KR101239342B1 - Storage tank for liquefied natural gas - Google Patents

Abstract

The present invention relates to a liquefied natural gas storage tank.
According to an aspect of the present invention, a liquefied natural gas storage tank, comprising: a first heat insulating part constituting an upper portion of the storage tank; And a second heat insulating part constituting a lower portion of the storage tank, wherein the first heat insulating part has a higher heat insulating performance than the second heat insulating part.

Description

Liquefied natural gas storage tank {STORAGE TANK FOR LIQUEFIED NATURAL GAS}

The present invention relates to a liquefied natural gas storage tank.

Liquefied Natural Gas (LNG) is obtained by liquefying natural gas (methane) based on methane (hereinafter referred to as "NG") by cooling it to about -162 ℃. Colorless, transparent liquid with a volume of about 1/600 compared to NG. Therefore, when liquefied and transported to LNG during NG transfer, it can be transported very efficiently. For example, an LNG carrier that can transport (transport) LNG to sea is used.

The LNG carrier is provided with a storage tank for storing LNG.

1 is a schematic cross-sectional view of a LNG storage tank according to the prior art.

Referring to FIG. 1, the conventional LNG storage tank 1 may be divided into a liquid part 3 in which LNG is filled and a gas part 2 in which NG is present, depending on the state of the fluid stored therein. Specifically, a predetermined region of the upper end of the storage tank 1 may correspond to the gas part 2, and the boundary between the liquid part 3 and the gas part 2 is variable according to the amount of NG.

The storage tank 1 including the liquid part 3 and the gas part 2 is manufactured to have a constant heat insulating performance, but cannot completely block heat introduced from the outside. That is, a certain level of heat is introduced into the storage tank 1, whereby the temperature of the LNG is increased to be vaporized into NG. This is called a boil-off gas (hereinafter referred to as "BOG").

BOG is a type of LNG loss and is an important issue in the transportation of LNG. In addition, there is a risk that the storage tank is damaged if the BOG accumulates in the storage tank and the pressure in the storage tank is increased. Therefore, various methods for treating BOG have been studied. Recently, a method of burning BOG into a gas combustor, a reliquefaction and returning to a storage tank, a method of using BOG as an energy source of an engine of a ship, etc. have been used. However, it is only a secondary measure to deal with the generated BOG, not a fundamental measure to suppress the generation of BOG.

In addition, the method of burning the BOG or using as an energy source of the engine is uneconomical by reducing the amount of LNG as a cargo, in particular, has a problem of causing environmental pollution due to the combustion of the BOG.

Applicant has applied for a method for reducing the amount of BOG by allowing the pressure of the LNG storage tank to increase the pressure in order to reduce the amount of BOG generated in the patent application No. 10-2009-0133870.

2 is a graph showing the relationship between the pressure of the storage tank disclosed in the above application and the generation amount of BOG.

In FIG. 2, X-ray is a pressure diagram inside the LNG storage tank, and Y-ray is a generation diagram of the boil-off gas generated inside the LNG storage tank.

The pressure inside the storage tank can be increased by vaporizing the LNG by external heat transferred to the storage tank.

On the other hand, the storage tank is connected to the above-described gas combustor, re-liquefaction apparatus, engine, etc., it is possible to adjust the BOG pressure inside the storage tank by consuming a portion of the BOG generated in the storage tank.

The storage tank is allowed to rise in pressure up to a certain level (P1) from the operating pressure P0, where it can be adjusted to allow rise to 0.7 atm (gauge pressure). As the pressure inside the storage tank rises, the saturation temperature increases, reducing the amount of BOG generated.

When the storage tank is maintained at the operating pressure P0, BOG may be generated according to the set design evaporation amount, and the BOG generation amount is BOR0. When the pressure of the storage tank is increased along the X-ray, the amount of BOG generated is changed in the order of BOR1, BOR2, and BOR3. Total BOG generation may correspond to the area of the hatched portion.

Here, in the section defined by BOR1, the amount of boil-off gas generated is reduced by 75% compared to BOR0, about 20% decrease in the section defined by BOR2, and the same amount of boil-off gas as in BOR3 is generated.

Embodiments of the present invention to provide a liquefied natural gas storage tank that can reduce the amount of boil-off gas.

According to an aspect of the present invention, a liquefied natural gas storage tank, comprising: a first heat insulating part constituting an upper portion of the storage tank; And a second heat insulating part constituting a lower portion of the storage tank, wherein the storage tank is configured to allow a pressure range of a predetermined range, and the first heat insulating part has a higher heat insulating performance than the second heat insulating part. It can provide a liquefied natural gas storage tank.

In addition, the heat insulating material of the first heat insulating part may provide a liquefied natural gas storage tank characterized in that it has a higher heat insulating performance than the heat insulating material of the second heat insulating part.

In addition, it is possible to provide a liquefied natural gas storage tank, characterized in that the thickness of the first heat insulating portion and the second heat insulating portion is formed in the same.

In addition, the first heat insulating portion may provide a liquefied natural gas storage tank, characterized in that formed thicker than the second heat insulating portion.

In addition, the first heat insulating part may provide a liquefied natural gas storage tank, characterized in that further comprises a reinforcing heat insulating material.

In addition, the first heat insulating portion may form a portion of the ceiling surface and the side surface of the storage tank, the side portion of the first heat insulating portion 1.5% to 2% of the total height of the storage space from the upper end of the storage space of the storage tank It can provide a liquefied natural gas storage tank, characterized in that extending to.

In addition, the storage tank may provide a liquefied natural gas storage tank, characterized in that to allow a pressure rise below the gauge pressure of 0.7 atm.

The first heat insulating part may be formed of an aerogel, and the second heat insulating part may be formed of a reinforced polyurethane foam.

Embodiments of the present invention can reduce the amount of generated boil-off gas by improving the thermal insulation structure of the liquefied natural gas storage tank, allowing the pressure rise.

1 is a schematic cross-sectional view of a liquefied natural gas storage tank according to the prior art,
2 is a graph showing the relationship between the pressure of the storage tank disclosed in Patent Application No. 10-2009-0133870 and the generation amount of BOG,
3 is a cross-sectional view of a liquefied natural gas storage tank according to the first embodiment of the present invention,
4 is a graph showing the relationship between the pressure of the liquefied natural gas storage tank according to the first embodiment of the present invention and the generation amount of BOG,
5 is a cross-sectional view of a liquefied natural gas storage tank according to a second embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

3 is a cross-sectional view of a liquefied natural gas storage tank according to the first embodiment of the present invention.

Referring to Figure 3, the storage tank 100 of liquefied natural gas according to the first embodiment of the present invention, liquefied natural gas (hereinafter referred to as "LNG", 110) and evaporated gas (hereinafter referred to as "BOG", 120 ) Is stored.

In general, the storage tank 100 may be loaded with LNG up to 98% to 98.5% of the total storage height. At this time, the height of the gas-liquid surface, which is an interface between the liquid LNG 110 and the gas BOG 120, may vary according to the amount of generation of the BOG 120. That is, when a lot of BOG is generated, the height of the interface is lowered, and when there is less BOG, the height of the interface may be increased.

The storage tank 100 may be composed of an upper first heat insulating part 130 and a lower second heat insulating part 140. That is, BOG is mainly in contact with the first heat insulating part 130, and LNG is mainly in contact with the second heat insulating part 140. Both the first heat insulating part 130 and the second heat insulating part 140 may be formed as a double barrier structure, for example, may be formed as a cargo hold structure of the membrane (membrane) type.

The first heat insulating part 130 is provided to have a higher heat insulating performance than the second heat insulating part 140. For example, a material having a higher heat insulating performance than the heat insulating material of the second heat insulating part 140 may be used as the heat insulating material of the first heat insulating part 130. For example, the first heat insulating part 130 may be an airgel ( aerogel), and the second heat insulating part 140 may be formed of a reinforced polyurethane foam.

In addition, the first heat insulating part 130 and the second heat insulating part 140 may be formed to have the same thickness.

In addition, the first heat insulating part 130 may form a portion of the ceiling surface and the side surface of the storage tank 100, and the side portion extends from the upper end of the storage space to 1.5% to 2% of the height of the entire storage space. Can be. In detail, in general, since the LNG can be loaded up to 98% to 98.5% of the total storage height in the storage tank 100, the empty space up to 1.5% to 2% of the total storage height is regarded as the BOG generation area. The first heat insulating part 130 may be formed. Therefore, up to 98% to 98.5% of the LNG load may be formed as the second heat insulating part 140.

On the other hand, the storage tank 100 is connected to the BOG delivery line (not shown), the BOG delivery line may be connected to the gas combustor, reliquefaction apparatus, dual fuel engine, etc. to consume the BOG. As a result, the pressure of the storage tank 100 may be maintained at a predetermined level.

For example, the storage tank 100 may be adjusted such that the internal pressure is allowed to rise to the maximum pressure (Fig. 4, P1) based on the operating pressure (Fig. 4, P0). Here, the difference between the operating pressure (P0) and the maximum pressure (P1) may be 0.7 atm (gauge pressure).

The first heat insulating part 130 and the second heat insulating part 140 of the storage tank 100 may be structurally more firmly formed to withstand such a pressure increase, or a material having high physical properties may be used.

Figure 4 is a graph showing the relationship between the pressure of the liquefied natural gas storage tank according to an embodiment of the present invention and the generation amount of BOG.

Referring to FIG. 4, X-rays are pressure lines inside the storage tank 100, and Y-rays are generation lines of the boil-off gas generated inside the storage tank 100.

When the storage tank 100 is maintained at the operating pressure P0, BOG may be generated according to the set design evaporation amount, and the BOG generation amount is BOR0. When the pressure in the storage tank 100 is increased along the X-ray, the amount of BOG generated is changed in the order of BOR1, BOR2, and BOR3. Total BOG generation may correspond to the area of the hatched portion.

Here, the biggest influence on the generation amount of the BOG in the BOR1 section is the amount of heat transferred from the outside of the storage tank 100 to the BOG 120 inside the storage tank 100.

In detail, the generation of BOG in a system having excellent thermal insulation performance, such as storage tank 100 is mainly generated in the gas-liquid surface that the BOG and LNG contact. In more detail, the heat introduced into the storage tank 100 through the first heat insulating part 130 is increased in temperature of the BOG 120 in the storage tank 100, and the gas-liquid surface that the BOG 120 and the LNG 110 contact each other. Contributes to the rise of the temperature of LNG and the direct evaporation of LNG near the gas-liquid level. On the other hand, the heat introduced into the LNG 110 inside the storage tank 100 through the second heat insulating part 140 contributes to the temperature rise and evaporation of the LNG.

At this time, in the BOR1 section, the LNG temperature rise near the gas-liquid surface due to the heat flowing into the first heat insulating part 130 is greater than the temperature rise of the LNG due to the heat flowing into the second heat insulating part 140. The heat flowing into the second heat insulating part 140 raises the temperature of the LNG, but does not move to the gas-liquid surface where evaporation occurs, thereby making a large flow in the LNG and contributing only to the temperature increase of the LNG continuously. However, in the section where the pressure is kept constant, the saturation temperature formed on the gas-liquid level does not increase any more, but since the temperature inside the LNG continuously rises with time, the temperature inside the LNG becomes closer to the saturation temperature. do. When the temperature inside the LNG approaches the saturation temperature, the flow inside the LNG moves near the gas-liquid level, and thus a large amount of BOG is generated near the gas-liquid level.

That is, when a large amount of heat is supplied to the BOG 120 from the outside of the storage tank 100, the BOG is more generated. The graph is moved as shown by the I curve, and when the supply is less, the BOG is generated less. Bars can be shifted as shown by curve II.

Therefore, as in the first exemplary embodiment of the present invention, when the thermal insulation performance of the first thermal insulation unit 130 mainly contacted by the BOG 120 is improved, the graph may be moved in the II curve direction. In other words, even allowing the same pressure rise, the amount of BOG can be further reduced (the area of the hatched portion is reduced).

In addition, without improving the overall thermal insulation performance of the storage tank 100, only the improvement of the thermal insulation performance of a portion of the first heat insulating portion 130 can greatly reduce the amount of generation of BOG, it is possible to obtain an effect without a significant cost increase.

Hereinafter, a liquefied natural gas storage tank according to a second embodiment of the present invention will be described with reference to the drawings. However, since the second embodiment has a difference in the first heat insulating part compared with the first embodiment, the difference will be mainly described, and the same reference numerals are used for description of the first embodiment.

5 is a cross-sectional view of a liquefied natural gas storage tank according to a second embodiment of the present invention.

Referring to FIG. 5, the first heat insulating part 130 ′ of the liquefied natural gas storage tank 100 according to the second embodiment of the present invention exhibits higher heat insulating performance than the second heat insulating part 140.

To this end, the first heat insulating part 130 ′ may be formed of the same heat insulating material as the second heat insulating part 140, but may be formed thicker. Or, it may be configured in the form that the reinforcing insulation is further attached to the outside.

Thereby, as in the first embodiment, since the amount of heat supplied to the BOG 120 during the pressure rise can be reduced, the graph of FIG. 4 can be moved in the II curve direction. That is, even if the pressure rise of the same width is allowed, the generation amount of BOG can be further reduced.

For example, a Mark-III membrane type cargo hold has a heat insulation formed from 270 mm reinforced polyurethane foam, with the first heat insulation 130 'extending from the top of the storage space to 1.5% to 2% of the total storage height. When formed to descend and formed of 300mm reinforced polyurethane foam, the amount of BOG generated in the BOR1 section can be reduced by up to 10%.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand. For example, a person skilled in the art can change the material, size and the like of each constituent element depending on the application field or can combine or substitute the embodiments in a form not clearly disclosed in the embodiments of the present invention, Of the range. Therefore, it should be understood that the above-described embodiments are to be considered in all respects as illustrative and not restrictive, and that such modified embodiments are included in the technical idea described in the claims of the present invention.

100: liquefied natural gas storage tank 110: liquefied natural gas (LNG)
120: BOG 130, 130 ': first heat insulating part
140: second heat insulation

Claims (8)

In a liquefied natural gas storage tank configured to allow a range of pressure rise,
A first heat insulating part constituting an upper portion of the storage tank; And
A second heat insulating part constituting the lower portion of the storage tank is included,
The first heat insulating part is formed of an airgel,
The second heat insulating part is formed of a reinforced polyurethane foam,
The first heat insulating part is a liquefied natural gas storage tank, characterized in that having a higher heat insulating performance than the second heat insulating part. The method of claim 1,
Liquefied natural gas storage tank, characterized in that the heat insulating material of the first heat insulating part has a higher heat insulating performance than the heat insulating material of the second heat insulating part. The method of claim 2,
Liquefied natural gas storage tank, characterized in that the first heat insulating portion and the second heat insulating portion have the same thickness. The method of claim 1,
The first heat insulating portion is liquefied natural gas storage tank, characterized in that formed thicker than the second heat insulating portion. The method of claim 1,
Liquefied natural gas storage tank, characterized in that the first heat insulating portion further comprises a reinforcing heat insulating material. 6. The method according to any one of claims 1 to 5,
The first heat insulating part may form a portion of the ceiling surface and the side surface of the storage tank,
Liquefied natural gas storage tank, characterized in that the side portion of the first insulating portion extends from the upper end of the storage space of the storage tank 100 to 1.5% to 2% of the total height of the storage space. The method of claim 1,
The storage tank is a liquefied natural gas storage tank, characterized in that to allow a pressure rise below the gauge pressure of 0.7 atm. delete

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