One thing that seems to cause more confusion than anything else amongst model steam enthusiasts is the bar-litre requirement of the rules. Consequently I think it is worth looking at in a bit more detail and examining a few examples to help you get a better idea of where your own boiler might sit as regards this requirement.
Definition
So what does bar-litre mean? Basically it is nothing more than a product, i.e. multiplied together, of two numbers which represent two size related aspects of a boiler. One is its capacity, or the internal volume, and the other is its working pressure. Looking at those two aspects a bit closer, the capacity is the total internal volume available for holding water. The easiest way of determining this is to fill it completely with water then pour it out into a measuring jug. The internal volume must be completely filled, with no air, and right to the top of the shell. This is then effectively the most accurate method of determining the internal volume as it automatically takes into account cross tubes and braces etc. For the sake of the bar-litre number this volume must, not really surprisingly, be quoted in litres. Consequently a boiler that holds 500ml of water has a capacity of 0.5 litres and a boiler that holds 750 ml has a capacity of 0.75 litres. If you want to work in traditional units and only have an imperial measuring jug then a boiler than holds ¾ of a pint would be a 0.43 litre capacity boiler.
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A manufactured boiler should be supplied with a certificate that should state an identification number and the working pressure on it, and this should be produced for the examiner who is checking the boiler. Any home made boiler will also need to have a working pressure, which will be determined from the physical dimensions, with a suitable factor of safety applied and the owners preference if he wants to operate the boiler at a lower pressure. If a boiler has been made from a published plan the plan should also state a maximum working pressure.
Working pressure must be quoted in ‘bar’, which is the SI unit of pressure and the only unit that can be used to determine the bar-litre number. So if now we have a boiler with a quoted working pressure we can determine the bar-litre number. For the ‘traditional’ fans 14.7 psi is the same as 1 bar so you would have to convert a working pressure quoted in psi to bar. For example a working pressure of 45 psi would equate to:
45 / 14.7 = 3.06 bar.
For most purposes using 15 rather than 14.7 is close enough so in the above example you have a 3 bar boiler.
The Product
So now we have two numbers, the capacity in litres and the working pressure in bar, all we do is multiply them together. A boiler of 0.5 litres capacity with a working pressure of 4.0 bar would be:
0.5 x 4.0 = 2.0
or a 2 bar-litre boiler.
Similarly a boiler of capacity 0.75 litres and a working pressure of 5.2 bar would be:
0.75 x 5.2 = 3.9
or a 3.9 bar-litre boiler.
From this you can see that the first boiler is lower than 3 so it is covered by the rules applying to “Boilers under 3 bar-litres”. Whereas the second boiler is above 3 bar-litres and is therefore covered by the rules applying to “Boilers 3 bar-litres to 1100 bar-litres”.
Examples
Let’s now have a look at what that means to us as marine steam modelers and how this relates to some common boiler configurations we can purchase.
2.5 inch boiler
Typically this size boiler has a capacity of around 0.29 litres and a stated working pressure of 4.0 bar:
0.29 x 4.0 = 1.16 bar-litres
This is below three so consequently this boiler is a below 3 bar-litre boiler.
3.5 inch boiler
Typically this size boiler has a capacity of around 0.75 litres and a stated working pressure of 4.0 bar:
0.75 x 4.0 = 3 bar-litres
This is right on the nose of the limit and, as the rules states that boilers are either below 3 bar-litres or they are from 3 bar-litres up to 1100 bar-litres, this boiler comes under the latter category.
6.0 inch boiler
Probably the biggest boiler you are likely to find in a model boat this boiler typically has a capacity of around 2.5 litres and a stated working pressure of 5.5 bar:
2.5 x 5.5 = 13.75 bar-litres
Not surprisingly this is well above the 3 bar-litre limit.
Just to compare with another couple of commonly found boilers
Hemmens Ribbersdale Horizontal Boiler
This boiler has a capacity of 0.5 litres and a working pressure of 5.3 bar:
0.5 x 5.3 = 2.65 bar-litres
This boiler therefore falls below the 3 bar-litre limit.
Finally the 3.0 inch boiler from Miniature Steam that is supplied with the Clyde engine.
The boiler has a capacity of 0.55 litres and a stated working pressure of 4.0 bar:
0.55 x 4.0 = 2.2 bar-litres
Again well below the limit.
