BOILERS.

Boilers are one of the most important item in any steam system, and seem to be the one that causes most concern to the builder.

It is extremely rare for a model boiler to explode, and indeed, I have not been able to find an authenticated example over the last twenty or so years, but it is a potential risk and so it behoves everyone involved to ensure that they are designed properly, constructed with care, and operated and maintained with reasonable "common sense".

Materials.
Boilers can be made from copper or steel. Copper is usual in the smaller sizes and steel for larger ones.
(Copper used to be used for full size loco's.)
The demarcation is not rigid, and some designs give the option of materials for the same size.
(But note that many design features differ so one cannot use a design intended for steel and make it of copper.)
The boiler tubes are usually of copper but can be steel in steel boilers.

Soft solder should never be used on a model boiler as it will prevent any subsequent brazing that may be required.

It is possible to use other materials, indeed, some early boilers were made from stone, but the only valid reason for deviating from the "tried and tested" is as an experiment, and there needs to be good justification for that.

Many people suggest stainless steel for model boilers. Although it can be used, there are many reasons why it is NOT the best material. Those of you who  are knowledgeable on metallurgy and welding will know the pitfalls and what should be done, to everyone else, my advice is to stick to conventional materials.

There are some excellent books on building model boilers, see the MORE INFORMATION page.

TESTING BOILERS.
The boiler should be carefully inspected to ensure that joints etc are sound, and that all openings are sealed. A large Certified pressure gauge (not the engines own gauge) should be coupled to the boiler.

The boiler must be COMPLETELY filled with water, and then slowly pumped up to twice the working pressure. If the boiler appears satisfactory and doesn't show any leaks or distortion, the pressure should be released, and then the test performed again, with the pressure held for 20 minutes..
It should then be tested under steam, which test should include the water gauge and safety valve (s).

Safety valves must be of sufficient size to maintain the pressure at less than 10% above working pressure under all  circumstances.
(The usual test is to get a good fire going, open the blower, and try to "overcome" the safety valves.) The pressure gauge fitted to the engine should then be marked to show the working pressure. This to be based on the pressure shown on the test gauge.

Any boiler that is built to other than a standard published design will need to show the design specification and calculations to the inspecting authority.
All boilers should be clearly and permanently marked to identify them.

Steel boilers.
Steel boilers should only be constructed from tested materials and the Test Certificates made available to the testing authority. The welding must be suitably qualified welder.

NOTE:
1. Local regulations may require a different procedure.
2. "Large" boilers (the definition of which varies in different localities), are subject to different, more stringent rules.
Your insurers, or local club, will advise what they require.
3. US rules on steel boilers that are to be to ASME regulations differ from those in other parts of the world. Check first!
4. Australian regulations are particularly strict.

There are many formulas for calculating the thickness of material for the boiler shell, or the working
pressure for a given thickness of material.
The following are useful guides, but the latest regulations (for example, the ASME codes, if you are in America) should also be consulted.

Two:-

P = (t-2) SJ
CD
Where:
C = 2.75 for double butt straps ; = 2.9 for double riveted lap joints ; 3.3 for single riveted lap joints.
D = inside diameter in inches
J = percentage strength of longitudinal seam (see below)
P = working pressure
S = tensile strength of material (in tons per square inch)
t = thickness in 32nds of an inch
T = thickness in inches

And, Or,
J = 100(p-d) J = 100 ANC
P Spt
Where:
a= cross sectional area of one rivet
C = 1 for single rivets; 1.8 for double rivet
d= diameter of rivet holes
n= number of rivets
p = pitch of rivets
S= tensile strength of plate
X= shear strength of rivets (usually taken to be 23 tons / sq. in

(Mainly applicable to full size boilers,

One:-

P = 2 x T x R x S
8 x D
Where:
D = diameter of boiler in inches
P = working pressure
R = 7.5 for riveted joint; 10 for welded and lapped joint
S = tensile strength of material in lbs. / sq. in.
T = thickness of plate in inches

More applicable to model boilers are the following;

4). For steel boilers: For silver soldered copper boilers:
P = Su x T x Q x 2 P = Su x T x A x 2
D x F D x F


(Note that these apply to silver soldered or welded boilers,
riveted construction will necessitate an additional allowance).

Where:
A = temperature allowance for copper boilers = 0.8 for pressure up to 100 psi,
0.7 for 100-150 psi, and 0.65 for pressure over 150 psi.
D = inside diameter in inches
F = factor of safety, to be between 6 and 10.
P = working pressure
Q = corrosion allowance for steel boilers = 0.5 for plate under ¼ ",
0.75 for plate over ¼" thick.
Su = tensile strength of material in lbs. per square inch,
usual figures are steel = 60,000; copper = 25,000
T = thickness in inches

4a). To find plate thickness from the previous formula,

T = D x P x F
Su x 1.6 x (either A or Q as appropriate)

5.)
also for model boilers,

P = T x A x B
D

Where,
A = 12,000 for steel or 4,000 for copper

B = 1 for seamless tube, 0.8 for brazed or
welded seam

P = working pressure
D = boiler diameter
T = thickness in inches