water treatment

ABSTRACT: According to the characteristics of sewage treatment control system in environmental protection, a distributed I/O system composed of S7-300 CPU and UniMAT I/O module is adopted to monitor and control the instruments and equipments on the spot. The results show that this way not only reduces the cost, but also achieves the desired effect.

Keywords: sewage treatment control system, UniMAT PLC

I. General Situation of the project

XX County Sewage Treatment Project is a municipal key project approved by the Development and Reform Commission. The design scale of the project is 100,000 tons of sewage per day. The control system of the sewage treatment project adopts UniMAT PLC module and Siemens S7-300 CPU control system. The upper control system adopts Siemens WINCC upper control system to feed the equipment of the sewage treatment plant. Line monitoring.

2. Control objects

This project mainly controls the opening, closing, stopping and running of field equipment (pumps, valves, etc.); the opening and closing of electric valves; the interlocking of key equipment; and realizes the automation of the whole sewage treatment process of sewage / coarse grille / fine grille / CASS tank / dosing room / sewage desliming room / landfill.

3. System configuration

The CPU 315-2DP and CP343-1 Lean modules of Siemens are adopted. The distributed I/O modules are all made up of UniMAT 300 series modules. The field control station and sludge dewatering room are connected by DP mode through EM277, and the field control station and dosing room are connected by MODBUS to DP mode. The whole system consists of 1 PLC field control stations, 1 operator stations and 1 engineer stations. The central control room and the field control station are connected by Ethernet. The operator station of the central control room provides the real-time information of the production process and all the alarm functions. The central control room monitoring system adopts Siemens WINCC configuration software. WINCC is the configuration software platform introduced by Siemens. It is easy to use, can construct flexible interface and strong function, and can develop strong combination picture with WINCC configuration software.

4. System functions and Implementation

1). system process

Total process screen

After the sewage is filtered through coarse grille, it enters the sewage regulating pool, then rises through the sewage lifting pump, passes through fine grille, then enters the sand sink, then enters the CASS pool for aeration treatment, then carries on the sedimentation, after the sedimentation, discharges the sludge to the sludge storage pool, and then carries on the sludge dewatering. The sewage from CASS pool is deposited and discharged to contact disinfection pool. Chemical treatment is carried out in the dosing room to reach the discharge standard. There are other reports, handover and other auxiliary interfaces.

2). system functions

The lowest layer of the system is the equipment control layer, which mainly completes the field control and monitoring of the equipment; the second layer is the monitoring layer, which mainly completes the on-line monitoring of the sewage treatment system in the whole plant and sends control instructions to the equipment control layer. The system includes monitoring the main interface and multiple sub interfaces. The main interface reflects the total process of sewage treatment, and the sub-interface monitors and controls each sub-process.

CASS pool control screen

The system is mainly controlled by the operator in the central control room, which has a server, two clients and UPS. The central control room uses WinCC to configure the upper computer system to generate the upper computer monitoring system, and processes the data collected by PLC. It is responsible for data management, data acquisition, alarm, fault, trend chart, data recording and report forms, and real-time display of equipment running status, running time, fault alarm processing, report output, etc., to achieve the monitoring of all equipment in the plant.

Its main functions are as follows:

A.Control operation: In the central control room, the system can be controlled on-line, such as starting and stopping a certain equipment, manual or automatic sequential control, PLC control mode switching and field PLC parameter setting value modification.

B. Display function: the real-time display of the operating conditions of the controlled equipment of each PLC station by graphics; dynamic display of various analog signals, switching signals, various cumulative signals and other values, through buttons, switches, signal lights, colors, percentages, filling and other means vividly expressed.

C.data management: it can set up information base such as runtime, alarm, fault, trend chart, system parameters and so on.

D.Data processing: Processing the digital and analog data uploaded by PLC and displaying them on the PC system as required.

E. alarm function: monitoring the status of all control equipment in sewage treatment plant, beyond the allowable range, that is, listed in the alarm table, and display the location and alarm level.

F. Fault Function: To monitor all the control equipment units in the sewage treatment plant. If there is a fault, it is listed in the fault table and the name and location of the equipment are displayed.

G. report function: generating runtime reports, influent / effluent flow reports, online COD reports, etc.

3)PLC control statio

According to the process requirements, after PLC power-on self-check, according to the signal of the host computer monitoring system to control the plant equipment start-stop, forward and reverse action. PLC completes the data acquisition of all the equipment of sewage treatment plant, such as water pump, coarse grille, fine grille, etc. After simple operation, it is uploaded to the upper computer monitoring system. When circuit breaks, short circuit or overload occurs, the data collected by PLC will exceed the set threshold, and the PLC will stop the control equipment and send an alarm signal to the upper computer monitoring system. All sensors adopt on-line instruments. The sampled data are converted into 4-20m standard current signal by the transmitter. The sampled data are directly transmitted to SM331 and 332 (analog input and output module), and converted to 0-27648 digital quantity by analog/digital conversion. The input and output of switch input is SM321 and 322 (digital input and output module). Among them, digital input (321-1BL00), output module (322-1BL00), analog input (331-7KF02), output module (332-5HF00), interface module (153-1AA03) all adopt domestic UniMAT series modules, which are stable in use, accurate in measurement and meet the requirements of the system.

4) data communication

This system connects the central control room and PLC control station into a network through industrial Ethernet, and realizes the data communication between PLC and WinCC. Separate 1 sets of S7-200 PLC systems are used for sludge dewatering. The system is connected with the EM277 of S7-200 in the sludge dewatering room, and the EM277 is hung on the DP sub-network of the system to communicate, thus controlling the start-up and stop of the equipment in the sludge dewatering room. The start and stop of the pump in the dispensing room is connected to the system DP network through the MODBUS to DP module, which is controlled in the central control room.

5. Use effect analysis

This PLC system adopts Siemens CPU and remote I/O station of Uygur UniMAT to form a centralized-distributed network structure control system, which not only ensures the stability of the control system, but also greatly saves the hardware investment and related construction costs. Since its completion and commissioning, the operation has been stable and reliable, and the control accuracy fully meets the design requirements, which not only saves costs, but also reduces energy consumption, improves efficiency, fully achieves the desired goal, and meets the requirements of environmental protection.

1.the advantages of constant temperature and constant pressure water supply.

Constant temperature and pressure water supply system is very important for life, for example, in the process of housing water supply, if the tap water supply pressure is insufficient or short-term water cut-off, it may affect the lives of residents. If the water supply pressure is insufficient or there is no water supply, the fire can not be extinguished quickly, which may cause heavy losses and casualties. Constant temperature water supply ensures that the user's water is kept within the set temperature range. Therefore, it is of great economic and social significance to adopt the dual constant pressure water supply system of life/fire fighting in living quarters.

Based on the above situation, our company developed a water supply system for a hotel, using Siemens 224 CPU plus our own UniMAT digital quantity module and temperature module as the main control unit, using Siemens inverter, according to the system state can quickly adjust the working pressure of the water supply system to achieve the purpose of constant pressure water supply.

2. the basic control strategy of water supply control system.

The control system is composed of motor speed regulating device and programmable logic controller (PLC), which optimizes the speed regulating operation of the pump group and automatically adjusts the number of pumping units. The closed-loop control of the water supply pressure is completed. The purpose of stabilizing the water supply pressure and saving electric energy is achieved when the flow of the pipe network changes. The control target of the system is the outlet pressure of the main pipe of the pumping station. The value of the feed water pressure set by the system is compared with the actual value of the feedback pressure of the main pipe. After the difference is input into the CPU operation, the control command is issued to control the number of pump motors running and the speed of variable pump motors running, so that the pressure of the main pipe of the pumping station is stable. On the set of pressure values. Constant pressure water supply is a closed loop control of industrial process realized by PID or PI function of frequency converter. The pressure signal (4-20mA) measured by the pressure control point is directly input into the frequency converter, which is compared with the pressure value set by the user. The result is converted into the frequency regulation signal to adjust the power frequency of the pump motor by the built-in PID operation of the frequency converter.

The communication between frequency converter and PLC calls only USS library program, and programming is simple and clear. UniMAT Temperature Module (231.7PD22) temperature measurement error control within 1 C, can effectively ensure the detection of water temperature, the digital input channels to detect low water level signals, the output channel according to the set procedures to control the operation of the switches and water valves, to ensure the correct operation of the system.

3.the basic composition of the water supply system

Frequency conversion constant pressure water supply system uses a Siemens inverter to drive four 0.5KW power motors, which can operate in both frequency conversion and power frequency mode. The user sets the water pressure through the man-machine interface, and monitors the electrode speed, frequency, running state and water pressure through the touch screen.

Starting mode: In order to avoid the impulse current when starting, the motor uses frequency conversion starting mode, from the inverter output terminal to gradually increase the frequency and voltage. Before starting, the inverter must be reset.

Frequency conversion speed regulation: according to the water supply pipe network flow, pressure change automatic control inverter output frequency, so as to adjust the motor and pump speed, to achieve constant pressure water supply. If the output voltage and frequency of the equipment can not meet the requirements of water supply when the output voltage and frequency rise to the power frequency, the PLC issued instructions No. 1 pump automatically switched to the power frequency power operation, until No. 1 pump completely quit the frequency conversion operation, after resetting the inverter, No. 2 pump into frequency conversion operation.

Multi-pump switching: according to the need of constant voltage, adopt the principle of no primary and secondary switching, that is, "start first stop" access and exit. In the PLC program, by setting the working number of the frequency conversion pump and the number of power frequency pumps, the pump increase or decrease can be judged by whether the given frequency reaches the upper or lower limit frequency. When the water consumption is small, the auxiliary pump is used.

The water supply system is divided into three parts, one is cold water, the other is heated hot water. The two parts of water are collected to the third part by water valve control. The part is equipped with water temperature detector, which feeds back the temperature signal to UniMAT temperature control module, and then controls the switch of the first two parts of the water valve by PLC to control the third part of water. The temperature is kept within a set range.

In order to prevent a pump from working for a long time, any pump can not run continuously for more than 3 hours. When the number of power frequency pumps is zero and one pump is running in the state of frequency conversion, start the timer, when it reaches 3 hours, the pump number of frequency conversion pump changes, that is, switch to another pump. When a pump runs in the power frequency state or the auxiliary pump starts, the timer stops counting and zeroing.

Fault handling: it can alarm the lower limit of water level, frequency converter, PLC fault and so on. PLC failure, the system automatically transferred to manual mode.

Monitor program: System main form of four button controls, respectively, parameter settings, real-time monitoring, start and close the system, users can modify the actual situation of pressure, temperature settings.

The main control functions include:

(1) pressure setting value range: 0.30-0.60Mpa; water temperature setting range: 5 C -60 C.

(2) correction factor: mainly for correction of pressure display, so that pressure display is consistent with the pressure gauge.

(3) reset button: press down in operation, which will restart the system and return the parameters to the initial settings.

(4) Setting button: Press this button after the text box input pressure settings and correction coefficients, pressure settings and correction coefficients can be sent to the lower computer through the serial port.

1. introduction

In the water supply system, constant pressure water supply means that the outlet pressure remains unchanged when the water consumption changes in the water supply network. In this paper, computer (PC), programmable logic controller (PLC) and frequency converter are used to compose the frequency conversion constant pressure water supply monitoring system, which can realize constant pressure water supply and meet the requirements of energy saving and consumption reduction through frequency conversion and speed regulation, and is beneficial to realize the automation and remote monitoring of production. The change of water consumption is stochastic, and the water pressure is insufficient when the peak water is used. Frequency conversion constant pressure water supply system can automatically adjust the increase and decrease of pump, the running mode of pump motor and the speed of motor according to the pressure change of public pipe network by PLC and frequency converter to realize constant pressure water supply, which not only prevents energy consumption, but also avoids the impact of impulse current on equipment when motor starts.

2.working principle

Frequency conversion constant pressure water supply system uses a frequency converter to drive two high-power motors, which can operate in both frequency conversion and power frequency; a low-power motor, as an auxiliary pump motor.

Starting mode: In order to avoid the impulse current when starting, the motor uses frequency conversion starting mode, from the inverter output terminal to gradually increase the frequency and voltage. Before starting, the inverter must be reset.

Frequency conversion speed regulation: according to the water supply pipe network flow, pressure change automatic control inverter output frequency, so as to adjust the motor and pump speed, to achieve constant pressure water supply. If the output voltage and frequency of the equipment can not meet the requirements of water supply when the output voltage and frequency rise to the power frequency, the PLC issued instructions No. 1 pump automatically switched to the power frequency power operation, until No. 1 pump completely quit the frequency conversion operation, after resetting the inverter, No. 2 pump into frequency conversion operation.

Multi-pump switching: according to the need of constant voltage, adopt the principle of no primary and secondary switching, that is, "start first stop" access and exit. In the PLC program, by setting the working number of the frequency conversion pump and the number of power frequency pumps, the pump increase or decrease can be judged by whether the given frequency reaches the upper or lower limit frequency. When the water consumption is small, the auxiliary pump is used.

In order to prevent a pump from working for a long time, any pump can not run continuously for more than 3 hours. When the number of power frequency pumps is zero and one pump is running in the state of frequency conversion, start the timer, when it reaches 3 hours, the pump number of frequency conversion pump changes, that is, switch to another pump. When a pump runs in the power frequency state or the auxiliary pump starts, the timer stops counting and zeroing.

Fault handling: it can alarm the lower limit of water level, frequency converter, PLC fault and so on. PLC failure, the system automatically transferred to manual mode.

3.PLC control circuit

The system uses S7-200PLC as the slave computer. S7-200 PLC hardware system contains a certain number of input/output (I/O) points, but also can expand the I/O module and various functional modules, in order to ensure the stability of the system on the basis of reducing system costs, we choose UniMAT expansion module after the CPU. The input point is 6, and the upper and lower limit signals are I0.0 and I0.1 respectively. The output point is 10, and O0.0-O1.0 corresponds to the output terminal of PLC. The reset of the inverter is realized by the contact point of the output point O1.0 through an intermediate relay KA. According to the I / O point and address assignment of the control system, the system has five input points of switching quantity, nine output points of switching quantity, one input point of analog quantity and one output point of analog quantity. You can choose CPU224PLC (14DI/10DO) and expand a UniMAT analog module EM235 (4AI/1AO).

4.electronic control program design

Design and analysis of 4.1 pumping station software

(1) management of the working group quantity required by the "constant pressure" requirement.

In order to keep the water pressure constant, the output frequency of the frequency converter should be increased when the water pressure is lowered, and the second one should be started when one pump can not meet the constant pressure requirement. There is a criterion to determine whether a new pump needs to be started, that is, whether the output frequency of the inverter reaches the set frequency upper limit. This function can be achieved by comparing instructions. In order to judge the certainty that the frequency of frequency converter reaches the upper limit, the upper limit of frequency fluctuation caused by accidental factors should be filtered out, and time filtering should be considered in the program.

(2) management specification for pumping stations in pumping stations

Because the inverter pumping station hopes that every start motor is soft start, there is a rule that each pump must be used alternately, so the operation of many pumping station pumping set needs a management specification. In this design, the control requirement stipulates that any pump should not run continuously for more than 3 hours. Therefore, it is reasonable to use the new pump as frequency conversion when starting a new pump or switching the frequency conversion pump. In the specific operation, the converter is removed from the converter and connected to the power frequency power supply for operation, and the converter is reset and used for the start-up of the new pump. In addition, there is another problem of pump group management is the pump working cycle control, in this design, the use of pump number plus 1 method to achieve the frequency converter cycle control that is 3 plus 1 to 0 logic, the total number of power frequency pumps combined with pump number to achieve power frequency pump rotation.

4.2 the structure of the program and the realization of its program functions.

According to the foregoing, PLC has many functions in the constant pressure water supply system. Because the analog unit and PID adjustment need to compile initialization and interruption program, this program can be divided into three parts: main program, subroutine and interruption program.

(1) Some of the initialization of the system is done in the initialization subroutine, which saves scanning time. Use time

Initialization program

(2) the main program flowchart is shown in Figure 2. It has the most functions, such as the generation of the switch signal of the pump, the synthesis of the logic control signal of the contactor of the pump group and the alarm processing, etc. in the main program. The two constant values of life and fire dual constant pressure are directly programmed in digital way. The water supply system is set at 70% of the full scale, and the system is set to 90% of the full scale of the fire water supply system. The gain and time constants of the system are: gain Kc = 0.25, sampling time Ts = 0.2s, integral time Ti = 30min.

Master control program

(3) The interrupt program is shown in Figure 3. Its function is mainly used for the corresponding calculation of PID. It works under the function of PLC's normally closed relay SM0.0.

5. concluding remarks

Constant pressure water supply technology uses frequency converter to change the frequency of the motor power supply, so as to adjust the pump speed to change the pump outlet pressure, than by regulating the valve to control the pump outlet pressure, with the effect of reducing pipe resistance greatly reduce the loss of interception. Because the variable displacement pump works in the frequency conversion condition, when its outlet flow is less than the rated flow rate, the pump speed reduces, reduces the bearing wear and heat, and prolongs the mechanical life of the pump and motor. To realize constant pressure automatic control, operators are not required to operate frequently, which reduces labor intensity and saves manpower.

The pump motor adopts soft start mode, accelerates according to the set acceleration time, avoids the impact of the current when the motor starts, and causes fluctuation of the grid voltage. At the same time, it avoids the surge of the pump system caused by the sudden acceleration of the motor.

Because the variable displacement pump works in the frequency conversion state, its speed is determined by the external water supply during its operation, so the system can save considerable electric energy in the operation process, and its economic benefit is very obvious. Because of this, the system can recover investment quickly, and the long-term benefits, its social benefits are enormous.

In practical application, the constant pressure water supply can be controlled by PLC, and the control program can be easily modified at any time to change the working time and working conditions of each component to meet different requirements. With relay or hardware.