lines, as shown in figure 4-31, view B, are long and irregular or curved excessively. Drag lines of this type indicate a poor cutting procedure that could result in the loss of the cut (fig. 4-31, views B and C). Draglines are the best single indication of the quality of the cut made with an oxygas torch. When the draglines are short and almost vertical, the sides smooth, and the top edges sharp, you can be assured that the slag conditions are satisfactory.

SIDE SMOOTHNESS

A satisfactory oxygas cut shows smooth sides. A grooved, fluted, or ragged cut surface is a sign of poor quaility.

TOP EDGE SHARPNESS

The top edges resulting from an oxygas cut should be sharp and square (fig. 4-31, view D). Rounded top edges, such as those shown in view E of figure 4-31, are not satisfactory. The melting of the top edges may result from incorrect preheating procedures or from moving the torch too slowly.

SLAG CONDITIONS

An oxygas cut is not satisfactory when slag adheres so tightly to the metal that it is difficult to remove.

SAFETY PRECAUTIONS

In all cutting operations, you must ensure that hot slag does not come in contact with combustible material. Globules of hot slag can roll along the deck for long distances. Do not cut within 30 to 40 feet of unprotected combustible materials. If you cannot remove the combustible materials, cover them with sheet metal or other flameproof guards. Keep the fuel gas and oxygen cylinders far enough away from the work so hot slag does not fall on the cylinders or hoses.

Many of the safety precautions discussed in chapters 5 through 8 of this manual apply to cutting as well as to welding. Be sure you are completely familiar with all the appropriate safety precautions before attempting oxygas cutting operations.

BACKFIRE AND FLASHBACK

Improper operation of the oxygas torch can cause the flame to go out with a loud snap or pop. This is called a "backfire." Close the torch valves, check the connections, and review your operational techniques before relighting the torch. You may have caused the backfire by touching the tip against the work, by overheating the tip, or by operating the torch with incorrect gas pressures. A backfire also may be caused by a loose tip or head or by dirt on the seat.

A flashback occurs when the flame burns back inside the torch, usually with a shrill hissing or squealing noise. You should close the torch oxygen valve that controls the flame to stop the flashback at once. Then you should close the gas valve and the oxygen and gas regulators. Be sure you allow the torch to cool before relighting it. Also, blow oxygen through the cutting tip for a few seconds to clear out soot that may have accumulated in the passages. Flashbacks may extend back into the hose or regulators. Flashbacks indicate that something is wrong, either with the torch or with the way it is being operated. Every flashback should be investigated to determine its cause before the torch is relighted. A clogged orifice or incorrect oxygen and gas pressures are often responsible. Avoid using gas pressures higher than those recommended by the manufacturer.

CYLINDERS

Gas cylinders are made of high-quality steel. High-pressure gases, such as oxygen, hydrogen, nitrogen, and compressed air, are stored in cylinders of seamless construction. Only nonshatterable high-pressure gas cylinders may be used by ships or activities operating outside the continental United States. Cylinders for low- pressure gases, such as acetylene, may be welded or brazed. Cylinders are carefully tested, either by the factory or by a designated processing station, at pressures above the maximum permissible charging pressure.

Identification of Cylinders

Color warnings provide an effective means for marking physical hazards and for indicating the location of safety equipment. Uniform colors are used for marking compressed-gas cylinders, pipelines carrying hazardous materials, and fire protection equipment.

Five classes of material have been selected to represent the general hazards for dangerous materials,