Gas cylinder

For mechanical devices used to impart a force from a pressurized liquid or gas, see pneumatic cylinder.

A Gas cylinder or tank is a pressure vessel used to store gases at high pressure.

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Applications

Applications for gas cylinders include:

• pneumatic reservoirs in transport and industry
• storage of medical gases
• storage of breathing gases in diving cylinder
• storage of gaseous fuels for internal combustion engines, heating equipment and cooking such as LP gas, butane and propane
• storage of gases used for oxy-fuel welding and cutting.

Regulations and Testing

The transportation of high pressure cylinders is regulated by many governments throughout the world. Various levels of testing are generally required by the governing authority for the country in which it is to be transported. In the United States this authority is the United States Department of Transportation (DOT), for Canada this authority is Transport Canada (TC). Cylinders may have additional requirements placed on design and or performance from independent testing agencies such as Underwriter's Laboratory (UL). Each manufacturer of high pressure cylinders is required to have independent quality agent that will inspect the product for quality and safety.

There are a variety of tests that may be performed on various cylinders. Some of the most common types of tests are hydrostatic test, burst test, tensile strength, Charpy impact test and pressure cycling.

During the manufacturing process, vital information is usually stamped or permanently marked on the cylinder. This information usually includes the type of cylinder, the working or service pressure, the serial number, date of manufacture, the manufacture's registered code and sometimes the test pressure. Other information may also be stamped depending on the regulation requirements.

High pressure cylinders that are used multiple times--as most are--are hydrostatically or ultrasonically tested and visually examined every few years. In the U.S., hydrostatic/ultrasonic testing is required either every five years or every ten years, depending on cylinder and its service. Helium Gas tanks have the highest pressures possible when full around 1000 atmospheres.

Valve Connections

When gases are supplied in gas cylinders, the cylinders have a stop angle valve at the end on top. Often, gas cylinders are somewhat long and narrow and may stand upright on a flattened bottom at one end with the valve at the top. During storage, transportation, and handling when the gas is not in use, a cap may be screwed over the protruding valve to protect it from damage or breaking off in case the cylinder were to fall over. Instead of a cap, cylinders commonly have a protective collar or neck ring around the service valve assembly.

When the gas in the cylinder is ready to be used, the cap is taken off and a pressure-regulating assembly is attached to the stop valve. This attachment typically has a pressure regulator with upstream (inlet) and downstream (outlet) pressure gauges and a further downstream needle valve and outlet connection. The upstream pressure gauge indicates how much gas is left in the cylinder according to pressure. The regulator could be adjusted to control the flow of gas out of the cylinder according to pressure shown by the downstream gauge. The outlet connection is attached to whatever needs the gas supply, such as a balloon for example.

The valves on industrial, medical and diving cylinders are usually of different size and type, as are the valves for different products, making it more difficult to mistakenly misuse a gas.

In the US, valve connections are sometimes referred to as 'CGA connections,' since the Compressed Gas Association (CGA) publishes guidelines on what connections to use for what products (e.g., In the USA, an argon cylinder will have a CGA 580 connection on the valve).

• Note: if the nut on a CGA connection has a notch in it, it uses a left-handed thread.

High purity gases will sometimes use CGA-DISS ('Diameter Index Safety System') connections.

In the EU, DIN connections are more common than in the US.

Color Coding

Gas cylinders are often color coded, but the codes are not standard across different jurisdictions, and sometimes are not regulated. Cylinder color should not be used for positive product identification. Gas cylinders have labels affixed to them which identify the product they contain and the label alone should be used for positive identification. When the identification provided by the label is dubious, it should be marked Contents Unknown and returned to the manufacturer.

Safety

Because the contents are under 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. Installing and replacing gas cylinders should be done by trained personnel.

In a fire, the pressure in a gas cylinder rises in direct proportion to its temperature (see the ideal gas law). If the internal pressure exceeds the mechanical limitations of the cylinder and there are no means to safely vent the pressurized gas to the atmosphere, the vessel will fail mechanically. If the vessel contents are ignitable, this event may result in a "fireball". If the cylinder's contents are liquid but become a gas at ambient conditions, this is commonly referred to as a Boiling Liquid Expanding Vapour Explosion (BLEVE).

Medical gas cylinders in the UK and other countries have a seal of Wood's metal between the valve block and the cylinder body. This seal melts at a comparatively low temperature (70°C) and allows the contents of the cylinder to escape in a controlled fashion, lessening the risk of explosion.

More common pressure relief devices are of a simple burst disc type. In these, a small burst disc is installed in the back of the valve. A burst disc is a small metal gasket engineered to rupture at a pre-determined pressure. Some of these burst disc are backed with a low-melting-point metal, so that the valve must be exposed to excessive heat before the burst disc can rupture.

If the valve of a compressed air cylinder is broken or sheared off, the released pressure will cause the cylinder to act like a rocket, shooting away quickly.

The Compressed Gas Association 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.)

See also

• Bottled gas
• Storage tank