Liquid level detection system and liquid level detection method

Abstract

This liquid level detection system comprises: a differential pressure detector that detects a differential pressure between a first height position and a second height position higher than the first height position in a liquefied gas tank; a pressure detector that detects an internal pressure of the liquefied gas tank; a first temperature detector that detects a first temperature that is a temperature of a liquid layer part of a liquefied gas; a second temperature detector that detects a second temperature that is a temperature of a gas layer part of the liquefied gas; and a processing circuit. The processing circuit: determines a liquid layer part density from the first temperature; determines a gas layer part density from the second temperature and the internal pressure; obtains, as liquid level information on a liquid level, information on a tank liquid layer part height or a tank gas layer part height in the liquefied gas tank on the basis of the liquid layer part density, the gas layer part density, an interior height of the liquefied gas tank, and the differential pressure; and outputs the liquid level information.

Description

Liquid level detection system and liquid level detection method 【0001】 The present disclosure relates to a liquid level detection system and a liquid level detection method. 【0002】 As sensors for detecting the liquid level of a liquefied gas tank that stores liquefied gases such as liquefied hydrogen and liquefied natural gas, various level sensors such as float type level sensors and radar type level sensors are known. However, such level sensors that directly measure the liquid level are required not to attach the head part to the liquid, and are difficult to use for tanks where the liquefied gas reaches near the ceiling of the liquefied gas tank. Also, if the head part is installed outside the tank and the probe is installed inside the tank, a hole will be made in the liquefied gas tank. For this reason, the location where the hole is made in the liquefied gas tank may become a leakage point of the liquefied gas, which is not desirable. In addition, level sensors that directly measure the liquid level are relatively expensive. 【0003】 As a method for detecting the liquid level of a liquefied gas tank where it is difficult to use a level sensor that directly measures the liquid level, there is a method of calculating the liquid level from the differential pressure between the liquid layer part and the gas layer part in the liquefied gas tank. For example, in Patent Document 1 below, the liquid level is calculated from the differential pressure and the respective densities of the gas layer part and the liquid layer part. 【0004】 Japanese Patent Application Laid-Open No. 2007-322292 【0005】 In Patent Document 1 above, although the densities of the gas layer part and the liquid layer part are determined based on pressure, it is not accurate. Therefore, the configuration of Patent Document 1 has room for improvement in more accurately detecting the liquid level in a liquefied gas tank using differential pressure. 【0006】 The present disclosure has been made in view of the above problems, and an object thereof is to provide a liquid level detection system and a liquid level detection method capable of simply and highly accurately detecting the liquid level in a liquefied gas tank. 【0007】A liquid level detection system according to one aspect of the present disclosure is a liquid level detection system for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: a differential pressure detector for detecting the differential pressure between a first height position and a second height position higher than the first height position in the liquefied gas tank; a pressure detector for detecting the internal pressure of the liquefied gas tank; a first temperature detector for detecting a first temperature which is the temperature of the liquid layer portion of the liquefied gas; a second temperature detector for detecting a second temperature which is the temperature of the gas layer portion of the liquefied gas; and a processing circuit including a memory, wherein the memory stores data on the height in the liquefied gas tank, data on the density of the liquid layer portion of the liquefied gas, and data on the gas layer The processing circuit stores data for the liquid layer density, the data for the liquid layer density is data showing the correlation between the first temperature and the liquid layer density, and the data for the gas layer density is data showing the correlation between the second temperature and the internal pressure and the gas layer density. The processing circuit determines the liquid layer density from the first temperature, determines the gas layer density from the second temperature and the internal pressure, obtains information regarding the height of the liquid layer or gas layer in the liquefied gas tank from the liquid layer density, the gas layer density, the height inside the liquefied gas tank, and the differential pressure, and outputs the liquid level information. 【0008】A liquid level detection method according to another aspect of the present disclosure is a liquid level detection method for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: detecting the differential pressure between a first height position and a second height position higher than the first height position in the liquefied gas tank; detecting the internal pressure of the liquefied gas tank; detecting a first temperature which is the temperature of the liquid layer portion of the liquefied gas; detecting a second temperature which is the temperature of the gas layer portion of the liquefied gas; obtaining data on the height in the liquefied gas tank, data on the density of the liquid layer portion of the liquefied gas, and data on the density of the gas layer portion of the liquefied gas; and the liquid layer density The first data shows the correlation between the first temperature and the density of the liquid layer, and the second data shows the correlation between the second temperature, the internal pressure and the density of the gas layer. The first temperature determines the density of the liquid layer, the second temperature determines the density of the gas layer, and the differential pressure determines information regarding the height of the liquid layer or gas layer in the liquefied gas tank, which is obtained as liquid level information related to the liquid level. 【0009】 According to this disclosure, the liquid level in a liquefied gas tank can be detected easily and with high accuracy. 【0010】 Figure 1 is a schematic diagram showing the general configuration of a liquid level detection system according to one embodiment of the present disclosure. Figure 2 is a table showing the relationship between temperature, pressure, and hydrogen density. Figure 3 is a flowchart showing the flow of the liquid level detection process in this embodiment. Figure 4 is a graph showing the relationship between differential pressure and liquid level under a certain internal pressure. 【0011】 An embodiment will be described in detail below with reference to the drawings. In the following, the same or corresponding elements will be denoted by the same reference numerals throughout all the drawings, and redundant explanations will be omitted. 【0012】Figure 1 is a schematic diagram showing the general configuration of a liquid level detection system according to one embodiment of the present disclosure. The liquid level detection system 1 in this embodiment is a system for detecting the liquid level of a liquefied gas tank 2 in which liquefied gas is stored. The liquefied gas tank 2 is, for example, a marine liquefied gas fuel tank installed on a ship that uses liquefied gas as fuel. Alternatively, the liquefied gas tank 2 may be a storage tank installed in a bunkering facility that temporarily stores liquefied gas before introducing it into the marine liquefied gas fuel tank. In this embodiment, the liquefied gas tank 2 is horizontally cylindrical. However, the liquefied gas tank 2 may be vertically cylindrical. Alternatively, the liquefied gas tank 2 may be spherical or substantially rectangular parallelepiped. The liquefied gas includes, for example, liquefied hydrogen, liquefied natural gas, liquefied petroleum gas, liquefied ammonia, etc. 【0013】 In this embodiment, the liquefied gas tank 2 has a double-shell structure including an inner tank 21 and an outer tank 22. Liquefied gas is introduced and stored in the internal space of the inner tank 21. The space between the inner tank 21 and the outer tank 22 is maintained under vacuum. As a result, the internal space of the inner tank 21 in which the liquefied gas is stored is vacuum-insulated from the outside. Note that the insulation method in the liquefied gas tank 2 is not limited to vacuum insulation; other insulation methods, such as using insulating materials, may be employed. 【0014】 The liquid level of the liquefied gas stored in the liquefied gas tank 2 serves as an indicator of the remaining fuel, for example, when the liquefied gas is used as fuel. Therefore, accurately knowing the liquid level is important. 【0015】 The liquid level detection system 1 includes a differential pressure detector 11, a pressure detector 12, a first temperature detector 13, a second temperature detector 14, and a processing circuit 15. The differential pressure detector 11 detects the differential pressure ΔP between a first height position Q1 and a second height position Q2 that is higher than the first height position Q1 in the liquefied gas tank 2. 【0016】The differential pressure detector 11 is connected to a first pressure sensing pipe 16 that introduces the liquid layer pressure at a first height position Q1 within the liquefied gas tank 2, and a second pressure sensing pipe 17 that introduces the gas layer pressure at a second height position Q2 within the liquefied gas tank 2. The differential pressure detector 11 outputs the differential pressure ΔP between the liquid layer pressure of the first pressure sensing pipe 16 and the gas layer pressure of the second pressure sensing pipe 17. For example, the first height position Q1 is a position near the bottom surface of the inner tank 21 of the liquefied gas tank 2, and the second height position Q2 is a position near the ceiling of the inner tank 21 of the liquefied gas tank 2. In this disclosure, the first height position Q1 and the second height position Q2 specify positions in the height direction within the liquefied gas tank 2, and do not specify positions in the horizontal direction. Therefore, the horizontal position of the inlet of the first pressure sensing pipe 16 and the horizontal position of the inlet of the second pressure sensing pipe 17 are not particularly limited. 【0017】 The pressure detector 12 detects the internal pressure P of the liquefied gas tank 2. For example, the pressure detector 12 is installed near a second height position Q2 in the height direction. That is, the pressure detector 12 is positioned at approximately the same height as the inlet of the second pressure guiding pipe 17. The pressure detector 12 may detect the pressure in the gas layer or the pressure in the liquid layer. For example, the pressure detector 12 may be installed near a first height position Q1 in the height direction. 【0018】 The first temperature detector 13 detects a first temperature, which is the temperature of the liquid layer of the liquefied gas inside the liquefied gas tank 2. To this end, the first temperature detector 13 detects a first temperature T1 near a first height position. For example, the first temperature detector 13 is installed at the bottom of the liquefied gas tank 2. The first temperature detector 13 may be installed on the outer surface or the inner surface of the inner tank 21 at the bottom of the liquefied gas tank 2. Note that "near the first height position" may include a height position at the same height as the first height position, a position a predetermined distance higher than the first height position, and a position a predetermined distance lower than the first height position. The predetermined distance is set to a distance within a range in the height direction where the temperature inside the liquefied gas tank 2 can be considered to be approximately the same as the temperature at the first height position. 【0019】The second temperature detector 14 detects the second temperature, which is the temperature of the gas layer of the liquefied gas inside the liquefied gas tank 2. To this end, the second temperature detector 14 detects the second temperature T2 near the second height position. For example, the second temperature detector 14 is installed on the ceiling of the liquefied gas tank 2. The second temperature detector 14 may be installed on the outer surface or the inner surface of the inner tank 21 on the ceiling of the liquefied gas tank 2. Note that "near the second height position" may include the same height position as the second height position, a predetermined distance higher than the second height position, and a predetermined distance lower than the second height position. The predetermined distance is set to a distance within a range in the height direction where the temperature inside the liquefied gas tank 2 can be considered to be approximately the same as the temperature at the second height position. 【0020】 As described above, the first height position Q1 and the second height position Q2 do not specify horizontal positions. Therefore, the horizontal positions of the pressure detector 12, the first temperature detector 13, and the second temperature detector 14 are not particularly limited. For example, at least one of these detectors 12, 13, and 14 may be positioned at a horizontal distance from the inlet of the first pressure suction tube 16 or the second pressure suction tube 17. 【0021】 The processing circuit 15 includes a computer such as a microcontroller, personal computer, or PLC (Programmable Logic Controller). More specifically, the processing circuit 15 includes a processor 19, a memory 18, and peripheral circuits. The processor 19 includes, for example, a CPU or MPU. The memory 18 includes ROM, RAM, registers, non-volatile storage, etc. Peripheral circuits include input / output interfaces, etc. Furthermore, the processing circuit 15 may include an input device for user operation input and an output device such as a monitor that outputs the control status. 【0022】The functions of the elements disclosed herein can be performed using circuits or processing circuits, including general-purpose processors, dedicated processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuits. In this specification, a circuit, unit, means, or part is hardware that performs the enumerated functions, or hardware programmed to perform the enumerated functions. The hardware may be hardware disclosed herein, or other known hardware that is programmed or configured to perform the enumerated functions. If the hardware is a processor, which is considered a type of circuit, then the circuit, unit, or means is a combination of hardware and software, and the software is used to configure the hardware and / or the processor. 【0023】 The memory unit 18 stores a processing program. The processor 19 reads the processing program from the memory unit 18, acquires data from each detector based on the processing program, and calculates the liquid level from the acquired data. The memory unit 18 also stores data on the height Ht in the liquefied gas tank 2, data on the density ρl of the liquid layer of the liquefied gas, and data on the density ρg of the gas layer of the liquefied gas. This data is input from an input device or the like based on the specifications of the liquefied gas tank 2 and the type of liquefied gas stored in the liquefied gas tank 2, and stored. The memory unit 18 that stores this data may be housed in a different enclosure from the processor 19. For example, the processing circuit 15 may include the processor 19 and a memory unit 18 such as a server that is communicated to the processor 19 via a predetermined network. 【0024】Figure 2 is a table showing the relationship between temperature, pressure, and hydrogen density. As shown in the table in Figure 2, both the hydrogen density ρl in the liquid layer and the hydrogen density ρg in the gas layer change with temperature and pressure. In particular, the change in temperature has a greater effect on the change in the hydrogen density ρl in the liquid layer than the change in pressure. A similar trend is observed for other liquefied gases besides liquefied hydrogen. For this reason, in this embodiment, the first temperature T1 in the liquid layer is detected by the first temperature detector 13, and the second temperature T2 in the gas layer is detected by the second temperature detector 14. 【0025】 The data for the liquid layer density ρl stored in the memory 18 is data showing the correlation between the first temperature T1 and internal pressure P and the liquid layer density ρl. Similarly, the data for the gas layer density ρg stored in the memory 18 is data showing the correlation between the second temperature T2 and internal pressure P and the gas layer density ρg. These data may be configured as a data table from which density can be read from temperature and pressure, as shown in the table in Figure 2, or as function data from which density can be calculated by inputting temperature and pressure. 【0026】 Figure 3 is a flowchart showing the flow of the liquid level detection process in this embodiment. As shown in Figure 3, the processing circuit 15 obtains the internal pressure P of the liquefied gas tank 2 from the pressure detector 12 (step S1). The processing circuit 15 also obtains the first temperature T1 and the second temperature T2 from the first temperature detector 13 and the second temperature detector 14 (step S2). 【0027】 The processing circuit 15 uses data on the liquid layer density ρl to determine the liquid layer density ρl from the first temperature T1 and internal pressure P. Similarly, the processing circuit 15 uses data on the gas layer density ρg to determine the gas layer density ρg from the second temperature T2 and internal pressure P (step S3). 【0028】In this embodiment, the density ρg of the gas layer is determined based on a third temperature T3, which is the average of the second temperature T2 detected by the second temperature detector 14 and the saturation temperature Ts of the liquefied gas. For this purpose, the memory 18 stores data on the saturation temperature Ts of the liquefied gas. The saturation temperature Ts is the temperature at the boundary between the liquid layer and the gas layer in the density table at each pressure shown in Figure 2. For example, when the pressure is 0.2 [MPaG], the saturation temperature Ts is 24 [K], and when the pressure is 0.4 [MPaG], the saturation temperature Ts is 27 [K]. 【0029】 Thus, the saturation temperature Ts changes according to the internal pressure P. Therefore, in this embodiment, the saturation temperature Ts data stored in the memory 18 is data that shows the correlation between the internal pressure P and the saturation temperature Ts. The saturation temperature Ts data may be configured as a data table from which the saturation temperature Ts can be read from the internal pressure P, or it may be configured as function data from which the saturation temperature Ts can be calculated by inputting the internal pressure P. 【0030】 The processing circuit 15 determines the saturation temperature Ts from the internal pressure P using the saturation temperature Ts data. The processing circuit 15 calculates the third temperature T3 by averaging the second temperature T2 and the saturation temperature Ts. For example, the processing circuit 15 calculates the third temperature T3 using T3 = (T2 + Ts) / 2. However, the calculation formula for the third temperature T3 is not limited to this. For example, the processing circuit 15 may calculate a weighted average by multiplying the second temperature T2 and the saturation temperature Ts by predetermined weighting coefficients. The processing circuit 15 determines the gas layer density ρg from the third temperature T3 and the internal pressure P. Therefore, in this embodiment, the data of the gas layer density ρg stored in the memory 18 may be data showing the correlation between the third temperature T3 and the internal pressure P and the gas layer density ρg. 【0031】Within the liquefied gas tank 2, temperature stratification exists in the gas layer, i.e., the gas region where the liquefied gas has vaporized. That is, in the gas layer of the liquefied gas tank 2, a temperature difference occurs between the temperature near the ceiling of the liquefied gas tank 2, which is the upper part of the gas layer, and the temperature near the liquid surface of the liquefied gas, which is the lower part of the gas layer. For this reason, the second temperature T2 detected by the second temperature detector 14 installed on the ceiling of the liquefied gas tank 2 cannot necessarily be said to be representative of the temperature in the gas layer within the liquefied gas tank 2. 【0032】 Therefore, in this embodiment, the third temperature T3, obtained by averaging the second temperature T2 and the temperature near the liquid surface of the liquefied gas, is determined as the temperature representing the gas layer in the liquefied gas tank 2. The region near the liquid surface of the liquefied gas in the gas layer can be said to be the region where the liquefied gas vaporizes. Therefore, the temperature near the liquid surface of the liquefied gas can be said to be approximately equal to the saturation temperature Ts. Accordingly, the saturation temperature Ts is adopted as the temperature near the liquid surface of the liquefied gas. 【0033】 Furthermore, the processing circuit 15 obtains the differential pressure ΔP from the differential pressure detector 11 (step S4). The processing circuit 15 uses the liquid layer density ρl, the gas layer density ρg, and the differential pressure ΔP to determine the tank liquid layer height Hl (step S5). 【0034】 The processing circuit 15 determines the height Hl of the liquid layer in the liquefied gas tank 2 from the following equations 1 and 2. In equation 1, Hg is the height of the gas layer in the tank, and in equation 2, G is the acceleration due to gravity. Note that a value other than the exact value of the acceleration due to gravity, for example 9.81, may be used as the value of G. For example, in equation 2, a predetermined coefficient K may be used instead of the acceleration due to gravity G. Ht = Hl + Hg ... (Equation 1) ΔP = (ρl・Hl + ρg・Hg)・G ... (Equation 2) 【0035】 The processing circuit 15 solves the simultaneous equations including the first and second equations for Hl to determine the height Hl of the tank liquid layer, and outputs liquid level information with the tank liquid layer height Hl as the liquid level (step S6). For example, the liquid level information includes the liquid level itself. 【0036】The height Ht inside the liquefied gas tank 2 obtained by the above simultaneous equations is precisely equal to the length between the first height position Q1 where the inlet of the first pressure sensing pipe 16 of the differential pressure detector 11 is located and the second height position Q2 where the inlet of the second pressure sensing pipe 17 is located. Therefore, it is preferable that the first height position Q1 coincides with the height of the bottom surface inside the liquefied gas tank 2. Similarly, it is preferable that the second height position Q2 coincides with the height of the ceiling surface inside the liquefied gas tank 2. In other words, the closer the inlet of the first pressure sensing pipe 16 is to the bottom surface inside the liquefied gas tank 2, the more accurately the liquid level can be calculated. Similarly, the closer the inlet of the second pressure sensing pipe 17 is to the ceiling surface inside the liquefied gas tank 2, the more accurately the liquid level can be calculated. However, the first height position Q1 does not necessarily have to coincide with the height of the bottom surface inside the liquefied gas tank 2, and similarly, the second height position Q2 does not necessarily have to coincide with the height of the ceiling surface inside the liquefied gas tank 2. 【0037】 According to this embodiment, the density ρl in the liquid layer and the density ρg in the gas layer of the liquefied gas are determined using a first temperature T1, which is the temperature of the liquid layer in the liquefied gas tank 2, a second temperature T2, which is the temperature of the gas layer, and the internal pressure P of the liquefied gas tank 2. Since the density ρl in the liquid layer and the density ρg in the gas layer are determined according to the internal pressure P and temperatures T1 and T2 in the liquefied gas tank 2, the differential pressure ΔP detected separately by the differential pressure detector 11 can be described more accurately using an equation that expresses the tank liquid layer height Hl. Therefore, the liquid level in the liquefied gas tank 2 can be detected easily and with high accuracy. 【0038】 Here, we show a comparison between this embodiment and an embodiment that does not consider the density ρg of the gas layer. Figure 4 is a graph showing the relationship between differential pressure and liquid level under a certain internal pressure. In Figure 4, we assume that there is no change in temperature T1 and T2. The comparative example graph Fc is a graph of liquid level L against differential pressure ΔP when ΔP = ρl・G・Hl is used instead of the second equation used in this embodiment. On the other hand, graph Fe is a graph of liquid level L against differential pressure ΔP obtained based on this embodiment. Liquid level Lm indicates the upper limit of liquid level in the liquefied gas tank 2, and the differential pressure at this time is expressed as ΔPm. 【0039】As shown in Figure 4, when the differential pressure detected by the differential pressure detector 11 is ΔPx, the liquid level calculated in the comparative example, which does not consider the gas layer, is Lxc, but the liquid level calculated in this embodiment is Lxe, which is lower than Lxc. When the liquid level L is high, the difference ΔL between the liquid level Lxc based on graph Fc in the comparative example and the liquid level Lxe based on graph Fe in this embodiment is relatively small. However, as the differential pressure ΔP decreases, that is, as the liquid level L decreases, the difference ΔL between the obtained liquid levels Lxe and Lxc increases. When the internal pressure P or temperatures T1, T2 change, the graphs Fc and Fe become different from the graphs in Figure 4, but the relationship between the two graphs Fc and Fe at the same internal pressure P and the same temperatures T1, T2 shows the same trend as in the example in Figure 4. 【0040】 Thus, in the comparative example that does not consider the gas layer, a liquid level Lxc higher than the actual liquid level is output, which may lead to the misconception that there is still liquefied gas remaining in the liquefied gas tank 2 even when the actual liquid level approaches zero. In contrast, according to this embodiment, a liquid level Lxe closer to the actual liquid level can be output. 【0041】 Furthermore, in this embodiment, the gas layer density ρg is determined based on a third temperature T3, which is the average of the second temperature T2 detected by the second temperature detector 14 and the saturation temperature Ts of the liquefied gas. As a result, even if temperature stratification exists in the gas layer of the liquefied gas tank 2, the gas layer density ρg can be determined to a value corresponding to the average temperature of the gas layer. Therefore, the liquid level in the liquefied gas tank 2 can be detected with high accuracy. 【0042】 Furthermore, in this embodiment, the saturation temperature Ts used to calculate the average temperature of the gas layer is determined to a value corresponding to the internal pressure P. As a result, the density ρg of the gas layer can be determined using the saturation temperature Ts, which changes according to the internal pressure P, and the liquid level in the liquefied gas tank 2 can be detected with high accuracy. 【0043】In addition to or instead of outputting the liquid level itself as liquid level information, the processing circuit 15 may output various values calculated based on the liquid level. For example, the processing circuit 15 may output the volume Vl of the liquid layer part of the liquefied gas in the liquefied gas tank 2 as liquid level information. In this case, the memory 18 stores data indicating the correlation between the liquid level and the volume Vl of the liquid layer part of the liquefied gas in the liquefied gas tank 2. The data of the volume Vl of the liquid layer part may be configured as a data table from which the volume Vl of the liquid layer part can be read out from the liquid level, or may be configured as function data that can calculate the volume Vl of the liquid layer part by inputting the liquid level. The data of the volume Vl of the liquid layer part is determined based on the internal shape of the liquefied gas tank 2. 【0044】 In this case, the processing circuit 15 uses the data of the volume Vl of the liquid layer part with the height Hl of the liquid layer part calculated as described above as the liquid level to determine the volume Vl of the liquid layer part from the liquid level. The processing circuit 15 outputs the determined volume Vl of the liquid layer part as liquid level information. The volume Vl of the liquid layer part corresponds to the storage amount of the liquefied gas stored in the liquefied gas tank 2. Thereby, the remaining amount of the liquefied gas stored in the liquefied gas tank 2 can be grasped more accurately. 【0045】 Alternatively, the processing circuit 15 may output at least any one of the weight Ml of the liquid layer part of the tank, the weight Mg of the gas layer part of the tank, and the total weight Mt of the tank, which is the sum thereof, as liquid level information. Also in this case, the memory 18 stores data indicating the correlation between the liquid level and the volume Vl of the liquid layer part of the liquefied gas in the liquefied gas tank 2. 【0046】 For example, the processing circuit 15 uses the data of the volume Vl of the liquid layer part with the height Hl of the liquid layer part calculated as described above as the liquid level to determine the volume Vl of the liquid layer part from the liquid level. The processing circuit 15 calculates the weight Ml of the liquid layer part of the liquefied gas in the liquefied gas tank 2 from the determined volume Vl of the liquid layer part and the density ρl of the liquid layer part. That is, the weight Ml of the liquid layer part is calculated by Ml = ρl × Vl. 【0047】Further, for example, the processing circuit 15 uses the data of the tank liquid layer volume Vl with the height Hl of the tank liquid layer part calculated as described above as the liquid level, and determines the tank gas layer volume Vg from the liquid level and the volume of the liquefied gas tank 2. The volume of the liquefied gas tank 2 can be given as the maximum value Vlmax of the tank liquid layer volume when the liquid level is at the maximum value in the tank liquid layer volume Vl. Alternatively, the volume of the liquefied gas tank 2 may be stored in the memory 18. The processing circuit 15 uses the data of the tank liquid layer volume Vl to determine the tank liquid layer volume Vl from the liquid level, and subtracts the determined tank liquid layer volume Vl from the volume of the liquefied gas tank 2 to determine the tank gas layer volume Vg. Or, data indicating the correlation between the tank gas layer height Hg and the tank gas layer volume Vg of the liquefied gas in the liquefied gas tank 2 may be stored in the memory 18, and the tank gas layer volume Vg may be determined from the tank gas layer height Hg. 【0048】 The processing circuit 15 calculates the tank gas layer weight Mg of the liquefied gas in the liquefied gas tank 2 from the determined tank gas layer volume Vg and the gas layer density ρg. That is, the tank gas layer weight Mg is calculated by Mg = ρg × Vg. 【0049】 Further, for example, the processing circuit 15 calculates the total of the tank liquid layer weight Ml and the tank gas layer weight Mg calculated as described above to calculate the tank internal weight Mt = Ml + Mg of the liquefied gas. 【0050】 According to this, by calculating the tank liquid layer weight Ml, the tank gas layer weight Mg, or the tank internal weight Mt, regardless of the change in volume due to the change in temperature, that is, the change in density, the remaining amount of the liquefied gas stored in the liquefied gas tank 2 or the amount of the vaporized gas present in the liquefied gas tank 2 can be grasped more accurately. 【0051】As described above, the liquid level detection system 1 in this embodiment exhibits the specific effects described above whether the liquefied gas stored in the liquefied gas tank 2 is liquefied hydrogen or a liquefied gas other than liquefied hydrogen. However, in particular, in the case of liquefied hydrogen, there is a strong correlation between temperature and density. Therefore, applying the liquid level detection system 1 in this embodiment is more useful when the liquefied gas stored in the liquefied gas tank 2 is liquefied hydrogen. 【0052】 While embodiments of this disclosure have been described above, this disclosure is not limited to the embodiments described above, and various improvements, changes, and modifications are possible without departing from the spirit of this disclosure. 【0053】 [Other Embodiments] For example, in the above embodiment, an example was given in which the density ρg of the gas layer is determined based on a third temperature T3 obtained by averaging the second temperature T2 detected by the second temperature detector 14 and the saturation temperature Ts of the liquefied gas. However, the density ρg of the gas layer may be determined without using the saturation temperature Ts. Also, in the above embodiment, an example was given in which the data for the saturation temperature Ts is data showing the correlation between the internal pressure P and the saturation temperature Ts. However, the embodiment is not limited to this. For example, the data for the saturation temperature Ts may be a fixed value. 【0054】 Furthermore, in the above embodiment, the processing circuit 15 is shown as determining the liquid layer density ρl from the first temperature T1 and internal pressure P using data on the liquid layer density ρl. However, instead, the processing circuit 15 may determine the liquid layer density ρl from the first temperature T1 alone. That is, the data on the liquid layer density ρl may be data that shows the correlation between the first temperature T1 and the liquid layer density ρl, and may not depend on the internal pressure P. As mentioned above, the change in liquid layer density ρl is more significantly affected by the change in the first temperature T1 than by the change in internal pressure P. For this reason, the data on the liquid layer density ρl may be data that does not depend on the internal pressure P. 【0055】Furthermore, in the above embodiment, an example was given in which the first height position, where the differential pressure detector 11 detects pressure, is the bottom of the liquefied gas tank 2, and the second height position is the top of the liquefied gas tank 2. However, the embodiment is not limited to this as long as the second height position is positioned higher than the first height position. However, it is preferable that the second height position is located above the maximum liquid level in the liquefied gas tank 2. Similarly, the height position of the second temperature detector 14 is not limited to the top, but it is preferable that it is located above the maximum liquid level in the liquefied gas tank 2. 【0056】 Furthermore, while the above embodiment illustrates the method of determining the tank liquid layer height Hl using the first and second equations, it is not limited to this. For example, the tank liquid layer height Hl may be determined using the following third equation: ΔP = (ρl・Hl + ρg・(Ht-Hl))・G ... (third equation) 【0057】 Alternatively, instead of determining the tank liquid layer height Hl, the tank gas layer height Hg may be obtained as liquid level information. In this case, when the tank gas layer height Hg exceeds a predetermined value, it can be determined that the liquefied gas in the liquefied gas tank 2 is nearly empty. 【0058】 Furthermore, the output liquid level information may be the height itself, as illustrated in the above embodiment, or it may be the ratio [%] of the height of the liquid layer in the tank Hl or the height of the gas layer in the tank Hg to the height Ht in the liquefied gas tank 2. 【0059】 Furthermore, in the above embodiment, the processing circuit 15 is shown as outputting the liquid level itself as liquid level information, but it may also output information based on the liquid level as liquid level information. For example, the liquid level information may include an alarm indicating that the liquid level has fallen below a predetermined reference value. In this case, the processing circuit 15 determines whether the height Hl of the tank liquid layer has fallen below the reference value. The processing circuit 15 outputs an alarm as liquid level information when the height Hl of the tank liquid layer has fallen below the reference value. Alternatively, the processing circuit 15 may determine whether the height Hg of the tank gas layer has risen above the reference value, and output an alarm as liquid level information when the height Hg of the tank gas layer has risen above the reference value. 【0060】[Summary of the Disclosure] [Item 1] A liquid level detection system according to one aspect of the Disclosure is a liquid level detection system for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: a differential pressure detector for detecting the differential pressure between a first height position in the liquefied gas tank and a second height position higher than the first height position; a pressure detector for detecting the internal pressure of the liquefied gas tank; a first temperature detector for detecting a first temperature which is the temperature of the liquid layer portion of the liquefied gas; a second temperature detector for detecting a second temperature which is the temperature of the gas layer portion of the liquefied gas; and a processing circuit including a memory, wherein the memory contains data on the height in the liquefied gas tank, data on the density of the liquid layer portion of the liquefied gas, and data on the gas layer The processing circuit stores data for the liquid layer density, the data for the liquid layer density is data showing the correlation between the first temperature and the liquid layer density, and the data for the gas layer density is data showing the correlation between the second temperature and the internal pressure and the gas layer density. The processing circuit determines the liquid layer density from the first temperature, determines the gas layer density from the second temperature and the internal pressure, obtains information regarding the height of the liquid layer or gas layer in the liquefied gas tank from the liquid layer density, the gas layer density, the height inside the liquefied gas tank, and the differential pressure, and outputs the liquid level information. 【0061】 According to the above configuration, the density of the liquid layer and the gas layer of the liquefied gas are determined using a first temperature, which is the temperature of the liquid layer in the liquefied gas tank, a second temperature, which is the temperature of the gas layer, and the internal pressure of the liquefied gas tank. Since the density of the liquid layer and the gas layer are determined according to the internal pressure and temperature of the liquefied gas tank, the differential pressure detected separately by a differential pressure detector can be described more accurately using an equation that expresses it in terms of the height of the liquid layer in the tank. Therefore, the liquid level in the liquefied gas tank can be detected simply and with high accuracy. 【0062】[Item 2] In the liquid level detection system of Item 1, the processing circuit may determine the height of the tank liquid layer or the tank gas layer from the following equations 1 and 2, and output information based on the height of the tank liquid layer or the gas layer as the liquid level information. Ht = Hl + Hg ... (Equation 1) ΔP = (ρl・Hl + ρg・Hg)・G ... (Equation 2) Here, Ht is the height inside the liquefied gas tank, Hl is the height of the tank liquid layer, Hg is the height of the tank gas layer, ΔP is the differential pressure, ρl is the density of the liquid layer, ρg is the density of the gas layer, and G is the acceleration due to gravity. 【0063】 [Item 3] In the liquid level detection system of Item 1 or 2, the first temperature detector may be configured to detect the temperature near the first height position, and the second temperature detector may be configured to detect the temperature near the second height position. 【0064】 [Item 4] In any of the liquid level detection systems described in Items 1 to 3, the liquid level information may include an alarm indicating that the liquid level has fallen below a predetermined reference value. 【0065】 [Item 5] In any of the liquid level detection systems of Items 1 to 4, the memory may store data of the saturation temperature of the liquefied gas, and the processing circuit may calculate a third temperature by averaging the second temperature and the saturation temperature, and determine the density of the gas layer from the third temperature and the internal pressure. This makes it possible to determine the density of the gas layer to a value corresponding to the average temperature of the gas layer, even if temperature stratification exists in the gas layer of the liquefied gas tank. Therefore, the liquid level in the liquefied gas tank can be detected with high accuracy. 【0066】 [Item 6] In the liquid level detection system of Item 5, the saturation temperature data is data showing the correlation between the internal pressure and the saturation temperature, and the processing circuit may determine the saturation temperature from the internal pressure. This makes it possible to determine the density of the gas layer using the saturation temperature which changes according to the internal pressure, and thus enable high-precision detection of the liquid level in the liquefied gas tank. 【0067】[Item 7] In any of the liquid level detection systems described in Items 1 to 6, the data on the liquid layer density is data showing the correlation between the first temperature, the internal pressure, and the liquid layer density, and the processing circuit may determine the liquid layer density from the first temperature and the internal pressure. This allows for a more accurate determination of the liquid layer density. 【0068】 [Item 8] In any of the liquid level detection systems described in Items 1 to 7, the liquefied gas may include liquefied hydrogen. 【0069】 [Item 9] In any of the liquid level detection systems described in Items 1 to 8, the first temperature detector may be installed at the bottom of the liquefied gas tank, and the second temperature detector may be installed at the top of the liquefied gas tank. 【0070】 [Item 10] In any of the liquid level detection systems of Items 1 to 9, the memory may store data showing the correlation between the liquid level and the volume of the liquid layer of the liquefied gas in the liquefied gas tank, and the processing circuit may determine the volume of the liquid layer from the liquid level and output the determined volume of the liquid layer. This makes it possible to more accurately grasp the remaining amount of liquefied gas stored in the liquefied gas tank. 【0071】[Item 11] In any of the liquid level detection systems of Items 1 to 10, the memory stores data showing the correlation between the liquid level and the volume of the liquid layer of the liquefied gas in the liquefied gas tank, the processing circuit determines the volume of the liquid layer of the tank from the liquid level, calculates the weight of the liquid layer of the liquefied gas in the liquefied gas tank from the determined volume of the liquid layer and the density of the liquid layer, and outputs the weight of the liquid layer of the tank as the liquid level information, or the liquid level and the volume in the liquefied gas tank Alternatively, the volume of the tank gas layer of the liquefied gas in the liquefied gas tank may be determined from the determined tank gas layer volume and the gas layer density, the weight of the tank gas layer of the liquefied gas in the liquefied gas tank may be calculated, and the weight of the tank gas layer may be output as the liquid level information. 【0072】 [Item 12] A vessel according to another aspect of the present disclosure comprises a liquefied gas tank for storing liquefied gas and a liquid level detection system as described in any of items 1 to 11, and uses the liquefied gas as fuel. 【0073】[Item 13] A liquid level detection method according to another aspect of the present disclosure is a liquid level detection method for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: detecting the differential pressure between a first height position in the height direction within the liquefied gas tank and a second height position higher than the first height position; detecting the internal pressure of the liquefied gas tank; detecting a first temperature which is the temperature of the liquid layer portion of the liquefied gas; detecting a second temperature which is the temperature of the gas layer portion of the liquefied gas; obtaining data on the height within the liquefied gas tank, data on the density of the liquid layer portion of the liquefied gas, and data on the density of the gas layer portion of the liquefied gas, and the liquid layer portion The density data is data showing the correlation between the first temperature and the density of the liquid layer, and the gas layer density data is data showing the correlation between the second temperature and the internal pressure and the density of the gas layer. The density of the liquid layer is determined from the first temperature, and the density of the gas layer is determined from the second temperature and the internal pressure. Information regarding the height of the liquid layer or the gas layer in the liquefied gas tank is obtained from the liquid layer density, the gas layer density, the height inside the liquefied gas tank, and the differential pressure as liquid level information relating to the liquid level. 【0074】 1. Liquid level detection system 2. Liquefied gas tank 11. Differential pressure detector 12. Pressure detector 13. First temperature detector 14. Second temperature detector 15. Processing circuit 18. Memory unit

Claims

1. A liquid level detection system for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: a differential pressure detector for detecting the differential pressure between a first height position and a second height position higher than the first height position in the liquefied gas tank; a pressure detector for detecting the internal pressure of the liquefied gas tank; a first temperature detector for detecting a first temperature which is the temperature of the liquid layer of the liquefied gas; a second temperature detector for detecting a second temperature which is the temperature of the gas layer of the liquefied gas; and a processing circuit including a memory, wherein the memory stores data on the height in the liquefied gas tank, data on the density of the liquid layer of the liquefied gas, and data on the density of the gas layer of the liquefied gas, the data on the density of the liquid layer being data showing the correlation between the first temperature and the density of the liquid layer, the data on the density of the gas layer being data showing the correlation between the second temperature and the internal pressure and the density of the gas layer, and the processing circuit determining the density of the liquid layer from the first temperature and determining the density of the gas layer from the second temperature and the internal pressure. A liquid level detection system that obtains information regarding the height of the liquid layer or the gas layer in the liquefied gas tank from the density of the liquid layer, the density of the gas layer, the height inside the liquefied gas tank, and the differential pressure, as liquid level information relating to the liquid level, and outputs the liquid level information.

2. The liquid level detection system according to claim 1, wherein the processing circuit determines the height of the tank liquid layer or the tank gas layer from the following first and second equations, and outputs information based on the height of the tank liquid layer or the gas layer as the liquid level information. Ht = Hl + Hg ... (First equation) ΔP = (ρl・Hl + ρg・Hg)・G ... (Second equation) Here, Ht is the height inside the liquefied gas tank, Hl is the height of the tank liquid layer, Hg is the height of the tank gas layer, ΔP is the differential pressure, ρl is the density of the liquid layer, ρg is the density of the gas layer, and G is the acceleration due to gravity.

3. The liquid level detection system according to claim 1, wherein the first temperature detector is configured to detect the temperature near the first height position, and the second temperature detector is configured to detect the temperature near the second height position.

4. The liquid level detection system according to claim 1, wherein the liquid level information includes an alarm indicating that the liquid level has fallen below a predetermined reference value.

5. The liquid level detection system according to claim 1, wherein the memory stores data of the saturation temperature of the liquefied gas, the processing circuit calculates a third temperature by averaging the second temperature and the saturation temperature, and determines the density of the gas layer from the third temperature and the internal pressure.

6. The liquid level detection system according to claim 5, wherein the saturation temperature data is data showing the correlation between the internal pressure and the saturation temperature, and the processing circuit determines the saturation temperature from the internal pressure.

7. The liquid level detection system according to any one of claims 1 to 6, wherein the data of the liquid layer density is data showing the correlation between the first temperature and the internal pressure and the liquid layer density, and the processing circuit determines the liquid layer density from the first temperature and the internal pressure.

8. The liquid level detection system according to any one of claims 1 to 6, wherein the liquefied gas includes liquefied hydrogen.

9. The liquid level detection system according to any one of claims 1 to 6, wherein the first temperature detector is installed at the bottom of the liquefied gas tank, and the second temperature detector is installed at the top of the liquefied gas tank.

10. A liquid level detection system according to any one of claims 1 to 6, wherein the memory device stores data showing the correlation between the liquid level and the volume of the liquid layer of the liquefied gas in the liquefied gas tank, and the processing circuit determines the volume of the liquid layer of the tank from the liquid level and outputs the determined volume of the liquid layer of the tank as liquid level information.

11. The memory device stores data showing the correlation between the liquid level and the volume of the liquid layer of the liquefied gas in the liquefied gas tank; the processing circuit determines the volume of the liquid layer from the liquid level, calculates the weight of the liquid layer of the liquefied gas in the liquefied gas tank from the determined volume and density of the liquid layer, and outputs the weight of the liquid layer as the liquid level information; or determines the volume of the gas layer of the liquefied gas in the liquefied gas tank from the liquid level and the volume of the liquefied gas tank, calculates the weight of the gas layer of the liquefied gas in the liquefied gas tank from the determined volume and density of the gas layer, and outputs the weight of the gas layer as the liquid level information; or A liquid level detection system according to any one of claims 1 to 6, comprising: calculating the weight of the liquid layer portion of the tank and the weight of the gas layer portion of the tank; calculating the weight of the liquefied gas inside the tank by adding the weight of the liquid layer portion of the tank and the weight of the gas layer portion of the tank; and outputting the weight inside the tank as liquid level information.

12. A ship comprising a liquefied gas tank for storing liquefied gas and a liquid level detection system according to any one of claims 1 to 6, wherein the liquefied gas is used as fuel.

13. A liquid level detection method for detecting the liquid level of liquefied gas stored in a liquefied gas tank, comprising: detecting the differential pressure between a first height position and a second height position higher than the first height position within the liquefied gas tank; detecting the internal pressure of the liquefied gas tank; detecting a first temperature which is the temperature of the liquid layer portion of the liquefied gas; detecting a second temperature which is the temperature of the gas layer portion of the liquefied gas; obtaining data on the height within the liquefied gas tank, data on the density of the liquid layer portion of the liquefied gas, and data on the density of the gas layer portion of the liquefied gas; the data on the density of the liquid layer portion being data showing the correlation between the first temperature and the density of the liquid layer portion; the data on the density of the gas layer portion being data showing the correlation between the second temperature and the internal pressure and the density of the gas layer portion; determining the density of the liquid layer portion from the first temperature; and determining the density of the gas layer portion from the second temperature and the internal pressure. A liquid level detection method that obtains information relating to the height of the liquid layer or the gas layer in the liquefied gas tank from the density of the liquid layer, the density of the gas layer, the height inside the liquefied gas tank, and the differential pressure, as liquid level information relating to the liquid level.

Images