Table 5.1 DOT Signal Words

Danger: Warns the user that release of the gas in use to the atmosphere would pose an immediate hazard to health and/or property.

Warning: Warns the user that release of the gas in use to the atmosphere would not necessarily pose an immediate hazard, but would nonetheless be extremely hazardous to health and/or property under certain conditions.

Caution: Warns the user that there is no danger from the gas in use beyond the dangers associated with using any gas that is under high pressure.

use, and they also discuss appropriate responses. In addition to the general potential dangers of compressed gas tanks, you should also be knowledgeable to the potential dangers of the gas you are using.

5.1.5 Safety Practices Using Compressed Gases

Always Work in a Well-Ventilated Area. Perhaps the most subtle danger when working with compressed gases is asphyxiation. If one is working in an area with poor (or nonexistent) ventilation and there is a leak of a non-harmful gas (nonflammable, nonpoisonous), then there will be a resulting decrease of the percentage (within air) of available oxygen.* This decrease can lead to asphyxiation. Remember, propane is heavier than air, so in a basement situation, it settles and forces out the remaining air.

Table 5.5 Signal Words and Their Potential Hazards

Extremely hazardous gas and/or liquid

Flammable

Danger Harmful if inhaled

Poisonous

Radioactive

Corrosive gas or liquid

Extremely cold liquid and/or gas under pressure

Warning Extremely irritating gas

Liquid causes burns

Vigorously accelerates burning

High concentration in the atmosphere can cause immediate asphyxiation

Caution

High pressure

Liquefied gas under pressure

The most insidious thing about asphyxiation is that it comes on without warning. There can be feelings of sleepiness, fatigue, lassitude, loss of coordination, errors of judgment, confusion, and even euphoria preceding unconsciousness. Regardless, the victim will not be in a mental state to realize the danger and will be incapable of reacting properly, which involves getting out of the area to better air.

IN CASE OF EMERGENCY: Anyone going into a room suspecting low oxygen concentrations should (1) arrange for a buddy system and (2) bring in portable breathing equipment. Gas masks or filters will not help in an oxygen-poor environment. Obviously, if you see someone lying on the floor there is a desire to run and help them. Unfortunately, that only places you in equal danger. Under no circumstances should you go into the room without calling the fire department, getting help, and attaching a rope onto yourself so your backup can drag you out.

Because most modern labs have adequate ventilation, asphyxiation is seldom a problem. However, when work is being done in a basement, in rooms that do not have an adequate ventilation system, or when a gas is heavier than air, every precaution should be taken such as the use of the buddy system, having portable air supplies available, and intercom systems with regular check-in times.

Always Secure Compressed Gas Tanks. A compressed gas tank should never be allowed to stand free. Instead, it should be supported by an approved tank support and the screws should be tightened by a wrench, not just by hand. If lying horizontal, the compressed gas tank should be prevented from rolling.

Just about everyone has heard stories of compressed gas tanks being knocked over, thus snapping off the main valve, turning the compressed gas tank into a rocket. The Compressed Gas Association claims that this orifice is too small for the tank to become a projectile. However, if the tank contains a flammable gas, the question of whether the tank will turn into a rocket is of secondary importance.

Even when a compressed gas tank is empty, it should be strapped down. The standard compressed tank used in most laboratories weighs about 100 and could be quite destructive if it were to fall over onto something or someone. While the west coast is vulnerable to earthquakes, any lab is susceptible to the clumsy or accidental actions of a person knocking over a tank. It is now a safety standard for compressed gas tanks to be supported by two straps about 2-3 feet apart, as opposed to the previous standard of a single strap. The reason for this is that occasionally the tank's footing would slip, allowing the tank to slip out from underneath the single strap. This second strap should be mandatory in earthquake areas. It has been the author's experience that the standard clamp-on design of com-

Oxygen content in the atmosphere is normally about 21%.Because the partial pressure varies with the atmospheric pressure, the percentage of oxygen at higher altitudes will decrease relative to sea level. The flame of a candle will be extinguished when oxygen concentration is reduced to about 15% to 16%. A human will be rendered unconscious when oxygen concentration is reduced to about 12%. Prolonged exposure within a low-oxygen-percentage environment can lead to brain damage or even death.

String for

tying onto tank.

Card has serrations for

easy tearing

off.

Fig. 5.2 Suggested card to be placed on compressed gas tanks.

pressed gas tank support proved to be very unreliable in a powerful earthquake. Permanently bolted, double-strapped supports have shown to be very reliable.

Keep Electric Lines Free from Compressed Gas Tanks. Compressed gas tanks are made of metal. Keeping electric lines away from compressed gas tanks helps to prevent spark development and potential shock to the user.

If you should come upon someone who is frozen by electric shock to an electrified compressed gas tank, do not handle the person with your bare hands. First, if possible, quickly ascertain the source of the electricity; and if it is safe and easy to stop the current, do so. Otherwise, be sure you are not standing on, or in, water, and use something that is nonconducting (e.g., a broom handle) to pull the victim from the electrified tank. Call for medical attention immediately.

If a tank regulator is connected to metal tubing that in turn is connected to any type of equipment, the tubing should be grounded (for example, to metal piping). This grounding can be done with bushing wire, or any heavy-gauge wire, and a screw clamp.

Keep Compressed Gas Tanks Away from High Heat. Compressed gas cylinders should not be subjected to atmospheric temperatures above 130°F. A compressed gas cylinder should never be subjected to a direct flame. If defrosting an iced or frozen tank, water above 130°F should not be used. If compressed gas tanks need to be left outside, they should be kept in a shaded environment.

Compressed Gas Tanks Should be Properly Moved. Compressed gas tanks should never be dragged, slid, or allowed to bang against one another. One should use a proper hand-truck or other suitable device for transporting the specific type of compressed gas tank. Compressed gas tanks should never be lifted by their caps, by magnets, or by ropes, chains, or slings unless the manufacturer has provided attachments (such as lugs) on the tanks.

Compressed Gas Tanks Should Be Properly Stored. When a variety of different gases are to be stored together, they should be separated by type. For example, combustible gases should never be stored directly next to sources of oxygen. They should also be separated as to whether they are full or empty. After a tank has

272 Compressed Gases

been emptied,* it should be marked to make its status easily identifiable. One technique is to take a chalk, or crayon, and write "M T" on the tank signifying that the tank is "empty." Unfortunately, not all gas companies wipe this mark off; thus after the tank is returned, a full tank could be displayed as "empty." A better technique is to loosely tape a piece of paper saying "empty" onto the tank. This paper will be removed by the gas company. Some compressed gas suppliers provide tags that can be hung on tanks which provide convenient identification of the gassupply (see Fig. 5.2). If you cannot obtain these tags from your supplier, they are very easy to make.

When you empty a tank, other people's confusion and frustration can be prevented if you detach the regulator and identify the tank as empty before you call your gas people requesting a fresh tank. Likewise, if you need to remove the regulator on a partially emptied compressed gas tank, replace the protective capf and label the tank to indicate it is not a full tank, noting the remaining partial pressure.

Although it is not dangerous to store acetylene tanks on their side, such positions may allow solvent loss, which in turn can decrease the flame quality.

5.1.6 In Case of Emergency

In the United States there is an organization called CHEMTREC (Chemical Transportation Emergency Center) that has a 24-hour toll-free number to call for advice on chemical transportation emergencies [(800)-424-9300]. For written or general questions, write:

Chemical Transportation Emergency Center

1300 Wilson Blvd.

Arlington VA 22209 or call (703) 741-5000

For information on the rules and regulations of transporting various gasses, contact:

Department of Transportation

Office of Hazardous Materials

400 7th St. SW

Washington DC, 20590

(800) 467-4922

A tank with less than 50 psig should be considered empty. It is not recommended to totally empty a tank because a completely emptied tank can develop condensation that can prematurely age the tank. fA tank should have either a regulator or a cap on. No tank should be left with the valve opening exposed to the environment.