Discussion on some technical problems of the flow

2022-08-04
  • Detail

Discussion on some technical problems of heat meter flow sensor

brief introduction: in many household heat meter products at home and abroad, due to many factors such as price and power consumption, small caliber mechanical hot water meter is widely used as the flow sensor of heat meter, and the requirements for flowmeter in the heat meter industry standard CJ of the Ministry of construction basically adopt the same requirements as the performance of current hot water meter products

the application and research practice show that there are a series of problems to be solved when the small caliber mechanical hot water meter is directly used as the flow sensor of the heat meter. According to the research experience of the heat meter flow sensor, we found that the main problems of the small caliber mechanical hot water meter as the heat meter flow sensor are: the range problem, the difference of the cold and hot water flow coefficient, the problem of reducing the starting flow and improving the accuracy under the condition of small flow, the problem of magnetic interference in the magnetic transmission mode, the problem of out of step at high temperature, and the adaptability to the water quality of the heating system in China. According to the research and analysis results, we have made a preliminary analysis of the above problems, and put forward some solutions to discuss with peers in the industry, in order to develop a flow sensor suitable for the heat meter and jointly improve the development level of heat meters in China. 1. Measuring range of heat meter flow sensor 1.1 measuring range of heat meter flow sensor article 4.3.3 of the heat meter industry standard CJ of the Ministry of construction stipulates: "the common flow of heat meters shall meet the requirements of gb/778.3 cold water meters. The ratio of common flow to minimum flow shall be 10, 25, 50 or 100. For heat meters with nominal diameter ≤ 40mm, the ratio of common flow to minimum flow must be 50 or 100." The flow range of hot water meters with different diameters in a certain plant (which is widely used by heat meter manufacturers at present) is shown in Table 1: Table 1 nominal diameter

dn

common flow

m3/h

maximum flow

m3/h

minimum flow

l3/h (take 1/50)

boundary flow

l3/h150.61.212481 Article 5.3.5 of CJ standard stipulates that error limit of level 3 flow sensor: eq= ± (3+0.05qp/q)

indication error is 2% between boundary flow (included) and maximum flow, and 5% between boundary flow and minimum flow. At the same time, it is stipulated that the maximum error limit of flow sensors at all levels shall not exceed 5%. Taking the widely used DN20 heat meter as an example, its measurement error curve is shown in Figure 1. 1.2 flow design range of building heating system according to relevant data, the flow design range of energy-saving and non energy-saving building heating systems in northern cities of China is shown in Table 2 and table 3. According to the empirical data of changing the hydraulic oil according to the actual service conditions, when the user realizes the compartment adjustment, the working flow will be reduced to 50% of the flow of the transmission electromechanical meter of the auxiliary unit. 1.3 analysis conclusion and improvement measures according to the above data, the surface in direct contact with the sample is connected with soft rubber, and the small caliber mechanical hot water meter is used as the flow sensor of the heat meter. The following conclusions can be drawn: table 2 Design scope of heating system for energy-saving buildings building area m2 maximum design flow kg/h minimum design flow kg/h Table 3 flow design scope of heating system for non energy-saving buildings building area m2 maximum design flow kg/h minimum design flow kg/ha The common flow of hot water meter is too large, which is more than 10 times of the design flow of building heating system; b. Most heat meters will work below the boundary flow, and heat meters with diameters above DN 20 will even work near the minimum flow; c. The flow sensor of the heat meter will work in the high error area most of the time. If it works below the minimum flow, the actual measurement error will exceed the requirements of CJ standard by 2% ~ 5%; d. The unmodified hot water meter is not suitable to be directly used as the flow sensor of the heat meter; e. When the hot water meter is used as the flow sensor of the heat meter, the problems to be solved are: reducing the common flow to make it close to the design flow range of the building heating system; Improve the sensitivity of the flow sensor, reduce the starting flow, and further reduce the minimum flow to meet the requirements of 1/100 of the current common flow; Take effective measures to improve the measurement accuracy of the measurement range below the boundary flow. In the process of developing HM-1 heat meter, we have taken a series of technical measures to solve the above problems. Including: removing the gear technical mechanism of the original hot water meter to reduce the resistance of the impeller (according to the experimental data: after removing the gear counting mechanism of the original hot water meter, the resistance can be reduced by about 30%), so as to improve the sensitivity, reduce the starting flow, and further reduce the minimum flow to achieve the purpose of 1/100 of the current commonly used flow; The flow sensors are calibrated one by one and the dynamic measurement correction measures are taken to realize one meter and one coefficient, and effectively improve the measurement accuracy of the measurement range below the boundary flow. Design a new flow sensor to reduce the common flow and make it close to the design flow range of the building heating system. In order to reduce the pressure loss of the system, we can take measures to change the diameter. These should be the key technical problems to be solved at present. 2. it is well known that the difference of flow coefficient between hot and cold water. The flow coefficient kV of rotor type mechanical magnetic transmission hot water meter is related to the design, manufacturing accuracy and production commissioning. Within the whole flow range of the hot water meter, its indication error changes with the flow (flow rate), as shown in Figure 1. The results show that the flow coefficient kV also changes with the change of water temperature, especially in the small flow area below the boundary flow. It is not difficult to understand that as the water temperature increases, the water density decreases, its viscosity decreases, and the impeller resistance decreases; When the water temperature rises, the shell and impeller will expand. Due to their different manufacturing materials and expansion coefficients, the gap between the shell cavity and the impeller will change. The calculation results show that the impact of this change on the flow coefficient kV can not be ignored. In addition, when the water temperature rises, the resistance between the impeller and the bearing will also change. The comprehensive influence of the above factors causes the flow coefficient kV to change with the change of water temperature. For different hot water meters, the change law will be different. Table 4 shows the change of flow coefficient kV of a hot water meter at 85 ℃ and normal temperature. Table 1 Table 2 Table 3 table 4 average K coefficient of each point 5032.538.9838.7239.0438.74438

Copyright © 2011 JIN SHI