1 Introduction
In recent years, most newly built or renovated large and medium-sized cement production lines in my country have adopted computer control systems. The control method has shifted from the past use of instruments, relays, or small standalone computer systems to centralized computer operation and distributed control of the entire production line. However, most of the electrical control products for lubrication oil stations配套 with the main equipment in China still rely on traditional control methods, which are incompatible with current production management methods and control levels. Traditional control cabinets use relays with hard-wired connections for electrical control and interlocking protection, resulting in problems such as low reliability, limited interlocking protection functions, and high failure rates. Signal interlocking with the computer-based full-line monitoring system is rare or nonexistent, affecting on-site inspections and centralized operation management. Furthermore, changes in on-site conditions during actual use make the installation and commissioning of control cabinets using traditional methods quite difficult. To address this situation, we have adopted programmable logic controllers (PLCs) to control lubrication oil stations, designing and developing the CPO-OSC (Cement Process Optinum Control - Oil Station Control) series lubrication oil station control system. This system has been successfully implemented on production lines at cement plants such as Shandong Lubi and Fujian Nanping, achieving satisfactory application results.
This article takes the electrical control of the large equipment lubrication oil station on the 1000t/day cement clinker production line of Shandong Lubi Building Materials Co., Ltd. as an example to introduce the use of PLC in the control of these oil stations.
2. Control scheme for lubrication station system
Lubi Building Materials Co., Ltd.'s cement production line has a daily clinker production capacity of 1000 tons. The entire line is monitored by a Honeywell MicroTDC3000 distributed control system (DCS) from the United States, consisting of three field control stations for raw materials, calcination, and coal milling, with centralized operation and management in the central control room. The production line has five large equipment lubrication stations: the raw material mill head bearing, the raw material mill tail bearing, the raw material mill main reducer, the raw material mill main motor, and the high-temperature fan lubrication station.
2.1 Raw material mill lubrication station
The raw material mill adopts a Φ3.5m ×10m central discharge drying mill system, manufactured by Tangshan Cement Machinery Plant, with a production capacity of 75t/h. To protect the main bearing of the mill, a lubrication station is set up at both the front and rear bearings, using a forced circulation lubrication method. Each oil supply station is equipped with one high-pressure pump and two low-pressure pumps, and the two lubrication stations are controlled by a CPO-OSC system.
2.2 Lubrication oil station for main reducer and main motor
The main parameters of the main reducer and main motor are as follows:
Main motor: Model YR1400-8, power 1400kW
742 r/min
Voltage 6000V, voltage frequency 50Hz
Main reducer: Model JS110-B
Input speed 742 r/min, output speed 16.9 r/min, input power 1400 kW, transmitted power 1400 kW.
To protect the safe operation of the main reducer and main motor, forced lubrication stations are also installed, each equipped with two low-pressure pumps. The two lubrication stations are controlled by a CPO-OSC system.
2.3 High-Temperature Fan Lubricating Oil Station
The high-temperature fan motor has a capacity of 630kW and a speed of 1430r/min, and is equipped with:
1) YDT63/15 hydraulic coupling, with a power transmission capacity of 350～650kW.
2) The air inlet is equipped with a louvered valve and an electric actuator.
3) The high-temperature fan bearing lubrication station is equipped with two oil pumps for forced lubrication.
To ensure the safe operation of large main equipment and improve system reliability, we adopted the CPO-OSC lubrication station control system, developed with SIEMENS S7PLC as the main controller, to realize the electrical control and interlocking protection of the lubrication stations of the raw material mill reducer and main motor, the grinding head and grinding tail bearings, and the high-temperature fan.
Due to poor production site conditions and high electrical noise interference, the system's input and output modules all use high-voltage modules to avoid malfunctions caused by interference signals, which could damage the system.
To achieve data sharing and facilitate monitoring and management in the central control room, we connected the lubricating oil station control system with other optimized control systems (raw material quality control system and kiln optimized control system) into a computer communication network .
3. Composition and Functions of the Control System
3.1 Signals provided by the lubrication station to the TDC3000
1) Prepare signals
2) Enable start signal
3) Interlocking stop signal
3. Signals provided by 2TDC3000 to the lubrication station
Start/Stop Signal
3.3 Signal Processing
3.3.1 Prepare signals
Includes the following:
1) In the "Centralized/Local" control mode, the selector switch is set to the "Centralized" position;
2) The auxiliary contacts of the main power circuit breaker are closed;
3) Control the closing of the auxiliary contacts of the power supply circuit breaker;
4) The auxiliary contacts of the air switch in the main circuit of the oil pump are closed.
3.3.2 Enable Start Signal
The "start-enabled signal" should consist of the following conditions:
1) High pressure is established and reaches the set value;
2) The low-pressure level is established and reaches the set value;
3) The oil temperature should not be lower than the lower limit;
4) The oil flow rate shall not be lower than the lower limit;
5) The oil level shall not be lower than the lower limit.
3.3.3 Interlocking Stop Signal
In typical oil station control systems, the interlocking shutdown signal is established based on the "low pressure" condition during operation. Because this control system utilizes a powerful and flexible PLC, we have also incorporated the protection functions for large equipment into the system. For example, in the reducer oil station system, we have incorporated the upper limit alarm signal for reducer temperature into the interlocking shutdown; in the mill main bearing and high-temperature fan oil station, we have incorporated the upper limit alarm signal for bearing temperature into the interlocking shutdown. Considering oil pressure fluctuations and transient interference in temperature detection, we have added a 10-second delay to the trip signal before sending it out.
3.3.4 Standby Pump Control
The automatic commissioning of low-pressure standby pumps is generally handled using pressure signals, including standby pump start-up pressure signals and standby pump stop-down pressure signals. When the oil pressure is lower than the standby pump start-up pressure value, the standby pump is started; when the oil pressure is higher than the standby pump stop-down pressure value, the standby pump is shut down.
3.4 Other Key Points
The start-up and shutdown of the lubrication station in a centralized manner should be entirely decided by the central control personnel. Under normal circumstances, the lubrication station can only obediently serve the main unit and has no constraints on higher-level control, such as automatically shutting down the lubrication station after the main unit stops.
Because the SSR output modules of DCS systems generally have excessive leakage current, resulting in voltage still existing when the signal is turned off (this is present in systems such as MicroTDC3000 and N-90), gas station start and stop signals sent by the TDC3000 system are isolated using intermediate relays.
3.5 Communication Network
In general applications, the CPO-OSC series lubricating oil station control system can be used independently. However, the CPO-OSC control system has excellent openness and networking capabilities, and can be connected to different types of communication networks according to the user's actual needs. At Lubi Company, we have established an MPI control network including 8 stations, which introduces the field oil station signal parameters into the upper-level monitoring system in the central control room.
The Multipoint Interface (MPI) is a communication interface integrated into the SIMATIC S7-300 CPU. The MPI can simultaneously connect to the following sites:
1) IBM PC compatible machine;
2) Programmer (PG);
3) Operator Interface (HMI);
4) S7-300, M7-300;
5) S7-400, M7-400.
Networked CPUs can periodically exchange data with each other using the "Global Data Communication" service. It can connect up to 32 MPI stations with a transmission speed of 187.5 bps, and the maximum distance between adjacent MPI stations can be 9100 m (using 10 repeaters).
During use, we found that the laying of communication cables should be taken seriously. If communication cables are laid together with high-voltage cables, interference will occur, causing communication errors. Therefore, communication cables should be laid separately to improve the reliability of the system.
4. Supervisory Control System
The host computer is a PC bus industrial control computer equipped with a SIMATIC MPI communication network card and a Chinese Windows operating system, with a communication baud rate of 1.5 Mbps. It monitors data from each gas station via data communication and displays the current operating status in real time on the host computer, allowing operators to address on-site situations promptly.
The MPI network is equipped with two operator stations, which run the raw mill optimization system (CPO-QCS) and the kiln optimization system (CPO-KOS) respectively.
The lubricating oil station monitoring system operates on the raw material mill optimization operation station, and its monitoring screen is shown in Figure 2.
5 Engineer Workstations
To facilitate future system maintenance and monitoring, such as modifying or downloading programs in the on-site PLC control cabinet, we utilized an existing Lenovo Pentinum 166MHz commercial machine from Lubi Company, configured it with a CP5411 communication card, connected it to the MPI network, and turned it into an online engineer workstation. The engineer workstation uses the Windows 95 operating system and has the STEP7 software package installed.
STEP7 is an application software package for programming S7 series PLCs, including:
1) SIMATIC Manager;
2) Symbol editor, used to define symbol names, data types, etc.;
3) Hardware configuration, used for configuring the automation system and setting parameters for each template;
4) Communication, used to define data transmission via MPI, PROFIBUS, or Industrial Ethernet;
5) Program editor.
It allows you to write programs using any of the ladder logic diagrams (LAD), statement lists (STL), and function blocks (FBD), and to perform online debugging and monitoring.
6 Conclusion
The practical application of programmable logic controllers (PLCs) for lubrication station control on several cement production lines has proven that using PLCs to control lubrication stations in large equipment has become an inevitable trend. Compared with traditional relay control cabinets, PLCs offer significantly improved reliability and scalability, achieving good application results. However, some problems were also discovered during use. For example, the cabinet design lacked a rear door, causing difficulties in on-site instrument debugging. Furthermore, due to the inaccuracy of oil flow, oil level, and differential pressure signals from the field instruments, these signals were not taken into account during implementation to avoid delays in material feeding due to difficulty in establishing a start-up signal. Because many of the electrical contact pressure gauges used in the Lubi Company's on-site oil station were damaged, we only started and stopped the standby pump based on the normal pressure signal. If the pressure did not reach the set value, the standby pump was started; if the pressure reached the set value, the standby pump was stopped. Using this control method, if the working pump body is damaged during operation, frequent start-stop oscillations of the standby pump will occur, easily damaging the pump body and motor. Therefore, the following improvements are proposed for the future control system: If the "drive signal" has been sent and the selected "working" pump has started working, if the pressure still cannot be established within a specified time, such as 20 seconds, the "standby" pump will automatically switch to the main pump and start operation, and the original "working" pump will switch to standby.
