It positions and holds the welding head where the joint needs it, on a boom extending from a powered column. For circumferential welds it holds the head still at the top of a rotating vessel; for longitudinal welds it travels the head along a stationary one. Lift, boom travel and column rotation between them place the arc anywhere in the working envelope.

From the largest work in scope: boom reach must span from the column to the far end of the longest seam, and lift must clear the largest diameter on its rolls, with allowance for the head, flux hopper and any tracking sensor. Internal seams and overlay need the head to pass into the bore, which usually governs the reach figure.

Because deflection moves the arc. A boom at full extension carrying a head, hopper and cable loads deflects under its own weight, and any deflection that changes during travel shifts the bead off the joint centreline. Manufacturers rate capacity against a maximum head load for that reason, and overloading a boom shows up as wandering beads.

Supporting a cylindrical vessel and rotating it under a stationary head, so a circumferential seam is welded continuously in the flat position. The drive roll's speed is the weld's travel speed, so it needs fine, stable control. Self-aligning designs adjust their spacing to diameter automatically as the vessel is loaded.

Turning rolls rotate a cylinder about its own horizontal axis and support it on its circumference. A positioner clamps the work to a faceplate and both rotates and tilts it, so it suits assemblies that are not cylindrical or that need several different joints brought to the flat. Many vessel shops run both.

Yes. The same column and boom will carry gas metal arc, flux-cored or strip cladding heads, and many shops swap heads according to the job. What changes is the service load on the boom and the ancillary equipment carried - flux hopper and recovery for submerged arc, gas and cooling for other processes.

Coordinated control of boom travel or rotation speed with the welding parameters, since travel speed is a weld variable. Modern machines integrate manipulator, rolls and power source under one controller so a procedure can be recalled complete, and support a cross-slide for seam tracking adjustment during the weld.