Introduction
Flow measurement is an important part of measurement science, and at the same time promotes and supports the development of the national economy. However, flow meters are one of the few instruments that are more difficult to use than to manufacture. Therefore, when performing flow measurement for a real industrial and mining occasion, we should proceed from the actual situation and comprehensively consider the safety, accuracy, and economy of the measurement, so as to finally achieve better flow measurement.
Select the pipeline flowmeter according to the actual working conditions of the sewage treatment unit
A sewage treatment unit, as a link to receive the sewage discharged from the pollution source with a high total amount or concentration of pollutants, which does not meet the discharge standard requirements or does not meet the environmental capacity requirements, and artificially strengthens it, needs flow information at any time to determine The operating status of the system and the upload of sewage information. In order to maximize the comprehensive benefits, the selection of sewage flowmeters in sewage treatment plants should be checked one by one from the indicators of safety, accuracy, and economy.
Security
Where flow meters need to be installed in sewage treatment plants, there are generally no complicated and dense mechanical devices and electrical devices, so there will be no accidents caused by mechanical strength or electrical circuit failure. Sewage treatment units will produce a large amount of gas during operation, such as methane, CO, hydrogen sulfide, etc. When some of these gases reach a certain concentration, there are potential safety hazards. Therefore, when selecting a flowmeter, the explosion-proof performance of the flowmeter should be considered.
Accuracy
In order to achieve the accuracy of sewage measurement, the working objects and characteristics of the flowmeter installed in the sewage treatment unit should be comprehensively analyzed to find the basis for selecting the flowmeter and the precautions when using it.
Measured medium
The treated medium of sewage treatment units is sewage, but the sources of sewage are different, and the parameters of various indicators of sewage from different sources are also different, so the source of sewage should be considered first.
(1) Domestic sewage is water that has been used by people in daily life and polluted by domestic waste. The water quality and quantity vary with the seasons. Generally, the water is relatively more in summer and the concentration is low; in winter, the corresponding amount is less and the concentration is high. Domestic sewage generally does not contain toxic substances, and the main pollutants are organic matter, inorganic salts, etc., and there are many suspended solids.
(2)Industrial wastewater is wastewater generated in industrial and mining production activities. Industrial wastewater can be divided into production wastewater and production wastewater. Production wastewater is formed during the production process and is polluted by raw materials, semi-finished products, or finished products, as well as thermally polluted water (referring to water generated during the production process with a water temperature exceeding 60°C); production wastewater is formed during the production process but does not directly participate in the production process, is not polluted by raw materials such as production raw materials, semi-finished products or finished products, or only has a small rise in temperature. Production wastewater needs purification treatment; production wastewater does not need purification treatment or only needs simple treatment, such as cooling treatment. Production sewage is related to the production industry, and the pollutants in different production industries or different production processes in the same industry are also very different. The main pollutants in the production of sewage are nothing more than organic aerobic substances, chemical poisons, inorganic solid suspended solids, heavy metals, acids, alkalis, plant nutrients, pathogens, etc.(3) Polluted rainwater mainly refers to the initial rainwater. Since the initial rainwater has washed away various pollutants on the surface, the pollution level is very high, so it is suitable for purification treatment.
It can be seen that for sewage treatment units, the medium to be measured is a mixed medium containing certain suspended matter and other dirt. Because it contains a large number of substances, it often has certain acidity, alkalinity, corrosiveness, and conductivity. Due to the corrosiveness of suspended substances in sewage, for contact flow meters such as turbine flow meters, positive displacement flow meters, differential pressure flow meters, rotor (float) flow meters, etc., if long-term measurement, The dirt in the sewage may block the flowmeter, corrode the working components of the flowmeter, and make the flowmeter unable to work normally. Therefore, when measuring the sewage flow, it is best to use a non-contact flowmeter to avoid being eroded by sewage and unable to work normally for a long time.
Temperature of the measured medium
Sewage treatment units usually operate at normal temperature and pressure, so the temperature of the measured medium can be considered as normal temperature, so there is no need to consider too much when selecting a flowmeter.
Pressure of measured medium
Sewage treatment units usually operate at normal temperature and pressure, so the temperature of the measured medium can be considered normal pressure, so there is no need to consider too much when selecting a flowmeter.
Flow rate of the measured medium
For different sewage treatment units, their treatment capacity is different, and their pipe diameters are also different, so it is necessary to carry out actual analysis according to the specific sewage treatment unit. When the pipe diameter is small, most flow meters are adequate. Because some flowmeters, such as differential pressure flowmeters, require that there should be several times the length of the straight pipe diameter before and after the installation point during the installation, but when the pipe diameter is large, this is difficult to achieve. Electromagnetic flowmeters, ultrasonic flowmeters, and insertion flowmeters have great advantages when measuring the flow of large-diameter pipelines. However, combined with the above-mentioned analysis of the sewage itself containing suspended solids and having certain corrosive characteristics, insertion flowmeters are generally less used in such working conditions as sewage treatment units and are not considered.
Economy
In terms of measurement accuracy, the electromagnetic flowmeter is higher than the ultrasonic flowmeter, but the price of both will increase with the increase of the pipe diameter. The measurement accuracy of the ultrasonic flowmeter will become more accurate as the pipe diameter increases. In actual application, the decision should be made after fully considering the actually required accuracy, pipe diameter, and other factors.
According to the above analysis, the measurement accuracy of the electromagnetic flowmeter is higher, while the measurement accuracy of the ultrasonic flowmeter will become more accurate with the increase of the pipe diameter. Considering the generality, it is better to face different sewage treatment units, so this paper gives electromagnetic flowmeter and ultrasonic flowmeter as alternatives for the flowmeter selection of sewage treatment units. For specific sewage treatment units, electromagnetic flowmeters and ultrasonic flowmeters should be flexibly selected according to actual working conditions, such as the size of the pipe diameter and the required measurement accuracy, to achieve better results.
Electromagnetic flowmeter and ultrasonic flowmeter
Electromagnetic flowmeter
The measurement of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When the conductive medium cuts the magnetic force line in the electromagnetic field, the conductor will generate an electromotive force Ed, and the electromotive force Ed is proportional to the flow rate (velocity) of the measured medium. Because the electromotive force is only related to the flow rate of the medium, the electromagnetic flowmeter is generally not disturbed by other external factors such as temperature, pressure, viscosity, and density. In addition, the initial signal detected by the flow element is a voltage that changes linearly with the average flow velocity of the fluid, which is independent of other properties of the fluid and has great advantages.
According to the characteristics of sewage with large flow changes, impurities, low corrosion, and certain electrical conductivity, electromagnetic flowmeters are a good choice for measuring the flow of sewage. It is compact in structure, small in size, and easy to install, operate and maintain. In recent years, the electromagnetic flowmeter measurement system has gradually become intelligent, and the overall sealing of the instrument has been strengthened to ensure normal operation in harsh environments. In view of the certain corrosiveness of sewage, neoprene lining, and molybdenum-containing stainless steel (OCrI8Ni12Mo2Ti) electrodes can be selected for the installation of electromagnetic flowmeters, which can meet the requirements of sewage flow measurement.
Ultrasonic flowmeter
At present, ultrasonic flowmeters are roughly divided into two types: one is Doppler ultrasonic flowmeter, and the other is a transit-time ultrasonic flowmeter. The Doppler flowmeter is a method of measuring the flow rate by using the phase difference method, that is, when a sound wave of a known frequency moves in water because the liquid itself has a moving speed, the ultrasonic wave between the two receivers The relative change of frequency or phase occurs, and the liquid velocity can be obtained by measuring this relative change, so as to measure the flow rate; the time-difference flowmeter uses the time difference method to measure the flow rate, that is, the sound wave of a certain speed is caused by the fluid to make it flow in the two receivers. The velocity of propagation changes between the two and the fluid velocity can be obtained by measuring this relative change. In practical applications, we mostly use time-difference flowmeters.
It is suitable for homogeneous liquids that can transmit ultrasonic waves and gases with high flow rates. The medium temperature range is -10～100℃, and the medium pressure is not limited. It has the characteristics of a wide range ratio, is not affected by the viscosity and density of the fluid, and can measure strong corrosive media. The real advantage of the ultrasonic flowmeter is that it is used to measure the flow of large-diameter pipes, such as sewage treatment plants, users with large water consumption, and large pipe diameters.
Flowmeter selection case analysis
Selection of pipeline flowmeter for a sewage treatment plant
The sewage treatment plant is the largest sewage treatment plant in Hangzhou. Its sewage mainly comes from industrial and domestic sewage from the three sewage main pipes in Hangzhou, the Municipal Economic and Technological Development Zone, and Yuhang District. The sewage it treats comes from a wide range of sources, including Production wastewater from industrial sites and domestic sewage from citizens are representative. Sewage contains certain suspended matter and other pollutants and has certain acidity, alkalinity, corrosiveness, and conductivity.
The treatment plant can process 600,000 tons of sewage per day at full capacity, which is equivalent to about 40% of Hangzhou's current sewage discharge. It can be seen that the sewage treatment capacity of the treatment plant is relatively strong, so this also determines that the diameter of the pipeline transporting sewage in the sewage treatment process is also relatively large. After actual investigation, it is known that the diameters of the pipelines in front of the lift pump are 1800-2200mm.According to the actual characteristics of the treatment plant, when selecting the flowmeter, the main references are the pipe diameter of the treatment plant, the length of the straight pipe before and after the lift pump, and the fluid medium. The medium in the pipeline of the sewage treatment plant is conductive and contains suspended particles, so in principle, both electromagnetic flowmeters and ultrasonic (Doppler) flowmeters can be applied.
Although the electromagnetic flowmeter is higher than the ultrasonic flowmeter in terms of measurement accuracy, its price tends to be faster than the ultrasonic flowmeter with the increase of the pipe diameter, and its economy is poor when measuring the flow of a large-diameter pipe. In view of the large diameter of the treatment plant, electromagnetic flowmeters, and ultrasonic (Doppler) flowmeters can be used as alternatives. If there are 5-8m (minimum 3-5m) straight pipe sections before and after the installation position of the flow meter, you can consider using an electromagnetic flowmeter; if you cannot guarantee enough straight pipe sections or the pipeline construction is inconvenient, then consider using ultrasonic (Doppler) ) flow meter.
Selection of pipeline flowmeter for a dairy company
The sewage produced by the dairy company is mainly dairy wastewater, which has the characteristics of high organic matter concentration, high oil content, and high nutrient concentration. The content of organic matter in its discharged sewage is relatively high, but according to the flow chart of the sewage treatment plant provided by it, it can be known that its sewage also contains certain domestic sewage and sewage, and has certain conductivity.
According to the information provided by it, it can be known that the pipe diameter behind the pump of its submersible pump is 80mm, which is relatively small. According to the flow data provided by the dairy company: the flow data in the data report of the dairy company from April to June 2011, it can be seen that the average flow during this period is small, but because of intermittent drainage, the data cannot reflect the average flow of the pipeline. Instantaneous flow and maximum instantaneous flow. According to general experience, the flow velocity of the outlet pipe of the submersible pump is generally not greater than 3m/s; or the outlet flow can be estimated according to the brand name data of the submersible pump, and the flow meter range can be selected accordingly.
According to the actual working conditions of the dairy company, the cost of choosing an ultrasonic flowmeter is too high; it is more appropriate to choose an electromagnetic flowmeter. Due to its small diameter, the cost of using an electromagnetic flowmeter is not very high, and using an electromagnetic flowmeter can achieve more accurate measurement results.