Using Propane Safely e-learning reference material

Guide Book Series

Cylinders and Valves

Cylinders and Valves Design, construction and operation

Propane cylinders are pressure vesels and should be treated with care: Never repaint, change markings or remove product labels Never interfere with cylinder valve threads Never remove a valve guard or modify a cylinder Never mix gases in a cylinder Never transfer gas from one cylinder to another Never apply PTFE to cylinder valves or regulators Never scrap a cylinder you do not own Never allow cylinders to become too hot Never use a cylinder as a container for other substances Just as a small volume of boiling water will produce a lot of steam, 1 volume of liquid propane will produce approximately 270 volumes of gas when vaporised. For this reason propane cylinders are fitted with a pressure relief valve which operates at approximately 26 bar. Should the take-off rate from the cylinder become excessive the boiling liquid needs more heat. This comes from the surrounding atmosphere trying to heat the cylinder. Condensation of the moisture from the relatively warmer atmosphere in contact with the cold cylinder surface results in frost appearing on the cylinder. If this is allowed to develop the boiling rate slows and the pressure within the cylinder may fall. If the operator requires a high flow rate of propane gas then it may be necessary to manifold two or more cylinders together to supply the required pressure. In normal use, as the gas is used the liquid “boils off”, reducing the cylinder liquid content. When the cylinder valve is closed the pressure in the cylinder builds and the liquid stops boiling. In other words the pressure in the cylinder will always be approximately 7.5 bar despite the fact that the gas is being used.

The first gas cylinders were manufactured in 1874. Since then manufacturers have been constantly improving their methods of production and ultimately the capacities and capabilities of gas cylinders. Today there is a wide variety of cylinder types that meet the needs of all applications. Permanent gases such as oxygen, nitrogen, argon, hydrogen and helium are filled into gas cylinders that have been manufactured from a single cylindrical block of steel called a billet. This type of cylinder is a hollow tube with a closed concave base that permits it to stand upright. The opposite end is tapered to a small opening that is threaded to accommodate the installation of a valve. This type of cylinder contains gas molecules and may be operated either with a spindle key or a handwheel

Propane cylinders look different and are different to other gas cylinders. The pressure at which propane forms liquid is much lower than other gases, this means that the cylinders are lighter, have thinner walls and are of a welded construction. Propane is stored as a liquid under approximately 7.5 bar (100 psi) pressure.

The boiling point of propane is -42.1°C. Since the cylinder is not insulated sufficient heat can be absorbed through the cylinder wall to generate the heat needed for the liquid to boil thus giving off a vapour, i.e. gas. When the cylinder control valve is opened to release the pressure the liquid will boil in the same way that water boils. As the liquid boils it uses ‘available’ heat from the cylinder wall. Over time the amount of usable heat will fall and the cylinder shell will cool.

Gas cylinders are fitted with a cylinder valve which must not be removed or tampered with. Valves fitted to non-flammable gas cylinders have conventional right-hand threads whilst valves fitted to flammable gas cylinders mostly have left-hand threads. An exception is ppropane cylinders used on forklift trucks which are fitted with right- hand connections

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