The practical application of variable frequency speed regulation technology in box type non negative pressure water supply system
The box type non negative pressure water supply system pump is the most important energy consuming equipment in urban water supply systems. According to the investigation of operating pumps in multiple water supply pump rooms, it was found that the actual operating head of most pumps is less than the nameplate head of the pump, and some even differ by more than 30%. Therefore, this has led to a decrease in the operating efficiency of the water pump, serious energy waste, and frequent overloading of the motor. The reasons for this phenomenon are not only due to the advanced nature of municipal infrastructure and overestimation of water volume, but also due to unreasonable pump selection methods and improper pump matching.
At present, most water supply networks are direct supply networks without water towers. Existing regulations stipulate that pump head selection should be calculated based on the highest daily and hourly flow rate. However, in the vast majority of operating conditions outside of the highest hour, the required head is lower than the calculated head at the highest hour. When the operating point of the water pump under the highest working condition is in the high-efficiency zone, the water pump will move outside the high-efficiency zone for most of the time, resulting in lower operating efficiency and energy waste. The improvement method is that when selecting water pumps, the head should be mainly calculated based on the average hourly water consumption. The number of water pumps should not be too small and different models of water pumps should be matched with each other. This article will explore how to properly select water pumps to reduce energy consumption and meet the changing needs of urban water consumption through examples.
2、 Example of Box type Non negative Pressure Water Supply System:
The original design water supply capacity of a certain water plant was 100000 m3/day. The water supply pump room selected 5 single-stage double suction open centrifugal pumps, including 3 RDL500-790AIS pumps, with a pump head of 57.5M, a flow rate of 3084M3/h, and a motor power of 630KW; 2 14sh-9B pumps, with a pump head of 55M, a flow rate of 1080M3/h, a pump efficiency of 78%, and a motor power of 250KW. Currently, the actual water demand in the city is between 2.6-3.6000 m3/day, with an hourly flow rate between 1600M3/h and 3000M3/h, a pump outlet pressure of about 0.4MPa, and an hourly flow rate of about 2000M3/h. There are 8 months in the year.
1. Problem: Due to the large water volume in the original design, one small water pump, namely 14SH-9B, is usually needed to meet the demand. However, due to the high head in the original design, the actual required head is relatively low (factory pressure 0.40MPa). When the outlet valves of the water pump are fully opened and the pipeline pressure is low, the hourly flow rate of the water pump can reach 1600M3/h, resulting in cavitation phenomenon. The efficiency of the water pump seriously deviates from the high-efficiency zone, and the motor runs overloaded, causing the motor to heat up and easily burn out. When the water demand reaches 1700-2000M3/h, two small 14SH-9B water pumps must be turned on. At this time, both water pumps need to close their outlet valves to adjust the flow rate and meet the pressure requirements of the pipeline network. At this time, the efficiency of the two water pumps is extremely low, a large amount of energy is consumed on the valves, and the valves are severely damaged.
2. Solution to the problem: Firstly, we will modify 14SH-9B. Without replacing the original motor, we will select one KQSN400-N13 water pump with a head of 45M, a flow rate of 1632M3/h, and an efficiency of 87%. This way, one water pump can meet the water demand and operate in the high-efficiency zone; Once again, the original large RDL500-790AIS water pump was renovated (without a matching motor installed). One KQSN600-M13 water pump with a head of 42M, a flow rate of 2980M3/h, an efficiency of 85%, and a motor of 450KW was replaced. A variable frequency speed regulation device was installed on this unit to adapt to various water consumption changes. Through the above renovation, it can fully meet the demand for urban water consumption changes in the next 3-5 years.
3. Energy consumption calculation of box type non negative pressure water supply system:
According to the different water supply volumes, under the condition of a head of 40M, the motor efficiency is calculated at 92% based on the performance curve of the water pump. The unit consumption of the water pump before and after the renovation is calculated according to the formula W=QH/102 η (KWh). The calculation results are shown in the table, which shows the effect before and after the renovation.
In the formula, Q represents the water output of the pump station (L/s);
H - actual operating head of the water pump (M);
η - comprehensive efficiency of the water pump, η=η1·η2
η 1- Operating efficiency of water pump;
η 2- Efficiency of water pump motor;
3、 Conclusion
Based on the above calculation and analysis results, the following principles for selecting water pumps are proposed:
1. At present, the vast majority of urban water supply networks in China do not have water towers, and are directly supplied by pump stations to meet users' water needs. In order to adapt to different water demand, the number of water pumps selected should not be too small, and the size and model of water pumps should be reasonably matched, implementing a stepped water supply system to ensure urban water demand.
2. Using a combination of large and small water pumps, the flow rate of the small pump should be 40% to 60% of the flow rate of the large pump. If it is too small or too large, it will cause uneven water flow steps between different combinations of water pumps, thereby reducing the operating efficiency of the water pump.
3. Whenever possible, speed control devices should be used for motors, and the speed control devices should be installed on larger units, with a speed range controlled between 60% and 100%.
Overview of Box type Non negative Pressure Water Supply System
The box type non negative pressure water supply system is developed based on a combined stainless steel water tank and variable frequency constant pressure water supply, and is equipped with control devices such as anti negative pressure, anti backflow, and anti water quality deterioration. Compared with ordinary variable frequency water supply equipment and non negative pressure water supply equipment, this device has the advantages of significant energy saving, low noise, low occupancy, and high reliability, and is currently the most advanced water supply mode.
The box type non negative pressure water supply system is the seventh generation product developed by upgrading the existing variable frequency constant pressure water supply equipment and non negative pressure water supply equipment. It consists of a stainless steel water tank, a dedicated water pump, an intelligent variable frequency control cabinet, a non negative pressure device, a boosting device, a water diversion device, and a stabilizing tank. It is a new type of electromechanical integrated water supply equipment that combines computer technology, variable frequency technology, and motor pumps. To meet the requirements of urban water supply network, which not only does not generate negative pressure during pumping, but also has a certain capacity for regulating and storing water, and continuously stops water supply during peak periods. Using a microcomputer controlled variable frequency pump to operate at a constant speed and utilizing the existing pressure of tap water can ensure that the pressure required by the user is constant. It has the advantages of energy saving, environmental protection, low noise, and constant water supply pressure.
The main technical indicators of the boost pressure range, flow rate, and energy-saving indicators are all higher than other reported products in China, and the structural design is innovative, with an overall level that is leading domestically and advanced internationally. Therefore, it has a great competitive advantage in the market. In summary, the main characteristics of intelligent box type non negative pressure water supply equipment are to protect pipeline pressure, regulate peak water use, save energy, be safe, environmentally friendly, provide high-quality water supply (solve secondary water supply pollution), and have low noise.
With the development of secondary water supply pressurization technology, the box type non negative pressure water supply system fundamentally solves these problems. According to the "Water Supply Equipment Promotion Center", box type non negative pressure water supply equipment does not require the construction of a water tower, has small investment and occupies less land. It adopts automatic water and gas regulation, automatic operation, energy saving, and automatic grid connection with tap water. It can still supply water after a power outage and does not require supervision for several years after commissioning. Compared to building a water tower, it saves 70% of investment, and compared to building a high-level water tank, it saves 60% of investment, greatly reducing civil engineering investment.
The box type non negative pressure water supply system is widely used for production, living, and office water in enterprises, residential areas, and rural areas. There are 20-2000 water supply households. The daily water supply is between 20-50000m3, and the water supply height reaches 150 meters, which is a 50 story building.
Working principle of box type non negative pressure water supply system
When the pressure of the public water supply network is ≤ 0.2Mpa (can be freely set to 0.2-0.4Mpa), the non negative pressure device will be turned off and the non negative pressure inlet device will be opened, and water will be supplied from the water tank. Conversely, when the pressure of the public water supply network is ≥ 0.2Mpa, a delay of 10 minutes (time adjustable) will be applied. The non negative pressure device will be turned on and the non negative pressure inlet device will be turned off, and water will be supplied from the public water supply network. The water stored in the non negative pressure water tank is intelligently controlled to circulate every 6 hours to ensure fresh and pure water quality.
The variable frequency pump of the box type non negative pressure water supply system operates at a certain speed, utilizing the original pressure of tap water to achieve superposition, ensuring the pressure required by the user and a constant pressure. The inlet of the variable frequency pump for the non negative pressure water tank is connected to the non negative pressure device and the non negative pressure inlet device. By opening and stopping the non negative pressure device, the tap water pipe network does not generate negative pressure while the non negative pressure device starts and stops? The negative pressure inlet device will perform the opposite action as the negative pressure inlet device.
The non negative pressure device of the tap water network automatically shuts off when the water is stopped, and the non negative pressure inlet device of the water tank automatically opens to supply water from the water tank. When the liquid level in the water tank drops to a certain level, the non negative pressure inlet device will automatically shut down, the equipment will automatically stop, and it will automatically start running when power is restored.
Specific description of box type non negative pressure water supply system
1. Ensure pipeline pressure: Adjust peak water usage, and use intelligent solenoid valves at the inlet of the water tank to control inlet pressure and water level. When the pressure in the pipeline decreases and the booster pump draws water from the water tank, it can be intelligently controlled. When the water level in the tank is below a certain level, the solenoid valve is opened to release water from the pipeline, ensuring a certain pressure at the water intake. This measure can avoid peak water usage and have a peak shaving effect. Secondly, it avoids zero pressure discharge from ordinary water tanks and provides good protection for pipeline pressure.
2. Unique configuration: The equipment introduced by the "Water Supply Equipment Promotion Center" is at the forefront of leading the development of water supply equipment, using imported international advanced technology and key equipment, combined with dedicated pumps. It uses water-cooled motors, water-cooled bearings, fully enclosed absolute drip proof, with a noise level of less than 50 decibels, to prevent small water tank space and conventional motors from overheating and burning out motors and equipment.
3. Operational reliability: When there is a peak in water usage or when the tap water stops, water is supplied from the water tank, greatly improving the reliability of user water usage and fully complying with self supplied water supply standards.
4. Environmentally friendly and pollution-free: The water tank, water pump, pipelines, valves and other water related materials in this complete set of equipment are all made of food grade stainless steel materials, sealed for water supply, and will not produce secondary pollution, ensuring hygiene and environmental protection.
5. Energy saving: When the tap water pressure can meet the user's water requirements, the booster pump is controlled to be in a sleep state through frequency conversion. When non peak water usage occurs and the tap water pressure cannot reach the user's water pressure, the water pump can add pressure boosting on top of the tap water pressure (0.2-0.5MPa), making full use of the excess pressure to save electricity and achieve significant energy-saving effects.
Scope of application of box type non negative pressure water supply system
1. Applicable geographical scope: Suitable for pressurized water supply in areas with sufficient pressure in urban pipeline networks
2. Applicable engineering types: production and domestic water for industrial and mining enterprises, large intermediate booster pump stations for water supply in water plants
3. Applicable engineering location: Secondary pressurization system for water supply
4. Other applicable scope: Domestic water supply for newly constructed, renovated, expanded residential buildings, office buildings, hotels, restaurants and other public buildings
5. Types of using units: design unit, construction unit, supervision unit
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