Introduction of Level Wave Guide Radar
In the industrial production process, the material level is one of the key parameters. There are many ways to measure the level of different materials. According to the different working conditions and media, the corresponding measurement method should be used. For material level measuring instruments, the commonly used ones mainly involve pontoon type, static pressure type, heavy hammer type, ultrasonic, etc., which all involve different characteristics and application ranges.
Guided wave radar level gauge can realize non-contact measurement by virtue of radar measurement technology, has strong aging resistance and wear resistance, and will not be affected by pressure, temperature, vacuum, and other factors. Advanced two-wire system technology and digital output are adopted, with high measurement resolution (± 1mm) and strong reliability. At the same time, guided wave radar level gauge has been widely used because of its advantages of normal operation in environments such as easy scarring, high steam, high temperature, and strong corrosion.
Principle of Level Wave Guide Radar
Guided wave radar level gauge refers to a radar level gauge based on Time Domain Reflectometry (TDR). Its electromagnetic pulse mainly propagates along the steel cable or the probe at the speed of light. Once the radar level gauge meets the surface of the medium to be measured, it will reflect some pulses, forming echoes, and then return to the pulse transmitter along the same route.
The distance between the two is proportional to the propagation time of the pulse between them. The liquid height is obtained by calculation. In addition, the technical advantage involved in the guided wave radar level gauge is that it can measure the continuous material level of liquid, particle, and slurry, and in the actual application process, the guided wave radar level gauge will not be disturbed by medium, temperature changes, foam, steam, dust, etc. At the same time, the guided wave radar level gauge has an accuracy of 5 mm and a measuring range of 60 m. It can work normally in a high-pressure environment of 250 ° and 40 kg and is suitable for explosive hazardous areas. In addition, the guided wave radar level gauge can also be used in solid particles, acid and alkali storage tanks, small oil storage tanks, and conductive, non-conductive, corrosive media, such as acid tanks, ash silos, etc.
Model selection and on-site installation
Precautions for model selectionMaterial hanging problem: for viscous and easy-to-crystallize media, it is not applicable in guided wave radar mode, and can measure general liquid and liquid hydrocarbons;
Tensile strength: 4 mm antenna shall be applied during liquid measurement, which can be cut off, and the tension can reach 1 T;
Corrosion: at this stage, for the guided wave radar level gauge, the antenna and flange are made of 316 material, which can meet the requirements of most media. For some special corrosive media, special fabrication is required;
Minimum dielectric constant: select the corresponding level gauge according to the different dielectric constants involved in different media;
Installation size: based on the specific needs of the site, define the flange form, size, and thread size;
Antenna selection: refer to the measurement of different types of containers, different installation methods, media characteristics, etc., and select a single cable, double cable, single pole, double pole, coaxial cable, etc.
Precautions for installation
For the guided wave radar level gauge, whether it can play a role depends on the reflected wave signal. If the installation position and liquid level cannot smoothly reflect the electromagnetic wave back to the radar antenna, or there are interfering substances in the signal wave range reflecting the interference wave to the radar level gauge, the radar level gauge will not be able to reflect the specific liquid level. In this regard, in order to effectively avoid obstacles to the radar level gauge and maintain a stable operating state, it is necessary to ensure the rationality of the installation work. The scientific selection of installation location is closely related to the role of the guided wave radar level gauge.
The following points should be paid attention to during the specific installation:
(1) During the measurement of liquid materials, the antenna axis of the guided wave radar level gauge must be perpendicular to the liquid level reflecting surface, as well as the sensor axis and the medium surface. In the process of measuring solid materials, the sensor should be slightly tilted because of the stacking angle of solid media, so as to prevent the existence of relevant devices that may cause false reflection in the emission angle;
(2) It is worth noting that the obstacle device should be as far away from the nearest one-third of the conical transmission area of the antenna as possible. This is mainly because the closer the obstacle device is, the stronger the false transmission signal will be. If it is unavoidable, the stronger false reflection signal can be refracted away through the refracting plate to reduce the false echo energy density, so that the sensor can easily filter out the false signal;
(3) When selecting the position of the guided wave radar level gauge, a certain distance shall be kept from the outlet and inlet to avoid false reflection;
(4) During the installation of the sensor, the central area of the arch tank shall not be selected, otherwise the false echo received by the sensor will be strengthened, and the sensor shall not be close to the tank wall. The sensor shall be installed in half of the container radius as far as possible; For metal covers and plastic pipes, no collision is allowed in the range. For circular and oval containers, they should be installed in the area half of the distance from the center as far as possible, and should not be installed in the center, otherwise the radar wave will have multiple reflections, and gradually concentrate to the center of the top of the container, forming a strong interference wave, which is not conducive to the measurement accuracy;
(5) Do not install in places with strong eddy currents. In case of obvious chemical reaction or stirring, the guided wave tube, and bypass tube can be used for measurement;
(6) If the sensor is directly installed on the connecting pipe, the antenna can be directly drawn out of the connecting pipe. The length of the horn antenna extending out of the connecting pipe shall not be less than 10 mm. The length of the rod antenna connecting pipe shall not exceed 100 mm or 250 mm. The minimum diameter of the connecting pipe shall be 250 mm. It is worth noting that it is better to increase the nozzle diameter to effectively weaken the interference echo caused by the nozzle;
(7) The inner wall of the waveguide tube is always in a smooth state, and the lower open waveguide tube must meet the requirements of the minimum liquid level so that it can be measured in the pipeline;
(8) For the sensor, its type plate must be aligned with the axis of the waveguide tube opening. If the measured dielectric constant is less than 4, the reflective plate must be installed at the end of the waveguide tube, or the end of the waveguide tube can be bent so that the reflected echo can be effectively refracted;
(9) The antenna shall not be in close contact with the short pipe or pipe wall. The antenna must be firmly tightened and fixed at the lower end of the container, and appropriate width shall be reserved;
(10) For the antenna signal sensing ring, no structural parts shall be installed. If various liquid level meters are installed in the same container, the distance between antennas shall be more than 1 m.
Solutions to probe scarring and frequent failures
First of all, the position of the guided wave radar level gauge needs to be properly raised during the installation. If it cannot be raised due to external factors, it is necessary to use the method of interlocking the liquid level measurement value with the pump of the tank to solve this problem and reduce the maximum liquid level setting value by 0.5 m. If the liquid level rises to the maximum value, stop or start the discharge pump.
Treatment of antenna scar
For hanging materials with a small dielectric constant, the measurement will not be affected in the dry state, but if the dielectric constant is high, the measurement will be affected. In this regard, it can be purged by compressed air or washed with clean water. At the same time, the cooled compressed air can reduce the temperature of flanges and electrical components. In addition, an acid-cleaning solution can also be used to clean alkaline scars, but the liquid level cannot be measured during cleaning.
The way to solve this problem is to change the guided wave radar level gauge to the guided wave tube type measurement. The guided wave tube-type radar level gauge is still installed at the original opening position. It should be noted that the guided wave tube needs to be 0.2 m higher than the exhaust pipe. After using this method, when the liquid slurry overflows from the exhaust pipe, it will not cause the level gauge antenna to be submerged by the slurry. At the same time, it can prevent the influence of the agitator vortex and the steam from emerging from the probe area, reducing the damage to the probe, And under the good focusing effect of the waveguide tube, the received radar signal can be stronger, and the real and reliable measurement effect can be obtained. Therefore, using the method of guided wave tube measurement can comprehensively optimize the measurement conditions of the meter, promote the overall improvement of the measurement performance of the meter, and has a high promotion value.
Case analysis of fault handling
The radar liquid level gauge of a light waste oil tank is weak in echo signal, which makes it difficult to carry out the liquid level measurement. During the process of material delivery, the liquid level of the tank does not exceed 5 m, and the DCS shows that the liquid level is maintained at 7 m. The steps of fault analysis and handling mainly involve the following:
(1) Check the communication. The radar configuration data and echo intensity were checked by debugging software, but no abnormality was found;
(2) Check the oscillogram. Combined with the waveform diagram, it can be concluded that there are chaotic waveforms and many interferences. The echo signal is weak, not more than 200 mV, and it is very easy to cause the liquid level signal to jump. The wave peak value is maintained at 7 m, which is inconsistent with the specific liquid level;
(3) The process medium shall be comprehensively checked. Based on the analysis of the components involved in the cleaning oil in combination with the production process, the waste oil is relatively complex in terms of the medium, which may cause uneven bubbles and floating waste oil on the surface of the medium, and affect the radar wave, leading to the weakening of the echo signal. In addition, the receiving medium is mainly crude gasoline, and light hydrocarbon components are easy to volatilize and condense, which weakens the radar antenna signal. However, due to the large process risk involved, there is no judgment on opening the cover temporarily;
(4) Replace the electronic unit. The failure of the electronic unit will lead to the weakening of the transmitted radar wave signal, or it is difficult to achieve normal echo processing communication. After the replacement of the new electronic unit, the echo is still not restored to normal;
(5) Check the radar head and the aluminum waveguide sleeve connected to the antenna. For this sleeve, it is mainly to keep the radar wave in a stable state, and then conduct it to the antenna. After inspection, it is found that the aluminum sleeve is missing, and the detection echo does not change significantly after addition;
(6) Antenna inspection. After disassembling the flange and removing the antenna, it was found that the array antenna had some cracks on its surface. After it was replaced, the echo returned to normal and the liquid level measurement value was correct.