Safe Handling of Compressed Gases in the Laboratory and Plant  (click on the title to view the complete article from Matheson)

The procedures adopted for the safe handling of compressed gases are mainly centered on containment of the material, to prevent its escape to the atmosphere, and proper  control of pressure and flow. All rules and regulations are directed toward these ends. Emergency procedures are usually only necessary because a basic rule of handling has been broken. It is far better to observe the rules and avoid the need for emergency measures.

It is of the utmost importance that the user be well aware of those properties of a compressed gas that represent hazards (such as flammability, toxicity, chemical activity, and corrosive effects). Every attempt should be made to learn these various properties before the gas is put to use. It is sometimes difficult to determine the major hazard of any one gas, since this factor is influenced a great deal by how the gas is used. In a laboratory hood in the presence of an open flame, the flammability of carbon monoxide  might well be the major hazard, whereas in a pilot-plant run using carbon monoxide as a reactant, leakage, and therefore toxicity, may represent the major hazard.

As the content of a cylinder of nonliquefied gas is discharged, the cylinder pressure decreases by an amount proportional to the amount withdrawn. The cylinder should be considered empty while positive pressure (25 psig or greater) still remains, in order to prevent reverse flow and contamination. Failure to close the valve on an empty cylinder will allow air and moisture to be drawn into the cylinder as it “breathes” during temperature changes; an explosive mixture may build up if the gas is flammable; and an  extremely corrosive condition will be created in cylinders that contain chlorine, hydrogen chloride, or other acid-forming or corrosive gases. As the vapor phase of a liquefied gas is withdrawn from a cylinder, the cylinder pressure or vapor pressure will remain constant as long as any liquid is present.

This condition holds true if the temperature does not vary. If, however, the material is withdrawn from the cylinder at a rapid rate, the material itself will supply the heat for vaporization, and upon subsequent cooling, the vapor pressure will be lowered. It is, therefore, impossible to determine the content of a cylinder containing a liquefied gas, except by weighing. A scale such as the one shown in Figure 13 makes this convenient. Cylinders containing liquefied gases are stamped or tagged with the tare weight in
order to allow the content to be determined.

An indication of cylinder-content depletion for some high pressure liquefied gases such as carbon dioxide, ethane, and nitrous oxide can be obtained by noting the cylinder pressure. After depletion of the liquid phase, the cylinder pressure will decrease below the normal vapor pressure, as long as the contents have not been withdrawn rapidly before the cylinder pressure is noted. A cylinder containing carbon dioxide will have approximately 20% of its original content remaining after depletion of the liquid phase. As with cylinders of nonliquefied gases, cylinders containing liquefied gases should never be completely emptied, in order to prevent reverse flow and contamination of the cylinder.

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