A compressed gas cylinder is a high-strength container designed to store various materials that are in a gaseous state at normal atmospheric conditions. The internal pressure of these cylinders is significantly higher than ambient pressure, which allows a large quantity of gas to be contained in a portable volume. This pressure is typically measured in pounds per square inch (psi) and can reach thousands of pounds per square inch, presenting a substantial amount of stored energy. The exact pressure within a cylinder varies widely, depending on the specific gas and the method used to store it.
How Gases Are Stored: The Difference Between Permanent and Liquefied Gases
The physical state of a gas inside the cylinder dictates its pressure behavior and storage method. Compressed gases are broadly categorized into permanent gases and liquefied gases. This distinction is based on the gas’s critical temperature—the temperature above which it cannot be turned into a liquid by pressure alone.
Permanent gases have a critical temperature too low to be liquefied at typical ambient temperatures. Examples include Oxygen, Nitrogen, Helium, and Argon. These are stored solely as highly compressed gases, and their pressure directly correlates with the amount of gas remaining in the cylinder. As the gas is used, the cylinder pressure drops linearly from its maximum fill pressure down to zero.
Liquefied gases, such as Propane, Carbon Dioxide (CO2), and Nitrous Oxide, have a higher critical temperature, allowing them to exist as both liquid and gas vapor inside the cylinder at room temperature. The pressure in these cylinders is determined by the vapor pressure of the liquid. Consequently, the pressure remains relatively stable as the gas is drawn out, only beginning to fall once nearly all the liquid has evaporated.
A third, unique category is dissolved gas, with Acetylene being the most common example. Acetylene is chemically unstable under high pressure and can decompose explosively. To stabilize it, the cylinder is packed with a porous filler material saturated with a solvent, typically acetone, into which the acetylene gas is dissolved.
Standard Working Pressures of Common Cylinders
High-pressure permanent gas cylinders are typically designed to hold gas at pressures between 2,000 psi and 2,640 psi, though some specialized cylinders can go up to 6,000 psi. For common industrial and medical gases like Oxygen, Nitrogen, and Argon, a standard full pressure is often around 2,200 psi at 70°F (21°C).
Liquefied gases operate at much lower pressures. A full Carbon Dioxide cylinder, for instance, shows a pressure of 800 to 1,000 psi at a standard room temperature. Propane and other liquefied petroleum gases (LPG) are considered low-pressure cylinders, with pressures ranging from 100 psi to 250 psi depending on the ambient temperature.
The pressure for a full Acetylene cylinder is typically around 250 to 275 psi (up to 19 bar). This moderate pressure is necessary to maintain the gas’s stability within the acetone solvent and porous medium.
Managing Extreme Pressure: Units, Regulation, and Safety Factors
Pressure is commonly expressed in pounds per square inch (psi) in the United States, but international and scientific contexts often use the metric units of bar or kilopascals (kPa). One atmosphere of pressure is roughly equivalent to 14.7 psi, 1 bar, or 101.3 kPa.
A pressure regulator connects to the cylinder valve to reduce the high internal pressure to a safe, controlled “working pressure” suitable for the application. These regulators are gas-specific to prevent improper connections and ensure precise pressure control.
Cylinders are constructed from robust materials like steel or aluminum. Government regulations, such as those from the Department of Transportation (DOT), mandate periodic hydrostatic testing to confirm the structural integrity of the cylinder. During this test, the cylinder is filled with water and pressurized to a level significantly higher than its maximum working pressure, often 1.5 times its rated service pressure. This over-pressurization ensures that the cylinder can withstand unexpected stress and provides a large safety margin.