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Bending superheater tubes?
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Author:  Ed Walters [ Sun Jun 17, 2012 3:36 pm ]
Post subject:  Bending superheater tubes?
Hi folks,

I was wondering if anyone had any advice to offer on bending superheater elements?

While the boiler in question is not directly in the realm of this forum (it's for my Dad's steam wagon in the UK), presumably the method would be similar! It's a vertical water tube boiler and the elements are expanded into a header that sits in the top of the boiler with the tubes hanging down.

The tube in question (specified as 11/16"OD x 0.095" wall thickness 'solid drawn steel tube') needs to be bent into a series of tight bends. I've attached part of the drawing below.

One concern we've got is the thinning of the tubes at the outside of the bend, as this is also where they are exposed to the fiercest flame. We were wondering if heating the side of the tube that will become the inside surface may help that? Or whether we just have to bend away and hope for the best?

We were also wondering if anyone had any thoughts on grades/specs of steel to use? The previous set were ASTM A106 Gr.B.

Thanks!

Ed Walters

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Author:  Dave [ Sun Jun 17, 2012 6:36 pm ]
Post subject:  Re: Bending superheater tubes?
Check your PMs, Ed. A friend over there has a solution for you.

dave
Author:  dinwitty [ Mon Jun 18, 2012 2:30 pm ]
Post subject:  Re: Bending superheater tubes?
if I recall right like 765 the tube ends were not bent, they were cast turns, welded on.
Your right bending them like such may weaken the metal structure you don't want that in high heat, high pressure especially close to the fire full of toxic elements ready to eat thru any metal.
Note that firebox temperatures can get high enough to melt metal, its the water cooling stopping that.
Author:  jasonsobczynski [ Mon Jun 18, 2012 2:40 pm ]
Post subject:  Re: Bending superheater tubes?
There is a tube bending process in this country called either "hydro" or "hydra" bending. One of their claims to fame is that of being able to bend tubing with minimal loss of wall thickness. I have seen samples which were bent using this process to a radius fire type E superheater units. The wall thickness was only reduced by about 15-20%...

The units you need to have made, having a much larger radius, would likely have a lesser reduction in wall.

Cheers, Jason
Author:  David Johnston [ Mon Jun 18, 2012 4:56 pm ]
Post subject:  Re: Bending superheater tubes?
With the right dies and the right material, I think you could bend something that looked like your drawing with a Hossfeld bender. But if may not give you the product you need for steam pressure service. I use to do business with a company associated with Philadelphia Pipe Bending Company, http://www.philapipebend.com . This is the most competent tube bending company I have ever been associated with.

As for the temperature rating of the material, there are some very good high temperature steel alloys used in the furnaces in the oil refining industry. The problem would be getting something as small and thin as you want. I would look to the trade associations and manufactres to provide recommendations.
Author:  Dave [ Mon Jun 18, 2012 5:52 pm ]
Post subject:  Re: Bending superheater tubes?
dinwitty wrote:
if I recall right like 765 the tube ends were not bent, they were cast turns, welded on.


756 has a HFT boiler. This is for a VWT boiler. Different style, different application......not relevant data.

dave
Author:  Bobk [ Mon Jun 18, 2012 6:06 pm ]
Post subject:  Re: Bending superheater tubes?
180 degree return bends are readily available in about any radius you need. They are end prepped with the correct 37.5 degree bevel for butt welding. Material is also correct. You may search "weldolet"
Author:  Ed Walters [ Mon Jun 18, 2012 6:37 pm ]
Post subject:  Re: Bending superheater tubes?
Thanks all for the suggestions! They are much appreciated! I've attached an image below showing the boiler and superheater. The superheater is on the floor at the bottom right, and you can see the way the tubes fit (they all run parallel to each other, with the tubes getting shorter and shorter).

When mounted, the superheater fits in the top of the boiler. Incidentally, if anyone's wondering how the firebox sides are stayed - they aren't. They are a corrugated pressing, so they are self supporting.

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