Liquid level meters account for a large proportion of wastewater treatment applications and are used in almost every part of wastewater treatment. In sewage treatment, it is necessary to measure the liquid level with waste water, sludge, chemical solution, and the like. In the sewage treatment system using automatic control, in addition to measuring the liquid level gauge, the liquid level meter also involves the start and stop of the interlocking pump in the automatic control and the opening and closing of the control valve. Therefore, the proper selection of the level gauge plays a very important role in reasonably meeting the requirements of the production process.
First, the magnetic flap level gauge
Principle: The level gauge works according to the principle of buoyancy and the principle of 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, and the red and white flip columns are turned over by 180°. When the position rises, the column turns from white to red. When the liquid level drops, the column turns from red to white. The red and white boundaries of the indicator are the actual height of the medium level in the container, thus achieving the liquid level instructions.
Features:
(1) The structure is simple, the display is clear, the reading is intuitive, and it is especially suitable for on-site display.
(2) The equipment has few openings, and the magnetic flap level gauge with remote output is generally used to enable monitoring on site and remotely.
(3) According to the condition of the medium, such as the dirt and easy to block the medium, it is necessary to clean the main pipe regularly to remove the deposits in the pipe to ensure the accuracy of the measurement.
In the sewage treatment process, magnetic flap level gauges are commonly used for the measurement of liquid levels in chemical solvent tanks, acid tanks, and alkali tanks.
Second, the ultrasonic level gauge
The ultrasonic level gauge works by using the ultrasonic principle to reflect the liquid level deviation at the liquid level, that is, applying the principle of echo measurement distance. When the ultrasonic probe emits a short ultrasonic pulse to the liquid surface, after the time t, the probe receives the echo pulse reflected from the liquid surface, so the distance from the probe to the liquid surface can be obtained by the following formula: setting the ultrasonic probe to the bottom of the container The distance is h, then the actual level. In the formula, v is the speed at which the ultrasonic wave propagates in the medium to be measured (that is, the sound velocity m/s), and it can be seen that as long as the sound velocity v is known, the height H of the liquid level can be obtained by accurately measuring the time t.
2.1 Characteristics of ultrasonic level gauge
(1) The ultrasonic level gauge can be used for non-contact measurement, and the operation is stable and reliable: the ultrasonic level gauge is installed above the silo and the liquid tank, and does not directly contact the material, overcoming the direct contact with other types of liquid (material) gauges. And the drawbacks that come with it.
(2) The range that can be measured is large, liquid, block, and powder level can be measured.
(3) Continuous measurement can be fixed at a fixed point, and the measurement signals of telemetry and remote control can be conveniently provided.
(4) Installation is simple and convenient, and does not require safety protection.
2.2 shortcomings of ultrasonic level gauge
(1) There is a blind zone in the ultrasonic level gauge measurement. When installing, it is necessary to avoid the blind zone. When the liquid level enters the blind zone, the ultrasonic transmitter cannot measure the liquid level, so when determining the range of the ultrasonic level gauge, it must be Leave the margin in the blind zone. When installing, the transmitter probe must be higher than the blind spot of the highest liquid level. This will ensure accurate monitoring of the liquid level and ensure the safety of the ultrasonic level gauge.
(2) Ultrasonic level gauge In the case of foam, since the sound wave cannot penetrate the foam, the sound wave will be reflected back on the foam, so that the measurement has a large deviation from the actual liquid level. An antifoaming agent can be added to the foamed tank container to reduce the generation of foam and ensure accurate measurement.
(3) The ultrasonic level gauge will be affected by the agitator in the container with the agitator, causing the reflected false reflection echo, resulting in inaccurate measurement. By reducing the speed of the agitator, the center of the agitator is removed when the level gauge is installed, which reduces the effect of the agitator on the measurement of the ultrasonic level gauge.
(4) Measuring the temperature of the medium also affects the ultrasonic level gauge. Especially in a closed container, when there is a temperature difference between the temperature of the medium and the surrounding temperature, the water droplets will condense around the probe, which will affect the accuracy of the measurement. By installing a ultrasonic level gauge, a compressed air tube is blown against the probe to reduce the influence of water droplets condensed by the temperature difference between the medium and the container and the probe.
Third, static pressure level gauge
The static pressure level measurement method is realized according to the principle that the liquid column static pressure is proportional to the liquid column height. The liquid level measurement is realized by measuring the static pressure generated by the liquid column height, and the differential pressure level gauge is utilized. When the liquid level in the vessel changes, the static pressure generated by the liquid column also works accordingly.
The static pressure level gauge measures the liquid level by measuring the static pressure generated by the liquid level of the liquid. According to P = ρgh, the density ρ of the liquid and the gravitational acceleration g are known, and as long as the pressure P is measured, the liquid level h of the liquid can be determined.
A pressure gauge for liquid level measurement is an input level gauge, that is, a medium that puts a liquid level measuring instrument into a liquid level to be tested, and a pressure of diffusion silicon in the pressure transmitter as the liquid level changes. The detecting component converts the static pressure into a resistance signal for liquid level detection, and the investment level gauge can directly invest in the measured medium.
The characteristics of the input pressure level gauge.
(1) The structure is simple, adopts solid structure, and has no movable parts.
(2) It is quite convenient to install and use, and has a long service life.
(3) The measuring range is relatively wide, and it can measure the phase from water and oil to the viscosity.
(4) It is not affected by the foaming, deposition and electrical holding properties of the measured medium, no material fatigue wear, and is not sensitive to vibration and impact.
(5) The price is cheap and the reliability is relatively high.
(6) When installing, pay attention to the choice of fluids that are relatively stable and have small fluctuations. If water flow impact is not avoided, it is necessary to install isolation pipes to reduce the impact of water flow and ensure the accuracy and stability of the measurement.
(7) Installation of the input liquid level timing, the Zui is better from the bottom of the pool or the bottom of the tank 100mm to 200mm, in order to reduce the sediment due to sludge and medium at the bottom of the pool or the bottom of the tank, affecting the accuracy of the measurement.
(8) In the environment of excessive water quality, especially when there is a lot of suspended matter in the medium, when it is impurity, it is easy to block the pressure-receiving hole and affect the measurement accuracy. The level gauge should be cleaned regularly to clear the pressure tap to ensure accurate and stable measurement.
Pressure level gauges are suitable for use in processes with good water quality, such as supernatant pools, filters, clear water tanks, effluent tanks and SBR tanks. By adding an isolation pipe to avoid sludge impurities at the bottom of the tank, it can also be used for a concentrating tank conditioning tank with agitation.
Fourth, radar level gauge
The antenna of the radar sensor emits an electromagnetic wave signal in the form of a beam, and the emitted 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 beams is acquired by ultrasonic sampling. After the signal is processed by the intelligent processor, the distance between the medium and the probe is obtained, and the terminal display is sent for display, alarm, operation, and the like. The distance D from the surface of the material is proportional to the time travel T of the pulse:
D=C×T/2
Where C is the speed of light because the distance E of the empty can is known, then the level L is: L=E-D
Radar level gauge features:
(1) The radar level gauge adopts integrated design, no moving parts, no mechanical wear and long service life.
(2) Due to the characteristics of electromagnetic waves, it is not affected by the environment. Therefore, the application of the measurement is relatively wide. The probe of the radar level gauge has no contact with the surface of the media and is a non-contact measurement that can accurately and quickly measure different media. The probe is almost immune to temperature, pressure, gas, and the like. It can be used in situations where the working conditions are bad, the change is large, and the ultrasonic level gauges such as water vapor, steam, and foam are not competent.
(3) The radar level gauge is also suitable for use in situations where there is a stirrer and the liquid level changes and changes.
(4) The radar level gauge is relatively expensive, but it can be applied to almost every process level control process of wastewater treatment.
Summary
In summary, because the process of sewage treatment involves many occasions that need to measure the liquid level, and also because of the characteristics of the sewage treatment process, in the selection process, it is necessary to select the appropriate liquid for each process flow and the characteristics of the medium. The position meter has a great influence on the accuracy and reliability of the process measurement and the economical service life. It is recommended to choose a level gauge with high precision, low maintenance and long service life when funding is allowed. The liquid level measurement and control of the entire sewage treatment is operated reliably, stably and safely.
Read more:
How to adjust the static pressure error of differential pressure transmitter
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