KR100422186B1 - Oxygen blowing lance for the vacuum refining apparatus - Google Patents

Abstract

The present invention relates to a RH vacuum degassing apparatus including a ladle in which molten steel is stored, a vacuum tank positioned above the ladle, and a deposition tube forming a circulation flow path of the molten steel between the ladle and the vacuum chamber. An oxygen injection lance which penetrates the side wall of the vacuum chamber and supplies oxygen to be supplied at a predetermined inclination angle, the oxygen supply tube which blows oxygen into the vacuum chamber, surrounds an outer surface of the oxygen supply tube, and is disposed inside the vacuum chamber. A tubular body extending at a predetermined length from the tip end of the exposed oxygen supply pipe toward the outside of the vacuum chamber and sealed at both ends, and in a sealed storage space formed between the inner surface of the tubular body and the outer surface of the oxygen supply pipe. The volatile liquid material is stored, the volatile liquid material is vaporized by the temperature inside the vacuum chamber Since the vaporized gaseous material is condensed by the temperature outside the vacuum chamber, the tip of the oxygen injection lance does not decrease the degree of vacuum of the vacuum chamber of the RH vacuum degassing apparatus and there is no risk of explosion due to the leakage of cooling water. Can be effective cooling.

Description

Oxygen blowing lance for the vacuum refining apparatus}

The present invention relates to the oxygen injection lance of the RH vacuum degassing refinery for refining molten steel in the furnace refining process of the steelmaking process to produce ultra low carbon steel, and more specifically to decarburize the molten steel in the RH vacuum degassing refinery. In order to prevent the oxygen injection lance for injecting oxygen into the molten steel without damaging the degree of vacuum in the vacuum chamber without being damaged or melted by the temperature of the molten steel is installed on the side wall of the vacuum chamber of the RH vacuum degassing refinery device An oxygen injection lance for a vacuum degassing refinery device having cooling means capable of cooling.

In general, the RH vacuum degassing and refining apparatus 10 used to produce ultra low carbon steel having a carbon content of 70 ppm or less is a ladle 12 in which molten steel which is stepped out of the converter in a non-deoxidation state is stored. And a vacuum tube 14 positioned above the ladle, and a deposition tube 16 forming a circulation path through which the molten steel circulates between the ladle and the vacuum chamber. The immersion pipe 16 is a rising pipe 16a in which the molten steel in the ladle 12 rises to the vacuum chamber 14 and a falling pipe 16b in which the molten steel decarburized in the vacuum chamber 14 descends to the ladle 12. It consists of, the vacuum chamber 14 is connected to the vacuum pump 18.

When the reflux gas is supplied from the reflux gas supply means 22 to the riser 16a and the pressure of the vacuum chamber 14 is reduced to several to several tens of torr by the operation of the vacuum pump 18, the ladle ( The molten steel stored in 12) is introduced into the ladle 12 through the downcomer 16b after a decarburization reaction in which carbon in the molten steel reacts with oxygen is performed while ascending to the vacuum chamber 14 through the upcomer 16a. . As a result, the carbon content in the molten steel is lowered.

On the other hand, an oxygen supply device for forcibly supplying oxygen to the molten steel introduced into the vacuum chamber 14 in order to accelerate the decarburization reaction described above is known. This oxygen supply device has an oxygen injection lance 24 exposed inside the vacuum chamber 14 to inject oxygen as shown in FIG. Then, cooling means for preventing the oxygen injection lance 24 from being damaged or damaged by the high temperature in the vacuum chamber 14 is provided. According to such cooling means, the oxygen injection lance 24 is divided into a water-cooled injection lance for supplying cooling water to its outer surface and an air-cooled injection lance for supplying cooling gas.

In this case, when the water-cooled injection lance is locally damaged or melted by heat inside the vacuum chamber, there is a problem that the cooling water flows into the molten steel and may explode the vacuum chamber.

On the other hand, the air-cooled injection lance 30 is composed of an inner tube 32 and an outer tube 34 surrounding the outer surface of the inner tube 32 for injecting oxygen as shown in FIG. 3 and the inner tube and the outer tube have the same central axis. . At this time, the end of the injection lance 30 exposed inside the vacuum chamber is cooled by injecting a cooling gas made of an inert gas into the vacuum chamber through a communication space between the inner tube outer surface and the outer surface inner surface. In addition, after the supply of oxygen through the inner tube 32 is stopped, the cooling gas continues through the communication space to prevent the melt from the molten steel in the vacuum chamber 14 coming into contact with the end of the injection lance 30. While spraying, argon gas is injected through the inner tube 32. However, in the case where the air-cooled injection lance 30 is used, the cooling gas continues to be supplied into the vacuum chamber 14 through the communication space even after the oxygen supply through the inner tube 32 is stopped as described above. 14) A problem of lowering the degree of vacuum inside occurs.

The present invention has been made to solve the conventional problems as described above, in order to improve the decarburization efficiency from the molten steel in the RH vacuum degassing apparatus for producing ultra low carbon steel without reducing the vacuum degree of the vacuum chamber and cooling water It is an object of the present invention to provide an oxygen injection lance for a vacuum degassing refinery device having cooling means capable of effectively cooling an oxygen injection lance for forcibly supplying oxygen without outflow.

1 is a view showing a general RH vacuum degassing refinery.

2 is a view showing a state in which the oxygen injection lance is installed in the vacuum chamber of the refining apparatus according to the conventional embodiment.

3 is a cross-sectional view showing the distal end of a typical oxygen injection lance.

Figure 4 is a view of the RH vacuum degassing apparatus is provided with an oxygen injection lance according to the present invention on the side wall of the vacuum chamber.

5 is a view showing a state in which the oxygen injection lance is installed on the side wall of the vacuum chamber according to the present invention.

6 is a cross-sectional view of the oxygen injection lance in accordance with the present invention.

FIG. 7 is a sectional view taken along the line A-A shown in FIG. 6; FIG.

Figure 8 is an enlarged cross-sectional view showing a state in which a bellows type coupling is installed in the cut portion of the oxygen injection lance according to the present invention.

9 is a graph measuring the temperature change of the oxygen injection lance installed on the side wall of the vacuum chamber according to the present invention over time.

<Description of the code for the main part of the drawing>

40: oxygen injection lance

42: oxygen supply pipe

43: screen

44: tubular body

45: spring

50: volatile liquid substance

52: bellows type coupling

54: pipe

In order to achieve the above object, according to the present invention, a ladle in which molten steel is stored, a vacuum chamber positioned above the ladle, and an immersion tube forming a circulation flow path of the molten steel between the ladle and the vacuum chamber In the RH vacuum degassing and refining apparatus, the oxygen injection lance penetrates the side wall of the vacuum chamber at a predetermined inclination angle to supply oxygen, and surrounds an oxygen supply pipe for injecting oxygen into the vacuum chamber, and an outer surface of the oxygen supply pipe. A tubular body extending at a predetermined length from the distal end of the oxygen supply tube exposed to the inside of the vacuum chamber toward the outside of the vacuum chamber and sealed at both ends, and formed between the inner surface of the tubular body and the outer surface of the oxygen supply tube. It is made of a volatile liquid material stored in a sealed storage space, the volatile liquid material is inside the vacuum chamber The gas material vaporized by the vaporizing temperature and is characterized in that the vacuum chamber by the condensation of the external temperature.

Hereinafter, preferred embodiments of the present invention will be described with reference to FIGS. 4 to 9, and the same components as those in the prior art are assigned the same reference numerals.

The RH vacuum degassing and refining apparatus 10 for manufacturing ultra low carbon steel includes a ladle 12 in which molten steel is stored, a vacuum chamber 14 positioned above the ladle 12, and a molten steel in the ladle 12 is vacuumed. It has a circulation pipe 16 which consists of a rising pipe 16a which rises to the tank 14, and a falling pipe 16b which decarburizes the molten steel degassed in the vacuum tank 14 to the ladle 12. As shown in FIG. 5, the vacuum chamber 14 includes a lower container into which molten steel flows from the upper vessel 14a and the ladle 12 maintained in a vacuum state by the vacuum pump 18 along the riser tube 16a. 14b), the outer wall of the vacuum chamber 14 is made of steel shell, and a refractory is built in the steel shell.

According to the present invention, the oxygen injection lance 40 for forcibly supplying oxygen into the vacuum chamber 14 penetrates the refractory of the vacuum chamber 14 at a predetermined inclination angle θ at the lower portion of the upper container 14a. Is installed.

And, the oxygen injection lance 40 according to the present invention, as shown in Figure 6, to guide the flow of oxygen to blow the oxygen supplied from the oxygen supply tank (not shown) toward the interior of the vacuum chamber 14 It has an oxygen supply pipe 42. The cylindrical tubular body 44 whose both ends are sealed by the upper plate 44a and the lower plate 44b which are fixed to the outer surface of the oxygen supply pipe 42 located in the refractory layer of the vacuum chamber 14 is provided. do.

A predetermined amount of storage space is provided between the outer surface of the oxygen supply pipe 42 and the inner surface of the tubular body 44, and a volatile liquid material 50 is provided in the storage space. On the other hand, the upper plate 44a is provided with an exhaust pipe 46 connected to a vacuum pump (not shown) for exhausting the air in the storage space so that the storage space can be maintained at a pressure below atmospheric pressure, particularly in a vacuum state. do. The exhaust pipe 46 is provided with a first valve 46a. In addition, the upper plate 44a is provided with a volatile supply pipe 48 for supplying the volatile liquid material 50 to the vacuum space. The volatile supply pipe 48 is provided with a second valve 48a.

As described above, in the oxygen supply pipe 42 in which the tubular body 44 is fixed by the upper plate 44a and the lower plate 44b, a part of the oxygen supply pipe 42 located in the storage space is It is kept in a cut state to prevent damage by thermal stress or the like described in FIG. Then, the bellows type coupling 52 is connected to each other at the cut portions of the cut first oxygen supply pipe 42a and the second oxygen supply pipe 42b. At this time, the cutting to close the gap (C) in order to prevent the outflow of oxygen through the gap (C) between the cut portion of each of the first oxygen supply pipe (42a) and the second oxygen supply pipe (42b) facing each other The site is provided with a pipe 54 located inside the bellows type coupling 52.

Referring to FIG. 8, a bellows type coupling is provided to assist the smooth stretching of the oxygen supply pipe 42 by thermal stress and to prevent the first oxygen supply pipe 42a from being separated from the second oxygen supply pipe 42b. Both ends of 52 are supported by first stoppers 56a and 56b provided on the outer surfaces of each of the first oxygen supply pipe 42a and the second oxygen supply pipe 42b. And, in order to effectively prevent the outflow of oxygen through the gap (C), the lower end of the pipe 54 is provided with a second stopper 56c provided on the outer surface of the first oxygen supply pipe 42a inside the bellows type coupling 52. Supported by).

As described above, since the oxygen supply pipe 42 is installed in the refractory layer of the vacuum chamber 14 at a predetermined inclination angle, the oxygen supply pipe 42 is formed in the storage space formed between the outer surface of the oxygen supply pipe 42 and the inner surface of the tubular body 44. The volatile liquid substance 50 to be stored is located on the lower end portion of the oxygen supply pipe 42, that is, the lower plate 44a, which is exposed to the inside of the vacuum chamber 14 by gravity. At this time, the volatile liquid substance 50 is vaporized by absorbing heat in the vacuum chamber 14, in particular heat around the lower end of the oxygen supply pipe 42, the resulting gas is the upper portion of the storage space, that is, the upper plate ( 44b). At this time, the upper plate (44b) is located adjacent to the steel shell outside the relatively low temperature vacuum chamber 14 and is kept at a low temperature state, so that the gas moved to the upper plate (44b) side condensed and the resulting liquid material It moves toward the lower plate 44a along the outer surface of the oxygen supply pipe 42 and / or the inner surface of the tubular body 44.

As such, the volatile liquid substance which absorbs external heat and is volatilized is selected in consideration of physical properties such as specific gravity, surface tension, viscosity and latent heat of vaporization, price, operating temperature, material of tubular body and oxygen supply pipe.

Table 1 below shows the melting point, boiling point and volatile temperature range of volatiles.

TABLE 1

matter Melting Point (℃) Boiling point (℃ under 1 atmosphere) Temperature range Freon 11 -111 24 -40 to 120 Methanol -98 64 10 to 130 water 0 100 30 to 200 Potassium (K) 62 774 500 to 1000 Sodium (Na) 98 892 600 to 1200 Iridium (Li) 179 1340 1000-1800 Silver (Ag) 960 2212 1800-2300

On the other hand, the relative value that each material can move the heat is represented by a merit number (M; number) can be expressed by the following equation.

Where ρ is the specific gravity of the volatile liquid substance, σ is the surface tension of the volatile liquid substance, λ is the latent heat of vaporization, and μ is the viscosity of the volatile liquid substance.

In order to select volatile liquid materials as described above, not only physical properties but also prices, operating temperatures, materials of tubular bodies and oxygen supply tubes, etc. should be considered. The higher the number, the more expensive the alloy to withstand the use of lithium. On the contrary, when sodium (Na) or potassium (K) is used as a volatile liquid material, relatively inexpensive stainless steel can be used.

Table 2 below shows suitable materials for volatile liquids.

TABLE 2

Volatile Liquid Substance material Methanol Copper, stainless steel water Copper, monel Potassium (K) Nickel, Stainless Steel, Inconel, Titanium Sodium (Na) Stainless Steel, Nickel, Inconel 800 Iridium (Li) Nb-1% Zr alloy, molybdenum (Mo) Silver (Ag) W-26% Rh

Therefore, as described above, the material for limiting the storage space for storing the volatile liquid substance is selected in consideration of compatibility with the volatile liquid substance, thermal conductivity, convenience of processing, wettability, and the like. .

Meanwhile, the volatile liquid substance stored in the storage space is charged to occupy about 15-20% of the total volume of the storage space.

Hereinafter, the operation of the oxygen injection lance 40 according to the present invention.

First, the oxygen injection lance 40 according to the present invention is installed through the side wall of the vacuum chamber 14 of the RH vacuum degassing and refining apparatus 10. The exhaust pipe 46 is connected to a vacuum pump (not shown) and the volatile supply pipe 48 is connected to a volatile storage tank (not shown). In the state in which the first valve 46a is opened, the storage space formed between the oxygen supply pipe 42 and the tubular body 44 by the operation of the vacuum pump is evacuated to maintain the vacuum therein. At this time, when the second valve 46b is opened while the first valve 46a is closed, the volatile liquid substance 50 flows into the storage space from the volatile storage tank. When the target amount of volatile liquid 50 is introduced, the second valve 48a is closed.

At this time, the volatile liquid material 50 may use water, methanol, sodium, potassium and the like depending on the temperature used as described above. According to the present invention, the volatile liquid substance 50 used sodium liquid in consideration of the internal temperature of the vacuum chamber 14 of the RH vacuum degassing and refining apparatus 10, that is, about 800 ° C to 1200 ° C. In general, sodium has a melting point of about 98 ° C. and a boiling point of about 892 ° C. at atmospheric pressure. Therefore, the boiling point of sodium is lowered in the storage space maintained at a pressure below atmospheric pressure as described below. At this time, the amount of the sodium liquid 50 to be used is charged by the amount that can occupy about 10 to 25% of the total storage space in the fully condensed state. This is because when the amount of sodium is less than 10%, a large amount of liquid is present on the side of the storage space in the process of evaporation and condensation, and there may be no sodium liquid that can evaporate from the bottom. When charged into, the space left for evaporating and evaporating sodium liquid is reduced.

Thus, the sodium liquid 50 introduced into the storage space is located adjacent to the interior of the vacuum chamber 14 by its own weight. Then, the sodium liquid 50 is vaporized by the heat in the vacuum chamber 14, the sodium gas moves to the upper portion of the storage space, that is, the upper plate 44a side. That is, the sodium gas is moved to the upper side by the difference between the gas pressure in the upper surface of the sodium liquid 50 and the gas pressure in the upper portion of the storage space. At this time, since the upper portion of the storage space is maintained at a relatively low temperature compared to the lower portion thereof, the sodium gas condenses into the sodium liquid 50, and the condensed sodium liquid 50 is along the side defining the storage space. It moves to the lower plate 44b side. By repeatedly performing the vaporization and condensation process as described above, the sodium liquid in the vacuum absorbs heat in the evaporation process to cool the surroundings, and the evaporated gas moves to the upper side by the internal pressure difference, and then the cold portion at the upper side Heat is released while condensing again in contact with, and the cycle of falling sodium liquid condensed by gravity back down continues to cool the hot spot at the bottom.

In this way, the oxygen injection lance 40 can be prevented from being melted by blowing oxygen from the oxygen storage tank into the vacuum chamber through the oxygen injection lance 40 having a lowered temperature.

On the other hand, reference numeral 43 in the drawings is a stainless steel screen fixedly installed on the outer surface of the oxygen supply pipe (42). The screen 43 guides the sodium liquid 50 condensed at the top of the reservoir to flow down evenly over the entire outer surface of the oxygen supply pipe 42. The size of this screen 43 is suitably 150 mesh or less. This is because installing a screen 43 having too fine mesh prevents sodium gas from passing through the screen and coming into contact with the outer surface of the relatively cold oxygen supply pipe 42.

In addition, reference numeral 45 is a spring installed adjacent to the inner surface of the tubular body 44. The spring 45 is made of a diameter capable of contacting the inner surface of the tubular body 44 with a pipe having a diameter of about 6 mm. This spring 45 guides the evaporated sodium gas to move along the helical path to the upper side of the reservoir and also to allow the condensed sodium liquid 50 to be evenly present throughout the inner surface of the tubular body 44. do. Therefore, the evaporated sodium gas condenses by contacting the outer surface of the oxygen supply pipe 42 and the inner surface of the tubular body 44, and the condensed sodium liquid 50 is oxygenated through the screen 43 and the spring 45. The outer surface of the supply pipe 42 and the inner surface of the tubular body 44 will be evenly present. In particular, the upper surface of the sodium liquid 50 stored in the lower portion of the storage space by the spring 45 ascends upwardly along the inner surface of the tubular body 44, and consequently hot on the tubular body 44. It is possible to prevent the occurrence of hot spots.

The oxygen supply pipe 42, the tubular body 44, and the upper and lower plates 44a and 44b used in the oxygen injection lance 40 according to the present invention are made of stainless steel. As shown in Table 2, a material compatible with sodium liquid used as a volatile liquid material may be stainless steel, nickel, or incoal 800, but stainless steel is most suitable in consideration of workability and price of the material. In addition, the screen 43 fixed to the outer surface of the oxygen supply pipe 42 is also made of stainless steel.

When the oxygen injection through the oxygen supply pipe 42 is stopped, about 60 Nm ³ / hr to prevent the blockage of the oxygen supply pipe injection end by slag or molten steel scattering in the vacuum chamber 14 and to cool the injection end. Inert gas of, for example, argon is injected. When the inert gas is blown through the oxygen supply pipe 42 as described above, the temperature of the oxygen supply pipe 42 and the tubular body 44 varies depending on the conditions inside the vacuum chamber of the RH vacuum degassing and refining apparatus 10. In general, the temperature is maintained at about 500 ° C. to 650 ° C., so that the temperature difference is almost uniform. In particular, the temperature of the oxygen supply pipe 42 and the tubular body 44 is maintained almost uniform by the sodium liquid in the storage space.

However, during the RH vacuum degassing and refining operation, when aluminum is added to the upper part of molten steel while injecting oxygen to raise the temperature, the operation of raising the molten steel using the heat generated during the oxidation reaction of aluminum is intermittently performed.

The tip inner portion B of the oxygen supply pipe 42 is processed to have a relatively narrow diameter so that oxygen can be blown at supersonic speed under oxygen flow conditions (1200 Nm ³ / hr). This is because oxygen is easily melted into the molten steel by spraying at supersonic speed. That is, oxygen dissolved in the molten steel may react with aluminum to generate heat, or may react with carbon in the molten steel to increase decarburization efficiency.

In general, when gas is injected at a supersonic speed or higher through a nozzle, the temperature of Ma (Mach number) and the injected gas may be expressed by the following relationship.

,

Here, T ₀ is the initial temperature of the gas, T is the gas temperature at the nozzle outlet, Ma is Mach number (multiple of sound speed), and k is the specific heat ratio (1.4).

From the above equation, when the Ma number exceeds 1, the right term of the above equation becomes 1 or more, so that the gas temperature T at the nozzle outlet is lower than the initial temperature T ₀ of the gas. The above equation means that when the gas is injected at supersonic speed, the gas temperature is drastically lowered and is rapidly cooled around the nozzle.

As described above, the temperature of the oxygen supply pipe 42 decreases the gas temperature due to the adiabatic expansion of gas in the above-mentioned supersonic nozzle while the oxygen is blown about 20 times larger than the blowing amount of argon used when oxygen is not blown. The temperature drop is much faster than the tubular body 44 due to the effect and the cooling effect by the large amount of oxygen itself. Accordingly, the internal stress is generated due to the difference in the linear expansion of the oxygen supply pipe 42 and the tubular body 44, in which case the weak spot, for example, the weld spot is broken.

Therefore, according to a preferred embodiment of the present invention, a portion of the oxygen supply pipe 42 is cut and the cut portion is connected to the bellows type coupling 52. Then, the pipe 54 is installed in order to prevent the distortion caused by the movement due to the difference in the linear expansion of the oxygen supply pipe 42. Therefore, the damage which may be caused by the formation of the internal stress due to the difference in the linear expansion of the oxygen supply pipe 42 is prevented.

9 is a graph measuring the temperature change of the oxygen injection lance 40 installed on the side wall of the vacuum chamber 14 according to the present invention. The oxygen injection lance 40 was preheated for about 2 days and then temperature was measured from the end of preheating. After 1 hour and 40 minutes had elapsed from the temperature measurement, RH vacuum degassing and refining operation was started. The numerals shown in FIG. 9 indicate the position of the thermocouple away from the tip of the oxygen supply pipe. That is, 1 cm is a graph showing the external temperature of the tubular body 44 at a position 1 cm from the tip of the oxygen supply pipe over time. Although the internal temperature of the vacuum chamber is generally about 800 ° C to 1200 ° C, the temperature of the inner wall of the RH vacuum degassing apparatus is continuously increased to about 1000 ° C in the preheating process at the tip of the oxygen supply pipe. It can be seen that it is maintained at a temperature of 600 ° C or less.

As described above, even when argon gas of about 80 Nm ³ / hr is blown through the oxygen supply pipe 42 in the state in which the oxygen injection is stopped, the temperature of the portion 1 cm away from the tip of the oxygen supply pipe 42 is at the end of preheating. It can be seen that the drop from the level of about 570 ℃ to 520 ℃. However, compared to the tip temperature of the oxygen injection lance according to the conventional embodiment is usually 1000 ℃ or more in accordance with the present invention maintains the tip temperature of the oxygen injection lance less than 600 ℃ can effectively prevent oxidation or melting of the lance. have.

As described above, the front end of the oxygen injection lance can be effectively cooled without lowering the vacuum degree of the vacuum tank of the RH vacuum degassing refiner and without the risk of explosion due to leakage of the cooling water, thereby improving the life of the injection lance and improving the oxygen injection efficiency. Can be.

The foregoing is merely illustrative of the preferred embodiments of the present invention and those skilled in the art to which the present invention pertains may make modifications and changes to the present invention without departing from the spirit and gist of the invention as set forth in the appended claims. It should be recognized.

Claims (8)

In the RH vacuum degassing apparatus comprising a ladle in which molten steel is stored, a vacuum tank positioned above the ladle, and a deposition tube forming a circulation flow path of the molten steel between the ladle and the vacuum chamber. In the oxygen injection lance is supplied through the side wall at a predetermined inclination angle to supply oxygen, An oxygen supply pipe that blows oxygen into the vacuum chamber, A tubular body surrounding the outer surface of the oxygen supply pipe and extending at a predetermined length toward the outside of the vacuum bath from the distal end of the oxygen supply pipe exposed to the inside of the vacuum bath, the both ends being sealed; It is made of a volatile liquid material stored in a sealed storage space formed between the inner surface of the tubular body and the outer surface of the oxygen supply pipe, Wherein the volatile liquid material is vaporized by the temperature inside the vacuum chamber and the vaporized gaseous material is condensed by the temperature outside the vacuum chamber. The method of claim 1, The volatile liquid substance is oxygen injection lance, characterized in that the liquid having a boiling point of 600 ℃ to 800 ℃ under atmospheric pressure. The method of claim 2, The oxygen injection lance, characterized in that the volatile liquid substance is sodium. The method of claim 2, The volatile liquid substance is oxygen injection lance, characterized in that potassium. The method according to claim 3 or 4, The tubular body is oxygen injection lance, characterized in that made of stainless steel material. The method of claim 5, Oxygen injection lances adjacent to the inner surface further comprises a spring is installed along the longitudinal direction of the tubular body. The method of claim 1, And the oxygen supply pipe is cut at a predetermined position located in the storage space, and the cut oxygen supply pipes are connected by a bellows type coupling. The method of claim 1, And the storage space is evacuated to maintain the pressure below atmospheric pressure.