Frequency conversion speed control technology in water pump control system

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  Pump power consumption accounts for 30% - 50% of the cost of water plant water production, how to reduce electricity consumption to reduce costs is the problem of each water plant is committed to research, this paper through specific examples of preliminary discussion on how to reasonably match the pump to achieve the purpose of energy saving and consumption reduction.

  I. Preface:

  Water pump is the most important energy-consuming equipment in urban water supply system. According to the investigation of operating pumps in several water supply rooms, it is found that the actual operating head of most pumps is smaller than the nameplate head of pumps, and some even differ by more than 30%. Therefore, the pump operation efficiency is reduced, the energy is wasted, and the motor is often overloaded. The reason for this phenomenon is that in addition to the municipal infrastructure which should be ahead of time, the water forecast is large, the pump selection method is unreasonable, and the pump is improperly matched is also one of the important reasons.

  At present, most of the water supply network for no water tower direct supply network, the existing specifications according to the highest day maximum flow to calculate the head selection pump, and in the majority of working conditions other than the highest time, the required head is lower than the highest time to calculate the head. When the pump working point in the highest time working condition is in the high efficiency zone, the pump working point will move to the high efficiency zone outside most of the time in normal time, thus the pump operating efficiency is low, resulting in the waste of energy. The improvement method is to choose the pump, the head should be based on the average water consumption to calculate the head, the number of pumps should not be too few and different models of pumps should be matched with each other.

  This article will discuss how to appropriately select pumps to reduce energy consumption to meet the changing needs of urban water consumption through examples.

  II. Example:

  The original design water supply capacity of a water plant is 100,000M3/day, and five single-stage double-suction centrifugal pumps are used in the water supply room, namely, three 600S-75AT pumps, with a pump head of 58M and a flow rate of 3000M3/h, and a motor power of 630KW; two 350S-75B pumps, with a pump head of 55M and a flow rate of 1080M3/h, and a pump efficiency of 78%, and a motor power of 220KW. Power 220KW; At present, the actual water demand of the city is between 26--36,000M3/day, the hourly flow is between 1600M3/h---3000M3/h, the pump outlet pressure is around 0.4MPa, the hourly flow is around 2000M3/h for 8 months in a year.

  1、Problem: Due to the original design of water volume is large, at present, usually open 1 small pump that is 350S-75B can meet the demand, due to the original design of the pump head is high, the actual demand head is low (factory pressure 0.40MPa), when the pump outlet valve all open the low pressure of the pipe network, the pump hourly flow can reach 1600M3/h, cavitation phenomenon, the pump efficiency is seriously The efficiency of the pump is seriously deviated from the high efficiency zone, and the motor is overloaded, the motor is hot, and it is easy to burn the motor. When the water demand reaches 1700 - 2000M3/h, it is necessary to open 2 small 350S-75B pumps, then all 2 pumps have to close the small export valve to adjust the flow to meet the pressure demand of the pipe network, then the efficiency of the 2 pumps is extremely low, a lot of energy is consumed in the valve, and the valve is seriously damaged.   2, the solution to the problem: first of all, we transform 350S-75B, without replacing the original motor premise, re-select a KBS300-435AT pump head 43M, flow 1577M3/h, pump efficiency 86.5%, so that usually open a pump to meet the water demand, and the pump can also run in the high-efficiency area; again the original large 600S -75AT water pump (this pump is not installed with supporting motor), replace 1 KBS400-500 water pump with 42M head, 2980M3/h flow, 85% efficiency of the pump, 450KW motor, and install frequency conversion speed control device on this unit to adapt to the needs of various water consumption changes, through the above transformation, it can fully adapt to the needs of the city in the next 3-5 years. Through the above transformation, it can fully adapt to the needs of the changes of water consumption in the city in the next 3-5 years.

  3、Energy consumption calculation:

  According to the different water supply volume, under the condition of 40M head, according to the performance curve of the pump, the motor efficiency is calculated according to 92%, and the unit consumption of the pump before and after the transformation is calculated according to the formula W=QH/102η(KWh), and the calculation results are shown in the table.   Where Q - pump station water output (L/s); H - actual operating head of the pump (M); η - comprehensive efficiency of the pump, η = η1 -η1 - pump operating efficiency; η2 - pump motor efficiency; Conclusion According to the above calculation and analysis results, the following principles for reasonable selection of pumps are proposed:

  1、At present, most of the water supply networks in China's cities and towns do not have water towers and are directly supplied by pumping stations to meet the water demand of users. In order to meet the demand of different water consumption, the number of pumps selected should not be too small, and the size of the pumps should be reasonably matched to implement ladder-type water supply to ensure urban water demand.

  2, the use of large and small models with pumps, small number of pumps flow should be 40% to 60% of the flow of large pumps, too small or too large will make the water volume between different combinations of pumps uneven, thus reducing the efficiency of pump operation.

  3、Where possible, try to use speed regulating device for the motor, and the speed regulating device should be installed on the larger unit, and the range of speed regulating should be controlled between 60% - 100%.


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