CN103608632B - Utilize the LNG being used for fuel with the system and method for the LPG boil-off gas that liquefies - Google Patents

Abstract

The present invention relates to a method and system for liquefying LPG boil-off gas (BOG), said system comprising an LNG fuel supply system and an LPG cargo system, wherein said LNG fuel system comprises at least one LNG fuel tank 23, an LNG fuel pipeline 5 and a second Two LNG fuel pipelines 13, wherein the LPG cargo system includes at least one LPG cargo tank 20, BOG pipeline 1, at least one reliquefaction unit 100 and condensation pipeline 3; wherein the system also includes: provided between the LNG fuel tank 23 and the second At least one vaporizer 15, 22 on the LNG fuel pipeline 5 between the two LNG fuel pipelines 13, wherein the at least one vaporizer 15, 22 performs heat exchange with the LPG cargo system.

Description

System and method for liquefying LPG boil-off gas using LNG for fuel

technical field

The present invention relates to systems and methods for utilizing LNG for fuel to liquefy LPG boiloff.

Background technique

current technology

KR20100102872 discloses a method for simultaneous reception of LPG and LNG streams on complex vessels or floating production storage and offloading vessels. The invention described in KR20100102872 solves how to utilize the low temperature of the LNG boil-off gas to condense the LPG boil-off gas before compression and liquefaction to reduce the complexity of the LPG reliquefaction system.

KR20100102872 uses sensible heat of LNG boil-off gas to condense LPG boil-off gas in contrast to the present invention which uses a combination of latent and sensible heat of LNG in combination with an LNG fuel gas system to condense LPG boil-off gas.

US5860294 describes a method for condensing gaseous hydrocarbons from a mixture of inert gases and gaseous hydrocarbons, in particular as found in storage tanks of LPG/LEG ships (LEG=Liquefied Ethylene Gas).

US5860294 first describes a reliquefaction system identical to the reliquefaction unit 100 of the present invention, but with a downstream second condenser system for the purpose of recovering gaseous hydrocarbons in the gaseous mixture which would not condense in the normal cargo condenser.

In order for the second condenser in US5860294 to function, the main cargo condenser must be running and partial condensation must occur in this exchanger.

In the present invention, the second condenser (22) is located upstream of the cargo reliquefaction system, independent of the operation of the main cargo condenser.

KR20010077227 describes a method for reliquefaction of LNG boil-off gas, in which the LNG boil-off gas is liquefied by means of a part of the LNG stream sent to a vaporizer for export to eg the national natural gas distribution network. The vapor gas condensate is mixed with the LNG export line. No condensate was returned to the storage tank.

Liquid LNG is used here to control the pressure of the LNG storage tank by heat exchange/absorption of the boil-off gas into the LNG prior to vaporizing the combined LNG and condensate stream.

EP1990272 is similar in principle to KR20010077227, but the condensate is returned to the storage tank and the vaporized product is sent to the gas engine as fuel.

US3306660 describes a method and system for storing multi-component cryogenic fluids in which the more volatile components that evaporate naturally are captured, condensed and reintroduced into storage tanks. The pumped cryogenic liquid is used to condense the evaporated gas.

WO2011062505 describes a process for recovering boil-off gas from an LNG storage tank in which the LNG is sent to a recondensation tank where the LNG boil-off gas is absorbed.

US2795937 describes a process and apparatus for storing and transporting liquefied gases, in which one tank contains liquefied natural gas (LNG) and one tank contains a higher boiling point liquid. LNG is used as fuel on board ships, where it is vaporized and heated by heat exchange in tank liquids of higher boiling point liquids before entering the combustion engines. US2795937 teaches that this heat exchange prevents any evaporative gas from the surface of the liquid, so no evaporative reliquefaction system is described.

US3864918 describes a method in which boil-off gas from an LNG cargo tank is captured and split into two component streams. The first stream is compressed, cooled and liquefied. The second stream is used as fuel to drive the boat.

US2006/0053806 describes a system for supplying boil-off gas as fuel and pressure controlling cargo tanks on a marine LNG carrier.

Contents of the invention

Existing ships transporting LPG (Liquefied Petroleum Gas) mainly use low-speed diesel engines for main propulsion and various types of marine fuel oils to fuel the engines. However, new environmental restrictions (now called Emission Control Areas) limit the release of sulfur, nitrogen oxides and particulate matter to the environment. In the future, strict global emission limits are also expected. The industry has proposed several options to meet new needs, such as exhaust gas cleaning. Another option is to use clean, lean burning fuels such as methane. For the large fuel consumption and high range requirements that characterize the marine industry, methane must be stored in its most efficient form, namely in the liquid state known as LNG (Liquefied Natural Gas).

LNG fuel systems are generally known in the art, an exemplary system is disclosed in Norwegian Patent Application No. 20093272 entitled "ALNG fuel tank system for at least one gas engine used for ship propulsion".

LPG carriers typically have up to four cargo tanks that share a common vapor atmosphere when carrying one type of cargo. LPG carriers are constructed such that they can carry two different cargoes at the same time. This means that the piping systems on deck are paired to be able to complete the isolation of the two cargo systems. The LPG carrier may, for example, be loaded with propane in three tanks and butane in a fourth cargo tank. The three propane tanks have a communicating vapor space but are completely isolated from the butane vapor space. Each deck piping system is connected to multiple reliquefaction units, ensuring that the cargo tank pressure does not exceed the maximum allowable pressure. The reliquefaction unit compresses the boil-off gas to sufficient pressure so that the gas can condense against seawater. The condensate is returned to the cargo tank. In this way, the cargo tank pressure is maintained low. For a VLGC (Very Large Gas Carrier, cargo capacity typically about 82.000 m ³ ) fully loaded and sailing at maximum ambient temperature conditions, the fuel oil consumption (to the generator) of the liquefied boil-off gas is about 1900 kg/day. There is currently no solution to avoid this fuel oil consumption. Furthermore, due to the fact that LNG is approximately 50% less dense than marine diesel, LNG will require twice the storage volume to maintain the same amount of energy. Importantly, the increased fuel storage volume comes at the expense of the ship's cargo capacity. Another issue concerns the transport of LPG in eg the North Sea basin, where sailing times are short and thus berthing time is a significant contributor to the total round trip duration, so reducing berthing times is very important.

Another operating cost issue is service and repair of refrigeration compressors, since any rotating equipment has service intervals based on operating hours. Newly built ships typically have a docking interval of five years, while refrigeration compressors have a service interval of approximately 20,000 hours. Therefore, it is considered that service is required while sailing. Therefore, it would be advantageous to reduce the overall operating time, thereby extending the time between services and preferably having service intervals that match docking intervals.

It is therefore an object of the present invention to provide a system and method for utilizing LNG for fuel to liquefy LPG boil-off gas (BOG) which manages to solve at least one of the above problems.

In order to cope with the above problems, the present invention discloses a system for liquefying LPG boil-off gas (BOG), which includes an LNG fuel supply system and an LPG cargo system, wherein the LNG fuel system includes at least one LNG fuel tank 23, an LNG fuel Pipeline 5 and second LNG fuel pipeline 13; wherein said LPG cargo system includes at least one LPG cargo tank 20, BOG pipeline 1, at least one reliquefaction unit 100 and condensate pipeline 3; wherein said system also includes: At least one vaporizer 15, 22 on the LNG fuel line 5 between the tank 23 and the second LNG fuel line 13, wherein the at least one vaporizer 15, 22 is in heat exchange with the LPG cargo system.

The present invention also discloses a method for liquefying LPG boil-off gas (BOG) in a system comprising an LNG fuel supply system and an LPG cargo system, wherein the LNG fuel system comprises at least one LNG fuel tank 23, a first LNG fuel pipeline 5 and a second Two LNG fuel pipelines 13, wherein the LPG cargo system includes at least one LPG cargo tank 20, a BOG pipeline 1, at least one reliquefaction unit 100 and a condensation pipeline 3; At least one vaporizer 22, 15 on the first LNG fuel line between the LNG fuel lines 13; condense the BOG by vaporizing the LNG flowing from the LNG fuel tank; and route the condensed BOG to the LPG cargo tank. In general, the number of carburetors 22, 15 corresponds to the number of goods that can be transported simultaneously. This is usually two.

Description of drawings

Further advantageous embodiments of the invention are understood from the dependent patent claims and the following detailed description with reference to the accompanying drawings, in which:

Figure 1 is an overview diagram of a reliquefaction unit of the current technology;

Figure 2 is an overview diagram of one embodiment of the system according to the present invention;

Figure 3 is an overview diagram of another embodiment of the system according to the present invention;

detailed description

LPG is to be understood as a range of different varieties or products of petroleum gas stored and transported as a liquid cargo. Of the various petroleum gases, propane and butane are prime examples; propane typically includes ethane in any concentration from 0% to 5% by volume; butane in propane can be present in any concentration from 0% to 20% by volume . This mixture consisting mainly of propane (typically 70% to 98% by volume) is called commercial propane, and is hereinafter referred to as propane.

Butane can be any mixture of n-butane and isobutane with possible fractions of unsaturated hydrocarbons and is referred to hereinafter as butane.

In addition to propane and butane, LPG shall at least include the following varieties:

ammonia,

Butadiene,

butane-propane mixture (any mixture),

Butene,

ether,

Propylene,

vinyl chloride.

LPG stored and transported at sub-ambient temperatures naturally and continuously releases a certain amount of vapor. The normal method of maintaining pressure in a cargo tank is to extract this vapor, liquefy it and return it to the cargo tank as condensate. A reliquefaction unit is understood hereinafter to be a refrigeration unit which liquefies said vapors, the prefix "re" designating the liquefaction of vapors from liquefied gases.

LPG is transported in liquid form at a pressure greater than atmospheric pressure or at a temperature below ambient or a combination thereof. The present invention relates to an LPG carrier ship that transports liquefied cargo LPG at a temperature lower than the environment, called a fully refrigerated LPG carrier ship; an LPG carrier ship that transports liquefied cargo LPG at a pressure greater than atmospheric pressure and a temperature lower than the environment. The latter is called semi-refrigerated/semi-pressurized.

The cargo type is any one of the above-mentioned LPG varieties or products.

Condensate is to be understood as liquefied boil-off gas, where boil-off gas is the vapor emitted from the cargo due to constant heat loss into the cargo tanks.

Warm cargo is LPG loaded at a temperature above the LPG saturation temperature at the current cargo tank pressure.

Figure 1 shows a typical current art reliquefaction unit 100 connected to at least one cargo tank 20 for reference. Cargo loading lines are not shown. Boil-off gas from cargo tank 20 flows via boil-off gas line 1 to cargo compressor 40, which is typically the first stage of a multi-stage compressor, where the vapor is compressed to an intermediate pressure. Any amount of vapor not processed by the reliquefaction unit shown in Figure 1 flows via boil-off gas line 51 to a parallel operating unit, not shown.

Boil-off gas from cargo compressor 40 via line 46 enters an economizer 43 where the boil-off gas is brought close to its saturation temperature. The boil-off gas then flows from the economizer 43 to the cargo compressor 41 via line 47 , where the boil-off gas is compressed to a bubble pressure corresponding to the temperature available in the cargo condenser 42 . The cargo compressor 41 is typically the second stage of a multi-stage compressor. Occasionally more than two compression stages are required, and they are usually in series with 41.

The compressed boil-off gas then enters cargo condenser 42 via line 48 to be condensed against seawater or any cooling medium that is generally at a temperature above seawater. Seawater is by far the most common heat sink for the cargo condenser 42, but a mixture of water and heat sink is also possible. The heat sink can be any suitable diol.

Warm condensate (the resultant condensed boil-off gas from cargo condenser 42 ) leaves cargo condenser 42 and flows via line 49 to economizer 43 . Line 50 branches off from line 49, a small portion of which passes through level control valve 44 to provide the required interstage cooling and subcooling for the main portion of the warm condensate. Typically, a liquid receiver is mounted on line 49, not shown. Where the remaining warm condensate to be returned to one or more cargo tanks 20 branches off from 50 , it flows through coil 52 inside economizer 43 and exits coil 52 in a subcooled state. The now subcooled condensate flows back to the cargo tank via line 3 . Condensate production valve 45 regulates the amount of condensate flow returned to one or more cargo tanks 20 . Condensate from other reliquefaction units operating in parallel is connected in line 3, not shown.

Figure 2 illustrates two hitherto distinct, separate systems LNG fuel gas supply system (above dotted line) and LPG cargo system. The LNG fuel gas supply system is configured with one or more connected LNG fuel tanks 23 . The fuel gas supply system is also equipped with a valve 27 that controls the amount of LNG to be vaporized in the pressure booster heat exchanger 24 to ensure a more or less constant vapor pressure in the fuel tank 23, thereby ensuring sufficient fuel in the fuel line 5 Gas supply pressure. According to known principles, LNG is supplied to vaporizer 25 where it is vaporized and the resulting vapor flows to superheater 26 where it is heated to the preferred fuel gas temperature before being transferred via line 13 to the main engine. Figure 2 also shows a vapor line 8 and a valve 28 which allows vaporized LNG to be delivered to the superheater 26 if a pressure drop in the fuel tank 23 is required. However, in most cases there is no gas flow in vapor line 8 .

Engine fuel gas injection pressure depends on whether it is an Otto engine or a diesel engine, an Otto engine requires only a medium gas injection pressure of 4 bar, while a diesel engine requires a fuel gas injection pressure of 300 to 350 bar. This significant difference in pressure has limited impact on the present invention, except that the high pressure alternative requires a dedicated pump for the pressure boost. Additional pump systems are not shown as they are generally known to those skilled in the art.

For diesel engines that require higher fuel gas pressure, as described above, an additional LNG fuel pump connected to the fuel line 5 is required to boost the LNG pressure to the required pressure before entering the carburetor 25 . In this embodiment, the vapor line 8 is delivered to a safe location or alternatively to the user, in most cases there is no gas flow in the vapor line 8 .

As disclosed above with reference to FIG. 1 , the LPG cargo system of FIG. 2 is configured with a reliquefaction unit 100 and at least one LPG cargo tank 20 , a BOG line 1 for flowing boil-off gas to the reliquefaction unit 100 and returning condensate via valve 45 Condensate line 3 to cargo tank 20 .

The heat loss of a typical VLGC (Very Large Gas Carrier) into the cargo tank system at maximum ambient temperature conditions carrying propane is about 425 kW, considering normal voyages. The resulting LPG evaporation rate averaged about 3300 to 3800 kg/hour. Furthermore, a typical installed propulsion power source on a VLGC is 14MW, the typical energy consumption at maximum continuous rating is 7500kJ/kWh, and the amount of LNG required under these conditions is about 2120kg/hour. Thus, for all practical purposes, the refrigeration potential of LNG eliminates the need to operate the reliquefaction unit during load voyages, particularly for speeds below maximum design ambient temperature and/or maximum sustained rating. In fact, under these conditions, reliquefaction operations can be completely eliminated.

Therefore, the system for utilizing LNG for fuel to liquefy LPG boil-off gas according to the present invention includes at least one vaporizer 15 provided on the LNG fuel line 5 between the LNG fuel tank 23 and the second LNG fuel line 13, 22, wherein said at least one vaporizer 15, 22 is in heat exchange with the LPG cargo system, effectively integrating the LNG fuel system into the LPG boil-off gas system.

A dedicated blower installed on the boil-off gas line directly fed to the carburetor 22 is an obvious option, but at the expense of additional equipment when a cargo compressor is already installed. Therefore, this solution may not be preferred and is therefore not shown. Alternatively, a free flow of vapor to the vaporizer 22 is also possible, however, a return pump should be considered for this combination.

According to a first embodiment of the invention, at least one vaporizer 22 is a BOG condenser suitable for condensing BOG by vaporizing LNG flowing out of fuel tank 23 via line 5 by pressure. The condensate is sent back to cargo tank 20 via line 2 . Conversely, LNG is vaporized by means of boil-off gas, and depending on the amount of vaporization and fuel consumption, LNG can be partially vaporized, completely vaporized, or completely vaporized and superheated. The resulting product exits 22 and enters vaporizer 25 where any remaining LNG is vaporized. Vapor from vaporizer 25 flows to superheater 26 where the vapor is heated to the preferred fuel gas temperature. Where the amount of LNG vaporization and fuel consumption is such that the vapor leaving at least one vaporizer 22 is superheated, both the vaporizer 25 and the superheater may be omitted. The vaporizer 25 may be omitted where the amount of LNG vaporization and fuel consumption is such that the vapor leaving at least one vaporizer 22 is completely vaporized.

In the above embodiment, all of the LNG flowing from the fuel tank 23 passes through the vaporizer 22 . However, in some situations (eg, not enough gas is vaporized or there is no boil-off gas), it may be advantageous to control the flow of LNG through the vaporizer. According to one embodiment of the invention, two valves 29, 30 are arranged respectively before and parallel to the vaporizer 22, so that when there is no BOG in the BOG line 1, the valve 29 is open and the valve 30 is closed and vice versa. The valves 29 , 30 regulate the amount of LNG flowing to the vaporizers 22 , 15 to the available BOG present in the BOG line 1 and the current LNG consumption. This embodiment ensures that the resulting product leaving the vaporizer 22, 15 is completely vaporized, allowing the vapor to be transferred to the superheater 26 where the vapor is heated to the preferred fuel gas temperature. Adjustment is performed by known principles, not shown. Any excess LNG not required in vaporizers 22, 15 is diverted directly to vaporizer 25 where it is vaporized and then transferred to superheater 26. The valves 29, 30 could equally well be replaced by three-way connections, not shown. Accordingly, the valve arrangements 29, 30 represent any alternative valve arrangements providing the same functionality.

Piping is arranged around the cargo compressors 40, 41 in the reliquefaction 100 unit so that they operate in single stage compression. Arranging the cargo compressors 40, 41 in a single-stage compression would be obvious to a person skilled in the art and what is proposed is one of several possible solutions. In single stage compression, the capacity is higher and generally only one compressor needs to be run, thus resulting in an overall reduction in total compressor run time. The boil-off gas from the LPG cargo tank 20 then flows in line 1 directly to the cargo compressors 40, 41 where the pressure of the boil-off gas is raised moderately. At a convenient location on line 48 or 49 , the boil-off gas branches off via line 61 and flows to vaporizer 22 where it is condensed and returned to cargo tank 20 via line 2 . Valves 71 and 72 regulate the position at which evaporative gas branching should take place.

In yet another embodiment of the invention, at least one evaporator 15 is a condensate subcooler provided on the condensate line 3 between the reliquefaction unit 100 and the cargo tank 20, the condensate subcooler 15 being suitable for To subcool the LPG condensate by vaporizing the LNG from the fuel tank 23 . Referring to the BOG condenser 22, a condensate subcooler alone may be used as in the embodiments described above.

In a preferred embodiment of the present invention, as disclosed in more detail below, both vaporizer 15 and vaporizer 22 are provided, increasing operational flexibility and utilization of excess refrigerant capacity in LNG when available. In this embodiment, valves 16 and 17 are provided to be able to bypass or pass vaporizer 15 .

According to an exemplary embodiment of the present invention, the boil-off gas discharged from the LPG cargo tank 20 flows in line 1 directly to the cargo compressors 40, 41 where the pressure of the boil-off gas is raised moderately. Piping is arranged around the cargo compressors 40, 41 in the reliquefaction 100 unit so that they operate in single stage compression. Moderately compressed boil-off gas from cargo compressors 40 , 41 flows via line 2 to vaporizer 22 where it is condensed and returned to cargo tank 20 .

In the vaporizer 22 BOG is condensed by vaporizing the LNG flowing from the fuel tank 23 through the line 5 under pressure. The condensed BOG is sent back to cargo tank 20 via line 2 . Conversely, LNG is vaporized by means of boil-off gas, which can be partially vaporized, fully vaporized, or fully vaporized and superheated, depending on the amount of vaporization present and general fuel consumption. The resulting product exits vaporizer 22 and enters vaporizer 25 where any remaining LNG is vaporized. Vapor from vaporizer 25 is transferred via line 12 to superheater 26 where it is heated to the preferred fuel gas temperature. Where the amount of LNG vaporization and fuel consumption is such that the vapor leaving at least one vaporizer 22 is superheated, both the vaporizer 25 and the superheater may be omitted. Where the LNG boil-off gas and fuel consumption are such that the vapor leaving at least one vaporizer 22 is completely vaporized, the vaporizer 25 may be omitted.

In the above embodiment, all of the LNG flowing from the fuel tank 23 passes through the vaporizer 22 . However, in some situations (eg, not enough gas is vaporized or there is no boil-off gas), it may be advantageous to control the flow of LNG through the vaporizer. According to one embodiment of the invention, two valves 29, 30 are provided respectively before and in parallel with the vaporizer 22, so that the valve 29 is open and the valve 30 is closed when there is no BOG in the BOG line 1 and vice versa. Both valves 29 , 30 also regulate the amount of LNG flowing to the vaporizers 22 , 15 to the amount of available BOG present in the BOG line 1 and the current LNG consumption. The LNG flowing to the vaporizers 22, 15 is completely vaporized and the vapor is transferred to the superheater 26 where it is heated to the preferred fuel gas temperature. Any excess LNG not required in the vaporizers 22 , 15 is diverted directly to the vaporizer 25 where it is vaporized and then transferred to the superheater 26 . Both valves 29, 30 may be replaced by any three-way providing the same functionality.

The exemplary embodiments disclosed above are primarily concerned with the use of the system during loaded voyages when LNG fuel consumption is high. However, during loading, the boil-off gas rate is at its maximum and the LNG fuel consumption is on the low side, therefore, the amount of LNG available to liquefy BOG is limited. Therefore, the reliquefaction unit 100 must be put into operation.

Under these conditions, it is natural to consider utilizing the reduced amount of LNG available during optimal loading in the liquefaction unit precooler 22 . However, this is not an optimal solution due to smoke problems eg at the compressor inlet (especially when loading warmer cargo).

Instead of precooling the boil-off gas, subcooling the condensate reduces flash gas generation in the cargo tank 20, thus increasing system performance by not having to recycle the flash gas back to the cargo compressor.

During loading, all the boil-off gas is sent via line 1 directly to the reliquefaction unit where it is condensed and returned via line 3 to the reliquefaction unit. Condensate subcooler 15 is located on line 3 and uses LNG as coolant. The condensate is returned to the cargo tank after the temperature of the condensate is lowered to preferably not below -50°C. The condensate temperature depends on the type of cargo and can be higher for warmer cargo types.

As previously mentioned, LPG carriers are constructed such that they can simultaneously carry two different cargoes, which may be propane and butane. The vaporizers 22, 15 are then run in series order such that first the LNG is vaporized with propane in at least one vaporizer 22 and then the vaporized LNG flows via line 74 to line 80 on the parallel unit running butane. The resulting warmer boil-off LNG is returned via line 5 to the fuel gas system. Figure 3 shows an arrangement by which it is possible to have a system with one tank charged with butane and three tanks charged with propane. Typically, four reliquefaction units 100 are installed, but usually two are spared during a voyage, especially when using LNG as fuel according to the invention. As can be seen from the figure, parallel operation of the carburetors 22 is also possible. Figure 3 does not show the spare reliquefaction unit.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, and are not intended to limit the invention to the disclosed implementation plan. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Reference signs in the claims shall not be construed as limiting the invention.

Claims (7)

1. the system for LPG boil-off gas (BOG) that liquefy, described system comprises LNG fuel system and LPG cargo system, wherein said LNG fuel system comprises at least one LNG fuel tank (23), LNG burning line (5) and the 2nd LNG burning line (13), at least one liquefaction unit (100) again that wherein said LPG cargo system comprises at least one LPG cargo tank (20), BOG pipeline (1) and is configured with condensate line (3);

It is characterized in that described system also comprises:

Be provided in the first vaporizer (15) on the described LNG burning line (5) between described LNG fuel tank (23) and described 2nd LNG burning line (13) and the second vaporizer (22), and be arranged at least one the 3rd vaporizer (25) in described first vaporizer (15) and the second vaporizer (22) downstream;

Described second vaporizer (22) above and first valve (29) in parallel with described second vaporizer (22) and the second valve (30), the amount of the LNG flowing through described second vaporizer (22) is made to be adjusted in the middle available BOG of existence of described BOG pipeline (1) and the amount of current LNG fuel consumption;

Wherein:

When described first valve (29) is closed and described second valve (30) is opened, described second vaporizer (22) is adapted to pass through the BOG condenser (22) with the heat exchange of described LPG cargo system, the LNG from described LNG fuel tank (23) being vaporized to carry out condensation BOG; Described system also comprises at least one liquefaction unit (100) again, the goods compressor (40,41) wherein arranged with single stage fashion is arranged in before described BOG condenser (22), be suitable for making stream fully to pressurize to get back to described product tank (20) as condensate transport from the BOG of described product tank (20), described 3rd vaporizer (25) process is not by excessive LNG that described condenser (22) is vaporized; And

When described first valve (29) is opened and described second valve (30) cuts out, described first vaporizer (15) is adapted to pass through the LNG from described fuel tank (23) to be vaporized the condensate subcooler (15) making LPG condensate excessively cold, in the described condensate line (3) described in described condensate subcooler (15) is positioned at again between liquefaction unit (100) and described product tank (20).

2. system according to claim 1, is wherein positioned with loopback pump after described BOG condenser, is suitable for the BOG of condensation to be transported to described product tank (20).

3. the method for LPG boil-off gas (BOG) that liquefies in the system comprising LNG fuel system and LPG cargo system, wherein said LNG fuel system comprises at least one LNG fuel tank (23), a LNG burning line (5) and the 2nd LNG burning line (13), at least one liquefaction unit (100) again that wherein said LPG cargo system comprises at least one LPG cargo tank (20), BOG pipeline (1) and is configured with condensate line (3);

It is characterized in that described method comprises:

-the first vaporizer (15) and the second vaporizer (22) are provided, it is positioned on the described LNG burning line (5) between described LNG fuel tank and described 2nd LNG burning line (13), and provides at least one the 3rd vaporizer (25) being arranged in described first vaporizer (15) and the second vaporizer (22) downstream;

-in BOG condenser by making stream carry out condensation BOG from the LNG of described LNG fuel tank vaporization, described second vaporizer (22) is described BOG condenser;

-arrange the goods compressor (40,41) of at least one liquefaction unit (100) more described, described goods compressor is arranged in before described BOG condenser with single stage fashion, wherein makes stream fully pressurize to transport back described product tank (20) as condensate from the BOG of described product tank (20); And

-BOG of condensation is transported to described LPG cargo tank.

4. method according to claim 3, described LNG stream wherein from described LNG fuel tank (23) is free stream, described method provides loopback pump after being also included in described BOG condenser, and the BOG of described condensation is pumped to described LPG cargo tank (20) by described loopback pump.

5. method according to claim 3, wherein makes the residue LNG received from described first vaporizer (15) and the second vaporizer (22) by least one the 3rd vaporizer (25) vaporization described; And

-provide at least one superheater (26) at least one the 3rd vaporizer (25) downstream described, wherein make the steam heated received from least one the 3rd vaporizer (25) described to preferred fuel gas temperature.

6. method according to claim 3, wherein said method also comprises:

-provide at least two product tank (20) for delivering the first and second different goods simultaneously,

-make first LNG at least one second vaporizer (22), rely on described first goods to be vaporized with series sequence described first vaporizer (15) of operation and the second vaporizer (22), then to flow to the pipeline (80) on the parallel units run on described second goods via pipeline (74) through the LNG of vaporization, and

-via pipeline (5), the LNG through vaporization warm for gained is transmitted back to described LNG fuel system.

7. method according to claim 6, wherein said first goods is propane and described second goods is butane.