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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
black letters. Signs should be posted conspicuously in areas in which flammable compressed gases are stored, identifying the substances and appropriate precautions, for example:
NO SMOKING-NO—OPEN FLAMES
6.D.1.2 Handling and Use
Gas cylinders must be handled carefully to prevent accidents or damage to the cylinder. The valve protection cap should be left in place until the cylinder is secured and ready for use. Gas cylinders should not be dragged, rolled, slid, or allowed to strike each other forcefully. Cylinders should always be transported on wheeled cylinder carts with retaining straps or chains. The plastic mesh sleeves sometimes installed on cylinders by vendors are intended only to protect the paint on the cylinder and do not serve as a safety device.
Compressed gas cylinders should be secured firmly at all times. A clamp and belt or chain, securing the cylinder between "waist" and "shoulder" to a wall, are generally suitable for this purpose. In areas of seismic activity, gas cylinders should be secured both toward the top and toward the bottom. Cylinders should be individually secured; using a single restraint strap or chain around a number of cylinders is often not effective. Pressure-relief devices protecting equipment that is attached to cylinders of flammable, toxic, or otherwise hazardous gases should be vented to a safe place. (See Section 6.D.2.2.1 for details.)
Standard cylinder-valve outlet connections have been devised by the Compressed Gas Association (CGA) to prevent the mixing of incompatible gases due to an interchange of connections. The outlet threads used vary in diameter: some are internal and some are external; some are right-handed and some are left-handed. In general, right-handed threads are used for nonfuel and water-pumped gases, and left-handed threads are used for fuel and oil-pumped gases. Information on the standard equipment assemblies for use with specific compressed gases is available from the supplier. To minimize undesirable connections that may result in a hazard, only CGA standard combinations of valves and fittings should be used in compressed gas installations; the assembly of miscellaneous parts (even of standard approved types) should be avoided. Use of an "adapter" or cross-threading of a valve fitting should not be attempted. The threads on cylinder valves, regulators, and other fittings should be examined to ensure that they correspond to one another and are undamaged.
Cylinders should be placed so that the rotary cylinder valve handle at the top is accessible at all times. Cylinder valves should be opened slowly, and only when a proper regulator is firmly in place and the attachment has been shown to be leak-proof by an appropriate test (see Chapter 5, section 5.H). The cylinder valve should be closed as soon as the necessary amount of gas has been released. Valves should be either completely open or completely closed. Flow restrictors should be installed on gas cylinders to minimize the chance of excessive flows. The cylinder valve should never be left open when the equipment is not in use. This precaution is necessary not only for safety when the cylinder is under pressure, but also to prevent the corrosion and contamination that would result from diffusion of air and moisture into the cylinder after it has been emptied.
Most cylinders are equipped with hand-wheel valves. Those that are not should have a spindle key on the valve spindle or stem while the cylinder is in service. Only wrenches or other tools provided by the cylinder supplier should be used to remove a cylinder cap or to open a valve. In no case should a screwdriver be used to pry off a stuck cap or should pliers be used to open a cylinder valve. Some valve fittings require washers or gaskets, and the materials of construction should be checked before the regulator is fitted.
If the valve on a cylinder containing an irritating or toxic gas is being opened outside, the worker should stand upwind of the cylinder with the valve pointed downwind, away from him or herself, and should warn those working nearby in case of a possible leak. If the work is being done inside, the cylinder should be opened only in a fume hood or specially designed cylinder cabinet. A differential pressure switch with an audible alarm should be installed in any hood dedicated for use with toxic gases. In the event of hood failure, the pressure switch should activate an audible alarm warning the user of hood failure.
Preventing and Controlling Leaks
Cylinders, connections, and hoses should be checked regularly for leaks. To check for leaks, a flammable gas leak detector (for flammable gases only) or soapy water, or a 50% glycerin-water solution, is used to look for bubbles. At or below freezing temperatures, the glycerin solution should be used instead of soapy water. When the gas to be used in the procedure is a flammable oxidizing or highly toxic gas, the system should be checked first for leaks with an inert gas (helium or nitrogen) before introducing the hazardous gas.
The general procedures discussed in Chapter 5, section 5.C, can be used for relatively minor leaks, when