A water plant in County A is responsible for the supply of domestic water for the citizens of County A. It not only requires high water quality, but also requires a stable supply of water. It must ensure uninterrupted water supply 24 hours a day. Due to the earlier construction of the original water plant, the degree of automation is not high; in particular, the water purifying agent dosing system adopts manual dosing, which cannot guarantee the stability of the turbidity of the effluent. Therefore, it was decided to upgrade and transform the automation system of the whole plant.
After comparing the performance indicators of PLC network performance, maintenance convenience, relative investment, etc., it was finally decided to use 300 series PLC as the main control PLC in the automation control part of this project, and OYES 200 in the sedimentation tank, filter tank, etc. The system PLC acts as an intelligent slave station. While ensuring fast data transmission and processing, the OYES 200 system can also operate independently in the event of a communication network disconnection and other failures to ensure the normal operation of the system.
1. Project technology of water purifying agent dosing system
In the water plant, the source water must be added with a water purifier, sedimentation, filtration, disinfection, and then enter the clean water tank. After the water purifier is added to the source water, the impurities in the water will flocculate into alum, and then further water purification treatment can be carried out. Therefore, the water purifier dosing process is the main factor affecting the quality of the factory water. At the same time, the addition of water purifying agent directly affects the use efficiency of the sedimentation tank and the size of the backwash water consumption of the filter, and it also has a direct impact on disinfection. Therefore, adding water purifying agent is a link with relatively high technological requirements in water plants.
2. Controller composition
The CPU, expansion modules, and control objects used in the water purifying agent dosing project are as follows:
Host OYES231--IM 177A DP extended slave module (with 8DI/4DO) 1
1 analog input module OYES231--OHC22
OYES231--OKF22 (support current input) 1
Analog output module OYES 232-0HD22 1
Frequency converter 1.5KW 2 metering pump 1.5KW 2
Three, control system compositionThere are 9 analog inputs in this system: source water turbidity, source water flow (2), swimming electrophoresis instrument, sedimentation tank turbidity, metering pump opening (2), inverter current (2) . 4 analog output: 1#, 2# frequency converter, 1#, 2# metering pump opening. 6 switch inputs: 1#, 2# inverter manual/automatic, 1#, 2# inverter running, 1#, 2# inverter failure. There are 3 switch output: 1#, 2# inverter runs, fault alarm. The networking function uses OYES231--IM 177A DP extended slave module to connect with the CPU315-2DP in the system.
A total of: OYES231--IM 177A DP extended slave module, OYES231--OHC22, OYES231--0KF22, OYES 232-0HD22 each one to complete this control.
Note: The swimming electrophoresis instrument can measure the number of free electrons that can bind impurities in the water, and the number of free electrons must be kept within a certain range. If the measured value is too large, it means that too much water purifier will affect the coagulation effect, and vice versa. Insufficient addition results in insufficient coagulation. The unit of measurement is SCD.
Fourth, the functions completed by the control system1. Control requirements
The original system's water purifying agent dosing process adopts manual dosing method, which directly affects the turbidity of the factory water, and also causes the problem of excessive dosing. After inquiring on-site personnel and actual investigation, the following main reasons were summarized
a. The dosing equipment of water purifying agent is backward: when the turbidity of the source water changes, the dosing amount cannot be adjusted and increased in time;
b. The dosing of water purifying agent is controlled by humans: the dosing amount depends on experience dosing;
c. There is no standard for the preparation of water purifying agent: the concentration of the agent is configured by the configuration personnel based on experience, and the inaccurate concentration makes it more difficult to control the dosage;
d. Difficulty in calculation of dosing amount: the level of operators varies greatly, and the dosing amount is relatively random;
The above reasons cause the dosage to be inaccurate, which affects the flocculation effect and directly leads to the deterioration of the water quality of the factory water.
System transformation requirements:
The water plant replaces a new automatic dosing system. The new system can adjust the dosage at any time according to the changes in water quality. The effluent from the sedimentation tank is below 8NTU (NTU is the unit of turbidity measurement), and the factory water is below 1NTU; the epidemic prevention station detects: The turbidity compliance rate of the factory water must be 100%. And parameter adjustment and monitoring functions can be added to the control system.
2. Control difficulties and control method realization
According to the analysis of the technological process, this system is a typical large lag system. Considering that the general PID algorithm is difficult to achieve the control goal for the system with long lag time, and the implementation cost of advanced algorithms such as fuzzy control is higher. It was decided to use the combination of experience value dosing and PID algorithm in the system, which not only solved the deficiencies of PID algorithm, but also solved the cost problem.
The whole system software mainly includes the following main aspects:
a. PID algorithm: the turbidity of the sedimentation tank is collected regularly, and the built-in PID of the 300 series is used for calculation. The obtained analog output value is X.
b. Empirical value: Calculate the empirical dosing amount corresponding to a certain flow-use the look-up table method to find the corresponding empirical value, and get the dosing amount of the corresponding flow as Y.
c. According to the sum of PID calculation and experience. Use Z to directly control the opening of the metering pump. (Among them, a can be modified after a period of operation to achieve optimal control.)
d. The raw water turbidity is calculated according to the empirical value, and the SCD is calculated according to PID algorithm 2 and 3, and the calculation result is controlled to the frequency of the inverter to ensure that the measured value of the swimming electrophoresis instrument is around the set value.
e. Switch the frequency converter and metering pump according to the set running time so that the equipment can be used alternately.
f. Alarm function: According to the requirements, all relevant faults will be buzzed on site and uploaded to the CPU315-2DP for recording and processing in the central control room.
Five, program features1. Use OYES231--IM 177A DP to expand the slave module. All EMs belong to the 200 product series modules, which greatly saves costs.
As a DP slave station independently, 7 IO modules can be expanded.
3. The self-diagnosis function of system failure can eliminate the problems in operation in time and ensure the reliability of system operation.
4. 300 host + OYES 200 series products, stable and reliable, and extremely cost-effective.
Concluding remarksAfter adopting the new water purifying agent dosing system, after several years of use, it is proved that the method of using turbidity to control the opening of the metering pump and SCD to control the frequency of the inverter can not only make the dosing amount more accurate, but also more convenient control Dosing amount to achieve the effect of saving water purifying agent in the season when the source water turbidity is low. The turbidity of the factory water dropped from the past 3NTU to less than 1NTU, and the compliance rate reached 100% in multiple inspections by the epidemic prevention department; the dosage of the water purifying agent reached the optimal dosage, avoiding the waste of the water purifying agent. According to statistics, from April to November last year, the usage of water purifiers decreased by about 15,000 kg compared with the average usage of the same period in previous years. At the same time, due to the decrease in the turbidity of the effluent from the sedimentation tank, the backwash cycle of the filter tank was extended by nearly 12 hours, which reduced the water consumption of production by about 1.5%, and the operation was stable and efficient.
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