Oxygas Cutting Equipment
An oxygas cutting outfit usually consists of a cylinder of acetylene or MAPP gas, a cylinder of oxygen, two regulators, two lengths of hose with fittings, and a cutting torch with tips (fig. 4-1). An oxygas cutting outfit also is referred to as a cutting rig.
In addition to the basic equipment mentioned above, numerous types of auxiliary equipment are used in oxygas cutting. An important item is the spark igniter that is used to light the torch (fig. 4-2, view A). Another item you use is an apparatus wrench. It is similar in design to the one shown in figure 4-2, view B. The apparatus wrench is sometimes called a gang wrench because it fits all the connections on the cutting rig. Note that the wrench shown has a raised opening in the handle that serves as an acetylene tank key.
Other common accessories include tip cleaners, cyl-inder trucks, clamps, and holding jigs. Personal safety apparel, such as goggles, hand shields, gloves, leather aprons, sleeves, and leggings, are essential and should be worn as required for the job at hand.
Oxygas cutting equipment can be stationary or portable. A portable oxygas outfit, such as the one shown in figure 4-3, is an advantage when it is necessary to move the equipment from one job to another.
To conduct your cutting requirements, you must be able to set up the cutting equipment and make the required adjustments needed to perform the cutting operation. For this reason it is important you understand the purpose and function of the basic pieces of equipment that make up the cutting outfit. But, before discussing the equipment, lets look at the gases most often used in cutting: acetylene, MAPP gas, and oxygen.
Acetylene is a flammable fuel gas composed of carbon and hydrogen having the chemical formula C2H2.When burned with oxygen, acetylene produces a hot flame, having a temperature between 5700°F and 6300°F. Acetylene is a colorless gas, having a disagree-able odor that is readily detected even when the gas is highly diluted with air. When a portable welding outfit, similar to the one shown in figure 4-3 is used, acetylene is obtained directly from the cylinder. In the case of stationary equipment, similar to the acetylene cylinder bank shown in figure 4-4, the acetylene can be piped to a number of individual cutting stations.
Hazards Pure acetylene is self-explosive if stored in the free state under a pressure of 29.4 pounds per square inch (psi). A slight shock is likely to cause it to explode.
Acetone is a liquid chemical that dissolves large portions of acetylene under pressure without changing the nature of the gas. Being a liquid, acetone can be drawn from an acetylene cylinder when it is not upright. You should not store acetylene cylinders on their side, but if they are, you must let the cylinder stand upright for a minimum of 2 hours before using. This allows the acetone to settle to the bottom of the cylinder.
Acetylene is measured in cubic feet. The most com-mon cylinder sizes are 130-, 290-, and 330-cubic-foot capacity. A common standard size cylinder holds
225 cubic feet of acetylene. Just because a cylinder has a 225-cubic-foot capacity does not necessarily mean it has 225 cubic feet of acetylene in it. Because it is dissolved in acetone, you cannot judge how much acety-lene is left in a cylinder by gauge pressure. The pressure of the acetylene cylinder will remain fairly constant until most of the gas is consumed.
An example of an acetylene cylinder is shown in figure 4-5. These cylinders are equipped with fusible plugs that relieve excess pressure if the cylinder is exposed to undo heat. A common standard acetylene cylinder contains 225 cubic feet of acetylene and weighs about 250 pounds. The acetylene cylinder is yellow, and all compressed-gas cylinders are color-coded for identification. More on the color coding of cylinders is covered later in this lesson.
MAPP (methylacetylene-propadiene) is an all-purpose industrial fuel having the high-flame temperature of acetylene but has the handling characteristics of propane. Being a liquid, MAPP is sold by the pound, rather than by the cubic foot, as with acetylene. One cylinder containing 70 pounds of MAPP gas can accomplish the work of more than six and one-half 225-cubic-foot acetylene cylinders; therefore, 70 pounds of MAPP gas is equal to 1,500 cubic feet of acetylene.
MAPP is not sensitive to shock and is nonflammable in the absence of oxygen. There is no chance of an explosion if a cylinder is bumped, jarred, or dropped. You can store or transport the cylinders in any position with no danger of forming an explosive gas pocket.
The characteristic odor, while harmless, gives warnings of fuel leaks in the equipment long before a dan-gerous condition can occur. MAPP gas is not restricted to a maximum working pressure of 15 psig, as is acetylene. In jobs requiring higher pressures and gas flows, MAPP can be used safely at the full-cylinder pressure of 95 psig at 70°F. Because of this, MAPP is an excellent gas for underwater work.
Cylinder-filling facilities are also available from bulk installations that allow users to fill their cylinders on site. Filling a 70-pound MAPP cylinder takes one man about 1 minute and is essentially like pumping water from a large tank to a smaller one.
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 concentrations (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.
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 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 processconsists of running an electrical 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.
You can determine the amount of oxygen in a com-pressed gas cylinder by reading the volume scale on the high-pressure gauge attached to the regulator.
You must be able to reduce the high-pressure gas in a cylinder to a working pressure before you can use it. This pressure reduction is done by a regulator or reduc-ing valve. The one basic job of all regulators is to take the high-pressure gas from the cylinder and reduce it to a level that can be safely used. Not only do they control the pressure but they also control the flow (volume of gas per hour).
Regulators come in all sizes and types. Some are designed for high-pressure oxygen cylinders (2,200 psig), while others are designed for low-pressure gases, such as natural gas (5 psig). Some gases like nitrous oxide or carbon dioxide freeze when their pressure is reduced so they require electrically heated regulators.
Most regulators have two gauges: one indicates the cylinder pressure when the valve is opened and the other indicates the pressure of the gas coming out of the regulator. You must open the regulator before you get a reading on the second gauge. This is the delivery pres-sure of the gas, and you must set the pressure that you need for your particular job.
The pressures that you read on regulator gauges is called gauge pressure. If you are using pounds per square inch, it should be written as psig (this acronym means pounds per square inch gauge). When the gauge on a cylinder reads zero, this does not mean that the cylinder is empty. In actuality, the cylinder is still full of gas, but the pressure is equal to the surrounding atmos-pheric pressure. Remember: no gas cylinder is empty unless it has been pumped out by a vacuum pump.
There are two types of regulators that control the flow of gas from a cylinder. These are either single-stage or double-stage regulators.
Regulators are used on both high- and low-pressure systems. Figure 4-8 shows two SINGLE-STAGE regu-lators: one for acetylene and one for oxygen. The regu-lator mechanism consists of a nozzle through which the gases pass, a valve seat to close off the nozzle, a dia-phragm, and balancing springs. These mechanisms are all enclosed in a suitable housing. Fuel-gas regulators and oxygen regulators are basically the same design. The difference being those designed for fuel gases are not made to withstand the high pressures that oxygen regulators are subjected to.
In the oxygen regulator, the oxygen enters through the high-pressure inlet connection and passes through a glass wool falter that removes dust and dirt. Turning the adjusting screw IN (clockwise) allows the oxygen to pass from the high-pressure chamber to the low-pressure chamber of the regulator, through the regulator outlet, and through the hose to the torch. Turning the adjusting screw further clockwise increases the working pressure; turning it counterclockwise decreases the working pressure.
The high-pressure gauge on an oxygen regulator is graduated from 0 to 3,000 psig and from 0 to 220 in cubic feet. This allows readings of the gauge to deter-mine cylinder pressure and cubic content. Gauges are calibrated to read correctly at 70°F. The working pres-sure gauge may be graduated in psig from 0 to 150, 0 to 200, or from 0 to 400, depending upon the type of regulator used. For example, on regulators designed for heavy cutting, the working pressure gauge is graduated from 0 to 400.
The major disadvantage of single-stage regulators is that the working gas pressure you set will decrease as the cylinder pressure decreases; therefore, you must constantly monitor and reset the regulator if you require a fixed pressure and flow rate. Keeping the gas pressure and flow rate constant is too much to expect from a regulator that has to reduce the pressure of a full cylinder from 2,200 psig to 5 psig. This is where double-stage regulators solve the problem.
The double-stage regulator is similar in principle to the one-stage regulator. The main difference being that the total pressure drop takes place in two stages instead of one. In the high-pressure stage, the cylinder pressure is reduced to an intermediate pressure that was predetermined by the manufacturer. In the low-pressure stage, the pressure is again reduced from the intermediate pressure to the working pressure you have chosen. A typical double-stage regulator is shown in figure 4-9.
Problems and Safety
Regulators are precise and complicated pieces of equipment. Carelessness can do more to ruin a regulator than any other gas-using equipment. One can easily damage a regulator by simply forgetting to wipe clean the cylinder, regulator, or hose connections. When you open a high-pressure cylinder, the gas can rush into the regulator at the speed of sound. If there is any dirt present in the connections, it will be blasted into the precision-fitted valve seats, causing them to leak This results in a condition that is known as creep. Creep occurs when you shut of the regulator but not the cylin-der and gas pressure is still being delivered to the low-pressure side.
Regulators are built with a minimum of two relief devices that protect you and the equipment in the case of regulator creep or high-pressure gas being released into the regulator all at once. All regulator gauges have blowout backs that release the pressure from the back of the gauge before the gauge glass explodes. Nowadays, most manufacturers use shatterproof plastic instead of glass.
The regulator body is also protected by safety devices. Blowout disks or spring-loaded relief valves are the two most common types of devices used. When a blowout disk ruptures, it sounds like a cannon. Spring-loaded relief valves usually make howling or shrieking like noises. In either case, your first action, after you recover from your initial fright, should be to turn off the cylinder valve. Remove the regulator and tag it for repair or disposal.
When opening a gas cylinder, you should just crack the valve a little. This should be done before attaching the regulator and every time thereafter. By opening the cylinder before connecting the regulator, you blow out any dirt or other foreign material that might be in the cylinder nozzle. Also, there is the possibility of a regulator exploding if the cylinder valve is opened rapidly.
The hoses used to make the connections between the torch and the regulators must be strong, nonporous, light, and flexible enough to make torch movements easy. They must be made to withstand internal pressures that can reach as high as 100 psig. The rubber used in hose manufacture is specially treated to remove the sulfur that could cause spontaneous combustion.
Welding hose is available in single- and double-hose lengths. Size is determined by the inside diameter, and the proper size to use depends on the type of work for which it is intended. Hose used for light work has a 3/16 or 1/4 inch inside diameter and one or two plies of fabric. For heavy-duty welding and cutting operations, use a hose with an inside diameter of 5/16 inch and three to five plies of fabric. Single hose is available in the standard sizes as well as 1/2-, 3/4-, and 1-inch sizes. These larger sizes are for heavy-duty heating and for use on large cutting machines.
The most common type of cutting and welding hose is the twin or double hose that consists of the fuel hose and the oxygen hose joined together side by side. They are joined together by either a special rib (fig. 4-10, view A) or by clamps (fig. 4-10, view B). Because they are joined together, the hoses are less likely to become tangled and are easier to move from place.
The length of hose you use is important. The deliv-ery pressure at the torch varies with the length of the hose. A 20-foot, 3/16-inch hose maybe adequate for a job, but if the same hose was 50 feet long, the pressure drop would result in insufficient gas flow to the torch. Longer hoses require larger inside diameters to ensure the correct flow of gas to the torch. When you are having problems welding or cutting, this is one area to check The hoses used for fuel gas and oxygen are identical in construction, but they differ in color. The oxygen hose cover is GREEN, and the fuel-gas hose cover is RED. This color coding aids in the prevention of mishaps that could lead to dangerous accidents.
All connections for welding and cutting hoses have been standardized by the Compressed Gas Association. Letter grades A, B, C, D, and E plus the type of gas used correspond directly with the connections on the regulators. A, B, and C are the most common size connections. A-size is for low-flow rates; B-size for medium-flow rates; and C-size is for heavy-flow rates. D and E sizes are for large cutting and heating torches.
When ordering connections, you must specify the type of gas the hose will be carrying. This is because the connections will be threaded different y for different types of gas. Fuel gases use left-hand threads, while oxygen uses right-hand threads. The reason for this is to prevent the accidental hookup of a fuel gas to a life-support oxygen system or vice versa. The basic hose connection consists of a nut and gland. The nut has threads on the inside that match up with the male inlet and outlet on the torch and regulator. The gland slides inside the hose and is held in place by a ferrule that has been crimped. The nut is loose and can be turned by hand or a wrench to tighten the threaded nut onto the equipment.
Another important item that is often overlooked are check valves. These inexpensive valves prevent per-sonal injuries and save valuable equipment from flashbacks. When ordering, make sure you specify the type of gas, connection size, and thread design. The check valves should be installed between the torch connection and the hose.
The equipment and accessories for oxygas cutting are the same as for oxygas welding except that you use a cutting torch or a cutting attachment instead of a welding torch. The main difference between the cutting torch and the welding torch is that the cutting torch has an additional tube for high-pressure cutting oxygen.The flow of high-pressure oxygen is controlled from a valve on the handle of the cutting torch. In the standard cutting torch, the valve may be in the form of a trigger assembly like the one in figure 4-11. On most torches, the cutting oxygen mechanism is designed so the cutting oxygen can be turned on gradually. The gradual opening of the cutting oxygen valve is particularly helpful in operations, such as hole piercing and rivet cutting.
Each manufacturer makes many different types of cutting tips. Although the orifice arrangements and the tips are much the same among the manufacturers, the part of the tip that fits into the torch head often differs in design.
Because of these differences, there is the possibility of having two or three different types of cutting torches in your kits. Make sure that the cutting tips match the cutting attachment and ensure that the cutting attachment matches the torch body. Figure 4-13 shows the different styles of tips, their orifice arrangements and their uses. The tips and sears are designed to produce an even flow of gas and to keep themselves as cool as possible. The seats must produce leakproof joints. If the joints leak, the preheat gases could mix with the cutting oxygen or escape to the atmosphere, resulting in poor cuts or the possibility of flashbacks.
To make clean and economical cuts, you must keep the tip orifices and passages clean and free of burrs and slag. If the tips become dirty or misshapened, they should be put aside for restoration. Figure 4-14 shows four tips: one that is repairable, two that need replacing, and one in good condition. Since it is extremely important that the sealing surfaces be clean and free of scratches or burrs, store the tips in a container that cannot scratch the seats. Aluminum racks, plastic racks, and wood racks or boxes make ideal storage containers.
TIP MAINTENANCE. In cutting operations, the stream of cutting oxygen sometimes blows slag and molten metal into the tip orifices which partially clogs them. When this happens, you should clean the orifices thoroughly before you use the tip again. A small amount of slag or metal in an orifice will seriously interfere with the cutting operation. You should follow the recommendations of the torch manufacturer as to the size of drill or tip cleaner to use for cleaning the orifices. If you do not have a tip cleaner or drill, you may use a piece of soft copper wire. Do not use twist drills, nails, or welding rods for cleaning tips because these items are likely to enlarge and distort the orifices.
Clean the orifices of the cutting torch tip in the same manner as the single orifice of the welding torch tip. Remember: the proper technique for cleaning the tips is to push the cleaner straight in and out of the orifice. Be careful not to turn or twist the cleaning wire. Figure 4-15 shows a typical set of tip cleaners.
Occasionally the cleaning of the tips causes enlargement and distortion of the orifices, even when using the proper tip cleaners. If the orifices become enlarged, you will get shorter and thicker preheating flames; in addition, the jet of cutting oxygen will spread, rather than leave the torch, in the form of a long, thin stream. If the orifices become belled for a short distance at the end, you can sometimes correct this by rubbing the tip back and forth against emery cloth placed on a flat surface. This action wears down the end of the tip where the orifices have been belled, thus bringing the orifices back to their original size.
Obviously, this procedure will not work if the damage is great or if the belling extends more than a slight distance into the orifice. After reconditioning a tip, you may test it by lighting the torch and observing the preheating flames. If the flames are too short, the orifices are still partially blocked. If the flames snap out when you close the valves, the orifices are still distorted.
If the tip seat is dirty or scaled and does not properly fit into the torch head, heat the tip to a dull red and quench it in water. This will loosen the scale and dirt enough so you can rub it off with a soft cloth.
MAPP GAS CUTTING TIPS. Four basic types of MAPP gas cutting tips are used: two are for use with standard pressures and normal cutting speeds, and two are for use with high pressures and high cutting speeds. Only the standard pressure tips, types SP and FS, will be covered here since they are the ones that Steelwork-ers will most likely use. SP stands for standard pressure and FS for fine standard.
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy
Copyright © 2001-2006 SweetHaven Publishing Services