TPT January 2012

T echnology U pdate

Orbital tube-tubesheet welding

AXTON Incorporated, located on Annacis Island in the South Arm of the Fraser River Delta, near Vancouver, British Columbia, Canada, has been manufacturing heat exchangers and other industrial equipment since the 1980s. Its list of projects includes the structural engineering and fabrication of the Vancouver 2010 Cauldron for the Vancouver 2010 Olympic Games. In 2003 Axton purchased a Model 227 orbital welding Power Supply and Model 6 orbital tube-to- tubesheet weld head from Arc Machines Inc, California, USA. Although orbital welding is generally considered to be welding of tube or pipe, orbital welding is defined as “machine or automatic welding in which the arc rotates around a stationary weld joint” and this includes orbital tube-to-tubesheet welding. While Axton had some previous experience with orbital welding, they gladly accepted the opportunity to refresh their skills by having an AMI factory trainer come to their facility to train their lead welders. Axton is an international company that exports a significant amount of its production to countries outside of Canada including Mexico, Africa and others worldwide. Industries served include mining, oil and gas, chemical, acid manufacture, petrochemical, power, transportation and pulp and paper. It is currently fabricating a large stainless steel exchanger for export. The tubesheet on this unit has 990 tubes 1 3 / 4 " (44.45mm) diameter with a wall thickness of 0.084" (2.1mm). The vessel is 50 feet (15.24m) long and 140" (3.55m) in diameter with tubesheets on both ends. The Model 227 weld power supply is microprocessor controlled and stores weld programs, or schedules, consisting of all the programmable weld parameters in the power supply memory. The weld schedule specifies primary and background amperage, travel speed (rotation), pulse times, wire feed speed and other parameters. Thus, if other factors such as tube end preparation, cleanliness and purge flow are constant, there is a very high degree of repeatability from weld to weld. The weld head features a chill follower linked mechanically to the torch that is spring loaded to the ID of the tube. This causes the torch to maintain a constant distance from the tube OD even when there is some degree of ovality in the tubes. The chill follower as

well as the entire torch block is water cooled, which removes heat from the ID of the tube. This allows the application of additional heat to the weld resulting in better penetration without melting through the tubes. The power supply’s arc gap control (AVC) enables the torch to maintain a constant distance between the tungsten electrode and the weld joint. The Model 6 is installed using a separate locating fixture that mounts on the tubesheet. Efficiency is improved by having two fixtures so that the welding operator can position the second fixture while the weld head is completing the weld while mounted on the first fixture. There is virtually no downtime between welds. The Model 6 is a full-function weld head with wire feed capabilities that can weld joints with the tubes flush to the tubesheet surface, joints where the tubes project above the tubesheet surface or where the tubes are recessed. All the tubes are tack- welded in place prior to orbital welding. The weld sequence is initiated by the welding operator either directly from the power supply or from the operator pendant. The welding operator monitors the welding through the weld lens on the Heads-Up-Display (HUD) on the operator pendant. The patented HUD displays weld parameters such as Amps, wire feed and AVC. The operator can adjust selected

Orbital tube-tubesheet welding (above and below)

parameters from the pendant while observing the effect of the change on the weld puddle through the lens of the HUD. After welding, the welds get a post roll and dye penetrant and air pressure tests at low pressure. Tube-to-tubesheet welds are inspected to ASME (Section VIII). Axton has maintained ASME certification for more than twenty-five years and achieved ISO 9001 registration in 1997, and completely qualified to meet ASME, ISO or customer specifications. Arc Machines Inc – USA Website: www.arcmachines.com

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J anuary 2012

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