JPH0571840B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for preventing freezing of the ground surrounding a low-temperature underground tank for storing low-temperature substances such as liquefied natural gas.

<Conventional technology> If no measures are taken to prevent freezing,
For example, if liquefied natural gas is stored in an underground tank 50 meters in diameter, the surrounding ground will freeze over an area of 100 meters in diameter. The frozen ground puts pressure on the outer walls of the tank, causing cracks in the tank. For this reason, for example, as disclosed in Japanese Patent Application Laid-Open No. 54-119120, a heat fence consisting of buried pipes for heat medium such as hot water is installed to surround a low-temperature underground tank. is circulated to warm the surrounding ground and prevent it from freezing.

<Problems to be Solved by the Invention> Conventionally, heat fences have been vertically disposed along the vertical outer wall of an underground tank, and the distance between the tank outer wall and the heat fence has been constant. When the surrounding ground was heated with this heat fence, the temperature of the tank outer wall remained almost constant; instead, the temperature of the tank outer wall became lower as the depth went deeper. There are three possible reasons for this:

The closer the tank is to the ground, the greater the influence of outside temperature, and the shallower the tank, the higher the temperature on the outer wall of the tank.

The insulation attached to the outer wall of the tank needs to have higher strength toward the bottom, so materials with higher density are used for this purpose. If this happens, the lower the tank, the worse the insulation, and the cold heat inside the tank will easily reach the surrounding area. As a result, the temperature of the tank outer wall becomes lower as the depth increases.

Although it depends on the structure of the heat fence, the surface temperature of the heat fence becomes higher at the top, and the amount of heat reaching the outer wall of the tank increases at shallower parts. As a result, the shallower the temperature of the tank outer wall, the higher it becomes.

The synergistic effect of the above causes causes a temperature difference in the depth direction of the outer wall of the tank, which leads to the following problems.

Of course, widespread freezing of the ground around tanks is harmful. However, it is desirable for the outer wall of the tank to be kept below the freezing point (frozen) in order to maintain the liquid-tightness and airtightness of the tank. Therefore, even if the surrounding ground is prevented from freezing by heating with the above-mentioned heat fence, it is necessary to keep the overall temperature of the tank outer wall below the freezing point and to prevent the frozen state from spreading around the tank outer wall. is desirable.

However, in the conventional technology, as described above, a temperature difference occurs in which the temperature of the outer wall of the tank decreases as the depth increases. Therefore, if settings are made to keep the outer surface temperature of the tank outer wall at the freezing point at the deepest point, the outer surface temperature of the tank outer wall will be higher than the freezing point at shallower parts, making it impossible to maintain the wall in a frozen state, resulting in poor liquid-tightness and air-tightness. Sexuality becomes worse. Therefore, if the wall is set to completely freeze even in shallow areas, the frozen state will spread to the surrounding ground in deeper areas.

Furthermore, if the temperature difference in the depth direction of the outer wall of the tank is large, the temperature stress on the wall will also be large, which will cause problems in terms of the strength of the tank.

This invention was made in view of the above-mentioned conventional problems, and its purpose is to provide a method for preventing freezing of the ground surrounding a low-temperature underground tank, which makes it possible to reduce the temperature difference in the depth direction of the tank outer wall. It's about doing.

<Means for Solving the Problems> Therefore, in the present invention, the distance between the heat fence and the outer wall of the tank is made smaller as the depth increases, or the flow velocity of the heat medium flowing through the heat fence is made smaller as the depth becomes larger. Make it smaller,
The greater the depth, the greater the amount of heat transferred from the heat fence to the outer wall of the tank, thereby reducing the temperature difference in the depth direction of the outer wall of the tank.

<Function> In a shallow portion, the distance between the heat fence and the outer wall of the tank is large, or the flow velocity of the heat medium is large, so the amount of heat transmitted to the outer wall is small. On the other hand, in a deep part, the distance is short or the flow rate of the heat medium is low, so the amount of heat reaching the outer wall of the tank from the heat fence is large. As a result, the temperature difference in the depth direction of the outer wall of the tank becomes smaller.

<Embodiment> FIG. 1 shows a first embodiment of the present invention. A heat fence is constructed by burying a large number of rod-shaped heaters 2 in the vertical direction so as to surround the circular outer wall of a tank 1 provided underground. The rod-shaped heater 2 has an inner tube 2a with an open bottom end and an outer tube 2 with a closed bottom end.
The hot water is supplied from the upper part of the inner pipe 2a, the hot water fills the outer pipe 2b, and the hot water exits from the upper part of the outer wall 2b. A large number of rod-shaped heaters 2 are connected by connecting pipes 3, 3, and are connected in parallel to equipment for a pump for circulating hot water and a heater for heating hot water (not shown). Note that the heater 2 may be connected in series with the equipment.

A feature of this embodiment is that the rod-shaped heater 2 is buried at a slight angle to the vertical outer wall of the tank 1. At the lower end of the rod-shaped heater 2, the distance from the outer wall of the tank 1 is about 1 m, and near the ground, the distance between the rod-shaped heater 2 and the outer wall of the tank 1 is about 3 m.
It's becoming m. When hot water is circulated through this rod-shaped heater 2, heat is transmitted to the surrounding area.In the deep part, since the rod-shaped heater 2 and the tank 1 are close to each other, more heat is transmitted, and in the shallow part, the rod-shaped heater Relatively little heat is transferred from tank 2 to tank 1. As a result, the temperature difference in the depth direction due to the above-mentioned causes is corrected, and the temperature of the outer wall of the tank 1 becomes almost constant regardless of the depth. By appropriately setting the spacing between the rod-shaped heaters 2 and the temperature of the hot water circulated there, the surface temperature of the outer wall of the tank 1 can be maintained at approximately the freezing point regardless of the depth, and the temperature around the tank can be kept at about the freezing point. It is possible to create a situation that prevents the situation from spreading.

FIG. 2A shows a second embodiment of the invention.
In this embodiment, the rod-shaped heater 2 is connected to the tank 1.
The diameter of the outer tube 2b of the rod-shaped heater 2 increases in three steps from top to bottom. Therefore, the distance between the outer tube 2b and the tank 1 is reduced in three steps from top to bottom. In addition, the capacity of the outer tube 2b (which is also the heat capacity when filled with hot water) increases toward the lower part, and more heat reaches the lower outer wall of the tank 1 than the upper part, similar to the first embodiment described above. It is possible to prevent the frozen state from spreading to the surrounding ground, reduce the temperature difference in the depth direction of the tank outer wall, and maintain the temperature of the outer surface at approximately the freezing point.

In addition, in this embodiment, the thickness of the outer tube 2b increases in three steps from top to bottom, so the flow rate of hot water discharged from the inner tube 2a decreases as the depth increases, so each rod-shaped heater The amount of heat reaching the outer wall of the tank 1 from the tank 1 increases as the depth increases, and the same effect as in the first embodiment can also be obtained.

Based on this idea, as shown in Figure 2 A,
Without changing the distance between the outer tube 2b and the tank 1,
As shown in FIG. 2B, the heaters 2 may be installed at constant intervals.

FIG. 3 shows a fourth embodiment of the invention. Here, a continuous hot water pipe 4 is arranged in a spiral shape surrounding the tank 1 to form a heat fence, and hot water is circulated through the hot water pipe 4 from a pump/heater equipment 5. As shown here,
The winding diameter of the hot water pipe 4 is smaller in the deeper part and larger in the shallower part. Therefore, in the deep part, the distance between the outer wall of the tank 1 and the hot water pipe 4 is small,
This interval is large in shallow parts. As a result, the same effects as the first and second embodiments described above are achieved.

<Effects of the Invention> As explained in detail above, in the method for preventing freezing of the ground around a low-temperature underground tank according to the present invention, the distance between the heat fence buried so as to surround the underground tank and the outer wall of the tank is reduced to a depth. The larger the temperature, the smaller the temperature, and the larger the depth, the more heat is transferred from the heat fence to the outer wall of the tank, thereby reducing the temperature difference in the depth direction of the outer wall of the tank. It is possible to easily create the desired warming conditions to keep the wall at the freezing point, maintain liquid and airtightness by keeping the wall frozen, and prevent the frozen condition from spreading to the surrounding soil. can.

The flow velocity of the heat medium flowing through the heat fence is made smaller as the depth increases, so that the larger the depth, the more heat is transferred from the heat fence to the outer wall of the tank, in the depth direction of the tank outer wall. A similar effect can be obtained by reducing the temperature difference.

[Brief explanation of the drawing]

1A and 1B are a plan view and a vertical cross-sectional view of essential parts showing a first embodiment of the present invention, FIG. 2 is a schematic diagram showing the second and third embodiments, and FIG. 3 is a fourth embodiment. FIG. 2 is a schematic diagram illustrating an example. 1... Low temperature underground tank, 2... Rod heater, 2
a...Inner pipe, 2b...Outer pipe, 3...Connecting pipe, 4...
…Hot water pipes, 5…Pump/heater equipment.

Claims (1)

[Scope of Claims] 1. A heat fence including heat medium pipes buried so as to surround a low-temperature underground tank is provided,
In this method of heating the surrounding ground with a heat fence to prevent freezing, the distance between the heat fence and the outer wall of the tank is made smaller as the depth increases, and the distance from the heat fence to the outer wall of the tank increases as the depth increases. Increase the amount of heat,
A method for preventing freezing of the ground around a low-temperature underground tank, characterized by reducing the temperature difference in the depth direction of the outer wall of the tank. 2. Install a heat fence consisting of buried heat medium piping to surround the low-temperature underground tank.
In this method of heating the surrounding ground using a heat fence to prevent freezing, the flow velocity of the heat medium flowing in the heat fence is decreased as the depth increases, and as the depth increases, the flow rate of the heat medium flowing through the heat fence is increased from the heat fence to the outer wall of the tank. A method for preventing freezing of the ground surrounding a low-temperature underground tank, characterized by increasing the amount of heat reaching the tank and reducing the temperature difference in the depth direction of the outer wall of the tank.