1 Introduction
Boiler feedwater treatment in power plants requires the application of appropriate treatment technologies based on different water quality conditions. Untreated water contains various solid and liquid impurities, forming scale and a large amount of sediment, which affects the service life of the boiler. Therefore, impurities must be removed through physical, chemical, physicochemical, and biochemical methods. Standardizing boiler feedwater treatment in power plants can not only effectively prevent and reduce accidents caused by boiler scaling, corrosion, and steam quality deterioration, but also promote safer, more economical, energy-efficient, and environmentally friendly boiler operation. The clean treatment of boiler feedwater plays a crucial role in the overall operation of the boiler.
2ControlLogix series PLC application design
The boiler feedwater monitoring system consists of a power supply cabinet, a PLC control cabinet, and an operator station.
The boiler feedwater system uses Rockwell ControlLogix series PLCs. All modules communicating via the backplane are based on a producer/consumer model. Each module occupies a separate slot and can be inserted into any slot of various 1756 frames. Module replacement does not require disconnection; users connect the wiring to detachable terminal blocks (RTBs) and insert the terminal blocks into the front of the module. All modules are hot-swappable. Opto-isolation and digital filtering effectively reduce signal interference. As a diagnostic aid, status indicators are located on the front of the modules to indicate input/output and fault status. I/O modules can directly report fault conditions to the processor. Digital I/O modules cover a range from 10V to 265VAC and 10V to 146VDC, while relay contact output modules range from 10V to 265VAC or 5V to 150VDC. Analog signal voltage ranges include standard analog inputs and outputs, as well as direct thermocouple and RTD temperature input signals. Optional features of the analog module include digital filtering suitable for interference sources and environments, and range selection for each I/O channel to increase user flexibility. The analog module's comprehensive self-diagnostic function can monitor:
Input open-circuit/open-loop monitoring, board-level fault monitoring, two alarm levels (hi and hi-hi) for the upper limit plus an over-physical range alarm, and two alarm levels (lo and lo-lo) for the lower limit plus a low-physical range alarm. Engineering unit conversions make input and output analog signals easier to use. User-configurable output responses (final values or any user-defined values) for analog module faults ensure safety. The status area of the analog module provides information to the processor for alarm and fault diagnosis. A mechanical key lock for each module against the RTB prevents the application of inappropriate voltage to the module. An electronic key lock between each module and the Logix5555 processor prevents users from inserting the wrong module type or a different version of the module into the slot. Modules are configured via software rather than through DIP switches or jumpers. Analog module data precision can reach IEEE 32-bit floating-point or 16-bit integer data formats.
The redundant control system based on Rockwell ControlLogix series PLC is shown in Figure 1.
3 System Process Flow Interface Design
3.1 Process Flow
The process system employs a pretreatment system consisting of three parts: a chemical desalination system and a dosing system. Furthermore, the control of the pretreatment dosing system and the ammonia addition system is also integrated into the boiler feedwater treatment control system.
Pretreatment system process flow: raw water from main plant → mechanically stirred clarifier → raw water tank → raw water pump → high-efficiency fiber filter → activated carbon filter.
Chemical desalination system process flow: pretreated clean water → strong acid cation exchanger → carbon dioxide remover → intermediate water tank → strong alkali anion exchanger → mixed ion exchanger → demineralized water tank → demineralized water pump → main plant.
The dosing system refers to the storage and metering equipment for acids and alkalis used for regeneration in each ion exchanger of a chemical desalination system. The dosing system flow is as follows: low-level acid tank → high-level acid tank → acid metering tank → acid injector → strong acid cation exchanger (and mixed ion exchanger); low-level alkali tank → high-level alkali tank → alkali metering tank → alkali injector → strong alkali anion exchanger (and mixed ion exchanger).
3.2 Process Control
(1) Valve control logic: The valve control logic is shown in Figure 2.
(2) Control of the cation exchange inlet regulating valve: The regulating valve adopts PID negative feedback closed-loop automatic control. When the water level in the intermediate water tank is higher than the set value, the valve opening is reduced. When the water level in the intermediate water tank is lower than the set value, the valve opening is increased, as shown in Figure 3.
(3) Pump control logic: The pump control logic is shown in Figure 4.
3.3 Process Interface
Before switching the automatic/manual selector switch on the local valve control box from the manual position to the automatic position, the valve's open/close selector switch must be in the middle position. The main screen includes real-time data display, alarm display, report display, and historical curve display.
(1) Filter control interface
The filter control interface is shown in Figure 5. The instantaneous flow rate of each filter inlet valve is displayed in real-time on the left side of the screen. When the local control box switch is switched to local, the corresponding filter's solenoid valve cannot be operated via the computer. When switched to remote, the corresponding filter's solenoid valve can be operated via the computer.
(2) Dosing control screen
The dosing control screen is shown in Figure 6. For flocculant and coagulant aid dosing: Click the dosing control mode button to bring up the control screen and select the operating mode. Manual dosing: Click the manual button to dosing at the set frequency. Automatic dosing: Click the automatic button to dosing automatically according to the flow rate. Close the screen, click the motor icon to bring up the motor control screen, and start the motor. The dosing pump will then operate. For chlorine dioxide dosing: Click the chlorine dioxide dosing machine icon on the motor to bring up the control screen, start the dosing machine, and the equipment will operate automatically.
4. Conclusion
The boiler feedwater monitoring system employs a combination of a host computer and a programmable logic controller (PLC). The system includes an operator station, serving as the human-machine interface for system debugging and operation. This operator station is located in the control room of the water treatment workshop and also functions as an engineer's workstation. Control of the boiler feedwater monitoring system is achieved through the PLC. The PLC controller is a dual-machine redundant hot-standby system, meaning the entire PLC system includes a dual-machine hot-standby central processing unit (CPU), redundant power supply modules, and redundant communication modules. The PLC enables automatic, semi-automatic, remote manual, or local manual control of the entire chemical water treatment system.
