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I was thinking of making a bursk-disk steam cannon; the design I'm contemplating is to have a sealed chamber of water, which would be heated until it burst the disk and forced the projectile out of the barrel.

For the heating I was considering passing an electric current through the water using two electrodes. If I used wall current this should result in minimal electrolysis but rapid temperature increase, and would have the advantage of breaking the circuit as soon as the gun fired. Should I position the electrodes near the bottom so that they remain in contact with the water as steam forms?

Can it be expected that the barrel will heat up significantly? And, if so, how do I calculate the change in ID with temperature for a given coefficient of thermal expansion.

Finally, what are the formulae for calculating failure pressure for the chamber and burst pressure of the disk. I assume that chamber pressure can be derived from this graph:

http://en.wikipedia.org/wiki/File:Vapor ... _Water.png

Thanks for taking the time to read and (hopefully) answering my questions!

To heat the water use the heating element from a water heater. They can be puchased as replacements for worn out elements.
Mythbusters tried to build a steam cannon. It was heated by a coal fire. The pressure was released by a butterfly valve. It worked but took a long time to raise to the pressure that they wanted to achive.

12332123 wrote:Can it be expected that the barrel will heat up significantly? And, if so, how do I calculate the change in ID with temperature for a given coefficient of thermal expansion.

The relevance of this is greatly dependent on the type of barrel and projectile you intend to use, I doubt that in most cases the expansion would be suffient to be significant.

12332123 wrote: For the heating I was considering passing an electric current through the water using two electrodes. If I used wall current this should result in minimal electrolysis but rapid temperature increase, and would have the advantage of breaking the circuit as soon as the gun fired. Should I position the electrodes near the bottom so that they remain in contact with the water as steam forms?

I really wouldn't recommend that, it's definitely not a very effective method for heating water. you do realize that since your chamber is dealing with high temperatures and pressure, PVC and most plastic plumbing parts are unsuitable, in fact plain PVC pipe is not suited for hot water plumbing (CPVC is but in general its temperature tolerance is still low also it is more expensive, thinner, has a lower pressure rating and rarely comes in sizes greater than 3/4"), so that just leaves you with metal as a base material. any there are tricks you have to do to insulate wires. but first you got to rethink how you want to heat the water

12332123 wrote: Finally, what are the formulae for calculating failure pressure for the chamber and burst pressure of the disk. I assume that chamber pressure can be derived from this graph:

there is none, only testing can tell the actual fail pressure of a chamber, the quality of the product varies between manufactures, your fitting and pipe should come with pressure rating which is the recommended max pressure, the fail pressure is much greater and depend on the saftey factor and the (pressure rating)x(saftey factor)=(the bursting pressure)


why are you so focused on calculations and numbers? is this for a formal project of some sort?

iknowmy3tables wrote:so that just leaves you with metal as a base material. any there are tricks you have to do to insulate wires. but first you got to rethink how you want to heat the water

Think outside the box for a moment.... And imagine insulating the chamber walls. Easy to do and advantageous for multiple reasons in a case such as this one.

you mean not using the chamber as ground, why? what's the advantage?