A BOG heating cycle control system and method for VLCC ships

Abstract

The present invention discloses a BOG heating circulation control system and method for a VLCC ship, including a BOG pre-heater, a normal-temperature BOG compressor, a control valve, and a BOG cooler; an output end of a first flow channel of the BOG pre-heater is respectively communicated with the control valve and the normal-temperature BOG compressor, and the control valve is communicated with a dual-fuel boiler. The normal-temperature BOG compressor is communicated with a low-pressure buffer tank through a first flow channel of the BOG cooler; a high-temperature ethylene glycol water circulation system is communicated with another flow channel of the BOG pre-heater to form a circulation loop, and heat exchange is performed between the high-temperature ethylene glycol water and the first flow channel of the BOG pre-heater; a low-temperature ethylene glycol water circulation system is respectively communicated with another flow channel of the BOG pre-heater and another flow channel of the BOG cooler to form a circulation loop, and heat exchange is respectively performed between the low-temperature ethylene glycol water and the first flow channel of the BOG pre-heater and the first flow channel of the BOG cooler. The present invention achieves the purpose of saving costs and energy.

Description

Technical Field

[0001] The present invention relates to the field of LNG fuel supply systems, and particularly to a BOG heating circulation control system and method for VLCC ships. Background Art

[0002] When IG Topping up is required during the normal navigation of a VLCC ship, that is, using the exhaust gas generated by the boiler combustion to fill the cargo hold with inert gas, a large amount of natural gas needs to be burned at this time; at the same time, when the VLCC ship is in the unloading state, the unloading pump is steam-driven and requires a large amount of steam, so a large amount of natural gas for the boiler fuel is also needed; the rest is for the daily dual-fuel generators and daily dual-fuel boilers, and the consumption of natural gas is not too large.

[0003] The safety problem caused by BOG generated in the LNG fuel tank is one of the key technologies of LNG fuel ships. In order to effectively control the pressure and temperature in the LNG fuel tank, a BOG pre-heater needs to be set to meet the use requirements of the self-flow function of the normal temperature BOG compressor or dual-fuel boiler. The current BOG pre-heater designs mainly include the following two types: 1) Set independent BOG pre-heaters for the self-flow functions of the normal temperature BOG compressor and the dual-fuel boiler respectively. Although this design method has a relatively simple control logic for the LNG supply system, the cost increases; 2) Set a shared BOG pre-heater for the self-flow functions of the normal temperature BOG compressor and the dual-fuel boiler. For this design method of BOG pre-heater, it is difficult to control the heating temperature of BOG under different working conditions, and the thermal energy utilization gap is relatively large, which is easy to cause energy waste. Therefore, the above problems need to be solved urgently. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a BOG heating circulation control system and method for VLCC ships. By using the switching signal of the self-flow function of the dual-fuel boiler, through the control of the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system, the working state control of the BOG pre-heater under different working conditions of the VLCC ship is realized, effectively avoiding the cost increase problem caused by setting up independent BOG pre-heaters, and at the same time avoiding the problems of difficult precise control of BOG temperature and energy waste caused by the increase in the electric power of the LNG supply system when setting up a shared BOG pre-heater, thereby achieving the purpose of cost saving and energy saving.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions: A BOG heating circulation control system for a VLCC ship according to the present invention is characterized in that it includes a BOG pre-heater, a normal temperature BOG compressor, a control valve, a BOG cooler, a low-pressure buffer tank, a high-temperature ethylene glycol water circulation system, and a low-temperature ethylene glycol water circulation system; the BOG pre-heater is divided into two non-connected flow channels through its heat exchange surface, and the BOG cooler is divided into two non-connected flow channels through its heat exchange surface; one flow channel output end of the BOG pre-heater is respectively connected to the control valve and the normal temperature BOG compressor, and the control valve is connected to the dual-fuel boiler, the normal temperature BOG compressor is connected to one flow channel input end of the BOG cooler, and one flow channel output end of the BOG cooler is connected to the low-pressure buffer tank; the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system are respectively electrically connected to the control valve, and the high-temperature ethylene glycol water circulation system is connected to the other flow channel of the BOG pre-heater to form a circulation loop, and heat exchange is carried out between the high-temperature ethylene glycol water and one flow channel of the BOG pre-heater; the low-temperature ethylene glycol water circulation system is respectively connected to the other flow channel of the BOG pre-heater and the other flow channel of the BOG cooler, and then respectively forms a circulation loop, and heat exchange is carried out between the low-temperature ethylene glycol water and one flow channel of the BOG pre-heater and one flow channel of the BOG cooler respectively;

[0006] The high-temperature ethylene glycol water circulation system includes a first circulation pump, a heat exchanger I, a high-pressure vaporizer, a low-pressure vaporizer & heater, and a second switching valve; the heat exchanger I is divided into two non-connected flow channels through its heat exchange surface, and heat exchange is carried out between its two flow channels; the two first circulation pumps are arranged in parallel, and their output ends are hermetically connected to one flow channel input end of the heat exchanger I, the other flow channel of the heat exchanger I is connected to high-temperature steam to form a circulation loop, and then heat exchange is carried out between the high-temperature steam and the ethylene glycol water in one flow channel of the heat exchanger I; one flow channel output end of the heat exchanger I is respectively hermetically connected to the second switching valve and the high-pressure vaporizer, and the high-pressure vaporizer is connected in series with the low-pressure vaporizer & heater, and is hermetically connected to the input ends of the two first circulation pumps through the low-pressure vaporizer & heater, and then forms a circulation loop; the second switching valve is electrically connected to the control valve, and the opening and closing actions of the second switching valve are synchronous and the same as those of the control valve; the second switching valve is hermetically connected to the input end of the other flow channel of the BOG pre-heater, and the output end of the other flow channel of the BOG pre-heater is hermetically connected to the input ends of the two first circulation pumps, and then forms a circulation loop, and heat exchange is carried out between the high-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG pre-heater.

[0007] Preferably, the low-temperature ethylene glycol water circulation system includes a second circulation pump, a heat exchanger II, and a first switching valve; the heat exchanger II is divided into two non-connected flow channels through its heat exchange surface, and heat exchange occurs between the two flow channels; the two second circulation pumps are arranged in parallel, and their output ends are hermetically connected to the input end of one flow channel of the heat exchanger II, and the other flow channel of the heat exchanger II is connected to fresh water to form a circulation loop, so as to exchange heat between the low-temperature fresh water and the ethylene glycol water in one flow channel of the heat exchanger II; the output end of one flow channel of the heat exchanger II is hermetically connected to the first switching valve and the input end of the other flow channel of the BOG cooler respectively, and the output end of the other flow channel of the BOG cooler is hermetically connected to the input ends of the two second circulation pumps, so as to form a circulation loop, and the BOG in one flow channel of the BOG cooler is cooled by heat exchange with low-temperature ethylene glycol water; the first switching valve is electrically connected to the control valve, and the opening and closing actions of the first switching valve and the control valve are synchronous and opposite; the first switching valve is hermetically connected to the input end of the other flow channel of the BOG pre-heater, and the output end of the other flow channel of the BOG pre-heater is hermetically connected to the input ends of the two second circulation pumps, so as to form a circulation loop, and heat exchange is carried out between the low-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG pre-heater.

[0008] Preferably, it further includes an ethylene glycol water tank; the ethylene glycol water tank is hermetically connected to the input ends of the two first circulation pumps and the input ends of the two second circulation pumps respectively, so as to supplement ethylene glycol water to the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system respectively through the ethylene glycol water tank.

[0009] Preferably, the output end of the low-pressure buffer tank is hermetically connected to the dual-fuel generator and the dual-fuel boiler respectively, so as to transport the BOG that meets the inlet pressure requirements to the dual-fuel generator and the dual-fuel boiler.

[0010] A control method for the BOG heating circulation control system of a VLCC ship according to the present invention is characterized by including the following steps:

[0011] (1) First, the control valve judges its opening and closing state according to the switching quantity signal of the self-flow function of the dual-fuel boiler;

[0012] (2) When the VLCC ship is in the normal navigation process and is in the non-IG Topping up and unloading state, the control valve, the first circulation pump, and the second switching valve are closed, and the first switching valve and the second circulation pump are opened;

[0013] (3) Then, the BOG in the LNG fuel tank flows into one flow path of the BOG pre-heater and exchanges heat with the low-temperature ethylene glycol water in the other flow path of the BOG pre-heater to be heated up to 0°C to meet the requirement for entering the normal-temperature BOG compressor.

[0014] (4) Then, after the BOG is pressurized by the normal-temperature BOG compressor, BOG with a pressure of 7 bar and a temperature of 100°C is obtained. Then it flows into one flow path of the BOG cooler and exchanges heat with the low-temperature ethylene glycol water in the other flow path of the BOG cooler to be cooled down to 40°C, and finally is stored in the low-pressure buffer tank for daily use by the dual-fuel generator and the dual-fuel boiler.

[0015] (5) When the VLCC ship is in the IG Topping up and discharging state, the control valve, the first circulation pump, and the second switching valve are opened, and the first switching valve and the second circulation pump are closed. At this time, the BOG in the LNG fuel tank does not pass through the normal-temperature BOG compressor.

[0016] (6) Then, the BOG in the LNG fuel tank flows into one flow path of the BOG pre-heater and exchanges heat with the high-temperature ethylene glycol water in the other flow path of the BOG pre-heater to be heated up to 40°C, and then enters the dual-fuel boiler through the control valve.

[0017] Preferably, in step (3), the pressure of the BOG in the LNG fuel tank is max. 4 bar, and its temperature is -130°C to 120°C.

[0018] Preferably, the ethylene glycol water coming out of the BOG pre-heater and the BOG cooler is pumped into one flow path of the heat exchanger II by the second circulation pump and exchanges heat with the fresh water in the other flow path of the heat exchanger II to be cooled down, thereby ensuring the recycling use of the ethylene glycol water in the low-temperature ethylene glycol water circulation system.

[0019] Preferably, in step (6), the ethylene glycol water coming out of the BOG pre-heater is pumped into one flow path of the heat exchanger I by the first circulation pump and exchanges heat with the high-temperature steam in the other flow path of the heat exchanger I to be heated up, thereby ensuring the recycling use of the ethylene glycol water in the high-temperature ethylene glycol water circulation system. At the same time, the high-temperature ethylene glycol water passes through the high-pressure vaporizer and the low-pressure vaporizer & heater in sequence to forcibly evaporate and heat the LNG to meet the usage requirements of the dual-fuel generator and the dual-fuel boiler.

[0020] Advantages of the present invention: By using the digital quantity signal of the self-flow function of the dual-fuel boiler, the present invention controls the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system to achieve the control of the working state of the BOG preheater under different working conditions of VLCC ships, effectively avoiding the problem of increased costs caused by the independent setting of the BOG preheater. At the same time, it also avoids the problem of energy waste caused by the difficult precise control of the BOG temperature and the increase in the electric power of the LNG supply system due to the setting of a shared BOG preheater, thereby achieving the purpose of cost savings and energy conservation. Brief Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other accompanying drawings can be obtained based on these drawings without creative efforts. Figure 1 It is a schematic diagram of a BOG heating circulation control system for a VLCC ship according to the present invention.

[0022] Among them, 1 - BOG preheater; 2 - control valve; 3 - normal temperature BOG compressor; 4 - BOG cooler; 5 - low-pressure buffer tank; 6 - ethylene glycol water tank; 7 - first switching valve; 8 - second switching valve; 9 - first circulation pump; 10 - heat exchanger I; 11 - low-pressure vaporizer & heater; 12 - high-pressure vaporizer; 13 - second circulation pump; 14 - heat exchanger II. Detailed Embodiments

[0023] The technical solutions of the present invention will be clearly and completely described below through specific embodiments.

[0024] A BOG heating circulation control system for a VLCC ship according to the present invention includes a BOG preheater 1, a normal temperature BOG compressor 3, a control valve 2, a BOG cooler 4, a low-pressure buffer tank 5, a high-temperature ethylene glycol water circulation system, and a low-temperature ethylene glycol water circulation system; as Figure 1As shown in the figure, the BOG pre-heater 1 is divided into two non-connected flow channels through its heat exchange surface, and the BOG cooler 4 is divided into two non-connected flow channels through its heat exchange surface; one flow channel output end of the BOG pre-heater 1 is respectively connected to the control valve 2 and the normal temperature BOG compressor 3, and the control valve 2 is connected to the dual-fuel boiler, the normal temperature BOG compressor 3 is connected to one flow channel input end of the BOG cooler 4, and one flow channel output end of the BOG cooler 4 is connected to the low-pressure buffer tank 5; the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system are respectively electrically connected to the control valve 2, and the high-temperature ethylene glycol water circulation system is connected to the other flow channel of the BOG pre-heater 1 to form a circulation loop, and heat exchange is carried out between the high-temperature ethylene glycol water and one flow channel of the BOG pre-heater 1; the low-temperature ethylene glycol water circulation system is respectively connected to the other flow channel of the BOG pre-heater 1 and the other flow channel of the BOG cooler 4, and then respectively forms a circulation loop, and heat exchange is carried out between the low-temperature ethylene glycol water and one flow channel of the BOG pre-heater 1 and one flow channel of the BOG cooler 4. Among them, the BOG pre-heater 1 should not only meet the temperature requirements for the use of the normal temperature BOG compressor 3, but also meet the temperature requirements at the inlet of the self-flow function of the dual-fuel boiler.

[0025] The high-temperature ethylene glycol water circulation system of the present invention includes a first circulation pump 9, a heat exchanger I 10, a high-pressure vaporizer 12, a low-pressure vaporizer & heater 11 and a second switching valve 8; as Figure 1 shown in the figure, the heat exchanger I 10 is divided into two non-connected flow channels through its heat exchange surface, and heat exchange is carried out between its two flow channels; two first circulation pumps 9 are arranged in parallel, and their output ends are hermetically connected to one flow channel input end of the heat exchanger I 10, the other flow channel of the heat exchanger I 10 is connected to the high-temperature steam to form a circulation loop, and then heat exchange is carried out between the high-temperature steam and the ethylene glycol water in one flow channel of the heat exchanger I 10; one flow channel output end of the heat exchanger I 10 is respectively hermetically connected to the second switching valve 8 and the high-pressure vaporizer 12, and the high-pressure vaporizer 12 is connected in series with the low-pressure vaporizer & heater 11, and is hermetically connected to the input ends of the two first circulation pumps 9 through the low-pressure vaporizer & heater 11, and then a circulation loop is formed. By arranging two first circulation pumps 9 in parallel in the present invention, one of the first circulation pumps 9 can be used as a standby, so as to avoid the phenomenon of stopping operation due to the damage of the first circulation pump 9.

[0026] As Figure 1 shown in the figure, the second switching valve 8 is electrically connected to the control valve 2, and the opening and closing actions of the second switching valve 8 are synchronous and the same as those of the control valve 2; the second switching valve 8 is hermetically connected to the other flow channel input end of the BOG pre-heater 1, and the other flow channel output end of the BOG pre-heater 1 is hermetically connected to the input ends of the two first circulation pumps 9, and then a circulation loop is formed, and heat exchange is carried out between the high-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG pre-heater 1.

[0027] The low-temperature ethylene glycol water circulation system of the present invention includes a second circulation pump 13, a heat exchanger II 14, and a first switching valve 7; as Figure 1 shown, the heat exchanger II 14 is divided into two non-communicating flow channels through its heat exchange surface, and heat exchange occurs between the two flow channels; two second circulation pumps 13 are arranged in parallel, and their output ends are hermetically connected to the input end of one flow channel of the heat exchanger II 14. The other flow channel of the heat exchanger II 14 is connected to fresh water to form a circulation loop, and then heat exchange is carried out between the low-temperature fresh water and the ethylene glycol water in one flow channel of the heat exchanger II 14; the output end of one flow channel of the heat exchanger II 14 is hermetically connected to the first switching valve 7 and the input end of the other flow channel of the BOG cooler 4 respectively. The output end of the other flow channel of the BOG cooler 4 is hermetically connected to the input ends of the two second circulation pumps 13, thereby forming a circulation loop, and the BOG in one flow channel of the BOG cooler 4 is heat-exchanged and cooled by the low-temperature ethylene glycol water. By arranging two second circulation pumps 13 in parallel, one of the second circulation pumps 13 can be used as a standby, avoiding the phenomenon of stopping operation due to the damage of the second circulation pump 13.

[0028] As Figure 1 shown, the first switching valve 7 is electrically connected to the control valve 2, and the opening and closing actions of the first switching valve 7 and the control valve 2 are synchronous and opposite; the first switching valve 7 is hermetically connected to the input end of the other flow channel of the BOG preheater 1, and the output end of the other flow channel of the BOG preheater 1 is hermetically connected to the input ends of the two second circulation pumps 13, thereby forming a circulation loop, and heat exchange is carried out between the low-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG preheater 1.

[0029] As Figure 1 shown, the ethylene glycol water tank 6 is hermetically connected to the input ends of the two first circulation pumps 9 and the input ends of the two second circulation pumps 13 respectively, and then the ethylene glycol water tank 6 replenishes ethylene glycol water to the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system respectively. Since both the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system are independent closed loops, only when the ethylene glycol water expands and contracts thermally during the circulation process, the liquid level of the ethylene glycol water tank 6 will rise or fall, or there is leakage during the circulation process, ethylene glycol water is replenished into the circulation system.

[0030] As Figure 1 shown, the output end of the low-pressure buffer tank 5 is hermetically connected to the dual-fuel generator and the dual-fuel boiler respectively, and then the BOG meeting the inlet pressure requirements is transported to the dual-fuel generator and the dual-fuel boiler.

[0031] A control method for the BOG heating circulation control system of a VLCC ship according to the present invention, as Figure 1 shown, includes the following steps:

[0032] (1) First, the control valve 2 determines its opening and closing state according to the digital signal of the self-flow function of the dual-fuel boiler.

[0033] (2) When the VLCC ship is in normal navigation and not in the IG Topping up and unloading states, the control valve 2, the first circulation pump 9, and the second switching valve 8 are closed, and the first switching valve 7 and the second circulation pump 13 are opened.

[0034] (3) Then, the BOG in the LNG fuel tank flows into a first flow channel of the BOG pre-heater 1 and exchanges heat with the low-temperature ethylene glycol water in another flow channel of the BOG pre-heater 1 to be heated up to 0 °C to meet the requirement for entering the normal-temperature BOG compressor 3.

[0035] In the above steps, the pressure of the BOG in the LNG fuel tank is max. 4 bar, and its temperature is -130 °C to 120 °C.

[0036] (4) Then, after being pressurized by the normal-temperature BOG compressor 3, the BOG with a pressure of 7 bar and a temperature of 100 °C is obtained; then it flows into a first flow channel of the BOG cooler 4 and exchanges heat with the low-temperature ethylene glycol water in another flow channel of the BOG cooler 4 to be cooled down to 40 °C, and finally is stored in the low-pressure buffer tank 5 for daily use by the dual-fuel generator and the dual-fuel boiler.

[0037] Among them, the ethylene glycol water from the BOG pre-heater 1 and the BOG cooler 4 is pumped into a first flow channel of the heat exchanger II 14 by the second circulation pump 13 and exchanges heat with the fresh water in another flow channel of the heat exchanger II 14 to be cooled down, thereby ensuring the recycling of the ethylene glycol water in the low-temperature ethylene glycol water circulation system.

[0038] (5) When the VLCC ship is in the IG Topping up and unloading states, the control valve 2, the first circulation pump 9, and the second switching valve 8 are opened, and the first switching valve 7 and the second circulation pump 13 are closed. At this time, the BOG in the LNG fuel tank does not pass through the normal-temperature BOG compressor 3.

[0039] (6) Then, the BOG in the LNG fuel tank flows into a first flow channel of the BOG pre-heater 1 and exchanges heat with the high-temperature ethylene glycol water in another flow channel of the BOG pre-heater 1 to be heated up to 40 °C and enters the dual-fuel boiler through the control valve 2.

[0040] In the above steps, the ethylene glycol water coming out of the BOG pre-heater 1 is pumped into a first flow channel of the heat exchanger I 10 by the first circulation pump 9, and exchanges heat with the high-temperature steam in another flow channel of the heat exchanger I 10 to increase the temperature, thereby ensuring the recycling of ethylene glycol water in the high-temperature ethylene glycol water circulation system. At the same time, the high-temperature ethylene glycol water passes through the high-pressure vaporizer 12 and the low-pressure vaporizer & heater 11 in sequence to forcibly evaporate and heat the LNG to meet the usage requirements of the dual-fuel generator and the dual-fuel boiler.

[0041] Advantages of the present invention: The present invention utilizes the digital quantity signal of the self-flow function of the dual-fuel boiler, and through the control of the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system, realizes the control of the working state of the BOG pre-heater 1 under different working conditions of VLCC ships, effectively avoiding the problem of increased cost caused by the independent setting of the BOG pre-heater 1. At the same time, it also avoids the problems of difficult precise control of the BOG temperature and energy waste caused by the increase in the electric power of the LNG supply system due to the setting of a shared BOG pre-heater 1, thereby achieving the purpose of cost saving and energy conservation.

[0042] The embodiments described above are only described as the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various variations and improvements made by ordinary engineering and technical personnel in the field to the technical solutions of the present invention shall fall within the protection scope of the present invention. The technical content claimed by the present invention has been fully recorded in the claims.

Claims

1. A BOG heating cycle control system for VLCC ships, characterized in that: It includes a BOG pre-heater, a normal-temperature BOG compressor, a control valve, a BOG cooler, a low-pressure buffer tank, a high-temperature ethylene glycol water circulation system and a low-temperature ethylene glycol water circulation system; the BOG pre-heater is divided into two non-connected flow channels through its heat exchange surface, and the BOG cooler is divided into two non-connected flow channels through its heat exchange surface; one flow channel output end of the BOG pre-heater is respectively communicated with the control valve and the normal-temperature BOG compressor, and the control valve is communicated with the dual-fuel boiler, the normal-temperature BOG compressor is communicated with one flow channel input end of the BOG cooler, and one flow channel output end of the BOG cooler is communicated with the low-pressure buffer tank; the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system are respectively electrically connected with the control valve, and the high-temperature ethylene glycol water circulation system is communicated with the other flow channel of the BOG pre-heater to form a circulation loop, and heat exchange is carried out between the high-temperature ethylene glycol water and one flow channel of the BOG pre-heater; the low-temperature ethylene glycol water circulation system is respectively communicated with the other flow channel of the BOG pre-heater and the other flow channel of the BOG cooler, and then circulation loops are respectively formed, and heat exchange is carried out between the low-temperature ethylene glycol water and one flow channel of the BOG pre-heater and one flow channel of the BOG cooler respectively; The high-temperature ethylene glycol water circulation system includes a first circulation pump, a heat exchanger I, a high-pressure vaporizer, a low-pressure vaporizer & heater and a second switching valve; the heat exchanger I is divided into two non-connected flow channels through its heat exchange surface, and heat exchange is carried out between its two flow channels; Two of the first circulation pumps are arranged in parallel, and their output ends are hermetically communicated with one flow channel input end of the heat exchanger I, the other flow channel of the heat exchanger I is communicated with high-temperature steam to form a circulation loop, and then heat exchange is carried out between the high-temperature steam and the ethylene glycol water in one flow channel of the heat exchanger I; one flow channel output end of the heat exchanger I is respectively hermetically communicated with the second switching valve and the high-pressure vaporizer, and the high-pressure vaporizer is connected in series with the low-pressure vaporizer & heater, and is hermetically communicated with the input ends of the two first circulation pumps through the low-pressure vaporizer & heater, and then a circulation loop is formed; the second switching valve is electrically connected with the control valve, and the opening and closing actions of the second switching valve are synchronous and the same as those of the control valve; the second switching valve is hermetically communicated with the input end of the other flow channel of the BOG pre-heater, and the output end of the other flow channel of the BOG pre-heater is hermetically communicated with the input ends of the two first circulation pumps, and then a circulation loop is formed, and heat exchange is carried out between the high-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG pre-heater.

2. The BOG heating cycle control system for a VLCC ship according to claim 1, characterized in that: The low-temperature ethylene glycol water circulation system includes a second circulation pump, a heat exchanger II and a first switching valve; the heat exchanger II is divided into two non-connected flow channels through its heat exchange surface, and heat exchange is carried out between its two flow channels; The two second circulation pumps are arranged in parallel, and their output ends are hermetically connected to an input end of a flow channel of the second heat exchanger. The other flow channel of the second heat exchanger is in communication with fresh water to form a circulation loop, and thus heat exchange is performed between the low-temperature fresh water and the ethylene glycol water in one flow channel of the second heat exchanger; the output end of one flow channel of the second heat exchanger is respectively hermetically connected to the first switching valve and an input end of another flow channel of the BOG cooler. The output end of the other flow channel of the BOG cooler is hermetically connected to the input ends of the two second circulation pumps, and thus a circulation loop is formed, and the BOG in one flow channel of the BOG cooler is cooled by heat exchange with the low-temperature ethylene glycol water; the first switching valve is electrically connected to the control valve, and the opening and closing actions of the first switching valve and the control valve are synchronous and opposite; the first switching valve is hermetically connected to an input end of another flow channel of the BOG preheater, and the output end of the other flow channel of the BOG preheater is hermetically connected to the input ends of the two second circulation pumps, and thus a circulation loop is formed, and heat exchange is performed between the low-temperature ethylene glycol water and the natural evaporation BOG in one flow channel of the BOG preheater.

3. A BOG heating cycle control system for a VLCC ship according to claim 2, characterized in that: It further includes an ethylene glycol water tank; the ethylene glycol water tank is respectively hermetically connected to the input ends of the two first circulation pumps and the input ends of the two second circulation pumps, and thus the ethylene glycol water tank replenishes ethylene glycol water to the high-temperature ethylene glycol water circulation system and the low-temperature ethylene glycol water circulation system respectively.

4. A BOG heating cycle control system for a VLCC ship according to claim 3, characterized in that: The output end of the low-pressure buffer tank is respectively hermetically connected to the dual-fuel generator and the dual-fuel boiler, and thus the BOG meeting the inlet pressure requirement is transported to the dual-fuel generator and the dual-fuel boiler.

5. The control method of a BOG heating cycle control system for a VLCC ship according to claim 4, characterized in that It includes the following steps: (1) First, the control valve judges its opening and closing state according to the switching quantity signal of the self-flow function of the dual-fuel boiler. (2) When the VLCC ship is in the normal navigation process and is in the non-IG Topping up and unloading state, the control valve, the first circulation pump and the second switching valve are closed, and the first switching valve and the second circulation pump are opened. (3) Then, the BOG in the LNG fuel tank flows into one flow channel of the BOG preheater and exchanges heat with the low-temperature ethylene glycol water in the other flow channel of the BOG preheater to be heated up to 0 °C to meet the requirement for entering the normal-temperature BOG compressor. (4) Then, after the BOG is pressurized by the normal-temperature BOG compressor, the BOG with a pressure of 7 bar and a temperature of 100 °C is obtained; then it flows into one flow channel of the BOG cooler and exchanges heat with the low-temperature ethylene glycol water in the other flow channel of the BOG cooler to be cooled down to 40 °C, and finally it is stored in the low-pressure buffer tank for daily use by the dual-fuel generator and the dual-fuel boiler. (5) When the VLCC ship is in the IG Topping up and unloading state, the control valve, the first circulation pump and the second switching valve are opened, and the first switching valve and the second circulation pump are closed. At this time, the BOG in the LNG fuel tank does not pass through the normal-temperature BOG compressor. (6) Then, the BOG in the LNG fuel tank flows into one flow channel of the BOG pre-heater, exchanges heat with the high-temperature ethylene glycol water in the other flow channel of the BOG pre-heater, is heated to 40°C, and enters the dual-fuel boiler through the control valve.

6. The control method of a BOG heating cycle control system for a VLCC ship according to claim 5, characterized in that: In step (3), the pressure of the BOG in the LNG fuel tank is max. 4 bar, and its temperature is -130°C to 120°C.

7. The control method of a BOG heating cycle control system for a VLCC ship according to claim 5, characterized in that: The ethylene glycol water coming out of the BOG pre-heater and the BOG cooler is pumped into one flow channel of the heat exchanger II by the second circulation pump, exchanges heat with the fresh water in the other flow channel of the heat exchanger II to cool down, thereby ensuring the recycling of the ethylene glycol water in the low-temperature ethylene glycol water circulation system.

8. The control method of a BOG heating cycle control system for a VLCC ship according to claim 5, characterized in that: In step (6), the ethylene glycol water coming out of the BOG pre-heater is pumped into one flow channel of the heat exchanger I by the first circulation pump, exchanges heat with the high-temperature steam in the other flow channel of the heat exchanger I to heat up, thereby ensuring the recycling of the ethylene glycol water in the high-temperature ethylene glycol water circulation system. At the same time, the high-temperature ethylene glycol water passes through the high-pressure vaporizer and the low-pressure vaporizer & heater in sequence to forcibly evaporate and heat the LNG to meet the usage requirements of the dual-fuel generator and the dual-fuel boiler.

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