Fabrication Practices which is Cutting and Bevelling

Fabrication Practices

Now Piping and Fabrication will start again to write about all what we need in Piping System, and now we entering with Fabrication Practices which is Cutting and Beveling. 

The methods of cutting plate or pipe to length can be classed as mechanical or thermal. Mechanical methods involve the use of saws, abrasive discs, lathes, and pipecutting machines or tools. Thermal methods are oxyfuel gas cutting or electric arc cutting. Oxyfuel gas cutting is a process wherein severing of the metal is effected by the chemical reaction of the base metal with oxygen at an elevated temperature. In the cutting torch, a fuel such as acetylene, propane, or natural gas is used to preheat the base metal to cutting temperature. A high-velocity stream of oxygen is then directed at the heated area resulting in an exothermic reaction and severing of the material. Oxyfuel gas cutting is widely used for cutting carbon steels and low alloys. It does, however, lose its effectiveness with increasing alloy content. For higher alloy materials, some form of arc cutting is required. Plasma arc cutting is the process most frequently employed. It involves an extremely high temperature (30,000 to 50,000°K), a constricted arc, and a high-velocity gas. The torch generates an arc which is forced to pass through a small-diameter orifice and concentrate its energy on a small area to melt the metal. At the same time a gas such as argon, hydrogen, or a nitrogen-hydrogen mixture is also introduced at the orifice where it expands and is accelerated through the orifice. The melted metal is removed by the jetlike action of the gas stream.

Because oxyfuel gas and arc cutting involve the application of heat, preheating may be advisable in some cases. Weld end bevels can also be prepared by the mechanical or thermal methods just described. Both mechanical and thermal methods are used to apply the V bevel, which is used in the vast majority of piping applications. For compound and U bevels or those which may involve a counterboring requirement, horizontal boring mills are most appropriate. Various factors to be considered in selecting a weld end bevel are discussed in the section, ‘‘Welding Joint Design.’’