PLC control system and configuration for boiler feed water treatment in power plants

1 Introduction

The feed water treatment of power plant boilers needs to be carried out according to different water quality conditions and with the corresponding treatment technology. Untreated water contains a variety of solid and liquid impurities, which form scale and a large amount of sediment, affecting the service life of the boiler. Therefore, impurities must be removed by physical, chemical, physical and chemical, and biochemical methods. Standardizing the treatment of power plant boiler feed water can not only effectively prevent and reduce accidents caused by boiler scaling, corrosion and deterioration of steam quality, but also promote the safe, economic, energy-saving and environmental protection of power plant boiler operation. The clean treatment of power plant boiler feed water plays a vital role in the overall operation of the boiler.

2 ControlLogix series PLC application design

The boiler feed water monitoring system consists of a power cabinet, a PLC control cabinet, and an operator station.

The boiler feed water system uses Rockwell's ControlLogix series PLC. All modules that communicate through the backplane are based on the producer/consumer mode. Each module occupies a separate slot, and the module can be inserted in any slot of various 1756 frames. There is no need to disconnect the wiring when replacing the module. When wiring, the user connects the connection wires to the removable terminal block (RTBs) and inserts the terminal block into the front of the module. All modules can be plugged and unplugged with power on. Optical isolation and digital filtering can effectively reduce signal interference. As a troubleshooting aid, there is also a status indicator on the front of the module to indicate the input or output and fault status. The I/O module can report the fault condition directly to the processor. The digital I/O module covers the range from 10V to 265VAC and 10V to 146VDC, and the relay contact output module provided ranges from 10V to 265VAC or 5V to 150VDC. The voltage range of analog signals includes standard analog inputs and outputs, as well as direct thermocouple and RTD temperature input signals. Optional features of analog modules include digital filtering suitable for interference sources and interference environments, as well as range selection for each I/O channel to increase user flexibility. The comprehensive self-diagnostic function of the analog module can monitor: input open circuit/open loop monitoring, board-level fault monitoring, 2 alarm levels for the upper limit (hi and hi-hi) plus an over-physical range alarm, and 2 alarm levels for the lower limit (lo and lo-lo) plus a low physical range alarm. Engineering unit conversion makes input and output analog signals easier to use. User-configured output response (final value or any user-defined value) for analog module failures to ensure safety. The status area of ​​the analog module can provide information to the processor for alarm and fault diagnosis. The mechanical key lock for each module against the RTB prevents the application of inappropriate voltage to the module. The electronic key lock between each module and the Logix5555 processor prevents the user from inserting the wrong module type or a different version of the module into the slot. The module is configured by software rather than by dip switches or jumpers. The data accuracy of the analog template can reach IEEE 32-bit floating point or 16-bit integer data format.

The redundant control system based on Rockwell's ControlLogix series PLC is shown in Figure 1.

Figure 1 Redundant control system

3 System process flow interface design

3.1 Process flow

The process system adopts a pretreatment system consisting of three parts: a chemical desalination system and a dosing system. In addition, the control of the pretreatment dosing system and the ammonia dosing system is also included in the boiler feed water treatment control system.

Pretreatment system process flow: raw water from the main plant → mechanical stirring clarification tank → raw water tank → raw water pump → high-efficiency fiber filter → activated carbon filter.

Chemical desalination system process flow: pretreated clean water → strong acid cation floating bed → carbon dioxide remover → intermediate water tank → strong alkali anion floating bed → mixed ion exchanger → desalted water tank → desalted water pump → main plant.

Dosing system The dosing system refers to the storage and metering equipment of acid and alkali for regeneration of each ion exchanger in the chemical desalination system. The dosing system process is: low-level acid tank → high-level acid tank → acid metering box → acid injector → strong acid cation floating bed (and mixed ion exchanger); low-level alkali tank → high-level alkali tank → alkali metering box → alkali injector → strong alkali anion floating bed (and mixed ion exchanger).

3.2 Process Control

(1) Valve Control Logic: The valve control logic is shown in Figure 2.

Figure 2 Valve control logic

(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.

Figure 3 Cation exchange inlet regulation loop

(3) Water pump control logic: The water pump control logic is shown in Figure 4.

Figure 4 Water pump control logic

3.3 Process interface

Before the automatic/manual switch on the local valve control box changes from manual to automatic, the valve on/off switch must be placed in the middle position. The main screen contains real-time data display screen, alarm screen, report screen and historical curve screen.

(1) Filter control interface

The filter control interface is shown in Figure 5. The left side of each filter inlet valve on the screen displays the instantaneous flow rate of the filter when it is working in real time. When the local operation box switch is turned to local, the solenoid valve of the corresponding filter cannot be operated on the computer. When it is turned to remote, the solenoid valve of the corresponding filter can be operated on the computer.

Figure 5 Filter control interface

(2) Dosing control screen

The dosing control screen is shown in Figure 6. Dosing of coagulants and coagulants: Click the dosing control mode button, the control screen pops up, and the working mode is selected. Manual dosing: Click the manual button to add drugs according to the set frequency. Automatic dosing: Click the automatic button to automatically add drugs according to the flow rate. Close the screen, click the motor icon, the motor control screen pops up, and the motor is started. The dosing pump works. Chlorine dioxide dosing: Click the motor chlorine dioxide dosing machine icon, the control screen pops up, start the dosing machine, and the manufacturer's equipment works automatically.

Figure 6 Dosing control interface

4 Conclusion

The boiler feed water monitoring system adopts a combination of a host computer and a programmable controller (PLC). The boiler feed water monitoring system has an operator station as a human-machine interface for system debugging and operation. The operator station is set in the control room of the chemical water workshop. The operator station also has the function of an engineer station. The control of the boiler feed water monitoring system is realized by PLC. The PLC controller is a dual-machine redundant hot standby, that is, the programmable controller (PLC) is equipped with a dual-machine hot standby central processing unit (CPU), a redundant power module, and a redundant communication module. The entire chemical water treatment system can be automatically, semi-automatically, remotely manually or locally manually controlled through PLC.