Almost Frazer!!! but not quite!!!
... just teasing
Posted by: fcheslop
« on: Today at 07:28:58
T=2T x D
2t
For tubes subjected to INTERNAL pressure: - thickness T = P x D / 2t
And for maximum safe working pressure P = 2T x t / D
Your formula Element descriptions are correct though.
For tubes subject to EXTERNAL pressure this is a whole different ball game.
Most importantly... the safe stress figure for copper under EXTERNAL (crush) pressure is just a little less than 1/3rd of its INTERNAL (tension) stress value.
It is therefore necessary to adjust (REDUCE) the INTERNAL stress figure by a factor of 3.5.
This gives a revised figure for S of 3125 / 3.5 =
892.85psiThe formulae for EXTERNAL pressure are: -
For thickness = T= (PD / (2S + P)) + 0.005D
For Pressure = P = S[(2T - 0.01D) / (D - (T - 0.005D))]
Where T = Tube thickness.
P = Pressure in psi.
D = OUTSIDE diameter.
S = Safe working stress (892.85psi)
Ok thats got that cleared up I hope.
Robert,
I posted some figures on your other thread but I will repeat them here to save switching back and forth: -
Your original 4" x 1.6mm thick shell would be safe up to 100psi. working pressure.
Your 4" x 2mm thick shell will be safe up to 128psi. working pressure.
Your 38mm OD x 1.6mm thick centre flue will be safe up to 72.736psi. working pressure.
Since you are planning on 30 - 40psi then you will have no issues with tube strength.
I can provide the specific calculations if you need them. (PM if required)
Hydraulic test should be 2 x working pressure and using water at nominal room temperature (62deg F)... there is no need to use HOT water.
For steam testing the boiler should be fired at MAX burner rate and the safety valve MUST lift at the working pressure... once the safety has lifted the pressure should not be permitted to rise above working pressure + 10% with the burner at MAX... if it does rise more than this you will need to change the safety valve to one having a higher throughput (discharge rate)... or use a burner with a lower maximum output.
The cross tubes being horizontal should not be much of an issue for marine use, as you say, the boat will not be horizontal for much of the sailing period.
It is more of an issue with fixed (land based) installations.
Gas Jets.When I tried to purchase one, the gas shop man had no idea of the actual size and after consulting some chart he had deduced it would have a 5mm hole!!!!!!
I guess he don't know much about MODEL boilers and burners.
A number 8 jet is, as you say, 0.25mm jet hole size.
A number 12 jet is 0.3mm jet hole size.
For a 4" boiler you will need to increase the OD of your burner to about 3 1/2" but keep the other dimensions the same.
For 30- 40psi I would start off using a number 8 jet and see how it behaves... if you have difficulty keeping pressure with the engine running then you may be better off using a number 12.
Ok on the mounting arrangements... I was not sure what you had in mind from your posted drawings, since they did not show these features.
I look forward to seeing your build.
Best regards.
Sandy.