11.3 Acetylene Supply
Figure 11-13
Figure 11-13. Cutaway view of an acetylene cylinder, showing the porous filler. Note the safety fuse plugs at the top and bottom of cylinder.
Figure 11-13. Cutaway view of an acetylene cylinder, showing the porous filler. Note the safety fuse plugs at the top and bottom of cylinder.
Figure 11-14
Figure 11-14. Acetylene cylinder fuse plugs. The body of the plug is usually made of brass. The center is a fusible material that melts at about 212°F (100°C).
Figure 11-14. Acetylene cylinder fuse plugs. The body of the plug is usually made of brass. The center is a fusible material that melts at about 212°F (100°C).
Figure 11-15
Figure 11-15. A cutaway view of an acetylene cylinder valve. Note that the regulator attaches to a female thread.
Figure 11-16
Figure 11-16. Acetylene cylinders come in a variety of sizes to meet the needs of most users. Note the post-type valves on the B size and MC size cylinders at the bottom right. (Worthington Cylinders)
Figure 11-16. Acetylene cylinders come in a variety of sizes to meet the needs of most users. Note the post-type valves on the B size and MC size cylinders at the bottom right. (Worthington Cylinders)
The majority of oxyfuel gas welding and cutting is done using acetylene fuel gas. Acetylene is the fuel gas referred to in most of the following sections of this chapter when the term “fuel gas” is used. Either an acetylene storage cylinder or an acetylene generator can be used to supply acetylene for welding.
Acetylene is produced by the chemical reaction between calcium carbide and water. The chemical formula for acetylene is C2H2. (See Chapter 34 for further technical information concerning the chemical structure and nature of acetylene.)
Acetylene is produced by the chemical reaction between calcium carbide and water. The chemical formula for acetylene is C2H2. (See Chapter 34 for further technical information concerning the chemical structure and nature of acetylene.)
11.3.1 Acetylene Cylinders
Acetylene gas can be stored in specially designed cylinders. Prior to cylinder storage, the acetylene gas is first passed through filters and purifiers. An acetylene cylinder is a pressure vessel suitable for the transport of acetylene. It contains a porous mass along with acetone solvent for acetylene with a cylinder valve fixed to the cylinder. Storing acetylene in its gaseous form under pressure is not safe at pressures above 15 psig (103kPa).
The method used to safely store acetylene in cylinders is as follows:
Acetylene cylinders, like oxygen cylinders, are fabricated according to Interstate Commerce Commission (ICC) specifications. Figure 11-13shows the construction of an acetylene cylinder. The base of an acetylene cylinder is concave, and it usually has two plugs threaded into it. These pipe-threaded fuse plugs have a center made of a special metal alloy that will melt at a temperature of approximately 212°F (100°C). Fuse plugs may also be threaded into the top of the cylinder. Figures 11-13 and 11-14 show the typical locations of fuse plugs. If the cylinder is subjected to a high temperature, the plugs will melt and allow the gas to escape before the pressure builds up enough to burst the cylinder. In a fire, the acetylene escaping from the fuse plug will burn, but at a relatively slow rate. These precautions are needed because the pressure in an acetylene cylinder builds up rapidly with an increase of temperature.
Acetylene cylinder valves come in two types. One has a handwheel which should remain attached at all times. Figure 11-15 shows the construction of a handwheel type of acetylene cylinder valve. The other type is equipped with a 3/8″ (9.5mm) square shank. The square shank is turned with a 3/8″ (9.5mm) square box-end wrench. The regulator fitting on the cylinder can have either a male or a female thread.
It is recommended that the cylinder valve be opened only 1/4 to 1/2 turn. The wrench should be left in place on the valve stem whenever the acetylene cylinder valve is open. This is done so that the valve can be closed quickly if a hose or some other part catches fire.
The amount of acetylene in a cylinder cannot be estimated by the pressure in the cylinder, because the pressure of the acetylene gas coming out of the acetone solution will remain fairly constant (depending on the temperature) until most of the gas is consumed. The amount of acetylene in a cylinder can be determined accurately by weighing the cylinder. During the process of making an acetylene cylinder, it is weighed multiple times. After it is charged with acetone, it is again carefully weighed. By knowing the tare weight (weight of the cylinder without the acetylene gas), the amount of acetylene in the cylinder can be easily determined after the acetylene has been added. Subtracting the tare weight from the overall cylinder weight will give the weight of the acetylene gas remaining. Acetylene gas weighs one pound per 14 1/2 ft3 (1.10kg per m3).
Acetylene cylinders should always be stored and used in an upright position. If they are not, some of the acetone is likely to escape with the acetylene and contaminate the equipment and the flame. Each time that an acetylene cylinder is refilled, the tare weight is checked and acetone is added if necessary.
Acetylene cylinders are available in a variety of sizes from 10 ft3 to 420 ft3 (283L to 11894L). Two small-size acetylene cylinders are available for portable welding and cutting equipment. These are B size, 40 ft3 (1133L), and MC size, 10 ft3 (283L). The fittings on these two small size cylinders (sometimes called tanks) are called Presto-O-Lite (POL) fittings.
Figure 11-16 illustrates the various sizes of acetylene cylinders. The valves in the larger cylinders shown in Figure 11-16 are known as POL Commercial. Like oxygen cylinders, manufacturers can paint acetylene cylinders any color, since there is no standard color code. Some acetylene cylinders have a recessed top. This recess protects the cylinder valve, which has a female regulator connection or fitting.
All acetylene cylinders are designed and constructed according to Interstate Commerce Commission Specification No. 8. Each design must be tested by the Bureau of Explosives and must pass these tests before it can be used commercially.
As noted, acetylene is dissolved in acetone. Therefore, acetylene cannot be drawn from the cylinder any faster than it can be released from the acetone. This release is a kind of boiling action that occurs efficiently at room temperature. The maximum safe rate for drawing acetylene from a cylinder is one-seventh of the cylinder's capacity per hour. Thus, a single 290 ft3 (8213L) cylinder can supply acetylene at a rate of about 41 ft3 (1161L) per hour. If acetylene is drawn out of a cylinder too rapidly, a considerable amount of acetone may be drawn from the cylinder along with the acetylene.
An oxyacetylene flame consuming some acetone burns with a purple color. Acetone in the flame is not desirable, since it lowers the flame temperature and increases gas consumption. Moreover, the quality of the weld is negatively affected.
Low or freezing temperatures can cause the flow of acetylene to decrease, since heat is needed to boil off the acetylene. Another factor that will cause a slowdown in acetylene flow is a nearly exhausted cylinder. As the cylinder approaches the empty or discharged condition, acetylene is released more slowly. If a greater flow rate is needed, a number of cylinders can be connected to a manifold. This kind of installation makes it possible to use up more of the acetylene from each cylinder. This reduces waste and lowers the cost of welding operations.
The rate at which acetylene can be drawn off depends on:
The method used to safely store acetylene in cylinders is as follows:
- New empty cylinders are filled with an inert monolithic (massively uniform) filler material that cures to a porosity of 85%, as required by federal safety regulations. This material is made of a non-asbestos, alkaline-resistant glass fiber. It fills the entire cylinder volume and due to its porosity, enables the absorption of the acetylene gas with the acetone solvent.
- After the cylinders are baked to remove any water moisture, they are weighed. Valves and fuse plugs are installed, then the cylinders are shot-blasted and painted. After strength tests are run, the cylinders are charged with acetone and carefully weighed to determine if they are fully charged. Acetone is a colorless, flammable liquid with an excellent capability of absorbing acetylene. At a full cylinder pressure of 250 psi at 70°F (1724 kPa at 21°C), acetone can absorb over 400 times its own volume of acetylene.
- Acetylene gas is then pumped into the cylinder, where it is absorbed by the acetone and stored in the small spaces or pores in the monolithic filler material. The use of the monolithic filler and the acetone is essential in preventing the accumulation of a pocket of high-pressure acetylene.
Acetylene cylinders, like oxygen cylinders, are fabricated according to Interstate Commerce Commission (ICC) specifications. Figure 11-13shows the construction of an acetylene cylinder. The base of an acetylene cylinder is concave, and it usually has two plugs threaded into it. These pipe-threaded fuse plugs have a center made of a special metal alloy that will melt at a temperature of approximately 212°F (100°C). Fuse plugs may also be threaded into the top of the cylinder. Figures 11-13 and 11-14 show the typical locations of fuse plugs. If the cylinder is subjected to a high temperature, the plugs will melt and allow the gas to escape before the pressure builds up enough to burst the cylinder. In a fire, the acetylene escaping from the fuse plug will burn, but at a relatively slow rate. These precautions are needed because the pressure in an acetylene cylinder builds up rapidly with an increase of temperature.
Acetylene cylinder valves come in two types. One has a handwheel which should remain attached at all times. Figure 11-15 shows the construction of a handwheel type of acetylene cylinder valve. The other type is equipped with a 3/8″ (9.5mm) square shank. The square shank is turned with a 3/8″ (9.5mm) square box-end wrench. The regulator fitting on the cylinder can have either a male or a female thread.
It is recommended that the cylinder valve be opened only 1/4 to 1/2 turn. The wrench should be left in place on the valve stem whenever the acetylene cylinder valve is open. This is done so that the valve can be closed quickly if a hose or some other part catches fire.
The amount of acetylene in a cylinder cannot be estimated by the pressure in the cylinder, because the pressure of the acetylene gas coming out of the acetone solution will remain fairly constant (depending on the temperature) until most of the gas is consumed. The amount of acetylene in a cylinder can be determined accurately by weighing the cylinder. During the process of making an acetylene cylinder, it is weighed multiple times. After it is charged with acetone, it is again carefully weighed. By knowing the tare weight (weight of the cylinder without the acetylene gas), the amount of acetylene in the cylinder can be easily determined after the acetylene has been added. Subtracting the tare weight from the overall cylinder weight will give the weight of the acetylene gas remaining. Acetylene gas weighs one pound per 14 1/2 ft3 (1.10kg per m3).
Acetylene cylinders should always be stored and used in an upright position. If they are not, some of the acetone is likely to escape with the acetylene and contaminate the equipment and the flame. Each time that an acetylene cylinder is refilled, the tare weight is checked and acetone is added if necessary.
Acetylene cylinders are available in a variety of sizes from 10 ft3 to 420 ft3 (283L to 11894L). Two small-size acetylene cylinders are available for portable welding and cutting equipment. These are B size, 40 ft3 (1133L), and MC size, 10 ft3 (283L). The fittings on these two small size cylinders (sometimes called tanks) are called Presto-O-Lite (POL) fittings.
Figure 11-16 illustrates the various sizes of acetylene cylinders. The valves in the larger cylinders shown in Figure 11-16 are known as POL Commercial. Like oxygen cylinders, manufacturers can paint acetylene cylinders any color, since there is no standard color code. Some acetylene cylinders have a recessed top. This recess protects the cylinder valve, which has a female regulator connection or fitting.
All acetylene cylinders are designed and constructed according to Interstate Commerce Commission Specification No. 8. Each design must be tested by the Bureau of Explosives and must pass these tests before it can be used commercially.
As noted, acetylene is dissolved in acetone. Therefore, acetylene cannot be drawn from the cylinder any faster than it can be released from the acetone. This release is a kind of boiling action that occurs efficiently at room temperature. The maximum safe rate for drawing acetylene from a cylinder is one-seventh of the cylinder's capacity per hour. Thus, a single 290 ft3 (8213L) cylinder can supply acetylene at a rate of about 41 ft3 (1161L) per hour. If acetylene is drawn out of a cylinder too rapidly, a considerable amount of acetone may be drawn from the cylinder along with the acetylene.
An oxyacetylene flame consuming some acetone burns with a purple color. Acetone in the flame is not desirable, since it lowers the flame temperature and increases gas consumption. Moreover, the quality of the weld is negatively affected.
Low or freezing temperatures can cause the flow of acetylene to decrease, since heat is needed to boil off the acetylene. Another factor that will cause a slowdown in acetylene flow is a nearly exhausted cylinder. As the cylinder approaches the empty or discharged condition, acetylene is released more slowly. If a greater flow rate is needed, a number of cylinders can be connected to a manifold. This kind of installation makes it possible to use up more of the acetylene from each cylinder. This reduces waste and lowers the cost of welding operations.
The rate at which acetylene can be drawn off depends on:
- The temperature of the cylinder.
- The amount of charge still remaining in the cylinder.
- The number of cylinders providing the flow.