V(f). Chemical Safety - Compressed Gases
A. What Is A Compressed Gas?
Compressed gas cylinders are defined by the U. S. Department of Transportation (DOT) as any materials or mixtures in containers having an absolute pressure in excess of 40 psi at 20oC (70oF) or in excess of 104 psi at 54.5oC (130oF).
Compressed gas cylinders should be considered high energy sources regardless of the type of gas and all should be treated as potential explosives. Compressed gases have many properties that make them a unique hazard such as their pressure, diffusivity, low flash points for flammable gases, low boiling points, and, for some, no visual and/or odor warnings.
The following are general safety precautions for working with compressed gases.
B. Cylinder Storage
Delivery Area Storage
Large cylinders are stored in a designated location until they are needed in the laboratory, as are all empty cylinders. Storage of cylinders outside of the building is prohibited. Storage areas must be in securable areas, storage in hallways is not allowed.
- Store oxygen away from flammable gases. Oxygen and fuel gases must be separated by a distance of at least 25 feet or by a 5-foot high noncombustible wall when stored. As an alternative, oxygen can be moved directly to the area of use.
- The valve protection cap must be kept on at all times, except when a cylinder is in use.
- Each cylinder must be chained or strapped tightly in place to prevent it from falling over. Cylinders must be secured individually. The attachment point must be below the neck and tapered portion of the cylinder.
- Corrosive gases should be stored for the shortest possible time period: under three months is preferable.
Cylinder storage in the laboratory should consist primarily of lecture bottle size cylinders. These cylinders pose the same hazards as larger cylinders, and storage should comply with the four standards listed above for Delivery Area Storage.
C. Inspection Of Cylinders
Before moving cylinders they should be inspected to ensure that:
- the cylinder has:
- a valve protection cap
- a DOT or ICC label
- the date of the last hydrostatic test (usually stamped on the cylinder just below the serial number)
- labels or stenciling identifying the contents (color coding is not acceptable as a contents label because there is no universal color code for identifying gas cylinders)
- identity of the manufacturer
- the cylinder shows no signs of damage or corrosion.
D. Moving Cylinders
Faculty members, staff, or trained students will move all large cylinders from the Delivery Area to the laboratory.
- Always consider cylinders full and handle them accordingly; the same hazards exist even if the cylinder is only partially full.
- Use a hand truck to transport cylinders that cannot be easily carried. Do not drag, roll, or slide cylinders.
- The valve protection cap should remain on until the cylinder has been secured in its final position and is ready for use.
- Never drop a cylinder or permit cylinders to strike each other violently.
- Protect cylinders from any object that will produce a cut or abrasion in the surface of the metal.
- Mount cylinders so that the valve is easily accessible and the label is readable.
- Always chain or strap cylinders immediately. Cylinders must be secured individually. The attachment point must be below the neck and tapered portion of the cylinder. Do not leave a cylinder in a laboratory if equipment is not available to secure it.
E. Laboratory Use
- Attach the proper regulator designed for the particular gas that is being used. Cylinder valves have been standardized for specific families of gases to prevent the interchange of regulator equipment between gases that are not compatible. Do not force the fitting of a regulator to a cylinder. Be sure that all components of a distribution system are compatible with the gas in use. Corrosive gases require special attention to the materials in the distribution system.
- After connecting the regulator secure all hose connections with clamps, secure any loose hoses to prevent sudden movement when pressure is supplied, and place a trap between the regulator and the reaction vessel to prevent suckback.
- Bond and ground all cylinders and piping containing flammable gases to prevent the hazards caused by the buildup of static electricity.
- Start the gas flow with the following procedure:
- With the regulator secured to the cylinder valve outlet, turn the delivery pressure adjusting screw counterclockwise until it turns freely.
- Next slowly open the cylinder valve until the cylinder pressure gauge on the regulator reads the cylinder pressure. The cylinder valve should be opened by hand; never use a wrench or other tool unless the vendor supplies a special tool for that purpose.
- With the cylinder valve open and the flow control valve (the outlet from the regulator) in closed position, set the desired delivery pressure by turning the delivery-pressure adjusting screw clockwise until the desired pressure is reached.
- Flow from the cylinder can now be commenced by opening the flow control valve at the outlet of the regulator.
- Door signs should be posted in rooms in which flammable compressed gases are present.
- Never mix gases in a cylinder, unless it is a specially designed cylinder such as for calibration gases.
- Never completely empty a cylinder. Leave a slight pressure (about 25 pounds) to keep out contaminates that may react with the contents or corrode the cylinder.
F. Special Precautions
- Gaseous acetylene under pressure may also decompose with explosive force, and should not be used at pressures in excess of 15 psig (30 psi absolute pressure). Acetylene pressure gauges should have a warning red line at this point.
- Acetylene in cylinders is dissolved in a liquid (e.g. acetone) and should always be used in an upright position. Do not use a cylinder which has been stored or handled in a non-upright position until it has remained in an upright position for at least 30 minutes.
- The outlet line of an acetylene cylinder must be protected by a flash arrester.
- Use the correct kind of tubing to transport the gaseous acetylene. Some tubing materials, such as copper, form explosive acetylides.
Oxidizers under pressure (oxygen, chlorine, etc.) will rapidly oxidize organic material, such as oil or grease, resulting in an explosion. Never use oil or grease on valves or gauges intended for oxygen cylinders.