The industry standard for designing fired heaters is API 560
The refining and petrochemical industry largely use this standard to identify best practices for fired heater design, materials, fabrication, inspection, testing and other considerations. A fired heater by definition accordingly is an exchanger that transfers heat from the combustion of fuel to fluids contained in tubular coils within an internally insulated enclosure.  Constituting a wide variety of structures, fired heaters or process heaters contain burners arranged to properly heat fluids transferred in the coils of the radiant and convection sections. Whether arranged with a single pass or multiple passes, the coils will most likely contain some variation of return bend in the header box.
The return bend creates several unique challenges for replacement
The header box is a small internally insulated compartment, separated from the flue-gas stream, which is used to enclose a number of return bends. Return bends are U-shaped 180 degree fittings connecting tubes in the coils. These return bends can be short or long radius and sized appropriately to the design but most often between 4” and 12”. Due to the orientation of the return bends in the header box, there is typically very limited radial clearance between tubes. This creates several issues to consider including layout, sequencing, heat treatment, weldability and overall safety of welders.
AZZ’s machine return bend welding – a new solution to an old problem
Historically, the return bend replacements have been executed by skilled manual welders who usually make the tube to return bend weld in teams or even using a third person as a spotter. The clearances between tubes can be as little as 2”. The area is congested, the sight lines are difficult and due to elevated preheats, the welder is exposed to high heat conditions. In most cases, the failure rates of these welds are significantly higher due to the environment, layout, and welder proficiency.
With a focus on first time quality, AZZ created field machine return bend welding equipment utilizing the HP GTAW™ welding process. This proprietary welding process incorporates duel torch design to eliminate re-wrap and the associated time between weld beads. This feature greatly reduces the loss of preheat between weld passes. The extended reach torch design allows for maximum welding efficiency of the joint while in between adjacent return bends. By utilizing a proprietary system of wire pre-heating, precise dabbing mechanics, and process control the HP GTAW™ process decreases weld defects, improves mechanical properties, and is significantly less susceptible to fit up imperfections in field joints. This provides superior weld quality with predictable productivity to mitigate risk of failure. Lastly, potential for burns and heat exhaustion is greatly reduced because the weld operator is not required to be close proximity during welding. All of these advancements equate to first time quality, predictable schedules and superior safety in the field.
AZZ Specialty Welding
Whether the furnace needs to be repaired emergently or during a planned turnaround, AZZ has unparalleled innovative leadership in the nuclear, refining, power generation and petrochemical industries. AZZ has over 2000 ASME qualified procedures and over 1000 active welder certifications. Find out more about our engineered solutions and welding technologies at azz.com/WSI or follow us on LinkedIn.