Acetylene is a colorless, combustible gas with a distinctive garlic type odor. When acetylene is liquefied, compressed, heated, or mixed with air, it becomes highly explosive. Acetylene is used as the fuel component in oxy-acetylene welding and metal cutting.
Acetylene is a hydrocarbon consisting of two carbon atoms and two hydrogen atoms. Its chemical symbol is C 2 H 2 . For commercial purposes, acetylene can be made from several different raw materials depending on the process used.
These processes’ use high temperature to convert the raw materials into a wide variety of gases, including hydrogen, carbon monoxide, carbon dioxide, acetylene, and others. The other gases are the products of combustion with oxygen. In order to separate the acetylene, it is dissolved in a solvent such as water, anhydrous ammonia, chilled methanol, or acetone, or several other solvents depending on the process.
Storage and Handling
Because acetylene is highly explosive, it must be stored and handled with great care.
When acetylene is pressurized and stored for use in oxy-acetylene welding and metal cutting operations, cylinders are used. The cylinders are filled with an absorbent material, like diatomaceous earth, and a small amount of acetone. The acetylene is pumped into the cylinders at a pressure of about 300 psi, where it is dissolved in the acetone. Once dissolved, it loses its explosive capability, making it safe to transport. When the cylinder valve is opened, the pressure drop causes some of the acetylene to vaporize into gas again and flow through the connecting hose to the welding or cutting torch.
Oxygen is used in many industrial, commercial, medical, and scientific applications. It is used in blast furnaces to make steel, and is a component in the production of many synthetic chemicals, including ammonia, alcohols and various plastics. Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting. When oxygen is cooled below -297° F, it becomes a pale blue liquid that is used as a rocket fuel.
The Compressed Gas Association establishes grading standards for both gaseous oxygen and liquid oxygen based on the amount and type of impurities present. Gas grades are called Type I range from A, which is 99.0% pure, to F, which is 99.995% pure. Liquid grades are called Type II and also range from A to F, although the types and amounts of allowable impurities in liquid grades are different than in gas grades. Type I Grade B and Grade C and Type II Grade C are 99.5% pure and are the most commonly produced grades of oxygen. They are used in steel making and in the manufacture of synthetic chemicals.
Periodic sampling and analysis of the final product ensures that the standards of purity are being met.
Oxygen may be used for patients requiring supplemental oxygen via mask. Usually accomplished by a large storage system of liquid oxygen at the hospital which is evaporated into a concentrated oxygen supply, pressures are usually around 345-380 kPa (50-55 psi), UK 4 bar. This arrangement is described as a vacuum insulated evaporator or bulk tank. In small medical centers with a low patient capacity, oxygen is usually supplied by a manifold of multiple high pressure cylinders. In areas where a bulk system or high pressure cylinder manifold is not suitable, oxygen may be supplied by an oxygen concentrator. However, on site production of oxygen is still a relatively new technology.
Nitrous oxide is supplied to various surgical suites for its anesthetic functions during pre-operative procedures. It is delivered to the hospital in high pressure cylinders and supplied through the Medical Gas system. Some bulk systems exist, but are no longer installed due to environmental concerns and overall reduced consumption of Nitrous oxide. System pressures are around 345 kPa (50 psi), 4 bar UK.
Nitrogen is typically used to power pneumatic surgical equipment during various procedures, and is supplied by high pressure cylinders. Pressures range around 1.2 MPa (175 psi) to various locations.
Typically used for insufflation during surgery, and also used in laser surgeries. System pressures are maintained at about 345 kPa (50 psi), UK 4 bar. It is also used for certain respiratory disorders.
Medical gas mixtures
There are many gas mixtures used for clinical and medical applications. They are often used for patient diagnostics such as lung function testing or blood gas analysis. Test gases are also used to calibrate and maintain medical devices used for the delivery of anesthetic gases. In laboratories, culture growth applications include controlled aerobic or anaerobic incubator atmospheres for biological cell culture or tissue growth. Controlled aerobic conditions are created using mixtures rich in oxygen and anaerobic conditions are created using mixtures rich in hydrogen or carbon dioxide. Supply pressure is 4 bar.
Helium is one of the basic chemical elements. In its natural state, helium is a colorless gas known for its low density and low chemical reactivity. It is probably best known as a non-flammable substitute for hydrogen to provide the lift in blimps and balloons. Because it is chemically inert, it is also used as a gas shield in robotic arc welding and as a non-reactive atmosphere for growing silicon and germanium crystals used to make electronic semiconductor devices. Liquid helium is often used to provide the extremely low temperatures required in certain medical and scientific applications, including superconduction research.
Gaseous helium is distributed in forged steel or aluminum alloy cylinders at pressures in the range of 900-6,000 psi.
The Compressed Gas Association establishes grading standards for helium based on the amount and type of impurities present. Commercial helium grades start with grade M, which is 99.995% pure and contains limited quantities of water, methane, oxygen, nitrogen, argon, neon, and hydrogen. Other higher grades include grade N, grade P, and grade G. Grade G is 99.9999% pure.
Carbon dioxide is used by the food, oil industry, chemical and industrial industry for welding applications. Co2 is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor.
Carbon Dioxide (CO2) is the most common of the reactive gases used in MIG welding and the only one that can be used in its pure form without the addition of an inert gas. CO2 is also the least expensive of the common shielding gases, making an attractive choice when material costs are the main priority. Pure CO2 provides very deep weld penetration, which is useful for welding thick material; however, it also produces a less stable arc and more spatter than when it is mixed with other gases. It is also limited to only the short circuit process.
Argon is a noble gas which under normal circumstances doesn’t react with any other element. This is why it’s used as a shield gas during welding. Its use protects the metal that’s being worked on from the oxygen in the air.
Argon is a food preservative and is also used to fill the “dry” type of scuba diving suits.
Though other inert gases can fulfill the functions of argon, argon is especially attractive because it’s inexpensive and plentiful. It makes up nearly one percent of the atmosphere and can be obtained through the production of liquid oxygen and liquid nitrogen.
Because 100% argon can be used to TIG weld all metals and thicknesses you only need one type of gas in your shop to handle all of your welding projects. MIG welding aluminum is different than welding steel. For aluminum, 100% argon is the gas of choice.
Argon-Carbon dioxide (75% argon/25% CO2) is commonly used by hobbyists and in small-scale production. Limited to short circuit and globular transfer welding. Common for short-circuit gas metal arc welding of low carbon steel. 75/25 mixture is used when weld quality and appearance are required and also for reducing post-weld clean up. This mixture also allows the use of a spray transfer process, which can produce higher productivity rates and more visually appealing welds. Argon also produces a narrower penetration profile, which is useful for fillet and butt welds. If you’re welding a non-ferrous metal — aluminum, magnesium or titanium — you’ll need to use 100 percent Argon.