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First and second filter system brief introduction
The condensers of the second unit of Pingdian Company are double shells, double back pressure, double inlets and double outlets, and single-process horizontal layout. This transformation adopts a unit system, which is used in the A/B column of circulating water inlet pipes of condensers. Add a secondary filter system installed, the control system also uses a unit system, one by one corresponding control, independent of each other. 
1) The primary system consists of a rotating screen, a drain valve and piping (Figure 1). 
2) The secondary filter system process: filtration - pressure - backwash filter. The principle of step-down backwashing is used. In the backwashing process, the backwash rotor continues to rotate on the entire screen mesh surface. The pressure difference between the front and back of the filter is reduced by the impurities in the rotor covered area due to the mesh surface of the covered area. Not affected by the positive differential pressure of the cooling water (fully released), the fibrous impurities go into a free state. At the same time, due to the opening of the blowdown valve, the water flow in this area is reversed, resulting in a backwash flow, which flushes the impurities away from the screen plate and enters the circulating water outlet pipe of the condenser and is discharged into the Huaihe River. 
Second, the secondary filter program control system
2.1 Program Control System Components
The program control system is mainly composed of PLC programmable controller, human-machine interface intelligent terminal (HMI), backwash motor, electric sewage valve and differential measuring system. The core of this program-controlled system is Siemens' SIMATIC S7-300 series PLC as the front control host, SIMATIC CPU is selected as 313C, and its unit process control system configuration (Figure 2). 
SIMATIC S7-300 series PLC features:
(1) Flexibility: A variety of different performance CPUs and a full range of 1/0 modules and function modules can form different control systems, and modules can be mixed randomly. The same hardware structure supports different software. 
(2) High performance: The high-speed instruction processing system enables the program to run faster. According to different user programs, the I/O refresh rate ranges from 1 ms to 6000 ms, and is preset at 150 ms. 
(3) Adaptability: High electromagnetic compatibility and strong vibration, impact, temperature range from -25 °C ~ 60 °C, making it a higher industrial environment adaptability. 
2.2 Secondary filter control method
(1) Basic control mode - differential pressure control. When the differential pressure before and after the filter reaches the set value (backwash value), the program automatically starts the continuous backwash of the filter, and the backwashing time for each program control is 60sec. When the differential pressure value is lower than the backwash value, the program automatically stops the filter backwash. 
(2) security protection - time control. In the set time, whether the differential pressure reaches the set value (backwash value), the program automatically starts the filter backwash once. 
(3) Continuous backwashing method. When there is a lot of impurities, set the time for the program to control the filter backwashing for several hours in a row. Differential pressure system participates in interlock protection and alarm. 
(4) Manual operation mode. All operating programs can be started by the operating personnel through the function keys on the man-machine operation screen. The startup procedure is: open the drain valve first and then start the backwash rotor. The stop sequence is: stop the backwash rotor and close the drain valve. Differential pressure system does not participate in interlock protection, only alarm. 
2.3 System Interlock Protection
In the following special circumstances, the filter automatically backwash:
(1) Differential pressure protection. In the automatic mode, the differential pressure measurement system is responsible for monitoring the operation of the entire strainer system. When the differential pressure value of the filter reaches a high value, the program automatically delays for 10 seconds, alarms, and starts the filter continuous backwash (a time of 5min), automatically determine the system differential pressure value. 
(2) When the differential pressure sampling tube is dirty and flushed, the differential pressure sampling tube can be flushed through the set differential pressure system on the man-machine operation screen. At this time, the program control system automatically starts the filter for 10 consecutive minutes. Backwashing, and program-controlled system within 10min of differential pressure system flushing, the program will automatically release the system differential pressure lock protection. 
(3) The washing rotor is automatically blocked reverse protection, that is, once the program control system receives the signal that the backwashing rotor is jammed by debris, the program is not limited by the operation mode, and the backwashing rotor is automatically started to reverse the alarm. 
(4) When manually cutting to automatic, the program will automatically backwash the filter once.
2.4 Interface Signals
(1) In addition to increasing the Chinese man-machine interface touch screen communication signal and the interface signal of the field device, this modification still retains the interface signal with the DCS of the unit, which constitutes a complete secondary filter program control system.
For example, stall torque signal, backwash rotor safety switch signal, drain valve position and torque signal, filter differential pressure high signal, control system fault signal, remote start filter backwash signal, and filter differential pressure analog signal (4 to 20mnA)
(2) Human-machine interface touch screen (HNI): Human-machine interface The touch screen is a smart display terminal. It consists of three parts: liquid crystal display, function keyboard, and system keyboard (Figure 3). The human-machine interface touch screen and PLC in Chinese mode The communication between each other uses the MPI port communication mode. Since the man-machine interface has been integrated in the S7-300 operating system, the A7-300 operating system automatically completes data transmission. The operation and maintenance personnel can directly touch the human-machine interface. Complete the following tasks:
a. Parameter setting and modification: Through the soft keys of the human-machine interface touch screen F9~F14, complete the following settings: △p cleaning value, △p backwash value, △p high high value setting, each backwash time setting, Continuous backwashing time setting, regular backwashing time setting, drain valve actuation time setting, differential pressure signal backwashing delay setting, differential pressure signal high delay setting, differential pressure system flushing time setting, differential pressure system Periodic backwash time settings and more during failures. 
b, equipment start and stop control, status display and alarm and operation management.
2.5 Software Design
The program control system control software uses the SIMATIC STEP7V5.0 software that conforms to the IEC1121-1 standard. The program control system is configured and the program user software is written. The program software is mainly written using the FBD editor. The entire program is mainly divided into normal filter backwash under automatic mode, continuous backwash of filter under automatic mode, backwash of filter under manual mode, system protection and alarm display, and human-machine interface information display.  
After the installation and debugging of this system, all functions meet the system requirements and have received good economic benefits. Condenser fouling is reduced by 15 times compared to the same period in the previous year. If it takes 4 to 5 hours to decontaminate the dirt in previous years, the power loss is as high as 1.2 million kilowatt-hours. This alone, the on-grid electricity increased by more than 1,800 kilowatts over the same period last year. At the time, it saved a lot of fuel. In particular, since the occurrence of the largest flood in the history of the Huaihe River in summer this year, the condensers of Units #1 and #2 have not been fouled and the unit has been operating safely and stably. As of August 20, during the peak of this year’s peak of the summer, Condensing has also made a clean sweep. This avoids the unsafe risks to the crew and personnel due to frequent operation and maintenance of the equipment. In addition, due to the improvement of the circulating water quality of the condenser, the copper tube fouling, corrosion, and leakage of the condenser are reduced, which improves the Copper tube heat transfer efficiency, unit vacuum significantly improved, power coal consumption also reduced by 2 g/kWh, the annual economic benefits.
Application of PLC in Secondary Filter of 600 MW Unit Condenser
Since Anhui Pingyi Power Generation Co., Ltd. (hereinafter referred to as the “flat power companyâ€) units #1 and #2 have been put into production, since the two units are all designed as open-cycle operation, there is no secondary filter for the circulating water inlet pipe of the condenser. Every summer, due to the high temperature of the circulating water, the large amount of contaminants in the water, and the reduced vacuum of the unit, it was forced to reduce the load many times, and the concentrator's semi-detachment was carried out. During the flood, even more, it increased the operation and maintenance of the unit due to condensation. The internal harsh environment has seriously affected the health of maintenance personnel. Frequent unloading of the crew affected the peak of the summer and the safe and stable operation. From October 2002 to April 2003, the second filter screen was installed on the inlet pipe of the condenser water circulating water.  SIMATIC S7-300 series PLC is a small, modular, structured, dedicated computer with high data processing capabilities. It is mainly used in industrial real-time control systems. Since the PLC automatic control system of the S7-300 series is a full range, fully compatible, and highly reliable program control system, it has a good openness and a high cost performance ratio. Therefore, the program of the SIMATIC CPU 313C+OP170B man-machine interface was selected for this transformation. Control system, SIMATIC CPU313C mainframe, working memory up to 32KB, external memory card, maximum expandable to 4MB, using floating-point arithmetic.  1) filter differential pressure setting value parameter modification program 2) filter normal backwash program Third, economic analysis
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