Abstract: According to the actual characteristics of sewage treatment occasions, after comprehensively considering indicators such as sewage flow measurement safety, accuracy and economy, two pipeline flow meters were selected for the sewage treatment unit, and their performances were comprehensively compared.
1. Introduction
Flow measurement is an important part of metrology science, and it also promotes and supports the development of the national economy. However, flowmeter is one of the few instruments that is more difficult to use than to manufacture. Therefore, when measuring flow in a real industrial and mining situation, we should start from the actual situation and comprehensively consider the safety, accuracy and economy of the measurement, so as to finally achieve better flow measurement.
2. Select pipeline flowmeter according to the actual working conditions of sewage treatment units
Sewage treatment units receive sewage (waste) water with a high total amount or concentration of pollutants discharged from pollution sources, which does not meet the discharge standards or does not meet the environmental capacity requirements, and perform artificial intensive treatment on it. They need flow information to determine the operating status of the system and upload sewage information. In order to maximize the comprehensive benefits, the selection of sewage flow meters for sewage treatment plants should be examined one by one from the indicators of safety, accuracy and economy.
2.1 Security
Sewage treatment plants that need to install flow meters generally do not have complex and dense mechanical and electrical devices, so there will be no accidents caused by mechanical strength or electrical circuit failures. Sewage treatment units produce a large amount of gas during operation, such as methane, CO, hydrogen sulfide, etc. Some of these gases will pose a safety hazard when reaching a certain concentration, so when choosing a flow meter, the explosion-proof performance of the flow meter should be considered.
2.2 Accuracy
In order to achieve accurate sewage measurement, the working objects and characteristics of the flow meters installed in the sewage treatment units should be comprehensively analyzed to find the basis for selecting the flow meters and the precautions when using them.
2.2.1 Measured medium
The treated medium of the sewage treatment unit is sewage, but the sources of sewage are not the same, and the various index parameters of sewage from different sources are also different. Therefore, the source of sewage should be considered first.
(1) Domestic sewage is water that has been used by people in their daily lives and has been contaminated by domestic waste. Its water quality and quantity vary with the seasons. Generally, water consumption is relatively high in summer and the concentration is low; while the corresponding amount is low in winter and the concentration is high. Domestic sewage generally does not contain toxic substances. 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 in the production process and polluted by raw materials, semi-finished products or finished products, and also includes thermal pollution (referring to water generated in the production process with a water temperature exceeding 60°C); production wastewater is formed in the production process but is not directly involved in the production process, is not polluted by raw materials, semi-finished products or finished products, or has only a slight increase in temperature. Production wastewater needs to be purified; production wastewater does not need to be purified or only needs simple treatment, such as cooling. Production wastewater is related to the production industry. Different production industries or different production processes in the same industry have very different pollutants. The main pollutants in production wastewater are nothing more than: organic oxygen-demanding substances, chemical poisons, inorganic solid suspended matter, heavy metals, acids, alkalis, plant nutrients, pathogens, etc.
(3) Polluted rainwater mainly refers to initial rainwater. Since initial rainwater washes away various pollutants on the surface, the pollution level is very high and therefore it is advisable to purify it.
It can be seen that for sewage treatment units, the measured medium is a mixed medium, containing certain suspended matter and other dirt. Because it contains a large amount of substances, it often has certain acidity, alkalinity, corrosiveness, and conductivity. Since the suspended matter in sewage is corrosive, for contact flow meters such as turbine flow meters, volumetric flow meters, differential pressure flow meters, rotor (float) flow meters, etc., if long-term measurement is carried out, the dirt in the sewage may block the flow meter, corrode the working components of the flow meter, and make the flow meter unable to work normally. Therefore, when measuring sewage flow, it is best to use a non-contact flow meter to avoid being corroded by sewage and unable to work normally for a long time.
2.2.2 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 to be normal temperature, so there is no need to consider too much when selecting a flow meter.
2.2.3 Pressure of the measured medium
Sewage treatment units usually operate at normal temperature and pressure, so the temperature of the measured medium can be considered to be normal pressure, so there is no need to consider too much when selecting a flow meter.
2.2.4 Flow rate of the measured medium
Different sewage treatment units have different treatment capacities and different pipe diameters, so actual analysis needs to be performed based on the specific sewage treatment units. When the pipe diameter is small, most flow meters are competent. Some flow meters, such as differential pressure flow meters, require a straight pipe section several times the pipe diameter before and after the installation point during installation, but this is difficult to achieve when the pipe diameter is large. When measuring the flow rate of large-diameter pipes, electromagnetic flow meters, ultrasonic flow meters and insertion flow meters have greater advantages. However, combined with the above analysis of the characteristics of sewage itself containing suspended matter and having a certain degree of corrosiveness, insertion flow meters are generally less used in working conditions such as sewage treatment units and are not considered.
2.3 Economical
In terms of measurement accuracy, electromagnetic flowmeters are higher than ultrasonic flowmeters, but the price of both will increase with the increase of pipe diameter. The measurement accuracy of ultrasonic flowmeters will become more precise as the pipe diameter increases. In actual application, the decision should be made after fully considering the actual required accuracy, pipe diameter and other factors.
According to the above analysis, the electromagnetic flowmeter has a higher measurement accuracy, while the ultrasonic flowmeter's measurement accuracy will become more accurate as the pipe diameter increases. Considering the generality and being better oriented to different sewage treatment units, this article provides electromagnetic flowmeters and ultrasonic flowmeters as alternatives for flowmeter selection in 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, the required measurement accuracy, etc., to achieve better results.
3. Electromagnetic flowmeter and ultrasonic flowmeter
3.1 Electromagnetic flowmeter
The measurement of electromagnetic flowmeter is based on Farad's law of electromagnetic induction. When the conductive medium cuts the magnetic lines of force in the electromagnetic field, the conductor will generate an electromotive force Ed, which is proportional to the flow rate (flow velocity) of the measured medium. Because the electromotive force is only related to the flow velocity of the medium, the electromagnetic flowmeter is generally not affected by other external factors such as temperature, pressure, viscosity, density, etc. In addition, the initial signal detected by the flow element is a voltage that changes accurately linearly with the average flow velocity of the fluid. It has nothing to do with other properties of the fluid and has great advantages.
According to the characteristics of sewage, such as large flow rate change, impurities, low corrosiveness, and certain electrical conductivity, electromagnetic flowmeter is a good choice for measuring the flow of sewage. It has a compact structure, small size, and is easy to install, operate, and maintain. In recent years, the measurement system of electromagnetic flowmeter has been gradually intelligentized, and the overall sealing of the instrument has been strengthened to ensure normal operation in a harsh environment. In view of the certain corrosiveness of sewage, chloroprene rubber lining and molybdenum-containing stainless steel (OCrI8Ni12Mo2Ti) electrode can be selected when installing the electromagnetic flowmeter, so as to meet the requirements of sewage flow measurement.
3.2 Ultrasonic flow meter
At present, ultrasonic flowmeters are roughly divided into two types: one is Doppler ultrasonic flowmeter, and the other is time-difference ultrasonic flowmeter. Doppler flowmeter uses the phase difference method to measure flow velocity and thus flow rate, that is, when a sound wave of a known frequency moves in water, the liquid itself has a certain speed of movement, which causes the frequency or phase of the ultrasonic wave between the two receivers to change relative to each other. By measuring this relative change, the liquid velocity can be obtained, thereby measuring the flow rate; the time-difference flowmeter uses the time difference method to measure flow velocity, that is, a sound wave of a certain speed changes its propagation speed between the two receivers due to the fluid, and the fluid flow rate can be obtained by measuring this relative change. In practical applications, we mostly use the time-difference flowmeter.
It is suitable for uniform liquids that can transmit ultrasonic waves and gases with high flow rates. The medium temperature range is -10 to 100°C, and the medium pressure is not limited. It has a wide range ratio, is not affected by the viscosity and density of the fluid, and can measure highly corrosive media. The real advantage of ultrasonic flowmeters is that they are used to measure the flow of large-diameter pipelines, such as sewage treatment plants, which have large water consumption and large pipe diameters.
4. Case analysis of flow meter selection
4.1 Selection of pipeline flowmeter for a sewage treatment plant
This sewage treatment plant is the largest sewage treatment plant in Hangzhou. Its sewage mainly comes from the three sewage mains in Hangzhou, the industrial and domestic sewage in the Economic and Technological Development Zone and Yuhang District. The sewage it treats comes from a wide range of sources, including both industrial wastewater and domestic sewage generated by citizens, which is representative. The 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 every day when it is fully loaded, which is equivalent to about 40% of the current sewage discharge in Hangzhou. 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 for transporting sewage in the sewage treatment process is also relatively large. After actual inspection, it is learned that the diameter of the pipeline before the lifting pump is 1800~2200mm.
According to the actual characteristics of the treatment plant, the main reference for selecting the flow meter is the pipe diameter of the treatment plant, the straight pipe length before and after the lift pump, and the fluid medium. The medium in the sewage treatment plant pipe is conductive and contains suspended particles, so in principle, both electromagnetic flowmeters and ultrasonic (Doppler) flowmeters can be used.
Although the electromagnetic flowmeter has higher measurement accuracy than the ultrasonic flowmeter, its price increases faster than the ultrasonic flowmeter as the pipe diameter increases. When measuring the flow rate of large-diameter pipes, its economic efficiency is poor. In view of the large pipe diameter of the treatment plant, both the electromagnetic flowmeter and the ultrasonic (Doppler) flowmeter can be used as alternatives. If there is a 5-8m (at least 3-5m) straight pipe section before and after the flowmeter installation location, the electromagnetic flowmeter can be considered; if it is impossible to ensure sufficient straight pipe sections or the pipeline construction is inconvenient, the ultrasonic (Doppler) flowmeter can be considered.
4.2 Selection of pipeline flowmeter for a dairy company
The wastewater produced by the dairy company is mainly dairy wastewater, which has the characteristics of high organic matter concentration, high oil content, high nutrient concentration, easy to corrupt, and the scum will turn black and produce a bad smell after 24 hours. The organic matter content in its discharged wastewater is relatively high, but according to the wastewater treatment plant flow chart provided by it, it can be seen that its wastewater also contains a certain amount of domestic sewage and dirt, and has a certain conductivity.
According to the information provided, the diameter of the pipe behind the submersible pump is 80mm, which is relatively small. According to the flow data in the data report of the dairy company from April to June 2016, the average flow rate during this period is relatively small, but due to intermittent drainage, the data cannot reflect the average instantaneous flow rate and maximum instantaneous flow rate of the pipeline. According to general experience, the flow velocity of the outlet pipe of the submersible pump is generally not more than 3m/s; or the outlet flow rate can be estimated based on the 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 the small diameter of the pipe, the cost of using an electromagnetic flowmeter is not very high, and the electromagnetic flowmeter can achieve more accurate measurement results.
5. Conclusion
According to the flow selection principle, the actual working conditions of the sewage treatment unit are comprehensively considered, and electromagnetic flowmeter and ultrasonic flowmeter are proposed as alternatives for the selection of pipeline flowmeters for sewage treatment units. Electromagnetic flowmeters have high accuracy and can be used as settlement metering instruments; ultrasonic flowmeters have advantages in measuring large pipe flow, and their measurement accuracy increases with the increase of pipe diameter. In order to achieve better real-time monitoring of the flow of sewage treatment plants and issue more authoritative measurement results, comprehensive consideration should be given to the actual conditions of different sewage treatment units and specific problems should be analyzed in order to select the best sewage treatment unit sewage pipeline flow measurement device for specific occasions.