Automatic constant-pressure water supply system controlled by PLC and frequency converter

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【Abstract】This paper introduces a programmable controller-based constant-pressure water supply monitoring system. It emphasizes the operating principle of the system and the design method for constant-pressure water supply, and provides a block diagram of the automatic pressure regulation process. Electrical Technology [Thesaurus] Programmable controllers, Control, Water supply systems, CAD/CAM technology. Introduction: CAD/CAM technology. Constant-pressure water supply systems are very important for certain industries or special users; for example, in some production processes, if the pressure of the tap water supply is insufficient or there is a temporary shortage of water, it can affect product quality, and in severe cases, it may lead to defective products and damaged equipment. Similarly, in the event of a fire, if the water supply pressure is insufficient or there is no water available, it will be impossible to extinguish the fire quickly, which could lead to significant economic losses and casualties. Therefore, the adoption of constant-pressure water supply systems in certain water-using areas holds great economic and social significance. In old pressure supply systems, maintaining a constant water pressure is generally achieved by starting or stopping the pumps in the pressure station and adjusting the opening degree of the outlet valves. The control system uses relay contactors to operate the circuits; such systems have complex circuitry, are difficult to maintain, cumbersome to operate, require workers to be on duty 24 hours a day, and involve high levels of physical strain. Therefore, it is necessary to modify it in order to improve the level of automation. Electrical Technology: The monitoring system for rapid startup and constant-pressure water supply at a certain water pressure boosting station, as described in this article, is controlled by an FP3 programmable logic controller (PLC) produced by **ElecTronics, while HONOR industrial computers are used for monitoring. It features a high degree of automation – the entire operation process is carried out automatically, it allows for clear display of the real-time status of various devices, and it can adjust the water pressure automatically. This system also features various protections, such as water pressure overload alarms, valve failure alarms, water level overload detection and handling, as well as pump motor current overcurrent alarms and corresponding handling mechanisms. Electrical Technology: System Architecture and Control Requirements. Mechanical and Electrical Engineering Professional Network. The constant-pressure water supply system consists of a main water supply circuit, a backup circuit, 2 clean water tanks, and a pump room, as shown in Figure 1. Electrical technology: The pump house is equipped with a total of 6 pump units, numbered 1# to 6#, each with a capacity of 150 kW. There are also multiple (V1 ~ V23) electric gate valves that control each water supply circuit and water flow rate. The constant-pressure water supply system is required to have the following basic operating functions. Electrical technology, Mechanical technology: When the pressure of the municipal water supply is higher than the set value of 21.56?04 Pa, water is supplied directly from the municipal supply. Electrical technology: When the pressure of the municipal water supply is lower than the set value but not lower than 7.84?04 Pa, a direct pumping system is used to increase the pressure and supply water. That is, start the 2 pumps gradually to pressurize the pipeline network. When the pressure of the municipal water supply is detected to be above the set value, it is switched back to direct supply from the municipal water supply. Electrical technology: When the pressure of the tap water remains below 2.94×10^4 Pa or a negative pressure signal is detected, it is necessary to immediately switch to pumping water from the tank to generate pressure; however, it is important to ensure that the water level in the tank remains above the minimum required level. Mechanical technology: When direct water pumping or pumping water from a pool is used to supply pressure-driven water, it should be possible to automatically adjust the total outlet water pressure to a specified value, with an adjustment error of less than or equal to 10%. In electrical technology, CAD/CAM technologies are used in PLC control system design. The constant-pressure water supply system has numerous monitoring points and control parameters, making it a large-scale measurement and control system. Based on its characteristics, we selected the **FP3 programmable controller from Electrician as the control device. Compared to other programmable controllers, this controller has several distinct advantages: FP3 features a modular design that allows for flexible assembly according to actual needs, making it easy to use; moreover, the I/O allocation can be programmed freely ； It has a large capacity; the amount of programming is limited only by the scanning cycle, which can be adjusted within a certain range ； It features advanced units such as A/D, D/A, pulse output, and position control, enabling \"shared memory\" functionality” ； There are also some special features. The structure of the PLC system for constant-pressure water supply in mechanical technology is shown within the dashed box in Figure 1. The system includes a power supply unit, a CPU unit, a host computer connection unit, as well as an I/O unit and an A/D unit. The host computer uses an Advantech industrial PC along with configuration software from ABB, and the communication between the host computer and its units is carried out through a C#/.NET adapter. The industrial computer monitors the entire system; the display shows the structure of the entire pressurization system, the real-time status of various valves and water pumps, as well as parameters such as water pressure and flow rates, valve opening degrees, and tank water levels. It also provides real-time display of various alarms and records of faults. Electrical technology systems have both analog input and digital input. Analog signals are input through the A/D module, with a total of 27 channels. I/O has 96 points each. CAD/CAM technology, mechanical technology, software design for PLCs, electrical technology. In accordance with the requirements of constant-pressure water supply, the PLC control system must continuously monitor the conditions of the municipal water supply and the water supply outlet in order to decide whether to start the water pump, or whether to use a direct pumping method for pressurization or a method that involves drawing water from a reservoir for pressurization. The program of the control system is quite complex. The Mechanical and Electrical Professional Technology Network: In the control process, the automatic adjustment of water pressure at the water supply outlet is one of the important and distinctive design elements; this section focuses on the software design used to achieve automatic constant-pressure functionality. Due to the long length and large diameter of the water supply system pipes, the opening and closing of valves as well as the pressurization of the pipeline network are slow, making the system a highly delayed system. At the same time, since the modification was carried out on existing equipment, and those existing devices had to be utilized, speed and voltage regulation were not adopted; instead, various methods listed below were used to adjust the water pressure. First, the segmented adjustment method is employed to divide the water pressure deviation into four ranges: 10%, 20%, 30%, and 40%. When a small deviation is detected, the control amount output (the increment of the butterfly valve) is small, and the operation cycle is also large ； When the deviation is large, the output control amount is high and the operation period is short, allowing the deviation to be reduced rapidly while avoiding excessive overshoot. Furthermore, when the deviation is less than or equal to ±10%, fuzzy control is applied as well; based on the value of Dek = ek – ek–1, it is determined whether to adjust the opening degree of the butterfly valve in order to further reduce the error and ensure that it remains within the ±10% error limit. When adjusting the valve opening still does not bring the deviation within the allowable range, the water pressure is adjusted by starting or stopping one or more water pumps. By combining such various methods of adjusting water pressure, a satisfactory adjustment of the water pressure at the outlet can be achieved. Electrical technology, Mechanical technology. Conclusion: Mechanical technology. The PLC-based constant-pressure water supply monitoring system designed in this paper has been successfully applied in an industrial area. The operation results show that this system fully meets its design requirements; it boasts advantages such as easy operation, high reliability, accurate data recording, and timely monitoring. It also **reduces the workload of operators and shortens operation time, earning positive feedback from operators, maintenance staff, and management personnel. The successful design of this monitoring system also provides valuable experience for the retrofitting of old equipment in similar systems.