Thread Content
Application of Fieldbus in Soft Water Treatment Systems Author: Huang Hangchang, Longyan Cigarette Factory For many years, PROFIBUS has been popular around the world, being successfully used in manufacturing, building management, process control, and power plant automation. PROFIBUS products are widely accepted in the global market, with a market share of over 40% and an annual growth rate of 20%–30%. Currently, there are more than 1,500 products that support the PROFIBUS standard, manufactured by over 250 companies worldwide. More than 2 million PROFIBUS devices have been installed and in use worldwide. PROFIBUS is an international, open, manufacturer-independent fieldbus standard. Its bus access protocol is implemented at the second layer of the OSI reference model (the data link layer), and it includes technologies to ensure data reliability, as well as transmission protocols and message handling. The PROFIBUS series is composed of PROFIBUS-FMS, PROFIBUS-DP, and PROFIBUS-PA. PROFIBUS-FMS is primarily used at the unit level in factory and building automation to enable access to variables, program invocation, operation control, and event management ； PROFIBUS-DP is primarily used for high-speed data transmission at the field level, addressing the communication needs between automatic control systems such as PLCs and PCs and distributed field devices (I/O units, drivers, valves, etc.) via high-speed serial buses. PROFIBUS-PA is PROFIBUS-DP with the addition of a transmission technology preferred for field instruments, while retaining its communication protocol; it represents an extension of PROFIBUS-DP to the field. In many enterprises, water softening systems are an essential component for the proper operation of boilers; the quality of water treatment has a direct impact on the lifespan and safety of these boilers. Making full use of the advanced and mature PROFIBUS fieldbus technology to achieve automatic control of water treatment systems is an inevitable approach for production and information-based management. 1 System Control Functions 1.1 Process Flow of the Water Softening System The water softening system operates on the principle of sodium ion exchange. Tap water (with an inlet pressure of 0.2–0.3 MPa) is processed by the water softener and then sent to the water storage tank, from where it is delivered to the cooling circulation tanks, freezing tanks, and boiler rooms via a constant-pressure water supply system. After the water production volume is set, the water inlet switch is turned on, and the water softener operates in a cycle of 4 processes, as shown in the figure below: Figure 1 Block diagram of the water treatment system’s operation. The completion of these 4 processes is achieved by switching the operating position of the multi-way valve: once the water softener has produced the desired amount of water, it enters the backwashing process; after T1 time during backwashing, it proceeds to the regeneration process ； During the regeneration process of the water softener, it draws in saturated salt solution from the salt tank via siphoning to regenerate the resin inside the tank. Once regeneration is complete after a time period of T2, a rapid flushing phase begins; after the sediment in the tank is removed within time period T3, the system enters the next operating cycle. 1.2 Control Functions To be able to monitor the operating status of the equipment and changes in the quality of the soft water, ensure the stable operation of the equipment and a consistent supply of soft water, as well as to enable centralized monitoring of this equipment, the following control functions are proposed: Control is carried out using a reliable SIEMENS S7-300 PLC. Set the touch screen as the field operation panel to enable local display and operation. Electric valves and level gauges are installed to monitor and control the liquid level in the salt solution tank, thereby ensuring stable salt concentration. Achieve online monitoring of the water flow rate and hardness of the water output from the water softener. Implement emergency treatment for water quality abnormalities. Set up operational feedback for the multi-way valve and actuation feedback for the water outlet solenoid valve to achieve visualization of the working process. Add variable-frequency pipeline pumps and pressure transmitters to control the inlet water pressure. Implement interlock control between the water level in the soft water tank and the soft water treatment unit, stopping or starting the operation of the soft water treatment unit when the tank is near full capacity, in order to ensure a stable supply of soft water. Enable remote monitoring functionality. 2 System Hardware Composition Figure 2: Network structure of the control system 2.1 PLC System The input signals of the PLC system include fault signals from the frequency converter, position feedback signals from the multi-way valve, on/off feedback signals for the water outlet valve and the make-up water valve, on/off signals from the air switch, as well as information regarding the liquid level in the salt tank, water output flow rate, and inlet water pressure. The outputs of the PLC system include commands to start/stop the frequency converter and to set its operating frequency, as well as controls for the on/off operation of the multi-way valve and the water outlet and make-up water valves. The programmable controller (PLC) for this system is the SIEMENS S7-315 PLC. This series of PLCs comes equipped with 2 DP interfaces, offers strong functionality, is easy to expand, and provides good cost-performance, thus fully meeting the requirements for system control. Acting as the master station of the PROFIBUS-DP network, this PLC handles various system data and process control procedures, such as receiving control commands from the network to operate devices ； Collect information from the DP slave station and process it. 2.2 The color touch screen TP170B: To enable field operators to carry out control in a timely, flexible, accurate, and safe manner, the system is equipped with a Siemens TP170B touch screen in the control cabinet as a field operation panel, which is connected to the S7-315 via a DP interface. Operators at different levels are assigned different operation passwords, which allow them to start and stop the water treatment unit as well as set time and flow parameters according to their respective permissions. This touch screen is easy to use and allows for flexible input of control commands as well as monitoring of status; it takes up no space during installation. Its robust protective structure and professional design enable it to withstand the harsh conditions in field applications. 2.3 Inverter: The inverter is of the DANFOSS 2800 series; it is connected to the PLC via a DP interface, enabling a constant pressure supply of water. Meanwhile, operation and fault signals are transmitted to the PLC via the DP bus for processing. 2.4 DP/PA Coupler and PA Slave Stations: The DP/PA Coupler serves as both a slave station for the S7-315 PLC and a master station for the lower-level PA instruments. All signals from the PA slave stations (inlet pressure, outlet flow rate, outlet hardness, level) are transmitted to the PLC for processing via the PA interface through the DP/PA Coupler. 2.5 Operator Station: The system is equipped with a computer in the energy management room as the operator station, which is used for system management and remote monitoring. Operators at different levels are assigned different login passwords, allowing them to effectively monitor and manage the system according to their respective permissions. The operator station and the S7-315 PLC master station transmit data via the plant backbone network (industrial Ethernet). 3 System Software Design 3.1 Upper-Level Monitoring Software The upper-level monitoring software is developed using Intellution IFIX 2.6, an industrial control configuration software that is widely used worldwide. This software is highly functional; it operates on a Microsoft Windows interface and features VBA script editing capabilities. Programmers can take advantage of the software’s built-in expert system for design purposes, or they can use the VBA scripting functions to write programs flexibly. The software is divided into two parts: HMI and SCADA. The HMI features a rich set of graphic libraries and drawing tools, allowing programmers to create vivid, dynamic, and colorful graphics as well as user interfaces ； SCADA has a powerful driver function; it allows for the configuration of eight drivers simultaneously, with each driver capable of configuring any number of hardware devices from the same series at the same time. SCADA’s powerful driving function can meet any driving requirements on site. The monitoring software includes an administrator login function, an authorized password management function, a monitoring interface for the water treatment system, a real-time alarm function, and system management functions. 3.2 Field Operation Panel The monitoring software for the field operation panel was developed using SIEMENS HMI configuration software ProTool V5.2 in its Chinese configuration version. The operator panel exchanges data with the PLC through its own MPI/DP interface. The monitoring software includes interfaces such as “Password Management,” “System Monitoring,” “Standalone Device Monitoring,” and “System Settings,” in addition to features like real-time fault alerts. Here, settings for backwashing, regeneration, forward washing time, as well as water production volume, inlet pressure, and PID parameters are all available; forced regeneration can be initiated with just a simple press, and the regeneration process will be automatically carried out. 3.3 PLC Program As shown in Figure 2, the control of the equipment in the water softening system is carried out through a PLC. The PLC software handles the responses to and processing of various control commands from the operator panel and the upper-level computer, and it is used to control the variable-frequency pump, multi-way valve, outlet solenoid valve, and make-up water solenoid valve ； At the same time, their feedback signals are collected; the PLC processes these signals to display in real time on the on-site control panel and the main computer the operating status of each valve, the various stages of operation of the water softener, the flow rate of the treated water, the level of the salt tank, and the inlet water pressure. This enables automatic detection and alerting of any faults that occur during the treatment process, allowing operators to identify and address problems promptly, thereby preventing the discharge of untreated water and reducing waste of salt and water. The system can also automatically store the set values for the water production volume and the remaining water amount before a power outage, and it can resume operation automatically once power is restored ； It pauses operation when the inlet water pressure is too low (or there is a water supply interruption), remembers the operating position and flow rate, and resumes automatic operation once the inlet water pressure returns to normal ； Depending on operating conditions, it is possible to choose single-tank operation or combined parallel operation, and each tank can be operated independently without affecting one another. The system provides three control modes – manual, automatic, and forced regeneration – designed according to the process requirements, allowing operators to choose the appropriate mode based on the actual conditions on site. (1) Manual control: In manual control mode, the operator can conveniently and flexibly control the opening and closing of each valve on-site or from the control room, allowing the multi-way valve to be operated in any of the working, backwashing, regeneration, or rinsing modes, while simultaneously being able to monitor the status in real time. (2) Automatic control: In automatic control mode, the system automatically determines and carries out control based on the set mode (time control mode, flow control mode). In flow control mode, after the operator sets the water production volume and presses the operation button, the PLC will issue commands to the multi-way valve and various solenoid valves according to the preset program, automatically opening and closing these valves, thereby enabling the various operational processes such as operation, backwashing, regeneration, and forward washing as shown in Figure 1. In this mode, the system uses turbine flowmeters to measure the water production volume and control the processing process. To further enhance the reliability of the system, and given that time is linearly related to flow rate when the water pressure remains essentially constant, a time control mode has been added to the system for use in case of problems with the flow meter. In this mode, the control process is identical to that of the flow control mode; only the flow signal needs to be replaced by a time signal, which will not be discussed in detail here. (3) Forced regeneration: During the operation of the water softener, when it is detected that the quality of the effluent is not satisfactory and the resin in the tank needs to be regenerated, the forced regeneration procedure can be used. In this mode, after pressing the forced regeneration button, the system automatically switches from operating mode to backwashing mode; after a time of T1, it switches to regeneration mode, after another T2 of time it switches to normal washing mode, and after T3 of time it returns to the operating mode, thus completing the forced regeneration process. 4 Conclusions Since the system was put into operation, operational results show that it has high reliability and stable performance; it features a user-friendly interface, is simple to operate and maintain, and all its performance indicators meet the expected requirements, ensuring a consistent quality and steady supply of soft water. The implementation of remote system monitoring improves work efficiency, reduces the workload on manual labor, and facilitates the operation and management of equipment.