GB2158214A

GB2158214A - Method and system for insulating a cargo tank of a liquefied gas tanker

Info

Publication number: GB2158214A
Application number: GB08510660A
Authority: GB
Inventors: Eke Verschuur; Yokichi Shibamura; Yoshiaki Nakamura; Takao Kanazawa
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1984-05-04
Filing date: 1985-04-26
Publication date: 1985-11-06
Also published as: FR2563801B1; KR850007959A; JPH0243956B2; NO164761B; NO164761C; GB8510660D0; FR2563801A1; GB2158214B; JPS60234199A; NO851768L

Abstract

A cargo tank of liquefied gas tanker carrying a liquefied gas having a boiling point lower than - 40 DEG C at the atmospheric pressure comprises two barrier layers 6 and 9, a stack of wood timbers such as balsa wood for supporting the load of the cargo tank and for insulating the tank from external heat. Air is extracted from the space for creating vacuum pressure level therein, thereby reducing a pressure in the cells of the wood timbers. <IMAGE>

Description

SPECIFICATION Method and system for insulating a cargo tank of a liquefied gas tanker.

This invention relates to a method of tank insulation for a liquefied gas tanker, especially to a membrane tank type liquefied gas tanker adapted for transport of liquefied gas having boiling point lower than - 40"C at atmospheric pressure and a system therefor.

The conventional tank insulating system of the membrane tank type LNG vessel is constructed as follows.

In the conventional tank insulating method, joists are placed on the lower part of the inner hull of the vessel at a predetermined space and each space is filled with a mastic filler. On these joists installed in a heat insulating layer consisting of plural layers of wood timbers such as balsa wood timber. In the insulating layer is provided a secondary barrier of laminated wood for maintaining liquid tightness for a certain time period in case of leakage of cargo liquid due to the failure of membrane. The membrane is a metallic thin sheet and placed on the inner surface of the insulating layer. The membrane serves only for maintaining liquid tightness and the cargo load is transmitted to the inner hull through the heat insulating layer and joist. The space between the membrane and the inner hull is divided into two spaces.An interbarrier space (IBS) is defined as the space between the membrane and the secondary barrier. And an interground space (IGS) is defined as the space between the secondary barrier and the inner hull. Nitrogen gas as inert gas is filled in these spaces at a pressure slightly higher than the atmospheric pressure (0-20mm bar. gauge).

Recently, in liquefied gas carriers, a demand has been raised for improving heat insulating performance in order to reduce the amount of vaporized gases (boil-off gas) produced from the cargo during voyage from the standpoint of energy saving. According to the above mentioned conventional method, if the same insulating material is used, this demand can be met only by increasing the thickness of the insulating layer. However, this increase of the insulating layer is necessarily accompanied by increase in costs and reduction of the available capacity of the LNG tank.

It is an object of the present invention to improve the tank insulating performance of the membrane tank type liquified gas tanker adapted for transport of liquefied gas having a boiling point lower then - 40"C at atmospheric pressure.

The present invention resides in a method and system for heat insulation of a cargo tank used in a liquefied gas tanker of the membrane tank type adapted for transport of a liquefied gas having a boiling point lower than - 40" at the atmospheric pressure, characterized in that air is evacuated from the inside of the airtight space surrounding the cargo tank, thereby reducing the pressure in the minute cells of wood timber which are provided in the stack form vyithin the airtight space, so as to improve the heat insulating performance. It should be noted that said wood timber are of such nature as usable for heat insulation and for bearing the load of the cargo tank.

In the drawings: Figure 1 is a diagrammatic view for explaining the tank insulating method for a membrane tank type liquefied gas tanker; Figure 2 is an explanatory view showing the microscopic structure of the heat insulating material; Figure 3 is an explanatory view of the inventive method; Figure 4 is an explanatory view of the system according to a first embodiment; Figure 5 is a chart showing the relation between the amount of vaporized gas and absolute pressure in the first embodiment; Figure 6 is an explanatory view showing the system according to the second embodiment; and Figure 7 is a chart showing the relation between the amount of vaporized gas and absolute pressure in IBS and IGS.

Fig. 1 shows in overall schematic view of the conventional tank insulating system of the membrane tank type LNG vessel.

Referring to Fig. 1, in the conventional tank insulating method, joists 2 are placed on the inner hull 1 of the lower vessel. Numeral 3 designates the intermediary of a mastic filler and Numeral 3' designates the gap. A heat insulating layer 4 is installed on these joists 2. A heat insulating layer 4 consists of plural layers of wood timbers such as balsa wood timbers. In the insulating layer 4, is provided a secondary barrier 6 of laminated wood for maintaining liquid tightness for a certain time period in case of leakage of cargo liquid due to the failure of membrane 5. The membrane 5 is a metallic thin sheet and placed on the inner surface of the insulating layer 4. The membrane 5 serves only for maintaining liquid tightness and the cargo load is transmitted to the inner hull 1 through the heat insulating layer 4 and joist 2.The space between the membrane 5 and the inner hull is divided into two spaces. An interbarrier space (IBS) 7 is defined as the space between the membrane 5 and the secondary barrier 6. And an interground space (IGS) 8 is defined as the space between the secondary barrier 6 and the inner hull. Nitrogen gas as inert gas is filled in these spaces at a pressure slightly higher than the atmospheric pressure (0-20mm bar. gauge). Numeral 9 designates membrane corrugations and numeral 10 insulating filler of polyurethane foam. According to the above mentioned conventional method, if the same insulating material is used, this demand can be met only by increasing the thickness of the insulating layer. However, this increase of the insulating layer is necessarily accompanied by increase in costs and reduction of the available capacity of the LNG tank.

We, the inventors, have conducted various researches on the heat conductivity of the woods used as insulating materials and attained the present invention, which will be described as follows: When viewed microscopically, the lumber used as heat insulating material is composed of a solid portion 11 and a void portion 12, as shown in Fig. 2.Considering heat conduction through such lumber, its apparent heat conduction coefficient may be regarded as the sum of heat conduction coefficient through solid portion 11 and that through void portion 1 2. The apparent heat conduction coefficient in this case is represented by the formula (1) S, S2 A A1+ A2 (1) S1 + S2 S1 + S2 where A: apparent heat conduction coefficient of lumber (Kcal/hm"C) A,: heat conduction coefficient of solid portion 11 (Kcal/hm"C) A2: heat conduction coefficient of void portion 1 2 (Kcal/hm'C) S,: width of solid portion 11 S2: width of cell (void portion 12) In the above equation (1), let us consider A2 which is the heat conduction coefficient of the gas in the void.

In general, the heat insulating coefficient of a gas is not dependent on the pressure. However, according to the theory of vacuum insulation, as the pressure approaches to vacuum level and the free mean path of the gas is increased to the cell diameter d (several to tens of microns), the heat conduction coefficient is the function of pressure in such a manner that, for the general range of pressure (less than several to tens of microns), the heat conduction coefficient is reduced in direct proportion to the decrease in pressure. Therefore, even though the insulating material is lumber, the value of A2 of the second term of the equation (1) can be reduced if the pressure in the cell (in the void portion 12) could be decreased to sufficiently close to vacuum level. The above finding has led to the completion of the present invention.

Using balsa as insulating material, and applying the above theory, S, S2 - = 0.08 and = 0.92 S, + S2 S, + S2 Assuming that vacuum insulation is not applied and nitrogen gas is sealed at around atmospheric pressure as in the conventional practice, with At = 0.1 5 Kcal/hm C and A2 = 0.017 Kcal/hm C, with nitrogen gas being at - 50"C, from equation (1) we obtain A 0.027 Kcal/hm C.

Applying the present method, and reducing the pressure in the cell down to about 0.1 to several mbar (abs.), A2 can be reduced to close to 0.002, with A1 = 0.15 Kcal/hm C, so that the heat insulating coefficient of the heat insulating layer can be lowered to 0.014 Kcal/hm"C and the heat input is equal to 0.014/0.027 = 0.5 times of that for conventional method.

Turning to the problem of whether the pressure in the cell of the lumber can be lowered to vacuum level, it was considered in general that the cell structure is a closed system and hence pressure reduction is not possible. The present inventor has found that, if the lumber is balsa or similar material, and the end face of the heat insulating layer is maintained at vacuum level for several hours to several days, it is possible to reduce the pressure in the inside of the cell in the heat insulating layer to a vacuum level of the order of 0.5 to 3 mbar (abs.), depending on the structure of the heat insulating layer, as demonstrated by the following Examples.

It should be noted that the above holds true only in instances where the fiber direction (I) of the lumber is perpendicular to the heat transmitting direction (II) and that the vacuum of the above described order is hardly effective to realize improved insulation performance when the fiber direction is parallel to the heat transmitting direction.

In actual liquefied gas tankers, the insulating layer is formed by timbers arranged in plural tier and, in general, some timbers are necessarily placed so that the fiber direction thereof is parallel to the heat transmitting direction. Therefore, as a whole, the heat input in the case of vacuum insulation is approximately 60 to 70% related to the case of not using vacuum insulation, which represents about 30 to 40% increase in the heat insulating performance.

A preferred embodiment of the invention is described by referring to Fig. 3.

The Fig. 3 is an explanatory view showing the sectional structure of the cargo tank of the liquefied gas tanker and showing the flow diagram (B) of the piping and pressure sensors for creating vacuum pressure level in the heat insulation system.

Referring to the Fig. 3, portion A designates a liquefied gas tanker in section, portion B designates an enlarged view of a heat insulating system having the same construction as in Fig.

1, and portion C a device for reducing the pressure in the system down to vacuum pressure level and maintaining the vacuum pressure level.

The device for the present method is composed of a vacuum pump unit 1 3 for reducing the pressure in the balsa wood layer, down to vacuum pressure level a suction piping 14 and a pressure sensing unit 1 5 for the insulating layer.

In general, the vacuum pump unit 1 3 is preferably composed of a large capacity pump 1 3a for creating vacuum and a small capacity pump 1 3b for maintaining vacuum. However, only one vacuum pump may suffice. The suction piping 14 is provided to both end faces of the insulating layer, that is, an inner suction pipe 1 4a is led out from membrane corrugation 9 and an outer suction pipe 1 4b is led out from a gap 3' between the inner hull 1 and insulating filler of polyurethane foam 1 0. Additional inner and outer suction pipes may also be used depending on the tank structure.The pressure sensing unit 1 5 is composed of pressure gauges 1 5a, 1 sub leading to end faces of the insulating layer and a pressure gauge 1 sic leading to the inside of the insulating layer for indicating the pressure in the timber cell. The pressure gauge 1 sic is used when switching the operation from the large capacity pump 1 3a for establishing vacuum to the small capacity pump 1 3b for maintaining vacuum.

The practical examples which will be described later has demonstrated that an improvement of 30 to 40% in comparison with the prior art insulating system is realized in the present invention in tank insulation of the membrane tank liquefied gas tanker adapted for transport of liquefied gas having a boiling point lower than - 40" at atmospheric pressure.

The increased thickness of the insulating layer hetherto thought unavoidable in improving insulating performance is now avoided and one may expect tremendous effect from the viewpoint of cost reduction and tank capacity. Thus the costs for execution of the present method are low whereas the capacity for cargo may be increased due to reduced thickness of the insulating layer, thus resulting in considerable practical merits. Even in cases where the improvement of insulating performance is not required, one may still expect the above described effects, because the insulating layer may have smaller thickness than in the conventional system.

An additional advantage of the invention is that the present system may be used for sensing defects in the membrane tank.

In the presently built membrane tank type LNG vessel, in cases where minute cracks have been developed in the membrane, causing a cargo leakage, there is no alternative but to detect the cargo gas leakage by a detector provided in IBS (inter barrier space: space between the membrane and the secondary barrier). In the tanker adopting the present method, defects may be sensed instantaneously by the rapid rise iss IBS pressure which should normally be closed to nill (vacuum level). This is highly efficient from the viewpoint of assuring safety in LNG tankers.

The effect of the invention is presently described in the following practical examples.

Example 1 The device for carrying out the method of the invention is shown diagrammatically in Fig. 4.

A tank with L equal to 0.25m cubic filled with liquid nitrogen is lined on its perimeter by a balsa insulating layer 4 to provide a tank 21, which is then placed on a weighing apparatus 22, 23 denotes thermocouples for temperature measurement and 24 an outlet of the vaporized gas.

The balsa fiber in the insulating layer runs at right angles to the heat transmitting direction except at the corners. The pressure in the insulating layer is lowered from atmospheric to vacuum level and the amount of the evaporated gas is measured for each vacuum level. The result is shown in Fig. 5.

As indicated in the chart of Fig. 5 in which the absolute pressure (mbar) is plotted against the amount of vaporized gas in kg/H, the amount of vaporized gas is decreased from 1.1 kg/H for ambient pressure to 0.45kg/H for 1 mbar. The rate of decrease in vaporization is 0.45/1.1 x 100=4:1%.

Example 2 A tank with capacity of 70m3 filled with liquid nitrogen as shown in Fig. 6 and lined on its perimeter with a balsa insulating 0.285m thick to provide a tank (inner hull 21) (1:5. ism: h:4.7m) in used. The pressure in the layer 4 is reduced from atmospheric to vacuum level and the amounts of vaporized gas are measured at each pressure level. The result is plotted in Fig.

7. It may be seen from this figure that the amount of vaporized gas is 50.1 kg/H for ambient pressure but falls to 43.9 kg/H at 1-3 mbar, showing the decrease amount to be 6.2 kg/H.

It is to be noted that the pressure curve of Fig. 7 represents a pressure in IBS and IGS for pressure range from ambient pressure to 9 mbar and a pressure in IGS for a pressure lower than 9 mbar. At the pressure range lower than 9 mbar, the pressure in IBS was kept at 1-3 mbars.

The rate of decrease in vaporization is 6.2/50.1 X 100=12%.

In Example 2, to simulate the tank of a practical tanker, use is made of a so-called corner panel that is usually heterogeneous to the standard insulating layer and exhibits substantially nill vacuum insulating effect. Therefore, the insulating effect is lower than in the Example 1.

In the present Example, the corner panels are used at a markedly higher ratio than that used in actual vessel, which accounts for a relatively low effect of improvement of the insulating performance. In actual vessel, tank size is that large and the presence of corner panels is practically negligible so that the insulating performance much higher than heretofore may be expected as a result of vacuum insulation.

Claims

1. A method for insulating the cargo tank of a membrane type liquefied gas tanker wherein a stack of wood timbers are provided in a space defined between the corrugation membrane and the inner hull of the tanker and a plurality of joists are arranged in said space to support said stack of wood timbers, thereby supporting the cargo tank and effecting heat insulation, characterized in that said space is so constructed as to be an airtight space and air is extracted from said airtight space for creating vacuum level therein so as to reduce a pressure in the cells of said wood timbers.

2. The method as claimed in Claim 1, characterized in that the fiber direction of said stack of wood timbers is perpendicular to the heat transmitting direction.

3. The method as claimed in Claim 1, characterized in that said air is extracted from an interground space between the inner hull and said stack of wood timbers and from an interbarrier space between the corrugations of said membrane and said stack of wood timbers.

4. The method as claimed in Claim 1, characterized in that the pressure in said interbarrier space is reduced down to several mbars.

5. The method as claimed in Claim 1, characterized in that a balsa wood timber is used for said stack of wood timbers.

6. The method claimed in Claim 1, characterized in that a large capacity pump for creating vacuum and a small capacity pump for maintaining vacuum pump are used to effect the air extraction.

7. A system for insulating the cargo tank of membrane type liquefied gas tanker wherein a stack of wood timbers are provided in a space defined between the corrugation membrane and the inner hull of the tanker and a plurality of joists are arranged in said space to support said stack of wood timbers, thereby supporting the cargo tank and effecting heat insulation, characterized in that said space is so constructed as to be an airtight space; that vacuum means are provided to extract air from said airtight space so as to create vacuum level pressure in the cells of said wood timbers; that an interground space is formed between said stack of wood timbers and said inner hull; that an interbarrier space is formed between the corrugations of said membrane and said stack of wood timbers;; that suction pipe means communicate said interground space and said interbarrier space to said vacuum means; and that pressure sensor means are provided to detect an inner pressure in said space and an inner pressure in the cells of said wood timbers.

8. The system as claimed in Claim 7, characterized in that said vacuum means include a large capacity pump for creating vacuum and a small capacity pump for maintaining vacuum.

9. The system as claimed in Claim 7, characterized in that a foam synthetic resin member is provided in each space defined at intervals of said joists, that a gap is formed between said resin member and the surface of said inner hull, and that said suction pipe means include conduit means which communicate the corrugations of said membrane and said gap, respectively, to said vacuum means.

10. The system as claimed in Claim 7, characterized in that said pressure sensor means comprise a plurality of pressure meters which indicate an inner pressure in both sides of said stack of wood timbers within said space and an inner pressure in said cells of wood timbers, respectively.

11. The system as claimed in Claim 7, characterized in that the fiber direction of said stack of wood timbers is perpendicular to the heat transmitting direction.

1 2. A method for insulating the cargo tank of a membrane type liquefied gas tanker, substantially as hereinbefore described with reference to Figs. 3 to 7 of the accompanying drawings.

1 3. A system for insulating the cargo tank of a membrane type liquefied gas tanker, the system being substantially as hereinbefore described with reference to, and as illustrated in, any of Figs. 3, 4 and 6 of the accompanying drawings.