"Compressed Gas" is a generic term frequently used for different types of gas products, compressed gases, liquefied gases, refrigerated liquefied gases, and dissolved gases. OSHA classification criteria for gases under pressure include:
Compressed Gas - A gas which when under pressure is entirely gaseous at -50°C (-58°F), including all gases with a critical temperature1 ≤ 50°C (-58°F).
Liquefied Gas - A gas which when under pressure is partially liquid at temperatures above -50°C (-58°F). A distinction is made between:
- High pressure liquefied gas: a gas with a critical temperature between -50°C (-58°F) and +65°C (149°F); and
- Low pressure liquefied gas: a gas with a critical temperature above +65°C (149°F).
[The critical temperature is the temperature above which a pure gas cannot be liquefied, regardless of the degree of compression.]
Refrigerated Liquefied Gas [also known as Cryogenic liquids (cryogens)]- A gas which is made partially liquid because of its low temperature.
Dissolved gas - A gas which when under pressure is dissolved in a liquid phase solvent.
Compressed gases may be grouped into different hazard categories based upon their physical or health properties, or both. Any gas could be placed into more than one category. A gas could be corrosive, flammable, toxic, an oxidizer, or act as an asphyxiant by displacing oxygen. An additional hazard is due to the fact that gases are stored under high pressure.
Cryogens create unique hazards including fire, pressure, embrittlement of materials, and skin or eye burns upon contact with the liquid. Cryogens condense oxygen from air creating an oxygen rich atmosphere and increasing potential for fire if flammable or combustible materials and an ignition source are present. Pressure is a hazard because of the large expansion ratio from liquid to gas, causing pressure build up in containers. Many materials become brittle at extremely low temperatures. Brief bodily contact with materials at extremely low temperatures can cause burns similar to thermal burns.
- Never drop cylinders or permit them to strike each other violently.
- The valve-protection cap should be left on each cylinder until it has been secured against a wall, bench, or placed in a stand and is ready to be used.
- Avoid dragging, rolling, or sliding cylinders, even for a short distance. Use a cylinder hand truck to move them.
- Never tamper with safety devices in valves or cylinders.
- No part of a cylinder should be subjected to temperatures higher than 125° F. A flame should never be permitted to come in contact with any part of a compressed gas cylinder.
- Do not store full and empty cylinders together. Serious suck-back can occur when an empty cylinder is attached to a pressurized system.
- Bond and ground all cylinder, lines, and equipment used with flammable compressed gases.
- Use compressed gases only in well-ventilated areas. Toxic, flammable, and corrosive gases should be handled in a hood. Only small cylinders of toxic gases should be used.
- When discharging gas into a liquid, a trap or suitable check valve should be used to prevent liquid from getting back into the cylinder or regulator.
- When returning empty cylinders, close the valve before shipment, leaving some positive pressure in the cylinder. Replace any valve outlet and protective caps originally shipped with cylinder. Mark or label cylinder "empty" and store in a designated area for return.
- Before using cylinders, read all label information and safety data sheets associated with the gas being used. Check the SDS for required personal protective equipment and hazard Information before use.
- For flammable cryogens the precautions detailed in Procedures for Handling Flammable and Combustibles should be followed.
- When handling cryogens always wear safety goggle. If there is a splash or spray hazard, personnel protective clothing should also include a face shield, impervious apron or coat, trousers without cuffs, and shoes that cover the foot. Gloves should be impervious.
- Containers and systems containing cryogens should have pressure relief mechanisms.
- Containers and systems should be capable of withstanding extreme cold without becoming brittle.