Bottled gas is a term used for substances which are gaseous at standard temperature and pressure (STP) and have been compressed and stored in carbon steel, stainless steel, aluminum, or composite bottles known as gas cylinders.
There are four cases: either the substance remains a gas at standard temperature but increased pressure, the substance liquefies at standard temperature but increased pressure, the substance is dissolved in a solvent, or the substance is liquefied at reduced temperature and increased pressure. In the last case the bottle is constructed with an inner and outer shell separated by a vacuum (dewar flask) so that the low temperature can be maintained by evaporative cooling.
The substance remains a gas at standard temperature and increased pressure, its critical temperature being below standard temperature. Examples include:
The substance liquefies at standard temperature but increased pressure. Examples include:
The substance is dissolved at standard temperature in a solvent. Examples include:
The substance is liquefied at reduced temperature and increased pressure. These are also referred to as cryogenic gases. Examples include:
The general rule is that one unit volume of liquid will expand to approximately 800 unit volumes of gas at Standard temperature and pressure with some variation due to intermolecular force and molecule size compared to an ideal gas. Normal high pressure gas cylinders will hold gas at pressures from 200 to 400 bars (3,000 to 6,000 psi). An ideal gas pressurised to 200 bar in a cylinder would contain 200 times as much as the volume of the cylinder at atmospheric pressure, but real gases will contain less than that by a few percent. At higher pressures, the shortfall is greater.
Because the contents are under high pressure and are sometimes hazardous, there are special safety regulations for handling bottled gases. These include chaining bottles to prevent falling and breaking, proper ventilation to prevent injury or death in case of leaks and signage to indicate the potential hazards.
In the United States, the Compressed Gas Association (CGA) sells a number of booklets and pamphlets on safe handling and use of bottled gases. (Members of the CGA can get the pamphlets for free.) The European Industrial Gases Association and the British Compressed Gases Association provide similar facilities in Europe and the United Kingdom.
In the United States, 'bottled gas' typically refers to liquefied petroleum gas. 'Bottled gas' is sometimes used in medical supply, especially for portable oxygen tanks. Packaged industrial gases are frequently called 'cylinder gas', though 'bottled gas' is sometimes used.
The United Kingdom and other parts of Europe more commonly refer to 'bottled gas' when discussing any usage whether industrial, medical or liquefied petroleum. However, in contrast, what the United States calls liquefied petroleum gas is known generically in the United Kingdom as 'LPG'; and it may be ordered using by one of several Trade names, or specifically as butane or propane depending on the required heat output.
Different countries have different gas colour codes but attempts are being made to standardise the colours of cylinder shoulders:
The user should not rely on the colour of a cylinder to indicate what it contains. The label or decal should always be checked for product identification.
The colours below are specific shades, defined in the European Standard in terms of RAL coordinates. The requirements are based on a combination of a few named gases, otherwise on the primary hazard associated with the gas contents:
|Argon||dark green shoulder|
|Carbon dioxide||grey shoulder|
|Nitrous oxide||blue shoulder|
|Nitrogen||black shoulder||previously grey in the UK|
|Oxygen||white shoulder||previously black in the UK|
|Toxic or corrosive||yellow shoulder||ammonia, chlorine, fluorine, arsine, carbon monoxide, sulfur dioxide|
|Flammable||red shoulder||hydrogen, methane, ethylene, forming gas|
|Oxidising||light blue shoulder||nitrous oxide, oxygen-containing blends|
(nontoxic, nonflammable, nonoxidising)
|bright green||neon, krypton, xenon|
|Toxic and flammable or
Toxic and corrosive
|yellow and red shoulders (either two bands or quartered)|
|Toxic and oxidising or
Corrosive and oxidising
|yellow and light blue shoulders (either two bands or quartered)|
Diving cylinders are left unpainted (for aluminium), or painted to prevent corrosion (for steel), often in bright colors, most often fluorescent yellow, to increase visibility. This should not be confused with industrial gases, where a yellow shoulder means chlorine.
|Air||white and black quartered shoulder or white top and black band|
mixture of nitrogen and oxygen
|white and black quartered shoulder or white top and black band||green stripe on yellow bottom|
mixture of helium and oxygen
|white and brown quartered shoulder|
mixture of helium, nitrogen and oxygen
|white, black and brown segmented shoulder|