The high liquid level of the storage tank can easily cause safety accidents and lead to economic losses. In order to prevent the occurrence of high liquid level accidents in the storage tank, it is necessary to detect and alarm the liquid level of the storage tank. The oil storage tank should also be designed with a high liquid level alarm device. . Therefore, a liquid
level switch is essential. Usually liquid level switches include ultrasonic type, buoyancy type, immersed capacitive type, tuning fork type, pressure type liquid level switch, most of which are contact type. In view of the large volume of storage tanks, flammable and explosive, and some storage tanks are not allowed to open holes, touch the outside air and other environmental conditions, this requires a non-contact, non-perforated liquid level switch, and the externally attached ultrasonic liquid level switch Switches meet this need. 1. Working principle In the storage tank equipment, the externally attached ultrasonic liquid level switch utilizes the surrounding of the ultrasonic wave in the tank wall and the reflection characteristics of the wave to detect and alarm the liquid level. The high-frequency ultrasonic pulse generated by the externally attached ultrasonic liquid level switch and its
sensor (probe) can pass through the container wall, the pulse will propagate in the container wall and the liquid, and will be reflected back. Detection and calculation can determine whether there is liquid in the container at the monitoring point. At the same time, the liquid level switch can output relay signals to the back-end electrical appliances or other equipment, so as to realize the monitoring or control of the liquid level. 2. Applicability analysis and advantages and disadvantages of the instrument Reports, etc., which requires the level switch must have good reliability and applicability. At first, the non-contact explosion-proof ultrasonic liquid level switch was tried on the high-pressure spherical tank, and it was successful. However, with the actual use of the site, some problems were encountered. When the high-level alarm and high-level alarm were required to be added at the same time, the anti-interference ability was strong. The alarm signal is easy to be dry, and when it encounters other electromagnetic wave interference, it will produce flash alarms and false alarms; the variety is single, and it cannot meet the various needs of the scene. In order to overcome the above shortcomings, the intelligent externally attached ultrasonic liquid level switch series products came into being, effective The problem of false alarm has been solved. The advent of the intelligent externally attached ultrasonic liquid level switch satisfies the increasing demand for explosion-proof and non-perforated hard alarm devices in petrochemical plants. 2.2. Advantages ① Strong reliability: The externally attached ultrasonic liquid level switch adopts non-contact measurement, and does not directly contact the medium during the whole measurement process. This measurement method makes the device not affected by the temperature, specific gravity and corrosiveness of the medium. , especially toxic liquids, and effectively eliminate the influence of the medium hanging on the wall and the upper fog and foam, which better ensures the reliability of its work. ②Large measurement range and wide application: the measurement medium includes pure liquid, milky liquid, and suspended liquid, and can also be equipped with a variety of probes. type tank. ③ Simple installation and easy maintenance: non-contact installation outside the tank wall, no holes, no fire, small workload and low installation cost. In addition, it can be installed, maintained and used without stopping production operation. At present, the most common installation methods are the magnetic adsorption method and the adhesive installation method. ④The repeatability and stability are high, and it is not easy to cause false alarms. At present, the repeatability error has been increased from ±5mm to ±2mm, which effectively overcomes the flashing and false alarms caused by various disturbances such as high-alarm, low-alarm, and high-high alarms installed on a tank. ⑤It has relay normally open and normally closed alarm output, which can be directly sent to the host computer and interlocking device. ⑥ There are no moving parts, therefore, almost no wear, no corrosion, and long service life. ⑦The ambient temperature of the external field of the externally attached ultrasonic liquid level switch can be as high as 300 ℃, and has nothing to do with the pressure. ⑧At present, this device equipped with a composite probe can be automatically calibrated without debugging. 2.3. Disadvantages ① The selection of externally attached ultrasonic liquid level switch is related to the wall thickness of the tank. The externally attached ultrasonic liquid level switch is usually not suitable for tanks with a tank wall thickness greater than 60mm. ②The externally attached ultrasonic liquid level switch is only suitable for the tank wall such as metal, glass, plastic and other dense materials. At present, the externally attached ultrasonic liquid level switch cannot be used to monitor and alarm the liquid level.
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