I didn't want to hijack the other thread on superheater flues so I started a new one. Why not use 5" (5-1/2" OD), SA-106-B seamless pipe in lieu of SA-178-A tube for superheater flues? Pipe has been used for decades for the main furnace tube in stationary boilers. 5" pipe is a little hard to find, but much easier than 5-1/2" OD tube. ASME Sec. II, Part D shows both SA-178-A and SA-106-B are good for service up to 1000° F and SA-106-B has a higher allowable stress at 700° F (a common design temperature for fireboxes). Need a swaged end, but don't have the equipment to do it? Why not use an SA-234-WPB (wrought steel) concentric, butt-weld reducer? Thoughts?

What is the wall thickness? Isn't there a reason they call it "pipe" instead of a flue"

This is a good thought, but it appears the wall thickness would be an issue.

The minimum acceptable thickness stated in the other thread was 0.165". 5" NPS (nominal pipe size) pipe is listed in schedule 10, schedule 40, and heavier. Schedule 10 is only 0.134" wall thickness. Schedule 40 is a whopping 0.258" wall thickness. That might be too heavy to roll into the flue sheets and it would add a significant amount of weight to the boiler. You might even get into clearance issues with the superheater elements.

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It makes a lot of sense that, with all other variables the same, a thicker-walled Flue would run hotter. The thicker wall creates a longer path for the heat from the inside of the Flue to transfer to water in the boiler and so heat transfer is slowed. The thicker-walled material has a smaller ID, and so the interior surface for heat transfer from the exhaust gas to the Flue to occur is also smaller, and this further slows heat transfer from the interior to the exterior. A few questions:

1. Does using thicker-walled tubes for the Superheater Flues noticeably impact the flow of the hot gas through the Flue? If it does, than the slower flow rate of the hot gas through the Flue may also contribute to the Flue being hotter since there is more time for the thermal energy in the exhaust gas to get to the interior wall of the Flue.
2. If the thick-walled Flue reduces hot exhaust gas flow, does it impact firing?
3. Under most operating conditions found in preservation, is the Superheater Flue more important in heating boiler water or in superheating the steam in the Superheaters?

If the impact of using thicker-walled material on firing is minimal or it can be mitigated in how the locomotive is fired or by modification to the front end and, if the adverse impact on thermal transfer from the interior of the Flue to the boiler water is negligible or of secondary importance to the transfer of thermal energy into the Superheaters, then it may make more sense to use thicker-walled material for the Flue. A thicker-walled Flue would be less efficient at heating boiler water but more efficient at keeping the thermal energy in the Flue and so it may be that a lower firing rate would give the same Flue temperature and the same transfer of thermal energy to the Superheater elements. Whether or not changing the thermal characteristics of the Superheater Flues makes sense would likely depend on the design of the steam and superheated steam-producing components of the locomotive and the most likely service the locomotive is expected to see.

Two things about that I see. If your point is to slow gases to increase superheat, why not just adjust the front end damper? It also overlooks what Kelly said in his last post. It won't do you much good if the maintenance and replacement costs skyrocket from the damage caused by added flexing, or if (worst case) the shredded boiler is laying a few hundred feet down the track and the head end crew is roasted goo on the footplate. No one wants all that, and front end dampers are existing technology. Why try to reinvent the wheel if the problem is just the cost of having new spokes made?

Front end dampers were automatic devices operated by the opening of the throttle such that the superheater ELEMENTS - not flues - would be protected from the heat of the gasses on their exteriors while dry on their interiors. If you tried to use them - or developed or adopted other draft diverting technology - all you'd do in the long run is burn out and cinder cut your tubes while plugging your flues and lowering your superheater temperature and the efficiency of superheating.

Use the right stuff even if it's inconvenient.