Application of Radar Level Gauge in Measuring the Liquid Level of Wastewater Tank
Time:2022-12-26 Read:612
Reading guide: Many countries and enterprises are also paying more and more attention to environmental protection. Waste water tank plays the role of collecting and temporarily storing chemical wastewater in petrochemical enterprises, and the accuracy of their level measurement directly affects environmental protection achievements. Therefore, the article discusses the liquid level measurement of waste water tanks.

Working principle of Radar Level Gauge

The radar level gauge adopts the working mode of transmitting and reflecting receiving. The electromagnetic wave emitted by the radar antenna is reflected by the surface of the measured object and then received by the radar antenna. The distance between the radar level gauge and the surface of the object to be measured is proportional to the time from the radar antenna transmitting electromagnetic waves to receiving radar echoes, and the relationship is as follows:

Radar Level Gauge suppliers

Where: D - distance from radar level gauge to liquid level; C-speed of light; T-electromagnetic wave operation time


Features of radar level gauge

The radar level gauge adopts an integrated design, without movable parts, mechanical wear, and long service life. The radar wave emitted by the radar level gauge can pass through the vacuum without the transmission medium. The measurement of the radar level gauge is not affected by the steam, atmosphere, and volatile media. It can be used for the measurement of liquid levels in volatile media, such as crude benzene, and is applicable to the measurement of most liquid levels.

A conductive medium or even a micro conductive medium can reflect enough electromagnetic wave signals. For non-conductive media, when the dielectric constant is greater than 1.5, enough radar waves can be reflected to be detected by the level gauge. The larger the dielectric constant is, the stronger the radar-reflected wave signal is.

The radar level gauge is a non-contact instrument, which is not affected by the density, concentration, and other physical characteristics of the medium in the tank. The measuring range is wide, up to 35m. It can also be used for liquid level measurement in high temperature, high pressure, and other environments. The key parts of the radar level gauge, such as the antenna, are made of high-quality materials with strong corrosion resistance, so they can be used in highly corrosive environments.

The radar level gauge has the function of automatically identifying false echoes according to the instrument parameters entered by the user. These interference waves can be eliminated through software, manual operator, or simple operation keys. The radar level gauge can use the built-in simple operation key to set most parameters on the level gauge or use the corresponding FF or HART protocol communicator according to the model of the level gauge to set the communication remotely or locally. The operation is convenient and fast.


Factors affecting the measurement of radar level gauge

Impact of ambient temperature

When the measured medium is volatile, the ambient temperature may cause the measured medium to volatilize and attach to the antenna surface, which will seriously affect the antenna transmitting and receiving radar waves, causing abnormal fluctuations in the measured data.

Effect of steam

The waste water tank is generally an underground tank with a low temperature. When there is a large amount of steam in the waste water tank, the steam will condense into small water droplets on the inner wall when it is cold. The small water droplets will gradually increase as the number of steam increases and become small droplets, which will fall into the tank under the action of gravity, and there will be a cycle of small droplets forming, collecting, and dropping. Through analysis, it is found that the radar wave transmitted by the radar antenna will generate irregular reflection echo when it encounters the liquid drops on the tank wall. When the reflection echo signal is strong, the reflected echo of the drops will be fed back to the control room as the true liquid level, thus causing a false liquid level.

In addition, when a large number of water drops are collected on the radar antenna, the echo of the liquid drops on the antenna will also generate radar-reflected waves. When the echo intensity of the reflected waves exceeds the echo intensity of the actual liquid level, the radar level gauge mistakenly thinks that the condensate on the antenna surface is the actual liquid level, generating false signals.

Turbulence of liquid and influence of bubbles

The inflow of liquid in the underground tank and the pumping out of the pump will cause turbulence of the liquid level in the tank, and a large number of bubbles will be generated. The turbulence and bubbles on the liquid surface will not only absorb and scatter radar waves but also refract and reflect radar waves, reducing the signal strength of radar-reflected waves and seriously affecting the measurement accuracy.


Precautions for installation of radar level gauge

The measurement accuracy of radar level gauge depends on the signal strength of the reflected wave of the actual liquid level. If the installation position of the selected instrument, the radar antenna cannot receive the radar wave reflected by the measured liquid level, or there is strong interference within the range of the radar wave and can reflect a strong enough interference wave to the radar level gauge, the radar level gauge cannot effectively reflect the actual liquid level. Therefore, selecting a reasonable installation position is the key to the accuracy of radar level gauge measurement.

Precautions for installation of radar level gauge in an underground tank and other containers

During the installation of the liquid level gauge, the interference sources such as the feed inlet, agitator, and mixing valve shall be avoided as much as possible. At the same time, attention shall be paid to avoiding the adhesions of the tank wall, ladders, and other fixed objects to improve the reliability of the signal. The installation position of the radar liquid level gauge shall be no less than 30 cm from the tank wall and tank wall to avoid mistaking the echo signal on the inner wall for the echo signal of the real liquid level; When installing the radar level gauge, pay attention to the installation marks on the nuts or flanges. 

The installation marks are used to aim at the antenna position. Make sure that the marks point to the nearest tank wall when installing the radar level gauge; The radar antenna shall be installed perpendicular to the surface of the measured medium and parallel to the measuring slot wall, which is conducive to the propagation of radar waves. The maximum effective measuring range of a horn antenna that cannot be installed vertically may be reduced.

The horn antenna shall extend the installation short tube of the liquid level gauge to improve the measurement accuracy; For horn-type radar level gauge, the installation of the horn mouth shall be 10mm larger than the inner surface of the installation hole; When installing the rod level gauge, pay attention that the antenna must extend out of the mounting hole, and the length of the mounting hole cannot exceed 100 mm. When the level gauge is used to measure the liquid level of an elliptical or circular vessel, it should be installed at a distance of 1/2R from the center (R is the radius of the vessel). Be careful not to install it at the center of the top of the vessel. When the radar is installed at the center of the top of the vessel, it is easy for the radar wave to form multiple reflections on the vessel wall. Finally, the radar wave will converge at the center of the top of the vessel, forming a strong interference wave, affecting the accurate measurement of the radar level gauge.

Precautions for installation in waveguide tube

The calibration mark on the installation position of the antenna shall be aligned with the drainage groove; After installation, the meter case can rotate 350 °, making the wiring chamber and LCD screen easy to view; Full bore ball valves can be used for measurement.

The waveguide tube must be a metal tube, which can be a parallel or seamless welded stainless steel tube. The waveguide tube cannot be enamel coated; The pipe diameter shall be uniform, and the inner wall shall always be smooth. When the waveguide tube needs to be extended, it can be achieved by welding the flange or sleeve, but there shall be no gap of more than 1 mm at the connection; If holes are required on both sides of the waveguide tube, the positions of the holes shall be symmetrical and the included angle shall be 180 °;

When a drainage groove is needed, its maximum aperture and width are 1/10 of the pipe diameter. The location of the opening needs to be deburred. The number and length of the opening have no effect on the measurement;

Precautions for installation in the bypass pipe

When the material in the container fluctuates greatly, the level gauge with bypass pipe can be used to reduce the impact of medium fluctuation on measurement; Pay special attention that the calibration mark on the antenna installation position should be perpendicular to the can body connecting the pipe.

The bypass pipe must be made of metal pipe, without enamel coating or plastic; The pipe diameter shall be uniform and the inner wall shall always be smooth; No transition section of the bypass pipe shall have a gap of more than 1 mm; At ± 20 cm of the connecting pipe of the tank body, the measurement accuracy will be reduced.

Select the horn antenna with the largest aperture.

4.4 Judge whether the installation is appropriate

After the radar level gauge is installed, the echo curve of the level gauge can be checked with a PC equipped with communication software to judge whether the installation is appropriate. If not, the installation position of the level gauge needs to be further adjusted until the level gauge can produce a satisfactory echo curve. At this time, the liquid level displayed on the level gauge display screen should be consistent with the actual liquid level.


Parameter configuration of radar level gauge

Radar level gauge parameter setting

After the radar is installed and the wiring is completed, the following parameters need to be input: distance unit, container type, measuring medium, null standard, and full standard, and then echo suppression can be used for measurement. The instrument will automatically optimize and adjust the related parameters internally according to the data entered by the user to achieve the best measurement effect.


Suggestions on the selection of radar level gauge

The selection of radar level gauge shall fully consider the factors that affect the instrument measurement in all aspects, such as equipment structure, installation position, medium characteristics, especially when measuring the liquid level of volatile and crystallizable objects, the impact of volatile medium attached to the antenna shall be fully considered. According to the characteristics of different working conditions, appropriate instruments with anti-condensation, back flushing, or back flushing functions shall be selected.


Conclusion

Due to the particularity of the measured medium and its environment, the radar level gauge can ensure the accuracy of liquid level measurement. At the same time, the appropriate model should be selected according to the characteristics of the medium, which is a strong guarantee for the long-term stable operation of the instrument. It is hoped that through the discussion in this paper, the readers will have a more in-depth understanding of the radar level gauge measuring the liquid level in the wastewater pool, and provide some experience in troubleshooting and daily maintenance of the radar level gauge.