JP2019142542A - tank - Google Patents

Abstract

To provide a tank that can reduce the amount of tendon in the lengthwise direction.SOLUTION: An LNG tank has a dike of cylindrical shape and an inner tank is provided inside. Pre-stress is introduced on the dike through tension of the PC steel material in the circumferential direction and the PC steel material in the vertical direction. The PC steel material in the vertical direction, when the height direction of the dike and the section in the thickness direction are viewed, is arranged in a curved line from the lower end of the dike all the way up the entire height of the dike, the distribution in the height direction of the vertical in-plane bending moment M apostrophe that occurs in the dike when the LNG leaks is opposite in signals to the distribution in the height direction of the vertical in-plane bending moment M that constantly occurs in the dike, and the absolute values are equal.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a tank.

  Ground tanks are sometimes used as facilities for storing cryogenic liquids such as LNG (liquefied natural gas) and LPG (liquefied petroleum gas).

  FIG. 9 shows an example of an LNG tank 100 that stores LNG as an above-ground tank. The LNG tank 100 is a tank housing by fixing the liquid breakwater 2 on the bottom slab 5 supported by the pile 4 in the ground 7 and having an inner tank 3a and an outer tank 3b made of steel plates or the like inside. . The roof portion of the outer tub 3b is made of steel or concrete, and the side steel plates are formed integrally with the liquid barrier 2. LNG is stored in the inner tank 3a, and a heat insulating material is disposed between the inner tank 3a and the outer tank 3b to perform cold insulation.

  The breakwater 2 is a concrete cylindrical side wall provided to prevent leakage of LNG to the outside when the inner tank 3a is damaged, and is usually cylindrical. The breakwater 2 needs to have a structure capable of withstanding the hydraulic pressure of LNG at the time of leakage, and is therefore rigidly coupled to the bottom slab 5 and tension in a tank circumferential direction (hereinafter sometimes simply referred to as circumferential direction) not shown. Prestress due to the tension of the material and the longitudinal tension material is introduced to generate a predetermined compressive stress in the concrete (for example, see Patent Documents 1 and 2).

  The breakwater 2 is pre-stressed by a circumferential tension material and a longitudinal tension material at all times other than during leakage, and in addition to this, the LNG liquid load and Temperature load acts. The breakwater 2 is always required to be liquid-tight and air-tight at the time of leakage, and in either state, a region (compressive stress region) where compressive stress is generated in the circumferential direction and the vertical direction of the member. By leaving it, liquid-tightness and air-tightness are secured.

  According to the standard (LNG ground storage tank guidelines), the compressive stress area should be 10 cm or more in the thickness direction of the breakwater 2. The thicker the compressive stress area, the better the liquid and air tightness. ing. Regarding the stress in the circumferential direction, usually, a large amount of circumferential tension material is disposed on the breakwater 2 so that a compressive stress is always generated in the entire thickness of the breakwater 2 even when leaking (full cross-section compression). The problem is the stress in the vertical direction.

  FIG. 10 is a view showing a cross section of the liquid barrier 2 along the height direction and the thickness direction. A circumferential PC steel material 11 and a longitudinal PC steel material 13 (13a, 13b) are disposed in the breakwater 2. The circumferential direction PC steel material 11 is a tension material in the circumferential direction of the breakwater 2 and is arranged over the entire circumference of the breakwater 2. The longitudinal PC steel material 13 is a tension material in the height direction of the breakwater 2, the longitudinal PC steel material 13 a is arranged over the entire height of the breakwater 2, and the longitudinal PC steel material 13 b is the lower part of the breakwater 2. It arrange | positions at the haunch part 21 provided in the outer side.

  As described above, since a large amount of the circumferential PC steel material 11 is arranged in the breakwater 2, a large outside tension vertical in-plane bending moment (breakwater 2 is placed inside the breakwater 2 at the bottom of the breakwater 2 due to the influence. Bending moment) is always generated. Therefore, the amount of PC steel in the lower part of the breakwater 2 is increased by the longitudinal PC steel materials 13a and 13b to cancel the tensile stress due to the bending moment.

  FIG. 11 shows the normal vertical in-plane bending moment M of the breakwater 2 and the vertical in-plane bending moment at the time of leakage when the vertical in-plane bending moment by the longitudinal PC steel 13 (13a, 13b) is zero. It is a figure which shows M 'as a height from the lower end of the breakwater 2 on a vertical axis | shaft value of a bending moment and a horizontal axis | shaft. The bending moment is a bending moment per unit length in the circumferential direction of the breakwater 2 and is negative when an outward tensile bending moment is generated in the breakwater 2, and an outward compression bending moment (liquidproof) The case where a bending moment (which causes the bank 2 to be bent outward) occurs is positive.

  As described above, a large outside tension vertical in-plane bending moment M is always generated in the lower part of the breakwater 2. On the other hand, a vertical in-plane bending moment M ′ of outer compression occurs at the bottom of the breakwater 2 at the time of leakage, but the former bending moment M is much larger (absolute value) than the latter bending moment M ′. Become.

Patent No. 569662 Japanese Patent No.4847994

  At present, there is a need for a method that can reduce the longitudinal tension material by introducing prestress into the breakwater in a more efficient and rational manner. In this respect, when the vertical in-plane bending moments M and M ′ generated in the breakwater 2 at the time of liquid leakage are always unbalanced as shown in FIG. An amount of the longitudinal PC steel material 13 corresponding to the larger bending moment M is required, and waste is increased.

  The present invention has been made in view of the above problems, and an object thereof is to provide a tank or the like that can reduce the amount of tension material in the vertical direction.

  The present invention for solving the above-mentioned problem is a tank having a cylindrical side wall, wherein pre-stress due to tension of a circumferential tension material and a longitudinal tension material is introduced into the side wall, The tension material in the direction is arranged in a curved line over a predetermined range in the height direction from the lower end of the side wall when the cross section in the height direction and the thickness direction of the side wall is viewed. is there.

  The tank has an inner tank that stores liquid inside the side wall, and the longitudinal tension material has a distribution in a height direction of a bending moment in a vertical plane generated in the side wall in the predetermined range, The liquid moments are arranged so that the directions of the bending moments are opposite and the absolute values are equal at the time of liquid leakage and at all times other than the time of liquid leakage.

  In addition, it is preferable that a haunch portion is provided inside the lower portion of the side wall.

  The predetermined range is the lowest inflection point of the distribution in the height direction of the bending moment in the vertical plane generated in the side wall by the prestress by the circumferential tension material from the lower end of the side wall. It is desirable to include a range up to the height of the inflection point. Or the range from the lower end of the said side wall to the height of the 2nd inflection point which is the said inflection point just above the said 1st inflection point may be included. Furthermore, it may be a range extending over the entire height of the side wall.

  In the present invention, the vertical in-plane bending moment at the time of liquid leakage and the sign (bending moment) can be easily obtained by arranging the longitudinal tension members in a curved line on the side wall of the LNG tank such as a breakwater. Direction) and opposite values can be made equal. Thereby, it is possible to leave the compressive stress region on the side wall with the least amount of PC steel in the longitudinal direction with the most rational arrangement of the longitudinal tension members in order to leave the compressive stress region on the side wall.

  Further, in the present invention, the absolute value of the vertical in-plane bending moment is increased by the prestress due to the circumferential tension material by providing a haunch inside the lower portion of the side wall to increase the amount of eccentricity of the longitudinal tension material to the outside. Even at the lower side of the side wall, the bending moment is reduced, and it becomes easy to make the vertical in-plane bending moment at the time of normal and liquid leakage opposite in sign and equal in absolute value.

  The range of the curved arrangement of the tension members in the vertical direction only needs to include a range in which the vertical in-plane bending moment generated in the side wall due to the prestress by the tension material in the circumferential direction increases, for example, from the lower end of the side wall. The range up to the inflection point and the second inflection point can be included. Or it is good also as a curvilinear arrangement over the full height of a side wall.

  According to the present invention, it is possible to provide a tank or the like that can reduce the amount of tension material in the vertical direction.

The figure which shows the LNG tank 1. FIG. The figure which shows the breakwater 2a. The figure which shows the vertical in-plane bending moment M1 of the breakwater 2a. The figure which shows the vertical in-plane bending moments M and M 'of the breakwater 2a. The figure which shows the vertical in-plane bending moment M1-M4 of the breakwater 2a. The figure which shows eccentric amount d of the longitudinal direction PC steel material 15. FIG. The figure which shows the stress degree distribution of the thickness direction of the breakwater 2a by the vertical in-plane bending moments M and M '. The figure shown about the comparison of no eccentricity, curvilinear eccentric arrangement, and linear eccentric arrangement. The figure which shows the LNG tank 100. FIG. The figure which shows the breakwater 2. The figure which shows the vertical in-plane bending moments M and M 'of the breakwater 2.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

(1. LNG tank 1)
FIG. 1 is a diagram showing an LNG tank 1 according to an embodiment of the present invention. The LNG tank 1 is basically the same as the LNG tank 100 described with reference to FIG. 9 and the like, and the same components as those described with reference to FIG.

  The LNG tank 1 of the present embodiment differs from the LNG tank 100 described with reference to FIG. Similarly to the above, the liquid breakwater 2a is a concrete cylindrical side wall rigidly coupled to the bottom plate 5 and is cylindrical in this embodiment. An inner tank 3a and an outer tank 3b are disposed inside the liquid breakwater 2a, and the liquid breakwater 2a prevents LNG from leaking to the outside when the inner tank 3a is damaged.

(2. Breakwater 2a)
FIG. 2 is a diagram schematically showing the breakwater 2a, and is a view of a cross section along the height direction and the thickness direction of the breakwater 2a.

  A circumferential PC steel material 11 and a longitudinal PC steel material 15 (15a, 15b) are disposed on the breakwater 2a, and prestress (compression force) due to tension of these PC steel materials is introduced. A PC steel wire or the like is used for each PC steel, and these are arranged in a sheath (not shown) embedded in the liquid barrier 2a or the like. After the tension of the PC steel material, the sheath is filled (grouted) with a solidified material such as mortar. However, there are cases where unbonded PC steel is used as the PC steel and a sheath is not required. In addition, although the reinforcing bar (not shown) is also embed | buried in the breakwater 2a, description is abbreviate | omitted here.

  The circumferential PC steel material 11 is a tension material in the circumferential direction of the breakwater 2a (corresponding to the normal direction of the paper surface of FIG. 2), and is arranged over the entire circumference of the breakwater 2a. Both ends of the circumferential PC steel material 11 are fixed by outward projecting portions (pilasters) provided at predetermined positions in the circumferential direction of the liquid barrier 2a.

  The longitudinal PC steel material 15 (15a, 15b) is a tension material in the height direction of the liquid barrier 2a. The longitudinal direction PC steel material 15a is arranged over the entire height of the liquid barrier 2a, the lower end is fixed in the bottom plate 5, and the upper end is fixed at the top of the liquid barrier 2a. The longitudinal PC steel material 15b is disposed at the lower part of the liquid breakwater 2a, the lower end is fixed in the bottom plate 5, and the upper end is fixed in the middle of the liquid barrier 2a in the height direction.

  The longitudinal PC steel material 15a is meanderingly arranged and has a curved shape over almost the entire height. In the present embodiment, the longitudinal PC steel materials 15a are arranged in a curved shape with a plurality of curvatures.

  The longitudinal PC steel material 15b is meandering in the same shape as that of the longitudinal PC steel material 15a in the same height range, and has a curved shape with a single or a plurality of curvatures. The longitudinal PC steel material 15b can be omitted.

  At the lower part of the breakwater 2a, the hunch portion 21a is provided so as to project inside the breakwater 2a. The haunch portion 21a of the present embodiment has a right-angled triangular cross section, but may have a rectangular cross section.

  FIG. 3 shows an example of the vertical in-plane bending moment M1 of the breakwater 2a caused by prestress due to the tension of the circumferential PC steel material 11, the vertical axis is the value of the bending moment, and the horizontal axis is from the lower end of the breakwater 2a. It is a figure shown as height. As described above, the bending moment is the bending moment per unit length in the circumferential direction of the breakwater 2a, and negative when the outer tensile bending moment is generated in the breakwater 2a, and the outer compression bend is applied to the breakwater 2a. The case where a moment occurs is assumed to be positive. The same applies to FIGS. 4 and 5 described later.

  The height range b (see FIG. 2) of the haunch portion 21a of the present embodiment is a range from the lower end of the liquid barrier 2a to the first code change point shown in FIG. This first sign change point indicates the height at which the sign (the direction of the bending moment) of the vertical in-plane bending moment M1 is reversed first. However, the height range b of the haunch portion 21a is not limited to this.

(3. Eccentric arrangement of longitudinal PC steel 15)
As shown in FIG. 4, the vertical PC steel material 15 in the breakwater 2a is arranged at a height of the vertical in-plane bending moment M that occurs at all times other than at the time of leakage over the entire height of the breakwater 2a. It is determined that the sign of the distribution in the direction and the distribution in the height direction of the vertical in-plane bending moment M ′ generated at the time of leakage are opposite and the absolute values are equal. That is, the distribution shape in the height direction of the vertical in-plane bending moment M and the distribution shape in the height direction of the vertical in-plane bending moment M ′ are made symmetrical about the line of the bending moment 0.

  At this time, the vertical in-plane bending moment that needs to be generated in the breakwater 2a by the prestress due to the tension of the longitudinal PC steel material 15 is M2 in FIG. 5 (a). And the distribution of bending moments M and M ′ as shown in FIG. 4 is realized.

  FIG. 5B shows the vertical in-plane bending moment M3 generated in the breakwater 2a due to the liquid load (hydraulic pressure) when LNG leaks and the breakwater 2a due to the temperature load when LNG leaks. It is a figure which shows the perpendicular | vertical in-plane bending moment M4 which arises. As for the temperature load, when the temperature of the inner surface of the breakwater 2a is lowered by the extremely low temperature LNG when the LNG leaks, the concrete of the inner surface of the breakwater 2a is shrunk to the bottom plate 5 and other parts of the breakwater 2a. Is a load generated by restraining.

  The normal vertical in-plane bending moment M shown in FIG. 4 is expressed as the sum of the vertical in-plane bending moments M1 and M2, and the vertical in-plane bending moment M ′ at the time of leakage is the sum of the vertical in-plane bending moments M1 to M4. It has become. The vertical in-plane bending moments M and M ′ may be added with the in-vertical in-plane bending moment due to the weight of the roof portion of the breakwater 2a or the outer tub 3b, but the value is small and there is almost no influence. Also, there is almost no difference in temperature load depending on the season.

  In the present embodiment, the longitudinal PC steel material 15 is eccentric from the center a (see FIG. 2) in the thickness direction of the breakwater 2a so that a vertical in-plane bending moment M2 as shown in FIG. And place it. FIG. 6 is a diagram showing the eccentricity d (see FIG. 2) of the longitudinal PC steel material 15 required at this time, with the vertical axis indicating the eccentricity d and the horizontal axis indicating the height from the lower end of the breakwater 2a. It is. The amount of eccentricity d is positive when eccentric from the center a of the breakwater 2a to the outside, and negative when eccentric from the center.

  The vertical in-plane bending moment M2 is determined by the product of the amount of prestress due to the tension of the longitudinal PC steel material 15 and the amount of eccentricity d. Since the magnitude of the prestress is constant at each height of the longitudinal PC steel material 15, the eccentricity d is proportional to the vertical in-plane bending moment M2. Therefore, the distribution shape in the height direction of the eccentricity d is basically a shape along (proportional to) the distribution shape in the height direction of the vertical in-plane bending moment M2. Therefore, in the present embodiment, the longitudinal PC steel material 15 is arranged to meander in a curved shape in accordance with the distribution shape in the height direction of the vertical in-plane bending moment M2.

  In the graph of FIG. 6, the eccentricity d in the range from the lower end of the breakwater 2a to the height of 10 m is deviated from the eccentricity d in the higher height in this range. This is because the steel material 15b is additionally arranged. That is, by adding the longitudinal PC steel material 15b, the prestress by the longitudinal PC steel material 15b is added to the prestress by the longitudinal PC steel material 15a, resulting in a large longitudinal prestress. Is possible. For example, when a plurality (n) of longitudinal PC steel materials per unit length in the circumferential direction of the breakwater 2a are arranged in a similar curvilinear arrangement, the absolute value of the eccentricity d per one is 1 / In the graph of FIG. 6, the absolute value of the eccentricity d in the range from the lower end of the breakwater 2a to the height of 10 m is small.

  Here, n = 3, and the eccentricity d in the range from the lower end of the breakwater 2a to the height of 10 m in the graph of FIG. 6 is 1/3 when the longitudinal PC steel material 15b is not added. ing.

  The eccentric arrangement of the longitudinal PC steel material 15 can be performed, for example, by placing concrete of the breakwater 2a in a state where the sheath is fixed to a cradle (not shown) and then inserting the PC steel material into the sheath. When using an unbonded PC steel material, the concrete of the breakwater 2a may be placed in a state where the unbonded PC steel material is directly fixed to the cradle. The tension of the longitudinal direction PC steel material 15 is performed after the tension of the circumferential direction PC steel material 11.

  The breakwater 2a may be constructed in a state of being rigidly coupled to the bottom plate 5 from the beginning of construction, or the breakwater 2a can move relative to the bottom plate 5 when the circumferential PC steel 11 is tensioned by a mechanism such as a pin / slide. In this way, the longitudinal PC steel material 15 may be tensioned after the breakwater 2a and the bottom plate 5 are rigidly connected. The amount of eccentricity of the longitudinal PC steel material 15 varies slightly depending on the difference in these construction methods. In each construction method, the signs of the above-mentioned vertical in-plane bending moments M and M ′ are opposite and the absolute values are equal. It is determined to be.

  As described above, in the present embodiment, the vertical PC steel material 15 is eccentrically arranged in a curved shape in the breakwater 2a of the LNG tank 1, so that the vertical in-plane bending moment M at the time of leakage always easily. , M ′ can be opposite in sign and equal in absolute value. As a result, in order to leave the compressive stress region in the breakwater 2a, the longitudinal PC steel 15 is arranged in the most rational manner, and the amount of the longitudinal PC steel is increased to the larger bending moment M as described with reference to FIG. There is no need to design together. As a result, it becomes possible to leave a compressive stress region in the liquid breakwater 2a with the minimum amount of PC steel in the longitudinal direction, and waste is reduced.

  That is, in this embodiment, the vertical in-plane bending moments M and M ′ at the time of leakage and the liquid leakage are opposite in sign and equal in absolute value. When considering the stress distribution in the thickness direction in the liquid barrier 2a when liquid, the signs are opposite as shown in FIG. Therefore, in order to always compress the stress in the vertical direction to the entire cross section even at the time of leakage, the longitudinal PC steel material 15 applies prestress that only cancels the maximum tensile stress c generated by the vertical in-plane bending moments M and M ′. It only has to be added.

  On the other hand, when the vertical in-plane bending moments M and M ′ are unbalanced as shown in FIG. 11, the stress distribution in the thickness direction in the breakwater is unbalanced at all times and at the time of leakage as shown in FIG. In order to keep the balance in the vertical direction and always compress the entire cross-section at the time of leakage, pre-stress is applied by the longitudinal PC steel to cancel the maximum tensile stress c 'caused by the larger bending moment M. This increases the amount of PC steel required.

For example, as shown in Fig. 8 (a), in the LNG tank with a capacity of 230,000 KL, the range from the lower end of the 45m high breakwater A to the 5.4m height is the haunch part B, and the thickness of the lower end of the breakwater A is Is 1.0 m, the thickness above the haunch B is 0.65 m, the inner diameter of the breakwater A is 86 m, and the longitudinal PC steel material C is 1. No eccentricity 2. Curved eccentric arrangement (Make sure that the vertical in-plane bending moments M and M 'are always opposite to each other and have the same absolute value)
3. Linear eccentric arrangement (inclination along the outer surface of the haunch B)
Suppose that In this example, the haunch portion B has a right triangle shape and is provided outside the liquid breakwater A.

  In these cases, the magnitude of the prestress necessary for the vertical stress at the time of the leak and the leakage in the range of the haunch portion B to be the entire cross-sectional compression, the normal time when the prestress is applied, and the time of leakage FIG. 8B shows the values of the vertical in-plane bending moments M and M ′ at the lower end of the liquid barrier A.

  As shown in FIG. 8 (b), the magnitude of the prestress for compressing the entire cross section at all times and at the time of leakage (proportional to the amount of PC steel of the longitudinal PC steel C) is 2. In this case, it becomes 4044kN. 68% compared to the previous case (5979kN). It is about 82% smaller than the case (4903kN).

  Thus, the curvilinear eccentric arrangement of the present embodiment has an effect of reducing the amount of PC steel material not only when there is no eccentricity but also when compared with a linear eccentric arrangement. This can be arranged along the distribution shape of the above-mentioned vertical in-plane bending moment M2 by arranging the longitudinal PC steel material C in a curvilinear arrangement, which is wasteful compared to the case of linear eccentric arrangement. This is because it can be reduced. In practice, there may be a design in which a certain amount of tensile stress region is left in the thickness direction of the breakwater.

  Further, in the present embodiment, the absolute value of the vertical in-plane bending moment M1 is increased by providing the hunch 21a inside the lower part of the breakwater 2a to increase the amount of eccentricity to the outside of the longitudinal PC steel material 15. Even in the lower part of the liquid breakwater 2a, the bending moment M1 can be reduced, and the vertical in-plane bending moments M and M ′ at the time of leakage and liquid leakage can easily be made opposite to each other and have the same absolute value.

  However, the present invention is not limited to this. For example, in this embodiment, the longitudinal PC steel material 15 is arranged in a curved shape over the entire height of the breakwater 2a, and the eccentric amount d is always within the vertical plane at the time of leakage along the entire height of the breakwater 2a. The bending moments M and M ′ are opposite in sign and equal in absolute value, but the range (predetermined range) arranged in a curved line is not limited to this.

  For example, as shown in FIG. 3, the range in which the vertical in-plane bending moment M1 increases is the inflection point at the lowest end in the distribution in the height direction of the vertical in-plane bending moment M1 from the lower end of the breakwater 2a. Since it is considered to be the range from the first inflection point or the second inflection point immediately above it, the range arranged in a curved line includes at least the range from the lower end of the breakwater 2a to the first inflection point. Or the range from the lower end of the breakwater 2a to the said 2nd inflection point should just be included. By arranging the longitudinal PC steel material 15 in a curved shape with a single or multiple curvatures in these ranges, the vertical in-plane bending moments M and M ′ at the time of leakage and in the range are always opposite in sign and absolute. The effect of reducing the amount of the longitudinal PC steel material 15 is obtained with the same value.

  In the present embodiment, the example of the breakwater 2a of the LNG tank 1 has been described. However, the present invention can be applied to any tank having a cylindrical side wall for introducing prestress, such as LPG and water. It can also be applied to ground tanks that store liquids.

  Further, the present invention can be applied not only to constructing the breakwater 2a with concrete cast on the spot but also to constructing the breakwater 2a by stacking precast blocks vertically and horizontally.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. It will be apparent to those skilled in the art that various changes or modifications can be conceived within the scope of the technical idea disclosed in the present application, and these are naturally within the technical scope of the present invention. Understood.

DESCRIPTION OF SYMBOLS 1,100: LNG tank 2, 2a: Breakwater 3a: Inner tank 3b: Outer tank 4: Pile 5: Bottom plate 7: Ground 11: Circumferential PC steel materials 13, 13a, 13b, 15, 15a, 15b: Longitudinal direction PC steel materials 21 and 21a: Haunch part

Claims (6)

A tank having a cylindrical side wall,
In the side wall, prestress due to tension of a circumferential tension material and a longitudinal tension material is introduced,
The longitudinal tension member is arranged in a curved shape from a lower end of the side wall to a predetermined range in the height direction when a cross section in the height direction and the thickness direction of the side wall is viewed. tank. The tank has an inner tank for storing liquid inside the side wall,
The longitudinal tension material is in the predetermined range,
The distribution in the height direction of the bending moment in the vertical plane generated on the side wall is such that the direction of the bending moment is opposite between the time of leakage of the liquid and the normal time other than at the time of leakage, and the absolute values are equal. The tank according to claim 1, wherein the tank is disposed in the tank.   The tank according to claim 1, wherein a haunch portion is provided inside the lower portion of the side wall. The predetermined range is
From the lower end of the side wall, the inflection point of the distribution in the height direction of the bending moment in the vertical plane generated in the side wall due to the prestress by the tension material in the circumferential direction and the height of the lowest first inflection point The tank according to any one of claims 1 to 3, wherein the tank includes the above range. The predetermined range is
The tank according to claim 4, including a range from a lower end of the side wall to a height of a second inflection point that is the inflection point immediately above the first inflection point.   The tank according to any one of claims 1 to 5, wherein the predetermined range is a range over the entire height of the side wall.