Hi all!
So my 3″ Fowler is nearing the point where I have the start looking at building the boiler. For the record I have no intention what so ever of having someone else build it, thus it has to be in copper, however, the drawings i have from Bridport Foundry by H.R Plastow give a working pressure of 110psi. I have been told on good authority by members of my local club that this isn’t really enough for a compound engine and that 150psi would be a much better working pressure. I have asked Bridport Foundry and they don’t have drawings for an uprated version of the 3″ boiler.
I have done all the calculations as follows.
The formula given by K N Harris in his book “Model Boilers and Boilermaking” (recommended by Edgar Westbury) p.31 is as follows:

T = P x D / 2t or P = 2T x t / D

Where;

T = thickness of shell in inches which will be 0.187″

P = working pressure

D = internal diameter of shell – 6.375″

t = maximum safe stress of shell material in lbs / in2 (he gives 3125 lb/in2 allowing for a safety factor of 8)

Therefore:

(2×0.187)×3,125)÷6.375 = 183psi

Stays should not exceed a pitch = to

tx800
_____
W.P.

t = thicknes of inner firebox wrapper

(0.187×800)÷150 = 0.997″

Firebox stays should be twice the thicknes of the inner firebox wrapper.

The boiler tester at my club agrees with these calculations but the insurance company Walter Midgly don’t seem to want to help me unless the boiler drawings are signed off, for 150psi by a “qualified boiler engineer”. Does this mean I have to re draw the boiler with the new specifications and pay one of the boiler makers round the country to sign them off??
If anybody has any help it would be greatly appreciated.