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Safety of high pressure gas cylinders -- Wan Rong experiment

Jul. 12, 2021

1. Cylinder and pressure reducing valve

Gas cylinders are high pressure vessels. The bottle is filled with high pressure gas, but also to withstand handling, rolling and other external forces. Because of this, its quality requirements are strict, the material requirements are high, commonly used seamless alloy or manganese steel tube into cylindrical containers. Cylinder wall thickness 5 ~ 8mm, capacity 12 ~ 55m3. The bottom is hemispheric and usually has a steel base for easy erection. The top of the cylinder has an open and close valve (i.e. on/off valve), and the side joint of the valve (branch pipe) is connected with threads. For combustible gas, the thread should be left, and for non-combustible gas, the thread should be right. This is to eliminate the possibility of compression of combustible gases into cylinders containing air or oxygen and of accidental connection of combustible gas cylinders to devices with explosive risks.

All types of gas cylinder containers must comply with the provisions of "Gas Cylinder Safety Supervision Regulations" in Document no.12 [1989] of ministry of Labor, PRC. The cylinder shall be marked with a manufacturing stamp and a inspection stamp. The manufacturing steel seal shall be marked with the code number of the cylinder manufacturer, the cylinder number, the working pressure MPa, the actual weight kg, the actual volume L, the design wall thickness of the cylinder mm, the inspection mark of the manufacturer and the manufacturing date, the supervision and inspection mark and the cylinder mark used in cold areas. The inspection stamp shall mark the code of the inspection unit, the date of inspection, the date of the next inspection, etc. Because the pressure in the cylinder is generally very high, and the pressure required for use is often low, the amount of gas released can not be accurately and stably regulated by opening and closing the valve alone. In order to reduce the pressure and maintain a stable pressure, you need to install a pressure reducer. Different working gases have different unstressors. The appearance of different pressure reducer is painted with different colors to mark, which is consistent with the color mark of various gas cylinders. It is important to note that a pressure reducer for oxygen may be used on cylinders containing nitrogen or empty gas, and that a pressure reducer for nitrogen may be used on oxygen cylinders only after the grease has been fully removed. When loading and unloading the pressure reducer, attention must be paid to prevent the screw on the branch pipe joint from sliding teeth, so as not to install the screw and leakage or high pressure injection. When unloading, we should pay attention to light, properly stored, avoid impact, vibration, do not put in the place of corrosive substances, and prevent dust from falling into the table so that the blocking failure.

After each cylinder is used up, first close the cylinder valve, then loosen the pressure regulating screw and drain the gas body in the pressure reducer. If you do not loosen the pressure regulating screw, the spring long-term pressure, pressure gauge will make the pressure reducer failure.

2. Classification of gas in cylinders

(1) The compressed gas whose critical temperature is lower than -10℃ after high pressure compression is still in the gaseous state, which is called compressed gas, such as oxygen, nitrogen, hydrogen, air, argon, helium and other gas cylinders. This type of gas cylinder design pressure is greater than 12MPa (125kGF /cm2), known as high pressure cylinder.

(2) the critical temperature of liquefied gas ≥10℃ by high pressure compression, into liquid and its vapor in equilibrium is called liquefied gas, such as carbon dioxide, nitrous oxide,ammonia, chlorine, hydrogen sulfide, etc.

(3) Dissolved gas can produce decomposition, explosion and other dangerous gases by simply adding high pressure compression. It must be dissolved in appropriate solvent and absorbed by porous solid filler at the same time of adding high pressure. Under 15℃ pressure up to 0.2MPa is called dissolved gas, such as acetylene.

3. Colors and labels of high-pressure gas cylinders Table 2-96 lists the colors and labels of high-pressure gas cylinders. Table 2-96 Colors and labels of high-pressure gas cylinders

4. Properties and safe handling of several compressed combustible gas and auxiliary gas

(1) Acetylene acetylene cylinder is filled with granular activated carbon, charcoal, asbestos or diatomite and other porous substances in the cylinder, and then mixed with acetone, through acetylene gas to dissolve in acetone, until 15℃ pressure up to 1.52MPa (15.5kGF /cm2). Acetylene is a highly flammable and explosive gas. Acetylene - air mixtures containing 7% to 13% acetylene and acetylene - oxygen mixtures containing approximately 30% acetylene are most explosive. There may be small amounts of phosphine in unpurified acetylene. Phosphine spontaneous combustion point is very low, gaseous phosphine (PH3) at 100℃ will be

spontaneous combustion, and Liquid phosphine will spontaneously ignite even at less than 100℃.

Therefore, when acetylene contains air, the presence of phosphine may constitute an explosive acetylene - air mixture. Acetylene and copper, silver, mercury and other metals or salt contact, will generate explosive substances such as copper acetylene (Cu2C2) and silver acetylene (Ag2C2).

Therefore, any equipment (pipes and parts) used for acetylene should not be made of silver or copper alloys. Combustion and explosion occur when acetylene meets compounds such as chlorine and hypochlorite. Therefore, the use of carbon tetracloride fire extinguishers is prohibited when acetylene burns and catches fire.

Acetylene cylinder should be placed in a well-ventilated place, not stored in the laboratory building, should be stored in a separate bottle storage room outside the building, the room temperature should be lower than 35℃. When using acetylene by atomic absorption method, attention should be paid to the prevention of tempering, and the pipeline should be installed to prevent the return of firearms (valves). Before opening the acetylene cylinder, it is necessary to supply enough air to the burner before supplying acetylene gas. When turning off the gas, the acetylene gas should be turned off first, and then the air should be turned off. When the pressure in the acetylene cylinder drops to 0.3mpa (3kGF /cm2), it is necessary to stop using and change another bottle.

(2) Hydrogen is a flammable gas. Because of its small density, easy to leak out from micropores, and its diffusion rate is fast, easy to mix with other gases. The explosive limit for a mixture of hydrogen and air is 4.1% ~ 7.5% (volume) of hydrogen in the air. The combustion rate is about 2.7m/s at normal temperature and pressure. When checking whether the hydrogen conduit and valve leak, the soapy water inspection method must be used, and the open flame inspection is not allowed. Storage hydrogen bottles should be strictly prohibited fireworks.

(3) Nitrous oxide, also known as laughing gas, has anesthetic excitability, so special attention should be paid to ventilation when used. The vapor pressure of liquid nitrous oxide at 20℃ is about 5MPa (50kgf/cm2). Nitrous oxide is decomposed into a mixture of oxygen and nitrogen when heated and is a combustion gas.

When atomic absorption method is used for the determination of elements of high melting point or refractory salt compounds, nitrous oxide - acetylene flame is required to obtain a highertemperature, and its reaction is as follows:

In the above process, nitrous oxide is decomposed into a mixture containing 33.3% oxygen and 66.7% nitrogen, and acetylene is burned with the oxygen in it. The reactions that occur in nitrous oxide - acetylene flames are more complex than in ordinary flames. When burning, we should pay attention to prevent the suction of air from the drainage valve of the atomization chamber in the atomic absorption spectrophotometer, otherwise it will cause an explosion.

(4) Oxygen is a strong combustion gas, pure oxygen is especially active at high temperature. When the temperature is constant and the pressure is increased, oxygen and oil substances can occur violent chemical reaction and cause fever spontaneous combustion, resulting in explosion.

For example, industrial mineral oil can produce spontaneous combustion when it comes into contact with oxygen at pressure above 3MPa. Therefore, oxygen cylinders must be strictly prevented from contact with oil. Grease lubrication is prohibited for pressure reducer and valve.

The oxygen cylinder should not be mixed with other combustible gases, or be filled with oxygen by misuse of other combustible gas cylinders. Oxygen cylinders are generally filled at 20℃ and 15MPa under pressure conditions. The pressure of the oxygen cylinder will increase with the increase of temperature, so it is prohibited to expose the cylinder to strong sunlight, in order to avoid the explosion of excessive pressure in the bottle.

5. Common sense on cylinder safety

(1) Gas cylinders must be stored in ventilated, cool, dry, isolated from open fire, away from heat source, and prevent exposure to the sun. It should be managed by special personnel. There should be eye-catching signs, such as "acetylene dangerous, no fireworks" and other words.

Combustible gas cylinders are not allowed to enter the laboratory building. It is strictly prohibited to store acetylene cylinders, hydrogen cylinders, oxygen cylinders and chlorine cylinders together or transport them in the same car.

(2) When using the gas cylinder, it should be placed erect and fixed to prevent dumping.

(3) Handling cylinders to use special cylinder car, to take gently, to prevent throwing, knocking, rolling slip or violent shaking. The cylinder must be carried in advance to wear the cylinder helmet, in case of accidentally broken bottle mouth explosion accident. The cylinder must have two rubber shockproof rings. Acetylene bottle is strictly prohibited to lie down rolling.

(4) Cylinders should be subjected to pressure tests and checked regularly.

(5) The pressure reducer of the gas cylinder should be special, and the screw buckle should be tightened during installation, and the screw thread should be rotated into 7 circles without air leakage. When opening a high-pressure gas cylinder, the operator should stand on the side of the cylinder mouth, and the action should be slow to reduce the friction of air flow and prevent static electricity. 

(6) Flammable gas cylinders such as acetylene shall not be placed on insulators such as rubber to facilitate electrostatic release.

(7) Oxygen cylinders and their special tools are strictly prohibited from contact with oil substances, and operators should not wear work clothes and gloves stained with various oils or oil.

(8) The distance between a flammable gas cylinder such as hydrogen cylinder and an open flame should not be less than 10m.

(9) The gas in the bottle shall not be used up completely, generally, the residual pressure of 0.2 ~ 1MPa should be maintained.

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