Judging Cutting Quality
To know how good of a cutting job you are doing, you must understand know what constitutes a good oxygas cut. In general, the quality of an oxygas cut is judged by four characteristics:
- The shape and length of the draglines
- The smoothness of the sides
- The sharpness of the top edges
- The amount of slag adhering to the metal
Drag lines are line markings that show on the surface of the cut. Good drag lines are almost straight up and down, as shown in figure 4-31, view A. Poor drag 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 (figure 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.
Figure 4-31.—Effects of correct and incorrect cutting procedures.
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 (figure 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.
An oxygas cut is not satisfactory when slag adheres so tightly to the metal that it is difficult to remove.
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 other lessons in this course 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.
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 non-shatterable 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, while a sixth class has been reserved for fire protection equipment. A standard color has been chosen to represent each of these classes and is shown in table 4-2.
Table 4-2.—Standard Colors
Since you work with fuel gas and oxygen, you must become familiar with the colors of the cylinders in which these gases are contained. The fuel-gas cylinder is yellow, and the oxygen cylinder is green.
In addition to color coding, the exact identification of the material contained in a compressed-gas cylinder must be indicated by a written title that appears in two locations-diametrically opposite and parallel to the longitudinal axis of the cylinder. Cylinders, having a background color of yellow, orange, or buff have the title painted black Cylinders, having a background color of red, brown, black, blue, gray, or green, have the title painted white.
COLOR WARNINGS.— The appearance on the body, top, or as a band(s) on compressed-gas cylinders of the six colors specified should provide a warning of danger from the hazard involved.
CYLINDER COLOR BANDS.— Cylinder color bands appear upon the cylinder body and serve as color warnings when they are yellow, brown, blue, green, or gray. The bands also provide color combinations to separate and distinguish cylinders for convenience in handling, storage, and shipping.
DECALS.— Two decals may be applied on the shoulder of each cylinder. They should be diametrically opposite and at right angles to the titles. They should indicate the name of the gas, precautions for handling, and use. A background color corresponding to the primary warning color of the contents should be used.
SHATTERPROOF CYLINDERS.— A shatter-proof cylinder should be stenciled with the phrase “NON-SHAT’’ longitudinally 90 degrees from the titles. Letters must be black or white and approximately 1 inch in size.
SERVICE OWNERSHIP.— On cylinders owned by or procured for the Department of Defense, the bottom and the lower portion of the cylinder body opposite the valve end may be used for service ownership titles.
The six colors identified in Table 4-2 are used on the body and top of, or as a band on, a compressed-gas cylinder to serve as a warning of the hazard involved in handling the type of material contained in the cylinder.
Figure 4-32.—Titles and color codes for compressed-gas cylinders.
Figure 4-32 shows titles and color codes for compressed-gas cylinders most often found on construction sites or in a public works department where welders are working. Figure 4-33 shows how cylinders are identified by the overall painted color code and by the stenciled name of the gas. It should be noted that the color code of cylinders shown in figure 4-32 is military only; the commercial industry does not necessarily comply with these color codes. Commercial U.S. and ISO color-code standards have yet to be established.
Figure 4-33.—Identifying color patterns for gas cylinders.
Handling and Storing Gas Cylinders
Each compressed-gas cylinder carries markings indicating compliance with Interstate Commerce Commission (ICC) requirements. When the cylinders are at your work site, they become your responsibility. There are several things you should not do when handling and storing compressed-gas cylinders.
- Never fill your own cylinders. It requires special training and special equipment.
- Never alter or fix the safety devices on a cylinder. It is illegal and also stupid. The only personnel permitted to work on cylinder safety devices are the cylinder owners and suppliers.
- Never store cylinders near a heat source or in direct sunlight. Heat causes the gas inside a cylinder to expand. This could result in cylinder failure or fire.
- Never store cylinders in a closed or unventilated space. If one of the cylinders were to leak, it could cause an explosion or asphyxiate someone entering the space.
- Store cylinders in protected, well-ventilated, and dry spaces. Protect the cylinder valves and safety devices from ice and snow. A safety device may not work if it is frozen.
- Never store fuel cylinders and oxidizers within the same space. Oxidizers must be stored at least 50 feet from fuel cylinders. Use fire-resistant partitions between cylinder storage areas.
- Never mix empty cylinders with full cylinders.
- Do not mix cylinders that contain different gases.
- Always replace the cylinder cap and mark the cylinder “Empty” or “MT.” Store the cylinders in a cool, dry place ready for pickup by the supplier. Even in storage, chain the cylinders when they are stored in the upright position.
- Never drag a cylinder to move it. When available, use a cylinder truck. If at all possible, leave the cylinders on the hand truck and operate them from there; otherwise, tilt the cylinder slightly and roll it on the bottom edge. Always install the cylinder cap before moving the cylinder. Never use slings or magnets to carry cylinders. If you lift a cylinder upright by the cap, make sure that it is screwed on tightly. If the cylinder cap comes off, the cylinder could fall and either crush your foot or snap the valve off. If a cylinder is dropped and the valve breaks, it could launch itself like a rocket.
When cylinders have been stored outside in freezing weather, they sometimes become frozen to the ground or to each other. To free the cylinders, you can pour warm water (not boiling) over the frozen or icy areas. As a last resort, you can pry them loose with a prybar. If you use a prybar, never pry or lift under the valve cap or valve.