INSULATION OF VESSELS HAVING CURVED SURFACES

Abstract: A vessel (10) with a curved surface (13) is insulated with the first layer of insulation (14) formed of a plurality of closed loops (15) joined to each other. A second layer of insulating material (17) is disposed over the first layer (14) and is comprised of a plurality of closed loops (18) of insulating material with adjacent portions of the loops (18) being affixed to each other. Such arrangement allows thicker insulation without the requirement of increased space and heavier equipment.

Description: INSULATION OF VESSELS HAVING CURVED SURFACES In the transportation of cryogenic liquids such as liquefied natural gas, vessels that are utilized must have a relatively high degree of thermal insulation to prevent loss of the liquefied cargo during transpor- tation. A variety of ships have been designed for the transportation of liquefied natural gas. One of the more desirable designs for such vessels employs spherical tanks which are equatorially supported. Ships generally utilize a plurality of such spherical tanks having an equatorial flange, wherein the flange is supported by a generally cylindrical skirt which is affixed to the hull of the ship. The use of spherical tanks provide a minimal surface to volume ratio thereby minimizing the surface which must be provided with suitable thermal insulation to prevent undesirable vaporization of the liquefied cargo. Such spherical tanks are useful not only for shipboard applications but for land based storage also. Tanks for cryogenic liquids have been employed which have a variety of configurations such as rectangular, cylindrical and the like. Cryogenic tanks have been insulated by a variety of methods. One method that has found some favor is the so-called panel insulating method wherein the cryogenic tank has applied thereto a plurality of panels of insulating material and the adjacent panels are bonded to each other by means of a suitable adhesive, for example a foamed in place urethane foam. The panel insulating technique is labor intensive and for many applications is therefore undesirable. When employed with tanks of rectangular configuration, the panel system provides minimal difficulty in fitting. When the panel system is applied to non-rectangular tanks such as those of cylindrical or spherical configuration, the amount of effort in the cutting and fitting of the tanks is generally found to be undesirably time consuming. Vessels having rotational symmetry such as those of cylindrical or spherical configuration beneficially may be insulated using the so-called spiral generation process. The term "spiral generation" refers to the preparation of structures by the progressive deposition of an insulating foam or like construction material by a material depositing means which travels about a predetermined path to deposit successive loops or turns of the insulating material on the cylindrical or spherical vessel, wherein the progressive loops are adhered to adjacent loops. In effect, when the spiral generation technique is employed, a strip of insulating material is wound about the vessel to be insulated and the adjacent turns of the strip are effectively affixed to each other. Spiral generation techniques and variations thereof are set forth in the following U.S. Patents: 3,206,899; 3,337,384; 3,443,276; 3,507,735; 3,874,983; 3,879,254; 3,902,943; 3,919,034; 3,923,573; 3,924,039; 4,017,346; 4,050,607; 4,098,635; and 4,175,998. OMPI " U.S. 4,017,346 discloses an apparatus and a method for the application of foam thermoplastic insulatio to a cryogenic vessel of rotational symmetry. A composite strip or log is applied to the vessel, the log having a vapor barrier layer disposed remote from the vessel, the vapor barrier layer generally being a flexible aluminum sheet. The thermoplastic foam of the log is heat bonded to adjacent loops as is the aluminum skin bonded to its adjacent turns by means of heat sealing or crimping or like connecting means. Insulation of a vessel in accordance with the present invention is readily accomplished employing the method and apparatus as set forth in U.S. Patent 4,017,346. In a typical application of thermal insulation to a vessel having rotational symmetry by the spiral generation method, a log of about 20 centimeters square cross section may be employed. In general, the appli¬ cation of insulation by the spiral generation technique becomes easier as the cross section of the insulating strip is reduced. Generally as the size of the cross section configuration of the strip increases, the mass of equipment required to deposit such a strip also increases. In most instances it is very desirable that the size of the apparatus employed to deposit a strip of insulating material in the spiral generation process be maintained at a minimum. This is particularly true in the case of shipboard installations where generally a tank such as a spherical tank is installed at least partially within the hull of a ship and the insulation is applied to the tank after such installation. Oftentimes a weather shield is provided for such a tank when a portion of the tank protrudes above the hull of the ship. Advantageously, the mass and volume of the spiral generation equipment employed to apply insulation is maintained at a minimum volume and minimal weight. Ideally the spiral generation equipment is of relatively low volume and low weight which facilitates moving the equipment from one location to another and requires minimal space about the tank for its operation during the application of insulating material. A very critical element in cryogenic insulation is the vapor barrier which as disclosed in U.S. Patent No. 4,017,346 comprises an external aluminum skin, enclosing the vessel and the insulation material. In the event that water vapor has ready access to the insulation area of the cryogenic vessel, a build-up of ice can occur and result in a substantial and significant loss of the insulating value. The insulating system set forth in U.S. Patent No. 4,017,346 was very satis¬ factory for use in shipping liquefied natural gas when the value of liquefied natural gas was relatively low. The single layer of insulation and vapor barrier provided adequate insulation and protection of the cryogenic vessel even when liquefied natural gas which vaporized was conducted from the cryogenic tank to provide fuel for the ship. As the value of liquefied natural gas increased relative to heavy fuel oils such as Bunker C, it became desirable to have increased thermal insulation on vessels employed to transport and/or store liquefied natural gas. The present invention provides an improved cryogenic vessel having thermal insulation disposed thereabout which has improved vapor barrier characteristic ΪRl O PI The present invention also provides an improved method for the spiral generation application of thermal insulation to cryogenic vessels. More particularly, the invention provides for an improved method for the application of thermal insulation to crygenic vessels by the spiral generation technique which permits the application of thermal insulation having a relatively high resistance to the flow of heat utilizing equipment having minimal weight and volume. The present invention particularly resides in an improved insulated cryogenic vessel, comprising at least a first containment vessel having an exterior curved surface, a first layer of insulation disposed adjacent the exterior surface and covering at least a substantial portion thereof, the insulation being in the form of a plurality of loops or turns of a strip-like configuration, adjacent turns of the insulation being adhered to each other; a second layer of insulation being disposed over at least a portion of the first layer of insulation and external thereto, the second layer of insulation being in the form of a plurality of loops or turns of strip-like configuration, adjacent turns of the insulation being adhered to each other. The present invention also resides in a method for the insulation of cryogenic vessels wherein a thermally insulating material is deposited about the periphery of the vessel in the form of a plurality of closed loops to thereby envelop at least a portion of the vessel within a first insulating layer, subsequently O FI +> depositing at least a second insulating layer about the first insulating layer, the second insulating layer being in the form of a plurality of closed loops to thereby envelop at least a portion of the first insulating layer. Further features and advantages of the present invention will become more apparent from the following specification taken in connection with the drawing wherein Figure 1 is a fractional view of a vessel being insulated in accordance with the present invention; Figures 2, 3 and 4 depict cross sectional configurations of insulation applied in accordance with the present invention. In Figure 1, there is schematically depicted a fractional view of a vessel 10 being insulated in accordance with the present invention. The vessel 10 has a generally spherical configuration having an external surface 13. Adjacent the external surface 13 of the vessel 11 is a first layer of insulation 14 which is formed of a plurality of closed loops 15 of insulating material which are joined to each other. A second layer of insulating material 17 is disposed external to the layer 14 of insulating material. The layer 17 comprises a plurality of closed loops 18 of insulating material, the adjacent portions of the loops 18 being affixed to each other. A spiral generation depositing apparatus 20 is rotatably mounted adjacent the tank 11 and spaced therefrom. The spiral generation apparatus 20 comprises an arcuate boom 21 having an upper or axial end 22 and a lower or equatorial end 23. The ends 22 and 23 are supported by means of rollers 24 and 25 respectively. The upper end 22 of the boom 21 is rotatably fixed to an axially disposed pivot 27 affixed to a dome 12 of the tank 11. An insulation depositing head 28 is supported by the boom 21 and is adapted to move along the boom 21 between the ends 22 and 23. A strip or log of insulating material 31 is engaged by the depositing heat 28 and is effectively wound around the vessel 11 to form the second insulating layer 17. Both of the insulating layers 14 and 17 are deposited by the depositing apparatus 20 which rotates about the vessel 11 as the insulation is deposited. In Figure 2 there is schematically depicted a cross sectional view of a portion of insulation 40 in accordance with the present invention. The insulation 40 has a first or inner side 41 and a second or external side 42. The insulation 40 comprises a first or inner layer 43 and a second or outer layer 44. The inner layer 43 comprises a plurality of loops or turns of a synthetic resinous thermoplastic foam insulating material 45 each of which comprises a first or inner log portion 46 and a second or outer log portion 47. Adjacent portions of the turns 45 are joined together at joints 48. Advantageously, such joints 48 are formed between adjacent turns by heating the thermoplastic foam of log portions 46 and 47 to a temperature sufficiently high so that, in effect, the foam melts and the adjacent turns are heat bonded together. The inner surface 41 of the insulation 40 has disposed thereon and affixed thereto a reinforcing scrim 49. Advantageously, the reinforcing scrim 49 is an open weave glass cloth which provides mechanical reinforcement for the log portions 46. Each of the turns 45 defines slots 51 which extend from the inner face 41 partially inwardly and in a direction generally normal to the log portions 46. A reinforcing scrim 53 is disposed between log portions 46 and 47 and heat bonded therebetween. The scrim 53 advantageously is an open weave glass cloth and provides mechanical reinforcement for the turns 45. The log portions 47 remote from the inner face 41 have affixed thereon an optional glass reinforcing scrim 54 which, in turn, is adhered to a vapor or gas barrier layer 55, advantageously of aluminum. The vapor barrier layer 55 is wider than the turns 45 and is heat sealed by means of appropriate hot melt adhesives to adjacent portions of vapor barrier material 55 on adjacent turns 45. Each of the turns 45 adjacent its edges defines a recess 57 adjacent the inner or cold side 41 and recesses 58 adjacent the vapor barrier layer 55. Advantageously, the adjacent recesses 57 form a groove on the inner face 41 to provide clearance for a heat sealing platen when the joints or fused portions 48 are formed. The grooves 58 facilitate the sealing of adjacent portions of the vapor barrier layer 55 and folding of the sealed portions so that the sealed portions lie generally parallel to the major portions of the vapor barrier layer 55. The outer insulation layer 44 comprises a plurality of loops or turns 61. Each of the turns 61 has a body portion 62 of a synthetic resinous thermoplastic foam insulating material and an inner face 63, having affixed thereto an optional reinforcing scrim 64 such as an open weave glass cloth. The body portion 62 has an external or outer face 65 having adhered thereto a reinforcing scrim 66 which in turn has adhered thereto a gas or vapor barrier layer 67 generally equivalent to the vapor barrier layer 55.. The body portion 62 has first or inner recess 68. The adjacent recesses 68 form grooves opening toward the inner face 63. Similar recesses 69 are formed adjacent the outer face 65 and form outwardly opening grooves which face the vapor barrier layer 67. In the insulation 40 as depicted in Figure 2, the slots 51 provide stress relief when the inner face 41 is cooled to cryogenic temperature. The reinforcing scrim 53 provides an effective means of terminating any cracks which might result from shrinkage of the adjacent face 41 of log portion 46 and minimize the tendency of cracks to form which would run vertically, that is in the plane of the paper. The scrim 54 adjacent the vapor barrier layer 55 provides added mechanical reinforcement to resist thermal stress on cooling of the inner face 41 of log portion 46 to cryogenic temperatures. The outer insulating layer 44 provides additional thermal insulation and mechanical protection for the vapor barrier layer 55. Accordingly, the insulation 40 effectively provides two concentric vapor barriers which provide a high degree of reliability both from a containment point of view in the event of any leakage of the liquefied gas from the tank and from any water vapor permeation from the space external to the tank. The grooves in the insulation formed by the recesses 57, 58, 68 and 69 provide convenient paths for the flow of purge gas when desired. The thermoplastic foam used in the practice of the present invention may be any one of a variety of foams. However, particularly desirable are styrene polymer foams such as polystyrene foam having a density generally in the range of from 24 to 40 kg/m 3 (1.5 to 2.5 lb/ft 3 ), and advantageously for many cryogenic applications, it is desirable to flexibilize the foam. This is done by a controlled crushing of the foam to render it more easily bendable. Flexibilizing of such foams is well known and set forth in U.S. Patent Nos. 3,159,700 and 3,191,224. Flexibilizing the foam employed for insulation in accordance with the present invention generally facilitates handling of the foam as it is fed to the foam depositing head. Flexibilization also increases the elongation at break of the foam and therefore reduces the tendency of such foam to crack when subjected to cryogenic temperatures. In Figure 3, there is schematically depicted a cross sectional view of insulation in accordance with the present invention, generally designated by the reference numeral 70. The insulation 70 comprises a first insulating layer 71 and a second insulating layer 72. The insulating layer 71 is disposed adjacent the vessel to be insulated and is of the same construction as the insulating layer 43 of Figure 2. The layer 71 has an inner or cold side 73 and an outer or warm side 74. The second layer 72 is of similar construction to the layer 71 with the exception that an optional scrim beneath the vapor barrier layer 75 of the outer layer 72 has been omitted and the vapor barrier layer is directly adhered by means of a suitable hot melt adhesive 76 to the foam body portion forming the turns which make up the layer 72. The arrangement, as depicted in Figure 3, utilizing two layers of generally equal thickness provides a mechanically desirable insulation and minimizes equipment changes in the preparation of the laminate material forming the loops of each layer. On completion of the formation of the logs or insulating ΪREΛ OMPI strips which make up the turns of the layer 71, laminates for the second layer 72 are readily prepared by omitting the scrim adjacent the vapor barrier. Thus the materials for layer 71 and 72 may be prepared and deposited without undesirable equipment changes or adjustment. Generally, in the preparation of insulation in accordance with the present invention, it is often desirable to stagger, that is offset, the joints between the turns forming the successive layers. Such an offset or staggering minimizes the possibility of propagation of cracks in the fused joint between adjacent turns from one layer to another. In Figure 4, there is schematically depicted a fractional cross-sectional view of an alternate embodiment of insulation in accordance with the present invention. The insulation 80 comprises in cooperative combination a first or inner insulating layer 81, a second or intermediate insulating layer 82 and a third or outer insulating layer 83. The inner insulating layer 81 comprises a plurality of loops or turns 85 joined to each other in edge to edge relationship. Each of the turns 85 comprises a first or inner body portion 86 and a second or outer body portion 87. The body portions 86 and 87 are laminated together and have therebetween a reinforcing scrim 88 such as an open weave glass cloth. A generally similar scrim 89 is * affixed to the face of inner body portion 86 remote from outer body portion 87. The inner body portion 86 is provided with stress relief slots 91 which extend part-way inwardly into the body 86 from the face having scrim 89. The body portion 87 is provided with stress relief recesses, grooves, or slots 92 which extend part-way into the body portion 87 from the scrim 88. The recesses 92 may be in the form of relatively long and narrow slots, in the form of relatively short and wide grooves, or in the form of any other cross-sectional shape, i.e., rectangular, hemispherical, ellipsical or the like, to provide the desired stress relief in the inner insulating layer 81 and to prevent cracking of the insulation due to temperature stress induced in the insulating layer in a crack barrier zone which passes through the inner body portion 86 in the region of the reinforcing scrim 88. The recesses may extend inwardly as well as outwardly from the reinforcing scrim 88 part-way into the insulation and they may be provided in the inner and outer insulating layers in the two-layered structures, illustrated in Figures 2 and 3. It will be apparent that stress relief grooves (not shown) may also be provided in the intermediate insulating layer 82 illustrated in Figure 4 such that they extend part-way into the insulating material from one or opposite sides of the reinforcing scrim 107. Grooves corresponding to groove 91 may also be provided in the outer insulating layer 83 to extend part-way into the insulation from the inner side of the layer, i.e., from the side of the scrim layer 117. Each of the turns 85 has affixed thereto a reinforcing scrim 93 which is disposed on body portion 87 generally parallel to the scrims 88 and 89 and remote from scrim 88. The reinforcing scrim 93 has adhered thereto and to body portion 87 a vapor or gas barrier layer 94. The barrier layer 94 is sealed to adjacent barrier layers on the turns 85 of the first insulating layer 81. The edges of adjacent turns 85 define inwardly extending grooves 96, extending inwardly from the face having scrim 89. The adjacent turns 85 also define grooves 97 which ' O I extend inwardly from the face having the vapor barrier layer 94. The intermediate layer 82 is of the same construction as the inner layer 81 and comprises a plurality of loops or turns 85a. Each of the turns 85a comprises a first or inner foam body 101 having adjacent turns 85, a reinforcing scrim 102 affixed thereto. The body 101 defines a pair of stress relief slots 103 and 104 which extend inwardly from the scrim 102. A second generally rectangular foam body 106 is laminated to body 101 and a reinforcing scrim 107 is affixed between the bodies 101 and 106. The body 106 has affixed thereto an outer reinforcing scrim 108 which is generally parallel to scrims 107 and 102 and disposed remote therefrom. Affixed to the body 106 immediately adjacent the scrim 108 is a vapor barrier 109. The vapor barrier 109 is affixed and adhered to adjacent vapor barrier 109 of the loops or turns 85a. The turns 85a define therebetween first or inner slots or grooves 110 in the region of a fused joint 111 between the adjacent turns. The groove 110 depend generally inwardly from the scrim 102. Generally, similar grooves 112 are defined between adjacent turns 85a adjacent the scrim 108 and vapor barrier 109. The third or outer layer 83 comprises a plurality of loops or turns 115 of insulating material. Each of the turns 115 comprises a foam body 116 having a generally rectangular cross-section. The turns 115 have a reinforcing scrim 117 affixed generally adjacent the intermediate insulation layer 82. A vapor barrier layer 118 is affixed remote from and parallel to the reinforcing scrim 117. The vapor barriers 118 are adhered to the body 116 by means of an adhesive layer 119. Adjacent portions of the vapor barriers 118 of the turns 115 are adhered together and folded to lie generally at parallel to the scrim 117. Adjacent turns O PI 115 define therebetween first grooves 121 which are inwardly facing and which extend inwardly from the scrim 117. The adjacent turns 115 also define outwardly facing grooves 122 which extend inwardly toward the grooves 121 from vapor barrier 118. Insulation in accordance with the present invention provides considerable flexibility in the choice of materials. Generally, the innermost layer of insulating material can comprise a foam having relatively high elongation and insulating value whereas as one progresses toward the outermost layer of insulation, lower elongation in the foam may be employed depending upon the requirements of the particular insulation installation. The use of multiple vapor or gas barriers is a substantial advantage in that from a practical standpoint, it is difficult to insure the integrity of a single vapor barrier. The use of multiple vapor barriers permits the use of subatmospheric pressure adjacent the tank or container being insulated and such atmospheric pressure can be maintained adjacent the vessel employing a pump of minimal capacity. As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended claims.

Claims: 1. An insulated vessel comprising a fluid containment vessel having an exterior curved surface, a first layer of insulation disposed adjacent the exterior surface and covering at least a substantial portion thereof, the insulation being in the form of a plurality of loops or turns of a strip-like configuration wherein adjacent turns of the insulation are adhered to each other; a second layer of insulation disposed over the first layer of insulation and external thereto, the second layer of insulation being in the form of a plurality of loops or turns of strip-like configuration wherein adjacent turns of the insulation are adhered to each other. 2. The cryogenic vessel of Claim 1 wherein the first insulation layer has affixed thereto a first vapor barrier remote from said containment vessel. 3. The cryogenic vessel of Claim 1 or 2 wherein the second insulation layer has affixed thereto a second vapor barrier remote from said containment vessel. 4. The cryogenic vessel of Claim 1, 2 or 3 wherein at least the loops forming the first insulation layer define a plurality of longitudinally extending OMPI ^ξ NAΥl slots opening toward the exterior surface of the con¬ tainment vessel. 5. The cryogenic vessel of Claim 4 wherein the slots terminate within the strip-like configuration at a location generally adjacent a reinforcing scrim incorporated within the strip-like configuration. 6. The cryogenic vessel of any one of the preceding Claims wherein the strip-like configuration forming the first insulation layer has a reinforcing scrim affixed thereto generally adjacent the exterior curved surface of the containment vessel. 7. The cryogenic vessel of any one of the preceding Claims wherein adjacent turns of the strip-like configuration define inwardly facing grooves adjacent the exterior curved surface of the containment vessel. 8. The insulated vessel of Claim 7 wherein adjacent loops of the strip-like configuration define outwardly facing grooves. 9. The cryogenic vessel of any one of the preceding Claims having at least three generally concentric layers of insulation, each of the layers of the insulation being in the form of a plurality of loops and turns wherein adjacent turns of the insulation are adhered to each other. 10. The cryogenic vessel of Claim 9 wherein each of the insulation layers has a vapor barrier affixed thereto disposed generally remote from the containment vessel. O PI 11. The insulating vessel of any one of the preceding Claims, wherein at least one of said insulating layers comprises inner and outer body portions having a reinforcing scrim between said body portions, and stress relief means extending from at least one side of said reinforcing scrim part-way into the insulation of a body portion. 12. The insulating vessel of Claim 11, wherein said stress relief means comprises recesses extending from opposite sides of said reinforcing scrim part-way into the insulation of said inner and outer body portions. 13. The insulating vessel of Claim 11 or 12, wherein said stress relief means is provided in said first and second insulating layers. 14. The insulating vessel of Claim 13, wherein said first insulating layer is disposed adjacent the exterior surface of the vessel, said second layer is disposed over the first layer of insulation, and a third layer of insulation disposed over the second layer, and wherein each loop of the first and second layers is provided with a plurality of stress relief recesses extending from opposite sides of said reinforcing means part-way into the inner and outer body portions of said first and second layers. 15. A method for the insulation of cryogenic vessels wherein a thermally insulating material is deposited about the periphery of the vessel in the form of a plurality of closed loops to thereby envelop at least a portion of the vessel within a first insulating layer, subsequently depositing at least a second insulating layer about the first insulating layer, the second insulating layer being in the form of a plurality of closed loops to thereby envelop at least a portion of the first insulating layer. 16. The method of Claim 15 wherein the insulating layer is of a thermoplastic foam, and including the steps of heat fusing adjacent loops to one another, providing a vapor barrier layer on the fused loops and heat sealing adjacent vapor barrier layers on adjacent loops together. 17. The method of Claim 15 or 16 including the step of joining adjacent loops together to define inwardly and outwardly facing grooves between adjacent loops and forming each of the loops of a strip-like configuration of a thermoplastic foam defining longi¬ tudinally extending slots.

Inventors: EKKER EMIL, SMITH HUBERT STACY, TONOKAWA HIROSHI, EKKER, EMIL, SMITH, HUBERT, STACY, TONOKAWA, HIROSHI
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1984
Main classification: B65D88
IPC codes: F17C3, F17C13