Simple Boiler Construction


The pressure vessel for the steam plant was designed to operated at 30psi and provide the oscillating steam engine project with enough steam for 15 to 20 minutes of run time.

Ultimately the steam plant was to be used in a model boat and so the firebox housing was designed to have the lowest profile possible, whilst still providing enough space for solid fuel firing.


Steam plant
    
To start, the copper tube for the pressure vessel was cut to length and the ends filed as flat as possible. Exact straightness of this edge was not critical because the end caps would go over the ends of the tube.

The tube had a line marked along it's length and then holes were drilled along this line to take the safety valve bush and the steam take-off port.
   
The end plates were made from the same thickness of copper. In fact a piece of the copper pipe was cut, annealed and opened out to give a flat sheet to work from.
Two discs were cut by hand, which were the diameter of the copper tube plus 25mm. They were drilled in the centre.
   
To create the flanged ends a former was turned from aluminium which was of the same diameter as the copper tube. This former had a slight radiused edge (arrowed), to allow an easier forming process on the copper.

A top-cap or thick washer arrangement (shown resting on the left) was also made up to hold the part during forming and the whole assembly was located and secured by a suitable screw in the centre.

Flanged end plate former
    
The forming process was a repeated cycle of annealing, cooling, then forming. These steps were performed 10 times to get the final form of the end plates. A soft brass hammer was used to avoid marking the copper.

Annealing was achieved by heating the part to glowing red and then cooling. (Failure to anneal can lead to fracture of the copper which work hardens as it is formed).

Before removing the finished part from the jig, the jig was placed in the lathe and the edge of the flange tidied up with light cuts from the parting tool.

Finishing the first end plate
   
The centre stay was a simple turning job but a note should be taken that copper is rather "chewy" to turn. A sharp tool helped and light cuts were taken to get the best finish. The stay had a shoulder turned on each end to suit a 2BA thread.
    
Three bushes were turned on the lathe for the safety valve, steam take-off and the level plug. These were shouldered parts created to match the holes drilled in the boiler shell.

The safety valve was a homemade part with a thread of 5/16" x 32 tpi. The steam take-off was tapped 1/4" x 40 tpi to suit a purchased manifold valve. The level plug was smaller with an internal thread of 3/16" x 32tpi.

[If a safety valve is to be purchased, a Mamod part is suitable and therefore the bush should be tapped 1/4" x 26tpi.]

Steam take-off bush

As a general rule 40tpi threads are best for permanently fitted components and copper washers can be used to set component orientation.
32tpi threads are more suitable for components that require regular removal such as filling caps. These components can use fibre washers to seal them.
  
The end caps were assembled with flux paste and held in place with the centre stay. Bushes were mounted similarly and the whole assembly silver soldered.

Details on silver soldering can be found here. However the process is basically to clean all parts, flux and then heat to red hot before adding the silver.

Afterwards the parts were cleaned and all joints inspected for a neat fillet of silver, as shown in the image on the right.

Anything that looked suspect would need to be cleaned, fluxed and silver soldered again.

Steam take-off bush, Note the nice fillet. (Click to enlarge).
   
A level plug was turned from brass with a thread to match the boiler bush; and then cross drilled to take a silver steel handle.

The safety valve was produced to a design by Tubal Cain a drawing for which can be found here.

Safety valve and level plug.
   
After completion of the pressure vessel it was hydraulically tested to twice working pressure by turning some blanking plugs for the bushes.

In this example a boiler test pump was used, but a test can also be conducted using mains water pressure and a suitable gauge. Test pressure was 60psi and this was held for 10 minutes without change to prove the boiler safe and leak free.

Following this test, the safety valve was inserted and adjusted to leak water at 30psi.

Pressure vessel under hydraulic test.
  

  
The firebox was made from 0.5mm steel plate.

The profiles for the ends were cut using tin snips and a file to get final size.

Flanges on each side were formed in the vice with a block of wood and a hammer.



Finished end plates
  
The front plate required a hole for the boiler shell. This was cut on the lathe faceplace using a drill start followed by a boring tool.

The flat sheet was mounted on some wood on the face plate as shown and held with some bolts outside the profile of the finished part.

Front plate mounted for cutting
  
The square hole for the fire box was formed by making two saw cuts, one each side and then bending the centre tab of material back by 90. This tab was then cut off with the tin snips and hammered flat against the back of the plate to form a stiff, safe edge.
Firebox cut and folded edge.
  
The firebox ends were mounted on the pressure vessel to work out the final size of the side panels.
Housing end plates on boiler

Proposed side panel size
  
Both side panels were drilled with ventilation holes using a step drill in the drill press; and then bolted to the end panels using 6BA brass screws and nuts.
  
A copper heat shield was added to finish the top of the boiler housing. The only critical thing being the location of the two bush holes which were taken directly from the boiler part.

The ventilation holes were added to suit and the overall length was slightly longer than the distance between the end plates so it would rest in place more securely.

The heat shield was curved gently to shape and secured to the sides of the firebox with 6BA screws and nuts.

Heat shield prior to fitting
  
A burner tray was made from a piece of steel pipe sliced in half longways, with some flat plate ends.

The parts were silver soldered together including a brass wire to be used as a handle. The handle had a wooden dowel added for comfort.

Completed burner tray with enough capacity for 4 solid tablets.
     
A base for the boiler house and engine was made by screwing a sheet of painted steel on to a piece of plywood.

The base also contained two rails made from aluminium angle which would locate the burner tray under the boiler.

The burner guides were opened out towards the front to help guide the tray into place; and a brass spacer was added at the back to act as a hard stop.


Model base
    
The smoke stack (optional on the model), was made from a piece of 28mm plumbing copper. It was cross drilled so that it could be mounted on the boiler stay at the back of the firebox housing.

A special nut (inset) with a long shoulder was machined to both secure the chimney to the model and hold the pressure vessel in place on the firebox.

As the smoke stack was to be used as a condenser; the bottom was sealed by soldering a plate in place and a small copper tube was added to the side wall above the securing nut. This tube was connected to a silicone hose to make a flexible union back to the engine exhaust.

Smoke stack and securing bolt
     
Final assembly involved the firebox being assembled around the pressure vessel and then adding the heat shield over the top.

The whole assembly was then secured to the base using Meccano angle brackets to save construction time.
     

Finished steam plant.
   
The finished plant is shown above (pending an exhaust connection).
Note that due to a construction error, the steam valve and safety valve would be better swapped over to reduce the length of the steam connection.
  
To complete the exhaust connection the engine had to be modified. Instead of the simple exhaust output hole in the frame a short tube length was needed.

The exhaust hole was plugged using a short brass screw and a new side connection was drilled into the frame to take a soldered union.

The part labeled "Inlet" was the additional engine connection on this model.
The connections were plumbed this way round to get the required flywheel rotation. **


Engine connections and flywheel rotation
    
** Another benefit of this modification was that the engine direction could be reversed simply by swapping over the inlet and exhaust connections.
If the engine has been built to the plans then it should turn anti-clockwise when viewed from the flywheel side and this is suitable for a standard right handed boat propeller.
However because this engine was the development one, the ports were actually the wrong way round and it turned the opposite way. This could have been addressed by using a left handed propeller, but instead the connections were changed to suit as shown in the photo above.
   

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