Hi Guys,

KBC, those are very nice boilers you have built there, I assume they are to your own design.

Not so I am afraid... the pressure on the outside of the firetubes or centre flu is the same as the pressure on the rest of the boiler... there is NO drop in pressure involved.

60psi is 60psi... and would apply to all exposed internal surfaces within the boiler pressure vessel, either INTERNAL PRESSURE (outer shell or water tubes) or EXTERNAL PRESSURE (Firetubes or centre flu).

Superheaters are subject to the same INTERNAL PRESSURE as the main boiler, but are subject to much higher temperatures. Superheaters do not increase pressure... only the steam temperature.

As you already know, Harris, Evans, Farmer, Tubal Caine et al... all provide the necessary formulae for INTERNAL PRESSURE, But... they are a bit short on info when it comes to flu and firetube calculations for EXTERNAL PRESSURE, but they do give lists of typical sizes.

When it comes to designing your own boilers then it is necessary to provide the necessary calculations in order to satisfy the boiler testers that the design is safe. If you cannot provide these then the tester can refuse to test your boiler.

Not necessarily a problem for home use only, but vital if you need to use your boiler in a public place... so: -

The correct formulae for tubes subject to EXTERNAL PRESSURE are: -

For wall thickness: -

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

And for max safe pressure this would be: -

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

Where D = Outside dia.

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

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

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

which is the correct figure for use with INTERNAL PRESSURE calculations.

To obtain the correct figure for the EXTERNAL PRESSURE calculations above; this should be further divided by 3.5... Hence: -

S = 3125 / 3.5 = 892.857psi.

These calculations assume tubes are solid drawn, truly round and straight, with uniform wall thickness and no dents or other damage.

Hope this is of some help to you all.

Best regards.

Sandy

PS... I cannot remember where I eventually found the above formulae, it was a long time ago, but I am sure it was in an early edition of the boiler design codes from either the USA or possibly the Australian boiler design codes... these being the FULL issues, not the latest revised codes.

They have served me well for more years than I care to admit.:

:shrug: