Gas cylinder

A gas cylinder is a pressure vessel for storage and containment of gases at above atmospheric pressure. Gas storage cylinders may also be 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 or dished bottom end or foot ring, with the cylinder valve screwed into the internal neck thread at the top for connecting to the filling or receiving apparatus.
Gas cylinders may be grouped by several characteristics, such as construction method, material, pressure group, class of contents, transportability, and re-usability.
The size of a pressurised gas container that may be classed as a gas cylinder is typically 0.5 litres to 150 litres. Smaller containers may be termed gas cartridges, and larger may be termed gas tubes, tanks, or other specific type of pressure vessel. A gas cylinder is used to store gas or liquefied gas at pressures above normal atmospheric pressure. In South Africa, a gas storage cylinder implies a refillable transportable container with a water capacity volume of up to 150 litres. Refillable transportable cylindrical containers from 150 to 3,000 litres water capacity are referred to as tubes.
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 term propane tank is also used for cylinders for propane.
The United Kingdom and other parts of Europe more commonly refer to "bottled gas" when discussing any usage, whether industrial, medical, or liquefied petroleum. In contrast, what is called liquefied petroleum gas in the United States 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.
The term cylinder in this context is sometimes confused with tank, the latter being an open-top or vented container that stores liquids under gravity, though the term scuba tank is commonly used to refer to a compressed gas cylinder used for breathing gas supply to an underwater breathing apparatus.
Since fibre-composite materials have been used to reinforce pressure vessels, various types of cylinder distinguished by the construction method and materials used have been defined:
Assemblies comprising a group of cylinders mounted together for combined use or transport:
All-metal cylinders are the most rugged and usually the most economical option, but are relatively heavy. Steel is generally the most resistant to rough handling and most economical, and is often lighter than aluminium for the same working pressure, capacity, and form factor due to its higher specific strength. The inspection interval of industrial steel cylinders has increased from 5 or 6 years to 10 years. Diving cylinders that are used in water must be inspected more often; intervals tend to range between 1 and 5 years. Steel cylinders are typically withdrawn from service after 70 years, or may continue to be used indefinitely providing they pass periodic inspection and testing. When they were found to have inherent structural problems, certain steel and aluminium alloys were withdrawn from service, or discontinued from new production, while existing cylinders may require different inspection or testing, but remain in service provided they pass these tests.
For very high pressures, composites have a greater mass advantage. Due to the very high tensile strength of carbon fiber reinforced polymer, these vessels can be very light, but are more expensive to manufacture. Filament wound composite cylinders are used in fire fighting breathing apparatus, high altitude climbing, and oxygen first aid equipment because of their low weight, but are rarely used for diving, due to their high positive buoyancy. They are occasionally used when portability for accessing the dive site is critical, such as in cave diving where the water surface is far from the cave entrance. Composite cylinders certified to ISO-11119-2 or ISO-11119-3 may only be used for underwater applications if they are manufactured in accordance with the requirements for underwater use and are marked "UW".
Cylinders reinforced with or made from a fibre reinforced material usually must be inspected more frequently than metal cylinders, e.g., every 5 instead of 10 years, and must be inspected more thoroughly than metal cylinders as they are more susceptible to impact damage. They may also have a limited service life. Fibre composite cylinders were originally specified for a limited life span of 15, 20 or 30 years, but this has been extended when they proved to be suitable for longer service.
The Type 1 pressure vessel is a seamless cylinder normally made of cold-extruded aluminum or forged steel. The pressure vessel comprises a cylindrical section of even wall thickness, with a thicker base at one end, and domed shoulder with a central neck to attach a cylinder valve or manifold at the other end.
Occasionally other materials may be used. Inconel has been used for non-magnetic and highly corrosion resistant oxygen compatible spherical high-pressure gas containers for the US Navy's Mk-15 and Mk-16 mixed gas rebreathers, and a few other military rebreathers.
Most aluminum cylinders are flat bottomed, allowing them to stand upright on a level surface, but some were manufactured with domed bottoms.
Aluminum cylinders are usually manufactured by cold extrusion of aluminum billets in a process which first presses the walls and base, then trims the top edge of the cylinder walls, followed by press forming the shoulder and neck. The final structural process is machining the neck outer surface, boring and cutting the neck threads and O-ring groove. The cylinder is then heat-treated, tested and stamped with the required permanent markings.
Steel cylinders are often used because they are harder and more resistant to external surface impact and abrasion damage, and can tolerate higher temperatures without affecting material properties. They also may have a lower mass than aluminium cylinders with the same gas capacity, due to considerably higher specific strength. Steel cylinders are more susceptible than aluminium to external corrosion, particularly in seawater, and may be galvanized or coated with corrosion barrier paints to resist corrosion damage. It is not difficult to monitor external corrosion, and repair the paint when damaged, and steel cylinders which are well maintained have a long service life, often longer than aluminium cylinders, as they are not susceptible to fatigue damage when filled within their safe working pressure limits.
Steel cylinders are manufactured with domed (convex) and dished (concave) bottoms. The dished profile allows them to stand upright on a horizontal surface, and is the standard shape for industrial cylinders. The cylinders used for emergency gas supply on diving bells are often this shape, and commonly have a water capacity of about 50 litres ("J"). Domed bottoms give a larger volume for the same cylinder mass, and are the standard for scuba cylinders up to 18 litres water capacity, though some concave bottomed cylinders have been marketed for scuba. Domed end industrial cylinders may be fitted with a press-fitted foot ring to allow upright standing.
Steel alloys used for gas cylinder manufacture are authorised by the manufacturing standard. For example, the US standard DOT 3AA requires the use of open-hearth, basic oxygen, or electric steel of uniform quality. Approved alloys include 4130X, NE-8630, 9115, 9125, Carbon-boron and Intermediate manganese, with specified constituents, including manganese and carbon, and molybdenum, chromium, boron, nickel or zirconium.
Steel cylinders may be manufactured from steel plate discs stamped from annealed plate or coil, which are lubricated and cold drawn to a cylindrical cup form, by a hydraulic press, this is annealed and drawn again in two or three stages, until the final diameter and wall thickness is reached. They generally have a domed base if intended for the scuba market, so they cannot stand up by themselves.For industrial use a dished base allows the cylinder to stand on the end on a flat surface. After forming the base and side walls, the top of the cylinder is trimmed to length, heated and hot spun to form the shoulder and close the neck. This process thickens the material of the shoulder. The cylinder is heat-treated by quenching and tempering to provide the best strength and toughness. The cylinders are machined to provide the neck thread and o-ring seat (if applicable), then chemically cleaned or shot-blasted inside and out to remove mill-scale. After inspection and hydrostatic testing they are stamped with the required permanent markings, followed by external coating with a corrosion barrier paint or hot dip galvanising and final inspection.
A related method is to start with seamless steel tube of a suitable diameter and wall thickness, manufactured by a process such as the Mannesmann process, and to close both ends by the hot spinning process. This method is particularly suited to high pressure gas storage tubes, which usually have a threaded neck opening at both ends, so that both ends are processed alike. When a neck opening is only required at one end, the base is spun first and dressed inside for a uniform smooth surface, then the process of closing the shoulder and forming the neck is the same as for the pressed plate method.
An alternative production method is backward extrusion of a heated steel billet, similar to the cold extrusion process for aluminium cylinders, followed by hot drawing and bottom forming to reduce wall thickness, and trimming of the top edge in preparation for shoulder and neck formation by hot spinning. The other processes are much the same for all production methods.
The neck of the cylinder is the part of the end which is shaped as a narrow concentric cylinder, and internally threaded to fit a cylinder valve. There are several standards for neck threads, which include parallel threads where the seal is by an O-ring gasket, and taper threads which seal along the contact surface by deformation of the contact surfaces, and on thread tape or sealing compound.
Type 2 is hoop wrapped with fibre reinforced resin over the cylindrical part of the cylinder, where circumferential load is highest. The fibres share the circumferential load with the metal core, and achieve a significant weight saving due to efficient stress distribution and high specific strength and stiffness of the composite. The core is a seamless metal cylinder, manufactured in any of the ways suitable for a type 1 cylinder, but with thinner walls, as they only carry about half the load, mainly the axial load. Hoop winding is at an angle to the length axis of close to 90°, so the fibres carry negligible axial load.
Type 3 is wrapped over the entire cylinder except for the neck, and the metal liner is mainly to make the cylinder gas tight, so very little load is carried by the liner. Winding angles are optimised to carry all the loads (axial and circumferential) from the pressurised gas in the cylinder. Only the neck metal is exposed on the outside. This construction can save in the order of 30% of the mass compared with type 2, as the fibre composite has a higher specific strength than the metal of the type 2 liner that it replaces.
Type 4 is wrapped in the same way as type 3, but the liner is non-metallic. A metal neck boss is fitted to the shoulder of the plastic liner before winding, and this carries the neck threads for the cylinder valve. The outside of the neck of the insert is not covered by the fibre wrapping, and may have axial ridges to engage with a wrench or clamp for torsional support when fitting or removing the cylinder valve. There is a mass reduction compared with type 3 due to the lower density of the plastic liner.
A welded gas cylinder comprises two or more shell components joined by welding. The most commonly used material is steel, but stainless steel, aluminium and other alloys can be used when they are better suited to the application. Steel is strong, resistant to physical damage, easy to weld, relatively low cost, and usually adequate for corrosion resistance, and provides an economical product.
The components of the shell are usually domed ends, and often a rolled cylindrical centre section. The ends are usually domed by cold pressing from a circular blank, and may be drawn in two or more stages to get the final shape, which is generally semi-elliptical in section. The end blank is typically punched from sheet, drawn to the required section, edges trimmed to size and necked for overlap where appropriate, and hole(s) for the neck and other fittings punched. The neck boss is inserted from the concave side and welded in place before shell assembly.
Smaller cylinders are typically assembled from a top and bottom dome, with an equatorial weld seam. Larger cylinders with a longer cylindrical body comprise dished ends circumferentially welded to a rolled central cylindrical section with a single longitudinal welded seam. Welding is typically automated gas metal arc welding.
Typical accessories which are welded to the outside of the cylinder include a foot ring, a valve guard with lifting handles, and a neck boss threaded for the valve. Occasionally other through-shell and external fittings are also welded on.
After welding, the assembly may be heat treated for stress-relief and to improve mechanical characteristics, cleaned by shotblasting, and coated with a protective and decorative coating. Testing and inspection for quality control will take place at various stages of production.
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 while filled. 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 Underwriters Laboratories (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 Department for Transport (DfT) — implements the regulations and appointment of authorised cylinder testers is conducted by United Kingdom Accreditation Service (UKAS), who make recommendations to the Vehicle Certification Agency (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, ultimate 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 or ultrasonic testing is required either every five years or every ten years, depending on cylinder and its service.
Cylinder neck thread can be to any one of several standards. Both taper thread sealed with thread tape and parallel thread sealed with an O-ring have been found satisfactory for high pressure service, but each has advantages and disadvantages for specific use cases, and if there are no regulatory requirements, the type may be chosen to suit the application.
A tapered thread provides simple assembly, but requires high torque for establishing a reliable seal, which causes high radial forces in the neck, and has a limited number of times it can be used before it is excessively deformed. This can be extended a bit by always returning the same fitting to the same cylinder, and avoiding over-tightening.
In Australia, Europe and North America, tapered neck threads are generally preferred for inert, flammable, corrosive and toxic gases, but when aluminium cylinders are used for oxygen service to United States Department of Transportation (DOT) or Transport Canada (TC) specifications in North America, the cylinders must have parallel thread. DOT and TC allow UN pressure vessels to have tapered or parallel threaded openings. In the US, 49 CFR Part 171.11 applies, and in Canada, CSA B340-18 and CSA B341-18. In Europe and other parts of the world, tapered thread is preferred for cylinder inlets for oxidising gases.
Scuba cylinders typically have a much shorter interval between internal inspections, so the use of tapered thread is less satisfactory due to the limited number of times a tapered thread valve can be re-used before it wears out, so parallel thread is generally used for this application.
Parallel thread can be tightened sufficiently to form a good seal with the O-ring without lubrication, which is an advantage when the lubricant may react with the O-ring or the contents. Repeated secure installations are possible with different combinations of valve and cylinder provided they have compatible thread and correct O-ring seals. Parallel thread is more likely to give the technician warning of residual internal pressure by leaking or extruding the O-ring before catastrophic failure when the O-ring seal is broken during removal of the valve. The O-ring size must be correct for the combination of cylinder and valve, and the material must be compatible with the contents and any lubricant used.
Gas cylinders usually have an angle stop valve at one end, and the cylinder is usually oriented so the valve is 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 sometimes have a protective collar or neck ring around the valve assembly which has an opening for access to fit a regulator or other fitting to the valve outlet, and access to operate the valve. Installation of valves for high pressure aluminum alloy cylinders is described in the guidelines: CGA V-11, Guideline for the Installation of Valves into High Pressure Aluminum Alloy Cylinders and ISO 13341, Transportable gas cylinders—Fitting of valves to gas cylinders.
The valves on industrial, medical and diving cylinders usually have threads or connection geometries of different handedness, sizes and types that depend on the category of gas, making it more difficult to mistakenly misuse a gas. For example, a hydrogen cylinder valve outlet does not fit an oxygen regulator and supply line, which could result in catastrophe. 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, and are usually used for flammable gases.
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 gasses. For example, an argon cylinder may have a "CGA 580" connection on the valve. High purity gases sometimes use CGA-DISS ("Diameter Index Safety System") connections.
Medical gases may use the Pin Index Safety System to prevent incorrect connection of gases to services.
In the European Union, 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 contents).
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. For some purposes, such as shielding gas for arc welding, the regulator will also have a flowmeter on the downstream side.
The regulator outlet connection is attached to whatever needs the gas supply.
Because the contents are under pressure and are sometimes hazardous materials, handling bottled gases is 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 falls 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. They can also be stored in a safety cabinet.
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.
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.
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.
The Indian Standard for Gas Cylinder Color Code applies to the identification of the contents of gas cylinders intended for medical use. Each cylinder shall be painted externally in the colours corresponding to its gaseous contents.
The below are example cylinder sizes and do not constitute an industry standard.
(US DOT specs define material, making, and maximum pressure in psi. They are comparable to Transport Canada specs, which shows pressure in bars. A 3E-1800 in DOT nomenclature would be a TC 3EM 124 in Canada.)
For larger volume, high pressure gas storage units, known as tubes, are available. They generally have a larger diameter and length than high pressure cylinders, and usually have a tapped neck at both ends. They may be mounted alone or in groups on trailers, permanent bases, or intermodal transport frames. Due to their length, they are mounted horizontally on mobile structures. In general usage they are often manifolded together and managed as a unit.
Groups of similar size cylinders may be mounted together and connected to a common manifold system to provide larger storage capacity than a single standard cylinder. This is commonly called a cylinder bank or a gas storage bank. The manifold may be arranged to allow simultaneous flow from all the cylinders, or, for a cascade filling system, where gas is tapped off cylinders according to the lowest positive pressure difference between storage and destination cylinder, being a more efficient use of pressurised gas.
A gas cylinder quad, also known as a gas cylinder bundle, is a group of high pressure cylinders mounted on a transport and storage frame. There are commonly 16 cylinders, each of about 50 litres capacity mounted upright in four rows of four, on a square base with a square plan frame with lifting points on top and may have fork-lift slots in the base. The cylinders are usually interconnected by a manifold for use as a unit, but many variations in layout and structure are possible.