Understanding Oxy-Fuel Cutting and Pressure Regulation

Posted on Oct 7, 2024 in Chemistry

Pressure Regulators

Pressure reducers are essential for reducing gas cylinder pressure to the required working pressure in cutting operations. While oxygen consumption rate isn’t critical for successful underwater oxy-arc cutting, optimal flow rates enhance efficiency and reduce diver fatigue. Insufficient oxygen flow slows the operation, while excessive flow wastes oxygen, cools the cut, and strains the diver due to back pressure. A two-stage regulator, capable of providing 2 m3/min (70 ft3/min), is recommended. Electrode tip pressure (Table 4-2) is determined by adding 1 kg/cm2 (14.2 PSI) per 50 meters of hose and 1 kg/cm2 (14.2 PSI) per 10 feet of depth. The formula for regulator pressure is: Regulator Pressure (kg/cm2) = (Depth/10) + (Hose Length in meters/50) + Working Electrode Pressure.

General Oxy-Fuel Cutting

Flame cutting of ferrous metals relies on oxygen’s high affinity for iron at high temperatures, represented by the equation: 3Fe + 2O2 = Fe3O4 + 270 cal. This process produces black iron oxide, and the iron is said to be ‘burned’ due to its chemical reaction with oxygen at the ignition temperature. Oxy-fuel cutting is a localized combustion process using an oxygen stream, resulting in curved grooves on the cut faces.

Oxide Formation

Depending on the operating temperature, slag can be solid, liquid, or gaseous. Solid slag forms a barrier on the cutting surface, preventing contact between the metal and oxygen, thus stopping the reaction (e.g., aluminum forms Al2O3 with a high melting point). Gaseous oxides create an oxygen-poor atmosphere, hindering the reaction. Liquid oxides, however, catalyze the reaction by dissolving oxygen and metal oxide, intensifying the process. Gaseous oxides hinder the process, while solid oxides shield, and liquid oxides catalyze.

Metal Composition

Steel characteristics are influenced by additives (carbon, silicon, nickel, chromium, tungsten, cobalt, copper) and impurities (phosphorus, sulfur). These affect flame cutting through physical properties (specific heat, thermal conductivity) and chemical properties influencing oxidation. Here’s a summary of the influence of each element:

  • Carbon: Dissolved carbon aids oxygen cutting by reducing the ignition temperature. Free carbon (graphite) hinders cutting due to endothermic reactions and the formation of oxygen-depleting gases. Soft and medium-hard steels are easily cut, while high-carbon steels require more heat.
  • Silicon: High silicon content hinders cutting.
  • Chromium: Small percentages are acceptable, but high amounts hinder cutting.
  • Nickel: Similar to chromium, small amounts are acceptable.
  • Molybdenum: Cuts well in combination with chromium and nickel.
  • Copper, Cobalt, and Vanadium: Steels with these additives cut easily.
  • Sulfur and Phosphorus: Don’t influence cutting but produce toxic smoke.
Pure Oxygen

. Oxygen is obtained by distillation of liquid air, so the impurities are the rest of the gases that make up the air. The influence of impurities on oxygen does not affect the chemical nature, but in the decreased amount of oxygen involved in the reaction. The main interference comes from water vapor, which must not exceed 0.002% at 63 ยบ C. The purity of industrial oxygen is usually not less than 97.5%. Oxygen with a purity below 80% can not make the cut. You can cut 25 mm plate. with 85% being the third of the speed obtained with 99.5%.