and Technology Press, 1997.
