【Weekly Question 20090831】Advanced questions on Sinopec radar level gauges – answers now available
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This post was last edited by baodingzhai on 2009-9-7 09:25. Everyone is probably familiar with radar level gauges. Can you answer the following question? Which properties of the medium being measured affect the use of radar level gauges? What precautions should be taken when installing a radar level gauge? The answers will be released next Monday. Note: For multiple accounts on the same IP, only the first response indicating participation will be counted as valid! Copying the answer is invalid! Correct answer: 1. Since the measurement principle of radar level gauges is related to the propagation properties of microwaves, characteristics of the material being measured such as the relative dielectric constant of the medium, turbulence in the liquid, and bubbles can cause attenuation of the microwave signal; in severe cases, the gauge may not function at all. 2.1) The feed inlet, feed curtain, and vortices should be avoided, as the liquid injected generates spurious echoes with amplitudes much larger than those of the effective echoes reflected by the liquid being measured. At the same time, the irregular liquid levels caused by vortices scatter microwave signals, leading to a reduction in the strength of the effective signals; therefore, such vortices should be avoided. 2) For containers equipped with agitators, the radar level gauge should not be installed near the agitator, as the agitator creates irregular vortices that can cause attenuation of the radar signal. 3) When radar level gauges are used to measure the level of corrosive or crystallizing substances, in order to prevent the medium from affecting the sensor, manufacturers generally employ a design with a polytetrafluoroethylene measurement window and a separation flange structure.
1. The dielectric constant of the medium being measured affects the operation of radar level gauges. 2. When installing a radar level gauge, the following points should be noted: (1) The antenna should be parallel to the walls of the measurement tank to facilitate the propagation of microwaves. (2) The installation position should be more than 30 cm away from the groove wall to avoid mistaking false signals on the groove wall for echo signals. (3) Try to avoid interference sources such as the material feeding area and mixers, ensuring that there are no fixed objects within the beam range to improve the reliability of the signal. (4) The takeover diameter should be less than or equal to the length of the shield tube (100 mm or 250 mm).
Answer: 1. The relative dielectric constant of the medium being tested, as well as factors such as fluid turbulence and bubbles, can cause attenuation of microwave signals. Measurement errors occur, and in severe cases it cannot be used. 2. When installing a radar level gauge, it should be placed away from the feed inlet and vortices, as the liquid entering these areas can generate significant false echoes. At the same time, the irregular liquid level caused by vortices scatters microwave signals, leading to a reduction in the effective signal strength; therefore, such conditions need to be avoided. For containers equipped with agitators, the radar should not be installed near the agitator, as the stirring action creates irregular vortices that can lead to signal attenuation. At the same time, the blades of the mixer can also generate false echoes for the microwave signal, especially when the relative dielectric constant of the medium is low or at low liquid levels, where the effect is more pronounced. When using radar to measure the level of corrosive and crystalline materials, in order to prevent the medium from affecting the sensor, manufacturers generally employ a design with a PTFE measurement window and a separated flange structure. The temperature of these components must not be too high. To avoid the effects of high temperatures on the radar antenna, and to prevent crystals present on the diaphragm from interfering with the proper operation of the instrument, a safety distance of at least 100 mm to 800 mm is required between the flange face and the highest liquid level.
The following properties of the medium affect the use of guided-wave radar level gauges: 1. Dielectric constant； 2. Foam components ； 3. Corrosivity. Installation notes: 1. The location should be appropriate, and it must not exceed the allowable range of the beam angle ； 2. When used in tanks equipped with mixing equipment, the edge of the beam angle should be kept at a sufficient distance from the mixing equipment ； 3. Install at a horizontal position perpendicular to point 0 of the object under test ； 4. Guided wave tubes with a measurement range greater than 10 meters ； 5 When measuring solid media, it should be installed at a surface on which radar waves can reach in a way that provides meaningful references ； 6. Choose an installation location with as small a blind spot as possible.
Factors affecting measurement: 1. Bottom shape of the tank: If the tank bottom is concave or conical, measurement cannot be carried out when the liquid level is below this point. 2. Dielectric constant: If the dielectric material has a low dielectric constant, the tank appears lower when the liquid level is low; to ensure measurement accuracy, it is recommended to set the zero point at a lower height. 3. Corrosion and viscosity: Theoretically, it is possible to measure at the position of the antenna tip, but considering the effects of corrosion and adhesion, the end point of the measurement range should be at least 100 mm away from the antenna tip. 4. Antenna and power: The minimum measurement range is related to the antenna. 5. Foam: Depending on the concentration, foam can either absorb microwaves or reflect them, but it is possible to measure them under certain conditions. Installation notes: 1. Recommended distance from the wall to the outer wall of the installation stub: it should be at 1/6 of the tank’s diameter from the tank wall, with a minimum distance of 200 mm. 2. It cannot be installed above the feed inlet. 3. It cannot be installed in the center position; if installed there, multiple false echoes will be generated, and these interfering echoes can cause the signal to be lost. 4. If the distance between the instrument and the tank wall cannot be maintained, the medium on the tank wall will adhere, causing false echoes; false echoes should be stored during the calibration of the instrument.
The following properties of the medium affect the use of guided-wave radar level gauges: 1. Dielectric constant； 2. Foam components ； 3. Corrosivity. Installation notes: (1) The axis of the radar level gauge antenna should be perpendicular to the reflecting surface of the liquid level. (2) Stirring valves inside the tank, adhering substances on the tank walls, and objects such as steps can generate interfering reflected waves if they are within the signal range of the radar level gauge, thereby affecting liquid level measurement. When installing, choose an appropriate location to avoid interference from these factors. (3) The bell mouth of the horn-type radar level gauge must be at a certain distance (>10mm) from the inner surface of the mounting hole. The antenna of the rod-type level gauge must extend out of the mounting hole, which cannot be longer than 100 mm. For circular or elliptical containers, it should be installed at a distance of 1/2R from the center (where R is the radius of the container), and not at the center of the top of such containers. Otherwise, after being reflected multiple times off the container walls, the radar waves converge at the center of the container’s top, creating strong interference waves that affect the accuracy of measurements. (4) For measuring the level in containers with large level fluctuations, a level gauge equipped with a bypass pipe can be used to reduce the impact of these fluctuations.
Come to me if you need a specific answer; I am a manufacturer of radar products. Contact me and explain the reason. QQ:107433171
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