JPH05332498A - Holding of internal pressure in liquid helium storing tank in transfer of liquid helium and device therefor - Google Patents

Abstract

PURPOSE:To keep the internal pressure in a liquid helium storing tank to a sufficient pressure when the liquid helium is transferred without specially preparing the high purity helium gas. CONSTITUTION:When the liquid helium liquefied by a helium liquefier 10 is stored in a liquid helium storing tank 11, and the liquid helium is transferred from the liquid helium storing tank 11 to a liquid helium container 12 by utilizing the internal pressure difference between both, the pressure in the liquid helium storing tank 11 is held. When the liquefaction operation completes, the gas which circulates in the helium liquefier 10 is recovered in a pressurized state into a buffer tank 55 and when the helium gas is transferred, the helium gas is supplied into the liquid helium storing tank 11 through a helium supply pipe 66.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to maintain a good internal pressure of the liquid helium storage tank when transferring the liquid helium stored in the liquid helium storage tank to another liquid helium container. Method and apparatus.

[0002]

2. Description of the Related Art It is not preferable to directly supply liquid helium from a liquefaction machine each time liquid helium is needed, such as when conducting a small-scale experiment, in view of time constraints and operation efficiency of the liquefaction machine. .. Therefore, generally, the liquid helium generated by the liquefaction machine is temporarily stored, and the stored liquid helium is appropriately pumped out as needed.

FIG. 3 shows an example of a conventional helium liquefier and a liquid helium storage system. The operation contents of the helium liquefier 10 and the transfer procedure of liquid helium in this system are as follows.

1. Liquefaction operation Helium gas at room temperature is 15kg / cm ² with the helium compressor 30.
After the pressure is increased to G, the cool box 3 is installed along the high pressure line 33.
Heat exchanger 35g, 35e, 35d, 35c, 35 in 2
It is cooled down to 4.2K through b and 35a in order, and is further depressurized to 0 kg / cm ² G by the JT valve 52. At this time, a part of the helium gas becomes liquid helium and accumulates in the liquid helium storage tank 11, and the remaining gas flows along the low pressure line 34.
While cooling the gas in the high pressure line 33 by going backward through the heat exchangers 35a to 35g, the temperature of the helium compressor 3 becomes normal.
Return to 0. In addition, the expansion turbine 4 of the cold generation line 43
A part of the gas sent to 1, 42 is returned to the low pressure line 34 after generating cold. In this low pressure line 34,
A buffer tank 56 is installed, and the buffer tank 56 is opened with the valve 54 shown in the drawing.
Stable liquefaction operation is ensured by absorbing the pressure fluctuation of.

2. Helium gas purification Purification of helium gas is performed in parallel with the liquefaction operation. First, the gas in the gas cylinder 62 is temporarily stored in the surge tank 60 after impurities are removed by the refining device 63. Then, the gas depletion of helium due to the liquefaction operation is replenished from the surge tank 60 to the suction side of the helium compressor 11 via the valve 58, and this gas is exchanged with the liquid nitrogen passed through the heat exchangers 35f and 35g. After being cooled by heat exchange, it is further purified by an activated carbon adsorber (internal purifier) 37. This activated carbon adsorber 37 adsorbs and removes impurities that may have entered the liquefier piping, and when the helium gas is cooled to 4.2K, the impurities become solid and the expansion turbines 41, 42 have Operation is hindered and JT valve 5
It plays a role to prevent blocking 2. With the progress of purification by the activated carbon adsorber 37, impurities in helium are accumulated in the activated carbon adsorber 37.

3. End of liquefaction operation The end of liquefaction operation is performed as follows.

(A) The gas supply from the gas cylinder 62 is stopped, the outlet valve 58 of the surge tank 60 is closed, and the helium gas purification is completed.

(B) Surge tank 60 and helium compressor 3
The inlet valve 59 provided between the zero side and the discharge side is opened, and the helium gas is returned to the surge tank 60.

(C) Bypass valve 5 of liquid helium storage tank 11
After opening 0, its inlet valve 36 and outlet valve 38 are closed,
Stop flowing helium gas into the liquid helium storage tank 11.

(D) Inlet valve 44 of expansion turbines 41, 42
Is gradually closed, and the pressure of both turbines 41, 42 is reduced to atmospheric pressure.

(E) Open the valve 48 to extract a part of the gas from the front side of the JT valve 52, heat it to room temperature with a warmer 49, adjust the flow rate with a flow rate control valve 47, and simultaneously reduce the pressure to lower the pressure. Return to line 34. This operation is performed in order to accelerate the pressure drop in the cool box 32.

(F) The inlet valve 53 of the cool box 32 is gradually closed to reduce the pressure from 15 kg / cm ² G to atmospheric pressure.

(G) The pressure of the helium compressor 30 is set to 15 kg / cm ²
Stop from G to atmospheric pressure.

4. Liquid helium transfer Liquid helium liquefied by the helium liquefier 10 is temporarily stored in a liquid helium storage tank (liquefaction dewar) 11, and the helium gas evaporated in the liquid helium storage tank 11 passes through a valve 20 and a passage 15 to a gas bag 18 To release.
When liquid helium is used, such as when cooling the superconducting magnet, the liquid helium is transferred from the liquid helium storage tank 11 to the liquid helium container (small dewar) 12 having a capacity of 100 to 200 liters via the transfer pipe 14. ,
Again, the liquid helium container 12 is transferred to an experimental cryostat (not shown).

The liquid helium is transferred through the transfer pipe 14 with the valve 20 closed.
This is performed by utilizing the difference between the internal pressure of 1 (about 0.2 to 0.3 kg / cm ² G) and the internal pressure of the liquid helium container 12 (atmospheric pressure). If 100 liters of liquid helium is transferred, the liquid helium in the liquid helium storage tank 11 is about 130 liters.
The volume is reduced by 30 liters, of which 30 liters of liquid is evaporated and discharged from the liquid helium container 12, warmed to room temperature by the warmer 22, and then collected in the gas bag 18. After a certain amount of gas (for example, 40 m ³ or more gas) is accumulated in the gas bag 18, the gas is pressurized to a predetermined pressure (usually 150 kg / cm ² G) by a gas recovery compressor (not shown), It is collected in a gas cylinder and used as a liquefaction gas.

[0016]

When the liquid is transferred through the transfer pipe 14, the internal pressure of the liquid helium storage tank 11 decreases as the liquid level in the liquid helium storage tank 11 decreases. Here, when the volume of the liquid helium container 12 (that is, the extraction volume of the liquid helium) is extremely smaller than the volume of the liquid helium storage tank 11, the liquid helium storage tank 1
Since the ratio of the liquid reduction amount to the volume of the gas phase in 1 is small, the decrease in the internal pressure of the liquid helium storage tank 11 is very slight and does not pose a problem. For example, when liquid helium is transferred from a 2000 liter liquid helium storage tank 11 to a 100 liter liquid helium container 12, the internal pressure before liquid transfer is 0.30.
kg / cm ² G drops to 0.23 kg / cm ² G after transfer, which does not hinder liquid transfer.

However, when the volume of the liquid helium container 12 is relatively large with respect to the volume of the liquid helium storage tank 11, the internal pressure of the liquid helium storage tank 11 is large because the ratio of the liquid reduction amount to the volume of the gas phase is large. It drops considerably and can no longer be ignored. For example, 1000
When transferring liquid helium liter liquid helium vessel 12 from the liquid helium storage tank 11 100 liters of the internal pressure before liquid transfer 0.30 kg / cm ² G is transferred after 0.13 kg / cm ² G
This causes the following inconvenience.

(A) When the liquid helium is continuously transferred to the 100-liter liquid helium container 12 after the above transfer, until the internal pressure reaches 0.30 kg / cm ² by evaporation of the liquid helium in the liquid helium storage tank 11. I have to wait (about 2 hours).

(B) For a liquid helium container 12 having a large volume exceeding 100 liters, it becomes difficult to transfer liquid helium due to the above-mentioned decrease in internal pressure. For example, liquid helium cannot be transferred to a 250 liter liquid helium container 12.

In order to prevent such a decrease in internal pressure, conventionally, helium gas is supplied from a separately prepared helium gas cylinder into the liquid helium storage tank 11 to maintain the internal pressure of the liquid helium storage tank 11. There is. However, 100p for a normal helium gas cylinder
When the liquid helium storage tank 11 is replenished with impurities of about pm, the above impurities are contained in the liquid helium storage tank 1.
There is an inconvenience that it is cooled in 1 and solidifies and accumulates, and the narrow transfer pipe 14 is closed during transfer. Therefore, in reality, a high-purity helium gas cylinder, which is more expensive than the gas cylinder, must be used, and moreover, it is necessary to purchase the helium gas cylinder periodically to supplement the supplied helium gas. Specifically, it is necessary to purchase once every 1-2 months, which is a major obstacle to reducing running costs.

In view of such circumstances, the present invention provides a method and an apparatus capable of sufficiently maintaining the internal pressure of a liquid helium storage tank during the transfer of liquid helium without special preparation of high-purity helium gas. With the goal.

[0022]

According to the present invention, liquid helium produced by a liquefaction machine having an internal purifier in a cool box and circulating helium by a compressor is stored in a liquid helium storage tank, and A method for maintaining the internal pressure of the liquid helium storage tank when transferring the liquid helium from the liquid helium storage tank to the liquid helium container by utilizing the internal pressure difference between the two, wherein the internal refiner in the liquefier is used Is also a line on the downstream side, and the helium gas circulating in the liquefier is collected under pressure from a position located outside the cold-insulating box, and the helium gas is transferred to the liquid helium storage tank during transfer of the liquid helium. It is supplied to the inside (Claim 1).

Further, the present invention is an apparatus for carrying out the above method, which is connected to a part of the liquefaction machine which is located on the downstream side of the internal purifier and outside the cold insulation box. And a helium replenishing passage connecting the inside of the tank and the inside of the liquid helium storage tank with each other (claim 2).

As the tank, a buffer tank which is connected to the low pressure line of the liquefaction machine and absorbs the pressure fluctuation of the compressor is preferable (claim 3).

[0025]

According to the above construction, when liquid helium is transferred, the helium gas previously recovered in a pressurized state in the liquefier is supplied to the liquid helium storage tank, so that the liquid helium can be reduced by the transfer. The pressure in the liquid helium storage tank can be maintained at a sufficient pressure. Moreover, since the helium gas is collected from a position downstream of the internal purifier of the liquefier, impurities are removed in advance, and the invasion of impurities into the liquid helium storage tank can be prevented.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG. The helium liquefaction and storage system shown in this embodiment has the same basic configuration as that shown in FIG. 3, and therefore common components are designated by the same reference numerals.

In FIG. 1, 10 is a helium liquefier and 1
1 is a liquid helium storage tank (liquefaction dewar), 12 is a liquid helium container (small mouth dewar), and the liquid helium liquefied by the helium liquefier 10 is appropriately supplied into the liquid helium storage tank 11.

The liquid helium storage tank 11 and the liquid helium container 12 are connected to a common gas bag 18 through pipes 15 and 16, respectively. Valve 2 in the middle of pipe 15
0, a warmer 22 and a valve 24 are provided in the middle of the pipe 16 from the side closer to the gas bag 18, and the helium gas evaporated in the liquid helium storage tank 11 is the valve 20 and the pipe 1.
It is designed to be discharged to the gas bag 18 through 5.

The helium liquefier 10 is provided with a cold insulation box 32, and has a helium compressor 30 outside thereof. The discharge side of the helium compressor 30 is connected to the liquid helium storage tank 11 via a high pressure line 33, and the liquid helium storage tank 11 is connected to the suction side of the helium compressor 30 via a low pressure line 34. ..

Inside the cool box 32, heat exchangers 35a, 35b, 35c, 35d, 3 are arranged in order from the low temperature side as heat exchangers through which both the high pressure line 33 and the low pressure line 34 pass.
5e, 35f, 35g are installed and the heat exchanger 3 on the high temperature side
Liquid nitrogen for cooling is passed through 5f and 35g.

In the high pressure line 33, the heat exchanger 3
A JT valve 52 and an inlet valve 36 of the liquid helium storage tank 11 are arranged in series at a position on the outlet side of 5a, and the low pressure line 34
An outlet valve 38 is provided at a position on the outlet side of the liquid helium storage tank 11, and both lines 33 and 34 can be interconnected via a bypass valve 50 of the liquid helium storage tank 11. An inlet valve 53 is also provided at a position on the inlet side of the cold insulation box 32 in the high pressure line 33.

The heat exchanger 35 in the high pressure line 33
The position between d and 35e and the position between the heat exchangers 35a and 35b in the low-pressure line 34 are connected via a cold generation line 43 passing through the heat exchanger 35c, and the cold generation line 43 is cold. Two-stage expansion turbine 4 for generation
1, 42 are provided.

In the high pressure line 33, the heat exchanger 3
An activated carbon adsorber (internal purifier) 37 for adsorbing and removing an impure component in helium gas is provided in a portion located between 5e and 35f, and the heat exchangers 35a and J on the lowest temperature side are connected to each other.
The portion located between the T valve 52 and the bypass valve 46 is
It is connected to the suction side of the helium compressor 30 via. In the middle of the bypass line 46, the high pressure line 3
In order from the connection part with 3, the warmer 49 and the bypass valve 4
8 and a flow rate control valve 47 are provided in series.

The surge tank 6 is connected to the high pressure line 33 and the low pressure line 34 via valves 59 and 58, respectively.
0 is connected. A gas cylinder 62 for storing helium gas is connected to the surge tank 60,
A refining device 63 and a valve 64 are provided between the gas cylinder 62 and the surge tank 60.

A buffer tank 56 is connected to the low pressure line 34 via a valve 54. The buffer tank 56 absorbs pressure fluctuations in both lines 33 and 34. Further, as a feature of this device, the inside of the buffer tank 56 is a helium supply pipe (helium supply passage) 6
Is connected to the liquid helium storage tank 11 via 6,
A pressure control valve 68 is provided in the middle of the helium supply pipe 66. The pressure control valve 68 is a liquid helium storage tank 11 in which the pressurized helium gas in the buffer 56 is preset.
The pressure is reduced to the specified internal pressure.

FIG. 3 shows an example of a conventional helium liquefier and a liquid helium storage system. The operation contents of the helium liquefier 10 and the transfer procedure of liquid helium in this system are as follows.

1. Liquefaction operation Helium gas at room temperature is 15kg / cm ² with the helium compressor 30.
After the pressure is increased to G, the cool box 3 is installed along the high pressure line 33.
Heat exchanger 35g, 35e, 35d, 35c, 35 in 2
It is cooled down to 4.2K through b and 35a in order, and is further depressurized to 0 kg / cm ² G by the JT valve 52. At this time, a part of the helium gas becomes liquid helium and accumulates in the liquid helium storage tank 11, and the remaining gas flows along the low pressure line 34.
While cooling the gas in the high pressure line 33 by going backward through the heat exchangers 35a to 35g, the temperature of the helium compressor 3 becomes normal.
Return to 0. In addition, the expansion turbine 4 of the cold generation line 43
A part of the gas sent to 1, 42 is returned to the low pressure line 34 after generating cold. The valve 54 is opened, and the pressure fluctuation of the helium compressor 30 is absorbed by the buffer tank 56.

2. Helium gas purification Purification of helium gas is performed in parallel with the liquefaction operation. First, the gas in the gas cylinder 62 is temporarily stored in the surge tank 60 after impurities are removed by the refining device 63. Then, the gas depletion of helium due to the liquefaction operation is replenished from the surge tank 60 to the suction side of the helium compressor 11 via the valve 58, and this gas is exchanged with the liquid nitrogen passed through the heat exchangers 35f and 35g. After being cooled by heat exchange, it is further purified by the activated carbon adsorber 37. This activated carbon adsorber 37 adsorbs and removes impurities that may have entered the liquefier piping, and when the helium gas is cooled to 4.2K, the impurities become solid and the expansion turbines 41, 42 have It plays the role of preventing the operation from being blocked and the JT valve 52 from being blocked. This activated carbon adsorber 37
Impurities in helium are accumulated in the activated carbon adsorber 37 with the progress of purification by.

3. End of liquefaction operation The end of liquefaction operation is performed as follows.

(A) The gas supply from the gas cylinder 62 is stopped, the outlet valve 58 of the surge tank 60 is closed, and the helium gas purification is completed.

(B) Surge tank 60 and helium compressor 3
The inlet valve 59 provided between the zero side and the discharge side is opened, and the helium gas is returned to the surge tank 60.

(C) After opening the bypass valve 50 of the 1000-liter liquid helium storage tank 11, the inlet valve 36 and the outlet valve 38 thereof are closed to stop the flow of helium gas into the liquid helium storage tank 11.

(D) Inlet valve 44 of expansion turbines 41, 42
Is gradually closed, and the pressure of both turbines 41, 42 is reduced to atmospheric pressure.

(E) Open the valve 48 to extract a part of the gas from the front side of the JT valve 52, heat it to room temperature with a warmer 49, adjust the flow rate with the flow rate control valve 47, and simultaneously depressurize it to lower the pressure. Return to line 34. This operation is performed in order to accelerate the pressure drop in the cool box 32.

(F) The inlet valve 53 of the cool box 32 is gradually closed to reduce the pressure from 15 kg / cm ² G to atmospheric pressure.

(G) The pressure of the helium compressor 30 is set to 15 kg / cm ²
Stop from G to atmospheric pressure.

(H) The outlet valve 58 and the inlet valve 59 of the surge tank 60, the valve 51 on the suction side of the helium compressor 30, the bypass valve 50 and the outlet valve 38 of the liquid helium storage tank 11 are closed, and the liquid helium storage tank 11 is closed. The valve 20 for discharging the evaporative gas and the valve 54 of the buffer tank 56 are opened, and the gas that gradually rises in temperature and expands inside the liquefier is stored in the buffer tank (volume 4 m ^{3 in} this embodiment). When this operation was actually performed, the entire liquefier was heated to room temperature in 2 days, the gas increased by 8 m ³ and the pressure increased from atmospheric pressure to 2 kg / cm ²
It rose to G.

During this operation, the outlet valve 38 is closed and the valve 2
Since 0 is opened, gas does not flow into the liquid helium storage tank 11 and therefore the internal pressure thereof does not rise. Further, since the bypass valve 50 is closed, the activated carbon adsorber 3
The impure gas desorbed by the temperature rise of 7 is the buffer tank 56.
It never flows into. In addition, other valves 58, 51
Since the buffer tank 35 is also closed, only the low pressure line 34 is connected to the buffer tank 35. Therefore, only the highly purified helium gas which has already been purified is accumulated in the buffer tank 56.

4. Liquid helium transfer The liquid helium transfer from the liquid helium storage tank 11 to the liquid helium container 12 is performed by connecting the both with a transfer pipe 14 and closing the valve 20 and the internal pressure of the liquid helium storage tank 11 (about 0.2 to 0.3 kg). / cm ² G) and the internal pressure (atmospheric pressure) of the liquid helium container 12 is used. By taking out the liquid helium as described above, the liquid level of the liquid helium in the liquid helium storage tank 11 is lowered, and the internal pressure of the liquid helium storage tank 11 is about to be reduced accordingly.

However, at the time of this transfer, the pressure control valve 68 arranged on the helium supply pipe 66 is set to a preset internal pressure of the liquid helium storage tank 11 (0.3 k in this embodiment).
g / cm ² G) and by supplying the high-purity helium gas in the buffer tank 35 to the liquid helium storage tank 11, the internal pressure of the liquid helium storage tank 11 becomes a good pressure regardless of the extraction of the liquid helium. Can be held. Therefore, even immediately after the completion of the transfer, the liquid helium in the liquid helium storage tank 11 can be transferred to another liquid helium container, and it is not necessary to wait for the internal pressure in the liquid helium storage tank 11 to rise before the transfer. Further, transfer to a liquid helium container having a capacity of more than 100 liters (for example, 250 liters) can be performed without any inconvenience. In addition, since the internal pressure is maintained by supplying the helium gas stored in the existing buffer tank 35 and purified to a high purity by the existing internal purifier 37, it is expensive as in the conventional case. Liquid helium can be transferred at low cost without the need to purchase a high-purity helium gas cylinder.

When the transfer of the liquid helium was actually attempted in the apparatus of this embodiment, the transfer of the liquid helium was completed in 1.5 hours, and the transfer of the liquid helium to the next 100 liter liquid helium container 12 could be similarly performed. It could be confirmed.

Next, a second embodiment will be described with reference to FIG. Here, the tank 70 is connected via a valve 73 to a position between the valve 48 and the flow rate adjusting valve 47 in the bypass line 46 in the first embodiment. Then, this tank 70 is connected to the inside of the liquid helium storage tank 11 via the helium replenishment pipe 76, like the buffer tank 56 in the first embodiment, and in the middle of this helium supply pipe 76, in the first embodiment. A pressure control valve 77 similar to the pressure control valve 68 is provided.

Also in such a structure, helium gas is stored in the tank 70 in a pressurized state at the end of the liquefaction operation, and this gas is adjusted by the pressure control valve 77 when liquid helium is transferred.
The internal pressure of the liquid helium storage tank 11 can be maintained by replenishing it through the tank. Specifically, the recovery of the helium gas into the tank 70 is performed in place of the operation (e) of “3. end of liquefaction operation” in the first embodiment,
You can do the following:

(E) ′ The valves 47 and 73 are opened, and the purified high-purity helium gas is stored in the tank 70 by the activated carbon adsorber 37. The valve 73 is closed when the internal pressure of the tank 70 reaches the discharge pressure (15 kg / cm ² ) of the helium compressor 30.
After that, the inlet valve 53 of the cool box 32 is gradually closed, and the pressure in the cool box 32 is reduced from 15 kg / cm ² to atmospheric pressure.

As shown in this embodiment, in the present invention, even if the buffer tank 56 shown in the first embodiment is not used, a tank is connected to an appropriate position of the helium liquefaction machine 10 and helium is connected thereto. The effect can be obtained by containing the gas under pressure. Specifically, the connection position of this tank is a position outside the cold storage box 32 (that is, a position at approximately room temperature) and a position downstream of the internal refiner (the activated carbon adsorber 37 in the above-described embodiment), that is, already. It may be at any position where the internally purified high-purity helium gas flows.

The internal purifier in the present invention is not limited to the above activated carbon adsorber, but various means can be widely applied as long as it is a means for removing impurities in helium gas.

[0057]

As described above, according to the present invention, when the liquid helium is transferred from the liquid helium storage tank storing the liquid helium liquefied by the liquefier into the liquid helium container by utilizing the internal pressure difference between the two, the liquefaction is performed. In the machine, the helium gas circulating in the liquefying machine is recovered in a pressurized state from a line located on the downstream side of the internal refining device and located outside the cool box, and the helium gas is collected in the liquid helium. Since the internal pressure in the liquid helium storage tank is maintained well by replenishing the liquid helium storage tank during transfer, the transfer from the liquid helium storage tank to the liquid helium container is continuously performed a plurality of times. Further, it is possible to transfer liquid helium to a relatively large capacity liquid helium container without any inconvenience. Moreover, it is helium gas that flows at the room temperature or near it in the liquefier outside the cool box, and collects high-purity helium gas that has already been purified by the internal purifier, and uses it to maintain the internal pressure. There is no need to heat or purify this gas again, and there is an effect that liquid helium can be satisfactorily transferred at low cost.

Further, in the apparatus according to claim 3, by using a regulated buffer tank as the tank,
Liquid helium can be transferred at a lower cost.

[Brief description of drawings]

FIG. 1 is a flow sheet showing a liquid helium storage system according to a first embodiment of the present invention.

FIG. 2 is a flow sheet showing a liquid helium storage system according to a second embodiment of the present invention.

FIG. 3 is a flow sheet showing an example of a conventional liquid helium storage system.

[Explanation of symbols]

 10 Helium Liquefaction Machine 11 Liquid Helium Storage Tank 12 Liquid Helium Container 14 Transfer Pipe 22 Warmer (Warming Means) 30 Helium Compressor 32 Cooling Box 37 Activated Carbon Adsorber (Internal Purifier) 56 Buffer Tank 66,76 Helium Supply Pipe ( Helium supply passage)

Claims (3)

[Claims] 1. A liquid helium storage tank is provided with liquefied helium produced by a liquefaction machine having an internal purifier in a cold storage box and a helium is circulated by a compressor. A method for maintaining the internal pressure of the liquid helium storage tank when transferring liquid helium by utilizing the internal pressure difference between the two, which is a line on the downstream side of the internal refiner in the liquefaction machine. The helium gas circulated in the liquefier is collected under pressure from a position outside the cold storage box, and the helium gas is replenished into the liquid helium storage tank during transfer of the liquid helium. Method for maintaining internal pressure of liquid helium storage tank when transferring liquid helium. 2. A liquid helium storage tank containing liquefied helium produced by a liquefaction machine in which a refrigerating box has an internal purifier and circulates helium by a compressor. A device for maintaining the internal pressure of the liquid helium storage tank when transferring liquid helium by utilizing the difference in internal pressure between the two, and a line on the downstream side of the internal purifier in the liquefaction machine. A tank connected to a place located outside the cold-insulating box, from which the helium gas circulating in the liquefier is recovered under pressure, and a helium connecting the inside of the tank and the liquid helium storage tank. A liquid helium storage tank internal pressure holding device at the time of liquid helium transfer, comprising a supply passage. 3. The liquid helium storage tank according to claim 2, wherein the tank is a buffer tank which is connected to a low pressure line of the liquefaction machine and absorbs pressure fluctuations of the compressor. Internal pressure holding device.