Pump System

Abstract

Provided is a pump system that can start a submerged pump among a plurality of submerged pumps connected in series while preventing rotation of the impeller of a submerged pump that is not operating. [Solution] The pump system comprises a first pump 1A and a second pump 1B connected in series, a second suction container 2B that houses the second pump 1B, and a flow path switching device 5B disposed within the second suction container 2B. The flow path switching device 5B has a valve body 47 that selectively connects an outlet flow path 43 to either a pump-side flow path 41 or a container-side flow path 42. The pump-side flow path 41 communicates with the discharge port of the second pump 1B, the container-side flow path 42 communicates with the interior of the second suction container 2B, and the outlet flow path 43 communicates with a discharge port 8 of the second suction container 2B.

Description

[Technical Field]

[0001] The present invention relates to a pump system used to transport liquefied gases such as liquefied hydrogen, liquid nitrogen, liquefied ammonia, liquefied natural gas, liquefied ethylene gas, and liquefied petroleum gas, and in particular to a technique for starting and stopping a submersible pump while preventing the impeller of the other submersible pump from rotating when the pump is not in operation. [Background technology]

[0002] Natural gas is widely used in thermal power generation and as a chemical raw material. Hydrogen, meanwhile, is expected to be an energy source that does not produce carbon dioxide, a cause of global warming. Applications of hydrogen as an energy source include fuel cells and turbine power generation. Natural gas and hydrogen are in a gaseous state at room temperature, so they are cooled and liquefied for storage and transportation. Liquefied gases such as liquefied natural gas (LNG) and liquid hydrogen are first stored in liquefied gas tanks and then pumped to power plants, factories, etc.

[0003] FIG. 11 is a schematic diagram showing a conventional example of a pump device for pumping liquefied gas. Pump 500 is installed in a vertical suction vessel 505 connected to a liquefied gas storage tank (not shown) in which liquefied gas is stored. Liquefied gas is introduced into suction vessel 505 through suction port 501, and suction vessel 505 is filled with liquefied gas. Pump 500 is entirely immersed in liquefied gas. Therefore, pump 500 is a submersible pump that can operate in liquefied gas. When pump 500 is operating, the liquefied gas is discharged by pump 500 through discharge port 502. During operation of pump 500, part of the liquefied gas in suction vessel 505 evaporates and becomes gas, which is discharged from suction vessel 505 through vent line 503.

[0004] In order to pressurize the liquefied gas to the pressure required on the demand side, multiple pump devices may be connected in series as shown in Fig. 12. The liquefied gas is sequentially pressurized by the pumps 500 of the multiple pump devices. When starting up the multiple pump devices, they are started in order starting from the upstream pump 500, and when stopping them, they are stopped in order starting from the downstream pump 500. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Utility Model Application Publication No. 59-159795 [Patent Document 2] Jpn. Jpn. Appl. KOKAI Publication No. 62-031680 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when serially connected pump devices are started or stopped in sequence, the following problem occurs: When the first pump 500 is started, a flow of liquefied gas occurs in the stopped pump 500. As a result, the impeller of the stopped pump 500 is forced to rotate, damaging the sliding parts such as bearings.

[0007] When the pump 500 is operating, the liquefied gas is pressurized by the rotation of the impeller, so the thrust balance mechanism installed in the pump 500 functions to prevent excessive load from being placed on sliding parts such as bearings. However, when the pump 500 is stopped, the thrust balance mechanism does not function, and the liquefied gas pumped from other pumps 500 forcibly rotates the impeller, resulting in damage to sliding parts such as bearings. In particular, liquefied gas has low viscosity, and unintended rotation of the impeller easily wears sliding parts such as bearings. Furthermore, the same problem occurs when the downstream pump 500 is stopped because the upstream pump 500 is still operating. The same problem occurs when the pump 500 suddenly stops due to a malfunction of the pump 500.

[0008] Therefore, the present invention provides a pump system that can start and stop multiple submersible pumps connected in series while preventing the impeller of a submersible pump that is not in operation from rotating. [Means for solving the problem]

[0009] In one aspect, a pump system is provided, comprising a first pump and a second pump connected in series, a second suction container that houses the second pump, and a flow path switching device arranged in the second suction container, wherein the flow path switching device has a flow path structure having a pump side flow path, a container side flow path, and an outlet flow path, and a valve body arranged in the flow path structure that selectively connects the outlet flow path to either the pump side flow path or the container side flow path, wherein the pump side flow path is connected to the discharge port of the second pump, the container side flow path is connected to the interior of the second suction container, and the outlet flow path is connected to the discharge port of the second suction container.

[0010] In one aspect, a method for starting a plurality of pump devices including at least a first pump device and a second pump device connected in series is provided, the method including starting a first submersible pump disposed in a first suction container of the first pump device to transfer liquefied gas through a first flow path switching device in the first suction container to a second suction container of the second pump device, passing the liquefied gas through a second flow path switching device in the second suction container while bypassing a second submersible pump disposed in the second suction container, and then starting the second submersible pump.

[0011] In one embodiment, each of the first flow path switching device and the second flow path switching device includes a flow path structure having a pump side flow path, a container side flow path, and an outlet flow path, and a valve body disposed within the flow path structure that selectively connects the outlet flow path to either the pump side flow path or the container side flow path, wherein the pump side flow path is connected to the discharge port of the corresponding submersible pump, the container side flow path is connected to the interior of the corresponding suction container, and the outlet flow path is connected to the discharge port of the corresponding suction container.

[0012] In one aspect, there is provided a method for stopping operation of a plurality of pump devices including at least a first pump device and a second pump device connected in series, the method including: when a first submersible pump disposed in a first suction container of the first pump device is transferring liquefied gas to a second suction container of the second pump device through a first flow path switching device in the first suction container, stopping operation of a second submersible pump disposed in the second suction container, causing the liquefied gas to bypass the second submersible pump while passing the liquefied gas through the second flow path switching device in the second suction container, and then stopping operation of the first submersible pump.

[0013] In one embodiment, each of the first flow path switching device and the second flow path switching device includes a flow path structure having a pump side flow path, a container side flow path, and an outlet flow path, and a valve body disposed within the flow path structure that selectively connects the outlet flow path to either the pump side flow path or the container side flow path, wherein the pump side flow path is connected to the discharge port of the corresponding submersible pump, the container side flow path is connected to the interior of the corresponding suction container, and the outlet flow path is connected to the discharge port of the corresponding suction container. [Effects of the Invention]

[0014] When the submersible pump is started or stopped, the flow path switching device allows the liquefied gas to bypass the submersible pump when it is not in operation. Therefore, the impeller of the submersible pump does not rotate when it is not in operation, and as a result, damage to the sliding parts of the submersible pump, such as the bearings, can be prevented. [Brief explanation of the drawings]

[0015] [Figure 1] 1 illustrates an embodiment of a pumping device for transferring liquefied gas. [Figure 2] FIG. 2 is a cross-sectional view showing a detailed configuration of a flow path switching device according to an embodiment of the present invention. [Figure 3] This shows the state of the flow path switching device when the submersible pump is operating. [Figure 4] FIG. 1 is a schematic diagram illustrating an embodiment of a pump system including multiple pump devices connected in series. [Figure 5] 5 illustrates one embodiment of a method for sequentially starting the submersible pumps shown in FIG. 4. FIG. [Figure 6] 5A-5C illustrate the embodiment of the method for sequentially starting the submersible pumps shown in FIG. 4. [Figure 7] 5 is a diagram illustrating an embodiment of a method for sequentially stopping the operation of the plurality of submersible pumps shown in FIG. 4. FIG. [Figure 8] 5A and 5B are diagrams illustrating the embodiment of the method for sequentially stopping the operation of the plurality of submersible pumps shown in FIG. 4. [Figure 9] FIG. 10 is a schematic diagram illustrating another embodiment of a pump system having multiple pump devices connected in series. [Figure 10] FIG. 10 is a schematic diagram illustrating yet another embodiment of a pump system including multiple pump devices connected in series. [Figure 11] FIG. 1 is a schematic diagram showing a conventional example of a pump device for pumping up liquefied gas. [Figure 12] FIG. 1 is a schematic diagram illustrating an example of a plurality of pump devices connected in series. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing one embodiment of a pump device for transferring liquefied gas. Examples of liquefied gas that can be transferred by the pump device 100 shown in Fig. 1 include liquefied hydrogen, liquid nitrogen, liquefied ammonia, liquefied natural gas, liquefied ethylene gas, and liquefied petroleum gas.

[0017] As shown in Figure 1, the pump device 100 includes a submersible pump 1 for transferring liquefied gas, a suction vessel 2 housing the submersible pump 1, and a flow path switching device 5 for preventing rotation of the impeller 15 of the submersible pump 1 when the pump is not in operation. The suction vessel 2 has a suction port 7 and a discharge port 8. Liquefied gas is introduced into the suction vessel 2 through the suction port 7, and the suction vessel 2 is filled with the liquefied gas. When the submersible pump 1 is in operation, the entire submersible pump 1 is immersed in the liquefied gas. Therefore, the submersible pump 1 is configured to be able to operate in liquefied gas.

[0018] The submersible pump 1 includes an electric motor 11 having a motor rotor 9 and a motor stator 10, a rotating shaft 12 connected to the electric motor 11, a plurality of bearings 14 that rotatably support the rotating shaft 12, an impeller 15 fixed to the rotating shaft 12, and a pump casing 16 that houses the impeller 15. The flow path switching device 5 is disposed within the suction vessel 2. More specifically, the flow path switching device 5 is connected to both the discharge port 4 of the submersible pump 1 and the discharge port 8 of the suction vessel 2. The specific configuration of the flow path switching device 5 will be described later.

[0019] When power is supplied to the electric motor 11 through a power cable (not shown), the electric motor 11 rotates the rotating shaft 12 and the impeller 15 together. As the impeller 15 rotates, liquefied gas is sucked into the submersible pump 1 from the suction port 3 and discharged into the flow path switching device 5 through the discharge flow path 17 and the discharge port 4. The liquefied gas then passes through the flow path switching device 5 and is discharged through the discharge port 8 of the suction container 2.

[0020] A suction valve 22 is connected to the suction port 7, and a discharge valve 23 is connected to the discharge port 8. A drain line 25 is connected to the bottom of the suction vessel 2, and a drain valve 26 is connected to the drain line 25. The suction port 7 is provided on the side wall of the suction vessel 2 and is located higher than the bottom of the suction vessel 2. The discharge port 8 is provided on the top of the suction vessel 2 and is located higher than the suction port 7. When the submersible pump 1 is in operation, the suction valve 22 and the discharge valve 23 are open, and the drain valve 26 is closed.

[0021] A vent line 31 is connected to the top of the suction vessel 2. During operation of the submersible pump 1, part of the liquefied gas is vaporized due to heat generated by the submersible pump 1, and this gas is discharged from the suction vessel 2 through the vent line 31. A vent valve 32 is connected to the vent line 31. In one embodiment, this gas may be led to a gas treatment device (not shown) through the vent line 31. The gas treatment device is a device that treats gas (e.g., natural gas or hydrogen gas) vaporized from the liquefied gas. Examples of gas treatment devices include a gas incineration device (flaring device), a chemical gas treatment device, and a gas adsorption device.

[0022] FIG. 2 is a cross-sectional view showing a detailed configuration of one embodiment of the flow path switching device 5. As shown in FIG. 2, the flow path switching device 5 includes a flow path structure 45 having a pump-side flow path 41, a container-side flow path 42, and an outflow flow path 43, and a valve body 47 disposed within the flow path structure 45. The pump-side flow path 41 communicates with the discharge port 4 of the submersible pump 1, the container-side flow path 42 communicates with the interior of the suction container 2, and the outflow flow path 43 communicates with the discharge port 8 of the suction container 2. The valve body 47 is disposed so as to selectively communicate the outflow flow path 43 with either the pump-side flow path 41 or the container-side flow path 42. The configuration of the flow path switching device 5 is not limited to the embodiment shown in FIG. 2 as long as it can perform its intended function.

[0023] 2 shows the state of the flow path switching device 5 when the submersible pump 1 is not operating. The valve element 47 is pressed against the flow path structure 45 by the spring 50 to close the pump-side flow path 41. More specifically, the flow path structure 45 has a valve seat 51 formed around the outlet of the pump-side flow path 41, and the valve element 47 is pressed against the valve seat 51 by the spring 50. Therefore, while the valve element 47 is pressed against the valve seat 51, the pump-side flow path 41 is closed, and the container-side flow path 42 and the outflow flow path 43 are in communication. The container-side flow path 42 opens inside the suction container 2 and is in communication with the suction port 7 through the interior of the suction container 2.

[0024] 3 shows the state of the flow path switching device 5 when the submersible pump 1 is operating. When the submersible pump 1 is operating, liquefied gas is discharged from the discharge port 4 of the submersible pump 1 and flows into the pump-side flow path 41 of the flow path switching device 5. The liquefied gas flowing through the pump-side flow path 41 moves the valve element 47 against the force of the spring 50, opening the pump-side flow path 41 and closing the container-side flow path 42 with the valve element 47. As a result, the pump-side flow path 41 and the outflow flow path 43 are connected to each other.

[0025] When the submersible pump 1 stops operating, the valve element 47 is pressed against the valve seat 51 by the spring 50. As a result, as shown in Figure 2, the pump-side flow path 41 is closed, and the container-side flow path 42 and the outflow flow path 43 are connected. In this way, the flow path switching device 5 of this embodiment operates only by the spring 50 and the flow of liquefied gas.

[0026] In order to pressurize liquefied gas to a pressure required on the demand side, multiple pumping devices 100 may be connected in series. FIG. 4 is a schematic diagram showing one embodiment of a pumping system including multiple pumping devices 100A, 100B, and 100C connected in series. In FIG. 4, the multiple pumping devices 100A, 100B, and 100C have the same configuration as the pumping device 100 described with reference to FIGS. 1 to 6. In the following description, the submersible pump, suction container, and flow path switching device of pumping device 100A will be referred to as submersible pump 1A, suction container 2A, and flow path switching device 5A, respectively; the submersible pump, suction container, and flow path switching device of pumping device 100B will be referred to as submersible pump 1B, suction container 2B, and flow path switching device 5B, respectively; and the submersible pump, suction container, and flow path switching device of pumping device 100C will be referred to as submersible pump 1C, suction container 2C, and flow path switching device 5C, respectively.

[0027] Pump device 100A is disposed upstream of pump device 100B, which is disposed upstream of pump device 100C. Suction port 7 of pump device 100A is connected to liquefied gas storage tank 105, which stores liquefied gas therein. Pump device 100A is connected in series to pump device 100B by communication line 107, and pump device 100B is connected in series to pump device 100C by communication line 108. More specifically, discharge port 8 of pump device 100A is connected to suction port 7 of pump device 100B by communication line 107, and discharge port 8 of pump device 100B is connected to suction port 7 of pump device 100C by communication line 108.

[0028] Submersible pumps 1A, 1B, and 1C are connected in series in the order of submersible pump 1A, submersible pump 1B, and submersible pump 1C. Liquefied gas is sequentially pressurized by these submersible pumps 1A, 1B, and 1C. When submersible pumps 1A, 1B, and 1C are operating and transferring liquefied gas, flow path switching devices 5A, 5B, and 5C are in the state shown in Figure 3.

[0029] Next, an embodiment of a method for starting submersible pumps 1A, 1B, and 1C connected in series as shown in Fig. 4 will be described. Submersible pumps 1A, 1B, and 1C are started in order from the upstream side. That is, submersible pump 1A is started first, then submersible pump 1B, and finally submersible pump 1C.

[0030] 5 is a diagram illustrating a state in which the submersible pump 1A is started while the submersible pumps 1B and 1C are not operating. When the submersible pump 1A is started, the liquefied gas is transferred by the submersible pump 1A through the flow path switching device 5A to the suction vessel 2B of the pump device 100B. When the submersible pump 1A is operating and transferring the liquefied gas, the flow path switching device 5A is in the state shown in FIG.

[0031] At this stage, submersible pump 1B is out of operation, so flow path switching device 5B is in the state shown in FIG. 2. Therefore, the liquefied gas passes through flow path switching device 5B while bypassing submersible pump 1B (i.e., the liquefied gas does not flow through submersible pump 1B). The liquefied gas is further transferred from pump device 100B to suction vessel 2C of pump device 100C. Since submersible pump 1C is also out of operation, flow path switching device 5C is in the state shown in FIG. 2. Therefore, the liquefied gas passes through flow path switching device 5C while bypassing submersible pump 1C (i.e., the liquefied gas does not flow through submersible pump 1C).

[0032] Next, submersible pump 1B is started. Figure 6 is a diagram illustrating the state in which submersible pump 1B is started while submersible pump 1A is operating, and submersible pump 1C is not operating. When submersible pump 1B is started, liquefied gas is transferred by submersible pump 1B through flow path switching device 5B to suction vessel 2C of pump device 100C. When submersible pump 1B is operating and transferring liquefied gas, flow path switching device 5B is in the state shown in Figure 3.

[0033] At this stage, the submersible pump 1C is still out of operation, and the flow path switching device 5C is in the state shown in Fig. 2. Therefore, the liquefied gas passes through the flow path switching device 5C while bypassing the submersible pump 1C (i.e., the liquefied gas does not flow through the submersible pump 1C).

[0034] Next, submersible pump 1C is started. When submersible pump 1C is started, submersible pumps 1A and 1B are in operation. The state in which all of submersible pumps 1A, 1B, and 1C are operating is shown in Figure 4. In this way, submersible pumps 1A, 1B, and 1C are started in order from the upstream side.

[0035] When starting up the submersible pumps 1A, 1B, and 1C, the flow path switching device allows the liquefied gas to bypass the submersible pumps that are not in operation. Therefore, the impellers of the submersible pumps that are not in operation do not rotate, and as a result, damage to the bearings and other sliding parts of the submersible pumps can be prevented.

[0036] Next, a description will be given of one embodiment of a method for stopping the operation of submersible pumps 1A, 1B, and 1C connected in series as shown in Fig. 4. The operation of submersible pumps 1A, 1B, and 1C is stopped in order from the downstream side. That is, first, the operation of submersible pump 1C is stopped, then the operation of submersible pump 1B is stopped, and finally the operation of submersible pump 1A is stopped.

[0037] Figure 7 is a diagram illustrating a state in which the operation of submersible pump 1C is stopped while submersible pumps 1A and 1B are operating. As a result of submersible pump 1C being stopped, flow path switching device 5C is in the state shown in Figure 2. Therefore, the liquefied gas passes through flow path switching device 5C while bypassing submersible pump 1C (i.e., the liquefied gas does not flow through submersible pump 1C).

[0038] At this stage, submersible pumps 1A and 1B are in operation. Therefore, liquefied gas is transferred by submersible pump 1A through flow path switching device 5A to suction vessel 2B of pumping device 100B, and further, liquefied gas is transferred by submersible pump 1B through flow path switching device 5B to suction vessel 2C of pumping device 100C. When submersible pumps 1A and 1B are in operation and transferring liquefied gas, flow path switching devices 5A and 5B are in the state shown in Figure 3.

[0039] Next, submersible pump 1B is stopped. Figure 8 is a diagram illustrating a state in which submersible pump 1B is stopped while submersible pump 1A is operating and submersible pump 1C is not operating. As a result of submersible pump 1B being stopped, flow path switching device 5B becomes the state shown in Figure 2. Therefore, the liquefied gas passes through flow path switching device 5B while bypassing submersible pump 1B (i.e., the liquefied gas does not flow through submersible pump 1B).

[0040] At this stage, the submersible pump 1A is still in operation, and the flow path switching device 5A is in the state shown in Figure 3. Therefore, the liquefied gas is transferred by the submersible pump 1A through the flow path switching device 5A to the suction vessel 2B of the pump device 100B.

[0041] Next, the submersible pump 1A is stopped. When the submersible pump 1A is stopped, the submersible pumps 1B and 1C are not operating. In this manner, the submersible pumps 1A, 1B, and 1C are stopped in order from the downstream side.

[0042] When submersible pumps 1A, 1B, and 1C are shut down, the flow path switching device allows the liquefied gas to bypass the shut down submersible pumps. Therefore, the impellers of the shut down submersible pumps do not rotate, and as a result, damage to the bearings and other sliding parts of the submersible pumps can be prevented.

[0043] 4-8 includes three pump units 100A, 100B, and 100C connected in series, but the number of pump units is not limited to this embodiment. In one embodiment, the pump system may include only two pump units connected in series, or may include four or more pump units connected in series. The multiple submersible pumps connected in series are started and stopped in the same manner as in the above-described embodiments.

[0044] Fig. 9 is a schematic diagram showing another embodiment of a pump system including a plurality of pump devices connected in series. The configuration and operation of this embodiment, which will not be specifically described, are the same as those of the embodiment described with reference to Fig. 7, and therefore redundant description will be omitted. The pump system of the embodiment shown in Fig. 9 further includes pump devices 100D, 100E, and 100F connected in series, in addition to pump devices 100A, 100B, and 100C connected in series.

[0045] The pump device 100D comprises a suction container 2D, a submersible pump 1D arranged in the suction container 2D, and a flow path switching device 5D; the pump device 100E comprises a suction container 2E, a submersible pump 1E arranged in the suction container 2E, and a flow path switching device 5E; and the pump device 100F comprises a suction container 2F, a submersible pump 1F arranged in the suction container 2F, and a flow path switching device 5F.

[0046] Pump device 100D is connected in series to pump device 100E by communication line 109, and pump device 100E is connected in series to pump device 100F by communication line 110. More specifically, the discharge port of pump device 100D is connected to the suction port of pump device 100E by communication line 109, and the discharge port of pump device 100E is connected to the suction port of pump device 100F by communication line 110.

[0047] Pumping devices 100D, 100E, and 100F are arranged in parallel with pumping devices 100A, 100B, and 100C. The multiple pumping devices 100A, 100B, 100C, 100D, 100E, and 100F have the same configuration as pumping device 100 described with reference to FIGS. 1 to 3, and therefore redundant description thereof will be omitted. Pumping device 100A and pumping device 100D are connected to a liquefied gas storage tank 105 in which liquefied gas is stored. According to the embodiment shown in FIG. 9, liquefied gas is pumped by submersible pumps 1A to 1C of pumping devices 100A to 100C and submersible pumps 1D to 1F of pumping devices 100D to 100F, which are arranged in parallel.

[0048] Submersible pumps 1D, 1E, and 1F are started in order from the upstream side, similar to submersible pumps 1A, 1B, and 1C. That is, submersible pump 1D is started first, then submersible pump 1E, and finally submersible pump 1F.

[0049] The operation of submersible pumps 1D, 1E, and 1F is stopped in order from the downstream side, similar to the operation of submersible pumps 1A, 1B, and 1C. That is, the operation of submersible pump 1F is stopped first, then the operation of submersible pump 1E is stopped, and finally the operation of submersible pump 1D is stopped.

[0050] Fig. 10 is a schematic diagram showing yet another embodiment of a pump system including a plurality of pump devices connected in series. The configuration and operation of this embodiment, unless otherwise specified, are the same as those of the embodiment described with reference to Fig. 9, and therefore redundant description will be omitted. In the embodiment shown in Fig. 10, communication line 107 connecting pump device 100A and pump device 100B is connected to communication line 109 connecting pump device 100D and pump device 100E by intermediate header 111. Furthermore, communication line 108 connecting pump device 100B and pump device 100C is connected to communication line 110 connecting pump device 100E and pump device 100F by intermediate header 112.

[0051] As in the above-described embodiment, the submersible pumps 1A, 1B, and 1C are started in order from the upstream side, and the submersible pumps 1D, 1E, and 1F are also started in order from the upstream side. The operation of the submersible pumps 1A, 1B, and 1C is stopped in order from the downstream side, and the operation of the submersible pumps 1D, 1E, and 1F is also stopped in order from the downstream side.

[0052] The pumping units 100A to 100C are also connected in series to the pumping units 100D to 100F via the intermediate headers 111 and 112. As a result, various flows of liquefied gas are formed, enabling various operations of the pumping units 100A to 100C and the pumping units 100D to 100F. For example, it is possible to stop the operation of the pumping unit 100C or the pumping unit 100F for maintenance or in response to a pressure required by the demand side.

[0053] In the pump system shown in FIGS. 9 and 10, two rows of pump devices 100A to 100C and pump devices 100D to 100F are provided in parallel, but three or more rows of pump devices may be provided in parallel.

[0054] The above-described embodiments have been described for the purpose of enabling a person of ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments would be obvious to a person skilled in the art, and the technical concept of the present invention may be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope in accordance with the technical concept defined by the claims. [Explanation of symbols]

[0055] 1, 1A, 1B, 1C, 1D, 1E, 1F Submersible pump 2, 2A, 2B, 2C, 2D, 2E, 2F Suction vessel 3 Intake port 4 Discharge port 5, 5A, 5B, 5C, 5D, 5E, 5F Flow path switching device 7. Suction port 8 Discharge port 9 Motor rotor 10 Motor stator 11 Electric motor 12 Rotation axis 14 Bearings 15 Impeller 16 Pump casing 17 Discharge channel 22 Suction valve 23 Discharge valve 25 Drain line 26 Drain valve 31 Vent Line 32 Vent valve 41 Pump side flow path 42 Container side flow path 43 Outlet channel 45 Flow path structure 47 Valve body 50 springs 51 Valve seat 100, 100A, 100B, 100C, 100D, 100E, 100F Pump equipment 105 Liquefied gas storage tank 107, 108, 109, 110 Connecting lines 111,112 Intermediate header

Claims

1. a first pump and a second pump connected in series; a second suction vessel containing the second pump; a flow path switching device disposed in the second suction vessel; The flow path switching device a flow path structure having a pump-side flow path, a container-side flow path, and an outflow flow path; a valve body disposed in the flow path structure and selectively connecting the outflow path to either the pump-side flow path or the container-side flow path; A pump system, wherein the pump-side flow path is connected to the discharge port of the second pump, the container-side flow path is connected to the interior of the second suction container, and the outlet flow path is connected to the discharge port of the second suction container.

2. a first suction vessel containing the first pump; 2. The pump system of claim 1, further comprising a communication line connecting a discharge port of the first suction receptacle to a suction port of the second suction receptacle.

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