Liquid level meters account for a large proportion of sewage treatment applications, and are almost used in all aspects of sewage treatment. In sewage treatment, the liquid level that needs to be measured includes wastewater, sludge, chemical solutions, etc. In the sewage treatment system using automatic control, the liquid level gauge is not only used to measure the liquid level gauge, but many also involve the start and stop of the chain pump and the opening and closing of the control valve in the automatic control. Therefore, the proper selection of the liquid level gauge plays a very important role in reasonably meeting the requirements of the production process.
1. Magnetic flap level gauge
Principle: The liquid level gauge works according to the principle of buoyancy and magnetic coupling. When the liquid level in the container under test 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, which drives the red and white turning columns to turn 180°. When the level rises, the turning column turns from white to red, and when the liquid level drops, the turning column turns from red to white, and the red and white boundaries of the indicator are the actual height of the medium liquid level in the container, so as to realize the adjustment of the liquid level. instruct.
(1) Simple structure, clear display and intuitive reading, especially suitable for on-site display.
(2) There are few openings in the equipment, and a magnetic flap level gauge with remote output is generally used, so that both on-site and remote monitoring can be performed.
(3) According to the condition of the medium, such as the medium that is easy to be polluted and blocked, it is necessary to clean the main pipe regularly to remove the sediment in the pipe to ensure the accuracy of the measurement.
In the sewage treatment process, the magnetic flap level gauge is often used to measure the liquid level of chemical dissolving tanks, acid tanks, alkali tanks, etc.
2. Ultrasonic level gauge
The ultrasonic level gauge uses the principle of ultrasonic reflection at the liquid surface to detect the liquid level separation, that is, the principle of echo measurement distance is applied. When the ultrasonic probe emits short ultrasonic pulses to the liquid surface, after time t, the probe receives echo pulses reflected from the liquid surface, so the distance from the probe to the liquid surface can be calculated as follows: Set the ultrasonic probe to the bottom of the container The distance is h, the actual liquid level. In the formula, v is the propagation speed of the ultrasonic wave in the measured medium (that is, the speed of sound m/s). It can be seen that as long as the speed of sound v is known, the height H of the liquid level can be obtained by accurately measuring the time t.
2.1 Features 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, and does not directly contact the material, which overcomes the direct contact of other types of liquid (material) level gauges with the material. and the resulting disadvantages.
(2) The measurement range is large, liquid, block, powder level can be measured.
(3) It can be continuously measured at fixed points, and can easily provide measurement signals for telemetry and remote control.
(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 level gauge. It is necessary to avoid the blind area during installation. When the liquid level enters the blind area, the ultrasonic transmitter cannot measure the liquid level. Therefore, when determining the range of the ultrasonic level gauge, it must be Leave a margin for the dead zone. When installing, the transmitter probe must be higher than the dead zone of the highest liquid level. In this way, the accurate monitoring of the liquid level and the safety of the ultrasonic level gauge can be ensured.
(2) When the ultrasonic level gauge has foam, because the sound wave cannot penetrate the foam, the sound wave will be reflected back on the foam, so the measurement has a large deviation from the actual liquid level. Defoamer can be added to the tank container with foam to reduce the generation of foam and ensure accurate measurement.
(3) The ultrasonic level gauge will be affected by the stirrer in the container with the stirrer, resulting in the reflection of false reflection echoes, resulting in inaccurate measurement. By reducing the rotational speed of the agitator and installing the level gauge away from the center of the agitator, the influence of the agitator on the measurement of the ultrasonic 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 has a temperature difference with the surrounding temperature, water droplets will condense around the probe, which will affect the accuracy of the measurement. When installing the ultrasonic level gauge, connect the compressed air pipe and blow it against the probe to reduce the influence of water droplets condensed on the measurement due to the temperature difference between the medium, the container and the probe.
3. Hydrostatic level gauge
The static pressure liquid level measurement method is realized according to the principle that the static pressure of the liquid column is proportional to the height of the liquid column, and the liquid level measurement is realized by measuring the static pressure generated by the height of the liquid column. When the liquid level in the container changes, the static pressure generated by the liquid column also changes accordingly.
The hydrostatic level gauge measures the liquid level by measuring the static pressure generated by the liquid level height. According to P=ρgh, and the density ρ of the liquid and the acceleration of gravity g are known, as long as the pressure P is measured, the liquid level h of the liquid can be obtained.
A pressure instrument used for liquid level measurement is a submersible liquid level gauge, that is, the liquid level measurement instrument is put into the medium of the liquid level to be measured. The detection element converts the static pressure into a resistance signal for liquid level detection, and the drop-in level gauge can be directly dropped into the measured medium.
Features of the submersible pressure level gauge.
(1) The structure is simple, the solid structure is adopted, and there are no moving parts.
(2) It is very convenient to install and use, and has a long service life.
(3) The measurement range is relatively wide, and it can measure from water, oil to phase-like substances with high viscosity.
(4) It is not affected by foaming, deposition and electrical durability 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 choose a place where the fluid is relatively stable and the fluctuation is small. If the impact of water flow cannot be avoided, it is necessary to install an isolation pipe to reduce the impact of water flow and ensure the accuracy and stability of the measurement.
(7) Install the submersible liquid level meter, preferably 100mm to 200mm away from the bottom of the pool or tank, so as to reduce the accuracy of the measurement due to sludge and medium sediment at the bottom of the pool or tank.
(8) In an environment with poor water quality, especially when there are many suspended solids and impurities in the medium, it is easy to block the pressure taking hole and affect the measurement accuracy. The level gauge needs to be cleaned regularly and the pressure-taking hole should be dredged to ensure accurate and stable measurement.
Pressure level gauges are suitable for use in processes with better water quality, such as supernatant collection tanks, filters, clean water tanks, external drainage tanks and SBR tanks. It can also be used in the concentration tank adjustment tank with stirring to avoid sludge impurities at the bottom of the tank by adding an isolation pipe.
4. Radar level gauge
The antenna of the radar sensor transmits electromagnetic wave signals in the form of beams, the transmitted waves are reflected on the surface of the measured material, and the reflected echo signals are still received by the antenna. Each point in the transmitted and reflected beams is acquired using 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 proportional to the time travel T of the pulse:
Among them, 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 an integrated design, has no moving parts, has no mechanical wear, and has a long service life.
(2) Due to the characteristics of electromagnetic waves, it is not affected by the environment. Therefore, its measurement applications are relatively wide. The probe of the radar level gauge has no contact with the surface of the medium, which is a non-contact measurement and can measure different media accurately and quickly. The probe is almost unaffected by temperature, pressure, gas, etc. It can be used in situations where the working conditions are harsh, the changes are large, and there are moisture, steam, foam and other ultrasonic level gauges that cannot be used.
(3) The radar level gauge is also suitable for the occasions where there is a stirrer and the liquid level is fickle and changeable.
(4) The price of radar level gauge is relatively expensive, but it can be applied to almost every process level control process of sewage treatment.
To sum up, because the technological process of sewage treatment involves many occasions where liquid level needs to be measured, and also because of the characteristics of the sewage treatment process itself, in the selection process, it is necessary to select a suitable liquid according to the characteristics of each technological process and medium. The position meter will have a great impact on the accuracy, reliability and stability of the process measurement, as well as the economical service life. It is recommended to choose a liquid level gauge with high precision, low maintenance and long life when the funds allow. So that the liquid level measurement and control of the entire sewage treatment is reliable, stable and safe.
Post time: 09-06-22