Environmental pollution brought about by industrial production and the pressure on the environment due to the rapid growth of the urban population has become essential factors limiting the sustainable development of a region and a country. With the loss of blue sky and blue water, people’s happiness brought about by industrial production and technological development becomes negative. China has accumulated considerable wealth after more than 30 years of crude economic growth. Still, if environmental protection is not taken into account, it will eventually drag down the final economic development. Currently, the discharge of industrial wastewater and municipal sewage will also increase significantly. If this wastewater is discharged without treatment, it will cause serious pollution to the environment and drinking water. The need for a highly automated sewage treatment plant has therefore become a necessity.
In sewage treatment, how to correctly, reasonably, and economically select the flow meter will directly affect the level of self-control of the water treatment project, the project cost. Flow detection point is different; the measured media composition is different, the type of flowmeter used will also be other. The electromagnetic flowmeter and the ultrasonic flowmeter is two in the water measurement for the universal and applicable flowmeter, have their measurement advantages and characteristics, can be used for different measurement site, this paper is from the technical and economic aspects, the two flowmeter performance, price, etc. to explore, to help enterprises and technical design of material acquisition personnel correct product selection and use of maintenance help.
Process flow for in Sewage Treatment Industry
The process flow is shown in the diagram below.
This requires a different medium flow rate to be measured. At the same time, the market and constantly launches a new flowmeter for you to choose. Then, how to reasonably choose a suitable flowmeter for different media is very important.
As can be seen from the above diagram, the type of flowmeter selected for each flow detection point is not the same. The main reason is the different compositions of the measured medium. For example, in wastewater treatment plants, the general inlet flow rate is measured using venturi plus ultrasonic sensors, as ultrasonic sensors have easy installation and are not easily worn.
However, suppose the surface of the measured medium floats with a large amount of foam. In that case, the ultrasonic pulse from the white ultrasonic sensor may be absorbed so that the measurement is disturbed, seriously affecting the measurement accuracy. At this time, the hydrostatic sensor can be used instead.
Working principle of Ultrasonic Flowmeters
Different types of flow meters are used in different situations, mainly determined by the working principle of the flow meter. The following is a description of the flow meters commonly used in wastewater treatment plants.
1. Ultrasonic Flowmeters
Ultrasonic flowmeter transducers are installed in pairs on both sides of the pipe; two transducers are alternately used as the generator or receiver of the ultrasonic signal. The flow of liquid flow will carry sound waves, sound waves will be affected by the liquid flow rate, assuming that the ultrasonic signal from the transducer, flow propagation to the transducer required time from the transducer B counter-current propagation to the transducer A (called reverse) TD of time needed then:
In this way, the ultrasonic flowmeter can directly find the average line flow rate of the fluid on the sound channel by measuring ∆T and then use a proprietary integration method to calculate the average surface flow rate over the cross-sectional area of the pipe appropriate to the flow rate.
Factors affecting the accuracy of the Ultrasonic Flowmeters
- Mounting position of the transducer.
- Reynolds number of the liquid.
- The straight pipe section above and below the installation section of the transducer.
- The volume concentration of suspended particles and bubbles in the liquid.
- The ellipticity of the pipe in which the transducer is installed.
Characteristics of Ultrasonic Flowmeters and Electromagnetic Flowmeters
Although the ultrasonic flowmeter for the measured liquid working conditions is more demanding, the installation requirements themselves are more stringent, but compared with other flow meters have specific characteristics:
- It can measure general acoustically conductive fluids and liquid: high temperature, high pressure, highly corrosive, any conductive liquids.
- It can be installed in existing pipelines, and even transducers can be installed and serviced without stopping the flow of fluid in the channel.
- The transducer is mounted on the pipe’s outer wall, on the pipe’s division, and the pipe’s inner surface, basically causing no pressure loss to the flow field in dry isolation.
- Particularly suitable for the measurement of large-diameter flows, the cost of which is largely independent of the size of the bore.
- According to information, the current ultrasonic flowmeter accuracy: double channel has reached 1. 0 levels, four channels up to 0. 5 levels, fully meet the general measurement requirements.
The Electromagnetic Flowmeters
The basis of electromagnetic flowmeter measurement is the Faraday principle of electromagnetic induction. Excitation system by the coil and core group car group h} when energized to produce a working magnetic field, the measurement tube is placed in the magnetic field, in the inner wall of the tube is equipped with a pair of electrodes. When the conductive medium flows through the conduit, equivalent to the conductor in the magnetic field cut harm” magnetic lines of force, the resulting induced electric potential by the electrode detection, the flow rate can be derived from the following formula:
In the equation:
D———Diameter of the tube, i.e., the perpendicular cutting of the magnetic line of force-length of the conductor;
This can be seen: when the electromagnetic flowmeter is selected, D is a constant, as long as B remains unchanged, then Q and E of the question has a particular linear relationship, and thus the instrument has a uniform scale. At present, the electromagnetic flowmeter measurement accuracy is up to OS level or more.
The characteristics & features of the electromagnetic flowmeter are as below :
① measuring cross-section without shrinkage, so no pressure loss;
② measurement is not affected by the viscosity of the measured medium;
③ independent of the flow rate distribution;
① suitable for a wide range of pipe diameter ((6 a 3000}>;
⑤ has good linearity and stability;
⑥ shorter of the front and rear straight pipe section (before 5p after 3 D>o)
The Useage of electromagnetic flowmeters:
① small conductivity of the measured liquid > 1l} s / chatter
② large-diameter installation difficulties; ③ if there are deposits on the electrode in the measuring tube will affect the measurement accuracy;
① expensive, substantial diameter.
Electromagnetic flowmeter vertical bag and fluid should be white bottom and up, so that can ensure a full tube and ensure that the suspended solids sink. However, suppose the flowmeter is to be installed horizontally. In that case, the measuring electrode should be on the horizontal axis to avoid interference due to the generation of air bubbles, affecting the measurement error.
As the designer of the instrumentation system should discuss more with the process designer in the design and cooperate, sometimes by changing a particular measurement point, there will be unexpected gains, which may bring significant economic benefits. The selection of instruments may seem simple, but there is a lot of learning to do. For example, electromagnetic flow Hugh generally chooses the same caliber as the pipeline flowmeter, but considering the more comprehensive measurement range of the flowmeter, the price is higher. You can select a smaller caliber than the pipeline flowmeter, which can also increase the flow rate to prevent sediment generation. But since then, and will bring fluid pressure loss and pipe reduction costs to grow, so how much to choose the appropriate diameter of the flowmeter? This requires us to design personnel in practice to continue to explore and summarize the experience. Only then can the instrument have an economical and reasonable selection.