During boiler operation, good steam and water quality is required. Regular and irregular venting of impurities deposited in the steam drum is necessary to improve boiler water quality, prevent scaling, and ensure safe boiler operation. Previously, in thermal power units, periodic boiler blowdown was mostly handled by hardware-based programmable controllers. However, due to programming limitations that didn't adequately adapt to the power plant's specific conditions, certain problems arose during operation. The main reasons are: firstly, if a mechanical failure occurs in any of the scheduled blowdown valves, the system will stop operating, requiring all valves to be in good working order before normal operation can resume. Furthermore, many of these valves are aging and prone to jamming and other mechanical failures, preventing normal operation. Secondly, hardware-based systems are susceptible to component failures over time, which are difficult to repair. Therefore, they are rarely used, and manual blowdown is generally employed. Thus, improving the reliability and stability of the boiler steam drum periodic blowdown control system is imperative. Currently, DCS control systems are widely used in China, offering advantages such as good control performance, high reliability, low system maintenance, and ease of configuration. In the DCS retrofit project of the 2×100MW units at Huayingshan Power Plant, the boiler periodic blowdown control was successfully integrated into the DCS control system (hereinafter referred to as the periodic blowdown control). After careful design and debugging, the project was a success. The periodic blowdown control uses the HS2000 system software from Beijing Hollysys Engineering Co., Ltd. Based on the operating procedures and site requirements, it was configured and debugged within the DCS control system. After static and dynamic tests and trial operation, the periodic blowdown control fully achieved the expected functions and effects. Its main advantages are as follows: 1) Stable and reliable operation. After long-term trial operation on site, the start-up, shutdown, and various functions of the control system are all normal. 2) Simple structure. No dedicated control device is required; only the I/O lines of the electric doors need to be connected to the DCS cabinet. Since the operation and control functions are completed by software, there is no issue of hardware failure in the control device. 3) Convenient operation. A pop-up window is designed on the DCS system operation screen, allowing direct software operation. The pop-up window displays the operation status and running condition of each door, facilitating observation by operators. 4) Flexible operation. On the operation screen, faulty electric doors can be disconnected (not participating in program control), and normal electric doors can be put into program control, thus increasing operational flexibility. 5) Full functionality. Manual and automatic operation can be performed on the screen, and it also has display and alarm functions, so that operators can grasp the operation status and remind operators to troubleshoot in time. 6) Convenient parameter modification. The sewage discharge time of the electric doors can be modified in the configuration according to the actual needs of the site (generally considered to be 20s). 1 Strategy for Fixed Discharge Program Control Design 1.1 Design Scheme The fixed discharge program control device originally used in this plant has become outdated due to historical reasons (technological development) and is rarely used. Based on the actual situation of the plant, the following configuration scheme is proposed, with the following considerations: 1) During the operation of the program control, the fixed discharge program control will only be interrupted if two or more electric doors show an open/close timeout signal (in actual operation, it is rare for two electric doors to time out at the same time). After the program control operation is interrupted, the "program control stop" button will be displayed in red. If an electric door shows an open/close timeout signal, it will only alarm and not interrupt the program control, and the corresponding electric door will be displayed in yellow. 2) If the No. 0 electric gate (sewage main gate) experiences an opening/closing timeout signal, the scheduled drainage control will automatically interrupt, and the "Fault Display" button will turn red. 3) If a mechanical fault is known in a particular electric gate, clicking "Cut Off" on the screen will prevent that electric gate from participating in the control program. The program will "skip" to allow the normal electric gates to participate in the control program, thus increasing operational flexibility and eliminating the requirement that all gates must be in good working order to operate. 4) After the scheduled drainage control program is activated, sewage discharge will proceed in sequence. First, the No. 0 electric gate (sewage main gate) will open, and then electric gates 1 through 17 will automatically discharge sewage in sequence. After all the electric gates in the control program have discharged sewage, the No. 0 electric gate will be closed last, at which point the automatic control program will end. 5) If a scheduled drainage control fault interruption occurs, the "Scheduled Drainage Start" button must be clicked again after the electric gate fault is resolved to restart the program. This prevents the scheduled drainage control program from automatically activating. 1.2 Permitted Conditions for Opening Scheduled Drainage Electric Gates 1) The steam drum water level is higher than the lower value. 2) The automatic exhaust valve can only be opened if the fire extinguishing protection is not activated and the steam drum water level is not high. Otherwise, the valve will be locked. 3) The next automatic valve can only be opened after the previous one has been drained and closed. Simultaneous opening of two automatic valves is not permitted. 1.3 Automatic Exhaust Control Program Start-up Conditions The automatic exhaust control program can only be started if all of the following conditions are met: 1) Automatic valves 0-17 must be closed. 2) The automatic exhaust valves must meet the opening permission conditions. 3) The automatic exhaust control program button is engaged and the automatic exhaust control reset signal is 0. 4) Automatic exhaust control program start-up is prohibited when the automatic exhaust valves are not powered. 1.4 Automatic Exhaust Control Program Interruption Conditions The automatic exhaust control program will be interrupted if any of the following occurs: 1) During program operation, an automatic valve 0 open/close operation timeout signal is generated, or two or more automatic valves 1-17 open/close operation timeout signals are generated. 2) The MFT (Medium-terminal Flood Control) is activated and the steam drum water level is not high. 3) A low water level signal is generated in the steam drum. 4) Manual stop of the program control. 5) Boiler safety valve activation. Based on the above logical conditions, the system was first debugged using simulated faults, and then put into operation to verify the correctness of the logic. When the scheduled discharge control is interrupted, the "Start" button on the screen turns green, and the "Fault Display" button turns red. 2. Usage During the debugging process, the scheduled discharge control system underwent repeated modifications to the program and parameters after consulting with power plant operators, analyzing and resolving various potential adverse factors. After static and dynamic tests and on-site operation, the system demonstrated good performance and ease of use, leading to the conclusion that the project was a success. The project was characterized by a short construction period, low investment, and simple structure, demonstrating a certain degree of advancement and practicality. It significantly improved the level of automation and played a positive role in the safe and stable operation of the boiler.