FR2563801A1 - METHOD AND APPARATUS FOR INSULATING A LOADING TANK OF A LIQUEFIED GAS TRANSPORT VESSEL. - Google Patents

Abstract

THE INVENTION RELATES TO A METHOD AND SYSTEM FOR THERMALLY ISOLATING THE LOADING TANK OF A LIQUEFIED GAS TRANSPORT VESSEL OF THE MEMBRANE TANK TYPE, TRANSPORTING LIQUEFIED GAS HAVING A BOILING POINT LESS THAN -40C AT ATMOSPHERIC PRESSURE ; IN THE AIRTIGHT SPACE SURROUNDING THE TANK, A SET OF WOODEN TIMBER BOARDS, PARTICULARLY IN BALSA WOOD, IS PROVIDED TO SUPPORT THE LOAD PLACED IN THE TANK AND TO ENSURE THERMAL INSULATION OF THE SAME; AIR IS EXTRACTED BY MEANS OF A PUMP13 OF SAID SPACE TO CREATE A VACUUM THEREIN, THUS REDUCING THE PRESSURE IN THE CELLS OF WOODEN TIMBER, THIS PRESSURE BEING MEASURED BY MANOMETERS 15A, 15C.

Description

1 256380 1

  The present invention relates to a tank insulation method for a liquefied gas vessel, in particular for a liquefied gas vessel of the membrane tank type, suitable for transporting liquefied gas having a boiling point below -40 C at atmospheric pressure, the invention also relates to an apparatus

  for the implementation of this process.

  The conventional tank insulation apparatus of an LNG vessel of the membrane tank type is arranged from the

next way.

  In the conventional tank insulation process, beams are placed on the lower part of the ship's inner hull with a predetermined spacing and each gap is filled with a load of putty. On these beams, there is a thermally insulating layer consisting of several layers of wooden planks, like balsa planks. In the insulating layer, it is

  provided a secondary barrier in laminated wood for main-

  provide a liquid tightness for a certain period of time in the event of a leakage of the loading liquid due to a rupture of the membrane. The membrane is a thin metal sheet and it is placed on the inner surface of the insulating layer. The membrane is only used to maintain the liquid tightness and the force exerted by the load is transmitted to the inner shell through the thermally insulating layer and the beams. The space between

  the membrane and the inner shell is divided into two spaces.

  An inter-barrier space (IBS) is defined as the space existing between the membrane and the secondary barrier. In addition an inter-bottom space (IGS) is defined as the space

  existing between the secondary barrier and the inner shell.

  Nitrogen serving as inert gas is introduced into these spaces at a pressure slightly higher than the pressure

  atmospheric (barometric pressure 0-20 mm).

  Recently, in liquefied gas transport vessels, a need has arisen with regard to improving the thermal insulation capacity with a view to reducing the quantity of vaporized gas (gas produced by vaporization) produced by the load. during a

  travel, from the point of view of energy saving. Compliance-

  the conventional process mentioned above, if the same

  insulating material is used, this need cannot be met

  by increasing the thickness of the insulating layer. However, this increase in the insulating layer is necessarily accompanied by an increase in the cost and by a reduction in the available capacity of the transport vessel.

  An object of the present invention is to improve

  rer the possibilities of tank insulation of a liquefied gas transport vessel of the membrane tank type, suitable for the transport of liquefied gas having a point

  boiling below -40 C at atmospheric pressure.

  The present invention relates to a method and a system for the thermal insulation of a loading tank used in a liquefied gas transport vessel of the membrane tank type, suitable for transporting liquefied gas having a lower boiling point. at -40 C at atmospheric pressure, characterized in that air is evacuated from inside the airtight space surrounding the loading tank, thereby reducing the pressure in the small plank cells woods which are placed in a stacked form inside the airtight space, thereby improving the thermal insulation capacity. It should be noted that the aforementioned wooden planks are of such a nature that they can be used for insulation

  thermal and to support the load of the tank.

  Other characteristics and advantages of the invention will be highlighted in the rest of the

  description, given by way of nonlimiting example, in

  reference to the appended drawings in which: FIG. 1 is a schematic view for explaining the tank insulation process used for a liquefied gas transport vessel of the membrane tank type; FIG. 2 is an explanatory view showing the microscopic structure of the thermally insulating material; Figure 3 is an explanatory view concerning the method according to the invention; Figure 4 is an explanatory view of the system according to a first embodiment; FIG. 5 is a diagram giving the relationship between the quantity of vaporized gas and the absolute pressure, in the first embodiment; Figure 6 is an explanatory view showing the system according to the second embodiment of the invention, and Figure 7 is a graph showing the relationship between the amount of vaporized gas and the absolute pressure in space

  inter-barrier (IBS) and inter-bottom space (IGS).

  Figure 1 shows a schematic overview of the conventional tank insulation system of the liquefied gas transport vessel of the tank type

membrane.

  Referring to Figure 1, in the conventional tank insulation process, beams 2 are placed on the inner hull 1 of the ship. The reference numeral 3 designates an intermediate zone filled with a

  filler load and the reference 3 'designates the interval.

  A thermally insulating layer 4 is placed on the beams 2. This thermally insulating layer 4 is made up of several layers of wooden planks, like balsa planks. In the insulating layer 4, a secondary barrier 6 is made of laminated wood to maintain a liquid tightness for a certain period of time in the event of a leakage of the loading liquid under the effect of a rupture of the membrane. 5. The membrane 5 is a thin metal sheet which is placed on the inner surface of the insulating layer 4. The membrane only serves to maintain a liquid tightness and the force exerted by the load is transmitted to the

  inner shell 1 through the thermal layer

  insulation 4 and beams 2. The space between the

  membrane 5 and the inner shell is divided into two spaces.

  An inter-barrier space (IBS) 7 is defined as the space existing between the membrane 5 and the secondary barrier 6; an inter-bottom space (IGS) 8 is defined as the space

  existing between the secondary barrier 6 and the inner shell

  better. These spaces are filled with nitrogen, serving as an inert gas, at a pressure slightly higher than atmospheric pressure (barometric pressure 0-20 mm). The reference 9 designates undulations of the membrane and the reference 10

  an insulating filling of polyurethane foam. Compliance-

  ment to the conventional method mentioned above, if the same insulating material is used, this requirement can only be satisfied by increasing the thickness of the insulating layer, However this increase in the insulating layer is necessarily accompanied by an increase in cost and by reducing the available capacity of the tank

transport.

  The inventors carried out various researches concerning the thermal conductivity of wood used as insulating materials and they developed the

  present invention which will be described below.

  During a microscope examination, the log used as thermally insulating material is composed of a solid part 11 and a part formed by voids

  12, as shown in FIG. 2. Considering the transmission

  heat transfer through this log, its apparent heat transfer coefficient can be considered

  as being equal to the sum of the transmission coefficients-

  heat ions through the solid part 11 and through the part formed by the voids 12. The coefficient

  apparent heat transfer in this case is shown

with formula (1):

S 1 S2

S + 2S 1 + s 2 (1)

S1 +22 S 1 + 2

  o: apparent heat transfer coefficient of plank (Kcal / hm C) 1 heat transmission coefficient of the solid part 11 (Kcal / hm C) 2 heat transfer coefficient of 11 part formed by the voids 12 (Kcal / hm C) s1: width of the solid part Il

  s2: cell width (empty part 12).

  In equation (1) above, we will consider 2 which is the heat transfer coefficient of the gas

in the voids.

  In general, the coefficient of thermal insulation of a gas is not a function of pressure. However, in accordance with the theory of vacuum insulation, when the pressure approaches the vacuum level and when the mean free path of the gas is increased to the cell diameter d (from a few microns to several tens of microns), the heat transfer coefficient is a function of the pressure so that, for the general pressure range (from a few microns to several tens of microns), the heat transmission coefficient is reduced in direct proportion to the reduction pressure. Consequently, even if the insulating material is a wooden plank, the value of A2 intervening in the second term of equation (1) can be reduced if the pressure in the cell (in the vacuum part 12) can be reduced

  up to a value sufficiently close to the vacuum level.

  The result indicated above led to the development

of the present invention.

  By using balsa as an insulating material and applying the theory made above, we obtain:

S1 S2

S + = 0.08 and S + = 0.92

S1 + S2 1 + 2

  Assuming that vacuum insulation is not applied and that nitrogen is trapped at a pressure close to atmospheric pressure as in current practice, we obtain, for A1 = 0.15 Kcal / hm * C and for A2 = 0.017 Kcal / hm'C using nitrogen gas at

  -50OC, from equation (1); = 0.027 Kcal / hm C.

  By applying the present process and by reducing the absolute pressure in the cell to a value of between approximately 0.1 and a few millibars (1 bar = Pa), it is possible to reduce À2 to 0.002, with 1 = 0.15 Kcal / hmC, so that the thermal insulation coefficient of the thermally insulating layer can be reduced to 0.014 Kcal / hm'C and the incoming heat is 0.014 / 0.027 = 0.5 times the value corresponding to

classic process.

  Returning to the problem of defining whether the pressure in the wooden log cell can be reduced to a vacuum level, it has generally been considered that the cellular structure constitutes a closed system and that consequently a reduction in pressure is not possible. The inventors have found that, if the wooden plank was made of balsa wood or a similar material and if the extreme face of the thermally insulating layer was kept at a vacuum level for a period of between several hours and several days, it was possible

  to reduce the pressure inside the cell of the -

  thermally insulating layer at a vacuum level of the order of 0.5 to 3 millibars (1 bar = 10 5Pa), depending on the structure of the thermally insulating layer, like this

  is demonstrated by the following examples.

  It should be noted that the considerations given above only apply in cases where the direction of the fibers (I) of the wood is perpendicular to the direction of heat transmission (II) and the vacuum having the order of magnitude described above is very effective in improving insulation performance when fiber direction

  is parallel to the direction of heat transmission.

  In vessels carrying liquefied gas, the insulating layer is formed by planks distributed in several compartments and, in general, certain planks are necessarily placed so that the direction of their fibers is parallel to the direction of heat transmission. As a result, overall, the heat

  incoming in the case of vacuum insulation is approxima-

  tively from 60 to 70% of the value obtained in the case where vacuum insulation is not used, which represents an increase of about 30 to 40% in the thermal insulation capacity. A preferred embodiment of the invention

  will be described with reference to FIG. 3.

  Figure 3 is an explanatory view showing in section the structure of the loading tank of the liquefied gas transport vessel and it highlights in (B) the layout diagram of the piping and pressure sensors making it possible to create a level of vacuum in the

thermal insulation system.

  With reference to Figure 3, Part A represents

  sectional view of a liquefied gas transport vessel, part B represents an enlarged view of a thermal insulation system having the same structure as in FIG. 1 and part C represents a device for reducing the pressure in the system up to a vacuum level,

  and maintaining that vacuum level.

  The system for putting the process into practice

  die according to the invention is composed of a vacuum pumping unit 13 for reducing the pressure in the balsa wood layer to an appropriate vacuum level, a suction pipe 14 and a pressure detection unit 15

for the insulating layer.

  In general, the vacuum pumping unit 13 is preferably composed of a large capacity pump 13a to create a vacuum and a small capacity pump 13b to maintain the vacuum. However, a single vacuum pump may suffice. The suction pipe 14 is provided

  on the two end faces of the insulating layer, i.e.

  say that an internal suction pipe 14a starts from a

  membrane corrugation 9 and that an external suction pipe

  tion 14b starts from an interval 3 'existing between the hull

  inner 1 and the insulating filling of polyurea foam

  thane 10. Additional internal and external suction tubes can also be used depending on the structure of the tank. The pressure detection unit 15 is composed of pressure measurement gauges 15a, 15b leading to the end faces of the insulating layer and

  a pressure measurement gauge 15c leading to the interior

  of the insulating layer to indicate the pressure prevailing in the cells of the wooden plank. The pressure measurement gauge 15c is used when switching between the large capacity pump 13a used to establish the vacuum and

  the low capacity pump 13b used to maintain the vacuum.

  The practical examples which will be described

  in the following have highlighted that we obtained an improvement

  ration of 30 to 40%, by comparison with the insulation system of the prior art, by adopting the present invention for the insulation of the tank of a liquefied gas transport vessel with membrane tank, suitable for transporting liquefied gas having a boiling point below - 40 * C-at pressure

atmospheric.

  The increase in thickness of the insulating layer

  te, which was once considered unavoidable for improving the insulation capacity, is now avoided and significant consequences can be expected from the point of view of cost reduction and increase in tank capacity. Consequently, the costs of implementing the process according to the invention are low and the transport capacity can be increased due to the reduction in thickness of the insulating layer, which in practice results in considerable advantages. Even in cases where the improvement of the insulation capacity is not necessary, the effects described above can still be obtained since the insulating layer can be

  made much thinner than in conventional systems.

  An additional advantage of the invention is that the present system can be used to detect

  faults in the membrane tank.

  In a liquefied gas tanker of the membrane tank type built at present, it happens that, in cases where small cracks have been created in the membrane by producing a leakage of the loading liquid, there is no There is no way to detect the gas leak by a detector placed in the inter-barrier space (IBS), i.e. the space between the membrane and the secondary barrier. In the liquefied gas transport vessel adopting the process according to the invention, it is

  possible to instantly detect faults by increasing

  Rapid pressure in the IBS space, this pressure should normally be constantly at the vacuum level. This is extremely effective from the point of view of establishing safety in transport vessels

liquefied gas.

  The effectiveness of the present invention will now be demonstrated by means of the examples.

following practices.

Example 1

  The device for implementing the method according to the invention is shown diagrammatically in FIG. 4. A tank with a capacity equal to 0.25 cubic meters and filled with liquid nitrogen is packed on its periphery

  an insulating layer of balsa 4 so as to form a reser-

  see 21 which is then placed on a weighing device; the thermocouples for measuring the temperature are designated by 22 and 23 and by 24 an outlet for the vaporized gas. The

  balsa fibers in the insulating layer are oriented per-

  perpendicular to the direction of heat transmission, except in the corners. The pressure in the insulating layer is reduced from atmospheric pressure to a vacuum level and the amount of evaporated gas is measured for each vacuum level. The results are shown

in figure 5.

  As indicated in the diagram in FIG. 5, where the absolute pressure (millibar) is represented as a function of the quantity of vaporized gas, expressed in kg / h, the quantity of vaporized gas is reduced by 1.1 kg / h for the ambient pressure up to 0.45 kg / h for 1 millibar (1 bar = 5Pa). The vaporization reduction rate is

0.45 / 1.1 x 100 - 41%.

Example 2

  A tank with a capacity of 70 m3, filled with liquid nitrogen as shown in Figure 6 and filled on its periphery with an insulating layer of balsa of 0.285 m thick was used to form a tank or tank (shell interior 21) (1: 5.1 m; h: 4.7 m). The pressure in layer 4 was reduced from atmospheric pressure to a vacuum level and the amounts of vaporized gas were measured for each pressure level. The results are given in FIG. 7. It can be seen with reference to this figure that the quantity of vaporized gas is 50.1 kg / h for the ambient pressure but that it decreases up to 43.9 kg / h for a pressure between 1 and 3 millibars (1 bar - 1O0 Pa), which corresponds

at a decrease of 6.2 kg / h.

  It should be noted that the pressure curve in FIG. 7 represents a pressure in the IBS space and in the IGS space for a range of pressures between ambient pressure and 9 millibars as well as a pressure in the IGS space , worth less than 9 millibars. In the range of pressures lower than 9 millibars, the pressure

  in the IBS space was maintained between 1 and 3 millibars.

  The vaporization reduction rate is

6.2 / 50.1 x 100 * 12%.

  In example 2, to simulate the tank of a real ship, we used what is called a corner panel, which is usually heterogeneous compared to the standard insulating layer and which has a corresponding insulation effect. substantially at a void of zero value. In

  Consequently, the insulation effect is lower than in Example 1.

  In the example considered, the corner panels are used with a percentage significantly higher than that adopted in a real ship, which results in a relatively low degree of improvement in insulation performance. In a real ship, the dimensions of the tanks

  are large and the presence of corner panels is practical-

  ment negligible so that one can expect, as regards the vacuum insulation, to good performances

  better than what was possible before.

Claims (11)

  1. A method of isolating the loading tank of a membrane type liquefied gas transport vessel, in which a set of wooden planks (4) is arranged in a space defined between the corrugated membrane (5) and the inner hull (1) of the ship and several beams (2) are arranged in said space so as to support said set of wooden planks (4), thereby supporting the tank and ensuring thermal insulation, characterized in that said space is arranged so as to constitute an airtight space and in that air is extracted from said airtight space to create a vacuum level therein in order to reduce the pressure   in the cells (12) of said wooden planks (4).   2. Method according to claim 1, characterized in that the direction of the fibers of the wooden planks (4)   of said assembly is oriented perpendicular to the direction heat transmission.   3. Method according to claim 1, characterized in that the air is extracted from an inter-bottom space (IGS) formed between the inner shell (1) and said set of wooden planks (4) as well as '' an interbarrier space (IBS) formed between the corrugations (9) of said membrane (5)   and said set of wooden planks (4).   4. Method according to claim 1, characterized in that the pressure in said inter-barrier space (IBS) is reduced up to a few millibars (1 bar = Pa). 5. Method according to claim 1, characterized in that a balsa wood is used to form said set of wooden planks (4).   6. Method according to claim 1, characterized in that a large capacity pump (13a) for creating a vacuum and a small capacity pump (13b) for maintaining a vacuum are used to extract the air . 7. System for isolating the loading tank of a membrane type liquefied gas transport vessel, in which a set of wooden planks is arranged in a space defined between the corrugated membrane (5) and the inner hull (1 ) of the ship and o several beams (2) are arranged in said space so as to support said set of wooden planks, thereby supporting the tank and ensuring thermal insulation, characterized in that said space is arranged so as to constitute an airtight space, in that vacuum creating means (13; 13a, 13b) is provided for extracting air from said airtight space so as to create a vacuum level in the cells of said wooden planks (4), in that an inter-bottom space (IGS) is formed between said set of wooden planks (4) and said interior shell (1), in that an inter-space barrier (IBS) is formed between the corrugations (9) of said membrane (5) and said set of wooden planks (4 ), in that pipes (14) put said inter-bottom space (IGS) and said inter-barrier space (IBS) into communication with said generation means   vacuum (13; 13a, 13b) and that detectors are provided   pressure sensors (15; 15a, 15b, 15c) to detect a   internal pressure in said space and internal pressure   higher in the cells of said wooden planks (4).   8. System according to claim 7, characterized in that said vacuum generation means comprise a large capacity pump (13a) for creating a vacuum and a pump   small capacity (13b) to maintain a vacuum.   9. System according to claim 7, characterized in that a synthetic foam filling (10) is arranged in each space formed between said beams, in that an interval is formed between said resin filling and the surface of said shell interior (1) and in that said suction pipe comprises conduits communicating respectively the undulations of said membrane (5) and said gap with said means of   vacuum generation (13; 13a, 13b).   10. System according to claim 7, characterized in that said pressure detectors comprise several pressure measuring devices (15a, 15b, 15c) which indicate an internal pressure prevailing on the two sides of said set of wooden planks inside of said space and an internal pressure prevailing in said cells of the wooden planks (4).   11. System according to claim 7, characterized in that the direction of the fibers of the wooden planks (4) of said assembly is oriented perpendicular to the   direction of heat transmission.