Thread Content
Abstract: Safe, environmentally friendly, and high-performance instruments play a crucial role in ensuring the safe production of chemical enterprises. Taking the liquid chlorine tank area of Jinniu Potash and Alkali Branch of Xingtai Mining Group, where instruments are used under high-risk conditions, as an example, this article illustrates to the industry the outstanding performance of this advanced technical instrument – the external liquid level gauge – and shares it with enterprises in the sector. Keywords: external level gauge ； Liquid chlorine storage tank ； Application I. Introduction: On-site instrument measurements function akin to a human sensory judgment system, allowing for an intuitive assessment of various process changes. This enables chemical plant operators to promptly understand the relevant technical parameters, make judgments, and thus take effective corrective actions in a timely manner. Therefore, on-site instrument measurements play an important role in ensuring the safe production of enterprises and promoting their development. Especially under harsh conditions such as those involving highly toxic substances, strong corrosivity, or flammable and explosive environments, deadly dangers can arise at any time. The instruments currently in use have many shortcomings in these areas, and chemical companies urgently need a revolution in new technologies for primary instruments. How to eliminate or minimize the likelihood of such hazards occurring is a major challenge for the instrument monitoring industry. The liquid chlorine tank area in ion-exchange membrane potash production plants is of paramount importance for ensuring safe operation in such enterprises. The highly toxic nature of liquid chlorine, along with its strong corrosive properties when it comes into contact with moisture, poses a serious threat to human life. In the event of a chlorine leak, the consequences can be disastrous. Therefore, selecting the right control instruments for these operating conditions is essential for ensuring safe production in such enterprises. II. Level measurement of liquid chlorine storage tanks 1. Process conditions for level measurement of liquid chlorine storage tanks In ion-exchange membrane potash production plants, the operating temperature of the liquid chlorine storage tanks is -25°C, the pressure ranges from 0.2 MPa to 1.6 MPa, the measurement range is 0–2200 mm, and the viscosity is 0.6 mPa·s. Under such operating conditions, liquid level measurement becomes an important reference parameter in production; it is necessary that the control instruments can provide reliable readings within a certain accuracy range, and can sensitively and continuously track any changes in the liquid level. So that process operators can adjust the output and input of liquid chlorine in a timely manner according to the indicated parameters and process requirements. 2. Example Applications and Analysis: Analysis and comparison of various level gauges. Based on years of experience in working with instruments, level gauges can be generally classified into the following types: a. Servo level gauge: It uses a motor to drive a force sensor, thereby measuring the force acting on the float in order to determine changes in liquid level ； b. Radar level gauge: Based on the principle of radar electromagnetic wave ranging ； C. RF sodium-conducting level gauge: The probe is inserted into the liquid through an opening at the top of the tank; the liquid level is calculated based on changes in sodium conductivity and capacitance ； d. Differential pressure level gauge: Measures the pressure difference between the bottom of the liquid and the gas above it, thereby determining the liquid level ； e. Ultrasonic level gauge: Based on sonar principle, it measures the liquid level height using ultrasonic waves from an opening at the top of the tank. Level gauges based on the various principles mentioned above have been widely used in industries such as chemical engineering, petrochemicals, and metallurgy. They can be considered the main type of level gauges, and they have gained recognition for their high cost-performance ratio under normal operating conditions. However, under the process conditions used for measuring the liquid level in liquid chlorine storage tanks, their working principles and installation methods have serious flaws. They often lead to inaccurate measurements and frequent damage due to the following reasons; moreover, they fail to provide a reliable guarantee for safe operation due to chlorine leaks: 1. Making openings in pressure storage tanks and using flange connections creates potential risks for chlorine leaks ； 2. The key components of the instrument are in direct contact with the medium; corrosion of these components leads to damage ； 3. During maintenance, production must be halted and the tank emptied, which increases maintenance costs, delays production, and may even lead to the leakage of residual chlorine, posing a risk to human safety. Faced with such technical challenges, Jinniu Potash and Alkali Branch of Xingtai Mining Group in Hebei Province, as a chemical enterprise that has just entered the chlor-alkali industry, has been striving to find the best solution for measuring the liquid level in liquid chlorine storage tanks. In June 2005, we obtained information on the external level gauges produced by Xi’an Dinghua Electronics Co., Ltd. After careful study of their measurement principles and installation methods, we concluded that they are highly suitable for measuring the level in hazardous equipment such as chlorine storage tanks. III. Principle of the external level gauge: The ELL external level gauge utilizes harmless micro-vibration analysis to continuously determine the exact height of the liquid surface below it on the outside of the container wall, without any contact with the liquid or gas inside the container. Therefore, it is extremely safe and reliable, particularly convenient for installation and maintenance, and is an eco-friendly instrument. It can be widely used for the continuous and accurate measurement of liquid levels in various containers. The external level gauge is an intelligent field transmitter-type instrument, with its explosion-proof main unit installed near the container being measured. The instrument’s sensing head is pressed against the outer wall of the container, detecting tiny mechanical vibrations on the container wall, which are then converted into electrical signals and sent to the instrument’s main unit. Inside the host, the signal is converted to a digital form and sent to the CPU, where it is analyzed using Dinghua Company’s proprietary artificial intelligence algorithm to determine the liquid level height. The characteristics of the micro-mechanical vibration waveforms detected on the outside of the container wall are determined by the vibration properties of the liquid and those of the container. After being processed using Dinghua Company’s proprietary algorithm, this vibration waveform yields its liquid level characteristic value y. The measured height hm of the liquid surface is related to y and the liquid temperature t through the following mathematical model: hm = h(f(a, t)y), where a is the characteristic coefficient of the liquid, t is the liquid temperature, and y is a parameter characterizing the liquid level of the vibration waveform. This instrument consists of two main parts: the measuring head and the main unit. The measuring head is attached directly to the outside of the container wall and is responsible for collecting information ； The host is installed next to the container under test and is responsible for analysis and computation. The measuring head is connected to the main unit via a cable; the main unit is equipped with a junction box (an explosion-proof junction box is used in flammable and explosive environments) that connects the signal and power cables coming from the level and temperature measuring heads, as well as those from the main unit and the control room. The host is powered by DC24V, and it provides an isolated standard output of 4–20mA. It can be used in conjunction with PLCs, regulators, recorders, and DCS systems. The instrument is configured using a dedicated magnetic pen; five magnetic buttons located on the transparent window of the instrument are used to enter various parameters. IV. Performance Features of External Level Gauges The ELL (ELL-FI-SE and ELL-FI-AE models) series of external level gauges enable continuous and accurate measurement of the liquid level inside a tank from outside the tank, without any contact with the liquid or gases inside the tank, thus achieving true isolated measurement. 1. It features a wide measurement range of up to 30 meters, as well as high measurement accuracy of ±0.2% of the set full scale. 2. It can be used in the most demanding environments: it is capable of measuring liquids at any pressure. The liquid with the most severe measurable toxicity. It can measure the most corrosive liquids. It can measure liquids that require sterility or high purity. It can measure flammable, explosive, leak-prone, and polluting liquids. 3. Safety: When measuring liquids that are toxic, corrosive, under pressure, flammable, explosive, volatile, or prone to leakage, since both the measuring probe and the instrument are located outside the container, there is no contact with the liquids and gases inside the tank during installation, repair, or maintenance, making it very safe. Even when the instrument is damaged or under maintenance, there is absolutely no possibility of leakage. 4. Environmentally friendly: It does not leak any liquids or gases, thus never polluting the environment; it is an eco-friendly and green instrument. 5. Convenient and cost-effective: During installation, there is no need to drill holes in the container; no flanges or connecting pipes are required. There’s also no need for welding. Installation and calibration can be carried out at any time without shutting down production. All that’s needed is to attach the measuring head to the outer wall of the container using a magnetic holder or adhesive specifically designed for this purpose. After a simple wiring process, measurement can be performed. This makes installation and maintenance extremely convenient and cost-effective. It can also perform parameter calibration automatically and calculate the temperature compensation factor automatically; regardless of changes in the ambient temperature, liquid temperature, or the composition of the liquid being measured, the instrument maintains high measurement accuracy at all times. 6. Durable and reliable: The measuring head and instrument have no mechanical moving parts, and are tightly sealed to be isolated from the outside environment. It does not wear out or corrode, making it very durable and reliable. The maintenance workload is very small. 7. Precision: The ELL external level gauge continuously calibrates itself automatically, always ensuring the highest measurement accuracy. V. Application Examples: In July 2005, in order to ensure its effectiveness, after careful consideration, we decided to first test the external level gauge developed by Xi’an Dinghua Electronics in one of the liquid chlorine storage tanks, in order to assess its performance in actual use. If it met the technical requirements regarding safety, environmental protection, accuracy, and stability, we would implement it across all the liquid chlorine storage areas. In August 2005, after a 24V DC power supply was provided on site, the manufacturer carried out an on-site installation. Without affecting the normal operation of the equipment, it took only half a day for the instrument to be installed and put into regular use. After six months of data testing, the instrument has shown stable performance and accurate measurements, fully meeting the requirements for level measurement and control. After practical evaluation, and taking into account the process conditions for measuring the liquid level in chlorine storage tanks, we ultimately selected four ELL-FI-TE03M0.5 temperature-compensated, intrinsically safe external level gauges as the instruments for measuring the liquid level in those tanks. The technical parameters of this type of meter are as follows: measurement range: 0.2–3 meters ； Accuracy class: 0.5% F.S. ； Explosion protection rating: Exd IIBT6 ； Protection rating: WF1, IP65 ； External leads: four-wire system ； Power supply: 24V DC / 10W ； Analog output: Isolated 4–20mA standard signal ； Operating temperature of the instrument: -30°C to 55°C, which fully meets the process requirements for liquid chlorine tank level measurement. When installing this instrument, it is sufficient to place the level measurement probe at the bottom of the container; no holes need to be made in the container. This allows for accurate measurement of the liquid level of the medium inside the container. As a result, the risk of liquid chlorine leakage associated with other level gauges that require holes to be drilled in the tank is avoided, and potential hazards to human health that could arise during maintenance of such instruments are eliminated. This provides a solid guarantee for the company’s safe production and environmental protection. Conclusion: Over the past three years since its installation on chlorine storage tanks, the ELL external level gauge has operated stably and provided accurate measurements, offering a reliable basis for the process operations carried out at these sites. It helps to minimize contact between harmful substances and the instruments as well as maintenance personnel, thereby providing strong protection for the company’s safety, environmental protection, and occupational health.