Blast Pipe, the magazine of the Hutt Valley and the Maidstone model engineering societies, reported in its March 2018 issue that Dave Watts recently completed a new set of superheater elements for one of his locos which previously had a set of traditional copper and spearhead return bends for its first set. These had developed leaks around the bend area and steaming dropped off. Dave delved into the design, materials and manufacture for a new set.

Incoloy tubes were TIG welded to 304 stainless return bends and these should have a much longer life. The components of the return bends can be seen in the photo below.

As a word of warning, David Grant-Taylor prepared a note so that no one is tempted to silver solder stainless steel, especially in this application. The Blast Pipe editor explains that ‘Live superheater’ means that the superheater tubes are at boiler pressure as the regulator is in the smokebox and downstream of the elements. “They can be configured with the regulator valve before the elements, which seems to be a generally safer arrangement.”

Failure of brazed joints in live superheaters

By David Grant-Taylor

I recently heard of the failure of a joint in a live superheater on a model locomotive. The loco was being run at a private event and had a stainless header and copper tubes, joined together by a material which we are not able to identify with any real assurance but the joint had failed at the stainless surface. In our club I have several times voiced my disapproval for the use of Easiflo as a brazing medium for stainless steel in general.

The result of the failure, coupled with the live superheater, was quite alarming ejecting the contents of the firebox out of the firehole door and the grate. Fortunately, the driver was not present at the time or his most precious assets would have had a very considerable surprise.

Because of the general disbelief that seems to result from my comments, I have dug through the internet to see someone else warning of the risks associated with silver solder joints on the stainless steels. A Johnson Matthey document covers several silver solders, and two different groups of stainless steel. The conclusion is that if you must do it, use the very expensive brazing alloys containing nickel, gold or palladium. And if you think those are expensive ask about the cost of a repair on your nether regions.

Finally, we need to consider whether you now feel comfortable with a live superheater. The smokebox is a difficult environment anyway, as the boiler water is 170 degC (the saturation temperature for 7 bar absolute), and the fire side will by rather above that, maybe as much as 150 degC higher under hard driving conditions when the fire is burning hard, but perhaps only 50 to 100 degC hotter at rest. The superheater will be yet hotter as in this case at least, it was a radiant  superheater (protruding into the firebox), so all up the temperature of the joint will be as high as 320 degC , and probably more. All this is on a material which was placed at about 630 degC and that is a bit of a thin margin. Most definitely this will eventually become a problem.

Dave Watts’ new superheater elements.
Dave Watts’ loco. -

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