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How to use Dewar flasks (cans) safely

Views:5     Author:Site Editor     Publish Time: 2020-11-30      Origin:Site

Dewar flask (tank) is a stainless steel pressure vessel with super vacuum insulation. It is designed for storing, transporting and using liquid oxygen, liquid nitrogen, liquid argon or carbon dioxide. These containers are manufactured according to the DOT4L standard and are used for reliable and economical transportation and storage of cryogenic liquid gas, and for storing and supplying cryogenic liquid gas on-site in a wide range of applications.


1. Structure of Dewar flask (tank)

Dewar flasks (tanks) generally have four valves, and the pipes where the valves are located are marked in English. LIQULD is a valve for liquid use, USE (GAS USE) is a valve for gas use, VENT is a vent valve, and PB (PRESSUREBUILD UP) is a booster valve. According to experience, a small number of Dewars do not have gas service valves. In addition, there are gas pressure gauges and level gauges.


2. Use of Dewar flasks (cans)

If you are using a Dewar flask (tank) for liquid phase. Therefore, the first step is to use a high-pressure metal hose to connect the pipe where the liquid valve of the Dewar (tank) is located to the evaporator. After confirming the connection, the liquid valve can be opened. If the gas pressure to the workshop does not meet the requirements for use , Please turn on the booster valve, wait a few minutes for the pressure to rise and meet the requirements for use. Ready to use. When the Dewar (tank) has no liquid phase and the gas pressure is only 5kg, it means that the gas in the Dewar (tank) has been used up. Please close the liquid use valve and stop using it.


3. Advantages of Dewar flasks (cans)

  • 1. It is safe and reliable to use, convenient and quick to discharge liquid, and low loss of static liquid daily evaporation rate.

  • 2. Compared with compressed gas cylinders, it can hold a large amount of gas at a relatively low pressure.

  • 3. It provides an easy-to-operate cryogenic liquid source.

  • 4. Because the Dewar flask (tank) is sturdy and reliable, has a long holding time, and contains a gas supply system, the built-in vaporizer can continuously output normal temperature gas with a flow rate of up to 9.2m3/h, and the maximum constant output pressure of the gas can reach 1.2MPa ( Medium pressure type)/2.2 MPa (high pressure type), which fully meets the gas requirements under normal circumstances.

  • 5. It is equipped with multi-level safety protection device, which has better safety and simple and reliable operation.

  • 6. Large storage capacity can reduce storage space, reduce bottle changing operations, and improve work efficiency.

  • 7. It avoids the hidden quality hazards caused by secondary pollution due to many links in the process of filling, transporting and using cylinder gas.

  • 8. High gas utilization rate: After each use of the cylinder gas, there must be residual pressure, and the liquid gas supply of the hexagonal gas cylinder can avoid any waste and reduce the gas cost.


4. Dewar (tank) gas supply operation

【Ready to work】

1. Keep the Dewar bottle (tank) and the oxygen cylinder outside the safe distance (the distance between the two bottles should be kept at least 5 meters).

2. There is no open flame device around the bottle and tank, and there should be a fire prevention device nearby.

3. Check whether the Dewar bottle (tank) is well connected to the end user.

4. Check that all valves, pressure gauges, safety valves, and Dewar (tank) valve fixtures in the system should be complete and easy to use.

5. The gas supply system must not have grease and leakage.


【Operation Procedure】

In order to make the Dewar (tank) gas supply system a continuous, stable, safe and economical gas supply, when the Dewar (tank) gas supply system is in use, a single Dewar (tank) can be used to supply gas to the terminal. It is also possible to switch between two Dewar bottles (cans) or more than three Dewar bottles (cans) through the busbar. The specific steps are as follows:

1. Before connecting the hose, let out a part of the gas from the tracheal system to remove the air in the hose, and then connect it to the Dewar gas interface to measure the flow of the production gas. Connect the liquid inlet and outlet valves for large quantities ( LIQUID); Connect the gas outlet valve (VENT) with a small amount.

2. When the air supply pressure is insufficient during use, open the boost valve to increase the pressure, and adjust the pressure regulator to automatically control the pressure boost range. Under normal circumstances, the small liquid tank has been adjusted before leaving the factory, and the user does not need to adjust it by himself.

3. When the liquid or gas flows out, the booster pipe, exhaust pipe, valve, etc. will cool down, and each valve and joint may have air leakage. At this time, use tap water to shower and then tighten the joint.

4. The maximum working pressure of the small liquid bottle (tank) is 12Kg/cm2. If the pressure is exceeded, the safety valve will automatically vent. At this time, the booster valve should be closed and opened after the pressure drops.

5. During use, do not touch the ventilation hose, booster tube, etc. with your hands to avoid cold injury, let alone knock on valves, pipelines, etc., to avoid embrittlement and cracking.

6. When using, the small liquid bottle (tank) should be upright and not allowed to lie down. When opening the valve, the operator is not allowed to face the positive direction of the valve discharge (including the discharge direction of the safety valve).

7. If the liquid discharge valve (LIQUID) is used for production and the gas supply needs to be interrupted in the middle, after the discharge valve is closed, the stop valve on the gas pipeline is not allowed to be closed immediately, because the liquid gas in the metal hose is rapidly expanded and the gas cannot flow. There will be an explosion accident.

8. When the small liquid bottle (tank) is used up, first close the booster valve, and then close the Dewar (tank) inlet valve, after venting through the vent valve, remove the metal hose connected to the Dewar (tank) Connector.


【Emergency stop procedure】

When an emergency occurs in the system or workshop and the gas supply system needs to be stopped urgently, the following steps should be followed to stop.

1. Quickly close the gas valve and booster valve for the Dewar (tank).

2. Open the confluence drain valve quickly.

3. Open the ventilation device in the operation room quickly.


【Precautions】

1. When the Dewar bottle (tank) is stored and not in use and the booster valve cannot be opened, be sure to check whether the booster valve is closed, otherwise it will cause gas leakage in the tank.

2. After stopping use, remember to close the opened booster valve. The booster valve is only opened during use.

3. When the booster valve is fully opened during use, the pressure of the Dewar (tank) pressure gauge will no longer increase, which means that there is no liquid phase in the tank but only gas phase.

4. Check whether the Dewar flask (tank) leaks air at each connection (dynamic and static sealing points) during use. Frosting during use is a normal phenomenon, and it is easy to confuse the Dewar flask (tank) with air leakage.

5. After the booster valve is opened, the system pressure shall not be greater than the design specified pressure. When the system pressure is too high, closing the booster valve cannot be controlled; the vent valve should be opened quickly to relieve the pressure of the system, and the reason can be found before it can be used again.

6. Check the pressure of the Dewar (tank), and try to check it every half an hour. When the pressure of the Dewar (tank) is low, you should open or open the system booster valve to increase the pressure of the system and avoid The surface is low.


5. Precautions for filling Dewar flasks (cans)

Before filling the dewar (tank) with cryogenic liquid, first determine the filling medium and filling quality of the gas cylinder. The filling quality can be found in the product specification table. To ensure accurate filling, please use a weighing scale.

  • 1. Connect the gas cylinder inlet and outlet valve (DPW cylinder is the inlet valve) to the supply source with a special delivery hose, and tighten it to ensure no leakage.

  • 2. Open the discharge valve and the inlet and outlet valves of the gas cylinder, and then open the supply valve to start filling.

  • 3. During the filling process, monitor the pressure in the bottle through the pressure gauge and adjust the discharge valve to keep the pressure at 0.07~0.1MPa (10~15 psi).

  • 4. When the required filling quality is reached, close the inlet and outlet valves, discharge valves and supply valves.

  • 5. Remove the delivery hose and remove the gas cylinder from the scale.  

    Warning: It is strictly forbidden to overfill the gas cylinder.  

    Warning: Confirm the medium in the bottle and the filling medium before filling.  

    Warning: Filling should be carried out in a well-ventilated place, because the accumulation of gas is very dangerous.  

    Note: The pressure of a fully filled gas cylinder rises very quickly, which may cause the safety valve to open.  

    Note: Just after performing operations related to liquid oxygen or liquefied natural gas, since liquid oxygen or liquefied natural gas is likely to be splashed on the clothes, please be careful not to smoke or approach a fire immediately.  

    Note: When the saturation pressure of liquid carbon dioxide drops to 0.48MPa (70 psi), the liquid carbon dioxide will form solid dry ice. Therefore, when using DPL450-175H/195H/210H, DPL450-175V/195V/210V, DPW650-499H gas cylinders to fill liquid carbon dioxide, the carbon dioxide gas in the cylinder must be pressurized before filling.


6. Other matters needing attention in Dewar bottles (cans)

  • 1. When engaging in any work with cryogenic liquids, you must wear protective equipment to prevent frostbite.

  • 2. In the operation of liquid oxygen, the opening and closing of the valve should be carried out slowly. Sudden opening and closing will cause any pollution in the system to ignite.

  • 3. Except for liquid oxygen, all liquid vapors can suffocate people.

  • 4. The filling volume cannot exceed: liquid oxygen 192kg, liquid nitrogen 136kg, liquid argon 230kg.

  • 5. It is recommended to reduce the pressure in the bottle as much as possible when using liquid.

  • 6. Make sure the gas cylinder is upright during operation, and avoid tipping over. Never roll the gas cylinder horizontally.

  • 7. Open flames are strictly prohibited within 5m around the storage, vaporization, filling, and use places of liquid oxygen, and all fire sources should be eliminated, and there should be obvious fire prohibition signs.

  • 8. No openings leading to low places (such as basements, pits, underground wells, ditches) are allowed within at least 5m around the storage, vaporization, filling and use places of liquid oxygen; there must be a liquid retaining weir at the entrance of the trench .

  • 9. When filling or disposing of liquid oxygen, operators must not wear work clothes and personal protective equipment contaminated by grease. All clothing and protective equipment contaminated by grease must be replaced.


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