A device, a gas filling machine and a method for preventing LNG gas filling machine from precooling and gas theft

Abstract

The present invention discloses a device, a gas filling machine and a method for preventing gas theft during precooling of an LNG gas filling machine. The device comprises a flow detection unit and a first temperature detection unit arranged on a connection path between a liquid inlet and a gas filling gun in a precooling circulation loop of the LNG gas filling machine, a second temperature detection unit arranged on a connection path between a gun seat and a liquid return port of the LNG gas filling machine in a precooling circulation loop of the LNG gas filling machine, and a control module; the control module controls the opening or closing of the gas filling valve based on a flow signal output by the flow detection unit and a temperature signal output by the first temperature detection unit and the second temperature detection unit. The device combines the flow at the end of the gas filling gun in the precooling circulation loop and the temperature information at different positions to determine whether there is an abnormal connection, and can automatically, accurately and reliably detect abnormal connection operations such as inserting a gas filling gun into a vehicle-mounted bottle, which can effectively prevent gas theft and reduce economic losses caused by human errors.

Description

Technical Field

[0001] The invention relates to the technical field of LNG gas dispensers, and in particular to a device, a gas dispenser and a method for preventing LNG gas dispenser pre-cooling gas theft. Background Art

[0002] LNG (Liquefied Natural Gas) is the abbreviation of liquefied natural gas. It is a clean and efficient energy source, mainly composed of methane. LNG is a liquid obtained by cooling gaseous natural gas to -162℃ under normal pressure.

[0003] LNG filling machine is a device used to fill liquefied natural gas into the gas cylinder of natural gas vehicles, and has the function of metering and pricing. Before filling the LNG filling machine, the pipeline (such as the output pipeline) and the core component of the filling machine, the mass flowmeter, need to be fully pre-cooled, also known as the large cycle. The purpose is to achieve the actual discharge of the gasified LNG in the pipeline, ensure the metering accuracy of the flowmeter, and achieve slow cooling to extend the service life of the pipeline. During pre-cooling, the filling gun needs to be inserted into the gun holder. If it is operated in violation of regulations and the filling gun is inserted into the vehicle bottle for large-cycle pre-cooling operation, the pre-cooling process medium will flow into the vehicle bottle instead of circulating back to the storage tank, resulting in LNG loss, which seriously affects the economic benefits of the gas station owner.

[0004] In the prior art, a Chinese patent with patent application number CN201810396776.8 discloses a method of preventing gas theft from LNG filling machines through pressure sensors. The patent proposes to add a pressure sensor to the liquid inlet and circulation loop of the LNG filling machine respectively, and to determine whether gas theft occurs by pressure difference. However, this method is prone to misjudgment, mainly due to the following reasons: (1) The pressure in the pipeline is lower during precooling (compared to when adding liquid), and the pressure value detection accuracy is low; (2) During the precooling process, large pressure fluctuations often occur, and the pressure detection value is unstable; (3) There are multiple one-way valves on the filling gun and the circulation pipeline. When LNG flows, there is a certain pressure difference between the inlet and outlet of the one-way valve, and the pressure difference value is not fixed when the flow rate changes. Summary of the invention

[0005] The present invention aims to at least solve the technical problems existing in the prior art, and in particular innovatively proposes a device, a gas dispenser and a method for preventing gas theft during pre-cooling of an LNG gas dispenser.

[0006] In order to achieve the above-mentioned purpose of the present invention, according to the first aspect of the present invention, the present invention provides a device for preventing gas theft during pre-cooling of an LNG filling machine, comprising a flow detection unit and a first temperature detection unit arranged on a connecting passage between a liquid inlet and a gas filling gun in a pre-cooling circulation loop of the LNG filling machine, a second temperature detection unit arranged on a connecting passage between a gun seat and a liquid return port of the LNG filling machine in the pre-cooling circulation loop of the LNG filling machine, and a control module respectively connected to the flow detection unit, the first temperature detection unit and the second temperature detection unit; the control module controls the opening or closing of the gas filling valve based on a flow signal output by the flow detection unit and a temperature signal output by the first temperature detection unit and the second temperature detection unit.

[0007] The above technical solution: during pre-cooling, inserting the gas gun into the gun holder is a normal connection, and other operations such as inserting the gas gun into the vehicle-mounted bottle are abnormal connections. The device combines the flow at the gas gun end in the pre-cooling circulation loop and the temperature information at different positions to determine whether there is an abnormal connection. It can automatically, accurately and reliably detect abnormal connection operations such as inserting the gas gun into the vehicle-mounted bottle, which can effectively prevent gas theft and reduce economic losses caused by human errors.

[0008] In a preferred embodiment of the present invention, the control module obtains a flow signal output by a flow detection unit. If the flow signal is less than or equal to a flow threshold, the control module controls the gas filling valve to continue to open for a pre-cooling cycle. If the flow signal is greater than the flow threshold, the control module obtains temperature information output by the first temperature detection unit, recorded as the first temperature. The control module obtains temperature information output by the second temperature detection unit, recorded as the second temperature. The control module calculates the difference between the second temperature and the first temperature. If the difference is greater than a preset temperature difference threshold, the control module closes the gas filling valve. If the difference is not greater than the preset temperature difference threshold, the control module controls the gas filling valve to continue to open for a pre-cooling cycle.

[0009] The above technical solution: the control module makes a preliminary judgment on the flow signal output by the flow detection unit to obtain a judgment result on whether the pre-cooling circulation loop is started and / or whether the pre-cooling circulation loop is operating normally. On the basis of the preliminary judgment, the temperature information output by the first temperature detection unit and the second temperature detection unit is compared to further accurately judge abnormal connections such as the gas filling gun being inserted into the vehicle-mounted bottle, thereby avoiding the loss of LNG, reducing economic losses caused by human errors, and realizing automated detection.

[0010] In a preferred embodiment of the present invention, the flow detection unit and the first temperature detection unit are integrated into one.

[0011] The above technical solution: makes the structure more compact.

[0012] In a preferred embodiment of the present invention, the control module includes a first comparator, a second comparator, a differential amplifier, a NAND gate, a first reference power supply and a second reference power supply; the output end of the flow detection unit is connected to the positive input end of the first comparator, the output end of the first reference power supply is connected to the negative input end of the first comparator, and the output end of the first comparator is connected to the first input end of the NAND gate; the output end of the second temperature detection unit is connected to the positive input end of the differential amplifier, the output end of the first temperature detection unit is connected to the negative input end of the differential amplifier, the output end of the differential amplifier is connected to the positive input end of the second comparator, the output end of the second reference power supply is connected to the negative input end of the second comparator, the output end of the second comparator is connected to the second input end of the NAND gate, and the output end of the NAND gate is connected to the control end of the gas filling valve; the voltage value output by the first reference power supply is the signal value output by the flow detection unit when the flow on the connection path between the liquid inlet and the gas filling gun reaches a preset flow threshold; the voltage value output by the second reference power supply is the voltage value output by the differential amplifier when the temperature difference detected by the second temperature detection unit and the first temperature detection unit reaches a preset temperature difference threshold.

[0013] The above technical solution: the control module adopts a hardware structure, so that the device does not require software costs.

[0014] In a preferred embodiment of the present invention, an alarm unit is further included, and the alarm unit is connected to the control module.

[0015] The above technical solution can automatically alarm and warn when gas is stolen.

[0016] In order to achieve the above-mentioned purpose of the present invention, according to the second aspect of the present invention, the present invention provides a gas filling machine, including a pre-cooling circulation loop and the device for preventing LNG gas filling machine from pre-cooling and gas theft as described in the present invention; the pre-cooling circulation loop includes a liquid inlet, a mass flow meter, a gas filling valve, a gas filling gun, a gun seat and a liquid return port in sequence; the flow detection unit and the first temperature detection unit of the device for preventing LNG gas filling machine from pre-cooling and gas theft are integrated into the mass flow meter.

[0017] The above technical solution: by detecting the flow rate and temperature through the mass flow meter in the liquid inlet pipeline of the existing gas dispenser, the cost is greatly reduced and the structure is simplified. The gas dispenser can automatically, accurately and reliably detect abnormal connection operations such as the gas gun being inserted into the vehicle-mounted bottle, which can effectively prevent gas theft and reduce economic losses caused by human errors.

[0018] In order to achieve the above-mentioned purpose of the present invention, according to the third aspect of the present invention, the present invention provides a method for preventing gas theft during pre-cooling of LNG gas filling machine, including: step S1, starting pre-cooling of LNG gas filling machine; step S2, obtaining the flow rate on the passage between the gas filling gun and the return liquid port in the pre-cooling circulation loop of the LNG gas filling machine, if the flow rate is less than or equal to the flow rate threshold, returning to execute step S2, if the flow rate is greater than the flow rate threshold, entering step S3; step S3, obtaining the temperature on the passage between the gas filling gun and the liquid inlet in the pre-cooling circulation loop of the LNG gas filling machine, recorded as the first temperature, obtaining the temperature on the passage between the gun seat and the return liquid port in the pre-cooling circulation loop of the LNG gas filling machine, recorded as the second temperature, calculating the difference between the second temperature and the first temperature, if the difference is greater than the preset temperature difference threshold, turning off the pre-cooling of the LNG gas filling machine, if the difference is not greater than the preset temperature difference threshold, returning to step S2.

[0019] The above technical solution: makes a preliminary judgment on the flow signal output by the flow detection unit to obtain a judgment result on whether the pre-cooling circulation loop is started and / or whether the pre-cooling circulation loop is operating normally. Based on the preliminary judgment, the temperature difference output by the second temperature detection unit and the first temperature detection unit is judged and compared, and abnormal connections such as the gas filling gun being inserted into the vehicle-mounted bottle can be accurately judged, thereby avoiding the loss of LNG, reducing economic losses caused by human errors, and realizing automated detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of a pre-cooling circulation loop of a gas dispenser in a specific embodiment of the present invention when it is normally connected;

[0021] Figure 2 It is a schematic diagram of a pre-cooling circulation loop of a gas dispenser when it is abnormally connected in a specific embodiment of the present invention;

[0022] Figure 3 It is a system block diagram of a device for preventing gas theft by precooling of an LNG filling machine in a specific embodiment of the present invention;

[0023] Figure 4 is a schematic diagram of a hardware circuit of a control module in a specific implementation manner of the present invention;

[0024] Figure 5 It is a flow chart of a method for preventing gas theft during precooling of an LNG filling machine in a specific embodiment of the present invention.

[0025] Reference numerals:

[0026] 1 liquid inlet; 2 mass flow meter; 3 gas filling valve; 4 gas filling gun; 5 gun seat; 6 second temperature detection unit; 7 liquid return port; 8 vehicle-mounted bottle. DETAILED DESCRIPTION

[0027] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0028] In the description of the present invention, it is necessary to understand that the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0029] In the description of the present invention, unless otherwise specified and limited, it should be noted that the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal connection between two components. It can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances.

[0030] The present invention discloses a device for preventing LNG filling machine from precooling and stealing gas. In a preferred embodiment, Figure 3 As shown, it includes a flow detection unit and a first temperature detection unit arranged on the connection path between the liquid inlet 1 and the gas filling gun 4 in the pre-cooling circulation loop of the LNG gas filling machine, a second temperature detection unit 6 arranged on the connection path between the gun seat 5 and the LNG gas filling machine return liquid port 7 in the pre-cooling circulation loop of the LNG gas filling machine, and also includes a control module connected to the flow detection unit, the first temperature detection unit and the second temperature detection unit 6 respectively; the control module controls the opening or closing of the gas filling valve 3 based on the flow signal output by the flow detection unit and the temperature signal output by the first temperature detection unit and the second temperature detection unit 6.

[0031] In this embodiment, the flow detection unit and the first temperature detection unit are preferably, but not limited to, arranged near the gas filling gun 4. The second temperature detection unit 6 is preferably, but not limited to, arranged near the gun seat 5. The flow detection unit is preferably, but not limited to, the AJWL turbine flowmeter for LNG provided by Shanghai Anjun Intelligent Technology Co., Ltd.; the first temperature detection unit and the second temperature detection unit 6 are preferably, but not limited to, the temperature sensor TR02091 for LNG natural gas. Preferably, the flow detection unit and the first temperature detection unit are integrated into one, such as selecting the existing mass flow meter 2 product, so as to make the device structure more compact. The mass flow meter 2 is preferably, but not limited to, the Emerson CNG filling machine mass flow meter CNG050290NCAPMZZX, and the LNG series low-temperature mass flow meter of Zhonglong Instrument. The control module can reuse the controller of the gas filling machine to save costs and simplify the structure, or it can not reuse the controller of the gas filling machine. When not reused, the control module can be a processor such as a single-chip microcomputer, or a hardware combination circuit.

[0032] In a preferred embodiment, the control module obtains the flow signal output by the flow detection unit. If the flow signal is less than or equal to the flow threshold, the control module controls the gas filling valve 3 to continue to open for a pre-cooling cycle. If the flow signal is greater than the flow threshold, the control module obtains the temperature information output by the first temperature detection unit, recorded as the first temperature, and the control module obtains the temperature information output by the second temperature detection unit 6, recorded as the second temperature. The control module calculates the difference between the second temperature and the first temperature. If the difference is greater than a preset temperature difference threshold, the control module closes the gas filling valve 3. If the difference is not greater than the preset temperature difference threshold, the control module controls the gas filling valve 3 to continue to open for a pre-cooling cycle.

[0033] In this embodiment, the flow threshold can be preset. The flow threshold can be 0.1%-10% of the normal loading flow. When the flow signal is greater than the flow threshold, it can be considered that pre-cooling is started and the subsequent temperature judgment steps are started; the flow threshold can also be the average flow in the loop when the pre-cooling circulation loop is normally connected. When LNG is loaded, the internal air pressure of the vehicle bottle 8 is smaller than the air pressure at the return liquid port 7. If the gas gun 4 is inserted into the vehicle bottle 8, under the effect of the higher pressure difference, the flow signal will be larger than when the pre-cooling circulation loop is normally connected. Therefore, when the flow signal is greater than the flow threshold, it can be preliminarily determined that the gas gun 4 is inserted into the vehicle bottle 8, and further temperature judgment is required to accurately detect abnormal connections; the flow threshold can also be It includes a first flow threshold and a second flow threshold, the first flow threshold is less than the second flow threshold, the first flow threshold can be 0.1%-10% of the normal loading flow, the second flow threshold can be the average flow in the loop when the pre-cooling circulation loop is normally connected, when the flow signal is less than or equal to the first flow threshold, it is considered that pre-cooling is not started, when the flow signal is greater than the first flow threshold and less than or equal to the second flow threshold, it is considered that the pre-cooling circulation loop is normally connected and pre-cooling continues, when the flow signal is greater than the second flow threshold, it is preliminarily considered that the pre-cooling circulation loop is abnormally connected, and the next step of temperature comparison and judgment is required, which can greatly reduce misjudgment and improve the accuracy of abnormal connection detection.

[0034] In this embodiment, the temperature difference threshold is preferably but not limited to greater than 5° C., or is the temperature difference between the second temperature detection unit 6 and the first temperature detection unit in the circuit when the precooling circulation circuit is normally connected.

[0035] In this embodiment, the precooling circulation loop of the gas filling machine includes a liquid inlet 1, a mass flow meter 2, a gas filling valve 3, a gas filling gun 4, a gun seat 5 and a liquid return port 7 in sequence. When the gas filling gun 4 is inserted into the gun seat 5, that is, when it is normally connected, a precooling circulation loop is formed. Figure 1 As shown, the temperature difference in the entire pre-cooling cycle is not large, and the temperature difference output by the second temperature detection unit 6 and the first temperature detection unit is not greater than the preset temperature difference threshold. Figure 2 As shown, the pre-cooling circulation loop is disconnected, and LNG enters the vehicle-mounted bottle 8 from the liquid inlet 1 through the gas gun 4. Since no low-temperature LNG flows into the circuit where the gun seat 5 and the return liquid port 7 are located, the temperature output by the second temperature detection unit 6 is greater than the temperature output by the first temperature detection unit, and the temperature difference between the two is greater than the preset temperature difference threshold.

[0036] In a preferred embodiment, Figure 4As shown, the control module includes a first comparator V1, a second comparator V2, a differential amplifier A1, a NAND gate, a first reference power supply and a second reference power supply; the output end of the flow detection unit is connected to the positive input end of the first comparator V1, the output end of the first reference power supply is connected to the negative input end of the first comparator V1, and the output end of the first comparator V1 is connected to the first input end of the NAND gate; the output end of the second temperature detection unit 6 is connected to the positive input end of the differential amplifier A1, the output end of the first temperature detection unit is connected to the negative input end of the differential amplifier A1, and the output end of the differential amplifier A1 is connected to the positive input end of the differential amplifier A1. The positive input terminal of the second comparator V2 is connected, the output terminal of the second reference power supply is connected to the negative input terminal of the second comparator V2, the output terminal of the second comparator V2 is connected to the second input terminal of the NAND gate, and the output terminal of the NAND gate is connected to the control terminal of the gas filling valve 3; the voltage value output by the first reference power supply is the signal value output by the flow detection unit when the flow on the connecting passage between the liquid inlet 1 and the gas filling gun 4 reaches a preset flow threshold; the voltage value output by the second reference power supply is the voltage value output by the differential amplifier A1 when the temperature difference detected by the second temperature detection unit 6 and the first temperature detection unit reaches a preset temperature difference threshold.

[0037] In this embodiment, the first comparator V1 and the second comparator V2 are preferably, but not limited to, LM324. The differential amplifier A1 is preferably, but not limited to, an existing differential operational amplifier circuit constructed by an operational amplifier, which is not described in detail here. The NAND gate is preferably, but not limited to, 74LS01. The first reference power supply and the second reference power supply can be existing voltage reference chips (such as voltage reference chips provided by TI, ADI and other companies) and a resistor voltage divider network connected to the output end of the voltage reference chip, which is a conventional technology in the art and is not described in detail here.

[0038] In this embodiment, when the flow signal is less than or equal to the flow threshold, or the flow signal is greater than the flow threshold and the temperature difference is less than or equal to the temperature difference threshold, the NAND gate outputs a high level and continues to open the gas filling valve 3. When the flow signal is less than or greater than the flow threshold and the temperature difference is greater than the temperature difference threshold, the NAND gate outputs a low level and the gas filling valve 3 is closed.

[0039] In a preferred embodiment, Figure 3 As shown, it also includes an alarm unit, which is connected to the control module.

[0040] In this embodiment, the alarm unit is preferably, but not limited to, an audible and visual alarm.

[0041] The present invention also discloses a gas dispenser. In a preferred embodiment, the gas dispenser includes a precooling circulation loop and the above-mentioned device for preventing the LNG gas dispenser from precooling and gas theft; the precooling circulation loop includes a liquid inlet 1, a mass flow meter 2, a gas filling valve 3, a gas filling gun 4, a gun seat 5 and a liquid return port 7 in sequence; the flow detection unit and the first temperature detection unit of the device for preventing the LNG gas dispenser from precooling and gas theft are integrated into a mass flow meter 2.

[0042] The present invention also discloses a method for preventing LNG filling machine from pre-cooling and gas theft. In a preferred embodiment, the flow chart of the method is as follows: Figure 5 As shown, it specifically includes: step S1, starting the LNG filling machine precooling; step S2, obtaining the flow rate on the passage between the gas filling gun 4 and the return liquid port 7 in the precooling circulation loop of the LNG filling machine, if the flow rate is less than or equal to the flow rate threshold, returning to execute step S2, if the flow rate is greater than the flow rate threshold, entering step S3; step S3, obtaining the temperature on the passage between the gas filling gun 4 and the liquid inlet 1 in the precooling circulation loop of the LNG filling machine, recorded as the first temperature, obtaining the temperature on the passage between the gun seat 5 and the return liquid port 7 in the precooling circulation loop of the LNG filling machine, recorded as the second temperature, calculating the difference between the second temperature and the first temperature, if the difference is greater than the preset temperature difference threshold, turning off the LNG filling machine precooling, if the difference is not greater than the preset temperature difference threshold, returning to step S2.

[0043] In this embodiment, the method can be used to check whether LNG is circulating normally in the circulation loop during the large-cycle precooling process of the LNG filling machine; by detecting the flow S1 and temperature T1 at the liquid inlet 11; detecting the temperature T2 behind the gun seat 5 in the circulation loop by the second temperature detection unit 6; when the LNG filling machine is precooling and the flow S1 is greater than the set flow threshold S, calculate the difference △T between T2 and T1; compare △T with the preset temperature difference threshold T3 between the two under normal circulation conditions; if △T is greater than T3, the large cycle of the LNG filling machine stops running, and if △T is not greater than T3, the large cycle of the LNG filling machine continues to run.

[0044] In an application scenario of this embodiment, the flow chart is as follows: Figure 3 As shown, when connected normally, the large cycle precooling LNG flow direction is as follows Figure 1As shown, the key of the present invention is to detect the temperature of the pipeline here by adding a second temperature detection unit 6 behind the gun seat 5 at the return liquid port 7. When the large circulation precooling is connected normally, the LNG liquid flows through: liquid inlet 1-mass flowmeter 2-gas valve 3-liquid outlet-hose-gas gun 4-gun seat 5-second temperature detection unit 6-return liquid port 7, the mass flowmeter 2 detects the flow S1 and temperature T1, and the temperature value detected by the second temperature detection unit 6 is T2. When S1>S (flow threshold), it is considered that LNG flows through the pipeline, and the difference between T1 and T2 is calculated, △T=T2-T1. If △T is less than the difference T3 between the two under the normal circulation state, it is determined that liquid flows through the return liquid port 7, and the gas dispenser is in normal precooling. If in this process, the gas gun 4 is not inserted into the gun holder 5 but is directly inserted into the vehicle bottle 8, LNG flows through: liquid inlet 1—mass flow meter 2—gas filling valve 3—liquid outlet—hose—gas gun 4—vehicle bottle 8. LNG flows through the mass flow meter 2 but does not pass through the second temperature detection unit 6. The second temperature detection unit 6 will quickly heat up and approach the ambient temperature due to the absorption of environmental heat due to the lack of continuous cooling of LNG. However, the mass flow meter 2 maintains a low temperature due to the continuous circulation of LNG. Then △T=T2-T1 will be much greater than T3, and the gas filling machine will immediately stop the large-circulation pre-cooling by closing the gas filling valve 3 and other operations.

[0045] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0046] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A device to prevent LNG filling machine from pre-cooling gas theft, characterized in that: It includes a flow detection unit and a first temperature detection unit provided on a connection passage between a liquid inlet and a gas filling gun in a precooling circulation loop of an LNG gas filling machine, a second temperature detection unit provided on a connection passage between a gun seat and a liquid return port of the LNG gas filling machine in a precooling circulation loop of the LNG gas filling machine, and a control module connected to the flow detection unit, the first temperature detection unit and the second temperature detection unit respectively; the control module controls the opening or closing of the gas filling valve based on a flow signal output by the flow detection unit and a temperature signal output by the first temperature detection unit and the second temperature detection unit; The control module obtains the flow signal output by the flow detection unit. If the flow signal is less than or equal to the flow threshold, the control module controls the gas filling valve to continue to open for a pre-cooling cycle. If the flow signal is greater than the flow threshold, the control module obtains the temperature information output by the first temperature detection unit, recorded as the first temperature, and the flow threshold is the average flow in the loop when the pre-cooling cycle loop is normally connected. The control module obtains the temperature information output by the second temperature detection unit, recorded as the second temperature, and the control module calculates the difference between the second temperature and the first temperature. If the difference is greater than the preset temperature difference threshold, the control module closes the gas filling valve. If the difference is not greater than the preset temperature difference threshold, the control module controls the gas filling valve to continue to open for a pre-cooling cycle.

2. The device for preventing LNG filling machine from precooling and gas theft as claimed in claim 1, characterized in that: The flow detection unit and the first temperature detection unit are integrated into one.

3. The device for preventing LNG filling machine from precooling and gas theft as claimed in claim 1, characterized in that: The control module includes a first comparator, a second comparator, a differential amplifier, a NAND gate, a first reference power supply and a second reference power supply; The output end of the flow detection unit is connected to the positive input end of the first comparator, the output end of the first reference power supply is connected to the negative input end of the first comparator, and the output end of the first comparator is connected to the first input end of the NAND gate; the output end of the second temperature detection unit is connected to the positive input end of the differential amplifier, the output end of the first temperature detection unit is connected to the negative input end of the differential amplifier, the output end of the differential amplifier is connected to the positive input end of the second comparator, the output end of the second reference power supply is connected to the negative input end of the second comparator, the output end of the second comparator is connected to the second input end of the NAND gate, and the output end of the NAND gate is connected to the control end of the gas filling valve; The voltage value output by the first reference power supply is the signal value output by the flow detection unit when the flow on the connecting passage between the liquid inlet and the gas gun reaches a preset flow threshold; the voltage value output by the second reference power supply is the voltage value output by the differential amplifier when the temperature difference detected by the second temperature detection unit and the first temperature detection unit reaches a preset temperature difference threshold.

4. The device for preventing LNG filling machine from precooling and gas theft as claimed in claim 1, characterized in that: It also includes an alarm unit, which is connected to the control module.

5. A gas filling machine, characterized in that: It comprises a precooling circulation loop and a device for preventing gas theft during precooling of an LNG filling machine as claimed in any one of claims 1 to 4; The precooling circulation loop includes a liquid inlet, a mass flow meter, a gas filling valve, a gas filling gun, a gun seat and a liquid return port in sequence; The flow detection unit and the first temperature detection unit of the device for preventing LNG filling machine from pre-cooling and gas theft are integrated into the mass flow meter.

6. A method for preventing LNG filling machine from precooling gas theft, characterized in that: include: Step S1, start precooling of LNG filling machine; Step S2, obtaining the flow rate on the passage between the gas filling gun and the liquid inlet in the precooling circulation loop of the LNG filling machine, if the flow rate is less than or equal to the flow rate threshold, returning to step S2, if the flow rate is greater than the flow rate threshold, entering step S3, the flow rate threshold is the average flow rate in the loop when the precooling circulation loop is normally connected; Step S3, obtain the temperature of the passage between the gas gun and the liquid inlet in the pre-cooling circulation loop of the LNG gas filling machine, recorded as the first temperature, obtain the temperature of the passage between the gun seat and the return liquid port in the pre-cooling circulation loop of the LNG gas filling machine, recorded as the second temperature, calculate the difference between the second temperature and the first temperature, if the difference is greater than the preset temperature difference threshold, turn off the LNG gas filling machine pre-cooling, if the difference is not greater than the preset temperature difference threshold, return to step S2.

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