A gas cylinder or tank is a pressure vessel for storage and containment of gases at above atmospheric pressure. High-pressure gas cylinders are also called bottles. Inside the cylinder the stored contents may be in a state of compressed gas, vapor over liquid, supercritical fluid, or dissolved in a substrate material, depending on the physical characteristics of the contents. A typical gas cylinder design is elongated, standing upright on a flattened bottom end, with the valve and fitting at the top for connecting to the receiving apparatus.
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 by using one of several trade names, or specifically as butane or propane depending on the required heat output.
For a detailed discussion about the materials for gas cylinders see pressure vessel.
Design codes and application standards and the cost of materials dictated the choice of steel with no welds for most gas cylinders, treated to be anticorrosive. Some newly developed lightweight gas cylinders have been made from stainless steel and composite materials. Due to the very high tensile strength of carbon fiber, these vessels can be very light, but are much more difficult to manufacture.
Regulations and cylinder 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). Similarly in the UK, the European transport regulations (ADR) are implemented by the Department for Transport (DfT). 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 an independent quality agent that will inspect the product for quality and safety.
Within the UK the "competent authority" — the DfT — implements the regulations and appointment of authorised cylinder testers is conducted by UKAS, who make recommendations to the VCA for approval of individual bodies.
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 — can be hydrostatically or ultrasonically tested and visually examined every few years. In the United States, hydrostatic/ultrasonic testing is required either every five years or every ten years, depending on cylinder and its service. Helium gas cylinders have the highest pressures possible when full, around 1000 atmospheres.
Gas cylinders have a stop angle valve at the end on 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 to be used at low pressure, 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. For gases that remain gaseous under ambient storage conditions, the upstream pressure gauge can be used to estimate how much gas is left in the cylinder according to pressure. For gases that are liquid under storage, e.g., propane, the outlet pressure is dependent on the vapor pressure of the gas, and does not fall until the cylinder is nearly exhausted although it will vary according to the temperature of the cylinder contents. The regulator is adjusted to control the downstream pressure, which will limit the maximum flow of gas out of the cylinder at the pressure shown by the downstream gauge. The outlet connection is attached to whatever needs the gas supply, such as a balloon for example. For some purposes, such as welding, the regulator will also have a flowmeter on the downstream side.
The valves on industrial, medical and diving cylinders are usually of different sizes and types, as are the valves for different categories of gases, making it more difficult to mistakenly misuse a gas. For example, a hydrogen cylinder does not fit an oxygen supply line which would end in catastrophic failure. Some fittings use a right-hand thread, while others use a left-hand thread; left-hand thread fittings are usually identifiable by notches or grooves cut into them.
In the United States, 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 United States, an argon cylinder will have a CGA 580 connection on the valve.
High purity gases will sometimes use CGA-DISS ("Diameter Index Safety System") connections.
Medical gases may use the pin-index system to prevent incorrect connection of gases to services.
In the EU, DIN connections are more common than in the United States.
In the UK, the British Standards Institution sets the standards. Included among the standards is the use left-hand threaded valves for flammable gas cylinders (most commonly brass, BS4, valves for non corrosive cylinder contents or stainless steel, BS15, valves for corrosive contents). Non flammable gas cylinders are fitted with right-hand threaded valves (most commonly brass, BS3, valves for non corrosive components or stainless steel, BS14, valves for corrosive components).
|Gas type||CGA Valve Outlet (USA)||BS Valve Outlet (UK)|
|Argon||580,718,680(3500 psi),677(6000 psi)||3|
|Nitrogen||580,718,680(3500 psi),677(6000 psi)||3|
|Oxygen mixtures (>23.5%)||296||Other guides apply|
Safety and standards
Because the contents are under pressure and are sometimes hazardous materials, handling bottled gases are regulated. Regulations may include chaining bottles to prevent falling and damaging the valve, proper ventilation to prevent injury or death in case of leaks and signage to indicate the potential hazards If a compressed gas cylinder tips over, causing the valve block to be sheared off, the rapid release of high-pressure gas may cause the cylinder to be violently accelerated, potentially causing property damage, injury, or death. To prevent this, cylinders are normally secured to a fixed object or transport cart with a strap or chain.
In a fire, the pressure in a gas cylinder rises in direct proportion to its temperature. 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 flammable, this event may result in a "fireball". Oxidisers such as oxygen and fluorine will produce a similar effect by accelerating combustion in the area affected. 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 some other countries have a fusible plug of Wood's metal in the valve block between the valve seat and the cylinder. This plug melts at a comparatively low temperature (70 °C) and allows the contents of the cylinder to escape to the surroundings before the cylinder is significantly weakened by the heat, lessening the risk of explosion.
More common pressure relief devices are a simple burst disc installed in the base of the valve between the cylinder and the valve seat. A burst disc is a small metal gasket engineered to rupture at a pre-determined pressure. Some burst discs are backed with a low-melting-point metal, so that the valve must be exposed to excessive heat before the burst disc can rupture.
The Compressed Gas Association publishes a number of booklets and pamphlets on safe handling and use of bottled gases.
International and national standards
There is a wide range of standards relating to the manufacture, use and testing of pressurised gas cylinders and related components. Some examples are listed here.
- ISO 11439: Gas cylinders — High-pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles 
- ISO 15500-5: Road vehicles — Compressed natural gas (CNG) fuel system components — Part 5: Manual cylinder valve 
- US DOT 3/4/8???: e-CFR (Electronic Code of Federal Regulations) Title 49, part 178, Subpart C - Specification for Cylinders 
- US DOT Aluminum Tank Alloy 6351-T6 amendment for SCUBA, SCBA, Oxygen Service - Visual Eddy inspection 
- AS 2896-2011:Medical gas systems—Installation and testing of non-flammable medical gas pipeline systems pipeline systems (Australian Standards).
Gas cylinders are often color-coded, but the codes are not standard across different jurisdictions, and sometimes are not regulated. Cylinder color can not safely be used for positive product identification; cylinders have labels to identify the gas they contain.
Common cylinder sizes
In scuba diving, the United States measures cylinder volume by the amount of free air that can be compressed into the cylinder; Europe and most of the rest of the world measure the cylinder volume as the internal volume of the cylinder: e.g. United States 19 cubic feet = European 3 liter at 180 bar.
The below are example cylinder sizes and do not constitute an industry standard.
|Cyl. size||Diameter × height (inches), includes 5.5 inches for valve and cap||Nominal tare weight (lb), includes 4.5 lb for valve and cap||Water capacity (lb)||Internal volume @ 70 °F (21 °C), 1 atm liters (cubic feet)||U.S. DOT Specs|
|2HP||9 × 51||187||43.3 (1.53)||3AA3500|
|K||9.25 × 60||135||110||49.9 (1.76)||3AA2400|
|A||9 × 51||115||96||43.8 (1.55)||3AA2015|
|B||8.5 × 31||60||37.9||17.2 (0.61)||3AA2015|
|C||6 × 24||27||15.2||6.88 (0.24)||3AA2015|
|D||4 × 18||12||4.9||2.24 (0.08)||3AA2015|
|AL||8 × 53||52||64.8||29.5 (1.04)||3AL2015|
|BL||7.25 × 39||33||34.6||15.7 (0.55)||3AL2216|
|CL||6.9 × 21||19||13||5.9 (0.21)||3AL2216|
|XL||14.5 × 50||75||238||108 (3.83)||4BA240|
|SSB||8 × 37||95||41.6||18.9 (0.67)||3A1800|
|10S||4 × 31||21||8.3||3.8 (0.13)||3A1800|
|LB||2 × 15||4||1||0.44 (0.016)||3E1800|
|XF||12 × 46||180||60.9 (2.15)||8AL|
|XG||15 × 56||149||278||126.3 (4.46)||4AA480|
|XM||10 × 49||90||120||54.3 (1.92)||3A480|
|XP||10 × 55||55||124||55.7 (1.98)||4BA300|
|QT||3 × 14 (includes 4.5 inches for valve)||2.5 (includes 1.5 lb for valve)||2.0||0.900 (0.0318)||4B-240ET|
|LP5||12.25 × 18.25||18.5||47.7||21.68 (0.76)||4BW240|
|Medical E||4 × 26 (excludes valve and cap)||14 (excludes valve and cap)||4.5 (0.16)||3AA2015|
|Wikimedia Commons has media related to Gas cylinders.|
- Bottled gas
- Pressure vessel
- Carbon fiber
- Composite overwrapped pressure vessel
- Filling carousel
- Industrial gas
- Storage tank
- UN Recommendations on the Transport of Dangerous Goods
- See Composite overwrapped pressure vessel for details
- Henderson, N. C.; Berry, W. E.; Eiber, R. J.; Frink, D. W. (1970). "Investigation of scuba cylinder corrosion, Phase 1". National Underwater Accident Data Center Technical Report Number 1. University of Rhode Island. Retrieved 2016-01-11.
- BS 341-3:2002, British Standards Institution, 389 Chiswick High Road, London, W4 4AL
- "Incident Insights - Trust But Verify". Divers Alert Network.
- "ISO 11439:2000 - Gas cylinders -- High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles".
- "ISO 15500-5:2001 - Road vehicles -- Compressed natural gas (CNG) fuel system components -- Part 5: Manual cylinder valve".
- "CNG Cylinder Valve ISO 15500 -".
- US DOT e-CFR (Electronic Code of Federal Regulations) Title 49, part 178, Subpart C - Specification for Cylinders - eg DOT 3AL = seamless aluminum
- Federal Register / Vol. 71, No. 167 / Tuesday, August 29, 2006 / Rules and Regulations Title 49 CFR Parts 173 and 180 Visual Edddy