Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety
Gases in their compressed state, and particularly compressed air, are almost indispensable to modern industry, and are also used widely for medical purposes, for the manufacture of mineral waters, for underwater diving and in connection with motor vehicles.
For purposes of the present article, compressed gases and air are defined as being those with a gauge pressure exceeding 1.47 bar or as liquids having a vapour pressure exceeding 2.94 bar. Thus, consideration is not given to such cases as natural gas distribution, which is dealt with elsewhere in this Encyclopaedia.
Table 1. Gases often found in compressed form
*These gases are flammable.
All the above gases present either an irritant, asphyxiant or highly toxic respiratory hazard and may also be flammable and an explosive when compressed. Most countries provide for a standard colour-coding system whereby different coloured bands or labels are applied to the gas cylinders to indicate the type of hazard to be expected. Particularly toxic gases, such as hydrogen cyanide, are also given special markings.
All compressed gas containers are so constructed that they are safe for the purposes for which they are intended when first put into service. However, serious accidents may result from their misuse, abuse or mishandling, and the greatest care should be exercised in the handling, transport, storage and even in the disposal of such cylinders or containers.
Characteristics and Production
Depending on the characteristics of the gas, it may be introduced into the container or cylinder in liquid form or simply as a gas under high pressure. In order to liquefy a gas, it is necessary to cool it to below its critical temperature and to subject it to an appropriate pressure. The lower the temperature is reduced below the critical temperature, the less the pressure required.
Certain of the gases listed in table 1 have properties against which precautions must be taken. For example, acetylene can react dangerously with copper and should not be in contact with alloys containing more than 66% of this metal. It is usually delivered in steel containers at about 14.7 to 16.8 bar. Another gas that has a highly corrosive action on copper is ammonia, which must also be kept out of contact with this metal, use being made of steel cylinders and authorized alloys. In the case of chlorine, no reaction takes place with copper or steel except in the presence of water, and for this reason all storage vessels or other containers must be kept free from contact with moisture at all times. Fluorine gas, on the other hand, although reacting readily with most metals, will tend to form a protective coating, as, for example, in the case of copper, where a layer of copper fluoride over the metal protects it from further attack by the gas.
Among the gases listed, carbon dioxide is one of the most readily liquefied, this taking place at a temperature of 15 °C and a pressure of about 14.7 bar. It has many commercial applications and may be kept in steel cylinders.
The hydrocarbon gases, of which liquefied petroleum gas (LPG) is a mixture formed mainly of butane (about 62%) and propane (about 36%), are not corrosive and are generally delivered in steel cylinders or other containers at pressures of up to 14.7 to 19.6 bar. Methane is another highly flammable gas that is also generally delivered in steel cylinders at a pressure of 14.7 to 19.6 bar.
Storage and transport
When a filling, storage and dispatch depot is being selected, consideration must be given to the safety of both the site and the environment. Pump rooms, filling machinery and so on must be located in fire-resistant buildings with roofs of light construction. Doors and other closures should open outwards from the building. The premises should be adequately ventilated, and a system of lighting with flameproof electrical switches should be installed. Measures should be taken to ensure free movement in the premises for filling, checking and dispatch purposes, and safety exits should be provided.
Compressed gases may be stored in the open only if they are adequately protected from the weather and direct sunlight. Storage areas should be located at a safe distance from occupied premises and neighbouring dwellings.
During the transport and distribution of containers, care must be taken to ensure that valves and connections are not damaged. Adequate precautions should be taken to prevent cylinders from falling off the vehicle and from being subjected to rough usage, excessive shocks or local stress, and to prevent excessive movement of liquids in large tanks. Every vehicle should be equipped with a fire extinguisher and an electrically conductive strip for earthing static electricity, and should be clearly marked “Flammable liquids”. Exhaust pipes should have a flame-control device, and engines should be halted during loading and unloading. The maximum speed of these vehicles should be rigorously limited.
The main dangers in the use of compressed gases arise from their pressure and from their toxic and/or flammable properties. The principal precautions are to ensure that equipment is used only with those gases for which it was designed, and that no compressed gases are used for any purpose other than that for which their use has been authorized.
All hoses and other equipment should be of good quality and should be examined frequently. The use of non-return valves should be enforced wherever necessary. All hose connections should be in good condition and no joints should be made by forcing together threads that do not exactly correspond. In the case of acetylene and combustible gases, a red hose should be used; for oxygen the hose should be black. It is recommended that for all flammable gases, the connection-screw thread shall be left-handed, and for all other gases, it shall be right-handed. Hoses should never be interchanged.
Oxygen and some anaesthetic gases are often transported in large cylinders. The transfer of these compressed gases to small cylinders is a hazardous operation, which should be done under competent supervision, making use of the correct equipment in a correct installation.
Compressed air is widely used in many branches of industry, and care should be taken in the installation of pipelines and their protection from damage. Hoses and fittings should be maintained in good condition and subjected to regular examinations. The application of a compressed air hose or jet to an open cut or wound through which air can enter the tissues or the bloodstream is particularly dangerous; precautions should also be taken against all forms of irresponsible behaviour which could result in a compressed air jet coming in contact with any openings in the body (the result of which can be fatal). A further hazard exists when compressed air jets are used to clean machined components or workplaces: flying particles have been known to cause injury or blindness, and precautions against such dangers should be enforced.
Labelling and marking
4.1.1. The competent authority, or a body approved or recognized by the competent authority, should establish requirements for the marking and labelling of chemicals to enable persons handling or using chemicals to recognize and distinguish between them, both when receiving and when using them, so that they may be used safely (see paragraph 2.1.8 (criteria and requirements)). Existing criteria for marking and labelling established by other competent authorities may be followed where they are consistent with the provisions of this paragraph and are encouraged where this may assist uniformity of approach.
4.1.2. Suppliers of chemicals should ensure that chemicals are marked and hazardous chemicals are labelled, and that revised labels are prepared and provided to employers whenever new relevant safety and health information becomes available (see paragraphs 2.4.1 (suppliers’ responsibilities) and 2.4.2 (classification)).
4.1.3. Employers receiving chemicals that have not been labelled or marked should not use them until the relevant information is obtained from the supplier or from other reasonably available sources. Information should be obtained primarily from the supplier but may be obtained from other sources listed in paragraph 3.3.1 (sources of information), with a view to marking and labelling in accordance with the requirements of the national competent authority, prior to use. ...
4.3.2. The purpose of the label is to give essential information on:
- (a) the classification of the chemical;
- (b) its hazards;
- (c) the precautions to be observed.
The information should refer to both acute and chronic exposure hazards.
4.3.3. Labelling requirements, which should be in conformity with national requirements, should cover:
(a) the information to be given on the label, including as appropriate:
- trade names;
- identity of the chemical;
- name, address and telephone number of the supplier;
- hazard symbols;
- nature of the special risks associated with the use of the chemical;
- safety precautions;
- identification of the batch;
- the statement that a chemical safety data sheet giving additional information is available from the employer;
- the classification assigned under the system established by the competent authority;
(b) the legibility, durability and size of the label;
(c) the uniformity of labels and symbols, including colours.
Source: ILO 1993, Chapter 4.
Labelling and marking should be in accordance with standard practice in the country or region in question. The use of one gas for another by mistake, or the filling of a container with a gas different from that which it previously contained, without the necessary cleaning and decontamination procedures, may cause serious accidents. Colour marking is the best method of avoiding such errors, painting specific areas of containers or piping systems in accordance with the colour code stipulated in national standards or recommended by the national safety organization.
For convenience in handling, transportation and storage, gases are commonly compressed in metal gas cylinders at pressures that range from a few atmospheres overpressure to 200 bar or even more. Alloy steel is the material most commonly used for the cylinders, but aluminium is also widely used for many purposes—for example, for fire extinguishers.
The hazards met with in handling and using compressed gases are:
- normal hazards entailed in handling heavy objects
- hazards connected with pressure (i.e., the amount of stored energy in the gases)
- hazards from the special properties of the gas content, which may be flammable, poisonous, oxidizing and so on.
Cylinder manufacture. Steel cylinders may be seamless or welded. The seamless cylinders are made from high-quality alloy steels and carefully heat-treated in order to obtain the desired combination of strength and toughness for high-pressure service. They may be forged and hot-drawn from steel billets or hot-formed from seamless tubes. Welded cylinders are made from sheet material. The pressed top and bottom parts are welded to a cylindrical seamless or welded tube section and heat-treated to relieve material stresses. Welded cylinders are extensively used in low-pressure service for liquefiable gases and for dissolved gases such as acetylene.
Aluminium cylinders are extruded in large presses from special alloys that are heat-treated to give the desired strength.
Gas cylinders must be designed, produced and tested according to strict norms or standards. Every batch of cylinders should be checked for material quality and heat treatment, and a certain number of cylinders tested for mechanical strength. Inspection is often aided by sophisticated instruments, but in all cases the cylinders should be inspected and hydraulically tested to a given test pressure by an approved inspector. Identification data and the inspector’s mark should be permanently stamped on the cylinder neck or another suitable place.
Periodic inspection. Gas cylinders in use may be affected by rough treatment, corrosion from inside and outside, fire and so on. National or international codes therefore require that they shall not be filled unless they are inspected and tested at certain intervals, which mostly range between two and ten years, depending on the service. Internal and external visual inspection together with a hydraulic pressure test is the basis for the approval of the cylinder for a new period in a given service. The test date (month and year) is stamped on the cylinder.
Disposal. A large number of cylinders are scrapped every year for various reasons. It is equally important that these cylinders be disposed of in such a way that they will not find their way back into use through uncontrolled channels. The cylinders should therefore be made completely unserviceable by cutting, crushing or a similar safe procedure.
Valves. The valve and any safety attachment must be regarded as a part of the cylinder, which must be kept in good working condition. Neck and outlet threads should be intact, and the valve should close tight without the use of undue force. Shut-off valves are often equipped with a pressure-relief device. This may be in the form of a resetting safety valve, bursting disc, fuse plug (melt plug) or a combination of bursting disc and fuse plug. The practice varies from country to country, but cylinders for low-pressure liquefied gases are always equipped with safety valves connected to the gas phase.
Different transport codes classify gases as compressed, liquefied or dissolved under pressure. For the purpose of this article, it is useful to use the type of hazard as a classification.
High pressure. If cylinders or equipment burst, damage and injuries may be caused by flying debris or by the gas pressure. The more a gas is compressed, the higher is the stored energy. This hazard is always present with compressed gases and will increase with temperature if the cylinders are heated. Hence:
- Mechanical damage to the cylinder (dents, cuts and so on) should be avoided.
- Cylinders should be stored away from heat and not in direct sun.
- Cylinders should be removed from fires.
- Cylinders should only be connected to equipment suitable for the intended use.
- The cylinder valve should be protected with the cap during transport.
- Cylinders should be secured in use against falling, which may knock off the valve.
- Tampering with safety devices should be avoided.
- Cylinders should be handled with care to avoid mechanical shocks in very cold climates, since steel may become brittle at low temperature.
- Corrosion, which reduces the strength of the shell, should be avoided.
Low temperature. Most liquefied gases will evaporate rapidly under atmospheric pressure, and may reach very low temperatures. A person whose skin is exposed to such liquid may sustain injuries in the form of “cold burns”. (Liquid CO2 will form snow particles when expanded.) Correct protective equipment (e.g., gloves, goggles) should therefore be used.
Oxidation. The hazard of oxidation is most evident with oxygen, which is one of the most important compressed gases. Oxygen will not burn on its own, but is necessary for combustion. Normal air contains 21% oxygen by volume.
All combustible materials will ignite more easily and burn more vigorously when the oxygen concentration is increased. This is noticeable with even a slight increase in oxygen concentration, and utmost care must be taken to avoid oxygen enrichment in the working atmosphere. In confined spaces small oxygen leaks may lead to dangerous enrichment.
The danger with oxygen increases with increasing pressure to the point where many metals will burn vigorously. Finely divided materials may burn in oxygen with explosive force. Clothing that is saturated with oxygen will burn very rapidly and be difficult to extinguish.
Oil and grease have always been regarded as dangerous in combination with oxygen. The reason is that they react readily with oxygen, their existence is common, the ignition temperature is low and the developed heat may start a fire in the underlying metal. In high-pressure oxygen equipment the necessary ignition temperature may easily be reached by the compression shock that may result from rapid valve opening (adiabatic compression).
- Valves should be operated slowly.
- All oxygen equipment should be kept clean and free from oil and dirt.
- Only materials that are proven to be safe with oxygen should be used.
- Workers should refrain from lubricating oxygen equipment.
- Entering confined spaces where oxygen may exist in higher concentration should be avoided.
- The atmosphere should be checked and the use of oxygen instead of compressed air or some other gas should be strictly avoided.
Flammability. The flammable gases have flashpoints below room temperature and will form explosive mixtures with air (or oxygen) within certain limits known as the lower and upper explosion limits.
Escaping gas (also from safety valves) may ignite and burn with a shorter or longer flame depending on the pressure and amount of gas. The flames may again heat nearby equipment, which may burn, melt or explode. Hydrogen burns with an almost invisible flame.
Even small leaks may cause explosive mixtures in confined spaces. Some gases, such as liquefied petroleum gases, mostly propane and butane, are heavier than air and are difficult to vent away, as they will concentrate in the lower parts of buildings and “float” through channels from one room to another. Sooner or later, the gas may reach an ignition source and explode.
Ignition may be caused by hot sources, but also by electrical sparks, even very small ones.
Acetylene takes a special place among the combustible gases because of its properties and wide use. If heated, the gas may start to decompose with the development of heat even without the presence of air. If allowed to proceed, this may lead to cylinder explosion.
Acetylene cylinders are, for safety reasons, filled with a highly porous mass which also contains a solvent for the gas. Outside heating from a fire or welding torch, or in certain cases internal ignition by strong backfires from welding equipment, may start a decomposition within the cylinder. In such cases:
- The valve should be closed (using protective gloves if necessary) and the cylinder should be removed from fire.
- If part of the cylinder becomes hotter, it should be put into a river, canal or the like to cool down or cooled with water sprays.
- If the cylinder is too hot to be handled, it should be sprayed with water from a safe distance.
- Cooling should continue until the cylinder stays cool by itself.
- The valve should be kept closed, because gas flow will accelerate decomposition.
Acetylene cylinders in several countries are equipped with fuse (melting) plugs. These will release the gas pressure when they melt (usually at about 100 °C) and prevent cylinder explosion. At the same time there is a risk that the released gas may ignite and explode.
Common precautions to observe in respect of combustible gases are as follows:
- Cylinders should be stored separately from other gases in a well ventilated area above ground level.
- Leaking cylinders or equipment should not be used.
- Liquid gas cylinders should be stored and used in an upright position. Larger quantities of gas will come out if liquid is expelled through the safety valves instead of gas. The pressure will be reduced more slowly. Very long flame will result if the gas ignites.
- In case of leaks, any possible ignition source should be avoided.
- Smoking where flammable gases are stored or used should be prohibited.
- The safest way of extinguishing a fire is usually to stop the supply of gas. Merely extinguishing the flame may cause the formation of an explosive cloud, which may re-ignite in contact with a hot object.
Toxicity. Certain gases, if not the most common, may be toxic. At the same time, they may be irritating or corrosive to the skin or eyes.
Persons who handle these gases should be well trained and aware of the danger involved and the necessary precautions. The cylinders should be stored in a well ventilated area. No leaks should be tolerated. Suitable protective equipment (gas masks or breathing equipment) should be used.
Inert gases. Gases such as argon, carbon dioxide, helium and nitrogen are widely used as protective atmospheres to prevent unwanted reactions in welding, chemical plants, steel works and so on. These gases are not labelled as being hazardous, and serious accidents may happen because only oxygen can support life.
When any gas or gas mixture displaces the air so that the breathing atmosphere becomes deficient in oxygen, there is a danger of asphyxiation. Unconsciousness or death may result very rapidly when there is little or no oxygen, and there is no warning effect.
Confined spaces where the breathing atmosphere is deficient in oxygen must be ventilated before entering. When breathing equipment is used, the person entering must be supervised. Breathing equipment must be used even in rescuing operations. Normal gas masks give no protection against oxygen deficiency. The same precaution must be observed with large, permanent firefighting installations, which are often automatic, and those who may be present in such areas should be warned of the danger.
Cylinder filling. Cylinder filling involves the operation of high-pressure compressors or liquid pumps. The pumps may operate with cryogenic (very low-temperature) liquids. The filling stations may also incorporate large storage tanks of liquid gases in a pressurized and/or deeply refrigerated state.
The gas filler should check that the cylinders are in acceptable condition for filling, and should fill the correct gas in not more than the approved amount or pressure. The filling equipment should be designed and tested for the given pressure and type of gas, and protected by safety valves. Cleanliness and material requirements for oxygen service must be observed strictly. When filling flammable or toxic gases, special attention should be given to the safety of the operators. The primary requirement is good ventilation combined with correct equipment and technique.
Cylinders which are contaminated with other gases or liquids by the customers constitute a special hazard. Cylinders with no residual pressure may be purged or evacuated before filling. Special care should be taken to ensure that medical gas cylinders are free from any harmful matter.
Transport. Local transport tends to become more mechanized through the use of fork-lift trucks and so on. Cylinders should be transported only with the caps on and secured against falling from the vehicles. Cylinders must not be dropped from trucks directly onto the ground. For hoisting with cranes, suitable lifting cradles should be used. Magnetic lifting devices or caps with uncertain threads should not be used for lifting cylinders.
When cylinders are manifolded into larger packages, great care should be taken to avoid strain on the connections. Any hazard will be increased because of the greater amount of gas involved. It is good practice to divide larger units into sections and to place shut-off valves where they can be operated in any emergency.
The most frequently occurring accidents in cylinder handling and transport are injuries caused by the hard, heavy and difficult-to-handle cylinders. Safety shoes should be worn. Trolleys should be provided for longer transport of single cylinders.
In international transport codes, compressed gases are classified as dangerous goods. These codes give details about which gases may be transported, cylinder requirements, allowed pressure, marking and so on.
Identification of content. The most important requirement for safe handling of compressed gases is the correct identification of the gas content. Stamping, labelling, stencilling and colour marking are the means that are used for this purpose. Certain requirements for marking are covered in International Organization for Standardization (ISO) standards. The colour marking of medical gas cylinders follows the ISO standards in most countries. Standardized colours are also used in many countries for other gases, but this is not a sufficient identification. In the end only the written word can be regarded as a proof of the cylinder content.
Standardized valve outlets. The use of a standardized valve outlet for a certain gas or group of gases strongly reduces the chance of connecting cylinders and equipment made for different gases. Adapters should therefore not be used, as this sets aside the safety measures. Only normal tools and no excessive force should be used when making connections.
Safe Practice for Users
The safe use of compressed gases entails applying the safety principles outlined in this chapter and the ILO Code of Practice Safety in the Use of Chemicals at Work (ILO 1993). This is not possible unless the user has some basic knowledge of the gas and the equipment that he or she is handling. In addition the user should take the following precautions:
- Gas cylinders should only be used for the purpose for which they are intended and not as rollers or work supports.
- The cylinders should be stored and handled in such a way that their mechanical strength is not reduced (e.g., by severe corrosion, sharp dents, cuts and so on).
- The cylinders should be removed from fires or excessive heat.
- Only the necessary number of gas cylinders should be kept in working areas or occupied buildings. It is preferable for them to be kept near doors and not in emergency escape routes or difficult-to-reach areas.
- Any cylinders that have been exposed to fires should be clearly marked and returned to the filler (owner), since the cylinders may have become brittle or lost their strength.
- Cylinders should be stored in a well-ventilated place, away from rain or snow and any combustible storage.
- Cylinders in use should be secured against falling.
- Gas content should be positively identified before use.
- Labels and instructions should be carefully read.
- Cylinders should only be connected to equipment meant for the particular service.
- Connections should be kept clean and in good order; their condition should be checked at regular intervals.
- Good tools (e.g., normal length, fixed wrenches) should be used.
- Loose valve keys should be left in place when the cylinder is in use.
- Valves should be kept closed when cylinders are not in use.
- Cylinders or connected equipment should be removed from confined spaces when not in use (even during short breaks).
- The atmosphere should be checked for oxygen content and, if possible, for flammable gases before confined spaces are entered and during prolonged working periods.
- It should be kept in mind that heavy gases may concentrate in lower areas and that they may be difficult to remove by ventilation.
- Cylinders should be protected against contamination from pressurized equipment, since backflow of other gases may lead to serious accidents. Proper non-return valves, block-and-bleed arrangements and the like should be used.
- Empty cylinders should be returned to the filler with the valves closed and the caps in place. A little residual pressure should always be left in the cylinder to prevent contamination with air and moisture.
- The filler should be notified of any faulty cylinders.
- Acetylene should only be used at a correctly reduced pressure.
- Flame arrestors should only be used in acetylene lines where acetylene is used with compressed air or oxygen.
- Fire extinguishers and heat-protecting gloves should be available with gas welding equipment.
- Liquid gas cylinders should be stored and used in an upright position.
- Poisonous and irritating gases, such as chlorine, should be handled only by well-informed operators with personal safety equipment.
- Unidentified cylinders should not be kept in stock. Fixed installations, with the gas cylinders connected in separate gas centrals, are safest where gases are used regularly.