Tube-to-tubesheet weld increased by 80% per shift

Tube-to-tubesheet Weld Increased By 80% Per Shift

AXTON, A HEAT EXCHANGER MANUFACTURER IN BRITISH COLUMBIA, PERFORMS ORBITAL TUBE-TO-TUBESHEET WELDING WITH EQUIPMENT FROM ARC MACHINES, INC.
Barbara K. Henon, Ph.D. Arc Machines, Inc.

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Figure 1. Axton Lead Welder (left), and Fabrication Foreman explain welding of the
tube-to-tubesheet to the author while the Model 6 Weld Head continues welding.


Axton Incorporated,
located on Annacis Island near Vancouver British Columbia, Canada, has been man­ufacturing heat exchangers and other industrial equipment since the 1980s. Their list of projects includes the structural engineering and fabrication of the Vancouver 2010 Cauldron for the Vancouver 2010 Olympic Games. Axton is a truly international company that exports a significant amount of their 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.

In 2002-2003 Axton’s owners decided to update some of their operations and, after careful consideration, they purchased a Model 227 orbital welding Power Supply and Model 6 orbital tube-to-tubesheet weld head from Arc Machines, Inc. in Pacoima, 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 the company 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 are currently fabricating a large stainless steel exchanger for export. The tubesheet on this unit has 990 tubes 1-3/4 inch (44.45 mm) diameter with a wall thickness of 0.084 inch (2.1 mm). The vessel is 50 feet (15.24 m) long and 140 inches (3.55 m) in diameter with tubesheets on both ends.

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Figure 2. Orbital welding system including Model 227 Power Supply and Water Cooling Unit (left.). Weldling operator is installing Model 6 Weld Head on a tubesheet..


Orbital welding equipment
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 (Figure 2.). The operator enters the pro­gram each time the power supply is turned on. 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, purge flow, etc., 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 welding operator may make small changes in the AVC setting during the day to adjust for changes in ambient temperature or humidity.

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Figure 3. Mounting the Model 6 onto fixture. The fixture is universal so it can fit different patterns of tube spacing.

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. When the weld is complete, the operator moves the head from one fixture to the other so there is virtually no downtime between welds (Figure 3).

Weld Joint Configuration
The Model 6, which is a full-function weld head with wire feed capabilities, 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 below the surface. All the tubes are tack welded in place prior to welding (Figure 4).


Figure 4. Finished weld on tubesheet (top center). Tubes are projected and pretacked
in place prior to orbital welding.


If the tubes are projected, Axton prefers to make three fill passes. They do a fusion pass and then two fill passes. If the fill is insufficient to meet the required “t” they may do a third, but this is left up to the welder.

Welding
The weld sequence is initiated by the welding operator either directly from the power supply or from the operator pendant. During welding the welding operator monitors the welding through the weld lens on the Heads-Up-Dis­play (HUD) (Figure 5.) which is mounted on the operator pendant. The pat­ented HUD displays weld parameters such as Amps, wire feed and AVC. The operator can adjust selected parameters by using the buttons on the pendant to increase or decrease a parameter while observing the effect of the change on the weld puddle through the lens of the HUD.


Figure 5. Welding operator observes welding of tubes through M227 Heads-Up-Display (HUD). The Model 227 power supply controls the weld parameters, but minor changes to amperage, arc gap (AVC), or wire feed can be made from the operator pendant.


QA/QC

After welding has been completed the welds will get a post roll and then be subjected to a dye penetrant and an air pressure test 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 so are completely qualified to meet ASME, ISO or customer specifications.

Conclusion
Using automated orbital welding for this application Axton can produce up to 80% more welds in a 10 hour shift than by manual welding. The orbital welders experience less fatigue in their shift than they would if welding manually, thereby improving their health, productivity and accuracy.

Acknowledgements
Photos by Richard Shilling Contract Employee Arc Machines, Inc.