JPH0243956B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention relates to a tank heat insulation method for a liquefied gas tanker, specifically a membrane tank type liquefied gas tanker that transports liquefied gas having a boiling point of -40°C or less at atmospheric pressure. It is.

[Conventional technology]

Among tankers that transport liquefied gas, the conventional tank heat insulation method for membrane tank type liquefied gas tankers (LNG carriers) will be described based on Figure 1. Figure 1 shows the conventional membrane tank system
FIG. 1 is a schematic explanatory diagram of a tank heat insulation system for an LNG carrier.

In FIG. 1, a conventional tank heat insulation system has the following configuration. Joists 2 are laid at regular intervals, forming the hull 1, with mastic fillings 3 interposed therebetween, and on the joists 2 a heat insulating layer 4 made of a wood layer, for example balsa wood, etc. is provided. Further, between the joists 2 there is a heat insulating filling layer 10 made of polyurethane, and a gap 3' is provided between the polyurethane layer 10 and the inner shell 1.

A secondary barrier 6 made of plywood is provided within the heat insulating layer 4 to maintain liquid tightness for a certain period of time in the event that the membrane 5 is torn. In addition, the membrane 5 is a metal thin film spread on the inner surface of the heat insulating layer 4, and the membrane 5 only maintains liquid tightness, and the cargo load is transferred to the hull 1 through the heat insulating layer 4.
There is a system in place to communicate this information to In the heat shielding layer 4, a space (interbarrier space IBS) 7 between the membrane 5 and the secondary barrier 6 and a space between the secondary barrier 6 and the inner shell 1, an interground space (IGS) 8, are formed, respectively. , nitrogen gas is used as an inert gas at a pressure slightly higher than atmospheric pressure (0~
20m bar.gauge). Note 9
1 is a corrugated part of the membrane, and 10 is a heat insulating filling layer made of polyurethane.

In recent years, there has been an increasing demand for improved thermal insulation performance for liquefied gas carriers in order to reduce the amount of evaporated gas (boil-off gas) generated from cargo during operation from the perspective of energy conservation. With conventional heat insulation methods, as long as the same heat insulation material is used, there is no other way than to increase the thickness of the heat insulation layer, and this method of increasing the thickness of the heat insulation layer not only increases costs, but also reduces the effective LNG tank volume. It was inevitable.

[Summary of the invention]

An object of the present invention is to improve the tank heat insulation performance of a membrane tank type liquefied gas tanker that transports liquefied gas having a boiling point of -40°C or less at atmospheric pressure.

The gist of the invention is that a laminate of wood is installed in a space defined between the corrugated membrane and the inner shell of the tanker, a second barrier of laminate wood is provided within the laminate of wood, and a plurality of In a method for thermally insulating a cargo tank of a membrane tank type liquefied gas tanker, in which a shaped steel is placed in the space to support the wood laminate, supporting the load of the cargo tank and thermally insulating the cargo tank, the fiber direction of the wood laminate is is perpendicular to the direction of heat transfer, said space is configured to be air-tight, and air is evacuated from said air-tight space by forming a vacuum to reduce the pressure within the cells of said wood. A tank heat insulation method for a liquefied gas tanker is characterized by improving heat insulation performance.

When wood as a heat insulating material is viewed microscopically, as shown in FIG. 2, it can be divided into a solid portion 11 and a cellular portion 12 with voids inside. When considering the heat conduction of this wood, the apparent thermal conductivity is
Heat conduction in this solid part 11 and void part 1
It can be considered that it is the sum of the heat conduction and the heat conduction of 2. In this case, the apparent wood thermal conductivity λ is as follows (1)
Expressed by the formula.

λ=S ₁ /S ₁ +S ₂ λ ₁ +S ₂ /S ₁ +S ₂ λ ₂ …(1) In equation (1), λ: apparent wood thermal conductivity
(Kcal/hm℃) λ ₁ : Thermal conductivity of solid part 11
(Kcal/hm℃) λ ₂ : of void part 12
(〃) S ₁ : Width of the solid part 11 S ₂ : Width of the cell 12 Now, in the above equation (1), let's focus on λ ₂ which is the thermal conductivity of the gas in the void.

Generally, the thermal conductivity of gas is unrelated to pressure, but according to vacuum adiabatic theory, when the pressure approaches the vacuum level, the mean free path of the gas
When the cell diameter increases to the cell diameter d (several to several tens of microns), the thermal conductivity becomes a function of the pressure, and when the pressure is generally several to several tens of meters bar abs or less, the thermal conductivity increases in proportion to the pressure. It's coming down. Therefore, even if the heat insulating material is made of wood, the inside of the cell (void part 1
If the pressure in 2) can be sufficiently lowered to the vacuum level, the value of λ ₂ in the second term in equation (1) can be reduced. The present invention has been completed from the above viewpoints.

When balsa is used as a heat shield and the above theory is applied, S ₁ /S ₁ + S ₂ = 0.08, S ₂ /S ₁ + S ₂ = 0.92. When confined, λ ₁ = 0.15Kcal/hm℃ λ ₂
If = 0.017Kcal/hm°C (nitrogen gas, -50°C), then λ = 0.027Kcal/hm°C from equation (1).

On the other hand, by implementing the method of the present invention, the inside of the cell was measured by 0.1 to several meters.
If the vacuum is applied to bar ads, it can be lowered to λ ₂ ≒ 0.002, and since λ1 = 0.15Kcal/hm℃, λ
It is possible to lower the thermal conductivity of the heat barrier layer to about 0.014Kcal/hm℃, and the amount of heat intrusion becomes 0.014/0.027≒0.5, or 50%.

Regarding whether or not it is possible to reduce the pressure inside wood cells to a vacuum level, it was generally thought that the cell structure is a closed system and that the pressure could not be reduced. When using a vacuum, if the end face of the heat barrier layer is kept in a vacuum for several hours to several days, the inside of the cell inside the heat barrier layer will also be 0.5 to 3 m bar, depending on the structure of the heat barrier layer.
It was confirmed through the examples described later that it is possible to achieve a vacuum level of approximately abs.

However, as shown in Figure 2, the above is applicable when the wood fiber direction () and the heat transfer direction () are perpendicular, and when the fiber direction is parallel to the heat transfer direction, the above degree In a vacuum, almost no vacuum insulation effect can be expected.

In actual liquefied gas tankers, the heat insulation layer is composed of a combination of multiple layers of wood, and generally some of the fibers are parallel to the direction of heat transfer, so overall, the insulation layer is The amount of heat intrusion is about 60 to 70% of that without vacuum insulation, or about 30 to
This is a 40% improvement in heat insulation performance.

〔Example〕

The present invention will be described based on FIG. 3, which is an embodiment example.

FIG. 3 is a diagram for explaining a cross-section of a cargo tank of a liquefied gas tanker, and a flow (part B) of piping and pressure detection for reducing the pressure of the heat-insulating structure system around the tank to a vacuum level. In Figure 3, A is the liquefied gas tanker and B is the first tanker.
The heat insulation system has the same structure as shown in the figure, and the device in section C is a device for forming and maintaining a vacuum.

The apparatuses used in the method of the present invention include vacuum apparatuses 13, 13a for evacuating the wood layer made of balsa wood;
13b, suction pipes 14, 14a, 14b, and heat-insulating layer pressure detection devices 15, 15a, 15b.

It is generally preferable that the vacuum device 13 consists of a large capacity pump 13a for generating vacuum and a small capacity pump 13b for maintaining vacuum, but it is also possible to use only one vacuum pump. The suction pipe 14 extends to both end surfaces of the heat-insulating layer, that is, a conduit 14a from the corrugated part 9 of the membrane on the inside, and a conduit 14a from the gap 3' between the inner shell 1 and the polyurethane layer 10 provided on the inner shell on the outside.
At 4b, suction is carried out through a conduit directed to one tube, respectively.
However, there may be two or more main suction conduits 14 depending on the structure of the tank. The pressure detection device 15 is composed of pressure gauges 15a and 15b on the end face of the heat insulation layer and a pressure gauge 15c inside the heat insulation layer that indicates the pressure inside the wood cells, and 15c is a large capacity pump 13a for generating vacuum.
It is used when switching the operation from the pump 13b to the small capacity pump 13b for vacuum maintenance.

Next, the effects of the present invention will be illustrated by examples.

Example 1 Figure 4 shows an outline of the implementation apparatus. The circumference of a 0.25 m cubic tank containing liquid nitrogen is t ₁ 0.05 m.
The tank 21 covering the balsa heat-insulating layer 4 is placed on the weighing device 22, and the pressure inside the heat-insulating layer is lowered from normal pressure to the vacuum level, and the amount of evaporated gas at each pressure level is measured. The results shown in the figure were obtained. In FIG. 4, 23 is a thermocouple for temperature measurement, and 24 is a gas evaporation port. In addition, the fiber direction of the balsa in the heat-insulating layer was perpendicular to the heat transfer direction except for the corner portions.

Figure 5: Absolute pressure (m.bar) and evaporation amount (Kg/
As is clear from the graph, the amount of evaporated gas was 1.1Kg/H at normal pressure, but at 1m bar
It dropped to 0.45Kg/H. Evaporation reduction rate is 0.45/1.1×
100≒41%.

Example 2 Next, a 70 m ² tank filled with liquid nitrogen as shown in Figure 6
In the tank 21 (L: 5.1 m, H: 4.7 m) whose circumference was covered with a balsa heat insulating layer with a thickness of 0.285 m, the pressure inside the heat insulating layer 4 was lowered from normal pressure to the vacuum level in the same manner as in Example 1. The amount of evaporated gas was then measured at each pressure level, and the results shown in Figure 7 were obtained. In other words, the amount of evaporated gas is 50.1Kg/H at normal pressure, but it is 1 to 3m.
At bar, it drops to 43.9Kg/H, and the reduction in evaporation is 6.2
It was Kg/H. The vacuum pressure curve in Figure 7 shows the pressure between IBS and IGS from normal pressure to 9m bar.
Pressures below bar are IBS pressures. Evaporation reduction rate is 6.2/50.1×100≒12% In Example 2, in imitation of an actual tanker tank, a corner panel is used at the corner, which is a material that has almost no vacuum insulation effect and is different from a normal heat insulation layer. It was used. Therefore, compared to Example 1, the heat insulation effect is less.

In addition, the ratio of corner panels in the equipment used in this example was much larger than that on an actual ship, so the improvement in heat insulation performance was small; however, on an actual ship, the tank size is large and there is almost no corner panel. Since this can be ignored, applying vacuum insulation can be expected to improve thermal insulation performance by approximately 30 to 40%.

〔Effect of the invention〕

According to the present invention, the heat insulation performance of the tank of a membrane tank-type liquefied gas tanker that transports liquefied gas with a boiling point lower than -40°C at atmospheric pressure is improved by 30 to 40% compared to conventional heat insulation performance. This was proven by the above examples.

It is possible to avoid the conventional necessity of increasing the thickness of the heat-insulating layer in order to improve heat-insulating performance, and a greater effect can be expected from the viewpoint of cost reduction and tank volume. That is, the power cost for carrying out the method of the present invention is small, and in comparison, the benefit of increasing the cargo carrying capacity due to the small thickness of the heat-insulating layer is enormous. Furthermore, even if it is sufficient to maintain the same heat insulation performance as in the past, it is possible to make the thickness of the heat insulation layer smaller than in the past, and it goes without saying that the same effect as above can be obtained.

Furthermore, an additional effect of the present invention is that the present vacuum apparatus can be used for detecting defects in membrane tanks.

If a slight crack occurs in the membrane of the current membrane tank type LNG carrier,
The only way to detect leaked cargo gas is with a gas detection device installed in the IBS (interbarrier space, space between the membrane and the secondary barrier), but in the case of tankers using the method of the present invention, under normal conditions The sudden rise in IBS pressure in a vacuum makes it possible to instantly detect the occurrence of a gap, which is extremely useful from the standpoint of ensuring the safety of LNG tankers.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram for explaining the tank insulation method for a membrane type liquefied gas tanker, Fig. 2 is an explanatory diagram of the microstructure of the heat insulating material, Fig. 3 is an explanatory diagram of the method of the present invention, and Fig. 4 is An explanatory diagram of the device of the first embodiment,
Fig. 5 is a graph of the relationship between the amount of evaporated gas and the absolute pressure in the first embodiment, Fig. 6 is an explanatory diagram of the device in the second embodiment, and Fig. 7 is a graph showing the relationship between the amount of evaporated gas and the amount of evaporated gas in the IBS and IGS. This is a relationship graph. 1...Inner shell, 2...Joist, 4...Heatproof layer, 5...Membrane, 6...Secondary barrier, 7...Interbarrier space, 8...Interground space, 9...Corrugated part, 11...Solid part, 12...Cell part ,1
3... Vacuum pump, 14... Vacuum piping, 15... Pressure detection device, 21... Tank, 22... Weighing device, 24...
Thermo cutlet. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

Claims: 1 a stack of wood is installed in a space defined between the corrugated membrane and the inner shell of the tanker, a second barrier of wood stacks is provided within the stack of wood;
A method for heat insulating a cargo tank of a membrane tank type liquefied gas tanker, wherein a plurality of shaped steels are arranged in the space to support the wood laminate, and perform load support and thermal insulation of the cargo tank, comprising: the fiber direction of is perpendicular to the direction of heat transfer, the space is configured to be air-tight, and air is evacuated from the air-tight space to form a vacuum to reduce the pressure within the cells of the wood. A tank heat insulation method for liquefied gas tankers characterized by: 2. The air is exhausted from an interground space between the inner shell and the wood laminate and an interbarrier space between the corrugations of the membrane and the wood laminate. The tank heat insulation method for a liquefied gas tanker according to item 1. 3. The tank heat insulation method for a liquefied gas tanker according to claim 1, wherein the pressure within the interval barrier is several mbar or less. 4. The tank heat insulation method for a liquefied gas tanker according to claims 1 to 3, wherein balsa wood is used for the wood laminate.