MECHANISATION AUTOMATION - Esab

scope of a single welding tool relatively broad.The clamp-on tool is locked onto the pipe in the weldingposition by a single movement of the hand, using the“self-locking pliers” principle. PRC welding tools canalso be provided with the AVC function (AutomaticVoltage Control of the arc length) and with a weavingaction mechanism - both needed in the multi-run weldingof thick-walled pipes.Clamp-on pipe welding tools can be either open (opentools) or enclosed (closed tools). Enclosed heads coverthe entire weld area within a space filled with shieldinggas, figure 3. This is to prevent the hot weld zone fromoxydation. The weld surface is protected in the same wayas the root of a stainless pipe weld is protected by backinggas. Enclosed tools are used in particular for weldsrequiring extreme purity, such as pipes used in thepharmaceutical industry or titanium pipes.For pipe diameters above 170 mm, orbital welding canFigure 2. Three PRC welding tools for pipe diameters Ø 17-170mm. This onward development of the PRB head features wea vingand arc voltage control, providing higher productivity and betterweld quality, especially with thick-walled tubes.Figure 3. Three enclosed PRH welding tools for pipe diametersØ 3-76 mm. The rotating part and the tungsten electrodeare enclosed by a gas chamber formed by the outer casing.This water-cooled head is designed for welding thin-walledstainless steel and titanium tubes.be achieved by using a welding carriage that travels thecircumference of the pipe along a track, figure 4. ESAB’sPRD 100 carriage is particularly low (75 mm), whichmeans that it will fit into confined spaces. Welding headsfor narrow-gap welding of thick-walled pipes are alsoavailable (to be discussed later).Mechanisation requires accuracyMechanised welding requires a greater accuracy ofgroove preparation and fit-up than manual welding -where the welder can use his skills to “patch-up” anyshortcomings. In orbital welding, the machine only doeswhat it has been programmed to do, without compensatingfor joint irregularities. The time spent on machining ajoint preparation with adequate precision, therefore, oftenFigure 4. PRD 100 carriage.Figure 5. Pipe groove cutter MF3i for pipe diametersØ 25-152 mm.exceeds the actual welding time. Efficient machiningtools are available from ESAB, figure 5.Also narrow gap weldingNarrow-gap welding with orbital TIG welders and specialwelding heads is a process adopted over recent years,figure 6. By narrowing the cross section of the joint, the4 • Svetsaren no. 1 • 2005