calc for swp of fire tubes

[fcheslop]

Help Ive lost my memory in my case its an old leather bound note book
I need to work out the swp for copper fire tubes in a Scotch return flue boiler I know its not the same formulae as the boiler barrel but cannot remember what or where I originally found it. Oh to have a retentative mind
many thanks
frazer

[SandCam]

Hi Frazer,

Here are the calcs you need for tubes subject to external pressure: -

The correct formula for minimum flue tube thickness is: -

T = (PD / (2S+P)) + 0.005D

And for Max allowable pressure this would be: -

P = S [(2T - 0.01D) / (D - (T - 0.005D))]

Where T = Thickness.
   
   P = Working pressure

   D= Outside diameter.

S = Maximum allowable stress value of the design material at design temperature.

Note… Annealed Copper has a typical Tensile Strength of 25000 PSI so divided by 8 (safety Factor) gives 3125 PSI.

For copper @ temperatures below 450deg F it is reasonable/acceptable to use: -

S = Tensile Strength / 3.5

For copper with an 8 times safety factor this would amount to: -

 S = 3125 / 3.5 = 892.857psi.

Hope this helps.

Best Regards.

Sandy.

[fcheslop]

Thanks Sandy 
I have some drawings from George for the boiler and just want to calc the tubes for a different pressure.
I'm planning a fast boat build and hope to get 20 mph out of her.I know its nothing spectacular compared to the flash guys but it will be quick enough for me.
Just not sure yet what boiler to build I still may go for a one along the lines of Mr Raymons. The engine will be a triple with piston valves.
best wishes
frazer