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.

Nomenclature

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.
A pressure receptacle is a general term for cylinders, bundles, tubes and pressure drums for the storage and transportation of compressed or liquefied gases with a water capacity from 0,5 L to 3000 L, and may be refillable or non refillable. Alternative terms include "transportable gas containers" and "transportable pressure containers".
In the United States, "bottled gas" typically refers to liquefied petroleum gas. "Bottled gas" is sometimes used in medical supply, especially for portable high pressure oxygen cylinders. 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.

Components

  • Cylinder – Either the shell or the complete assembly of shell and all accessories directly attached to the shell, depending on context.
  • Shell – The pressure vessel as a whole, excepting accessories.
  • *Shoulder – The end of the shell with a neck or boss into which the valve is fitted.
  • **Neck – A coaxial cylindrical extension of the shoulder with a threaded hole into which the cylinder valve or a gas pipe connection is fitted.
  • **Boss – A sturdy insert, usually in the centre of the shoulder, into which a valve or gas pipe connection is fitted.
  • *Base or foot – The end of the shell opposite the shoulder.
  • *Liner – The core on which filament windings are laid. The core may be structural, and share the pressure loads, or purely to separate composite wrapping from the cylinder contents,.
  • Cylinder valve – a shutoff valve directly coupled to the cylinder shell at the neck or boss which is opened to allow gas flow into or out of the cylinder, and closed to prevent such flow. It usually has a threaded inlet/outlet opening to which other equipment can be connected, but in some cases may have an integral pressure regulator on the outlet side, and a separate inlet opening for filling.
  • Foot ring – A permanently attached ring fitted to the base on which the cylinder can stand.
  • Valve guard – A fitting screwed or clamped to the shoulder, defending the valve from impact during transport, and in some cases, when in use.
  • Permanent markings – Information identifying the cylinder and its specification, stamped into the outside of the shoulder on metal cylinders. On composite cylinders permanent makings can be a printed label encapsulated under the resin or covered by a permanent transparent coating on the shoulder or side wall of the cylinder.

    Types

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:
  • Type 1: Metal only. Mostly seamless forged metal, but for lower working pressure, e.g., liquefied butane, welded steel vessels are also used.
  • Type 2: Metal vessel, hoop wrapped with a fibre composite only around the cylindrical part of the "cylinder".
  • Type 3: Thin metal liner ' fully wrapped with fibre composite material.
  • Type 4: Metal-free liner' of plastic, fully wrapped with fibre composite material. The neck of the cylinder which includes the thread for the valve is a metal insert.

    Cylinder assemblies

Assemblies comprising a group of cylinders mounted together for combined use or transport:
  • Bank – A group of cylinders connected to a gas distribution system for bulk storage, where the individual cylinders may be used together or separately, but are not necessarily supported by a structure which can be used to transport them as a group.
  • Cascade – A bank when used in cascade.
  • Quad or bundle, also occasionally gas pack or gas battery – A bank of high pressure gas storage cylinders, typically mounted upright on a rectangular frame for transport, and manifolded together. A pallet is a similar appearing group of cylinders on a lifting frame with no functional connections. The maximum combined cylinder volume for a bundle is 3000 litres for non-toxic gases and 1000 litres for toxic gases. Gas bundles are specified by ISO 10961:2019 Gas cylinders — Cylinder bundles — Design, manufacture, testing and inspection.
  • Rack – A structure to hold cylinders safely upright or horizontal while in use, for transport, or in storage.

    Materials

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 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.

Design

The primary design focus for seamless metal high pressure cylinders is on cylindrical wall thickness. This is calculated using the Lamé-von Mises formula for thick walled cylinders. Where the application may impose significant bending, torsional or local loads, these stresses must also be considered. Cylinder ends are generally approximately hemispherical, and require consideration for openings for internal inspection, filling and access to the contents.a A corrosion allowance may be added to the design thiskness to allow for surface corrosion over the service life where applicable, and the stresses are generally limited so that the test pressure stress is within the fatigue limit for steel cylinders.
The end dome thickness is usually determined by manufacturing process constraints, and for forged and spun ends the thickness tends to be greater than the cylindrical wall, though the pressure stresses in a spherical shell are roughly half those in a cylinder, making the ends much heavier than they need to be to theoretically withstand the pressure load. This extra thickness makes the ends relatively strong and able to support local stress concentrations due to stamp markings and neck threads. In seamless metal cylinders the thicker end wall and relatively small internal volume of the ends make the ends considerably heavier than the cylindrical section for the internal volume contribution, so this type of cylinder tends to be lighter per unit volume when the cylindrical part is longer.
  • Thin-walled cylinder stress. See: Cylinder stress#Thin-walled assumption
  • Thick-walled cylinder stress. See: Cylinder stress#Thick-walled vessels