Turbine expansion liquefying system utilizing natural gas pressure difference and liquefying method of turbine expansion liquefying system

Abstract

The invention discloses a turbine expansion liquefying system utilizing natural gas pressure difference and a liquefying method of the turbine expansion liquefying system. An outlet of a high-pressure natural gas pipeline is connected with a first compressor and an inlet of a differential pressure power generation device through pipelines, a pressure sensor is arranged on the pipeline from the outlet of the high-pressure natural gas pipeline to the inlet of the differential pressure power generation device, and an outlet of the differential pressure power generation device is connected with an inlet of a low-pressure output client through a pipeline via a heat exchange device. An outlet of the first compressor is communicated with the heat exchange device through a pipeline, the heat exchange device is connected with the pressure supplementing device and the liquefied natural gas storage tank through pipelines, the liquefied natural gas storage tank is connected with an outlet of the high-pressure natural gas pipeline through a pipeline, and the pressure supplementing device is arranged on the pipeline from the liquefied natural gas storage tank to the outlet of the high-pressure natural gas pipeline. And the pressure supplementing device is electrically connected with the differential pressure power generation device. Cooling capacity is obtained through the natural gas pressure difference, energy storage is conducted, cooling and liquefaction are achieved, the energy of the natural gas in the expansion process is recycled, the overall energy consumption is reduced, and the system efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas production, and in particular to a turbine expansion liquefaction system utilizing natural gas pressure difference. Background Art

[0002] Since the consumption of natural gas in cities is not stable and the gas consumption fluctuates periodically between peaks and valleys, city gas companies generally use gas storage devices to store part of the natural gas during the low gas consumption period, and transport the stored natural gas into the pipeline network for peak regulation during the peak gas consumption period. As the consumption of natural gas increases, the required gas storage and peak regulation devices not only require large investments, but also occupy a large amount of land, and the peak regulation cost is high. In order to reduce the peak regulation cost, city gas companies build their own natural gas liquefaction devices, which is an effective way to reduce the peak regulation cost. The high-pressure natural gas in the upstream pipeline needs to pass through the city distribution station, gate station and pressure regulating station for pressure reduction treatment. The traditional pressure regulation method mainly relies on the throttle valve to reduce the pressure, which leads to a large amount of pressure energy and heat energy waste. Therefore, the present invention discloses a turbine expansion liquefaction system that utilizes the natural gas pressure difference, which organically combines the utilization of the pipeline network pressure energy with the gate station pressure regulation demand to maximize the energy utilization efficiency. Summary of the invention

[0003] In view of the deficiencies in the prior art, the present invention proposes a turbine expansion liquefaction system that utilizes the pressure difference of natural gas to reduce the pressure of the high-pressure natural gas in the pipeline and transport it to the urban pipeline network, while utilizing the cold energy after the expansion of the natural gas to obtain liquefied natural gas.

[0004] The technical solution adopted by the present invention is:

[0005] The present invention includes a high-pressure natural gas pipeline, a pressure sensor, a first compressor, a pressure difference power generation device, a pressure compensation device, a heat exchange device, a liquefied natural gas storage tank and a low-pressure output client;

[0006] The outlet of the high-pressure natural gas pipeline is connected to the first compressor and the inlet of the pressure difference power generation device through pipelines respectively. A pressure sensor is provided on the pipeline from the outlet of the high-pressure natural gas pipeline to the inlet of the pressure difference power generation device. The outlet of the pressure difference power generation device is connected to the inlet of the low-pressure output client through a pipeline via a heat exchanger. The outlet of the first compressor is connected to the heat exchanger through a pipeline. The heat exchanger is connected to the pressure compensating device and the liquefied natural gas storage tank through pipelines respectively. The liquefied natural gas storage tank is connected to the outlet of the high-pressure natural gas pipeline through a pipeline. A pressure compensating device is provided on the pipeline from the liquefied natural gas storage tank to the outlet of the high-pressure natural gas pipeline, and the pressure compensating device is electrically connected to the pressure difference power generation device.

[0007] The pressure compensation device includes a phase change energy storage device, a gas mixing tank and a second compressor. The heat exchange device is connected to the gas mixing tank and the liquefied natural gas storage tank through pipelines. The gas mixing tank, the second compressor and the phase change energy storage device are sequentially arranged on the pipeline from the liquefied natural gas storage tank to the outlet of the high-pressure natural gas pipeline. The phase change energy storage device is electrically connected to the pressure difference power generation device.

[0008] The pressure difference power generation device includes an expander and a generator. The outlet of the high-pressure natural gas pipeline is connected to the first compressor and the expander inlet through pipelines respectively. The expander is transmission-connected to the generator, the generator is electrically connected to the phase change energy storage device, and the expander outlet is connected to the low-pressure output client inlet through a pipeline through a heat exchange device.

[0009] The heat exchange device comprises a heat exchanger and a gas-liquid mixing tank, wherein the heat exchanger is provided with a first heat exchange tube and a second heat exchange tube, the gas-liquid mixing tank has openings at the upper part, the top and the bottom of the inner cavity, and a third heat exchange tube is provided in the tank and directly penetrates the inner cavity;

[0010] The first compressor outlet is connected to the first heat exchange tube inlet in the heat exchanger through a pipeline, the first heat exchange tube outlet is connected to the upper opening of the inner cavity of the gas-liquid mixing tank through a pipeline, the top opening of the inner cavity of the gas-liquid mixing tank is connected to the gas mixing tank through a pipeline, the bottom opening of the inner cavity of the gas-liquid mixing tank is connected to the liquefied natural gas storage tank through a pipeline, the expander outlet is connected to the third heat exchange tube inlet in the gas-liquid mixing tank through a pipeline, the third heat exchange tube outlet is connected to the second heat exchange tube inlet in the heat exchanger through a pipeline, and the second heat exchange tube outlet is connected to the low-pressure output client inlet through a pipeline.

[0011] The high-pressure natural gas pipeline outlet is connected to the first compressor and the pressure difference power generation device inlet through a three-way valve, and a pressure sensor is provided on the pipeline from the high-pressure natural gas pipeline outlet to the three-way valve.

[0012] Stop valves are arranged on the pipeline between the gas-liquid mixing tank and the gas mixing tank, on the pipeline between the gas mixing tank and the second compressor, and on the pipeline between the liquefied natural gas storage tank and the gas mixing tank. A throttle valve is arranged on the pipeline between the gas-liquid mixing tank and the liquefied natural gas storage tank.

[0013] A heater is provided on the pipeline between the second heat exchange tube and the low-pressure output client.

[0014] A turbine expansion liquefaction process using natural gas pressure difference:

[0015] Set the gas pressure threshold. The pressure sensor detects the pressure of the high-pressure natural gas flowing through the pressure sensor:

[0016] When the pressure sensor detects that the high-pressure natural gas pressure exceeds the set threshold, the stop valve is controlled to close and the system operates in accordance with the state of sufficient high-pressure natural gas pressure;

[0017] When the pressure sensor detects that the high-pressure natural gas pressure exceeds, is lower than or equal to a threshold value, the shut-off valve is controlled to open and operate in accordance with the state of insufficient high-pressure natural gas pressure.

[0018] The operation according to the high-pressure natural gas sufficient pressure state is specifically as follows:

[0019] High-pressure natural gas is first divided into two paths through a three-way valve from a high-pressure natural gas pipeline. The first path of high-pressure natural gas flows into the first compressor through a pipeline for pressurization, and then is transported to the first heat exchange tube in the heat exchanger through a pipeline for the first heat exchange, and then is input into the gas-liquid mixing tank through a pipeline;

[0020] The second high-pressure natural gas is expanded and decompressed by the expander to obtain cold energy. During the operation of the expander, the coaxial generator is driven to generate electricity, and the generator drives the phase change energy storage device to store heat energy. Then the natural gas output by the expander is transported to the third heat exchange tube in the gas-liquid mixing tank through a pipeline. The natural gas in the third heat exchange tube input from the expander to the gas-liquid mixing tank undergoes a second heat exchange with the high-pressure natural gas in the first heat exchange tube input into the gas-liquid mixing tank through the first heat exchange tube in the gas-liquid mixing tank, so that more of the high-pressure natural gas input into the gas-liquid mixing tank through the first heat exchange tube is converted into liquid. The natural gas output through the third heat exchange tube is transported to the second heat exchange tube through a pipeline for a first heat exchange treatment, and then heated to the required temperature of the client by the heater and transported to the low-pressure output client.

[0021] The high-pressure natural gas input into the gas-liquid mixing tank through the first heat exchange pipe in the first path is transported to the liquefied natural gas storage tank for storage through the pressure regulation of the throttle valve.

[0022] The operation according to the insufficient pressure state of high-pressure natural gas is specifically as follows:

[0023] On the basis of the same processing as the high-pressure natural gas sufficient pressure state, the following processing is set:

[0024] The gaseous part of the high-pressure natural gas in the gas-liquid mixing tank input by the first heat exchange pipe flows out from the opening on the top of the inner cavity of the gas-liquid mixing tank, and is then transmitted to the gas mixing tank via the stop valve. The gaseous part in the liquefied natural gas storage tank is transmitted to the gas mixing tank via the stop valve. Subsequently, the natural gas in the gas mixing tank flows through the stop valve and is transmitted to the second compressor for pressurization. The pressurized high-pressure natural gas flows into the phase change energy storage device to exchange heat with the stored heat energy, is heated to be isothermal with the high-pressure natural gas in the high-pressure natural gas pipeline, and is transmitted to the pipeline between the high-pressure natural gas pipeline and the three-way valve for pressure replenishment.

[0025] The beneficial effects of the present invention are:

[0026] 1. The present invention uses a turbine expander to replace the traditional throttle valve, utilizes the pressure difference of high-pressure natural gas to generate electricity, and realizes the liquefaction and capture of natural gas;

[0027] 2. The present invention utilizes a turbine expander to obtain cooling capacity and exchanges heat with high-pressure natural gas to improve liquefaction efficiency. It has the advantage of reducing energy waste and achieving better energy-saving effects.

[0028] 3. The present invention adds a phase change heat storage device to achieve effective temperature regulation and control and improve gas supply stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the principle of the high-pressure natural gas in the sufficient pressure state of the present invention;

[0030] Figure 2 This is a schematic diagram of the principle of the high-pressure natural gas under insufficient pressure of the present invention;

[0031] Figure numerals: high-pressure natural gas pipeline 1, pressure sensor 2, first compressor 3, expander 4, generator 5, phase change energy storage device 6, heat exchanger 7, gas-liquid mixing tank 8, liquefied natural gas storage tank 9, gas mixing tank 10, second compressor 11, three-way valve S1, stop valve F1, stop valve F2, stop valve F3, throttle valve J1, heater 12, low-pressure output client 13. DETAILED DESCRIPTION

[0032] The present invention is further described below in conjunction with the accompanying drawings and embodiments. The embodiments of the present invention include but are not limited to the following embodiments.

[0033] A turbine expansion liquefaction system utilizing natural gas pressure difference Figure 1 As shown, it includes a high-pressure natural gas pipeline 1, a pressure sensor 2, a first compressor 3, a pressure difference power generation device, a pressure compensation device, a heat exchange device, a liquefied natural gas storage tank 9 and a low-pressure output client 13;

[0034] The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the pressure difference power generation device through pipelines respectively. A pressure sensor 2 is provided on the pipeline from the outlet of the high-pressure natural gas pipeline 1 to the inlet of the pressure difference power generation device. The outlet of the pressure difference power generation device is connected to the inlet of the low-pressure output client 13 through a pipeline via a heat exchanger. The outlet of the first compressor 3 is connected to the heat exchanger through a pipeline. The heat exchanger is connected to the pressure compensating device and the liquefied natural gas storage tank 9 through pipelines respectively. The liquefied natural gas storage tank 9 is connected to the outlet of the high-pressure natural gas pipeline 1 through a pipeline. A pressure compensating device is provided on the pipeline from the liquefied natural gas storage tank 9 to the outlet of the high-pressure natural gas pipeline 1, and the pressure compensating device is electrically connected to the pressure difference power generation device.

[0035] The liquefied natural gas storage tank 9 is provided with an LNG loading and unloading interface and is externally connected to an LNG loading and unloading device for transporting LNG.

[0036] The pressure compensation device includes a phase change energy storage device 6, a gas mixing tank 10 and a second compressor 11. The heat exchange device is connected to the gas mixing tank 10 and the liquefied natural gas storage tank 9 through pipelines. The gas mixing tank 10, the second compressor 11 and the phase change energy storage device 6 are sequentially arranged on the pipeline from the liquefied natural gas storage tank 9 to the outlet of the high-pressure natural gas pipeline 1. The phase change energy storage device 6 is electrically connected to the pressure difference power generation device.

[0037] The pressure difference power generation device includes an expander 4 and a generator 5, specifically a turbine expander. The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the expander 4 through pipelines, the working shaft of the expander 4 is transmission-connected to the rotating shaft of the generator 5, the generator 5 is electrically connected to the phase change energy storage device 6, and the outlet of the expander 4 is connected to the inlet of the low-pressure output client 13 through a pipeline through a heat exchange device.

[0038] The low-pressure output client refers to the user's natural gas load equipment.

[0039] The heat exchange device includes a heat exchanger 7 and a gas-liquid mixing tank 8. The heat exchanger 7 is provided with a first heat exchange tube and a second heat exchange tube. The upper part, top and bottom of the inner cavity of the gas-liquid mixing tank 8 are all provided with openings. A third heat exchange tube is directly arranged through the inner cavity in the tank. The third heat exchange tube is not connected to the inner cavity of the tank, and plays a role of mutual heat exchange. ;

[0040] The outlet of the first compressor 3 is connected to the inlet of the first heat exchange tube in the heat exchanger 7 through a pipeline, the outlet of the first heat exchange tube is connected to the upper opening of the inner cavity of the gas-liquid mixing tank 8 through a pipeline, the top opening of the inner cavity of the gas-liquid mixing tank 8 is connected to the gas mixing tank 10 through a pipeline, the bottom opening of the inner cavity of the gas-liquid mixing tank 8 is connected to the liquefied natural gas storage tank 9 through a pipeline, the outlet of the expander 4 is connected to the inlet of the third heat exchange tube in the gas-liquid mixing tank 8 through a pipeline, the outlet of the third heat exchange tube is connected to the inlet of the second heat exchange tube in the heat exchanger 7 through a pipeline, and the outlet of the second heat exchange tube is connected to the inlet of the low-pressure output client 13 through a pipeline.

[0041] The outlet of the high-pressure natural gas pipeline 1 is connected to the first compressor 3 and the inlet of the pressure difference power generation device through a three-way valve S1, and a pressure sensor 2 is provided on the pipeline from the outlet of the high-pressure natural gas pipeline 1 to the three-way valve S1.

[0042] The pipeline between the gas-liquid mixing tank 8 and the gas mixing tank 10, the pipeline between the gas mixing tank 10 and the second compressor 11, and the pipeline between the liquefied natural gas storage tank 9 and the gas mixing tank 10 are all provided with stop valves F1, F2, and F3. Specifically, the pipeline between the gas-liquid mixing tank 8 and the gas mixing tank 10 is provided with a first stop valve; the pipeline between the liquefied natural gas storage tank 9 and the gas mixing tank 10 is provided with a second stop valve. The pipeline between the gas mixing tank 10 and the second compressor 11 is provided with a third stop valve; the pressure sensor 2 is electrically connected to the stop valves F1, F2, and F3, and is used to control the opening and closing of the stop valves F1, F2, and F3. The pipeline between the gas-liquid mixing tank 8 and the liquefied natural gas storage tank 9 is provided with a throttle valve J1, which is used to adjust the pressure of the liquefied natural gas storage tank 9.

[0043] A heater 12 is provided on the pipeline between the second heat exchange tube and the low-pressure output client 13 for adjusting the temperature of the natural gas input into the low-pressure output client 13 .

[0044] The present invention has two states: the two states are respectively a state in which the high-pressure natural gas pressure is sufficient and a state in which the high-pressure natural gas pressure is insufficient.

[0045] Set the pressure threshold, and pressure sensor 2 detects the pressure of the high-pressure natural gas flowing through pressure sensor 2:

[0046] When the pressure sensor 2 detects that the high-pressure natural gas pressure exceeds the set threshold, it considers that this state is a state of sufficient high-pressure natural gas pressure, and controls the shut-off valves F1, F2, and F3 to close, and operate according to the state of sufficient high-pressure natural gas pressure;

[0047] When the pressure sensor 2 detects that the high-pressure natural gas pressure exceeds, is lower than or equal to the threshold value, it considers that this state is a state of insufficient high-pressure natural gas pressure, and controls the shut-off valves F1, F2, and F3 to open and operate according to the state of insufficient high-pressure natural gas pressure.

[0048] In the turbine expansion liquefaction system that utilizes the pressure difference energy of natural gas, a part of the high-pressure pipeline generates electricity after passing through the turbine expander, and then converts the excess electricity into heat energy for storage. The other part of the high-pressure natural gas passes through the compressor and exchanges heat with the low-temperature natural gas after the turbine expansion, so that this part of the natural gas is liquefied and stored in the LNG storage tank. The low-pressure loop natural gas after heat exchange enters the low-pressure natural gas pipeline after heat exchange, achieving efficient utilization of system energy. At the same time, when the gas supply pressure on the high-pressure natural gas side is insufficient, the high-pressure pipeline can be pressurized through the LNG storage tank and the phase change energy storage device.

[0049] Specifically, if Figure 1 As shown in the figure, the bold lines in the figure represent the pipelines or lines used in this state, and the specific operation according to the high-pressure natural gas pressure sufficient state is as follows:

[0050] High-pressure natural gas is first divided into two paths by a three-way valve S1 from a high-pressure natural gas pipeline 1. The first path of high-pressure natural gas flows into the first compressor 3 through a pipeline for pressurization, and then is transported to the first heat exchange pipe in the heat exchanger 7 through a pipeline for first heat exchange cooling to become a gas-liquid two-phase state, and then is input into the gas-liquid mixing tank 8 through a pipeline;

[0051] The second high-pressure natural gas is expanded and decompressed through the expander 4 to obtain cold energy. During the operation of the expander, the generator 5 is coaxially driven to generate electricity, and the generator 5 drives the phase change energy storage device 6 to store heat energy. Then the natural gas output by the expander 4 is transported to the third heat exchange tube in the gas-liquid mixing tank 8 through a pipeline. The natural gas in the third heat exchange tube input from the expander 4 to the gas-liquid mixing tank 8 undergoes a second heat exchange in the gas-liquid mixing tank 8 with the high-pressure natural gas input from the first heat exchange tube into the gas-liquid mixing tank 8 through the first heat exchange tube, i.e., secondary heat exchange, so that the high-pressure natural gas input into the gas-liquid mixing tank 8 through the first heat exchange tube is more converted into liquid. At this time, the natural gas in the gas-liquid mixing tank 8 is in a gas-liquid two-phase state. After the second heat exchange, the temperature of the natural gas in the third heat exchange tube rises initially, and then flows through the heat exchanger 7 for preheating. The natural gas output through the third heat exchange tube is transported to the second heat exchange tube through a pipeline for the first heat exchange treatment, and then heated to the required temperature of the client by the heater 12 and transported to the low-pressure output client 13.

[0052] The high-pressure liquid portion of natural gas input into the gas-liquid mixing tank 8 through the first heat exchange pipe in the first path is expanded to the storage pressure through the pressure regulation of the throttle valve J1 and transported to the liquefied natural gas storage tank 9 for storage.

[0053] like Figure 2 As shown, the specific operation according to the insufficient pressure state of high-pressure natural gas is as follows:

[0054] On the basis of the same processing as the high-pressure natural gas sufficient pressure state, the following processing is set:

[0055] The gaseous part of the high-pressure natural gas in the gas-liquid mixing tank 8 input by the first heat exchange pipe flows out from the opening on the top of the inner cavity of the gas-liquid mixing tank 8, and is then transmitted to the gas mixing tank 10 via the first stop valve. The gaseous part in the liquefied natural gas storage tank 9 is transmitted to the gas mixing tank 10 via the second stop valve. Subsequently, the natural gas in the gas mixing tank 10 flows through the third stop valve and is transmitted to the second compressor 11 for pressurization. The pressurized high-pressure natural gas flows into the phase change energy storage device 6 to exchange heat with the stored heat energy, is heated to be isothermal with the high-pressure natural gas in the high-pressure natural gas pipeline 1, and is transmitted to the pipeline between the high-pressure natural gas pipeline 1 and the three-way valve S1 for pressure replenishment.

[0056] When the pressure is replenished to a sufficient level, continue to operate as if the high-pressure natural gas pressure is sufficient.

Claims

1. A turbine expansion liquefaction system utilizing natural gas pressure difference, characterized in that: It comprises a high-pressure natural gas pipeline (1), a pressure sensor (2), a first compressor (3), a pressure difference power generation device, a pressure compensation device, a heat exchange device, a liquefied natural gas storage tank (9) and a low-pressure output client (13); The outlet of the high-pressure natural gas pipeline (1) is connected to the first compressor (3) and the inlet of the pressure difference power generation device through pipelines, respectively. A pressure sensor (2) is provided on the pipeline from the outlet of the high-pressure natural gas pipeline (1) to the inlet of the pressure difference power generation device. The outlet of the pressure difference power generation device is connected to the inlet of the low-pressure output client (13) through a pipeline via a heat exchange device. The outlet of the first compressor (3) is connected to the heat exchange device through a pipeline. The heat exchange device is connected to the pressure compensation device and the liquefied natural gas storage tank (9) through pipelines, respectively. The liquefied natural gas storage tank (9) is connected to the outlet of the high-pressure natural gas pipeline (1) through a pipeline. A pressure compensation device is provided on the pipeline from the liquefied natural gas storage tank (9) to the outlet of the high-pressure natural gas pipeline (1), and the pressure compensation device is electrically connected to the pressure difference power generation device.

2. The turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 1, characterized in that: The pressure compensation device comprises a phase change energy storage device (6), a gas mixing tank (10) and a second compressor (11); the heat exchange device is respectively connected to the gas mixing tank (10) and the liquefied natural gas storage tank (9) via pipelines; the gas mixing tank (10), the second compressor (11) and the phase change energy storage device (6) are sequentially arranged on the pipeline from the liquefied natural gas storage tank (9) to the outlet of the high-pressure natural gas pipeline (1); and the phase change energy storage device (6) is electrically connected to the pressure difference power generation device.

3. A turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 2, characterized in that: The pressure difference power generation device comprises an expander (4) and a generator (5); the outlet of the high-pressure natural gas pipeline (1) is connected to the first compressor (3) and the inlet of the expander (4) through pipelines respectively; the expander (4) is transmission-connected to the generator (5); the generator (5) is electrically connected to the phase-change energy storage device (6); and the outlet of the expander (4) is connected to the inlet of the low-pressure output client (13) through a pipeline via a heat exchange device.

4. A turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 3, characterized in that: The heat exchange device comprises a heat exchanger (7) and a gas-liquid mixing tank (8), wherein the heat exchanger (7) is provided with a first heat exchange tube and a second heat exchange tube, the gas-liquid mixing tank (8) has an inner cavity upper part, a top part and a bottom part provided with openings, and a third heat exchange tube is provided in the tank and is directly connected to the inner cavity; The outlet of the first compressor (3) is connected to the inlet of the first heat exchange tube in the heat exchanger (7) through a pipeline, the outlet of the first heat exchange tube is connected to the upper opening of the inner cavity of the gas-liquid mixing tank (8) through a pipeline, the top opening of the inner cavity of the gas-liquid mixing tank (8) is connected to the gas mixing tank (10) through a pipeline, the bottom opening of the inner cavity of the gas-liquid mixing tank (8) is connected to the liquefied natural gas storage tank (9) through a pipeline, the outlet of the expander (4) is connected to the inlet of the third heat exchange tube in the gas-liquid mixing tank (8) through a pipeline, the outlet of the third heat exchange tube is connected to the inlet of the second heat exchange tube in the heat exchanger (7) through a pipeline, and the outlet of the second heat exchange tube is connected to the inlet of the low-pressure output client (13) through a pipeline.

5. The turbine expansion liquefaction system utilizing natural gas pressure difference according to claim 1, characterized in that: The outlet of the high-pressure natural gas pipeline (1) is respectively connected to the first compressor (3) and the inlet of the pressure difference power generation device through a three-way valve (S1), and a pressure sensor (2) is provided on the pipeline from the outlet of the high-pressure natural gas pipeline (1) to the three-way valve (S1).

6. The turbine expansion liquefaction system using natural gas pressure difference according to claim 2, characterized in that: Stop valves (F1, F2, F3) are provided on the pipeline between the gas-liquid mixing tank (8) and the gas mixing tank (10), on the pipeline between the gas mixing tank (10) and the second compressor (11), and on the pipeline between the liquefied natural gas storage tank (9) and the gas mixing tank (10). A throttle valve (J1) is provided on the pipeline between the gas-liquid mixing tank (8) and the liquefied natural gas storage tank (9).

7. The turbine expansion liquefaction system using natural gas pressure difference according to claim 4, characterized in that: A heater (12) is provided on the pipeline between the second heat exchange tube and the low-pressure output client (13).

8. A turbine expansion liquefaction process using natural gas pressure difference according to any one of claims 1 to 7, characterized in that: The pressure threshold is set, and the pressure sensor (2) detects the pressure of the high-pressure natural gas flowing through the pressure sensor (2): When the pressure sensor (2) detects that the high-pressure natural gas pressure exceeds a set threshold, the stop valves (F1, F2, F3) are controlled to close and operate in accordance with a state where the high-pressure natural gas pressure is sufficient; When the pressure sensor (2) detects that the high-pressure natural gas pressure exceeds, is lower than or equal to a threshold value, the stop valves (F1, F2, F3) are controlled to open and operate in accordance with a state of insufficient high-pressure natural gas pressure.

9. The method for liquefaction of natural gas by turbine expansion using pressure difference according to claim 8, characterized in that: The operation according to the high-pressure natural gas sufficient pressure state is specifically as follows: High-pressure natural gas is first divided into two paths through a three-way valve (S1) from a high-pressure natural gas pipeline (1). The first path of high-pressure natural gas flows into a first compressor (3) through a pipeline for pressurization, and is then transported through a pipeline to a first heat exchange pipe in a heat exchanger (7) for a first heat exchange, and is then input into a gas-liquid mixing tank (8) through a pipeline; The second high-pressure natural gas is expanded and decompressed through the expander (4) to obtain cold energy. During the operation of the expander, the generator (5) is coaxially driven to generate electricity. The generator (5) drives the phase change energy storage device (6) to store heat energy. The natural gas output by the expander (4) is then transported to the third heat exchange tube in the gas-liquid mixing tank (8) through a pipeline. The natural gas input from the expander (4) into the third heat exchange tube of the gas-liquid mixing tank (8) undergoes a second heat exchange with the first high-pressure natural gas input into the gas-liquid mixing tank (8) through the first heat exchange tube. This allows more of the high-pressure natural gas input into the gas-liquid mixing tank (8) through the first heat exchange tube to be converted into liquid. The natural gas output through the third heat exchange tube is transported to the second heat exchange tube through a pipeline for a first heat exchange treatment. It is then heated to the temperature required by the client by the heater (12) and transported to the low-pressure output client (13). The high-pressure natural gas in the gas-liquid mixing tank (8) input through the first heat exchange pipe in the first path is transported to the liquefied natural gas storage tank (9) for storage through the pressure regulation of the throttle valve (J1).

10. The method for liquefaction of natural gas by turbine expansion using pressure difference according to claim 9, characterized in that: The operation according to the insufficient pressure state of high-pressure natural gas is specifically as follows: On the basis of the same processing as the high-pressure natural gas sufficient pressure state, the following processing is set: The gaseous part of the high-pressure natural gas in the gas-liquid mixing tank (8) input through the first heat exchange pipe flows out from the top opening of the inner cavity of the gas-liquid mixing tank (8), and is then transmitted to the gas mixing tank (10) through the stop valve. The gaseous part in the liquefied natural gas storage tank (9) is transmitted to the gas mixing tank (10) through the stop valve. Subsequently, the natural gas in the gas mixing tank (10) flows through the stop valve and is transmitted to the second compressor (11) for pressurization. The pressurized high-pressure natural gas flows into the phase change energy storage device (6) to exchange heat with the stored heat energy, is heated to be isothermal with the high-pressure natural gas in the high-pressure natural gas pipeline (1), and is then transmitted to the pipeline between the high-pressure natural gas pipeline (1) and the three-way valve (S1) for pressure replenishment.

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