1.0 Overview

According to the overall plan of XXX city, based on the prediction of sewage volume and the development prospects of the city, the construction scale of the sewage treatment plant is determined as a design scale of 20000 m3/d. Based on the sewage volume and investment situation, we consider the entire plant as a whole when configuring the system, and it can be easily expanded or upgraded. The system selects products that comply with international standards, have advanced technology and open structure, and can provide long-term technical support with guaranteed spare parts. At the same time, full consideration is given to economic applicability, investment savings, and connection with future projects. For control substations shared with the future, the number of control points is considered at once, and for independent parts in the future, separate control substations or remote control units are set up.

The self-control system of this sewage treatment plant adopts a layered and distributed topology structure of "centralized management, decentralized control, and data sharing", which is in line with the current development trend of industrial automation monitoring systems. It can achieve centralized monitoring of process parameters and equipment throughout the plant and automatic control of the production process.

The system includes: control system hardware equipment that meets the requirements, monitoring and programming software, auxiliary devices, as well as operation consoles, control cabinets, etc.

1.1 Design Principles
Centralized management, decentralized control, and data sharing;

Has high openness, reliability, stability, and security;

Has strong compatibility, scalability, and extensibility;

Easy to operate and use, modifiable;

All iconic, informative, warning, and display parts shall be in Simplified Chinese.

The automatic control instrument system must be designed based on the characteristics of the sewage treatment process in this project and in accordance with the advanced technology level of modern sewage treatment plants. In the design scheme, not only the progressiveness of operation and management level, but also the rationality and economy of high-tech application should be considered. On the premise of ensuring the production management requirements, investment should be saved as much as possible, good technical and economic indicators should be obtained, and long-term stable and efficient operation of the system should be ensured.

1.2 Overview of Automation System Functions
According to the design scale of XX sewage plant of 2 × 104m3/d and the characteristics of BAF process, the design is based on the principles of advanced technology, high cost performance, practicality and reliability. Based on the basic principle of centralized monitoring as the mainstay and decentralized control as a supplement, this project adopts a monitoring control and data acquisition system based on PLC (Programmable Logic Controller). Real time monitoring of various working conditions in the factory is carried out in the central control room using PC (industrial grade PC) machines, and signal alarm and interlocking facilities are provided to ensure normal production operation. The automatic control of the production process adopts independent control, that is, each sub station of the equipment control layer PLC and the upper monitoring computer are independent of each other, and can operate independently without relying on the upper computer, ensuring the independence and safety of the production process.

 

The distributed computer control system design adopted in this scheme meets the following requirements:

(1) Configure instruments and detection devices according to the process flow and equipment operation requirements, and establish a monitoring system;

(2) Adopting a decentralized control and centralized management approach, establish a central control system for the sewage treatment plant to manage the operation of the entire sewage treatment plant;

(3) The central control system adopts an open computer network that complies with the TCP/IP protocol and can be connected to the management system, as well as to the higher-level system and peripheral systems;

(4) The control of the main mechanical equipment adopts a three-layer control mode of local control, on-site control, and central control. The on-site control station is equipped with PLC and control operation human-machine interface;

(5) Other devices adopt a two-layer control mode of on-site control and central control;

(6) PLC based intelligent control devices are installed at each process node, and high-speed data communication networks such as fiber optic EtherNet are used to connect between PLCs and between PLCs and the central control system.

 

The following basic functions are implemented by the automation system of sewage treatment plant engineering:

(1) Real time monitoring of production process parameters (such as flow rate, liquid level, etc.), water quality parameters (such as pH value, SS, DO, etc.), and electricity parameters (such as current, voltage, power factor, active electricity, reactive electricity, etc.) of the entire factory, and collecting, processing, storing, displaying, and printing them; Real time monitoring of the operating status of major equipment in the entire factory (such as grid machines, lift pumps, blowers, valves, etc.), and collection and display of their signals. Accumulate the on/off frequency and operating time of important equipment in sewage treatment plants, such as lifting pumps, blowers, valves, etc., and generate equipment management reports to enable users to scientifically and reasonably arrange production equipment maintenance time.

(2) Fully automatic control of on-site equipment (such as grid machines, lift pumps, blowers, valves, etc.).

(3) In the central control room, monitoring and control of all equipment and instruments in the factory can be achieved.

(4) The upper computer adopts a fully Chinese operating interface. The interface is friendly and beautiful, easy to operate and learn, with fast response. It can achieve real-time dynamic display of trend charts of process parameters, water quality parameters, and electrical parameters; It can dynamically display the production process flow chart of the entire factory and the flow charts of each process unit, and can select to pop up multi-level detailed drawings on the flow chart. Capable of automatically generating various production statistics reports.

(5) It has automatic over limit alarm and equipment fault alarm, and can analyze the corresponding alarm data. Equipped with fault recall function, it can automatically record the status information of the system or a certain device before and during the fault process.

(6) This self-control system has the following three control modes

Manual mode: The start stop operation of the equipment is achieved through buttons on the local control box or MCC. This operation mode is mainly used for single machine debugging, single machine maintenance, or abnormal situations.

Remote control mode: The operator controls the on-site equipment through the monitoring screen of the control panel or central control system operation station using a mouse or keyboard, also known as "semi-automatic control". It mainly refers to the operator setting simple parameters or issuing control instructions to a certain link or device of the controlled object (system or process), and this link or device executes control according to the control requirements. The operator only needs to check its status and whether there is an alarm display. Such as remote air volume adjustment of blower, one-step control of lifting pump room, etc. Based on whether the operator is present at the controlled system or process site, semi-automatic control has two operation modes: remote (achieved through the central control room operator station) and local (achieved through the human-machine interface touch screen on the PLC control cabinet).

Automatic mode: also known as "fully automatic control", mainly refers to the operator setting or issuing control instructions for key operating parameters of the controlled object (system or process), and the system or process performs closed-loop automatic control according to requirements. The operator only needs to observe the status of the system or process and whether there are alarm displays, etc. For example, fully automatic coarse grid machine control, optimized control of lifting pump room units, automatic control of BAF biochemical treatment tank, etc. According to whether the operator is present at the controlled system or process site, fully automatic control has two operation modes: remote (achieved through the central control room operator station) and local (achieved through the human-machine interface touch screen on the PLC control cabinet).

The control levels of the three methods are from high to low: manual control, remote control, and automatic control.

 

1.3 System Configuration
1.3.1 Network Structure
Set up a central control room in the comprehensive building, which includes 1 # PLC pre-treatment control substation, 2 # PLCBAF biological filter treatment substation, 3 # PLC sludge dewatering system treatment substation, and 4 # PLC sewage central control treatment substation. The PLC processing substation is connected to the 1 # and 2 # workstations through an open fiber EtherNet ring network compatible with commercial Ethernet, and is connected to the upper management system through the workstations. The main equipment can be controlled to start and stop in the central control room. At the same time, the operation status of the equipment is also transmitted to the upper computer through EtherNet, and the operation status of the entire plant equipment is displayed and monitored on the computer.

The central control room is also equipped with Ethernet switches, which are connected to the Ethernet information network of the factory level management automation layer.

The self-control system is equipped with two sets of monitoring operation stations that are mutually hot standby, one faulty printer, and one chart printer. There are two sets of on-site control stations under the central control system.

According to the functions and equipment quantities of the controlled objects, and based on the process flow and layout plan of our factory, set up on-site control stations and central control rooms.

1.3.2 Specific Configuration (see Figure 1 for detailed configuration)
The central monitoring system consists of two industrial computers equipped with real-time monitoring software (hot standby for each other).

Programmable Logic Controller (PLC):

Adopting the latest products from world-renowned companies. Choose products from manufacturers with sufficient supply of Chinese materials, convenient spare parts, convenient technical services, and domestic repair centers. The selection of PLC shall fully consider its reliability, progressiveness and expandability, and shall meet the requirements of medium and high control performance and the strict requirements of industrial environment. The mean time between failures (MTBF) is 15 years.

Based on the technical specifications and requirements of the bidding document, as well as the overall considerations of the designer, Siemens' S7 300 series PLC is preferred.

The communication interfaces provided by the S7 300 system include EtherNet, ProfiBus, RS485, and RS-232. Its industrial bus EtherNet can reach 100Mbit/s.

In addition, Siemens is a world-renowned supplier of automation equipment, and its PLC products are in a leading position in the world with extremely high reliability. In China and Chengdu, users can receive comprehensive technical support and services.

This project uses S7 315-2DP with two DP ports as the main controller, and its main functional modules include EtherNet communication module CP 343-1, 32-bit digital input module SM 321, 16 point digital output module SM 322, 8-point analog input module SM 331, 4-point analog output module SM 332, etc. Please refer to Figure 1 and the equipment list for detailed configuration.

 

2 Control flowchart and detailed description of each part's functions
2.1 Production Process Monitoring System (Central Control Room)
Set up a central control room in the office building. There are two 21 "flat screen high-resolution computer workstations in the central control room, which are mutually backup (divided into the main operation station and the backup operation station). The main operation station is equipped with PLC programming software, which has remote programming function. The program can be easily downloaded to the designated on-site control station through the network, so that the program can be modified remotely in the central control room at any time during the debugging process. The central control room can perform system configuration management, system monitoring, real-time data monitoring, display, processing, and control of the status, communication, data, and information of each PLC substation for the entire distributed control system, and complete alarm and report printing. At the factory level management level, the results, benefit analysis, etc. can be sent to relevant departments through the Internet.

 

The central control subsystem mainly consists of the following parts:

1) Two industrial computers equipped with real-time monitoring software (as backup for each other), with the industrial control computer serving as the human-machine interface in the central control room and seamlessly connected to the industrial control system and the entire plant management network system through network adapters;

2) Ethernet card - a network communication interface adapter used to connect with the on-site PLC substation;

3) Data server for databases;

4) The report printer and accident alarm printer are mutually backup;

5) Ethernet switch;

6) Office printer;

7) Several computers used in the production management network;

8) Uninterrupted UPS power supply.

Its main functions:

Remote control of various PLC on-site sub stations, real-time reception of various data collected by PLCs, establishment of a factory wide detection parameter database, processing and display of various data;

Monitor the dynamic graphics of the entire factory's process flow and various details;

Display historical records and trend analysis curves as needed from the testing items;

The working conditions and accident alarms of the main parameters of important equipment, as well as printing and charting;

Compile and print production daily, monthly, and annual statistical reports;

Real time storage of various data;

Realize browsing of process flow, historical records, various equipment working status, reports, etc. through the server.

Application engineering software includes monitoring and control, online testing, offline testing, hardware testing software, and display functions for the entire sewage treatment plant.

It mainly completes the following functions:

Complete Internet functionality;

Adopting the standard Windows NT platform;

Contains various network drivers that support major hardware manufacturers around the world, supporting Ethernet and fieldbus monitoring;

There are security and confidentiality functions for system personnel and operators;

Support 1600 × 1280 high-resolution color graphics display;

Support various Windows standard printers and peripheral devices;

In order to facilitate the customization of user functions, the monitoring software is fully embedded with VBA, rather than just a subset of VBA;

Can support a friendly Chinese cultural interface;

Support open and ODBC compliant databases, and be able to share databases with programming software and other professional database software;

Provide users with rich and convenient graphic configuration and system configuration functions, making it easy to build various servers and graphic workstations;

Rich alarm functions, analysis report functions, online editing functions, and printing functions. Real time data, historical data analysis, comprehensive functions, and data recording and storage functions;

Easy to implement multi-user, multitasking, and multi terminal;

Online and offline switching function, automatic manual switching function.

The central control room can monitor the operation status of the entire sewage treatment plant at any time, display various detection values and parameters, and display current and historical values in the form of graphics and tables to provide various critical prompts and errors, over limit alarms, display and print panel reports, and send the results, benefit analysis, etc. to relevant departments through the Internet.

The communication between the central control room and the factory level management adopts 100Mbps Ethernet, which has email function and can send user-defined information, fault information, status information, etc. to the mail server. It can be connected to other control networks and on-site main lines through gateways (Ethernet switches) or PLCs.

The network has the following characteristics:

Transmission rate: 100Mbps;

Transmission protocols: TCP/IP, UDP/IP, FTP;

Transmission distance: Double comparison line 10Base-T-100 meters, fiber optic 100Base-F-40Km;

Transmission media method: fiber optic;

It can be connected to other control networks and fieldbus through PLC;

Uninterruptible Power Supply (UPS):

In order to ensure that the system has sufficient response time for emergency handling in case of power supply accidents, the central control room and each substation are designed with online uninterruptible power supplies (UPS). The UPS operates online, automatically switches to bypass work, and has no switching time.

2.2 Monitoring of Production Process (On Site) and Automatic Control System
2.2.1 1 1 # PLC Preprocessing Control Station
The 1 # PLC station is located in the pre-processing control room, with a monitoring range of coarse and fine grids, rotary sedimentation tanks, and reaction sedimentation tanks. It is implemented through on-site operation terminals (touch screens), programmable logic controllers (PLCs), instruments, meters, and transmitters.

I. Main equipment composition of 1 # PLC station:

a) One on-site display operation screen;

b) The PLC system used for automatic control, process parameter acquisition, and network connection on this site includes CPU modules, DI, DO, AI, AO modules, and various network communication interface adapters;

c) Overvoltage protectors and various isolators.

 

PLC serves as the controller, and the on-site display control screen serves as the human-machine interface to monitor and control process parameters, equipment operation status, and fault status. Send signals to the central control room computer operation station through the network to complete monitoring and management functions such as indication, recording, reporting, and alarm printing.