L-cng system and control method thereof

Abstract

The application discloses an L-CNG system and a control method thereof. The L-CNG system comprises a storage tank, a plunger pump pry provided with a plunger pump, a submerged pump pool provided with a submerged pump and an LNG liquid filling machine. The storage tank is used for storing low-temperature LNG liquid. A liquid inlet end of the plunger pump pry is communicated with a first liquid outlet end of the storage tank. A gas return end of the plunger pump pry is communicated with a first gas inlet end of the storage tank. A liquid outlet end of the plunger pump pry is provided with a pressure detection device. The installation height of the plunger pump pry is lower than that of the storage tank. A liquid inlet end of the submerged pump pool is communicated with a second liquid outlet end of the storage tank. A gas return end of the submerged pump pool is communicated with a second gas inlet end of the storage tank. A liquid inlet end of the LNG liquid filling machine is communicated with a liquid outlet end of the submerged pump through a first pipeline. The application can ensure that the plunger pump can be quickly filled and pressurized at any stage, and the BOG gas in the pump cavity of the plunger pump is reduced, thereby improving the pressurization efficiency of the plunger pump.

Description

Technical Field

[0001] This invention belongs to the field of L-CNG technology, and particularly relates to L-CNG systems and their control methods. Background Technology

[0002] Currently, L-CNG refueling stations typically pressurize cryogenic LNG stored in tanks into 20MPa cryogenic high-pressure LNG using a plunger pump, then reheat and vaporize it into 20MPa ambient temperature high-pressure CNG gas using a high-pressure vaporizer, and finally refuel vehicles using the station's CNG dispenser. However, due to an unreasonable process piping layout, numerous bends, long pipes, and significant pressure loss during installation, the plunger pump head is affected during liquid inlet and gas return. This often results in the plunger pump operating under no-load conditions, and the piston rings dry-wearing due to insufficient liquid pumping, thus affecting pressurization efficiency.

[0003] Among the issues, the sealing rings are prone to leakage due to dry grinding, thus affecting pressurization efficiency. When the pipeline inlet or outlet is obstructed, the plunger pump is prone to idling. On-site personnel need to manually or automatically open the outlet valve to vent and guide the LNG inlet to the plunger pump chamber, which can easily lead to increased on-site losses. Idle plunger pumps are prone to alarm shutdowns, causing the system to start frequently. However, each start-up is accompanied by automatic outlet gas venting to guide the flow, which increases the number of outlets and affects the operation of the gas station. In addition, the system needs to automatically discharge the high-pressure liquid or gas after the plunger pump before each start-up to form a low pressure in the plunger pump chamber, which is not conducive to energy conservation. Furthermore, the liquid level in the storage tank is low, and its net positive pressure head is small, making it more difficult for the plunger pump to inlet. This makes it more likely to experience frequent idling or low pressurization efficiency. The plunger pump head is prone to dry grinding and heat accumulation, which accelerates the further vaporization of the already small amount of LNG in the pump head, thus hindering the flow of cryogenic LNG into the pump head, further reducing the working efficiency of the plunger pump, further shortening the service life of the sealing rings, and increasing the maintenance and replacement costs at the gas station. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides an L-CNG system and its control method, which can improve the pressurization efficiency of a plunger pump.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] Firstly, an L-CNG system is provided, comprising:

[0007] Storage tanks are used to store cryogenic LNG liquids.

[0008] A plunger pump skid is equipped with a plunger pump. The inlet end of the plunger pump skid is connected to the first outlet end of the storage tank, and the return end of the plunger pump skid is connected to the first air inlet end of the storage tank. A pressure detection device is installed at the outlet end of the plunger pump skid. The installation height of the plunger pump skid is lower than the installation height of the storage tank.

[0009] Submersible pump pool, which is equipped with a submersible pump. The inlet end of the submersible pump pool is connected to the second outlet end of the storage tank, and the return end of the submersible pump pool is connected to the second air inlet end of the storage tank.

[0010] The LNG dispenser has its inlet end connected to the outlet end of the submersible pump via a first pipe.

[0011] The plunger pump skid's inlet and outlet are connected to the first pipeline via the second and third pipelines, respectively. The first outlet and first inlet of the storage tank are equipped with a first valve and a second valve, respectively. The first, second, and third pipelines are equipped with a third valve, a first regulating valve, and a second regulating valve, respectively. The two ends of the third valve are connected to the second and third pipelines, respectively. The plunger pump, pressure detection device, submersible pump, LNG dispenser, first valve, second valve, third valve, first regulating valve, and second regulating valve are all electrically connected to a controller.

[0012] The beneficial effects of adopting the above technical solution are as follows: When the LNG liquid level in the storage tank is high, the LNG liquid in the storage tank flows by gravity into the inlet end of the plunger pump skid, thereby avoiding the plunger pump from running unloaded and ensuring normal pressurization of the plunger pump; when the LNG liquid level in the storage tank is low, the submersible pump can intercept a portion of the LNG liquid and supply it to the inlet end of the plunger pump skid when the LNG dispenser is running, or it can run at a low frequency when the LNG dispenser is not running to directly supply the LNG liquid to the inlet end of the plunger pump skid, thereby avoiding the plunger pump from running unloaded and ensuring normal pressurization of the plunger pump; in summary, this L-CNG system can avoid the plunger pump from running unloaded and ensure normal pressurization of the plunger pump, which is conducive to improving the pressurization efficiency of the plunger pump.

[0013] In one embodiment, a check valve is provided between the first valve and the inlet end of the plunger pump skid.

[0014] In one embodiment, the second liquid outlet and the second air inlet of the storage tank are respectively provided with a fourth valve and a fifth valve.

[0015] In one embodiment, the first valve, the second valve, the third valve, the fourth valve, and the fifth valve are all pneumatic valves.

[0016] In one embodiment, the liquid outlet of the plunger pump skid is sequentially connected to a high-pressure vaporizer and a CNG dispenser.

[0017] Secondly, a control method for an L-CNG system is provided, which includes the following steps:

[0018] Collect current information from the plunger pump, operating information from the LNG dispenser, and pressure information from the plunger pump's liquid outlet end; determine whether the plunger pump is running under no-load based on its current information.

[0019] If the plunger pump is not running under no-load, keep the first, second, and third valves open, and close the first and second regulating valves to allow the LNG liquid in the storage tank to flow into the inlet end of the plunger pump skid by gravity; if the plunger pump is running under no-load, determine whether the submersible pump is running based on the operating information of the LNG dispenser.

[0020] If the submersible pump is running, the second valve is closed, the third valve is opened, and the first and second regulating valves are switched on and off alternately, with the first regulating valve opening a certain time earlier than the second regulating valve, so as to intercept a portion of the LNG liquid and supply it to the inlet end of the plunger pump skid when the LNG dispenser is running; if the submersible pump is not running, the submersible pump is opened, the second and third valves are closed, and the first and second regulating valves are switched on and off alternately, with the first regulating valve opening a certain time earlier than the second regulating valve, so as to directly supply the LNG liquid to the inlet end of the plunger pump skid when the LNG dispenser is not running.

[0021] The pressure information at the liquid outlet of the plunger pump is used to determine whether the LNG liquid pressure at the liquid outlet of the plunger pump meets the requirements. If the LNG liquid pressure at the liquid outlet of the plunger pump does not meet the requirements, the plunger pump continues to operate. If the LNG liquid pressure at the liquid outlet of the plunger pump meets the requirements, the plunger pump stops operating.

[0022] In one implementation, determining whether the plunger pump is running under no-load based on the plunger pump's current information includes:

[0023] If the current I of the plunger pump is less than or equal to 35A, the plunger pump will operate under no-load conditions.

[0024] In one embodiment, when the plunger pump is running under no-load and the submersible pump is running, the opening and closing times of the first regulating valve are 5s and 10s respectively each time. The opening time of the first regulating valve is 1s earlier than the opening time of the second regulating valve, and the closing time of the first regulating valve is the same as that of the second regulating valve.

[0025] The beneficial effects of adopting the above technical solution are as follows: This setting is conducive to the plunger pump head being filled with LNG liquid first and forming positive pressure, thereby pushing the BOG gas in the pump head back to the inlet end of the LNG dispenser through the second regulating valve.

[0026] In one embodiment, when the plunger pump is running under no-load and the submersible pump is not running, the opening and closing times of the first regulating valve are 5s and 20s respectively, the opening time of the first regulating valve is 1s earlier than the opening time of the second regulating valve, the closing time of the first regulating valve is the same as the closing time of the second regulating valve, and the operating frequency of the submersible pump is no greater than 40Hz.

[0027] The beneficial effects of adopting the above technical solution are as follows: This setting is conducive to the plunger pump head being filled with LNG liquid first and forming positive pressure, thereby pushing the BOG gas in the pump head back to the inlet end of the LNG dispenser through the second regulating valve.

[0028] In one embodiment, when the plunger pump is running under no-load, the opening degree of the first regulating valve is 100%, and the opening degree of the second regulating valve is 50% to 100%.

[0029] The beneficial effects of adopting the above technical solution are as follows: the opening degree of the second regulating valve is 50% to 100%, so that when the opening degree of the second regulating valve is 50%, the opening degree of the second regulating valve is gradually increased according to the obvious increase in current, until the opening degree of the second regulating valve is increased to 100%.

[0030] The beneficial effects of this invention are as follows:

[0031] When the LNG level in the storage tank is high, the LNG flows by gravity into the inlet of the plunger pump skid to avoid the plunger pump operating without load and to ensure normal pressurization. When the LNG level in the storage tank is low, the submersible pump can intercept a portion of the LNG and supply it to the inlet of the plunger pump skid when the LNG dispenser is running, or it can operate at a low frequency when the LNG dispenser is not running to directly supply the LNG to the inlet of the plunger pump skid, thereby avoiding the plunger pump operating without load and ensuring normal pressurization. In summary, this L-CNG system can avoid the plunger pump operating without load and ensure normal pressurization, which is beneficial to improving the pressurization efficiency of the plunger pump. Attached Figure Description

[0032] The invention will now be described in more detail with reference to embodiments and the accompanying drawings.

[0033] Figure 1 A schematic diagram of the structure of the present invention is shown;

[0034] In the accompanying drawings, the same parts use the same reference numerals. The drawings are not to scale.

[0035] Figure label:

[0036] 1-Storage tank, 2-Fourth valve, 3-First valve, 4-Check valve, 5-Submersible pump pool, 6-Submersible pump, 7-Plunger pump skid, 8-High-pressure vaporizer, 9-Pressure detection device, 10-Plunger pump, 11-Third pipeline, 12-Second regulating valve, 13-First regulating valve, 14-Second pipeline, 15-LNG dispenser, 16-First pipeline, 17-Third valve, 18-Second valve, 19-Fifth valve. Detailed Implementation

[0037] The invention will now be further described with reference to the accompanying drawings.

[0038] This invention provides an L-CNG system, such as Figure 1 As shown, it includes:

[0039] Storage tank 1 is used to store cryogenic LNG liquid;

[0040] A plunger pump skid 7 is provided with a plunger pump 10 and a buffer tank. The liquid inlet of the plunger pump skid 7 is connected to the first liquid outlet of the storage tank 1, and the gas return end of the plunger pump skid 7 is connected to the first gas inlet of the storage tank 1. A pressure detection device 9 is provided at the liquid outlet of the plunger pump skid 7. The installation height of the plunger pump skid 7 is lower than the installation height of the storage tank 1.

[0041] Submersible pump pool 5, submersible pump 6 is installed in submersible pump pool 5, the liquid inlet end of submersible pump pool 5 is connected to the second liquid outlet end of storage tank 1, and the air return end of submersible pump pool 5 is connected to the second air inlet end of storage tank 1.

[0042] LNG dispenser 15, the inlet end of LNG dispenser 15 is connected to the outlet end of submersible pump 6 through the first pipe 16;

[0043] The plunger pump skid 7 has its inlet and outlet ends connected to the first pipe 16 via the second pipe 14 and the third pipe 11, respectively. The first outlet and the first inlet of the storage tank 1 are equipped with the first valve 3 and the second valve 18, respectively. The first pipe 16, the second pipe 14, and the third pipe 11 are equipped with the third valve 17, the first regulating valve 13, and the second regulating valve 12, respectively. The two ends of the third valve 17 are connected to the second pipe 14 and the third pipe 11, respectively. The plunger pump 10, the pressure detection device 9, the submersible pump 6, the LNG dispenser 15, the first valve 3, the second valve 18, the third valve 17, the first regulating valve 13, and the second regulating valve 12 are all electrically connected to a controller.

[0044] Understandably, when the LNG level in storage tank 1 is high, the LNG in storage tank 1 flows by gravity into the inlet of plunger pump skid 7 to avoid plunger pump 10 operating without load and to ensure normal pressurization of plunger pump 10. When the LNG level in storage tank 1 is low, submersible pump 6 can intercept a portion of the LNG and supply it to the inlet of plunger pump skid 7 when LNG dispenser 15 is running, or it can operate at a low frequency when LNG dispenser 15 is not running to directly supply LNG to the inlet of plunger pump skid 7, thereby avoiding plunger pump 10 operating without load and ensuring normal pressurization of plunger pump 10. In summary, this L-CNG system can avoid plunger pump 10 operating without load and ensure normal pressurization of plunger pump 10, which is beneficial to improving the pressurization efficiency of plunger pump 10.

[0045] It should be noted that the controller can be a PLC controller, and the controller can be electrically connected to a touch screen for easy debugging; the controller will automatically collect the current of the plunger pump 10; the pressure detection device 9 can be a pressure sensor, which is used to detect the pressure information at the liquid outlet of the plunger pump skid 7.

[0046] In one embodiment, a check valve 4 is provided between the first valve 3 and the inlet end of the plunger pump skid 7.

[0047] In one embodiment, the second liquid outlet and the second air inlet of the storage tank 1 are respectively provided with a fourth valve 2 and a fifth valve 19.

[0048] In one embodiment, the first valve 3, the second valve 18, the third valve 17, the fourth valve 2, and the fifth valve 19 are all pneumatic valves.

[0049] In one embodiment, the liquid outlet of the plunger pump skid 7 is sequentially connected to a high-pressure vaporizer 8 and a CNG dispenser.

[0050] Understandably, the low-temperature, high-pressure LNG liquid discharged from the outlet of the plunger pump skid 7 can be reheated and vaporized into 20MPa room-temperature, high-pressure CNG gas by the high-pressure vaporizer 8, and then refueled in vehicles using a CNG refueling machine.

[0051] In addition, all pipes involved in the L-CNG system have been thickened.

[0052] The present invention also provides a control method for an L-CNG system, which includes the following steps:

[0053] The controller collects the current information of the plunger pump 10, the operating information of the LNG dispenser 15, and the pressure information at the outlet of the plunger pump skid 7; the controller determines whether the plunger pump 10 is running under no-load based on the current information of the plunger pump 10.

[0054] If the plunger pump 10 is running unloaded, the first valve 3, the second valve 18 and the third valve 17 are kept open, and the first regulating valve 13 and the second regulating valve 12 are closed so that the LNG liquid in the storage tank 1 flows into the liquid inlet of the plunger pump skid 7 by gravity; if the plunger pump 10 is running unloaded, the operation information of the LNG dispenser 15 is used to determine whether the submersible pump 6 is running.

[0055] If the submersible pump 6 is running, then the second valve 18 is closed, the third valve 17 is opened, and the first regulating valve 13 and the second regulating valve 12 are switched alternately, with the opening time of the first regulating valve 13 being earlier than the opening time of the second regulating valve 12, so as to intercept a portion of the LNG liquid and supply it to the inlet end of the plunger pump skid 7 when the LNG dispenser 15 is running; if the submersible pump 6 is not running, then the submersible pump 6 is opened, the second valve 18 and the third valve 17 are closed, and the first regulating valve 13 and the second regulating valve 12 are switched alternately, with the opening time of the first regulating valve 13 being earlier than the opening time of the second regulating valve 12, so as to directly supply the LNG liquid to the inlet end of the plunger pump skid 7 when the LNG dispenser 15 is not running;

[0056] The pressure information at the outlet of the plunger pump skid 7 is used to determine whether the LNG liquid pressure at the outlet of the plunger pump skid 7 meets the requirements. If the LNG liquid pressure at the outlet of the plunger pump skid 7 is less than 20MPa, the plunger pump 10 continues to operate. If the LNG liquid pressure at the outlet of the plunger pump skid 7 is not less than 20MPa, the plunger pump 10 stops operating.

[0057] It should be noted that when the LNG liquid level in storage tank 1 is at a high level, the LNG liquid in storage tank 1 flows into the inlet end of the plunger pump skid 7 by gravity, and the plunger pump 10 is not running under no-load. That is, it is only necessary to keep the first valve 3, the second valve 18 and the third valve 17 in the open state to avoid the plunger pump 10 running under no-load.

[0058] As the LNG liquid in storage tank 1 is sold externally, when the liquid level of the LNG liquid in storage tank 1 is low, the LNG liquid in storage tank 1 cannot flow into the inlet end of the plunger pump skid 7 by gravity, and the plunger pump 10 runs unloaded. At this time, the submersible pump 6 can intercept part of the LNG liquid and supply it to the inlet end of the plunger pump skid 7 when the LNG dispenser 15 is running, or it can run at a low frequency when the LNG dispenser 15 is not running, so as to directly supply the LNG liquid to the inlet end of the plunger pump skid 7, thereby avoiding the plunger pump 10 running unloaded.

[0059] It should also be noted that when the LNG dispenser 15 is used to refuel external vehicles with LNG, the submersible pump 6 will run. Therefore, the operation information of the LNG dispenser 15 can be used to determine whether the submersible pump 6 is running.

[0060] In one embodiment, determining whether the plunger pump 10 is running under no-load based on the current information of the plunger pump 10 includes:

[0061] If the current I of the plunger pump 10 is less than or equal to 35A, then the plunger pump 10 will operate under no-load conditions.

[0062] In one embodiment, when the plunger pump 10 is running under no-load and the submersible pump 6 is running, the opening and closing times of the first regulating valve 13 are 5s and 10s respectively each time. The opening time of the first regulating valve 13 is 1s earlier than the opening time of the second regulating valve 12, and the closing time of the first regulating valve 13 is the same as the closing time of the second regulating valve 12.

[0063] Understandably, this setup allows the plunger pump 10 to fill with LNG liquid first and create positive pressure, thereby pushing the BOG gas in the pump head back to the inlet of the LNG dispenser 15 through the second regulating valve 12.

[0064] In one embodiment, when the plunger pump 10 is running under no-load and the submersible pump 6 is not running, the opening and closing times of the first regulating valve 13 are 5s and 20s respectively each time. The opening time of the first regulating valve 13 is 1s earlier than the opening time of the second regulating valve 12. The closing time of the first regulating valve 13 is the same as the closing time of the second regulating valve 12. The operating frequency of the submersible pump 6 is no greater than 40Hz.

[0065] Understandably, this setup allows the plunger pump 10 to fill with LNG liquid first and create positive pressure, thereby pushing the BOG gas in the pump head back to the inlet of the LNG dispenser 15 through the second regulating valve 12.

[0066] In one embodiment, when the plunger pump 10 is running under no-load, the opening degree of the first regulating valve 13 is 100%, and the opening degree of the second regulating valve 12 is 50% to 100%.

[0067] It is understandable that the opening of the second regulating valve 12 is 50% to 100%. When the opening of the second regulating valve 12 is 50%, the opening of the second regulating valve 12 is gradually increased according to the obvious increase in current until the opening of the second regulating valve 12 is increased to 100%.

[0068] It should be noted that as the opening of the second regulating valve 12 increases, it can accelerate the continuous flow of cryogenic LNG liquid after the submersible pump 6 into the pump chamber of the plunger pump 10, and drive the BOG gas in the pump chamber back to the storage tank 1.

[0069] In summary, the PLC-based controller can automatically start and stop the submersible pump 6, open or close the first regulating valve 13, and open or close the second regulating valve 12. This ensures that the pump head of the plunger pump 10 is always filled with LNG cryogenic liquid, whether the LNG cryogenic liquid level in the storage tank 1 is high or low. This guarantees that the plunger pump 10 can quickly fill and efficiently pressurize LNG at any stage, reducing BOG gas in the pump chamber of the plunger pump 10. This improves the pressurization efficiency of the plunger pump 10, reduces cavitation during pressurization, and reduces damage to the sealing rings, thus reducing costs and increasing efficiency. It also significantly reduces the large amount of LNG loss caused by frequent venting of return gas when the plunger pump 10 is underpressurized. Furthermore, it reduces BOG emissions caused by underpressurization of the plunger pump 10 and reduces abnormal wear of the plunger pump 10, thereby lowering maintenance costs.

[0070] In the description of this invention, it should be understood that the terms "upper", "lower", "bottom", "top", "front", "rear", "inner", "outer", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0071] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A control method, characterized in that, An L-CNG system is adopted, the L-CNG system comprising: Storage tank (1), the storage tank (1) is used to store cryogenic LNG liquid; A plunger pump skid (7) is provided with a plunger pump (10). The liquid inlet of the plunger pump skid (7) is connected to the first liquid outlet of the storage tank (1). The gas return end of the plunger pump skid (7) is connected to the first gas inlet of the storage tank (1). A pressure detection device (9) is provided at the liquid outlet of the plunger pump skid (7). The installation height of the plunger pump skid (7) is lower than the installation height of the storage tank (1). Submersible pump pool (5), wherein a submersible pump (6) is provided in the submersible pump pool (5), the inlet end of the submersible pump pool (5) is connected to the second outlet end of the storage tank (1), and the return end of the submersible pump pool (5) is connected to the second air inlet end of the storage tank (1). LNG dispenser (15), the inlet end of the LNG dispenser (15) is connected to the outlet end of the submersible pump (6) through the first pipe (16); The inlet and outlet of the plunger pump skid (7) are connected to the first pipe (16) via the second pipe (14) and the third pipe (11), respectively. The first outlet and the first inlet of the storage tank (1) are respectively equipped with a first valve (3) and a second valve (18). The first pipe (16), the second pipe (14) and the third pipe (11) are respectively equipped with a third valve (17), a first regulating valve (13) and a second regulating valve (12). The two ends of the third valve (17) are respectively connected to the second pipe (14) and the third pipe (11). The plunger pump (10), the pressure detection device (9), the submersible pump (6), the LNG dispenser (15), the first valve (3), the second valve (18), the third valve (17), the first regulating valve (13) and the second regulating valve (12) are all electrically connected to a controller. The control method includes the following steps: Collect the current information of the plunger pump (10), the operation information of the LNG dispenser (15) and the pressure information of the outlet end of the plunger pump skid (7); determine whether the plunger pump (10) is running under no-load based on the current information of the plunger pump (10). If the plunger pump (10) is running unloaded, keep the first valve (3), the second valve (18) and the third valve (17) open, and close the first regulating valve (13) and the second regulating valve (12) so that the LNG liquid in the storage tank (1) flows into the inlet of the plunger pump (10) by gravity; if the plunger pump (10) is running unloaded, determine whether the submersible pump (6) is running based on the operating information of the LNG dispenser (15); If the submersible pump (6) is running, then the second valve (18) is closed, the third valve (17) is opened, and the first regulating valve (13) and the second regulating valve (12) are switched on and off at intervals. The opening time of the first regulating valve (13) is earlier than the opening time of the second regulating valve (12) by a certain amount of time, so as to intercept part of the LNG liquid and supply it to the inlet end of the plunger pump (10) when the LNG dispenser (15) is running. If the submersible pump (6) is not running, then the submersible pump (6) is opened, the second valve (18) and the third valve (17) are closed, and the first regulating valve (13) and the second regulating valve (12) are switched on and off at intervals. The opening time of the first regulating valve (13) is earlier than the opening time of the second regulating valve (12) by a certain amount of time, so as to directly supply the LNG liquid to the inlet end of the plunger pump (10) when the LNG dispenser (15) is not running. Based on the pressure information at the outlet of the plunger pump skid (7), determine whether the LNG liquid pressure at the outlet of the plunger pump skid (7) meets the requirements; if the LNG liquid pressure at the outlet of the plunger pump skid (7) does not meet the requirements, the plunger pump (10) continues to run; if the LNG liquid pressure at the outlet of the plunger pump skid (7) meets the requirements, the plunger pump (10) stops running.

2. The control method according to claim 1, characterized in that, A check valve (4) is provided between the first valve (3) and the inlet end of the plunger pump skid (7).

3. The control method according to claim 1, characterized in that, The storage tank (1) is equipped with a fourth valve (2) and a fifth valve (19) at its second liquid outlet and second air inlet ends, respectively.

4. The control method according to claim 3, characterized in that, The first valve (3), the second valve (18), the third valve (17), the fourth valve (2) and the fifth valve (19) are all pneumatic valves.

5. The control method according to claim 1, characterized in that, The outlet end of the plunger pump skid (7) is connected in sequence to a high-pressure vaporizer (8) and a CNG dispenser.

6. The control method according to claim 1, characterized in that, The method of determining whether the plunger pump (10) is running under no-load based on the current information of the plunger pump (10) includes: If the current I of the plunger pump (10) is less than or equal to 35A, then the plunger pump (10) will run under no-load conditions.

7. The control method according to claim 1, characterized in that, When the plunger pump (10) is running under no-load and the submersible pump (6) is running, the opening and closing times of the first regulating valve (13) are 5s and 10s respectively. The opening time of the first regulating valve (13) is 1s earlier than the opening time of the second regulating valve (12). The closing time of the first regulating valve (13) is the same as the closing time of the second regulating valve (12).

8. The control method according to claim 1, characterized in that, When the plunger pump (10) is running under no-load and the submersible pump (6) is not running, the opening and closing times of the first regulating valve (13) are 5s and 20s respectively. The opening time of the first regulating valve (13) is 1s earlier than the opening time of the second regulating valve (12). The closing time of the first regulating valve (13) is the same as the closing time of the second regulating valve (12). The operating frequency of the submersible pump (6) is not greater than 40Hz.

9. The control method according to claim 1, characterized in that, When the plunger pump (10) is running under no-load, the opening degree of the first regulating valve (13) is 100%, and the opening degree of the second regulating valve (12) is 50%~100%.

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