An LNG loading device and loading process applicable to small-scale LNG liquefaction plants

Abstract

The present invention relates to an LNG loading device and a loading process applicable to a small-scale LNG liquefaction plant. The device includes an LNG loading and unloading arm, an LNG liquid inlet pipeline, a BOG return gas pipeline, a safety relief pipeline, an LNG discharge pipeline, a nitrogen inlet pipeline, a nitrogen purge and vent pipeline, and a small-flow bypass pipeline for a flow regulating valve. According to the on-site environment and usage conditions of the small-scale liquefaction plant, the present invention develops an LNG loading process and device, which is applicable to the working conditions of a small-scale liquefaction plant. Only one set of loading processes and devices can achieve the function of switching between loading two tank trucks, improving the cost performance of the equipment.

Description

Technical Field

[0001] The present invention relates to an LNG loading device and a loading process applicable to a small-scale LNG liquefaction plant, which can realize the functions of automatic LNG loading and switching loading of tank trucks. Background Art

[0002] According to the implementation opinions of the General Administration of Work Safety of the State Council on the "Notice of the State Council on Further Strengthening the Work Safety of Enterprises", it is required that LNG filling be carried out using an LNG loading arm to replace the previously used metal hose, which puts higher requirements on the LNG loading device. Currently, in each LNG station, whether it is an LNG storage and peak shaving station or an LNG receiving station, an LNG loading device is required. Therefore, the safety, reliability, and user-friendliness of the entire LNG loading process need to be comprehensively considered to improve the competitiveness of the entire product in the industry. In current small-scale LNG liquefaction plant stations, usually one set of loading pipelines is configured with one set of LNG loading and unloading arms, which cannot meet the functional requirements of the station. Summary of the Invention

[0003] The present invention provides an LNG loading device and a loading process applicable to a small-scale LNG liquefaction plant, which realize one-key operation and quantitative loading functions, and are automatically controlled and adjusted according to the process to meet the loading requirements of small-scale liquefaction plant stations. When loading a tank truck, the automatic cut-off valve is used to automatically switch the LNG tank truck to start the loading process.

[0004] According to the first implementation scheme of the present invention, the present invention provides an LNG loading device, which includes an LNG loading and unloading arm, an LNG inlet pipeline, a BOG return gas pipeline, a safety relief pipeline, an LNG discharge pipeline, a nitrogen inlet pipeline, a nitrogen purge and vent pipeline, and a small-flow bypass pipeline of a flow regulating valve.

[0005] There are at least two LNG loading and unloading arms, for example, there can be 2 - 4 sets, preferably 2 sets. Each set of LNG loading and unloading arms includes a liquid phase arm (LNG inlet arm) with one end connected to the LNG inlet pipeline and the other end connected to the LNG inlet of the tank truck, and a gas phase arm (BOG return arm) with one end connected to the BOG outlet of the tank truck and the other end connected to the BOG return pipeline. Each set of LNG loading and unloading arms is connected in series. One set of LNG loading and unloading arms is connected to one tank truck. Preferably, two sets of LNG loading and unloading arms are provided, namely the first LNG loading and unloading arm and the second LNG loading and unloading arm. The first LNG loading and unloading arm includes a first gas phase arm and a first liquid phase arm, and the second LNG loading and unloading arm includes a second gas phase arm and a second liquid phase arm. Preferably, a first breakaway valve is provided on the first gas phase arm, a second breakaway valve is provided on the first liquid phase arm, a third breakaway valve is provided on the second gas phase arm, and a fourth breakaway valve is provided on the second liquid phase arm. The breakaway valves meet the emergency disconnection requirements and cut off the connection between the LNG loading and unloading arms and the tank truck; One end of the nitrogen purge and vent pipeline is connected to the nitrogen outlet nozzle of the LNG loading and unloading arm (in the case of including two loading and unloading arms, it is respectively connected to the nitrogen outlet nozzles of their respective gas phase arms and liquid phase arms), and the other end is connected to the field purge and vent pipeline. A third check valve is provided on the nitrogen purge and vent pipeline;

[0006] The LNG inlet pipeline is divided into at least two branches, and the outlet end of each branch is respectively connected to the LNG inlet of the liquid phase arm of one set of LNG loading and unloading arms. The inlet end of the LNG inlet pipeline is connected to the LNG cold box outlet pipeline. For example, the LNG inlet pipeline is divided into two branches, namely the first LNG inlet pipeline branch and the second LNG inlet pipeline branch. The first LNG inlet pipeline branch is connected to the first liquid phase arm, and the second LNG inlet pipeline branch is connected to the second liquid phase arm;

[0007] Preferably, along the LNG transportation direction on the LNG inlet pipeline, there are successively provided a first pipeline filter for filtering LNG impurities (for example, it can be an inlet conical filter for protecting the flowmeter and various valves on the LNG inlet pipeline), an LNG flowmeter for measuring the LNG flow rate (ensuring the measurement accuracy and realizing the function of LNG quantitative loading), a first flow regulating valve (by adjusting the opening degree of the first flow regulating valve, realizing the functions of small - flow precooling, large - flow loading and unloading of the vehicle, and small - flow closing of the device, ensuring the start - up precooling and slow - closing functions of the device, avoiding water hammer phenomenon, and ensuring the stable operation of the device), a first check valve (LNG inlet check valve, ensuring the one - way flow of the medium). Preferably, after passing through the first check valve, the LNG inlet pipeline is divided into at least two branches, and a cut - off valve is provided on each branch. For example, a first cut - off valve is provided on the first LNG inlet pipeline branch, and a second cut - off valve is provided on the second LNG inlet pipeline branch,

[0008] One end of the BOG return pipeline is connected to the gas-phase arm nozzle of the LNG loading and unloading arm, and the other end is connected to the BOG pipeline of the station, so that the BOG of the tank truck is discharged to the LNG storage tank. Preferably, the BOG return pipelines connected to each gas-phase arm converge into a main BOG return pipeline and then are connected to the BOG pipeline of the station. A cut-off valve is provided on each BOG return pipeline. Preferably, the BOG return pipelines are respectively the first BOG return pipeline and the second BOG return pipeline. After the first BOG return pipeline and the second BOG return pipeline converge, they are connected to the BOG pipeline of the station. The first BOG return pipeline is connected to the first gas-phase arm, and the second BOG return pipeline is connected to the second gas-phase arm. A third cut-off valve is provided on the first BOG return pipeline, and a fourth cut-off valve is provided on the second BOG return pipeline;

[0009] One end of the nitrogen inlet pipeline is connected to the nitrogen pipeline of the station, and the other end is connected to the nitrogen nozzle of the LNG loading and unloading arm, so that nitrogen enters the LNG loading and unloading arm for purging and replacement. A ninth stop valve is provided on the nitrogen inlet pipeline. If the rotary joint of the LNG loading and unloading arm requires positive pressure sealing with nitrogen, the nitrogen needs to be connected to the rotary joint for sealing to prevent moisture in the air from entering the rotary gap of the rotary joint and causing freezing, which would make the rotary joint unable to rotate;

[0010] One end of the LNG discharge pipeline is connected to the LNG inlet pipeline, and the other end is connected to the BOG return pipeline. When the loading and unloading of the vehicle stops, the remaining LNG liquid between the first flow regulating valve and the first or second cut-off valve, and between the first or second cut-off valve of the LNG inlet pipeline and the LNG loading and unloading arm is automatically drained, avoiding overpressure due to the expansion of the remaining LNG. The first LNG inlet pipeline branch and the second LNG inlet pipeline branch of the two branches of the LNG inlet pipeline and the LNG inlet pipeline respectively lead out the first drain pipeline, the second drain pipeline and the third drain pipeline (the three converge into the LNG discharge pipeline). A fifth cut-off valve, a sixth cut-off valve and a seventh cut-off valve are respectively provided on the first drain pipeline, the second drain pipeline and the third drain pipeline;

[0011] The small-flow bypass pipeline of the flow regulating valve is connected between the inlet and outlet of the first flow regulating valve and is opened during pre-cooling before the start of loading. At this time, the first flow regulating valve is closed. The small-flow bypass pipeline of the flow regulating valve is connected to the LNG discharge pipeline and enters the BOG return pipeline to keep the LNG inlet pipeline unblocked and ensure small-flow circulation. An eighth cut-off valve, such as a manual stop valve, can be provided on the small-flow bypass pipeline of the flow regulating valve;

[0012] The LNG loading device for a small LNG liquefaction plant of the present application is provided with a safety valve vent pipeline. One end of the pipeline is connected to the station vent pipeline, enabling overpressure relief gas to enter the station vent pipeline. The other end is respectively connected to safety valves provided at the rear of the shut-off valves on the LNG liquid inlet pipeline and its branch pipes. In the case of two branch pipes, the other end is connected to the first, second, and third safety valves provided at both ends of the first and second shut-off valves on the LNG pipeline, that is, the first, second, and third safety valve vent pipelines are respectively led out from the first LNG liquid inlet pipeline branch, the second LNG liquid inlet pipeline branch, and the LNG liquid inlet pipeline. The third safety valve, the second safety valve, and the first safety valve are respectively arranged on the first, second, and third safety valve vent pipelines, and the first, second, and third safety valve vent pipelines converge into the safety valve vent pipeline. The first, second, and third safety valves are arranged in such a way that pipelines are led out from the pipelines where the safety valves are installed to connect to the safety valve vent pipeline.

[0013] Furthermore, the inlet end of the first safety valve is connected to the LNG liquid inlet pipeline and discharges when the liquid or vaporized liquid between the first flow regulating valve and the first shut-off valve is overpressured. The inlet end of the second safety valve is connected to the second LNG liquid inlet pipeline branch and discharges when the liquid or vaporized liquid between the first shut-off valve and the first liquid phase arm is overpressured. The inlet end of the third safety valve is connected to the first LNG liquid inlet pipeline branch and discharges when the liquid or vaporized liquid between the second shut-off valve and the second liquid phase arm is overpressured. The outlet ends of the first, second, and third safety valves are aggregated and connected to the station safety relief pipeline.

[0014] Furthermore, after the loading and unloading of the first gas phase arm and the first liquid phase arm are completed, the fifth shut-off valve is opened to discharge the liquid between the first shut-off valve and the first liquid phase arm, and at the same time, the seventh shut-off valve is opened to discharge the liquid between the first flow regulating valve and the first shut-off valve to prevent overpressure due to LNG vaporization. After the loading and unloading of the second gas phase arm and the second liquid phase arm are completed, the sixth shut-off valve is opened to discharge the liquid between the second shut-off valve and the second liquid phase arm, and at the same time, the seventh shut-off valve is opened to discharge the liquid between the first flow regulating valve and the second shut-off valve to prevent overpressure due to LNG vaporization.

[0015] Further, one end of the nitrogen inlet pipeline is connected to the nitrogen pipeline of the station yard, and the other end is connected to the nitrogen gas port of the LNG loading and unloading arm, so that nitrogen enters the LNG loading and unloading arm for purging and replacement. When the first gas phase arm and the second liquid phase arm are working, the nitrogen inlet pipeline is connected to the first gas phase arm through the tenth cut-off valve, and nitrogen enters the first gas phase arm for purging. The purging gas enters the nitrogen purging and venting pipeline through the twelfth cut-off valve and is discharged; it is connected to the first liquid phase arm through the eleventh cut-off valve, enters the second liquid phase arm for purging, and the purging gas enters the nitrogen purging and venting pipeline (nitrogen outlet pipeline) through the thirteenth cut-off valve and is discharged. When the second gas phase arm and the second liquid phase arm are working, the nitrogen inlet pipeline is connected to the second gas phase arm through the fourteenth cut-off valve, and nitrogen enters the second gas phase arm for purging. The purging gas enters the nitrogen purging and venting pipeline (nitrogen outlet pipeline) through the sixteenth cut-off valve and is discharged; it is connected to the second liquid phase arm of the loading and unloading arm through the fifteenth cut-off valve, enters the second liquid phase arm for purging, and the purging gas enters the nitrogen purging and venting pipeline (nitrogen outlet pipeline) through the seventeenth cut-off valve and is discharged.

[0016] Further, the LNG loading and unloading arm is connected to the tank truck through a swivel joint, and the swivel joint structure is adopted to achieve flexible LNG loading. At present, there are two structures for the swivel joint of the LNG loading and unloading arm. One type of swivel joint is connected to the outside. If air enters the swivel joint, the moisture in it will cause the joint to freeze, and nitrogen sealing is required. One type of swivel joint is a self-sealing structure, which has no contact with the outside and does not require nitrogen positive pressure sealing. If the LNG loading and unloading arm uses a swivel joint with nitrogen sealing, then nitrogen needs to enter the swivel joint to maintain positive pressure sealing to prevent moisture in the air from entering the rotating gap of the swivel joint and causing freezing, making the swivel joint unable to rotate.

[0017] Further, the small flow bypass pipeline of the flow regulating valve is connected between the inlet and outlet of the first flow regulating valve. It is opened before precooling at the start of loading, connected to the LNG discharge pipeline, and enters the BOG return gas pipeline to achieve a small flow precooling cycle. A manual stop valve is provided on the small flow bypass pipeline of the flow regulating valve.

[0018] Further, the nitrogen inlet pipeline is respectively connected to the gas phase arm and the liquid phase arm through valves. One end of the nitrogen purging and venting pipeline is also respectively connected to the nitrogen outlets of the gas phase arm and the liquid phase arm through valves, and the other end of the nitrogen purging and venting pipeline is connected to the station yard venting pipeline. A ninth stop valve and a second pipeline filter are successively provided on the nitrogen inlet pipeline along the nitrogen flow direction.

[0019] Further, pressure transmitters and temperature transmitters are respectively provided on the LNG inlet pipeline and the BOG return gas pipeline.

[0020] Preferably, along the flow direction of LNG, a first pressure transmitter and a first temperature transmitter are sequentially arranged between the LNG inlet pipeline and the first check valve at the branch pipe outlet. A fourth pressure transmitter is arranged on the branch pipe of the first LNG inlet pipeline between the first cut-off valve and the first liquid phase arm. A second pressure transmitter is arranged on the branch pipe of the second LNG inlet pipeline between the second cut-off valve and the second liquid phase arm. A second temperature transmitter is arranged on the BOG return gas main pipeline after the confluence of each BOG return gas pipeline. A fifth pressure transmitter is arranged on the first BOG return gas pipeline between the fourth cut-off valve and the first gas phase arm. A third pressure transmitter is arranged on the second BOG return gas pipeline between the third cut-off valve and the second gas phase arm.

[0021] The pressure gauge of the LNG inlet pipeline, i.e., the first pressure transmitter, measures the pressure between the first regulating valve and the first cut-off valve. When the pressure exceeds the set value, it indicates that the seventh cut-off valve is not opened, and at this time, the seventh cut-off valve is opened; the second pressure transmitter measures the pressure between the second cut-off valve and the second liquid phase arm. When the pressure exceeds the set value, it indicates that the sixth cut-off valve is not opened, and at this time, the sixth cut-off valve is opened; the fourth pressure transmitter measures the pressure between the first cut-off valve and the first liquid phase arm. When the pressure exceeds the set value, it indicates that the fifth cut-off valve is not opened, and at this time, the fifth cut-off valve is opened; the third pressure transmitter measures the pressure between the third cut-off valve and the second gas phase arm. When the pressure exceeds the set value, it indicates that the third cut-off valve is not opened, and at this time, the third cut-off valve is opened; the fifth pressure transmitter measures the pressure between the fourth cut-off valve and the first gas phase arm. When the pressure exceeds the set value, it indicates that the fourth cut-off valve is not opened, and at this time, the fourth cut-off valve is opened. The first temperature transmitter measures the LNG temperature of the LNG inlet pipeline, displays the LNG liquid temperature, and can judge whether the LNG is a cryogenic liquid or a high-temperature liquid according to the temperature; the second temperature transmitter measures the BOG temperature of the BOG return gas pipeline. When the temperature of the second temperature transmitter ≤ -100 °C during loading, it represents that the precooling is completed, and the precooling speed can be monitored through the temperature.

[0022] In this application, all cut-off valves are preferably automatic cut-off valves, which fully automatically realize the opening and closing functions of each pipeline and realize the one-key start and stop of the LNG loading device.

[0023] Furthermore, check valves are respectively arranged on the nitrogen purging and venting pipeline and the nitrogen inlet pipeline; preferably, a nitrogen inlet check valve is arranged on the nitrogen inlet pipeline near the connection between the LNG loading and unloading arm and the nitrogen inlet pipeline; a nitrogen purging gas outlet check valve is arranged on the nitrogen purging and venting pipeline near the connection between the LNG loading and unloading arm and the nitrogen purging and venting pipeline to ensure the one-way flow of the medium.

[0024] According to the second embodiment of the present invention, the present invention further provides an LNG loading process using the above device, including the following steps:

[0025] 1) Nitrogen replacement step: Introduce nitrogen from the nitrogen inlet pipeline to the LNG loading and unloading arm (gas phase arm and liquid phase arm) of the first tank truck for nitrogen replacement. After replacement, discharge from the nitrogen purge vent pipeline. When loading, the LNG loading and unloading arm is connected to the tank truck to be loaded. Open the nitrogen replacement valve on the LNG loading and unloading arm to start nitrogen replacement to prevent air from remaining at the connection between the loading and unloading arm and the tank truck.

[0026] 2) Precooling step: LNG enters the first tank truck through the LNG liquid inlet pipeline from the LNG cold box, BOG comes out of the tank truck and returns to the BOG recovery device through the BOG return gas pipeline, LNG starts flow measurement through the LNG flowmeter, initially, the first flow regulating valve gives a 2-10% opening, preferably 3-6%, such as 5% opening, the cut-off valve of the first branch of the LNG liquid inlet pipeline is opened (for example, the first cut-off valve), so that LNG liquid enters the first tank truck through the LNG liquid inlet pipeline, the BOG gas of the first tank truck enters the BOG return gas pipeline connected to the tank truck, the cut-off valve on the BOG return gas pipeline is opened (for example, the fourth cut-off valve is opened), so that the LNG loading device starts precooling; temperature alarm (first and second temperature transmitters) and pressure alarm device (first to fourth pressure transmitters) may be provided; when the return gas temperature T2≤-100℃, the small flow loading is completed and the large flow loading is started;

[0027] 3) High-flow loading steps: When the temperature detected on the BOG return air pipeline reaches ≤-100℃, pre-cooling is completed, and high-flow loading to the LNG tank truck begins. At this time, the opening of the first flow regulating valve is gradually opened to make the loading flow reach 100% (the flow rate can be, for example, 0-80 m³ / h, preferably 0-60- m³ / h, usually the maximum is not allowed to exceed 80m³ / h, and the regulating valve opening is controlled by the maximum loading flow), and the high-flow loading process of the LNG tank truck begins; temperature alarms (first and second temperature transmitters) and pressure alarm devices (first to fourth pressure transmitters) may be provided. When the return air temperature T2≤-100℃, the low-flow loading ends and the high-flow loading starts; when the pressure and temperature exceed the set value, and when an abnormal situation occurs during the loading process, the system alarm is cut off;

[0028] 4) Small flow stop step: When the LNG tank truck is loaded with about 90%±5%, preferably 90%±2%, for example, 90%, the first flow control valve starts to adjust the opening to 5%±3%, further 5%±2% or 5%±1%, so that the device starts a small flow stop process. When the set flow reaches 100% (by collecting the flow meter signal and feeding it back to the batch controller, the batch controller outputs a signal to the flow control valve to adjust the flow, and the loading flow is set to 100% by sending a signal from the station loading management system), the loading stops to ensure stable parking and avoid water hammer;

[0029] 5) LNG discharging step: When the LNG loading onto the truck is completed, the first flow regulating valve and the first cut-off valve on the LNG liquid inlet pipeline automatically close, and the fifth cut-off valve and the seventh cut-off valve on the LNG discharging pipeline automatically open, starting the discharging of the LNG liquid inlet pipeline to avoid overpressure due to the expansion of the residual LNG.

[0030] 6) Truck switching step: Before the loading of the first truck is completed, for example, about ten minutes before the end, a truck switching prompt is given (prompted by the batch controller). At this time, the loading staff needs to check the valve opening status of the second truck to be loaded and wait for the start of loading. When the loading of the first truck is completed, the first and fourth cut-off valves automatically close, the fifth cut-off valve automatically closes, and after a delay of, for example, 1 minute, the second and third cut-off valves automatically open, and the sixth cut-off valve opens after a delay of, for example, 10 seconds, realizing the switching of the loading from the first truck to the second truck. At this time, the loading of the first truck is completed. Similarly to the first truck, the loading process of the second truck starts.

[0031] Advantages of the present invention:

[0032] The LNG loading process of the present invention not only saves the investment in equipment but also ensures the more stable operation of the system. For the LNG loading device for a small-scale LNG liquefaction plant described in the present invention, the yard operator only needs to connect the LNG loading and unloading device to the flange of the LNG truck and click to start according to the signal. At the same time, the device and process achieve automatic switching of LNG truck loading through double-arm switching, and LNG loading can be achieved without setting up an LNG storage tank and an LNG booster pump. It reduces the generation of BOG, the loading pressure is stable, the reliability is high, and the equipment investment is low. Description of the drawings

[0033] Figure 1 is a schematic diagram of an LNG loading device applicable to a small-scale LNG liquefaction plant described in the present invention.

[0034] Figure 2 is a schematic diagram of the rotary joint of the LNG loading arm.

[0035] Reference numerals:

[0036] 1 is the gas phase port of the first truck C1, connected to ZXB1; 2 is the liquid phase port of the first truck C1, connected to ZXB2; 3 is the boosting port of the first truck C1, which does not need to be connected to this device; 4 is the gas phase port of the second truck C2, connected to ZXB3; 5 is the liquid phase port of the second truck C2, connected to ZXB4; 6 is the boosting port of the second truck C2, which does not need to be connected to this device; 7 - rotary joint.

[0037] L1 - Nitrogen purging and venting pipeline, L2 - LNG inlet pipeline, L21 - First branch of LNG inlet pipeline, L22 - Second branch of LNG inlet pipeline, L3 - Safety valve venting pipeline (safety relief pipeline), L31 - First safety valve venting pipeline, L32 - Second safety valve venting pipeline, L33 - Third safety valve venting pipeline, L4 - BOG return gas pipeline, L41 - First BOG return gas pipeline, L42 - Second BOG return gas pipeline, L5 - Nitrogen inlet pipeline, L6 - LNG discharge pipeline, L61 - First drain pipeline, L62 - Second drain pipeline, L63 - Third drain pipeline, L7 - Small flow bypass pipeline of flow regulating valve;

[0038] C1 - First tanker truck, C2 - Second tanker truck;

[0039] FIT1 - LNG flowmeter; FV1 - First flow regulating valve;

[0040] V1 - First cut-off valve, V2 - Second cut-off valve, V3 - Third cut-off valve, V4 - Fourth cut-off valve, V5 - Fifth cut-off valve, V6 - Sixth cut-off valve, V7 - Seventh cut-off valve;

[0041] V8 - Manual stop valve (Eighth cut-off valve), V9 - Ninth cut-off valve; V10 - Tenth cut-off valve, V11 - Eleventh cut-off valve, V12 - Twelfth cut-off valve, V13 - Thirteenth cut-off valve, V14 - Fourteenth cut-off valve, V15 - Fifteenth cut-off valve, V16 - Sixteenth cut-off valve, V17 - Seventeenth cut-off valve;

[0042] E1 - First breakaway coupling, E2 - Second breakaway coupling, E3 - Third breakaway coupling, E4 - Fourth breakaway coupling;

[0043] ZXB1 - First gas phase arm, ZXB2 - First liquid phase arm, ZXB3 - Second gas phase arm, ZXB4 - Second liquid phase arm; F1 - First pipeline filter; F2 - Second pipeline filter;

[0044] P1 - First pressure transmitter, P2 - Second pressure transmitter, P3 - Third pressure transmitter, P4 - Fourth pressure transmitter, P5 - Fifth pressure transmitter;

[0045] T1 - First temperature transmitter, T2 - Second temperature transmitter;

[0046] CH1 - First check valve, CH2 - Nitrogen inlet check valve (Second check valve), CH3 - Third check valve. Detailed implementation manners

[0047] The present invention will be further described below with reference to the accompanying drawings.

[0048] Figure 1A multi-functional LNG loading and unloading vehicle device of the present invention is shown, which includes an LNG loading and unloading arm, a nitrogen purging and venting pipeline L1, an LNG liquid inlet pipeline L2, a safety venting pipeline L3, a BOG return gas pipeline L4, a nitrogen inlet pipeline L5, an LNG discharge pipeline L6, and a small flow bypass pipeline L7 of a flow regulating valve.

[0049] There are two sets of LNG loading and unloading arms, namely the first LNG loading and unloading arm and the second LNG loading and unloading arm. Each set of LNG loading and unloading arms includes a liquid phase arm with one end connected to the LNG liquid inlet pipeline L2 and the other end connected to the LNG inlet of the tank truck, which is the LNG liquid inlet arm, and a gas phase arm with one end connected to the BOG outlet of the tank truck and the other end connected to the BOG return gas pipeline L4, which is the BOG return gas arm. The first LNG loading and unloading arm and the second LNG loading and unloading arm are connected in series. The first LNG loading and unloading arm is connected to the first tank truck C1, and the second LNG loading and unloading arm is connected to the second tank truck C2. The first LNG loading and unloading arm includes a first gas phase arm ZXB1 and a first liquid phase arm ZXB2. The second LNG loading and unloading arm includes a second gas phase arm ZXB3 and a second liquid phase arm ZXB4. A first breakaway valve E1 is provided on the first gas phase arm, a second breakaway valve E2 is provided on the first liquid phase arm, a third breakaway valve E3 is provided on the second gas phase arm, and a fourth breakaway valve E4 is provided on the second liquid phase arm. The breakaway valves meet the emergency disconnection requirements and cut off the connection between the LNG loading and unloading arm and the tank truck.

[0050] One end of the nitrogen purging and venting pipeline L1 is connected to the nitrogen outlet nozzle of the LNG loading and unloading arm (in the case of including two loading and unloading arms, it is respectively connected to the nitrogen outlet nozzles of the respective gas phase arms and liquid phase arms), and the other end is connected to the field purging and venting pipeline. A third check valve CH3 is provided on the nitrogen purging and venting pipeline L1.

[0051] The LNG inlet pipeline L2 is divided into two branch pipes, namely the first LNG inlet pipeline branch pipe L21 and the second LNG inlet pipeline branch pipe L22. The outlet end of each branch pipe is connected to the liquid phase arm LNG inlet of a set of LNG loading and unloading arms. The first LNG inlet pipeline branch pipe L21 is connected to the first liquid phase arm ZXB2, and the second LNG inlet pipeline branch pipe L22 is connected to the second liquid phase arm ZXB4. The inlet end of the LNG inlet pipeline L2 is connected to the LNG cold box outlet pipeline. Along the LNG transportation direction on the LNG inlet pipeline L2, there are successively arranged a first pipeline filter F1 for filtering LNG impurities (for example, it can be an inlet conical filter to protect the flowmeter and various valves on the LNG inlet pipeline), an LNG flowmeter FIT1 for measuring the LNG flow rate (ensuring the measurement accuracy and realizing the function of quantitative loading of LNG), a first flow regulating valve FV1 (by adjusting the opening degree of the first flow regulating valve FV1, realizing the functions of pre-cooling with a small flow rate, loading and unloading vehicles with a large flow rate, and closing with a small flow rate of the device, ensuring the start-up pre-cooling and slow closing functions of the device, avoiding the occurrence of water hammer phenomenon, and ensuring the stable operation of the device), a first check valve CH1 (LNG inlet check valve to ensure the one-way flow of the medium). A first cut-off valve V1 is arranged on the first LNG inlet pipeline branch pipe L21, and a second cut-off valve V2 is arranged on the second LNG inlet pipeline branch pipe L22.

[0052] One end of the BOG return gas pipeline L4 is connected to the gas phase arm pipe orifice of the LNG loading and unloading arm, and the other end is connected to the station BOG pipeline, so that the BOG of the tank truck is discharged to the LNG storage tank. Preferably, the BOG return gas pipeline L4 is respectively the first BOG return gas pipeline L41 and the second BOG return gas pipeline L42. After the first BOG return gas pipeline L41 and the second BOG return gas pipeline L42 converge, they are connected to the station BOG pipeline. The first BOG return gas pipeline L41 is connected to the first gas phase arm ZXB1, and the second BOG return gas pipeline L42 is connected to the second gas phase arm ZXB3. A third cut-off valve V3 is arranged on the first BOG return gas pipeline, and a fourth cut-off valve V4 is arranged on the second BOG return gas pipeline.

[0053] One end of the nitrogen inlet pipeline L5 is connected to the station nitrogen pipeline, and the other end is connected to the nitrogen pipe orifice of the LNG loading and unloading arm, so that nitrogen enters the LNG loading and unloading arm for purging and replacement. A ninth stop valve V9 is arranged on the nitrogen inlet pipeline. If the rotating joint of the LNG loading and unloading arm requires positive pressure sealing with nitrogen, the nitrogen needs to be connected to the rotating joint for sealing to avoid moisture in the air entering the rotating gap of the rotating joint and causing freezing, making the rotating joint unable to rotate.

[0054] One end of the LNG relief pipeline L6 is connected to the LNG liquid inlet pipeline L2, and the other end is connected to the BOG return gas pipeline L4. When the loading and unloading vehicle stops, the LNG liquid remaining between the first flow regulating valve FV1 and the first or second shut-off valve, and between the first or second shut-off valve of the LNG liquid inlet pipeline and the LNG loading and unloading arm is automatically drained, avoiding overpressure caused by the expansion of the remaining LNG. The first LNG liquid inlet pipeline branch L21, the second LNG liquid inlet pipeline branch L22 of the two branches of the LNG liquid inlet pipeline L2, and the LNG liquid inlet pipeline L2 respectively lead out the first drain pipeline L61, the second drain pipeline L62, and the third drain pipeline L63 (the three converge into the LNG relief pipeline L6). The fifth shut-off valve V5, the sixth shut-off valve V6, and the seventh shut-off valve V7 are respectively provided on the first drain pipeline L61, the second drain pipeline L62, and the third drain pipeline L63.

[0055] The small flow bypass pipeline L7 of the flow regulating valve is connected between the inlet and outlet of the first flow regulating valve FV1 and is opened during pre-cooling before the start of loading. At this time, the first flow regulating valve FV1 is closed. The small flow bypass pipeline L7 of the flow regulating valve is connected to the LNG relief pipeline L6 and enters the BOG return gas pipeline L4 to keep the LNG liquid inlet pipeline L2 unobstructed and ensure small flow circulation. A manual stop valve V8 is provided on the small flow bypass pipeline L7 of the flow regulating valve.

[0056] The LNG loading device for a small LNG liquefaction plant of the present application is further provided with a safety valve vent pipeline L3. One end of it is connected to the station vent pipeline to allow the overpressure relief gas to enter the station vent pipeline, and the other end is connected to the first, second, and third safety valves TSV-01, TSV-02, and TSV-03 provided at both ends of the first shut-off valve V1 and the second shut-off valve V2 of the LNG pipeline (that is, the first, second, and third safety valve vent pipelines L31, L32, and L33 are respectively led out from the first LNG liquid inlet pipeline branch L21, the second LNG liquid inlet pipeline branch L22, and the LNG liquid inlet pipeline L2. The third safety valve TSV-03, the second safety valve TSV-02, and the first safety valve TSV-01 are respectively provided on the first, second, and third safety valve vent pipelines L31, L32, and L33. The first, second, and third safety valve vent pipelines L31, L32, and L33 converge into the safety valve vent pipeline L3). The first, second, and third safety valves are arranged in such a way that pipelines are led out from the pipelines where the safety valves are installed to connect to the safety valve vent pipelines.

[0057] The inlet end of the first safety valve TSV-01 is connected to the LNG liquid inlet pipeline L2 and discharges when the liquid or vaporized liquid between the first flow regulating valve FV1 and the first shut-off valve V1 is over-pressurized. The inlet end of the second safety valve TSV-02 is connected to the second LNG liquid inlet pipeline branch L22 and discharges when the liquid or vaporized liquid between the first shut-off valve V1 and the first liquid phase arm ZXB2 is over-pressurized. The inlet end of the third safety valve TSV-03 is connected to the first LNG liquid inlet pipeline branch L21 and discharges when the liquid or vaporized liquid between the second shut-off valve V2 and the second liquid phase arm ZXB4 is over-pressurized. The outlet ends of the first, second, and third safety valves are aggregated and connected to the station safety relief pipeline.

[0058] When the loading and unloading of the first gas phase arm ZXB1 and the first liquid phase arm ZXB2 from / to the tank truck are completed, open the fifth shut-off valve V5 to discharge the liquid between the first shut-off valve V1 and the first liquid phase arm ZXB2, and at the same time open the seventh shut-off valve V7 to discharge the liquid between the first flow regulating valve FV1 and the first shut-off valve V1 to prevent over-pressurization due to LNG vaporization. When the loading and unloading of the second gas phase arm ZXB3 and the second liquid phase arm ZXB4 from / to the tank truck are completed, open the sixth shut-off valve V6 to discharge the liquid between the second shut-off valve V2 and the second liquid phase arm ZXB4, and at the same time open the seventh shut-off valve V7 to discharge the liquid between the first flow regulating valve FV1 and the second shut-off valve V2 to prevent over-pressurization due to LNG vaporization.

[0059] Preferably, a rotary joint 7 (as Figure 2 shown) is used to connect the LNG loading and unloading arm to the tank truck, and the rotary joint structure is adopted to achieve flexible LNG loading.

[0060] The nitrogen inlet pipeline L5 is respectively connected to the nitrogen gas ports of the gas phase arms ZXB1 / ZXB3 and the liquid phase arms ZXB2 / ZXB4 through valves (the tenth valve V10, the eleventh valve V11, the twelfth valve V12, the thirteenth valve V13), so that nitrogen gas enters the LNG loading and unloading arm for purging and replacement. One end of the nitrogen gas purging and venting pipeline L1 is also respectively connected to the nitrogen gas outlets of the gas phase arms ZXB1 / ZXB3 and the liquid phase arms ZXB2 / ZXB4 through valves (the fourteenth valve V14, the fifteenth valve V15, the sixteenth valve V16, the seventeenth valve V17), and the other end of the nitrogen gas purging and venting pipeline is connected to the station venting pipeline. A second pipeline filter F2 may be provided on the nitrogen inlet pipeline L5. When the first gas phase arm ZXB1 and the second liquid phase arm ZXB2 are working, the nitrogen inlet pipeline L5 is connected to the first gas phase arm ZXB1 through the tenth cut-off valve V10, and nitrogen gas enters the first gas phase arm ZXB1 for purging. The purging gas enters the nitrogen gas purging and venting pipeline L1 through the twelfth cut-off valve V12 and is discharged; it is connected to the first liquid phase arm ZXB2 through the eleventh cut-off valve V11, and nitrogen gas enters the second liquid phase arm ZXB2 for purging. The purging gas enters the nitrogen gas purging and venting pipeline (nitrogen gas outlet pipeline) L1 through the thirteenth cut-off valve V13 and is discharged. When the second gas phase arm ZXB3 and the second liquid phase arm ZXB4 are working, the nitrogen inlet pipeline L5 is connected to the second gas phase arm ZXB3 through the fourteenth cut-off valve V14, and nitrogen gas enters the second gas phase arm ZXB3 for purging. The purging gas enters the nitrogen gas purging and venting pipeline (nitrogen gas outlet pipeline) L1 through the sixteenth cut-off valve V16 and is discharged; it is connected to the second liquid phase arm ZXB4 of the loading and unloading arm through the fifteenth cut-off valve V15, and nitrogen gas enters the second liquid phase arm ZXB4 for purging. The purging gas enters the nitrogen gas purging and venting pipeline (nitrogen gas outlet pipeline) L1 through the seventeenth cut-off valve V17 and is discharged.

[0061] Along the flow direction of LNG, a first pressure transmitter P1 and a first temperature transmitter T1 are successively provided between the LNG inlet pipeline L2 at the branch point and the first check valve. A fourth pressure transmitter P4 is provided on the branch pipe of the first LNG inlet pipeline between the first cut-off valve V1 and the first liquid phase arm ZXB2. A second pressure transmitter P2 is provided on the branch pipe of the second LNG inlet pipeline between the second cut-off valve V2 and the second liquid phase arm ZXB4. A second temperature transmitter T2 is provided on the BOG return gas main pipeline after the first BOG return gas pipeline and the second BOG return gas pipeline converge. A fifth pressure transmitter P5 is provided on the first BOG return gas pipeline L41 between the fourth cut-off valve V4 and the first gas phase arm. A third pressure transmitter P3 is provided on the second BOG return gas pipeline L42 between the third cut-off valve V3 and the second gas phase arm. A nitrogen gas inlet check valve CH2 is provided on the nitrogen gas inlet pipeline near the connection between the LNG loading and unloading arm and the nitrogen gas inlet pipeline to ensure the one-way flow of the medium.

[0062] The LNG inlet pipeline pressure gauge, i.e., the first pressure transmitter P1, measures the pressure between the first flow regulating valve and the first cut-off valve. When the pressure exceeds the set value, it indicates that the seventh cut-off valve V7 is not opened. At this time, the seventh cut-off valve V7 is opened; the second pressure transmitter P2 measures the pressure between the second cut-off valve V2 and the second liquid phase arm ZXB4. When the pressure exceeds the set value, it indicates that the sixth cut-off valve V6 is not opened. At this time, the sixth cut-off valve V6 is opened; the fourth pressure transmitter P4 measures the pressure between the first cut-off valve V1 and the first liquid phase arm ZXB2. When the pressure exceeds the set value, it indicates that the fifth cut-off valve V5 is not opened. At this time, the fifth cut-off valve V5 is opened; the third pressure transmitter P3 measures the pressure between the third cut-off valve V3 and the second gas phase arm ZXB3. When the pressure exceeds the set value, it indicates that the third cut-off valve V3 is not opened. At this time, the third cut-off valve V3 is opened; the fifth pressure transmitter P5 measures the pressure between the fourth cut-off valve V4 and the first gas phase arm ZXB1. When the pressure exceeds the set value, it indicates that the fourth cut-off valve V4 is not opened. At this time, the fourth cut-off valve V4 is opened. The first temperature transmitter T1 (installed on the LNG inlet pipeline L2) measures the temperature of LNG in the LNG inlet pipeline L2, displays the LNG liquid temperature, and can judge whether the LNG is a cryogenic liquid or a high-temperature liquid according to the temperature; the second temperature transmitter T2 (installed on the BOG return gas pipeline L4) measures the temperature of BOG in the BOG return gas pipeline L4. When the temperature of the second pressure transmitter T2 ≤ -100 °C during loading, it represents that the precooling is completed, and the precooling speed can be monitored through the temperature.

[0063] In this application, all cut-off valves are preferably automatic cut-off valves, which fully automatically realize the opening and closing functions of each pipeline and achieve one-key start and stop of the LNG loading device. Example 1

[0064] As Figure 1 shown, the LNG loading process using the LNG loading device for a small LNG liquefaction plant includes the following steps:

[0065] 1) Nitrogen replacement process: Nitrogen at 1 m³ / h is introduced from the nitrogen inlet pipeline L5 into the second LNG loading and unloading arm and the first LNG loading and unloading arm (including the first gas phase arm, the first liquid phase arm, the second gas phase arm, and the second liquid phase arm) for nitrogen replacement, and then discharged from the nitrogen purge and vent pipeline L1 after replacement. During loading, the first and second LNG loading and unloading arms are respectively connected to the second tank truck C2 and the first tank truck C1, and the nitrogen replacement valve on the LNG loading and unloading arm is opened to start nitrogen replacement to avoid air remaining at the connection between the loading and unloading arm and the tank truck.

[0066] 2) Precooling process: LNG enters the first tank car C1 from the LNG cold box through the LNG liquid inlet pipeline L2, and BOG comes out of the first tank car and returns to the BOG recovery device through the first BOG return pipeline L41. The specific process is: LNG starts flow measurement through the LNG flowmeter FIT1, the initial first flow regulating valve FV1 gives a 5% opening, the first shut-off valve V1 is opened, and the LNG liquid enters the first tank car through the LNG liquid inlet pipeline, the fourth shut-off valve V4 is opened, and the BOG gas of the first tank car enters the first BOG return pipeline L41, so that the LNG loading device starts precooling.

[0067] 3) High-flow loading process: When the temperature of the second temperature transmitter T2 on the BOG return gas pipeline L4 reaches -100℃, it means that precooling is completed, and high-flow loading begins at this time. The specific process is: the opening of the first flow control valve FV1 is gradually opened by 30%, 50%, and finally 100%, the LNG flow rate is 60m³ / h, and the high-flow loading process of the first tank truck C1 begins. There are temperature and pressure alarm devices during the loading process. When an abnormal situation occurs, the system alarm is cut off.

[0068] 4) Small flow stop process: When the first tank truck C1 is loaded about 90%, the first flow control valve FV1 on the LNG inlet pipeline L2 begins to adjust the opening to 5%, so that the device starts the small flow stop process, the LNG flow is 5m³ / h, and when the set loading volume reaches 100%, the loading stops. Ensure stable parking and avoid water hammer.

[0069] 5) LNG discharge process: When the first tank truck is loaded, the first flow regulating valve FV1 and the first shut-off valve V1 on the LNG inlet pipeline L2 are automatically closed, and the fifth or sixth and seventh shut-off valves on the LNG discharge pipeline L6 are opened to start the LNG pipeline discharge to avoid the expansion of residual LNG and overpressure.

[0070] 6) Tank truck switching process: Ten minutes before the first tank truck C1 is loaded, the tank truck switching is prompted. At this time, the loading staff needs to check the valve opening status of the second tank truck C2 to be loaded and wait for the loading to start. When the first tank truck C1 is loaded, the second and third shut-off valves are automatically opened, and the sixth shut-off valve is automatically closed. After a delay of 1 minute, the first and fourth shut-off valves are automatically closed, and the fifth shut-off valve is opened after a delay of 10 seconds, realizing the switching of tank trucks C1 and C2. At this time, the loading of the first tank truck C1 is completed, and the second tank truck C2 begins to load similarly to the first tank truck.

[0071] The present invention develops an LNG loading process and device according to the on-site environment and use conditions of a small liquefaction plant, which is suitable for the working conditions of a small liquefaction plant. Only one set of loading process and device can realize the double-tank truck loading switching function, thereby improving the equipment cost performance.

Claims

1. An LNG loading device applicable to small-scale LNG liquefaction plants, characterized in that, it includes LNG loading and unloading arms, a nitrogen purging and venting pipeline (L1), an LNG liquid inlet pipeline (L2), a safety valve venting pipeline (L3), a BOG return gas pipeline (L4), a nitrogen inlet pipeline (L5), an LNG relief pipeline (L6), and a small-flow bypass pipeline of a flow regulating valve (L7). There are at least two sets of LNG loading and unloading arms, namely the first LNG loading and unloading arm and the second LNG loading and unloading arm. Each set of LNG loading and unloading arms includes a liquid phase arm with one end connected to the LNG liquid inlet pipeline (L2) and the other end connected to the LNG inlet of the tank truck, and a gas phase arm with one end connected to the BOG outlet of the tank truck and the other end connected to the BOG return gas pipeline (L4). The first LNG loading and unloading arm and the second LNG loading and unloading arm are connected in series. The first LNG loading and unloading arm is connected to the first tank truck (C1), and the second LNG loading and unloading arm is connected to the second tank truck (C2). The first LNG loading and unloading arm includes a first gas phase arm (ZXB1) and a first liquid phase arm (ZXB2), and the second LNG loading and unloading arm includes a second gas phase arm (ZXB3) and a second liquid phase arm (ZXB4). A first breakaway valve (E1) is provided on the first gas phase arm, a second breakaway valve (E2) is provided on the first liquid phase arm, a third breakaway valve (E3) is provided on the second gas phase arm, and a fourth breakaway valve (E4) is provided on the second liquid phase arm. The breakaway valves meet the emergency disconnection requirements and cut off the connection between the LNG loading and unloading arms and the tank truck. One end of the nitrogen purging and venting pipeline (L1) is connected to the nitrogen outlet pipe orifice of the LNG loading and unloading arm, and the other end is connected to the station purging and venting pipeline. A third check valve (CH3) is provided on the nitrogen purging and venting pipeline (L1). The LNG liquid inlet pipeline (L2) is divided into at least two branches, namely the first LNG liquid inlet pipeline branch (L21) and the second LNG liquid inlet pipeline branch (L22). The outlet end of each branch is connected to the LNG inlet of the liquid phase arm of a set of LNG loading and unloading arms. The first LNG liquid inlet pipeline branch (L21) is connected to the first liquid phase arm (ZXB2), and the second LNG liquid inlet pipeline branch (L22) is connected to the second liquid phase arm (ZXB4). The inlet end of the LNG liquid inlet pipeline (L2) is connected to the LNG cold box outlet pipeline. Along the conveying direction of LNG on the LNG liquid inlet pipeline (L2), a first pipeline filter (F1) for filtering LNG impurities, an LNG flowmeter (FIT1) for measuring the LNG flow rate, a first flow regulating valve (FV1), and a first check valve (CH1) are successively provided. A first cut-off valve (V1) is provided on the first LNG liquid inlet pipeline branch (L21), and a second cut-off valve (V2) is provided on the second LNG liquid inlet pipeline branch (L22). One end of the BOG return gas pipeline (L4) is connected to the gas phase arm pipe orifice of the LNG loading and unloading arm, and the other end is connected to the station BOG pipeline to discharge the BOG of the tank truck to the LNG storage tank. The BOG return pipeline (L4) is respectively the first BOG return pipeline (L41) and the second BOG return pipeline (L42). After the first BOG return pipeline (L41) and the second BOG return pipeline (L42) converge, they are connected to the BOG pipeline of the station. The first BOG return pipeline (L41) is connected to the first gas phase arm (ZXB1), and the second BOG return pipeline (L42) is connected to the second gas phase arm (ZXB3). A third cut-off valve (V3) is provided on the first BOG return pipeline, and a fourth cut-off valve (V4) is provided on the second BOG return pipeline; One end of the nitrogen inlet pipeline (L5) is connected to the nitrogen pipeline of the station, and the other end is connected to the nitrogen gas port of the LNG loading and unloading arm, so that nitrogen enters the LNG loading and unloading arm for purging. A ninth stop valve (V9) is provided on the nitrogen inlet pipeline; One end of the LNG discharge pipeline (L6) is connected to the LNG inlet pipeline (L2), and the other end is connected to the BOG return pipeline (L4). When the loading and unloading of the vehicle stops, the LNG liquid remaining between the first flow regulating valve (FV1) and the first cut-off valve (V1) or the second cut-off valve (V2), and between the first cut-off valve or the second cut-off valve of the LNG inlet pipeline and the LNG loading and unloading arm is automatically drained to avoid overpressure caused by the expansion of the remaining LNG. The first LNG inlet pipeline branch (L21) and the second LNG inlet pipeline branch (L22) of the two branches of the LNG inlet pipeline (L2) and the LNG inlet pipeline (L2) respectively lead out the first drainage pipeline (L61), the second drainage pipeline (L62) and the third drainage pipeline (L63). The three converge into the LNG discharge pipeline (L6). A fifth cut-off valve (V5), a sixth cut-off valve (V6), and a seventh cut-off valve (V7) are respectively provided on the first drainage pipeline (L61), the second drainage pipeline (L62), and the third drainage pipeline (L63); The small flow bypass pipeline (L7) of the flow regulating valve is connected between the inlet and outlet of the first flow regulating valve (FV1), and is opened during precooling before the start of loading. At this time, the first flow regulating valve (FV1) is closed. The small flow bypass pipeline (L7) of the flow regulating valve is connected to the LNG discharge pipeline (L6) and enters the BOG return pipeline (L4) to keep the LNG inlet pipeline (L2) unobstructed and ensure small flow circulation. An eighth cut-off valve (V8) is provided on the small flow bypass pipeline (L7) of the flow regulating valve; One end of the safety valve vent pipeline (L3) is connected to the vent pipeline of the station, so that the overpressure relief gas enters the vent pipeline of the station, and the other end is respectively connected to the safety valves provided at the rear of the cut-off valves on the LNG inlet pipeline and its branches. The first safety valve vent pipeline (L31), the second safety valve vent pipeline (L32), and the third safety valve vent pipeline (L33) are respectively led out from the first LNG inlet pipeline branch (L21), the second LNG inlet pipeline branch (L22), and the LNG inlet pipeline (L2). The first safety valve (TSV-01), the second safety valve (TSV-02), and the third safety valve (TSV-03) are respectively arranged on the third safety valve vent pipeline (L33), the second safety valve vent pipeline (L32), and the first safety valve vent pipeline (L31). The first safety valve vent pipeline (L31), the second safety valve vent pipeline (L32), and the third safety valve vent pipeline (L33) converge into the safety valve vent pipeline (L3). When the loading and unloading of the first gas phase arm (ZXB1) and the first liquid phase arm (ZXB2) from the truck are completed, open the fifth cut-off valve (V5) to discharge the liquid between the first cut-off valve (V1) and the first liquid phase arm (ZXB2), and at the same time open the seventh cut-off valve (V7) to discharge the liquid between the first flow regulating valve (FV1) and the first cut-off valve (V1) to prevent overpressure caused by LNG vaporization. When the loading and unloading of the second gas phase arm (ZXB3) and the second liquid phase arm (ZXB4) from the truck are completed, open the sixth cut-off valve (V6) to discharge the liquid between the second cut-off valve (V2) and the second liquid phase arm (ZXB4), and at the same time open the seventh cut-off valve (V7) to discharge the liquid between the first flow regulating valve (FV1) and the second cut-off valve (V2) to prevent overpressure caused by LNG vaporization.

2. The LNG loading device applicable to a small-scale LNG liquefaction plant according to claim 1, characterized in that, the inlet end of the first safety valve (TSV-01) is connected to the LNG inlet pipeline (L2), and discharges when the liquid or the vaporization of the liquid between the first flow regulating valve (FV1) and the first cut-off valve (V1) is overpressured. the inlet end of the third safety valve (TSV-03) is connected to the first LNG inlet pipeline branch (L21), and discharges when the liquid or the vaporization of the liquid between the first cut-off valve (V1) and the first liquid phase arm (ZXB2) is overpressured. the inlet end of the second safety valve (TSV-02) is connected to the second LNG inlet pipeline branch (L22), and discharges when the liquid or the vaporization of the liquid between the second cut-off valve (V2) and the second liquid phase arm (ZXB4) is overpressured.

3. The LNG loading device applicable to a small-scale LNG liquefaction plant according to claim 1 or 2, characterized in that, the LNG loading and unloading arm is connected to the tank truck through a rotary joint (7).

4. The LNG loading device applicable to a small-scale LNG liquefaction plant according to claim 1 or 2, characterized in that, The nitrogen inlet pipeline (L5) is respectively connected to the nitrogen gas ports of the gas phase arm and the liquid phase arm through valves. One end of the nitrogen purging and venting pipeline (L1) is also respectively connected to the nitrogen gas outlets of the gas phase arm and the liquid phase arm through valves. The other end of the nitrogen purging and venting pipeline is connected to the station venting pipeline. The nitrogen inlet pipeline (L5) is respectively connected to the first gas phase arm (ZXB1), the first liquid phase arm (ZXB2), the second gas phase arm (ZXB3), and the second liquid phase arm (ZXB4) through the tenth shut-off valve (V10), the eleventh shut-off valve (V11), the fourteenth shut-off valve (V14), and the fifteenth shut-off valve (V15), so that nitrogen enters the LNG loading and unloading arm for purging and replacement. One end of the nitrogen purging and venting pipeline (L1) is also respectively connected to the nitrogen gas outlets of the first gas phase arm (ZXB1), the first liquid phase arm (ZXB2), the second gas phase arm (ZXB3), and the second liquid phase arm (ZXB4) through the twelfth shut-off valve (V12), the thirteenth shut-off valve (V13), the sixteenth shut-off valve (V16), and the seventeenth shut-off valve (V17). The other end of the nitrogen purging and venting pipeline (L1) is connected to the station venting pipeline. A second pipeline filter (F2) is provided on the nitrogen inlet pipeline (L5). When the first gas phase arm (ZXB1) and the first liquid phase arm (ZXB2) are working, the nitrogen inlet pipeline (L5) is connected to the first gas phase arm (ZXB1) through the tenth shut-off valve (V10), and nitrogen enters the first gas phase arm (ZXB1) for purging. The purging gas enters the nitrogen purging and venting pipeline (L1) through the twelfth shut-off valve (V12) and is discharged. It is connected to the first liquid phase arm (ZXB2) through the eleventh shut-off valve (V11), and nitrogen enters the first liquid phase arm (ZXB2) for purging. The purging gas enters the nitrogen purging and venting pipeline (L1) through the thirteenth shut-off valve (V13) and is discharged. When the second gas phase arm (ZXB3) and the second liquid phase arm (ZXB4) are working, the nitrogen inlet pipeline (L5) is connected to the second gas phase arm (ZXB3) through the fourteenth shut-off valve (V14), and nitrogen enters the second gas phase arm (ZXB3) for purging. The purging gas enters the nitrogen purging and venting pipeline (L1) through the sixteenth shut-off valve (V16) and is discharged. It is connected to the second liquid phase arm (ZXB4) of the loading and unloading arm through the fifteenth shut-off valve (V15), and nitrogen enters the second liquid phase arm (ZXB4) for purging. The purging gas enters the nitrogen purging and venting pipeline (L1) through the seventeenth shut-off valve (V17) and is discharged.

5. The LNG loading device applicable to a small LNG liquefaction plant according to claim 4, characterized in that The LNG inlet pipeline (L2) is successively provided with a first pressure transmitter (P1) and a first temperature transmitter (T1) along the LNG flow direction between the branch pipe outlet and the first check valve. A fourth pressure transmitter (P4) is provided on the branch pipe of the first LNG inlet pipeline between the first shut-off valve (V1) and the first liquid phase arm (ZXB2). A second pressure transmitter (P2) is provided on the branch pipe of the second LNG inlet pipeline between the second shut-off valve (V2) and the second liquid phase arm (ZXB4). A second temperature transmitter (T2) is provided on the BOG return gas main pipeline after the first BOG return gas pipeline and the second BOG return gas pipeline converge. A fifth pressure transmitter (P5) is provided on the first BOG return gas pipeline (L41) between the fourth shut-off valve (V4) and the first gas phase arm. A third pressure transmitter (P3) is provided on the second BOG return gas pipeline (L42) between the third shut-off valve (V3) and the second gas phase arm. A second check valve, namely a nitrogen inlet check valve (CH2), is provided on the nitrogen inlet pipeline near the connection of the LNG loading and unloading arm and the nitrogen inlet pipeline to ensure unidirectional flow of the medium.

6. The LNG loading device applicable to a small-scale LNG liquefaction plant according to claim 5, characterized in that the LNG inlet pipeline pressure gauge, i.e., the first pressure transmitter (P1) measures the pressure between the first flow regulating valve (FV1) and the first shut-off valve (V1). When the pressure exceeds the set value, it indicates that the seventh shut-off valve (V7) is not opened, and at this time, the seventh shut-off valve (V7) is opened; the second pressure transmitter (P2) measures the pressure between the second shut-off valve (V2) and the second liquid phase arm (ZXB4). When the pressure exceeds the set value, it indicates that the sixth shut-off valve (V6) is not opened, and at this time, the sixth shut-off valve (V6) is opened; the fourth pressure transmitter (P4) measures the pressure between the first shut-off valve (V1) and the first liquid phase arm (ZXB2). When the pressure exceeds the set value, it indicates that the fifth shut-off valve (V5) is not opened, and at this time, the fifth shut-off valve (V5) is opened; the third pressure transmitter (P3) measures the pressure between the third shut-off valve (V3) and the second gas phase arm (ZXB3). When the pressure exceeds the set value, it indicates that the third shut-off valve (V3) is not opened, and at this time, the third shut-off valve (V3) is opened; the fifth pressure transmitter (P5) measures the pressure between the fourth shut-off valve (V4) and the first gas phase arm (ZXB1). When the pressure exceeds the set value, it indicates that the fourth shut-off valve (V4) is not opened, and at this time, the fourth shut-off valve (V4) is opened.

7. The loading process using the LNG loading device applicable to a small-scale LNG liquefaction plant according to any one of claims 1-6, which comprises: 1) Nitrogen replacement step: Introduce nitrogen from the nitrogen inlet pipeline into the LNG loading and unloading arm of the first tank truck, including the gas phase arm and the liquid phase arm, for nitrogen replacement, and discharge it from the nitrogen purge and vent pipeline after replacement; 2) Precooling step: LNG enters the first tank truck through the LNG liquid inlet pipeline from the LNG cold box, BOG comes out of the tank truck and returns to the BOG recovery device through the BOG return pipeline, LNG starts to be measured by the LNG flowmeter, initially, the first flow regulating valve gives a 2-10% opening, the shut-off valve of the first branch pipe of the LNG liquid inlet pipeline is opened, so that the LNG liquid enters the first tank truck through the LNG liquid inlet pipeline, the BOG gas of the first tank truck enters the BOG return pipeline connected to the tank truck, the shut-off valve on the BOG return pipeline is opened, and the LNG loading device starts to precool; 3) High-flow loading steps: When the temperature detected on the BOG return gas pipeline reaches ≤-100℃, pre-cooling is completed, and high-flow loading to the LNG tank truck begins. The first flow regulating valve is gradually opened to make the loading flow reach 100%, and the high-flow loading process of the LNG tank truck begins; 4) Small flow stop step: When the LNG tank truck is loaded with 90%±5%, the first flow control valve starts to adjust the opening to 5%±3%, so that the device starts the small flow stop process. When the set flow reaches 100%, the loading stops; 5) LNG discharge steps: When LNG loading is completed, the first flow regulating valve and the first shut-off valve on the LNG inlet pipeline are automatically closed, and the fifth shut-off valve and the seventh shut-off valve on the LNG discharge pipeline are automatically opened to start the LNG inlet pipeline discharge to avoid residual LNG expansion and overpressure; 6) Tank truck switching steps: Before the loading of the first tank truck is completed, the tank truck switching is prompted. At this time, the loading staff needs to check the valve opening status of the second tank truck to be loaded and wait for the loading to start. After the loading of the first tank truck is completed, the first and fourth shut-off valves are automatically closed, and the fifth shut-off valve is automatically closed. After a delay, the second and third shut-off valves are automatically opened, and the sixth shut-off valve is opened with a delay to realize the switching of loading from the first tank truck to the second tank truck. At this time, the loading of the first tank truck is completed and the loading process of the second tank truck begins.

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