Oxygen

Figure 4-6.—Explosive limits of MAPP and acetylene in air. Figure 4-7.—Typical oxygen cylinder. figure 4-6, you can see these limits are narrow in com- parison with that of acetylene. MAPP gas has a highly detectable odor. The smell is detectable at 100 ppm, or at a concentration of 1/340th of its lower explosive limit. Small fuel-gas systems may leak 1 or 1 1/2 pounds of fuel or more in an 8-hour shift; bulk systems will leak even more. Fuel-gas leaks are often difficult to find and often go unnoticed; however, a MAPP gas leak is easy to detect and can be repaired before it becomes dangerous. MAPP toxicity is rated “very slight,” but high con- centrations (5,000 ppm) may have an anesthetic effect. Local eye or skin contact with MAPP gas vapor causes no adverse effect; however, the liquid fuel can cause dangerous frostlike burns due to the cooling caused by the rapid evaporation of the liquid. The identification markings on a MAPP cylinder are a yellow body with band “B” colored orange and the top yellow. corrosion of aluminum are all due to the action of atmospheric oxygen. This action is known as oxidation. Oxygen is obtained commercially either by the liquid-air process or by the electrolytic process. In the liquid-air process, the air is compressed and then cooled to a point where the gases become liquid (ap- proximately –375°F). The temperature is then raised to above –321 ‘F, at which point the nitrogen in the air becomes gas again and is removed. When the tempera- ture of the remaining liquid is raised to –297°F, the oxygen forms gas and is drawn off. The oxygen is further purified and compressed into cylinders for use. The other process by which oxygen is produced— the electrolytic process—consists of running an electri- cal current through water to which an acid or an alkali has been added. The oxygen collects at the positive terminal and is drawn off through pipes to a container. Oxygen is supplied for oxyacetylene welding in seamless steel cylinders. A typical oxygen cylinder is shown in figure 4-7. The color of a standard oxygen cylinder used for industrial purposes is solid green. Oxygen cylinders are made in several sizes. The size most often used in welding and cutting is the 244-cubic- foot capacity cylinder. This cylinder is 9 inches in di- ameter, 51 inches high, and weighs about 145 pounds and is charged to a pressure of 2,200 psi at 70°F. OXYGEN Oxygen is a colorless, tasteless, and odorless gas and is slightly heavier than air. It is nonflammable but supports combustion with other elements. In its free state, oxygen is one of the more common elements. The atmosphere is made up of about 21 parts of oxygen and 78 parts of nitrogen, the remainder being rare gases. Rusting of ferrous metals, discoloration of copper, and 4-5