Oil tank nitrogen seal oil gas recovery device and use method

Abstract

The invention belongs to the technical field of oil gas recovery treatment, and particularly provides an oil tank nitrogen seal oil gas recovery device which comprises an oil tank, a balance tank, a pressure tank, a supercharging device and a control device, the oil tank is connected with a breather valve, and the breather valve is connected with a first nitrogen input pipeline. The balance tank is connected with a second nitrogen input pipeline, and the control device is electrically connected with the oil tank, the balance tank, the pressure tank and the supercharging device. The problems that during traditional nitrogen sealing of an existing oil gas recovery system, nitrogen content in oil gas is large, treatment is difficult, air enters an oil tank when an oil tank breather valve sucks air, and potential safety hazards exist are solved. Nitrogen in recovered oil gas of the oil tank can be reduced, the recovered oil gas can be used as fuel gas, the energy-saving purpose is achieved, meanwhile, it can be guaranteed that an original breather valve of the oil tank can work normally, the breather valve is located in the nitrogen environment all the time, and potential safety hazards are eliminated.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas recovery and processing, and in particular relates to an oil tank nitrogen sealing oil and gas recovery device and a use method thereof. Background Art

[0002] At present, nitrogen sealing of oil tanks is generally done by injecting nitrogen into the oil tank to prevent air from entering the oil tank when a slight negative pressure is formed when the liquid level in the oil tank drops. This process may result in excessive injection of nitrogen, which may cause the oil tank to be over-pressurized and VOC (VOC is volatile organic compound) to be discharged. The nitrogen injected into the oil tank brings great difficulties to the subsequent oil and gas treatment.

[0003] Existing oil and gas recovery system technologies include condensation method, absorption method, adsorption method and membrane separation method. These recovery systems have complex structures, require more equipment, and have high investment and operating costs.

[0004] The current nitrogen sealing technology and oil and gas recovery technology have many defects. When the oil tank exhales, it contains more nitrogen, and the recovered oil and gas loses its utilization value. At the same time, it is difficult to handle. There is a risk of inhaling air when inhaling, which brings hidden dangers to production operations.

[0005] A Chinese patent document with publication number CN215901229U and publication date February 25, 2022, discloses an oil and gas recovery and processing system, which includes a cryogenic recovery system, an adsorption homogenization system and a biocatalytic oxidation system connected in sequence by pipelines, the cryogenic recovery system includes a cryogenic heat exchanger A and a cryogenic heat exchanger B in parallel, a liquid nitrogen tank, a condensate tank and a nitrogen buffer tank connected to the cryogenic heat exchanger A and the cryogenic heat exchanger B, the adsorption homogenization system includes an adsorption homogenization tank A and an adsorption homogenization tank B in parallel, the adsorption homogenization tank A and the adsorption homogenization tank B are connected to the oil and gas inlet pipeline arranged at the front end of the cryogenic recovery system through a desorption vacuum pump; the biocatalytic oxidation system includes a biocatalytic oxidation box and a nutrient solution spraying device and an aeration fan connected thereto, and biological fillers and biological bacteria attached to the biological fillers are arranged in the biocatalytic oxidation box. This document addresses the problem that existing devices require larger fire protection distances, have a greater risk of fire and explosion, and limit their use. It does not address the problem that the nitrogen content in the oil and gas during traditional nitrogen sealing in the oil and gas recovery system is high and difficult to handle, nor does it address the problem of air entering the oil tank when the oil tank breathing valve is inhaling, and has not eliminated safety hazards. Summary of the invention

[0006] The oil tank nitrogen sealing oil and gas recovery device and use method provided by the present invention have the following purposes: first, to overcome the problem that the nitrogen content in the oil and gas is high and difficult to handle during traditional nitrogen sealing of the oil and gas recovery system in the prior art; second, to overcome the problem that air enters the oil tank when the oil tank breathing valve is inhaling, posing a safety hazard.

[0007] To this end, the present invention provides an oil tank nitrogen sealing oil and gas recovery device, including an oil tank, which is provided with a breathing valve, and the oil tank nitrogen sealing oil and gas recovery device also includes a balancing tank, a pressure tank, a boosting device and a control device. The balancing tank is provided with an oil and gas inlet and a pressure oil and gas inlet, and the pressure tank is provided with an outlet 1, an outlet 2 and an outlet 3. The oil and gas inlet of the oil tank is connected to the oil and gas inlet, and the oil and gas outlet of the balancing tank is connected to the inlet of the pressure tank through the boosting device. The outlet 1 is connected to a low-pressure pipeline network 1, the outlet 2 is connected to a fuel gas pipeline network or a low-pressure pipeline network 2, the outlet 3 is connected to the pressure oil and gas inlet, the breathing valve is provided with a nitrogen input pipeline 1, the balancing tank is provided with a nitrogen input pipeline 2, and the control device is electrically connected to the oil tank, the balancing tank, the pressure tank and the boosting device respectively.

[0008] Preferably, the balancing tank is provided with an oxygen content analyzer, and the oxygen content analyzer is electrically connected to the control device.

[0009] Preferably, the top of the balancing tank is connected to a pressure transmitter 1, and the top of the oil tank is connected to a pressure transmitter 2, and both the pressure transmitter 1 and the pressure transmitter 2 are electrically connected to the control device.

[0010] Preferably, a cylinder is provided on the outside of the breathing valve, the breathing valve is sleeved in the cylinder and an oil tank is connected below the breathing valve.

[0011] Preferably, the cylinder is a coverless cylinder, and a canopy is provided and connected to the upper part of the cylinder.

[0012] Preferably, there is no valve between the oil and gas port and the oil and gas inlet of the oil tank.

[0013] Preferably, the oil tank nitrogen sealing oil and gas recovery device also includes an air supply pipeline, one end of which is connected to the fuel gas pipeline network or the low-pressure pipeline network 2, and the other end of the air supply pipeline is connected to the pipeline between the air outlet 3 and the pressure oil and gas inlet.

[0014] Preferably, the balance tank, pressure tank and booster device are skid-mounted as one.

[0015] Preferably, the bottom of the balance tank and the bottom of the pressure tank are both connected with sewage pipelines.

[0016] A method for using the oil tank nitrogen sealing oil and gas recovery device is characterized by comprising the following steps:

[0017] The oil and gas in the oil tank enter the balance tank;

[0018] When the pressure in the balance tank reaches the target value 1 and the oxygen content in the balance tank is normal, the booster device is started, the booster device sucks the oil and gas in the balance tank and compresses it into the pressure tank, and the pressure tank outputs the pressurized oil and gas;

[0019] When the pressure in the balance tank drops to target value 2, the booster device is turned off;

[0020] When the pressure in the balance tank drops from target value 2 to target value 3, oil and gas are added to the balance tank from the pressure tank;

[0021] When the pressure in the oil tank reaches the target suction pressure of the breathing valve, open the nitrogen input pipeline to inject nitrogen around or into the breathing valve;

[0022] When the oxygen content in the balance tank exceeds the standard, turn off the booster and open nitrogen input pipeline 2 to inject nitrogen into the balance tank to replace nitrogen and oxygen until the oxygen content in the balance tank reaches the standard.

[0023] Beneficial effects of the present invention:

[0024] 1. The oil tank nitrogen sealing oil and gas recovery device and use method provided by the present invention, the oil and gas in the oil tank enter the balance tank; when the pressure in the balance tank reaches the target value one and the oxygen content in the balance tank is normal, the booster device is started, the booster device sucks the oil and gas in the balance tank and compresses it to the pressure tank, and the pressure tank outputs the boosted oil and gas; when the pressure in the balance tank drops to the target value two, the booster device is turned off; when the pressure in the balance tank drops from the target value two to the target value three, the pressure tank replenishes the oil and gas to the balance tank; when the oxygen content in the balance tank exceeds the standard, the booster device is turned off and the nitrogen input pipeline two is opened to inject nitrogen into the balance tank to replace nitrogen and oxygen until the oxygen content in the balance tank meets the standard. It is achieved to reduce the nitrogen in the oil and gas recovered from the oil tank, and the recovered oil and gas can be used as fuel gas to achieve the goal of energy saving.

[0025] 2. The oil tank nitrogen sealing oil and gas recovery device and the use method provided by the present invention, when the pressure in the oil tank reaches the target suction pressure of the breathing valve, the nitrogen input pipeline is opened to inject nitrogen around the breathing valve or into the breathing valve; it ensures that the original breathing valve of the oil tank can work normally, and the breathing valve is always in a nitrogen environment, reducing the risk of corrosion and blockage. It solves the problem of air entering the oil tank when the breathing valve of the oil tank is inhaling, eliminating safety hazards.

[0026] 3. The oil tank nitrogen sealing oil and gas recovery device provided by the present invention reduces the investment and transformation of the oil tank and other oil and gas recovery (VOC control) operating costs while ensuring the original function of the oil tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described in detail below with reference to the accompanying drawings.

[0028] Figure 1 It is the structural principle diagram of the oil tank nitrogen sealing oil and gas recovery device.

[0029] Explanation of the accompanying drawings: 1. Oil tank; 2. Breathing valve; 3. Balance tank; 4. Pressure tank; 5. Pressurization device; 6. Oxygen content analyzer; 7. Pressure transmitter one; 8. Cylinder; 9. Pressure transmitter two; 2-1. Nitrogen input pipeline one; 10. Air supply pipeline; 11. Sewage pipeline; 12. Low-pressure pipeline network one; 13. Fuel gas pipeline network or low-pressure pipeline network two; 3-1. Oil and gas inlet; 3-2. Pressure oil and gas inlet; 3-3. Nitrogen input pipeline two; 4-1. Air outlet one; 4-2. Air outlet two; 4-3. Air outlet three. DETAILED DESCRIPTION

[0030] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples are only used to explain the present invention and are not used to limit the scope of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0031] Embodiment 1:

[0032] like Figure 1 As shown, an oil tank nitrogen sealing oil and gas recovery device comprises an oil tank 1, on which a breathing valve 2 is arranged and connected, and the oil tank nitrogen sealing oil and gas recovery device also comprises a balance tank 3, a pressure tank 4, a pressurizing device 5 and a control device, the balance tank 3 is provided with an oil and gas inlet 3-1 and a pressure oil and gas inlet 3-2, the pressure tank 4 is provided with an outlet 1 4-1, an outlet 2 4-2 and an outlet 3 4-3, the oil and gas port of the oil tank 1 is connected to the oil and gas inlet 3-1, the oil of the balance tank 3 is connected to the oil and gas inlet 3-1, and the oil of the balance tank 3 is connected to the oil of the balance tank 3. The gas outlet is connected to the inlet of the pressure tank 4 through the boosting device 5, the gas outlet 1 4-1 is connected to the low-pressure pipeline 12, the gas outlet 2 4-2 is connected to the fuel gas pipeline or the low-pressure pipeline 2 13, the gas outlet 3 4-3 is connected to the pressure oil and gas inlet 3-2, the breathing valve 2 is provided with a nitrogen input pipeline 1 2-1, the balance tank 3 is provided with a nitrogen input pipeline 2 3-3, and the control device is electrically connected to the oil tank 1, the balance tank 3, the pressure tank 4 and the boosting device 5 respectively.

[0033] The oil and gas in the oil tank 1 enter the balance tank 3; when in use, when the pressure in the balance tank 3 reaches the target value one and the oxygen content in the balance tank 3 is normal, the boosting device 5 is started, the boosting device 5 sucks the oil and gas in the balance tank 3 and compresses it to the pressure tank 4, and the pressure tank 4 outputs the boosted oil and gas; when the pressure in the balance tank 3 drops to the target value two, the boosting device 5 is closed; when the pressure in the balance tank 3 drops from the target value two to the target value three, the pressure tank 4 replenishes the oil and gas to the balance tank 3; when the pressure in the oil tank 1 reaches the target intake pressure of the bottom breathing valve, the nitrogen input pipeline one 2-1 is opened to inject nitrogen around the breathing valve 2 or into the breathing valve 2; when the oxygen content in the balance tank 3 exceeds the standard, the boosting device 5 is closed and the nitrogen input pipeline two 3-3 is opened to inject nitrogen into the balance tank 3 to replace nitrogen and oxygen until the oxygen content in the balance tank 3 meets the standard.

[0034] Specifically, the nitrogen input pipeline 1 2 - 1 and the nitrogen input pipeline 2 3 - 3 are set according to actual conditions to meet the requirements of nitrogen injection or nitrogen and oxygen replacement.

[0035] The low-pressure pipeline network 1 12, the fuel gas pipeline network or the low-pressure pipeline network 2 13 are existing gas pipeline networks, which will not be described in detail here; the low-pressure pipeline network 1 and the low-pressure pipeline network 2 can be low-pressure gas pipeline networks.

[0036] The oil tank nitrogen sealing oil and gas recovery device of the present invention can reduce the nitrogen in the oil and gas recovered from the oil tank 1. The recovered oil and gas can be used as fuel gas to achieve the goal of energy saving. It ensures that the original breathing valve of the oil tank 1 can work normally, and the breathing valve is always in a nitrogen environment, reducing the risk of corrosion and blockage. It solves the problem of air entering the oil tank 1 when the breathing valve 2 of the oil tank 1 is inhaling, eliminating safety hazards. At the same time, the oil tank nitrogen sealing oil and gas recovery device provided by the present invention, in addition to ensuring the original function of the oil tank 1, reduces the investment and transformation of the oil tank 1 and other oil and gas recovery (VOC control) operating costs.

[0037] Embodiment 2:

[0038] On the basis of Example 1, the balancing tank 3 is provided with an oxygen content analyzer 6, which is electrically connected to the control device. The oxygen content analyzer 6 is convenient for monitoring the oxygen content in the balancing tank 3.

[0039] Preferably, the top of the balancing tank 3 is connected to a pressure transmitter 7, and the top of the oil tank 1 is connected to a pressure transmitter 2 9, and the pressure transmitter 1 7 and the pressure transmitter 2 9 are both electrically connected to the control device. The pressure transmitter 1 7 is convenient for monitoring the gas pressure in the balancing tank 3, and the pressure transmitter 2 9 is convenient for monitoring the gas pressure in the oil tank 1.

[0040] Preferably, a cylinder 8 is provided outside the breathing valve 2 , the breathing valve 2 is sleeved in the cylinder 8 and the breathing valve 2 is connected to the oil tank 1 below.

[0041] That is, the breathing valve 2 is surrounded by a tube 8 (the size of the tube 8 is designed according to the normal working of the breathing valve 2. The tube can be a cylinder or other shapes, as long as it can cover the breathing valve 2). A nitrogen input pipeline 2-1 (nitrogen discharge pipeline) is installed at the bottom of the breathing valve 2, and an external nitrogen pipeline and electromagnetic regulating valve are connected. When the pressure in the oil tank 1 is close to the suction pressure of the breathing valve 2, the nitrogen regulating valve on the nitrogen input pipeline 2-1 is opened to inject nitrogen into the tube 8 to ensure that the breathing valve 2 inhales nitrogen, which plays an open nitrogen sealing role on the oil tank 1. Under normal circumstances, a small amount of nitrogen can be injected and tested regularly to keep the oil breathing valve 2 in a nitrogen environment to meet the requirements of open nitrogen sealing of the oil tank.

[0042] Preferably, the cylinder 8 is a coverless cylinder, and the upper part of the cylinder 8 is connected to a canopy.

[0043] The cylinder is filled with nitrogen on a daily basis. The uncovered cylinder does not affect the breathing of the breathing valve 2 , and the canopy prevents debris from entering the breathing valve 2 .

[0044] Preferably, there is no valve between the oil and gas port and the oil and gas inlet 3-1 of the oil tank 1. There is no valve control between the oil tank 1 and the balance tank 3, which facilitates the free entry and exit of oil and gas.

[0045] Preferably, the oil tank nitrogen sealing oil and gas recovery device also includes an air supply pipeline 10, one end of the air supply pipeline 10 is connected to the fuel gas pipeline network or the low-pressure pipeline network 13, and the other end of the air supply pipeline 10 is connected to the pipeline between the air outlet 3 4-3 and the pressure oil and gas inlet 3-2.

[0046] When the pressure of the oil tank 1 is close to the suction pressure of the breathing valve 2, and when the pressure in the balancing tank 3 drops to the target value 2, the boosting device 5 cannot be closed until the pressure in the balancing tank 3 drops from the target value 2 to the target value 3. The pressure is then replenished to the balancing tank 3 through the air replenishment pipeline 10 to meet the pressure requirement in the balancing tank 3 without affecting its use.

[0047] Preferably, the balance tank 3, the pressure tank 4 and the booster device 5 are skid-mounted as a whole.

[0048] The balance tank 3, the pressure tank 4 and the booster device 5 are skid-mounted as a whole, which can reduce the workload on site. Only the external connections of pipelines and electricity need to be completed, saving a lot of work in on-site installation of equipment. The skid-mounted equipment has a compact structure and occupies a small area, which greatly saves space. At the same time, the skid-mounted equipment can be moved as a whole, and the equipment reuse rate is high.

[0049] Preferably, the bottom of the balance tank 3 and the bottom of the pressure tank 4 are both connected with a sewage pipeline 11 .

[0050] The sewage pipeline 11 is convenient for discharging the sewage in the balance tank 3 and the pressure tank 4, so as to avoid the accumulation of sewage and affect the use of the balance tank 3 and the pressure tank 4. Specifically, the sewage pipeline 11 is set according to the actual situation to meet the sewage discharge demand.

[0051] Preferably, the boosting device 5 is a compressor or a fan, which has good practicability and applicability.

[0052] Preferably, the control device adopts a PLC control system, which can meet the real-time performance of the device, has high reliability, and has a simple and flexible system configuration. The PLC control system is configured according to actual conditions to meet the use requirements.

[0053] Embodiment 3:

[0054] On the basis of Example 2, a method for using the oil tank nitrogen sealing oil gas recovery device is provided, characterized in that it comprises the following steps:

[0055] The oil and gas in the oil tank 1 enter the balance tank 3;

[0056] When the pressure in the balancing tank 3 reaches the target value 1 and the oxygen content in the balancing tank 3 is normal, the booster 5 is started, the booster 5 sucks the oil and gas in the balancing tank 3 and compresses it to the pressure tank 4, and the pressure tank 4 outputs the boosted oil and gas;

[0057] When the pressure in the balance tank 3 drops to target value 2, the booster device 5 is closed;

[0058] When the pressure in the balance tank 3 drops from the target value 2 to the target value 3, the pressure tank 4 replenishes oil and gas to the balance tank 3;

[0059] When the pressure in the oil tank 1 reaches the target suction pressure of the bottom breathing valve, the nitrogen input pipeline 1-2-1 is opened to inject nitrogen around the breathing valve 2 or into the breathing valve 2;

[0060] When the oxygen content in the balance tank 3 exceeds the standard, the booster 5 is closed and the nitrogen input pipeline 2 3-3 is opened to inject nitrogen into the balance tank 3 to replace nitrogen and oxygen until the oxygen content in the balance tank 3 meets the standard.

[0061] The present invention solves the problem that the nitrogen content in the oil and gas of the traditional nitrogen sealing of the oil and gas recovery system is high and difficult to handle, and solves the problem that air enters the oil tank when the oil tank breathing valve is inhaling, thereby ensuring that the oil tank 1 does not enter air and eliminating safety hazards; the VOC of the oil tank is used as fuel gas to achieve the purpose of energy saving, and the volatile gas can be used as fuel to reduce VOC emissions and achieve the goal of carbon reduction; in addition to ensuring the original function of the oil tank, the investment in oil tank transformation and other oil and gas recovery (VOC control) operating costs are reduced; the oil tank breathing valve is placed in a nitrogen environment to reduce the risk of corrosion and blockage.

[0062] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "front", "inside", "right", etc. is based on the orientation or positional relationship shown in the drawings, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as a limitation on the present invention.

[0063] The above examples are merely illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. All designs that are the same or similar to the present invention fall within the protection scope of the present invention.

Claims

1. An oil tank nitrogen sealing oil gas recovery device, comprising an oil tank (1), the oil tank (1) is provided with a breathing valve (2) connected thereto, and is characterized in that: The oil tank nitrogen sealing oil and gas recovery device also includes a balance tank (3), a pressure tank (4), a booster device (5) and a control device. The balance tank (3) is provided with an oil and gas inlet (3-1) and a pressure oil and gas inlet (3-2). The pressure tank (4) is provided with an outlet port 1 (4-1), an outlet port 2 (4-2) and an outlet port 3 (4-3). The oil and gas inlet of the oil tank (1) is connected to the oil and gas inlet (3-1), and the oil and gas outlet of the balance tank (3) is connected to the outlet port of the pressure tank (4) through the booster device (5). The first air outlet (4-1) is connected to a low-pressure pipeline network (12), the second air outlet (4-2) is connected to a fuel gas pipeline network or a low-pressure pipeline network (13), the third air outlet (4-3) is connected to a pressure oil and gas inlet (3-2), the breathing valve (2) is provided with a nitrogen input pipeline (2-1), the balance tank (3) is provided with a nitrogen input pipeline (3-3), and the control device is electrically connected to the oil tank (1), the balance tank (3), the pressure tank (4) and the booster device (5).

2. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: The balance tank (3) is provided with an oxygen content analyzer (6), and the oxygen content analyzer (6) is electrically connected to the control device.

3. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: The top of the balancing tank (3) is connected to a pressure transmitter 1 (7), and the top of the oil tank (1) is connected to a pressure transmitter 2 (9). Both the pressure transmitter 1 (7) and the pressure transmitter 2 (9) are electrically connected to a control device.

4. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: A cylinder (8) is arranged outside the breathing valve (2), the breathing valve (2) is sleeved in the cylinder (8), and the bottom of the breathing valve (2) is connected to the oil tank (1).

5. The oil tank nitrogen sealing oil gas recovery device according to claim 4, characterized in that: The cylinder (8) is a coverless cylinder, and the upper part of the cylinder (8) is connected to a rain canopy.

6. The oil tank nitrogen sealing oil and gas recovery device according to claim 1, characterized in that: There is no valve between the oil and gas port of the oil tank (1) and the oil and gas inlet (3-1).

7. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: The oil tank nitrogen sealing oil and gas recovery device also includes an air supply pipeline (10), one end of which is connected to the fuel gas pipeline network or the low-pressure pipeline network 2, and the other end of which is connected to the pipeline between the gas outlet 3 (4-3) and the pressure oil and gas inlet (3-2).

8. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: The balance tank (3), the pressure tank (4) and the pressure boosting device (5) are skid-mounted as a whole.

9. The oil tank nitrogen sealing oil gas recovery device according to claim 1, characterized in that: The bottom of the balance tank (3) and the bottom of the pressure tank (4) are both connected with sewage discharge pipelines (11).

10. A method for using the oil tank nitrogen sealing oil gas recovery device according to any one of claims 1 to 8, characterized in that: The steps include: The oil and gas in the oil tank (1) enter the balance tank (3); When the pressure in the balancing tank (3) reaches the target value 1 and the oxygen content in the balancing tank (3) is normal, the booster device (5) is started, the booster device (5) sucks the oil and gas in the balancing tank (3) and compresses it into the pressure tank (4), and the pressure tank (4) outputs the pressurized oil and gas; When the pressure in the balance tank (3) drops to target value 2, the boosting device (5) is closed; When the pressure in the balance tank (3) drops from target value two to target value three, the pressure tank (4) replenishes oil and gas into the balance tank (3); When the pressure in the oil tank (1) reaches the target suction pressure of the bottom breathing valve, the nitrogen input pipeline 1 (2-1) is opened to inject nitrogen around the breathing valve (2) or into the breathing valve (2); When the oxygen content in the balance tank (3) exceeds the standard, the booster device (5) is closed and the nitrogen input pipeline 2 (3-3) is opened to inject nitrogen into the balance tank (3) to replace nitrogen and oxygen until the oxygen content in the balance tank (3) reaches the standard.

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