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3" Scale Fowler DCC boiler upgrade

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John Lintorn28/06/2017 15:55:48
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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

(0.187x800)÷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.
KWIL28/06/2017 16:50:04
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Why would a boiler maker sign off your drawings? They have neither designed nor offered to make and supply one to that drawing.

You really need a Qualified Boiler Inspector (ie via an insurance company) if that is the route you wish to take.

John Lintorn28/06/2017 17:04:22
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66 forum posts
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Surely they would do it because I would pay for their services and help?
The insurance company said I have to find a Qualified boiler engineer to sign off the drawings though?
JasonB28/06/2017 17:46:21
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I have a Feeling Paul at Southern boiler works will (for a fee) run your design through his spreadsheet, produce working drawins and get them insurance approved.

Not sure if he will doi a copper one over 120psi though, I know someone who had him redesign the boiler for a BB1 which is a compound and that now has a WP of 120psi. Probably need to go to steel for higher.

Are you rolling the barrel? probably need to factor the joint into the calcs if you do and depending on what you go for 0.7 to 0.85 would be the factor to use.

Just out of interest there is a current thread on Traction Talk regarding a new design boiler, admittedly steel and the inspections and approval were done by BES at a cost of £1200

John Lintorn28/06/2017 20:31:40
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66 forum posts
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Cheers Jason!
I've sent him an email to see if he wants to help me. I was always willing to pay for help though so I'll see what he says.
I will be rolling the shell yes. Although in my calculations is already a safety factor of 8.
I would consider a steel boiler if only we were allowed to weld it ourselves.

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