Several methods for measuring liquid level in desulfurization absorption towers

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Author: Qin Junfeng (Guangzhou Branch, Shanghai Civil Aviation New Era Airport Design and Research Institute Co., Ltd., Guangzhou, Guangdong). When reproducing this article, please acknowledge the copyright of the author. Abstract: The liquid level in the absorption tower is a very important parameter in the desulfurization system. The interlock protection conditions for key equipment such as the circulation pump, oxidation fan, and mixer in the system are all directly related to it. Therefore, the accuracy and stability of the liquid level measurement data in the absorption tower determine the stable operation of the desulfurization system, as well as the safe operation of other process systems associated with it. Introduction: At present, most flue gas desulfurization systems use the limestone-gypsum wet desulfurization technology. The absorption tower is the main equipment used for flue gas desulfurization, and its liquid level plays a crucial role in ensuring the safe and reliable operation of the desulfurization system. However, due to the special structure of the absorption tower itself, it is not possible to use the conventional level gauges available today for direct measurement. This article introduces several methods currently used to measure the liquid level in absorption towers, and analyzes the advantages and disadvantages of each method. The main equipment in the limestone-gypsum flue gas desulfurization system is the absorption tower. As shown in Figure 1, the absorption tower mainly consists of a slurry oxidation zone, an absorption zone, a spray layer, a demisting layer, an inlet flue, and an outlet flue. For level measurement in conventional containers, an ultrasonic level gauge, a radar level gauge, or a float level gauge can be installed at the top of the container, or a magnetic flap level gauge can be installed on the side wall for this purpose. For liquids whose density is not greatly affected by temperature, in the case of an open-container setup, a pressure transmitter can be installed at the bottom of the container; the height can then be calculated using the formula H=(P/ρg)+h ； In the case of a closed container, a differential pressure transmitter must be installed. The liquid level height can be calculated using the formula H = (ΔP/ρg) + h, where H represents the liquid level height, P is the pressure, ΔP is the differential pressure, ρ is the density of the liquid, and h is the installation height of the pressure transmitter or differential pressure transmitter. http://www.tx7878.cn/d/file/jingyan/2019-05-30/18909498cf80c999f1c7fc11aa6ce47c.jpg 1 Problem occurrence: The medium inside the desulfurization absorption tower is quite complex. In the slurry oxidation zone, it mainly consists of calcium sulfate slurry, calcium sulfite slurry, and oxidizing air; in the absorption zone, it is a mixture of flue gas under positive pressure and slurry. Since there is a large mixture of sprayed slurry and flue gas above the slurry tank in the absorption tower, it is not possible to install ultrasonic or radar level gauges at the top for measurement. Limestone-gypsum slurry has three main special features. (1) To ensure desulfurization efficiency, the solid content of the slurry is as high as 20%; even with continuous mixing to keep the slurry in motion, the density varies between the upper and lower layers of the slurry tank. (2) Calcium sulfite in the slurry has strong viscosity; if a sensor probe is inserted into it, calcium sulfite gradually adheres to the surface of the probe, thereby affecting the proper operation of the sensor and causing distortion in the measurement data. (3) The slurry contains a large amount of oxidizing air; the oxidizing air pipeline network is generally installed at a height of about 3 m from the bottom of the tower. As the bubbles rise, they expand gradually due to the decrease in slurry pressure, which further increases the density difference between the upper and lower layers of the slurry in the absorption tower. Due to the above characteristics of the slurry, it is not possible to obtain accurate liquid level measurements by installing pressure transmitters only on the side walls of the absorption tower. Furthermore, float level gauges and magnetic flap level gauges are even less capable of withstanding such harsh operating conditions. 2 Solutions: In order to measure the liquid level in the absorption tower with greater accuracy, domestic desulfurization systems currently use pressure transmitters to measure the pressure at the bottom of the absorption tower, and they also install devices for measuring the density of the slurry. The data collected is sent to a DCS (Distributed Control System) or PLC (Programmable Logic Controller) control system, after which the liquid level of the absorption tower is calculated using the formula H = (P/ρg) + h. Due to the various density measurement methods, each with its own characteristics and significant differences, these factors directly affect the cost of projects, the stability of the measuring devices, as well as the amount of maintenance work required during the operation of the system. Since the liquid level in the absorption tower is a very important parameter in the desulfurization system, the number of instruments is configured redundantly to meet the process requirements; therefore, the various measurement methods will not be discussed further here. (1) Unit 1——Mass flow meter + pressure transmitter measurement circuit. This method first uses a mass flow meter to measure the density of the slurry in real time, and then calculates the liquid level in the absorption tower based on the pressure value measured by a pressure transmitter. The density measurement circuit mainly consists of a gypsum slurry pumping pump (one in use and one as backup), valves (inlet valve, outlet valve, flushing valve, and discharge valve of the pumping pump), a mass flow meter, pressure gauges, and piping. The pressure measurement circuit is primarily composed of a pressure transmitter, valves, and flushing piping (Figure 2). When starting the density measurement circuit, it is necessary to first close the flushing valve, the discharge valve, and the outlet valve, then open the inlet valve. Once the pump is filled with slurry, start the pump; after successful startup, open the outlet valve and adjust it using the pressure gauge on the pump’s outlet to achieve an appropriate pressure. This ensures that the flow rate within the measurement tube meets the requirements for accurate measurement, while also preventing excessively high flow rates that could cause severe wear on the mass flow meter and shorten its service life. When the desulfurization system is shut down or the mass flow meter requires maintenance, the slurry extraction pump should be stopped first, followed by closing the inlet valve and opening the discharge valve. Once all the slurry in the measurement pipeline has been drained, the flushing valve should be opened to clean the pipeline with process water; after that, the flushing valve, discharge valve, and outlet valve can be closed. http://www.tx7878.cn/d/file/jingyan/2019-05-30/767e4fb83a7cfb398e32f378064d2e2a.jpg Pressure measurement instruments use integrated diaphragm-type pressure transmitters. The maintenance valve should be placed as close as possible to the side wall of the absorption tower, and the sampling tube should form an angle of about 60° with the side wall; this helps to reduce the accumulation of slurry in the measurement pipeline and prevents clogging of the sampling tube. In addition, a flushing line should be installed near the diaphragm of the pressure transmitter, to periodically flush the diaphragm of the pressure transmitter, the sampling tube, and the maintenance valves, thereby ensuring the unobstructed flow in the measurement lines. The liquid level in the absorption tower measured by this method should be calculated using the formula H=(P/ρg)+h. In the formula, H is the calculated liquid level value, P is the pressure, ρ is the density of the slurry as measured by the mass flow meter, g is the acceleration due to gravity, and h is the installation height of the pressure transmitter. The mass flow meter specified in this law features high accuracy, with a precision of up to 0.2%, which fully meets the operational requirements of desulfurization systems ； No requirement for straight pipe sections, making installation convenient ； High reliability and low maintenance rate. The slurry in the absorption tower is continuously drawn out using a slurry extraction pump for measurement, ensuring the real-time nature of the data collected. (2) Unit 2——Tuning fork densitometer + pressure transmitter measurement circuit. In this method, a fork-type density meter and a pressure transmitter are installed on the side wall at the bottom of the absorption tower; the fork-type density meter is used to measure the density of the slurry, while the pressure transmitter is used to measure the pressure at the bottom of the slurry tank, as shown in Figure 3. To ensure the reliability and stability of instrument measurements, during installation the instrument should be positioned at an angle of approximately 60° to the side wall of the absorption tower. Additionally, a flushing system should be installed to regularly flush the sampling tubes as well as the sensors of the tuning fork densitometer. The liquid level is calculated using the formula H = (P/ρg) + h. In the formula, H is the calculated liquid level value, P is the pressure, ρ is the slurry density, g is the acceleration due to gravity, and h is the installation height of the pressure transmitter. http://www.tx7878.cn/d/file/jingyan/2019-05-30/26b559944d7e6b45183acebb97958de7.jpg When using this method for measurement, the structure is simple, which reduces the rate of equipment failures and thus lessens the amount of maintenance work required. However, since the probe of the tuning fork densitometer is inserted into the absorption tower, it is not possible to install maintenance valves there. When maintenance is required for the tuning fork densitometer, it must be disassembled and sent for inspection only after the desulfurization system is shut down and the slurry in the absorption tower is emptied. Therefore, it is recommended to use a redundant configuration of tuning fork densitometers to increase the reliability of this system. Online plug-and-play ball valve assemblies can also be customized, thereby completely preventing any disruption to the operation of the process system during instrument maintenance. http://www.tx7878.cn/d/file/jingyan/2019-05-30/0ca55a177cda412c93989779c65dce29.jpg (3) Unit 3——Differential pressure transmitter + pressure transmitter measurement circuit. This system uses a differential pressure transmitter to measure the density of the slurry, and a pressure transmitter to measure the pressure at the bottom of the slurry tank; the liquid level in the absorption tower is then calculated indirectly using formulas, as shown in Figure 4. The differential pressure transmitter features a diaphragm-type split structure; two remote diaphragms are installed at appropriate positions on the side walls of the absorption tower (with the height difference generally kept between 3 and 5 meters), and these diaphragms are connected to the transmitter body via capillaries. When the desulfurization system is operating normally, the density of the slurry is kept at around 1120 kg/m3. Therefore, when the medium in the absorption tower’s slurry tank changes from process water to normal limestone-gypsum slurry, the reading of the differential pressure transmitter rises from 29.4 kPa to 32.9 kPa (with a diaphragm height of 3 m); the change is quite small, at approximately 3.5 kPa. If the gauge range is 50 kPa, this change represents only 7% of that range. Hence, it is necessary to use a high-precision differential pressure transmitter. Density calculation method: ρ = ΔP/(gΔH), which is obtained through calculation. In the formula, ρ is the calculated value of the slurry density, ΔP is the differential pressure, g is the acceleration due to gravity, and ΔH is the height difference between the two diaphragms of the differential pressure transmitter. Liquid level calculation method: H = (P/ρg) + h, as calculated. In the formula, H is the calculated liquid level value, P is the pressure, ρ is the calculated slurry density as per the density calculation formula, g is the acceleration due to gravity, and h is the installation height of the pressure transmitter. When using this device to measure the liquid level in a slurry tank, its structure is simple; the differential pressure transmitters and pressure transmitters employed are highly mature and reliable, and the cost is also low. Only the flushing pipeline needs to be installed to periodically flush the instrument diaphragm and sampling pipeline, resulting in relatively low maintenance requirements. 3 Comparison of measuring devices: The above 3 sets of devices are all commonly used liquid level measurement devices for absorption towers in current desulfurization systems, each with its own advantages and disadvantages. (1) The mass flow meter used in Unit 1 features high precision and good stability, as well as excellent data repeatability; therefore, the density measurements of the slurry are highly reliable, which improves the overall accuracy of the entire level measurement system. It was widely used when desulfurization technology was first introduced to China. However, the device itself has a complex structure, featuring specialized measurement pipelines, pumps, and numerous valves, which increases the number of potential failure points and thus raises the workload for maintenance. (2) Unit 2 incorporates some improvements over Unit 1; primarily, the dedicated density measurement pipeline has been removed, and the density measuring instrument is installed directly on the side wall of the absorption tower. A high-precision fork-type densitometer is used for density measurement, **which simplifies the measurement setup. The downside is that there are very few manufacturers of fork-type density meters suitable for desulfurization applications, and their prices are relatively high ； Moreover, there are no corresponding online maintenance valves, posing the risk of having to shut down the process system when maintaining the instruments. (3) In Unit 3, low-cost differential pressure transmitters are used in place of the expensive mass flow meters and fork-type density meters; through proper selection and installation design, it is still possible to meet the requirements for measuring the density of slurry, allowing its use in certain desulfurization units. Furthermore, since the density of the slurry in the absorption tower actually changes depending on the liquid level and the distribution of the oxidizing air, measuring the density at a fixed height within the absorption tower does not reflect the true density of the entire slurry pool. The two diaphragms of a differential pressure transmitter are separated by a significant distance, so the calculated density value represents the average value over that height range, which is theoretically closer to the actual density within the slurry pool. Another flaw of this device lies in the installation positions of the two diaphragms in the differential pressure transmitter; when the liquid level is below the diaphragm on the high-pressure side, the differential pressure transmitter reads zero. As a result, both the calculated density value ρ and the calculated liquid level value H are zero, failing to reflect the actual liquid level ； When the liquid level is between the diaphragms on the high-pressure and low-pressure sides, the calculated density value ρ increases gradually as the liquid level rises, but it remains lower than the actual density of the slurry; therefore, the calculated liquid level value H is also not useful for reference. The calculation results of this device are considered normal only when the liquid level rises above the diaphragm on the low-pressure side. Since the height difference between the two diaphragms is approximately 3 m, and the diaphragm on the high-pressure side is about 1 m away from the bottom of the tower, the measurement blind zone of this system is roughly around 4 m. It is recommended that when the liquid level operates within the blind zone, appropriate data processing in the DCS or PLC control system should be employed to ensure that the calculated values are as close as possible to the actual conditions. Additionally, enhanced manual inspections should be carried out to compensate for the shortcomings of the control system. The liquid level when the desulfurization system is operating normally is around 10 m, so this device is still suitable for the operating conditions of the desulfurization system. 4 Conclusion In summary, each desulfurization unit should, based on its own specific conditions – such as the technical skills of the operating staff, the workload requirements for those staff, and the impact of the shutdown of the desulfurization system on other process systems – conduct a comprehensive comparison in order to select an appropriate liquid level measurement device for the absorption tower. This will enable safe, stable, economical, and practical operation.