Hello
I've been asked to offer up a repair procedure for replacing a 4' section of 20" schedule 40 on a direct fired natural gas burner inside of a 1 million gallon asphalt tank. Burner rating is 2.5 million btu and checked annually for proper set up.
Tube length is 30' with the first 8' a fire brick lined combustion chamber. The problem is at the transition from lined to unlined. The first 2' of unlined area has buckled on top of the tube and fractured letting product into the extended combustion chamber. The tube looks a bit like a turned up banana. 1.5"/30'  Note product temps are held under 400 degrees. The kicker is this is the second time its happened, each tube in service for four years to the week?

The manufacturer ate the tube cost first time, my client paid the labor. On the initial claim the manufacturer responded that this material will not "melt" under normal use. They're adamant opinion was the tank product level was drawn down below the burner tube allowing it to overheat, warp, buckle and fracture. My client insisted it did not happen and reconfigured the tank outlet to maintain 2' of product over the burner tube. Then it just happened again. The outside has a coke like lamination that chips off easily to reveal a like new tube. No pits.

Is this a proper application for the A 53?
The area did not actually "melt" it was just hot enouph to allow the deformation. At what temp might this happen? Could this happen while immersed in asphalt?
Any thoughts will be greatly appreciated.
Randy