A Brief Discussion on the Automated Design of Tank Areas and Instrument Selection

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The commonly used level measurement instruments for oil storage tanks at home and abroad include； Float-type level gauges, differential pressure level gauges, magnetostrictive level gauges, servo-level gauges, ultrasonic level gauges, radar level gauges, and so on. Based on different measurement principles and methods, each measuring instrument has its own advantages and suitable application conditions. The development trend of liquid level instruments in tank farms is toward higher precision, more functionality, and greater automation. Moreover, since some tank farms are responsible for supplying materials to production facilities, a high level of measurement accuracy and automation is essential. 1 Tank area measuring instruments: These instruments are primarily used for measuring the product as it enters and leaves the tank. Commonly used flow meters include mass flow meters, turbine flow meters, twin-rotor flow meters, gear flow meters, and scraper flow meters. Since measuring instruments are directly related to cost accounting and economic efficiency, it is necessary to ensure measurement accuracy, while also taking into account stability and cost-performance ratio. Mass flow meters offer relatively better accuracy and performance metrics; although the initial investment is high, they are capable of measuring parameters such as the density, temperature, and pressure of the fluid ； Other types of flow meters, such as positive-displacement or velocity-type meters, can be selected based on actual needs in practical applications; turbine flow meters should be used with caution in situations involving high viscosity, corrosivity, or toxicity. 2 Tank area level instruments: For tanks at atmospheric pressure, a single-flange level transmitter can be used ； For pressure vessels, a double-flange level transmitter can be used for level measurement; this is the simplest, most practical, and cost-effective method. However, this approach requires calculating the level gauge drift, which is rather cumbersome. It is not very suitable for spherical tanks and large storage tanks, especially for measuring the liquid level (level of material) of media at high temperatures, as well as media that are subject to stirring, have high viscosity, or are toxic or corrosive. 2.1 External level gauge: The external level gauge is a new type of instrument for measuring liquid levels that has been independently developed in China in recent years. The ELL series invented and produced by Xi’an Dinghua Electronics Co., Ltd. The measuring probe of the external level gauge is directly attached to the outer wall of the container, and the characteristics of the detected minor mechanical vibration waves are determined by the vibration characteristics of the liquid level and those of the container. According to the computational relationships, the vibration signals detected by the sensitive elements of the measuring probe are converted into electrical signals and transmitted to the main unit. The main unit filters and evaluates the various types of vibrations collected, and after complex calculations, obtains the liquid level measurement value. To ensure higher accuracy in liquid level measurement, temperature compensation for the liquid level measurement can also be applied. 2.2 Radar Level Gauge (1) Principle of Radar Level Gauge The basic operating principle of a radar level gauge is transmission, reflection, and reception. The antenna of the radar sensor emits electromagnetic wave signals in the form of beams; the emitted waves are reflected by the surface of the material being measured, and the reflected echo signals are once again received by the antenna. Each point in the transmitted and reflected beams is acquired using ultrasonic sampling. After being processed by the intelligent processor, the signal determines the distance between the medium and the probe, which is then sent to the terminal display for display, alarm generation, operation, etc. Radar level gauges transmit and receive extremely short microwave pulses with very low energy via an antenna system. Radar waves travel at the speed of light. Operating time can be converted into a level signal through electronic components. A special time extension method can ensure stable and accurate measurements in an extremely short period of time. Even in complex operating conditions where false echoes exist, the latest microprocessing technology and debugging software can still accurately analyze the echoes corresponding to the liquid level. Current radar level gauges have two measurement methods: one is the frequency-modulated continuous wave type, and the other is the pulse type. Frequency-modulated continuous waves perform signal analysis in the frequency domain, while pulsed signals perform signal analysis in the time domain. Frequency-modulated continuous-wave systems have high precision, while pulsed systems have low precision. The widespread use of radar level gauges in the petrochemical industry in recent years is closely linked to the rapid development of radar measurement techniques. Radar level gauges that employ bottom-tracking technology are able to accurately detect the liquid level of media with lower dielectric constants. As is well known, the most important measurement parameter when selecting a radar level gauge is the dielectric constant. (2) Features of radar level gauges: When radar level gauges were first introduced to the market, they were not accepted by many people due to their high price. However, as people come to understand it better and its price decreases, it has gained favor with an increasing number of customers. Measuring the liquid level in tanks using radar instruments has the following advantages: a. Neither the transmitting nor the receiving antenna comes into contact with the medium ； b. High-frequency electromagnetic wave signals can be transmitted over long distances, allowing for measurement over a wide range ； c. The measurement is not affected by changes in the gas-phase conditions in the space above the liquid surface ； d. Radars with meter-level precision can be used for measurement. Radar measures liquid levels by emitting and receiving high-frequency (GHz range) electromagnetic energy, and by calculating the time it takes for the electromagnetic waves to reach the liquid surface and reflect back to the receiving antenna. It can propagate in the absence of air (vacuum) or in the presence of gaseous media, and fluctuations in gas pressure do not affect the speed of propagation of electromagnetic waves. Currently, radar level gauges are divided into three types: conventional space-transmitting radar level gauges powered by AC or DC, the new type of space-transmitting radar level gauge powered by 24VDC in a two-wire system, and guided-wave radar level gauges. Under normal operating conditions, both AC/DC power supply and two-wire radar level gauges can provide accurate measurement values. However, under special conditions such as a very low dielectric constant of the medium being measured, fluctuations and bubbles on the liquid surface, or the presence of obstacles, guided wave radar level gauges are required. Its basic principle is the same as that of a radar level gauge; the slight difference is that it takes advantage of the different electrical conductivity of guided waves in gases versus liquids to generate a signal corresponding to the liquid level. Additionally, there is a preset impedance at the top of the probe, which facilitates the generation of a reliable basic pulse, known as the baseline reflection pulse. The guided-wave radar level gauge detects the original pulse reflected by the liquid level and compares it with the baseline reflection pulse generated at the same time, thereby calculating the liquid level height of the medium. Furthermore, by taking advantage of the difference in the strength of the reflected waves from high-conductivity and low-conductivity media, it is possible to measure the interface between two liquids, provided that the dielectric constant of the liquid beneath the interface is much higher than that of the liquid on the interface. Currently, the price of two-wire radar level gauges is almost the same as that of ordinary level transmitters. As the measurement technology of radar level gauges continues to mature, the range of products suited for various operating conditions has also become increasingly comprehensive. Currently, in terms of liquid level measurement in domestic tank farms, radar level gauges are being utilized more and more across diverse operating conditions, thanks to their unparalleled advantages and performance. (3) Applications of radar level gauges: Radar levels can be classified according to the type of antenna ； Pyramidal antennas, parabolic antennas, rod antennas. http://www.dhechina.com/ueditor/php/upload/image/20220704/1656918793528796.png Cone antennas are suitable for installation on tank tops or waveguides, and are appropriate for applications that require a large measurement range. Parabolic antennas are suitable for measurement applications involving both liquids and solids. Because it has the largest diameter, it has the narrowest radar beam and can be used for measurements over extremely long distances. Parabolic antennas are also the type of antenna that is least sensitive to pollution. It can be used in application conditions with harsh antenna adhesives such as asphalt and liquid sulfur. The rod antenna is small in size, with only the polytetrafluoroethylene material exposed to the atmosphere inside the storage tank. Rod antennas are suitable for measurement applications involving hygiene requirements or corrosive chemical substances. For meter-grade high-precision radar level gauges, the measurement principle is based on Frequency-Modulated Continuous Wave (FMCW) technology; parabolic antennas are used for storage tanks with high viscosity and tendency to condense, such as those holding fuel oil and asphalt, while cone antennas are used for other types of tanks. In internally floating roof tanks, waveguides are designed to enable accurate measurement of the tank’s liquid level. And an on-site indicator is installed on the ground to display the liquid level. 3 Project Overview: The liquid caustic soda tank farm consists of 4 liquid caustic soda storage tanks. The liquid caustic soda from train tank cars and road tank cars is respectively pumped using caustic pumps to liquid caustic soda storage tanks for storage ； The stored liquid caustic soda is pumped by caustic pumps to the wharf for ship loading and to tank trucks for filling. 3.1 System Composition The automatic control system for tank farms differs from that of conventional production facilities. Firstly, the process flow in tank farms is intermittent; secondly, the system lacks control and regulation mechanisms; thirdly, the system does not have particularly high requirements regarding real-time performance. Since data acquisition in the automatic control system occurs continuously, reliability is the key factor determining the overall performance of the system. Based on the existing automatic control system model and network architecture of the port, the liquid caustic soda tank farm utilizes PLC field I/O stations. These stations are connected via dedicated communication cables to the corresponding operator stations in the central control room. Each storage tank is equipped with a radar level gauge and temperature sensors. The signals from the radar level gauge and the temperature sensors are combined via the bus of the radar electronics unit and sent through the junction box to one input port of the field communication unit located in the I/O station. The FCU has two output ports: one for sending signals to the PLC and another for sending signals to the customs (CIO) monitoring system; the signals from other conventional instruments are sent directly to the PLC. 3.2 Instrument Selection: Ball tanks are pressure vessels that store flammable and explosive materials; the composition of these materials is not fixed, which leads to changes in density. They also contain corrosive gases such as H2S, and operate over long periods of time. This requires that the instruments used be explosion-proof, highly reliable, require minimal maintenance, and be corrosion-resistant; preferably, they should be non-contact instruments. The selection of level measurement instruments, level switches, and emergency shut-off valves for spherical tanks is rather special, and a detailed explanation is provided here; for other instruments, conventional selection methods can be applied. (1) Level measuring instruments: Radar level gauges and external level gauges were used as level measuring instruments. After several years of operation, it has been proven that both types of instruments are suitable for measuring the liquid level in spherical tanks. (2) External level gauge: The fiber optic level gauge consists of a main unit, sensors, communication cables, explosion-proof junction boxes, etc. The vibration signals detected by the sensitive elements of the measuring probe are converted into electrical signals and transmitted to the main unit. The main unit filters and analyzes the various types of vibrations it receives; through complex calculations, it determines the liquid level measurement value and outputs a standard current signal to the PLC. Alternatively, it can be connected to the PLC via an RS-232 interface. (3) Radar level gauge: The principle behind using a radar level gauge to measure liquid levels is that an antenna emits high-frequency electromagnetic waves toward the liquid surface; these waves are reflected back when they hit the surface, and then captured by the antenna. The time difference between the emitted waves and the reflected waves is measured, and through an intelligent signal processor, this time period – which is proportional to the distance from the liquid surface – is converted into a standard 4-20mA signal, which is then sent to the PLC. 3.3 The two operation stations of the PLC used for controlling the instruments in the control room are located in the central control room of the tank farm. Dedicated communication cables are used to establish communication between the PLC control stations and the central control operation stations, thereby enabling centralized monitoring of the tank farm. 3.4 Safety detection systems: In flammable and explosive hazardous environments, combustible gas detectors and fire alarms are essential. At the same time, during design, proper placement of local manual alarm buttons and alarm panels in the control room should be considered to ensure rapid response in case of an emergency. If conditions permit, industrial TV monitoring systems should be installed in some key locations. 3.5 Computer Control Systems There are many design approaches for tank farm control systems, but each approach has its own suitability. In line with the requirements of low cost, high efficiency, and standardization, PLC systems can be installed in some large storage tank areas, making use of tank cluster control systems ； For some smaller tank farms, intelligent instruments in the control room can be used for control; in some cases, these tank farms are combined with loading docks and fuel delivery systems to form more sophisticated control systems.