Liquid level gauge accounts for a large proportion in the application of sewage treatment, and it is almost used in all links of sewage treatment. In sewage treatment, the liquid level needs to be measured, including wastewater, sludge, chemical solution, etc. In the sewage treatment system using automatic control, in addition to measuring the liquid level gauge, many liquid level gauges also involve the start and stop of interlocking pumps and the opening and closing of control valves in automatic control. Therefore, the proper selection of liquid level gauge plays a very important role in reasonably meeting the requirements of production process.
1、 Magnetic flap level gauge
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Principle: the liquid level gauge works according to the principle of buoyancy and magnetic coupling. When the liquid level in the measured container rises and falls, the float in the main pipe of the liquid level gauge also rises and falls, and the magnetic steel in the float is transmitted to the field indicator through magnetic coupling to drive the red and white column to turn 180 °. When the liquid level rises, the column turns from white to red, when the liquid level falls, the column turns from red to white, and the red and white of the indicator The white boundary is the actual height of the medium liquid level in the container, so as to realize the indication of the liquid level.
Characteristic:
(1) Simple structure, clear display and intuitive reading, especially suitable for on-site display.
(2) The equipment has few openings. Generally, the magnetic flap level gauge with remote output is selected to enable on-site and remote monitoring.
(3) According to the condition of the medium, such as the medium that is easy to be polluted and blocked, the main pipe needs to be cleaned regularly to remove the sediment in the pipe, so as to ensure the accuracy of measurement.
In sewage treatment process, magnetic flap level gauge is often used to measure the liquid level of chemical dissolving tank, acid tank, alkali tank and so on.
2、 Ultrasonic level gauge
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Ultrasonic liquid level gauge is based on the principle of ultrasonic reflection at the liquid level to detect the liquid level deviation, that is, the principle of echo measurement distance is applied. When the ultrasonic probe emits a short ultrasonic pulse to the liquid level, after time t, the probe receives the echo pulse reflected from the liquid level. Therefore, the distance from the probe to the liquid level can be calculated according to the following formula: if the distance from the ultrasonic probe to the bottom of the container is h, the actual liquid level will be determined. Where, V is the propagation speed of ultrasonic wave in the measured medium (i.e. sound speed M / s). It can be seen that as long as the sound speed V is known, the height h of liquid level can be obtained by accurately measuring the time t.
2.1 characteristics of ultrasonic level gauge
(1) The ultrasonic level gauge can achieve non-contact measurement, and the operation is stable and reliable: the ultrasonic level gauge is installed above the silo and liquid tank without direct contact with materials, which overcomes the disadvantages caused by the direct contact of other types of liquid (material) level gauges with materials.
(2) It can measure a wide range of liquid, block and powder levels.
(3) It can measure continuously at fixed points, and can easily provide telemetry and remote control measurement signals.
(4) The installation is simple and convenient, and no safety protection is required.
2.2 disadvantages of ultrasonic level gauge
(1) There will be a blind area in the measurement of the ultrasonic liquid level gauge, which needs to be avoided during installation. When the liquid level enters the blind area, the ultrasonic transmitter cannot measure the liquid level. Therefore, when determining the measuring range of the ultrasonic liquid level gauge, the margin of the blind area must be reserved. During installation, the probe of the transmitter must be about higher than the blind area of the highest liquid level. Only in this way can we ensure the accurate monitoring of liquid level and the safety of ultrasonic liquid level gauge.
(2) when the ultrasonic level gauge is in foam, the sound wave can not penetrate the foam, so the sound wave will be reflected on the foam, so the measurement will be greatly deviated from the actual liquid level. Defoamer can be added to foam tank tank to reduce foam generation and ensure accurate measurement.
(3) The ultrasonic liquid level gauge will be affected by the agitator in the container with agitator, resulting in false reflection echo and inaccurate measurement. By reducing the rotating speed of the agitator and leaving the center of the agitator when installing the liquid level gauge, the influence of the agitator on the measurement of the ultrasonic liquid level gauge can be reduced.
(4) The temperature of the measuring medium also affects the ultrasonic level gauge, especially in a closed container. When the temperature of the medium is different from the surrounding temperature, water droplets will condense around the probe, which will affect the accuracy of the measurement. When installing the ultrasonic liquid level gauge, connect the compressed air pipe to blow against the probe to reduce the impact of condensed water droplets on the measurement due to the temperature difference between the medium, the container and the probe.
3、 Static pressure level gauge
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The static pressure liquid level measurement method is based on the principle that the static pressure of the liquid column is directly proportional to the height of the liquid column. The liquid level measurement is realized by measuring the static pressure generated by the height of the liquid column. The differential pressure liquid level gauge works based on the principle that when the liquid level in the container changes, the static pressure generated by the liquid column also changes accordingly.
The static pressure liquid level gauge measures the liquid level by measuring the static pressure generated by the liquid level height. According to p= ρ GH, and the density of the liquid ρ， The gravitational acceleration g is known. As long as the pressure P is measured, the liquid level H of the liquid can be calculated.
A pressure instrument used for liquid level measurement is an input liquid level gauge, that is, the liquid level measuring instrument is input into the medium of the liquid level to be measured. With the change of liquid level, the pressure detection elements such as diffused silicon in the pressure transmitter convert the static pressure into a resistance signal for liquid level detection. The input liquid level gauge can be directly input into the measured medium.
Characteristics of input pressure level gauge.
(1) Simple structure, solid structure, no movable parts.
(2) The installation and use are quite convenient and the service life is long.
(3) The measurement range is relatively wide, which can measure from water and oil to phase substances with large viscosity.
(4) It is not affected by the foaming, deposition and electrical holding of the measured medium, has no material fatigue wear, and is not sensitive to vibration and impact.
(5) Low price and high reliability.
(6) When installing, pay attention to the place where the fluid is relatively stable and the fluctuation is small. If the impact of water flow cannot be avoided and the friction vibration, it is necessary to install an isolation pipe to reduce the impact of water flow and ensure the accuracy and stability of measurement.
(7) Install the input type liquid level gauge, Zui 100mm to 200mm away from the tank bottom or tank bottom, so as to reduce the influence on the accuracy of measurement due to the sediment of sludge and medium at the tank bottom or tank bottom.
(8) In the environment of poor water quality, especially when there are many suspended solids and impurities in the medium, it is easy to block the pressure tapping hole and affect the measurement accuracy. It is necessary to clean the liquid level gauge and dredge the pressure tapping hole regularly to ensure accurate and stable measurement.
The pressure type liquid level gauge is suitable for the process flow with good water quality, such as upper clean water collecting tank, filter tank, clean water tank, external drainage tank and SBR tank. By installing isolation pipe to avoid sludge impurities at the bottom of the tank, it can also be used in the concentration tank and regulating tank with stirring.
4、 Radar level gauge
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The antenna of the radar sensor transmits the electromagnetic wave signal in the form of beam. The transmitted wave is reflected on the surface of the measured material, and the reflected echo signal is still received by the antenna. Each point in the transmitted and reflected beam is collected by ultrasonic sampling. After the signal is processed by the intelligent processor, the distance between the medium and the probe is obtained and sent to the terminal display for display, alarm, operation, etc. The distance d from the material surface is directly proportional to the time travel t of the pulse:
D=C × T/2
Where C is the speed of light. Since the distance E of the empty tank is known, the material level L is: l = E-D
Features of radar level gauge:
(1) The radar level gauge adopts integrated design, without movable parts, no mechanical wear and long service life.
(2) Due to the characteristics of electromagnetic wave, it is not affected by the environment. Therefore, its measurement is widely used. The probe of the radar level gauge has no contact with the medium surface and belongs to non-contact measurement. It can accurately and quickly measure different media. The probe is hardly affected by temperature, pressure, gas, etc. It can be used under bad working conditions, large variation, and water level, steam, foam and other ultrasonic level gauges.
(3) Radar liquid level gauge is also suitable for occasions with agitator and changeable liquid level.
(4) The price of radar level gauge is relatively expensive, but it can be used in almost all process level control processes of sewage treatment.
summary
To sum up, because the process flow of sewage treatment involves many occasions where the liquid level needs to be measured, as well as the characteristics of the sewage treatment process itself, in the selection process, it is necessary to select the appropriate liquid level gauge according to the characteristics of each process flow and medium, which will have a great impact on the accuracy, reliability, stability, economy and service life of the process measurement. It is suggested to select the liquid level gauge with high precision, less maintenance and long service life as far as the funds allow. Make the liquid level measurement and control of the whole sewage treatment run under reliable, stable and safe conditions.
Post time: 08-03-22