Analysis of liquid ammonia storage tank level control methods

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Authors: Li Qi, Zhang Yujie (**Instrument and Meter Components Quality Supervision and Inspection Center). When reproducing this article, please acknowledge the copyright of the authors. Abstract: Liquid ammonia is an important chemical raw material with high hazards; therefore, it is necessary to control the liquid level in liquid ammonia storage tanks. The magnetic flap level gauge features a simple structure and easy viewing, and is widely used in industries such as petroleum, chemicals, and papermaking. For its remote control, two methods can be employed: the resistive sensor method and the differential pressure transmitter method. The text analyzes these two methods and presents application examples, providing a reference for selecting and applying liquid ammonia storage tank level control methods. Introduction: Liquid ammonia is a colorless and transparent liquid that is flammable, toxic, and has a strong pungent odor; it poses significant hazards to human health. Mixing with air can form an explosive mixture. It can catch fire and explode in the presence of open flames or high heat. The density of liquid ammonia decreases as the temperature rises; if liquid ammonia is exposed to heat, the pressure inside the container increases, posing a risk of cracking and explosion. Therefore, monitoring the liquid level in the liquid ammonia storage tank is very important. Low concentrations of ammonia irritate the mucosa, while high concentrations can cause tissue dissolution and necrosis. Properties of liquid ammonia: Liquid ammonia is an important chemical raw material, widely used in industries such as petroleum, chemicals, and papermaking. For ease of transportation and storage, gaseous ammonia is typically converted into liquid ammonia by pressurization or cooling, and stored in pressure-resistant steel cylinders or tanks. Cylinders or tank trucks used for canned transportation, as well as storage tanks for storage, etc., shall comply with relevant regulations such as the \"Regulations on Safety Supervision of Gas Cylinders\" and the \"Regulations on Safety Technical Supervision of Fixed Pressure Vessels\". The allowable weight filling factor is 0.52 kg/L. Since liquid ammonia is corrosive and volatile, accidents are likely to occur; therefore, liquid ammonia cylinders should be stored in a warehouse or on a covered platform. When stored outdoors, it should be covered with a tent to prevent direct sunlight. The liquid ammonia level in the storage tank must be closely monitored. Level control of 2-liquid ammonia storage tanks 2.1 Control methods The magnetic flap level gauge is a device specifically developed for level measurement, based on principles such as magnetic pole coupling and Archimedes’ law (law of buoyancy), along with the characteristics of mechanical transmission. This device features a simple structure, intuitive and clear viewing, no clogging or leakage, easy installation, and simple maintenance. It is currently widely used in industrial fields such as petroleum, chemicals, papermaking, and sewage treatment. To achieve remote monitoring, liquid level transmitters are added in accordance with relevant standards and requirements to output various electrical signals, thereby enabling remote measurement and monitoring. Among them, the 4–20 mA current signal is a commonly used type. While monitoring the liquid level on-site, the changes in liquid level are transmitted to the control room via transmitters, sensors, cables, and instruments, enabling remote monitoring and control. Resistive sensors and differential pressure transmitters are currently commonly used devices for liquid level measurement and control. Due to the different principles behind these two types of devices, it is necessary to select the appropriate measurement and monitoring methods based on the properties of the medium being measured and the environment in which it is located, in order to achieve the goals of measurement and monitoring. 2.2 Structure and Principle of the Remote Transmitted Magnetic Flip-plate Level Gauge The magnetic flip-plate level gauge is an instrument that measures liquid levels and provides on-site display based on the principles of buoyancy and magnetic coupling. As the liquid level in the container under test rises or falls, the magnetic float in the level gauge’s measuring tube rises or falls as well. Magnetic floats are manufactured based on the set density of the detected medium (as shown in Figure 1). The magnetic position plane of the float is always level with the surface of the medium liquid. The permanent magnet inside the float transmits a signal to the on-site indicator through magnetic coupling, causing the red and white flip columns to rotate 180 degrees. When the liquid level rises, the flip column changes from white to red; when the liquid level drops, it changes from red to white. The red and white boundaries of the indicator indicate the actual height of the liquid level in the container, thereby providing an on-site indication of the liquid level. http://www.huoyumi.com/d/file/zhishi/2016-10-10/7d611444d0f281c58c17f3ba94893a87.gif 2.2.1 Remote level control using resistive sensors (resistive type) A resistive or reed switch transmitter is composed of a resistive sensor (as shown in Figure 2), an R/I conversion module, and a protective cover; by combining a Sanchang magnetic flap level gauge with a remote level transmission device, a remote magnetic flap level gauge is formed (as shown in Figure 3). The resistance sensor of this transmitter is fixed in a bundled manner on the outside of the main tube of the Sanchang magnetic flap level gauge, placing it within the same magnetic coupling system as the level gauge. In a remote-transmission magnetic flap level gauge, as the magnetic float moves up and down with the liquid level, the reed switch located at the corresponding liquid level is attracted by the magnetic field generated by the float, causing a change in the resistance value of the resistance chain. The conversion module then transforms this changing resistance signal into a 4–20 mA DC standard signal, which can be easily used in conjunction with digital display instruments to achieve functions such as display, control, adjustment, and alarm. http://www.huoyumi.com/d/file/zhishi/2016-10-10/b79c7d9569e90c82826616ed662b4540.jpg 2.2.2 Using differential pressure transmitters (differential type) for remote level control: The measuring element of differential pressure transmitters usually adopts a differential capacitance structure (as shown in Figure 4). http://www.huoyumi.com/d/file/zhishi/2016-10-10/1273a4778ed7b77db465e12439f19b98.gif A remote-controlled magnetic flap level gauge is formed by combining a magnetic flap level gauge with a differential pressure transmitter, a fixing plate, and pipes for connecting positive and negative pressures; the fixing plate is fixed to the lower end of the main pipe. The positive pressure connection pipe is connected to both the lower flange connection pipe of the Sanchang magnetic flap level gauge and the positive pressure port of the differential pressure transmitter, while the negative pressure connection pipe is connected to both the upper flange connection pipe of the magnetic flap level gauge and the negative pressure port of the differential pressure transmitter (as shown in Figure 5). Based on the principle that hydrostatic pressure is proportional to height, the static pressure is converted into an electrical signal through the piezoresistive effect of sensitive elements. After temperature compensation and linear correction, it is transformed into a standard 4–20 mA DC current signal for output, thereby enabling display, control, adjustment, and alarm functions. http://www.huoyumi.com/d/file/zhishi/2016-10-10/27f5cc8a89d75fc10a01685a814f5903.jpg Analysis of two liquid ammonia storage tank level control methods 3.1 Common features of resistive control and differential pressure control: Local display requires no power supply; the display section is completely isolated from the medium, so contamination of the display strip by the medium does not affect readings. The switch point can be adjusted according to user requirements, and installation is straightforward. The level transmitter features a 2-wire 4–20 mA DC standard signal output, enabling remote monitoring or control. Low maintenance requirements and low repair costs. It is suitable for high-temperature and high-pressure measurements; in particular, the Sanchang magnetic flap level gauge can be used to detect the liquid level of media in various devices such as towers, tanks, vats, spherical vessels, and boilers. It enables high-level sealing and zero leakage, as well as level measurement in high-temperature and corrosive environments. It is safe and reliable, offers no blind spots throughout the measurement process, provides intuitive readings, and has a wide measurement range. Especially in the field indication section, since it does not come into direct contact with liquid media, it has advantages when dealing with high-temperature, toxic, harmful, and highly corrosive media. 3.2 Differences between resistive control and differential pressure control 3.2.1 Resistive control: The magnetic float moves up and down with the liquid level; on-site, the flip column is activated through magnetic coupling, while the remote signal is generated by the engagement of reed switches, thereby changing the resistance value of the resistance chain. The corresponding liquid level positions transmit on-site indication signals and remote transmission signals respectively. If the density of the container medium increases relative to the set density, the actual liquid level will be lower than the on-site indicated liquid level and the remote transmission signal liquid level (the indicated liquid level and the remote transmission signal liquid level are at the same height and synchronized). As the medium density decreases, the actual liquid level is higher than the on-site indicated liquid level and the remote signal liquid level (the indicated liquid level and the remote signal liquid level are at the same height and in sync). When the density difference remains constant, the indication and remote transmission errors are fixed, independent of the liquid level height. 3.2.2 Differential pressure control: The remote signal is determined by the difference between the pressure at the bottom of the liquid medium inside the container and the pressure at the surface of the liquid medium (the set density is the same as that of the float). If the density of the medium in the container increases relative to the set density, the actual liquid level is lower than the remote signal liquid level (the indicator level and the remote signal level are not at the same height). As the medium density decreases, the actual liquid level is higher than the remote signal liquid level (the indicator level and the remote signal level are not at the same height). When the density difference remains constant, the error in the remote transmission signal is not fixed. The difference increases proportionally as the liquid level rises from low to high. If the magnetic float fails (the float gets stuck or is damaged), the on-site indication will stop working, but the remote transmission signal can still be used. The differential pressure transmitter is malfunctioning, but the on-site indicator still works, which does not affect monitoring and enables dual-dimensional monitoring. 3.3 Application Example: The actual operating temperature range of a certain liquid ammonia storage tank is between -50 and 20°C; the density of the liquid ammonia inside it varies from 702.15 to 610.28 kg/m3, with a measurement height of 3,000 mm. The reference density is set at 652.02 kg/m3 at -10°C; the total length of the magnetic float is 360 mm, with the magnet located at 310 mm. The storage tank is filled with liquid ammonia with a density of 702.15 kg/m3. The actual liquid level is at 2000 mm, while the on-site indicator shows 2020 mm; the resistance-based signal also indicates 2020 mm, whereas the differential pressure-based signal shows 2154 mm. The actual liquid level is 500 mm, the on-site indication is 520 mm, the resistive signal shows 520 mm, and the differential pressure signal indicates 538 mm: The test results are shown in Table 1. http://www.huoyumi.com/d/file/zhishi/2016-10-10/f5bd751eec69683fe0359364471b5894.gif As can be seen from Table 1, there is a certain discrepancy between the level reading provided by differential pressure control and that shown by the Sanchang magnetic flap level gauge on site; this discrepancy is mainly caused by changes in temperature, which lead to variations in the density of liquid ammonia and thus to changes in its level. What a differential pressure transmitter measures is the pressure difference. Since oil storage tanks are generally cylindrical, the area S of their circular cross-section remains constant; therefore, the discharge volume is proportional to the height Ah. As the temperature changes, the density of liquid ammonia changes as well; the volume of liquid ammonia expands or contracts, which causes the actual liquid level to rise or fall. If it is necessary to display the actual liquid level, medium temperature compensation can also be introduced to address this issue. As can be seen from Table 1, A/B is relatively close to C; therefore, for temperature compensation of the liquid level, the ratio of densities can be used as a rough correction. For more accurate corrections, fitting the density variation curve can be employed. It can also be seen from Table 1 that the resistance-based control provides a level indication that is consistent with that of the on-site magnetic flap level gauge. This is because both the magnetic float and the sensor resistance move together, undergoing identical changes, which results in identical indications. From this perspective, resistance-based control yields a much more accurate liquid level reading than differential pressure-based control. However, if the magnetic float fails (gets stuck or damaged), both the on-site indication and the remote signal will fail, affecting monitoring. This situation does not occur with differential pressure transmitters, as measurement is comparative; if a difference between the two readings is detected, it is possible to identify instrument failures and changes in medium density early on, allowing for timely corrections. 4 Conclusion The level control of the liquid ammonia storage tank is achieved through a combination of a magnetic flap level gauge for on-site display and a remote level transmission device. The signal from the level transmitter is converted into a 4–20 mA DC standard two-wire signal, which can be easily used in conjunction with digital displays. This enables display, control, adjustment, and alarm functions, thereby enhancing the reliability of level monitoring in ammonia storage tanks. The two control methods introduced in this article have their respective advantages and disadvantages; however, from a safety perspective, it is recommended to use a magnetic flap level gauge in combination with a differential pressure transmitter to monitor the liquid ammonia tank level.