Application of Ultrasonic Externally Sticking Liquid Level Gauge in Safety Monitoring of Liquid Ammonia Storage Tanks

1. Introduction to the properties of liquid ammonia Ammonia is an important raw material in the chemical production process. Ammonia is a flammable gas and a toxic substance at the same time. It is easily soluble in water to form ammonia water under normal temperature and pressure, and is easily liquefied into a colorless liquid under pressure. In order to facilitate storage and transportation, the industry usually adopts normal temperature and high pressure or low temperature pressure to liquefy ammonia. The dangers of ammonia are as follows: (1) The physical properties of ammonia are colorless and irritating odor, molecular formula N H3, molecular weight 17.03, relative density 0.7714 g/L, melting point -77.7 ℃, boiling point 33.35℃, auto-ignition point is 651.1l℃, it will cause fire or explosion in case of fire source. The explosion limit of liquid ammonia vapor and air mixture is 16% to 25% (the most flammable concentration is 17%). It is soluble in chloroform and ether and is a good solvent for many elements and compounds. (2) The chemical properties of liquid ammonia are that the aqueous solution is alkaline. Liquid ammonia will attack some plastics, rubber and coatings. Liquid ammonia is difficult to ignite when exposed to heat and open fire, and its danger is low, but when the ammonia and air mixture reaches the concentration range of 16% to 25%, it will burn and explode when exposed to open fire. If oil or other flammable substances exist, it is dangerous. Sex is higher. Liquid ammonia cannot coexist with acetaldehyde, acrolein, boron, halogen, hydrogen peroxide and other substances. (3) Toxicology of liquid ammonia: Ammonia gas invades the human body through the respiratory tract, has alkaline irritating and corrosive effects on mucous membranes and skin, can cause tissue lysis and necrosis, and can cause reflex respiratory arrest and cardiac arrest at high concentrations. Eye contact with liquid ammonia or high-concentration ammonia gas can cause burns, and in severe cases, corneal perforation may occur. Skin contact with liquid ammonia can cause burns. When the concentration of ammonia in the air reaches 700 mg/;m, the human body can be poisoned by inhalation for 30 min, and when the concentration reaches 1750-4500 mg/m, it can be life-threatening. (4) In addition, the gas formed during the leakage of liquid ammonia and the mixture formed by dissolving in water have serious harmful effects on the environment, and can cause pollution to water, soil and atmosphere. To sum up, the leakage of liquid ammonia has serious hazards, and the liquid ammonia storage tank is a major source of danger. In recent years, various major accidents of acute poisoning caused by ammonia leakage have occurred frequently at home and abroad. Therefore, the monitoring of the liquid level of the liquid ammonia storage tank is very important. 2. Introduction of externally-mounted liquid level gauge The externally-mounted liquid level gauge adopts the principle of sonar ranging, uses a special probe attached to the outside of the container wall to collect sonar signals, takes special sonar wave processing technology as the core of the system, and adopts advanced high-speed Signal processing technology, using Shaanxi Kaidi's special algorithm to accurately calculate the liquid level in the container. The calculated liquid level value is displayed locally and the remote signal is output. The instrument can output the measurement results to the control center through (4-20) mA analog signal, HART, RS-485 and other interfaces, and can also conduct online debugging through HART, RS-485, infrared and other interfaces. As shown in Figure 1, when measuring the liquid level, the sonar wave signal is emitted from the probe, and the echo signal is detected by the probe after being reflected back by the liquid surface. The echo signal is processed by a proprietary algorithm to calculate the time t, and the system calculates the liquid level height according to the formula. H=v×(t/2-tg/2)×αH: liquid level height v: speed of sonar wave propagation in liquid t: time taken for sonar wave from launch to return tg: sonar wave transmission in the tank wall Time α: correction factor 3. Advantages of applying externally attached liquid level gauges in the installation of liquid ammonia storage tanks . The two-wire externally attached liquid level gauge achieves ultra-low power consumption and meets the standard of two-wire instrumentation in the industrial field, which simplifies the field wiring requirements and is more conducive to safety and explosion-proof. 2. Exclusive temperature calibration technology Liquid ammonia is a medium with a relatively large density change under the influence of temperature. When the liquid ammonia in the storage tank is less than half, if the side probe calibration mode is used, it will not be calibrated correctly, thus affecting the accuracy of the measurement results. The externally mounted liquid level gauge of our company's temperature calibration mode only needs to install a measuring probe at the bottom of the storage tank that can collect the real-time temperature of liquid ammonia inside the storage tank (no need to install the probe on the side), know the internal temperature of the storage tank, and then from Kaidi The calculation results of the sound velocity of liquid ammonia at this temperature are obtained from the large database of the software system, thereby ensuring the high precision of the measurement results. The temperature calibration mode not only greatly improves the accuracy, but also realizes full-scale real-time calibration, and the installation method is also simpler. In particular, there is no need to install probes on the side of the spherical tank, which greatly reduces the workload of on-site installers. 3. The measurement accuracy of the externally mounted liquid level gauge is super high, up to 1‰FS. The built-in bottom boundary false wave suppression function, automatic power control algorithm, automatic sampling rate control algorithm and other functions greatly improve the measurement accuracy. In some practical cases the absolute accuracy is 1cm. 4. The advanced automatic power control algorithm can realize small blind area or no blind area. When the liquid level is high, use higher power, when the liquid level is low, use lower power, and automatically switch the probe power. It not only increases the measurement range, but also reduces the measurement dead zone. Thereby achieving a small blind spot,Even measurements without blind spots. 5. Non-contact measurement of liquid level, safe realization of true non-contact measurement, making installation and maintenance greatly simple and convenient.
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