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Abstract: In the chlor-alkali production process, when liquid chlorine storage tanks experience inaccurate level measurement due to issues with the level gauges used, the ELL external-type level gauge produced by Xi’an Dinghua Company can be utilized to effectively address this problem of inaccurate level measurement in such tanks. The text compares the performance of frosty tube level gauges, radar level gauges, and ELL external measurement level gauges, concluding that using external measurement level gauges for level measurement is an advanced, scientific, safe, and effective method. Keywords: ELL external level gauge ； liquid chlorine ； Micro-vibration analysis 1. Introduction Zhongyan Jilantai Chlor-Alkali Chemical Co., Ltd. is a chemical enterprise in the chlor-alkali industry, with an annual production capacity of 400,000 tons of ion-exchange membrane caustic soda. In chlor-alkali plants, the production and temporary storage of liquid chlorine are inevitable. The chemical name of liquid chlorine is liquid chlorine; it is a yellow-green liquid with a boiling point of -34.6 °C and a melting point of -103 °C. It vaporizes into a gas at normal pressure, and inhalation can cause severe poisoning. It is highly irritating and corrosive, and can cause combustion and explosion when mixed with other flammable gases in the presence of sunlight. Liquid chlorine storage tanks are pressure vessels and also constitute major hazard sources; therefore, they are considered equipment that requires special management. Initially, the \"frosting tube level gauge\" was used for measurement; this method was relatively intuitive and belonged to the traditional category. However, judged by the two basic criteria for level gauges – accuracy and reliability – this measurement method cannot be considered a true level gauge. Later, radar level gauges were used. Since liquid chlorine is a liquefied gas, there is a phenomenon of signal loss during the transmission and reception of radar waves. Additionally, the presence of chloride sludge in liquid chlorine causes it to stick to the antenna, interfering with the transmission and reception of radar waves and resulting in unstable measurement results. Finally, by consulting design institutes and industry experts, we learned about the contactless level measurement instrument – the ELL external level gauge. After evaluation, one such instrument was installed in Tank A, and its measurement performance met the required standards. Finally, the level gauges of the remaining five liquid chlorine storage tanks were all replaced with ELL external level gauges. The most fundamental feature of this gauge is that it can measure the liquid level inside a liquid chlorine storage tank without the need to make any holes in the tank. This prevents problems such as leakage, dripping, escaping fluid, and seepage caused by too many openings, thereby improving the level of safety management ； Furthermore, since it can be installed and maintained online without coming into contact with the medium, it facilitates maintenance and ensures the safety of the instrument maintenance personnel. Therefore, when choosing a method for measuring the liquid level in liquid chlorine storage tanks, external, contactless measurement is the safest and most scientific approach. 2 Comparison of the effectiveness of different level gauges 2.1 Frost tube level gauge The frost tube level gauge is a relatively primitive measurement method, based on the principle of communicating vessels. Holes are made at the top and bottom of the storage tank’s head, and a seamless steel pipe is used to connect the tank to the steel pipe. Since the temperature of liquid chlorine is usually below zero, the water vapor present in the air will freeze at the locations where liquid chlorine is present. Figure 1 shows a schematic diagram of the installation location of the frost-forming pipe. The advantage of this method is its low cost and simplicity. The biggest drawback of the frosty tube level gauge is the lag in “display”. Feeding process: As the temperature on the outer surface of the frosting tube drops from room temperature to below zero degrees, water vapor near the outer surface of the tube begins to condense, changing from a liquid state to a solid state. Factors that affect frosting include ambient temperature, humidity, and the feeding speed of the liquid into the storage tank. As the company is located in Wuhai City, Inner Mongolia, it experiences drought throughout the year. The frosting rate is very slow; sometimes no frosting occurs at all, which affects observations. Discharge process: Ice at temperatures below zero is melted by the surrounding air; during this process, the readings of the frosty tube level gauge become severely delayed, so this type of level gauge cannot be used as a reference. 2.2 Radar level gauges: Their measurement principle is based on the time-domain reflection principle. The electromagnetic pulses emitted by these gauges travel at a certain speed along the surface of the medium. When they encounter the surface of the medium being measured, some of these pulses are reflected, forming echoes that return to the pulse transmitter along the same path. The distance between the transmitter and the surface of the medium is proportional to the time it takes for the pulses to travel between them; by calculating this value, the liquid level can be determined. Radar level gauges come into contact with the gas phase of the medium inside the tank; due to the polarity of this gas phase and the presence of numerous liquid droplets, it has a strong ability to absorb and reflect electromagnetic pulses, which sometimes leads to signal loss and results in inaccurate measurements. Additionally, a layer of water vapor mixture forms on the surface of the medium being measured; this layer is not taken into account in level calculations. Since the radar cannot penetrate this layer of waves, it can also lead to false level readings. 2.3 ELL external level gauge: Its principle involves attaching a level measurement probe to the outer wall at the bottom of the storage tank, in order to detect the slight vibrations generated by the tank. It converts the minute vibrations of the container under different liquid levels into electrical signals, which are then transmitted to the main instrument unit. After the main unit filters, identifies, and performs complex calculations on these vibration waves of various patterns, it obtains a continuous and accurate value of the liquid level. It is transmitted to the DCS system via 4 – 20 mA for monitoring; a high liquid level alarm is set according to the process control requirements of the branch plant, ensuring that the liquid level is strictly kept within the specified range and thus maintaining the safe and stable operation of the chlorine liquor system. The ELL external level gauge consists of two main parts: one is the level measurement probe as shown in Figure 3, and the other is the gauge main unit as shown in Figure 4. The liquid level measurement probe is attached directly to the outside of the bottom wall of the chlorine liquid storage tank, and it is responsible for detecting micro-vibration signals ； The host is installed next to the liquid chlorine storage tank and is responsible for analysis and calculations. The liquid level measurement probe is connected to the main unit via a cable. The host is powered by DC 24 V and outputs 4–20 mA signals; it also supports HART and Modbus protocols, allowing communication with secondary instruments such as DCS and PLCs. Instrument parameters can be set using a magnetic pen or a handheld operator. The ELL external level gauge is very easy to install; Figure 5 shows the installation location of its probe. During installation, it is only necessary to remove slag, paint, and similar substances from the bottom wall of the liquid chlorine tank, after which the liquid level measurement probe can be attached directly to the tank wall. Advantages of the ELL external level gauge: First, it is easy to install. There’s no need to drill holes in the storage tank; the level measurement probe can be attached to the outer wall of the tank to measure the liquid level. It can also be installed, calibrated, and maintained online ； Second, the external level gauge does not come into contact with liquid chlorine; there is absolutely no possibility of leakage, making it very safe ； Third, the instruments can be easily disassembled and maintained at any time in case of a failure. It eliminates steps such as replacement, evacuation, and analysis, thus avoiding any impact on normal production. 3 Conclusion Through the comparison and analysis of the measurement performance of the three types of level gauges used on the liquid chlorine storage tanks in our company, it is concluded that the current method of using external level gauges to measure the liquid level is the most advanced, scientific, and safe approach. The unique contactless method of external level gauges makes them suitable for use in areas where measuring the level of flammable, explosive, highly toxic, or similar substances is required.