Compressed Gases

stored in their liquid states. The region above a liquefied gas is an area called the head space, which is the gas form of the liquefied gas. The pressure of the gas in the head space depends on the vapor pressure of the gas, which, in turn, depends on the temperature of the liquid gas. The greater the temperature of the liquefied gas is and the greater the amount of gas released from the liquid, the greater the pressure in the head space. If the pressure is sufficient, the container can rupture. To prevent this, all tanks have some sort of pressure release valve that release excess pressure at some determined value.

To control the rate of flow and releasing pressure, gases are released from compressed gas cylinders by regulators (see Sec. 5.2) at a user-defined pressure. A regulator can display the amount of gas remaining in a compressed gas tank (in cubic feet of gas), but not in a liquefied gas tank.

The remaining liquid gas in a tank can be estimated by observing any floats that may exist [most commonly seen in cryogenic tanks (see Sec. 6.2.10)], but it can always be accurately determined by weighing the tank. First you must subtract the weight of the empty tank by the weight of the partially filled tank (known as the tare weight*) to determine the weight of any remaining liquid. Then you divide the difference by the weight of the liquid gas per liter to calculate the remaining volume.

Tank-weighing should be done with everything you plan to use with it (i.e., regulators, tubing, and so forth) attached, making sort of a "ready-to-use tare weight." This ready-to-use weight means you do not have to strip the tank of all equipment to make remaining gas determinations. Otherwise, if it was originally weighed without a regulator but later weighed with one, you might assume that you have more gas than really exists.

5.1.2 The Dangers of Compressed Gas

Compressing a gas allows a lot of gas to exist in a small amount of area for transportation, storage, and use. It is physically easier to deal with a compressed, gas tank that is 9 in. in diameter and 51 in. tall than to store the approximately several hundred cubic feet1^ of gas contained within.

Regardless of the inherent danger of any given gas, once a gas is compressed its potential for danger takes on a completely different light. A completely safe gas compressed in a container at some 2000 lb/in.2 could act like a bomb if improperly handled! Your safety, and the safety of your equipment, is therefore dependent on:

1.

*A11tanks that require weighing for volume content have their tare weight stamped on the side of the tank. This weight is exclusively the tank and does not include any regulators, valves, straps, or other fittings the user may have added.

fA tank of this size will contain (for example) 282 cu ft of oxygen, 257 cu ft of nitrogen, or 290 cu ft of ultrapure argon.

2.The materials and equipment selected to be used with the compressed gases

3.The proper use of the compressed gas cylinders and associated equipment

The first two points of the above list can best be controlled by strict industry standards and the conscientious matching of equipment to the needs and demands of the user. The third requires education, and unfortunately the user often does not have the opportunity (or desire) to learn everything there is to know about how to use compressed gases safely. Therefore, there have been a variety of industrystandard "idiot-proof controls to minimize the possibility of mistakes. To make compressed gas cylinders consistently safe, reliable, and as idiot-proof as possible, these strategies took the study, analysis, and deliberation of at least 18 different private and governmental agencies (see Table 5.1).

The first level of safety for quality and control is the construction of compressed gas cylinders. The specifications for their construction in North America is denned by Department of Transportation (DOT) and Canadian Transport Commission (CTC) regulations. Cylinders are made from carbon steel or alloy steel with seamless, brazed, or welded tubing that is formed by billeting (drawing flat

Table 5.1 Agencies involved

in Standardization of Compressed Gas Tanks

American Gas Assoc.

American Petroleum Inst.

American Society for Testing and Materials

American Welding Society

Association of American Railroads

Canadian Transport Commission (CTC)

Compressed Gas Association (CGA)

Compressed Gas Manufacturer's Assoc. Inc.

Connections Standards Commission of the CGA (This was first known as the Gas Cylinder Valve Thread Commission of the CGA. Later it became the Valve Standards Com. of the CGA, and in 1971 it received its current name.)

Department of Transportation (DOT)

Interdepartmental Screw Thread Commission

International Standards Organization (ISO)

National Institute of Standards and Technology (This is the new name of the National Bureau of Standards)

National Fire Prevention Assoc.

Standards Associations (representing Great Briton, Canada, and the U.S.)

U.S. Army

U.S. Dept. of Commerce

U.S. Navy

256 Compressed Gases

sheets to a cylindrical shape) or by using punch-press dies. Ends are sealed by forging or spinning at great heat. Alternatively, closed ends are drilled out and a metal piece is plugged into the hole. In the United States, one tank design can be used for a variety of gases (except for acetylene). In Europe, a tank can only be used for the gas it was designed for.

Generally, cylinders that are broad and squat in contour are for low-pressure service, such as the propane tanks used on automobiles or with campers. Those that are tall and thin are generally used for high-pressure containment, such as for oxygen, hydrogen, or nitrogen.

A series of letters and numbers are stamped on the shoulders of compressed gas tanks (see Fig. 5.2) to provide coded information for tank inspectors. These numbers tell under what codes the tank was made, the manufacturer, service pressure, serial number of the tank, and when it was last inspected and by whom. The manner in which the codes are laid out varies, and it may be difficult for an untrained person to interpret. However, to the compressed gas industry, they are important for preventing mistakes from the misuse of the containers.

5.1.3 CGA Fittings

To prevent gases from being attached to the wrong system, the Compressed Gas Association (CGA) implemented a variety of different fittings for attaching a regulator to a compressed gas tank. These fittings prevent a user from accidentally taking a regulator that is used (for example) for nitrogen and attaching it to a hydrogen tank. By itself, such an attachment may not seem too bad, but if the nitrogen regulator was already connected to a system expecting nitrogen, the consequences of hydrogen could be dramatic.

Table 5.2 lists a number of gases and the appropriate CGA fittings or threaded connections that a regulator must have to be attached to a tank of such a gas (in this table, when a second CGA number follows a first entry, it is the CGA fitting for a lecture bottle). Alternatively, if you have a regulator and want to know what gases it can be used with—or what other gases use the same regulator—see Table 5.3.

If, after looking at Table 5.2 or Table 5.3, you wish to know what a fitting looks like, Fig. 5.2 illustrates the 15 most common CGA fittings used on full-sized compressed gas tanks within the laboratory. Please note that right-handed threads close with CW rotation, and left-handed threads close with CCW rotation. Lefthanded thread fittings can be easily identified by the notch on the closing nut (see Fig. 5.2) as opposed to right-handed fittings, which are notchless.

Never spray or drip oil onto compressed tank cap threads to ease removal or to replace the tank cap. Minimally, the oil could contaminate the CGA fitting, but oil near compressed oxygen can explode! If you are unable to remove or replace a tank cap, obtain help from the manufacturer or the distributor of the compressed tank.