More than a dozen gases commonly used in industrial plants are supplied in pressurized cylinders. The gases are supplied in this way simply because more gas can be shipped, stored, and distributed to a work station under high pressure than can be at atmospheric pressure. For example, about 251 cu ft of oxygen at 2400 psi can be held by a 1.6 cu ft container. If the oxygen were delivered at atmospheric pressure (14.7 psi) to a plant at sea level, 150 cylinders would be required.

Gases under high pressure can be hazardous if not used properly. Pressure is one problem and volume is another. A volume of gas that supports combustion will support 150 times more combustion per cylinder at 2400 psig than at sea-level atmospheric pressure. And oxidation reactions occur more rapidly in pure oxygen than in air. Some substances that are safe in air, such as petroleum-based lubricants, will spontaneously ignite in pure oxygen.

A third problem is the nature of the contained gas.

Fuels burn. Oxygen supports combustion. Carbon dioxide has special characteristics. A slowly leaking cylinder of inert argon can deplete the oxygen in a room or work enclosure to the extent that people entering it will be asphyxiated without warning.

This chapter discusses many general rules for safe handling of compressed gases, and their cylinders and regulators. The second chapter will discuss special procedures that apply to certain gases and will outline problems that surround safe storage of compressed gases and safe use of cylinder manifolds, pipelines, and hose.

Most rules for the safe handling of high-pressure gases and their related equipment are based on common sense and experience. Rules are not static; they continue to be improved as more is learned. Although suppliers and distributors of compressed gases take steps to make their products and containers safe to use, certain rules must be followed in the plant.

Gases arrive at the plant in various forms and in different types of containers. For example, natural gas is delivered through a low-pressure distribution pipeline. Liquefied petroleum (LP) gases such as propane, propylene, and butane arrive as room-temperature liquids in low-pressure cylinders. Most room-temperature, lower-pressure gases are fuels. These are flammable and have a special set of safe-handling requirements.

But oxygen, nitrogen, argon, hydrogen, and helium, among others, are supplied in gaseous form in high-pressure cylinders. Or, they may be shipped as super cooled (cryogenic) liquids in highly insulated containers. At atmospheric pressure, these gases will liquefy only at very low temperature.

Carbon dioxide (CO2) can be supplied as a solid, a liquid, or a gas. It is unusual because it solidifies directly from its gaseous state at –109°F (-70°C) at atmospheric pressure. Liquid CO2, cannot exist unless it is colder than 87.4°F (30.8°C) and simultaneously under a pressure of at least 60.4 psig. Conditions in a liquid on-site CO2, receiver are usually zero°F (-18°C) and 300 psig.

Carbon dioxide gas will solidify when it is released quickly from a unheated cylinder regulator. As the pressure suddenly drops, the regulator freezes solid. Nitrous oxide behaves similarly. In both cases electrically heated cylinder regulators are necessary to keep the gas and the regulator from freezing.

Acetylene also has special needs. As a liquid, it is chemically unstable and will detonate at the slightest shock. Pure liquefied acetylene is never used in conventional plant operations, but gaseous acetylene can be safe as long as special handling and storage rules are followed. The most important rule is that acetylene gas is unsafe at delivery pressures higher than 15 psig.