Design and selection of oil depot radar level gauge

DHE-RD series radar level meter can be widely used in liquid and solid level measurement. Even under high pressure, strong corrosiveness, high hygiene requirements or other working conditions, it can continuously measure the level height of various media. DHE-RD series radar level meters are divided into non-contact radar level meters and contact radar (guided wave) level meters based on different measurement purposes, material characteristics and storage tank environments.

At present, the automatic tank level measurement system used for custody transfer measurement at home and abroad is usually a hybrid tank measurement system (HTMS, hybrid tank measurement system). Radar level gauge is commonly used in HTMS. The radar level gauge has no moving parts and no mechanical wear. In actual use, it is found that the working time can generally reach 10 years. It does not pollute the environment and is easy to install. It is very suitable when the medium in the tank is relatively viscous and corrosive. The radar level gauge has high accuracy, a wide operating temperature range, a maximum working pressure of up to 4MPa, and a measurement range of 0-40m. By using different antenna forms and waveguide applications, the requirements for measuring liquid level in most storage tanks can be met.

1. Working principle and classification

Radars that meet custody transfer measurement accuracy (±1mm) are all non-contact radar level gauges. From the measurement principle, it is basically divided into two categories:

(1) Based on the time difference method (time of flight): the antenna emits electromagnetic waves that propagate at the speed of light. When the electromagnetic waves hit the liquid surface after a delay, they will be reflected back. The distance is proportional to the lag time. The instrument determines the distance (air height) between the setting surface and the reflective surface by measuring the lag time from emission to reception.

(2) Composite frequency modulation principle (FMCW). The chirped constant-amplitude microwave signal is emitted through the antenna and is reflected back when it encounters an obstacle. After a delay, the instrument receives the reflected microwave signal and mixes it with part of the transmitted wave. The frequency of the mixed output signal is proportional to the measured distance. When the modulation frequency and frequency offset of the solid-state source are constant, as long as the difference frequency is measured, the distance (air height) can be calculated through signal processing technology.

2. Factors affecting liquid level measurement

2.1 The influence of temperature inside the tank on the measurement accuracy of the instrument

The electromagnetic waves emitted by radar antennas can also propagate in vacuum. When spreading in the air, temperature changes basically do not affect its propagation rate. When the temperature of the measured medium changes, the reflection time will change very little.

2.2 Effect of pressure inside the tank on instrument measurement accuracy

Microwave propagation is almost unaffected by changes in air density, so the radar level gauge can work normally in a vacuum or pressurized state. The microwave propagation rate in the vacuum state only changes by 0.029% relative to the air state; when the operating pressure is high to a certain When the range is exceeded, the error caused by pressure on measurement cannot be ignored.

2.3 Influence of medium characteristics on measurement

Factors such as the relative dielectric constant of the liquid medium, the stability of the liquid surface, and bubbles will affect the reflection of radar wave signals, which may cause the liquid level meter to fail to work properly.

When the radar level gauge measures the liquid level in the tank, it is best to have a stable and regular liquid level. However, when the liquid flows in and out, there will be waves that cause the liquid level to fluctuate. During the production process, boiling or foaming may occur, making the liquid level blurred.

When electromagnetic waves propagate in a medium, the smaller the dielectric constant of the medium, the greater the attenuation rate. The greater the attenuation rate, the smaller the reflectivity. Therefore, the dielectric constant is small, the reflectivity is weak, and the signal strength is low. When the relative dielectric constant of the measured medium is small to a certain value, the effective reflected signal of the radar wave is attenuated too much, and the reflected wave energy received by the instrument is too small. Coupled with the interference wave, the liquid level meter will not work properly. Radar level gauges from different companies have different requirements for the minimum relative permittivity. This is because the dielectric constant is related to temperature and electromagnetic fields. However, through other measures to reduce the energy loss of electromagnetic wave reflection or improve the signal processing technology inside the instrument, the instrument's rigid requirements for low values ​​of relative dielectric constant are becoming smaller and smaller. At present, the products of major manufacturers (such as Honeywell, Saab) can measure media with a relative dielectric constant of only 1.2.

For boiling or large-disturbed liquid levels or small dielectric constants of the measured medium, or to eliminate possible interference effects caused by the structural shape of storage tanks and containers, waveguides and other measures should be used to enhance the echo signal to ensure measurement accuracy. .

3. Antenna form and design selection

The structural form and material of the antenna should be determined based on the characteristics of the medium being measured, tank type, opening, tank temperature, pressure and other factors. Common forms include horn (cone) antenna, parabolic antenna, waveguide array (planar) antenna and rod antenna, etc.

For conventional vaulted tanks, the radar level gauge should be a horn-shaped (conical) antenna design, which eliminates the need for a guide tube and can be installed close to the tank wall to ensure measurement accuracy.

For dome tanks that are relatively viscous and require heat tracing, a parabolic antenna radar with anti-adhesion capabilities is very suitable, and a waveguide is usually not required.

For floating roof tanks, the radar level gauge should adopt a planar antenna design so that it can be easily installed on the still pipe of the oil tank.

For high-pressure spherical tanks, the antenna form is an extended bell mouth antenna with a waveguide, and an integrated ball valve is also provided to facilitate the maintenance of the radar level gauge under pressure. In order to perform measurement verification under pressurized operating conditions of the tank, reference equipment needs to be installed: a reference pin in the hole of the still pipe and a reflector with a reference ring at the end of the lower pipe provide reference at fixed and known distances echo.

4. Installation precautions

The radar liquid level should not be installed in the center of the top of the tank. This is because the electromagnetic waves emitted by the antenna propagate throughout the entire space inside the tank, and the electromagnetic waves reflected back from the tank wall and tank bottom will be concentrated in the center, causing great interference. It will drown out the normal electromagnetic waves reflected from the medium surface.

When using a horn antenna radar level gauge, the antenna extends out of the nozzle by at least 10mm. If the length of the nozzle on the container cannot meet the above requirements, you can use an antenna extension tube or consult the manufacturer to design a suitable nozzle length.

Radar level gauges without waveguides require different distances between the instrument installation connecting short pipe and the inner wall of the tank depending on the antenna form. Generally, the distance between the center of the short pipe connected to the liquid level gauge and the inner wall of the pipe should not be less than 1/2 of the beam width. Beam width = 2X measurement distance Xtan (beam angle/2), and the beam angle is the angle when the radar wave power density reaches half of the maximum value. When the microwave frequency emitted by the antenna is fixed, the larger the size, the smaller the beam angle. Parabolic antennas have much smaller beam widths than horn antennas.