Low temperature liquid tank

A low temperature liquid tank includes: a storage tank having a bottom portion obtained by joining a plurality of bottom plates; and a support portion supporting the bottom portion. The support portion includes: an outer support portion supporting a margin of the storage tank; and an inner support portion disposed inside the outer support portion and having a heat insulation in which creep occurs when a load is applied. An initial height of an upper surface of the inner support portion is set so that, during a service life of the low temperature liquid tank, maximum bending stress applied to the bottom plates due to a difference between a height of the upper surface of the inner support portion and a height of an upper surface of the outer support portion remains equal to or smaller than an allowable bending stress of the bottom plates.

TECHNICAL FIELD

Embodiments described herein relate to a low temperature liquid tank.

BACKGROUND ART

Tanks (low temperature liquid tanks) in which a low temperature liquid is stored, such as liquefied natural gas (LNG) tanks or liquefied petroleum gas (LPG) tanks, are each equipped with a storage tank in which the low temperature liquid is stored and a support portion that supports the storage tank. To prevent heat from being input from the ground, a heat insulation is included in the support portion (bottom cold insulating structure).

Conventionally, foam glass, which has high rigidity and in which the effect of creep caused by a load applied from above is negligible in a manner similar to concrete, has been used as the heat insulation included in the support portion. Further, in recent years, a technique in which a margin including a sidewall of a storage tank is formed of a material in which the effect of creep is negligible, such as concrete, and a water- or cyclopentane-foamed heat insulation having higher cold insulating performance as shown in Patent Documents 1 and 2 is arranged inside the margin has also been proposed.

CITATION LIST

Patent Documents

SUMMARY

Technical Problem

However, unlike foam glass, a water- or cyclopentane-foamed heat insulation does not have high rigidity. For this reason, there is a possibility of creep occurring during the service life of a low temperature liquid tank and of an upper surface of the support portion that supports the storage tank gradually sinking.

If the upper surface of the middle portion of the support portion including the water- or cyclopentane-foamed heat insulation sinks in this way, a great level difference occurs between the upper surface of the middle portion and the upper surface of portions supporting the margin of the storage tank. Due to the level difference, the bottom portion of the storage tank is bent. Thus, bending stress occurs, and a great load is applied to the bottom portion of the storage tank. For this reason, during the use of the low temperature liquid tank, a possibility of a need to perform large-scale maintenance on the bottom portion of the storage tank arising is increased.

In the tanks in which low-temperature liquids are stored at a low temperature with no change in temperature, including but not limited to LNG tanks and LPG tanks, the heat insulation is included in the support portion that supports the storage tank. Thus, when the water- or cyclopentane-foamed heat insulation is used as the heat insulation, the same problems occur.

The present disclosure has been made in consideration of the aforementioned problems, and an object of the present disclosure is to provide a low temperature liquid tank that inhibits a great load from being applied to a bottom portion thereof while in use.

Solution to Problem

The present disclosure employs the following structures as means of solving the above-described problems.

A first aspect of the present disclosure provides a low temperature liquid tank that includes: a storage tank having a bottom portion obtained by joining a plurality of bottom plates; and a support portion supporting the bottom portion, in which the support portion includes: an outer support portion supporting a margin of the storage tank including a sidewall of the storage tank; and an inner support portion disposed inside the outer support portion and having a heat insulation in which creep occurs when a load is applied to the heat insulation, and an initial height of an upper surface of the inner support portion is set so that, during a service life of the low temperature liquid tank, maximum bending stress applied to the bottom plates due to a difference between a height of the upper surface of the inner support portion and a height of an upper surface of the outer support portion remains equal to or smaller than an allowable bending stress of the bottom plates.

A second aspect of the present disclosure is configured such that, in the first aspect, the initial height of the upper surface of the inner support portion is set to be higher than that of the upper surface of the outer support portion.

A third aspect of the present disclosure is configured such that, in the first or second aspect, the inner support portion has a height setting plate that prescribes the initial height of the upper surface of the inner support portion.

A fourth aspect of the present disclosure is configured such that, in the third aspect, the height setting plate is a heat-resistant board disposed on the heat insulation.

A fifth aspect of the present disclosure is configured such that, in any one of the first to fourth aspects, an edge of the outer support portion which is adjacent to the inner support portion is chamfered.

A sixth aspect of the present disclosure is configured such that, in any one of the first to fifth aspects, the heat insulation is a rigid plastic foam.

Advantageous Effects

In the present disclosure, the initial height of the upper surface of the inner support portion is set such that the maximum bending stress applied to the bottom plates due to the difference between the height of the upper surface of the inner support portion and the height of the upper surface of the outer support portion during a service life of the low temperature liquid tank does not exceed the allowable bending stress of the bottom plates. For this reason, according to the present disclosure, the difference between the height of the upper surface of the inner support portion and the height of the upper surface of the outer support portion is not great enough to have an influence on the bottom plates during the service life of the low temperature liquid tank. Accordingly, according to the present disclosure, the low temperature liquid tank can inhibit a great load from being applied to the bottom while in use.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of a low temperature liquid tank according to the present disclosure will be described with reference to the drawings.

Note that in the drawings, a scale of each member is adequately changed such that each member has a recognizable size. Further, in the present embodiment, as the low temperature liquid tank, a liquefied natural gas (LNG) tank will be described by way of example.

FIG. 1is a sectional view schematically showing a general constitution of an LNG tank1of the present embodiment. As shown inFIG. 1, the LNG tank1of the present embodiment is a ground-type metal double shell tank, and is equipped with a base plate2, an outer tank3, a bottom cold insulating mechanism (support portion)4, an inner tank (storage tank)5, a blanket6, and a lateral cold insulation7.

The base plate2is a disc-like member formed of concrete, and supports the outer tank3, the bottom cold insulating mechanism4, the inner tank5, the blanket6, and the lateral cold insulation7. The outer tank3is a cylindrical container formed of carbon steel, and is erected on the base plate2so as to surround the bottom cold insulating mechanism4, the inner tank5, the blanket6, and the lateral cold insulation7. The bottom cold insulating mechanism4is disposed under the inner tank5inside the outer tank3, and is adapted to support the inner tank5. The bottom cold insulating mechanism4is a member equivalent to the support portion in the present disclosure, and details thereof will be described below.

The inner tank5is a cylindrical container in which LNG is stored, and is erected on the bottom cold insulating mechanism4. The inner tank5is made up of a bottom portion5aand a sidewall5bformed of nickel steel, an annular plate5cconnecting the bottom portion5aand the sidewall5b(seeFIGS. 2A and 2B), and a ceiling5dthat is formed of aluminum steel and is supported in a suspended state. The bottom portion5aof the inner tank5is formed in such a manner that a plurality of bottom plates5a1(seeFIGS. 2A and 2B) formed of nickel steel are joined. The annular plate5cis a part of the inner tank5as described above. However, in the present embodiment, the annular plate5cserves as a part of the support portion of the present disclosure. The blanket6is disposed to cover the sidewall5bof the inner tank5from the outside, has a cold insulating function, and absorbs thermal deformation of the inner tank5. The lateral cold insulation7is filled between the blanket6and the outer tank3, and is formed of, for example, perlite.

FIGS. 2A and 2Bare enlarged views of an area A ofFIG. 1. Note that it is shown inFIGS. 2A and 2Bthat each member is changed particularly in height among actual dimensions in order to emphasize a difference in the height of each member. As shown inFIGS. 2A and 2B, the bottom cold insulating mechanism4is made up of a peripheral section4adisposed under the sidewall5bof the inner tank5, and a midsection4bdisposed inside the peripheral section4a.

The peripheral section4asupports the annular plate5cof the inner tank5, is formed of concrete, and is provided along the sidewall5bof the inner tank5in an annular shape. The midsection4bis formed by a heat insulating layer4b1installed on the base plate2, and a plurality of calcium silicate boards4b2provided on the heat insulating layer4b1.

The heat insulating layer4b1is a member for preventing heat from being input to the inner tank5from the ground. In the present embodiment, the heat insulating layer4b1is formed of a rigid plastic foam, in which, unlike concrete or foam glass, creep occurs due to a load from above. To be more specific, the heat insulating layer4b1may be formed of a rigid urethane foam, a rigid polyisocyanurate foam, or a rigid polyvinyl chloride foam.

The calcium silicate boards4b2are heat-resistant boards, and upper surfaces4b3thereof serve as supporting surfaces which support the bottom plates5a1that form the bottom portion5aof the inner tank5. These calcium silicate boards4b2prevent a heat effect on the underlaid heat insulating layer4b1when the bottom plates5a1are welded to each other while the LNG tank1is under construction.

As shown inFIG. 2A, the bottom portion5a(i.e., the bottom plates5a1) of the inner tank5is in contact with an upper surface5c1of the annular plate5cat a margin of the inner tank5, and is in contact with the upper surfaces4b3of the calcium silicate boards4b2at the midsection of the inner tank5. That is, the bottom portion5aof the inner tank5is supported by the bottom cold insulating mechanism4and the annular plate5c. In the LNG tank1of the present embodiment, a structure made up of the bottom cold insulating mechanism4and the annular plate5cis referred to as a support portion10. Further, a peripheral section of the support portion10is made up of the peripheral section4aof the bottom cold insulating mechanism4and the annular plate5c, and supports the margin of the inner tank5including the sidewall5bof the inner tank5. Hereinafter, the peripheral section of the support portion10is referred to as an outer support portion11. In addition, a midsection of the support portion10is made up of the midsection4bof the bottom cold insulating mechanism4. Hereinafter, the midsection of the support portion10is referred to as an inner support portion12. That is, the LNG tank1of the present embodiment includes the outer support portion11that supports the margin of the inner tank5including the sidewall5bof the inner tank5, and the inner support portion12that is disposed inside the outer support portion11and that has the heat insulating layer4b1formed of the heat insulation in which creep occurs when a load is applied.

FIG. 2Ashows a state immediately after construction of the LNG tank1of the present embodiment is completed. As shown inFIG. 2A, in the LNG tank1of the present embodiment, an upper surface12a(i.e., the calcium silicate boards4b2) of the inner support portion12has an initial height set to be higher than a height of an upper surface11a(the upper surface5c1of the annular plate5c) of the outer support portion11. In the LNG tank1of the present embodiment, since the heat insulating layer4b1formed of rigid plastic foam is included in the bottom cold insulating mechanism4, when the heat insulating layer4b1receives a load from above due to weight of LNG stored in the inner tank5, creep occurs in the heat insulating layer4b1. For this reason, in the LNG tank1of the present embodiment, the heat insulating layer4b1is gradually compressed due to long-term use, and the upper surface12aof the inner support portion12sinks. As a result, after the service life of the LNG tank1has lapsed, the upper surface12aof the inner support portion12is, as shown inFIG. 2B, located below the upper surface11aof the outer support portion11.

Here, in the LNG tank1of the present embodiment, an extent value to which the upper surface12aof the inner support portion12sinks after the service life of the LNG tank1has lapsed is obtained through experimentation or simulation in a design step, and the initial height of the upper surface12aof the inner support portion12is set based on the obtained value so as not to affect a great effect on the bottom plates5a1. To be specific, a difference between the height of the upper surface12aof the inner support portion12and the height of the upper surface11aof the outer support portion11is obtained from an amount of sinkage of the upper surface12aof the inner support portion12. Maximum bending stress applied to the bottom plates5a1is obtained from this difference, and is compared with allowable bending stress of the bottom plates5a1(stress at which the bottom plates5a1can be estimated not to need repair during the service life of the LNG tank1). The initial height of the upper surface12ais set such that the maximum bending stress does not exceed the allowable bending stress of the bottom plates5a1. The initial height is naturally set such that the maximum bending stress applied to the bottom plates5a1by the difference between the height of the upper surface12aof the inner support portion12and the height of the upper surface11aof the outer support portion11at an initial stage does not exceed the allowable bending stress of the bottom plates5a1.

As described above, in the LNG tank1of the present embodiment, the initial height of the upper surface12aof the inner support portion12is set such that the maximum bending stress applied to the bottom plates5a1due to the difference between the height of the upper surface12aof the inner support portion12and the height of the upper surface11aof the outer support portion11during the service life of the LNG tank1remains equal to or smaller than the allowable bending stress of the bottom plates5a1. For this reason, according to the LNG tank1of the present embodiment, the difference between the height of the upper surface12aof the inner support portion12and the height of the upper surface11aof the outer support portion11does not become great enough to have an influence on the bottom plates5a1during the service life of the LNG tank1. Accordingly, according to the LNG tank1of the present embodiment, it is possible to inhibit a great load from being applied to the bottom portion5aof the inner tank5during the use of the LNG tank1.

Further, the initial height of the upper surface12aof the inner support portion12may be adjusted, for instance, by changing thicknesses of the components (i.e., in the present embodiment, the heat insulating layer4b1and the calcium silicate boards4b2) of the inner support portion12or by raising the base plate2. Also, the height of the upper surface12aof the inner support portion12may be adjusted by newly installing on the inner support portion12aheight setting plate for prescribing the height of the upper surface12a. However, since it is easy to adjust the thicknesses of the calcium silicate boards4b2, the calcium silicate boards4b2are preferably used as the height setting plate.

While a preferred embodiment of the present disclosure has been described with reference to the attached drawings, it goes without saying that the present disclosure is not limited to the above embodiment. All the shapes and combinations of the components shown in the aforementioned embodiment are only examples and can be variously modified based on design requirements without departing from the spirit and scope of the present disclosure.

For example, as shown inFIG. 3A, a constitution in which an edge11bof the outer support portion11which is adjacent to the inner support portion12is chamfered may also be employed. As a result of employing this constitution, as shown inFIG. 3B, even when the upper surface12aof the inner support portion12sinks and is located below the upper surface11aof the outer support portion11, the edge of the outer support portion11can be prevented from colliding with the bottom plates5a1and high stress can be prevented from being locally applied to the bottom plates5a1.

Also, in the above embodiment, the constitution in which the outer support portion11is made up of the peripheral section4aof the bottom cold insulating mechanism4and the annular plate5c, the bottom plates5a1are supported by the upper surface of the annular plate5c, and the annular plate5cand each bottom plate5a1overlap and are welded together is employed. However, the present disclosure is not limited to this constitution. For example, a constitution in which the bottom plates5a1are directly supported by the upper surface of the peripheral section4aof the bottom cold insulating mechanism4and each bottom plate5a1and the annular plate5care butted and welded may also be employed.

In this case, the bottom plates5a1are supported by the upper surface of the peripheral section4aof the bottom cold insulating mechanism4. For this reason, the outer support portion is configured of only the peripheral section4aof the bottom cold insulating mechanism4, and the upper surface of the peripheral section4abecomes the upper surface of the outer support portion.

Also, in the above embodiment, the constitution in which the inner support portion12is made up of the heat insulating layer4b1formed of the rigid urethane foam and the calcium silicate boards4b2is employed. However, the present disclosure is not limited to this constitution, and the inner support portion12may also have a different structure. For example, a constitution in which a second heat insulating layer formed of, for example, foam glass is included in the inner support portion12may be employed. Also, foam glass may be disposed at an upper layer, and the calcium silicate boards4b2may be removed. When the structure of the inner support portion12is changed, the component having a surface supporting the bottom plates5a1is also modified.

Also, in the above embodiment, the constitution in which the heat insulating layer4b1is formed of the rigid urethane foam has been described. However, the heat insulating layer is not limited to the rigid urethane foam, and any foamed plastic may be used as the heat insulating layer.

Also, in the above embodiment, the example in which the low temperature liquid tank of the present disclosure is applied to the LNG tank1has been described. However, the low temperature liquid tank of the present disclosure may also be applied to an LPG tank or other low temperature liquid tanks.

In addition, in the present disclosure, the initial height of the upper surface of the inner support portion is not necessarily higher than that of the upper surface of the outer support portion. For example, the initial height of the upper surface of the inner support portion may be flush with that of the upper surface of the outer support portion.

INDUSTRIAL APPLICABILITY

The low temperature liquid tank can inhibit a great load from being applied to the bottom portion while in use.

REFERENCE SIGNS LIST