Acetylene is a colorless gas widely used in industrial applications, most notably for oxy-acetylene welding and cutting, where it produces the hottest flame of common fuel gases. Acetylene has unique properties that require a specialized storage method, unlike gases that are simply compressed. This distinct engineering solution determines the relatively low pressure found inside a full cylinder, a condition established for safety and stability.
Standard Pressure in a Full Cylinder
A fully charged acetylene cylinder typically holds a pressure around 250 pounds per square inch gauge (psig) at a standard temperature of 70°F (21°C), equivalent to about 17 bar. This pressure is notably low compared to other compressed gases, such as oxygen or argon, which are often stored above 2,000 psig. This low pressure is a direct result of the gas’s instability, not an indication of low content. The pressure reading on the gauge represents only the acetylene gas that has escaped from its storage solution. Consequently, the pressure gauge is a poor indicator of how much total acetylene remains in the cylinder.
Why Acetylene Must Be Dissolved
The low storage pressure is necessary because free acetylene gas becomes dangerously unstable when compressed. When subjected to pressure above approximately 15 psig, acetylene is prone to spontaneous and explosive decomposition. This reaction is self-propagating and generates significant heat and pressure that can lead to cylinder rupture.
To safely contain the gas, a specialized method known as dissolving is used. The steel cylinder is filled with a porous internal material, such as calcium silicate or diatomaceous earth, which resembles a hard sponge. This porous mass is saturated with a solvent, most commonly acetone or sometimes Dimethylformamide (DMF). Acetylene gas is then pumped into the cylinder, where it dissolves into the solvent under pressure. The porous mass eliminates large void spaces, preventing the accumulation of free, unstable gaseous acetylene.
How Temperature and Usage Affect Pressure
The reading on the pressure gauge is significantly influenced by the cylinder’s ambient temperature. Since the acetylene is dissolved in a solvent, its solubility changes with temperature. Higher temperatures cause more gas to come out of the solution, resulting in a higher pressure reading. Conversely, when temperatures drop, the gas becomes more soluble, causing the pressure gauge to read lower, even if the cylinder is full. Users must take the storage temperature into account when interpreting the pressure reading.
The rate at which the gas is withdrawn from the cylinder also affects its performance. Drawing gas too quickly can lead to a phenomenon known as solvent carryover. This occurs when the rapidly escaping gas pulls the liquid solvent, typically acetone, out of the cylinder and into the regulator and hoses. Solvent carryover reduces the cylinder’s capacity and can damage equipment. To prevent this, cylinders must always be used in the upright position, which helps keep the solvent at the bottom. Unlike cylinders containing compressed gases, the pressure in an acetylene cylinder remains relatively constant until the gas content is almost depleted.