Abstract: This paper analyzes and introduces the possible selection of measurement schemes for Siemens' radar level measurement technology applied to the level measurement of various common oil products and liquefied petroleum gases. Keywords: Radar level measurement, oil tank, dielectric constant, liquefied petroleum gas. I. Introduction Storage tanks are storage equipment commonly used in oil refineries, oil depots, oil terminals, and major petrochemical enterprises. By measuring parameters such as temperature, pressure, level, and density of the liquid medium (various petrochemical products) in the storage tank, the volume and mass of the liquid in the tank can be calculated to meet the asset management needs of modern production. The requirements for level measurement of storage tanks vary and can generally be divided into process-grade and trade-grade. Trade-grade level gauges have very high accuracy requirements, requiring precise measurement of the contents of the tank, generally requiring a measurement accuracy of ±1mm; while process-grade level gauges generally have an accuracy requirement between ±5 and 15mm, with more stringent requirements for the reliability and stability of the level gauge measurement. The purpose of the measurement is generally to prevent accidents such as tank overflow and vacuuming. Various storage tanks vary in size and type, and the characteristics and process conditions of the measured media also differ. Therefore, only by starting from the customer's actual situation and rationally selecting the level gauge and designing the level measurement solution can the customer's investment be optimized. II. Storage Tank Level Measurement Technology Currently, there are many level measurement technologies used for storage tanks. Based on the measurement method, they can be divided into contact and non-contact methods. Contact measurement methods mainly include manual inspection rulers, float-type steel strip level gauges, servo level gauges, hydrostatic level gauges, magnetostrictive level gauges, and radio frequency admittance level gauges. Non-contact measurement methods mainly include ultrasonic level gauges and radar level gauges. Among these, the measurement methods commonly used in the market that meet the requirements of trade-grade level gauges include: magnetostrictive level gauges, servo level gauges, radar level gauges, and hydrostatic level gauges. Hydrostatic level measurement systems (HTG) are becoming increasingly less used due to their excessive complexity and high maintenance requirements. Radar level gauges, as a high-precision liquid level measurement method, are increasingly recognized and accepted by customers due to their advantages such as large measuring range, convenient installation, and low maintenance. Furthermore, in recent years, the price of radar level gauges has decreased year by year, leading many ordinary users to adopt them as a liquid level measurement solution for storage tanks. [align=center]Table 1. Comparison of Various Storage Tank Liquid Level Measurement Methods[/align][/align] III. Radar Liquid Level Measurement Principle and Characteristics Radar level gauges, also known as microwave level gauges (Radar), are an abbreviation derived from the initials of the English phrase "RAdio Detection And Ranging." A microwave level gauge emits electromagnetic waves towards a target, and the electromagnetic waves return to the source. A receiver installed at the source captures the reflected wave and compares it with the emitted wave to determine the presence of the target and its distance from the source. Currently, the most common working principles used in microwave level gauges on the market are FMCW (Continuous Frequency Modulation) and pulse. FMCW radar level gauges use linearly modulated high-frequency signals, typically 10GHz or 24GHz microwave signals. It is an indirect measurement method based on complex mathematical formulas, calculating the level distance from the spectrum. The antenna emits a linearly modulated continuous high-frequency microwave signal and scans the surface, while simultaneously receiving the return signal. The frequency difference between the emitted and returned microwave signals is proportional to the distance to the medium surface. Pulse radar level gauges, similar to ultrasonic technology, use the time difference principle to calculate the distance to the medium surface. The device transmits pulses at a fixed frequency, then receives and establishes an echo pattern. The signal propagation time is directly proportional to the distance to the medium. However, unlike ultrasound which uses sound waves, radar uses electromagnetic waves. It uses tens of thousands of pulses to "scan" the container and obtain a complete echo pattern. Typical frequency bands for radar level gauges are 5.8 GHz, 10 GHz, and 24 GHz. We usually refer to the 5.8 GHz (or 6.3 GHz) frequency as C-band microwave; the 10 GHz frequency as X-band microwave; and the 24 GHz (or 26 GHz) frequency as K-band microwave. The gain coefficient and beam angle of a radar level gauge are related to the wavelength of the microwave and the size of the gauge's horn. Therefore, an increasing number of radar level gauge manufacturers are developing high-frequency microwave technology to improve performance. Furthermore, using high-frequency technology allows for significant reduction in antenna size while improving radar performance, making installation easier. Since the microwaves used in radar level gauges are electromagnetic waves, they do not require a transmission medium during propagation, thus largely eliminating the need to consider the effects of volatile gases and vapors, temperature, pressure (vacuum), or even dust. Moreover, based on the time-of-flight (TOF) principle, radar level gauges are not subject to wear and tear, requiring virtually no maintenance. Top-mounting also simplifies installation. Due to the characteristics of microwaves, their accuracy typically reaches 0.1% (full range) compared to other level measurement methods. Because of these unparalleled advantages, more and more storage tank users are adopting radar level gauges to replace other technologies. IV. Solutions Since launching its first intelligent radar level gauge, the IQ160, in 1998, Siemens has successively introduced radar level gauges based on various working principles and using different frequencies, forming a relatively complete product line capable of meeting the measurement needs of various processes and media. The Sitrans LR series radar currently includes four product series: Probe LR, LR200, LR300, and LR400. The Probe LR is a radar level gauge with a very high cost-performance ratio. It is a two-wire loop-powered, 5.8GHz frequency radar level gauge used to measure the level and volume of liquids and slurries in storage tanks and process tanks. The Probe LR is ideal for conditions such as volatile chemicals, temperature gradients, vacuum, or atmospheric pressure, such as oil depots, chemical storage tanks, chemical reaction vessels, and long-range applications. Its measuring range is 20 m, and its accuracy is 0.1% of the total weighing volume (as shown in Figure 1). The LR200, while retaining all the advantages of the Probe LR, also offers various mounting options (including flange and threaded options) and antenna choices (including horn antennas, rod antennas, waveguide antennas, etc.), greatly meeting the needs of different process technologies. Both the Probe LR and LR200 utilize an integrated, fully enclosed instrument housing, providing excellent sealing, shock resistance, and corrosion resistance. The SITRANS LR 400 is a long-range, continuously frequency modulated (FMCW) radar level gauge suitable for measuring tanks up to 50 meters high. It also provides excellent measurement results even when measuring media with low dielectric constants (light oil, liquefied gas, etc.). Its 24 GHz frequency and high signal-to-noise ratio characteristics result in exceptional signal reflection, independent of the dielectric constant of the medium. It also features advanced echo processing algorithms, providing reliable measurements for harsh solid-state applications. Its measurement accuracy is ±5 mm (as shown in Figure 2). Based on the characteristics of the storage tank medium, the measurements of various storage tanks are divided into five categories: light oil and crude oil, heavy oil, asphalt, liquefied gas, and corrosive media. 1. Measurement of light oil: Typical light oil media include gasoline, naphtha, and petroleum. They generally produce some volatile gases and have a low dielectric constant (<3). Low-frequency radar (Probe LR/LR200 + waveguide). Direct measurement with a low-frequency (6GHz) radar level gauge may result in signal loss due to its weak signal; therefore, a waveguide is generally required. For example, the Sitrans LR200 with a waveguide configuration can meet the requirements for light oil level measurement (as shown in Figure 3). The waveguide's manufacturing requirements are: metal material (preferably 304 or 316 stainless steel); smooth inner wall with no obvious weld seams (seamless welding or sleeve welding is required); vent holes should be opened in appropriate locations, generally at the top of the waveguide, with the vent area determined by the medium's characteristics (viscosity); if necessary, an offset plate should be installed at the bottom of the tube to prevent erroneous signals from the bottom due to microwave signal penetration of the liquid surface. This solution is generally used in applications where the tank height is less than 5-10m. High-frequency radar (Sitrans...) LR400). Using a high-frequency radar level gauge, no auxiliary measures are needed (high-frequency radar is generally not recommended for waveguides due to its characteristics), it can measure the level of light oil. For some taller storage tanks, the cost of manufacturing and installing waveguides is high, and many long waveguides may deform under environmental changes, affecting the measurement effect and accuracy of the radar level gauge. Therefore, choosing the high-frequency Sitrans LR400 for measuring large-range light oil storage tanks will be more economical than a low-frequency radar solution. 2. Heavy oil measurement: Typical heavy oil media include diesel, whose dielectric constant is generally >3 and contains some volatile gases. Low-frequency Probe LR or LR200 can be used for direct measurement (as shown in Figure 4). 3. Asphalt measurement: Asphalt (liquid) also has a relatively low dielectric constant, generally around 3 (depending on temperature); moreover, liquid asphalt generally requires high temperatures to maintain, which usually involves some water vapor; asphalt is relatively viscous and easily causes some adhesion. In high-temperature (>100ºC) conditions, PTFE antennas are generally required, while PPS antennas (such as Probe LR) cannot be used. To prevent the influence of material buildup, horn-mouth antennas can be selected. However, water vapor generally leads to condensation. Experiments show that rod antennas are less sensitive to condensation than horn-mouth antennas because the effective electromagnetic wave transmission area of ​​a rod antenna is much larger than that of a horn antenna (the transmitting electrode is only the size of a pen tip). Asphalt has a low dielectric constant, and the small amount of material buildup generated by the antenna can generally be penetrated by the electromagnetic waves of the radar level gauge. Because the material buildup in asphalt is quite viscous, the effect of using a cleaning device is not very significant. Therefore, the author does not recommend using a cleaning device for measuring asphalt. Therefore, for asphalt applications where mixing and material adhering within the storage tank are very serious, an LR200 antenna with a flared mouth can be used, and the size of the flared mouth should not be less than DN150 (6); while for processes with higher temperatures and more obvious condensation, a full PTFE rod antenna is preferred: PTFE antennas with stainless steel shielding sections have a better shielding effect on installation risers with smaller diameters, but if the shielding length is too short, it cannot be extended; the shielding section length of a full PTFE antenna can be freely and conveniently extended according to the length of the installation riser, but it will affect the antenna's operation in installation risers with small diameters (≤DN80). Therefore, it is best to specify the diameter and length of the installation riser when installing a radar level gauge. 4. Measurement of liquefied gas Liquefied gas includes liquefied natural gas (LNG) and liquefied petroleum gas (LPG). Typical liquefied gas media include propane, butane, etc. The main characteristics are: generally very low dielectric constant (1.5-1.8); high-pressure and low-temperature storage; explosion-proof requirements; usually stored in spherical tanks, with tank heights generally ranging from 10 to 28 meters. Currently, there are two main methods for measuring liquefied gas (LPG) using radar level gauges. One method uses a bypass pipe and a low-frequency radar (Sitrans LR200) ​​for measurement. Because this method has very high requirements for the bypass pipe's manufacturing, poor quality bypass pipes can easily lead to poor performance; even in some large-range spherical tank applications, excessively long bypass pipes can easily become twisted, making the radar level gauge unable to measure. Solutions using the LR200 mounted on a bypass pipe or waveguide for LPG measurement have few successful cases due to various reasons. The other method utilizes the good transmission characteristics of high-frequency radar to low-dielectric-constant media to directly measure the LPG level. The LR400 can directly measure media with dielectric constants as low as 1.5, meeting the requirements for liquefied gas measurements. The installation location can be on top of a spherical tank, with a direct opening at the top. For ease of maintenance, the LR400 is generally installed on a riser with a ball valve (DN>150 recommended) for easy removal and maintenance; alternatively, it can be installed on a larger bypass pipe (diameter 6-10” or higher). The Sitrans LR400 has been used in the Dushanzi Petrochemical Refinery in China with excellent results. Field tests have shown that the LR400 achieves very good measurement accuracy and repeatability. 5. Measurement of Corrosive Media Typical corrosive media include: sulfuric acid, nitric acid, caustic soda, hydrochloric acid, etc. Most corrosive media have high dielectric constants and are accompanied by varying degrees of volatile gases (fumes). Different concentrations of corrosive media have different corrosiveness requirements for different radar antenna materials. Many corrosive media' volatile gases have a certain penetrating ability, and after a period of use, they may penetrate into the instrument's electronic components or the antenna's motor transmitter, damaging the radar level gauge. The choice of which radar antenna to use for measuring corrosive media generally needs to be confirmed beforehand by consulting a chemical compatibility table. [align=center]Table 2: Compatibility of various radar level gauges with common corrosive media[/align] Considering that the volatile gases of many corrosive media also have a certain degree of corrosivity and penetrability, an integrated flange gasket option is generally chosen to prevent volatile gases from corroding the flange and penetrating through the flange connection. In addition, many acid and alkali storage tanks produce a lot of water vapor and foam. This requires appropriate selection based on the actual situation: for applications with severe condensation from water vapor, a rod antenna can be considered; the thickness and composition of the foam play a crucial role in whether microwaves can penetrate the foam for accurate level measurement, requiring specific analysis and further experience. V. Conclusion Through the research on the application of radar level gauges in the petrochemical field in recent years, and Siemens' continuous research and development of new products and technologies, the application of Siemens radar level gauges in various storage tanks has become increasingly mature, with more and more successful cases. The author believes that with the development and maturation of radar technology, radar level gauges will gradually become the mainstream for tank level measurement. References: 1. Siemens (China) Co., Ltd., Level Measurement Technology, 2005, 1 2. Wang Xinsheng. Oil Tank Level Measurement Based on Microwave Method. Automation Instrumentation. 2001, 6, 27-30 3. Huang Bo. Measurement Experiment of Siemens Radar Level Gauge in Complex Working Conditions. Chemical Automation and Instrumentation. August 2005, No. 4, P80-81, Article Number: 100-3932 (2005) (04)-0080-02 4. Ji Xiaobo, Tu Yaqing, Ren Kaichun, Zhang Haitao. 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