Steam Launch Boiler

Simple Steam Launch - The Boiler

The boiler was manufactured form a length of 50mm diameter copper tubing with a wall thickness of 1.2mm. As this boiler was designed to be combined with the Beginner's machining project, it was designed to be simple but effective. For this reason it had no water tubes or fire tubes in the pressure vessel, but the housing was designed to get the most from the spirit burner.	Finished Boiler Assembly

Some calculations were made on the proposed copper tube to see if it was suitable to be made into a pressure vessel and to ensure a safe design - these are shown below. The formula used was P = (2T x s) / D Where : P = working pressure T = Thickness of the copper tube (inches) D = Internal diameter of tube (inches) s = Maximum safe stress of copper Therefore : P = (2x0.047" x 2500) / 2" = 118 psi The engine was found to perform best on 20 - 40 psi, any more than this and the cylinder was lifted from the port face. So this copper tube should be more than strong enough for this pressure. Copper tensile strength taken as 25,000 pounds force. ** Chosen factor of safety = 10, so Copper strength reduced to 25,000 / 10 = 2,500 pounds

It was planned to make the boiler end plates from the same thickness of copper, with a central stay through the middle, also made from copper. The stay diameter was calculated as follows. Working pressure 40psi. Copper tensile strength 25,000 pounds force. Area supported by stay = Pi x R² = Pi x 1² = 3.141 square inches. Multiplied by boiler pressure = 3.141 x 40 = 125psi x factor of safety (10) = 1,250 pounds. - So the stay would need to have a breaking stress of 1,250 pounds force. This was divided by the tensile strength of copper to give the area required of the boiler stay to ensure adequate strength = 1,250 / 25,000 = 0.05 square inches. So boiler stay radius would therefore be sqroot of (0.05 / Pi) = 0.126" or 3.2mm radius.

Based on these calculations the pressure vessel design was drawn up. The bushes for the safety valve and level plug were made over-long to allow them to protrude through the boiler housing.		click for larger image

The flanged end plates were made from discs cut from a plate of copper on the lathe and then formed over a metal former as shown. The end plates had a central hole for the boiler stay, this also helped locate the part on the former. In total 10 annealing and forming iterations were needed to get to the final shape.		Forming the boiler endplates.

		A bush for the safety valve and level plug were turned from brass bar. The steam dome was also turned from solid brass. During brazing the central stay was used to hold the two end plates in place with copper nuts.

Boiler under test			The finished boiler was hydraulically tested to twice the working pressure (80psi). After this the safety valve was fitted and set to release at 40psi.

Boiler Fittings	The boiler fittings are shown on the left. The safety valve was made to a design by Tubal Cain.

The boiler housing was made from 0.6mm sheet steel. The 2 ends were made first and then the top and sides were bent round them. The parts were joined with self-tapping screws. One end had an additional hole for level plug and the other had an opening to allow access to the fire. Both sides had holes along the bottom to allow the fire to breath. The top had holes cut for the safety valve and steam dome. Two chimneys were added made from 28mm plumbing copper. The sheet metal parts were the painted in some automotive heat resistant paint.		Boiler housing parts

The steam plant so far.........			The chimneys were covered with an aluminium sleeve to create a single 'ships funnel' shape.
** Note : At this point it was found that the steam dome wasn't tall enough to allow connection of the main steam pipe. This was addressed by plugging the holes in the side and adding a new outlet on the top. However a better fix would have been to make the dome taller (bad design!).

Burner tank components		The burner tank was made from 2 'u-shaped' sheets of brass with an outlet tube and filling nozzle.
Burner tank components		Burner components brazed
Unfortunately the brass sheet parts warped during the heating but the end results was liquid tight, although not cosmetically great.

		The completed tank and burner. To finish the plant, a connecting pipe was made and a stop valve fitted in line to speed control.

# The complete plant was 400mm long (click to enlarge)

The engine running under steam.

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