Model steam boiler

The boiler construction.

( a long overdue update for this blog)

The steam boiler for my Württembergische T3 locomotive is a standard coal fired copper boiler, designed for a steam pressure of 6 Bar (approx. 90 Psi) . All the flanged boiler plates are silver soldered and the stays are of the screwed and caulked type.

This design is used the past decades in the model engineering world with success, and when well-made it will give you an easy to maintain boiler that will last for a long time.

Boilermaking isn’t as difficult as it looks, but it's different than machine work and most tools used are used by hand. In both cases planning ahead and making the individual parts will take up most of the time. 

I started with the boiler parts in 2010. The drawing of the boiler was made in Solidworks and made according to suit the Dutch boiler design rules. (These are derived of British, Belgian and German  model locomotives regulations and the European PED)

The boiler barrel has a diameter of 133 mm, the total boiler is 630 mm long, the firegrate measures  105 mm wide  x 155 mm long. There are 16 fire tubes; 430 mm long and with a diameter of 15 x 13 mm. Two water gauges are fitted and a Stroudley regulator will supply the steam to the cylinders. A Ramsbottom safety valve is set to a working pressure of 6 Bar. Water supply for the boiler will be done by a live steam injector, an axle pump and steam pump.
The total boiler weighs  approx. 19 kilogram and holds 6 liters of water.

The drawing of the complete locomotive (including boiler) can be downloaded as Solidworks 2014 file. Kick here for Mediafire or here for Grabcad download site.

Stress analyse and calculations were done on paper and with a Finite Element Method (FEM) in Solidworks .

A static displacement calculation shows (842 times enlarged) the deformation at boiler pressure of 6 bars.

The fabrication was very simular  to the boiler I've made for my 5" gauge GWR 14xx loco. Only this boiler is a bit bigger. But not that much; a 5" gauge German BR64 has about the same dimensions as this 7¼" gauge loco.

Former plates were made of 25 mm thick aluminium on a professional CNC DMU 50 milling machine. 

The copper plates are 3 mm thick.  

For 7 ¼" gauge this will be a rather small boiler.  

The plates in a temporary fire hearth for annealing. Heating up was done by a large Sievert propane torch. The annealing was repeated several times, to get the final shape. The annealing itself is nothing more than heating the copper until it reaches a bright red color and let it cool down again. This softens the copper, so it won’t ‘crack’ during the forming process. 

The backhead plate on the former during the hammering process.

The inner firebox front plate during heating.

Due to the deformation the copper will get hard again; therefore several heatings are needed.

Especially in the corners (where there is a lot of material displacement) work hardening will occur more quickly and reheating is necessary. 

The firebox plates are getting into shape.

Almost there; one final heating and hammering is needed to complete this plate.

The flanges were made to length, after they were formed. In this case the blanks can be roughly cut to shape, when starting with fabrication of the plates. 

Filing to the final dimension (an overlap of 8 mm of the flanges) 

The inner and outer firebox plates. In this position the outer- and inner-firebox will be located once the boiler is finished. The water space on the sides of the firebox will be 11 mm and in the front and back will be 15 mm.

The throatplate is also made of  3 mm thick copper plate. 

A special formerplate was needed for making the outward flange. With a wooden tool the first bend was made. 

The first stage of the throatplate.

The finished plate after removing the excess of material.

The outer firebox shell (which was annealed first) was bend over the boiler barrel. 

Final shaping of the firebox shell between the bending rolls.

A test fitting of the outer boiler. 

The drilling and boring of the holes for the fire tubes was done on my milling machine. With the aid of the Digital Read Out is easy to locate the holes in the correct position, as determined with the CAD software.

The tubebank during test fitting.

The smokebox tubeplate. Boring 15 mm holes for the fire tubes. The real loco had no super-heater, so it will be omitted in the model as well. 

A rear view through the boiler.

A test set-up of the tube bank. The holes are all a little oversized and not, as stated in most of the available construction notes in the ME press, reamed to a close fit. This would be incorrect for soldering, as the tubes will expand during the process and than there would be no gap for the silver solder to penetrate. To prevent leaks around tubes, a rough surfaced hole and a little over dimension of the hole are ideal, so that solder will always be able to get in the joints. This was done by using a rough round file in the bored holes. 

Set-up for soldering the firebox tube plate.

As can be seen on these pictures, there is a good penetration of silver through the joints.

For bending the inner firebox shell a small bending tool was made. Although very basic, it did the job wonderful.

Checking the gaps

On my 14xx 0-4-2 loco the firehole is conical shaped.

Because I experienced on my 3.5" tank locomotive that your view on the fire is limited, once you are in the driving position behind the locomotive. A conical shaped firehole gives a far better view on the small firebox and it gives also the opportunity to control the fire. For groundlevel driving this is a welcome extra. 

 For this purpose a special conical ring was turned from copper bar. On the outside of the boiler backhead it is only 46 mm in diameter, but on the inside it is almost 62 mm in diameter.

A good sized firehole

Making the holes in the backhead plate for the boiler fittings.

Boring the holes for the safety valve and turret bushes in the outer shell.

Girder roof stays are used on the crown of the firebox to give the strength. 

To obtain a good fit in the boiler, a special former block was made for the production of these girders.

This former block was adapted for usage in the plate folding machine.

The first soldering operation on the inner firebox.

Soldering was mostly done with a Sievert propane burner and a oxy-fuel burner. The summer allowed me to work outside the workshop. With such a large boiler it is almost impossible to solder inside the workshop. The small parts I did solder in the workshop, rose the temperature very quickly up to 45° C.

When the parts were sufficient cooled down after soldering they were pickled in acid. The tank I used, normally functions as our chemical waste container.

After about ten minutes in the acid, the copper comes out of the pickling tank with a nice pink clean surface. This removes the oxides and brazing flux residue from the copper in a fast and efficient way and also prepares the copper for the next stage of soldering.

Aligning the boiler barrel and outer firebox wrapper. 

The photo shows the outer boiler just after it was soldered, with the flux still on it. Where there isn't any flux, the copper will turn black after cooling down. Pickling the boiler in acid will remove this black oxidation layer. 

The crown stays in this boiler are of the so called ‘girder stays’ type  This type of stay is relatively easy to fit and solder in the boiler and is easy to check if the stays did have a good fillet if silver solder all around. The water circulation on top of the firebox should be good with the holes provided.

A test fitting on the frames and between the side tanks and cab.

The foundation ring consists of four separate pieces of copper bar, that are riveted and soldered in position. 

The boiler is almost ready for the last soldering operation. 

A layout for the firebox stays.  It is vitally important that all flat plates in boiler are adequatley supported and strengthenend by means of stays. Firebox side stays are ftted to prevent the inner and outer plates bulging and collapsing due to the stresses induced by the steam pressure.  

The stays are of the screwed and caulked type. At a spacing of 20 mm, M4 holes were drilled and tapped, to take up the 4mm copper threaded stays.  I used this method on my other loco's; my 3½" loco is in service for almost 30 years.

Every stay was individually made and fitted. This takes up several evenings to complete the job of staying the entire boiler.

At a steam-up in Breda one of our club members found out that this method is not only easy during manufacturing, but is also a very good safety measure if the water level should drop to low. He had trouble with his locomotive and the water level fell dangerously low. The soft solder of the top row of the stays melted and water and steam extinguished the fire before any real harm was done. He dismantled the locomotive at home, cleaned the boiler, resoldered the stays and was back in steam the following event. 

Drilling the holes in the throat plate calls for an extended drill. Therefore the 3.4 mm drill was glued with Loctite in a 8 mm rod. Drilling this way went fine, although the long drill needed some guidance by hand. 

Once the boiler was finished is was ready for hydraulic testing. I used a small water pump for that, which was connected via a copper pipe to one of the bushes. All the others were blanked off, except for the dome top bush, to which a pressure gauge was attached. 

The boiler was completely filled with water and pumping began. Within a few strokes of the pump the pressure began to rise. 

 The testing pressure of 12 Bar (1.2 Mpa or 174 PSI) was reached. This pressure was kept for 30 minutes.

 I contacted the boiler inspector and made an appointment for the official test.
On the 9th of November 2012 the boiler was approved by the Inspector. However a boiler certificate will be issued after the steam test.

Starting with the pipework

Most of the pipework on the locomotive will be out of sight, but should be easy to fit and maintain. There are pipes needed to supply water from the tanks to the pumps, injector and from there into the boiler.

The connections between the side tanks were a bit of a puzzle. I had to decide in which way I wanted to assemble the locomotive. Tanks first and boiler fitted afterwards, or first the boiler on the frame and then the side tanks put in position on the running boards?

The connections between the tanks and the mechanical driven axle pump, the pipe work between the left and right tank and pipe work between bypass valves, and the injector and future steam pump were drawn (or sketched)  in Solidworks and studied on the model. 
It was decided that most of the pipe work shoud be fitted, before the boiler was on the frame. I read in the Haynes Manual that this methode was also used on the Tornado (by the way a very nice book to read).

This way I was able to fit (and access) most of the pipework, within the limited space between the frames and under the boiler, without too much trouble. The boiler is lifted in its position from the top and can than be connected to the supply pipes from the pumps and injector.

Home made connectors for the tanks were turned of brass with a M16 x 1 thread. The nut (hex. 19mm) was also made from solid brass. This design is big enough  for a 10 mm hole. The flat end pieces are screwed together and will provide the seal. This will be sufficient to prevent leakage, because there is no water pressure to take in account.

The space between the water tanks, here seen from rear to the front of the locomotive. The four connectors in place on the tanks. Only 1 mm of water will be left on the bottom of the tanks when  empty, this is due to the side ways mounting of the connectors to the tanks.

This is the double connection between the left and right tank (lying up side down in the picture).  Due to the opening in the side tanks (to make space for the weighshaft and mechanical lubricater) there is also a connection between the front and back of the tank.

A test set-up of the connecting pipes. The final soldering was done in position on the locomotive. 

This 22 mm pipe with a brass tee fitting (standard pipe fitting) contains a water filter. This can be easily removed for maintenance from the underside of the locomotive without disturbing the boiler.

Top view between the tanks. The pipes do not obstruct the weighshaft.

The pipe work is made from standard brass fittings and 12 mm copper pipe from the DIY store.

The water filters were made from fine brass gauze, soldered in between a brass ring and end cap. One filter will be for the axle pump, the other will be situated in the bottom of the left hand tank for the injector.

The filter assembly. Only three stainless steel M2 screws hold the filter in place.

The first stage completed: connections between the front and rear of the tank and between the left and right tanks. Filling water in either on the left or right tank shouldn't present any problems with water flow between the tanks. Two 12 mm pipes with a 10mm bore connect them together.  Both tanks combined hold a total of 7 litres of water.
On the real locomotive there were also tanks between the frames, even though the loco had only an action radius of 24 Kilometre.  On the model these tanks are omitted.

Stroudley Regulator

The regulator is the main stop valve of the locomotive. It will control the passage of the steam to the cylinders, so that you can control the speed of the engine at any time.  A standard Stroudley type disk regulator was drawn for this loco. I find them reliable and relatively easy to make and the large dome is very suitable for this type of regulator. The range of rotation between closed and fully open is 60 degrees, which allow a gradual opening and hence good control over the driving speed.  Although this is a German locomotive, the regulator is closed, moving the regulator handle to the right and opened by moving it to the left (like most of the English locos) This was typical for the Württembergische Staatsbahnen.

The main body was milled  from a 20 thick bar. Straight forward milling job with a large end mill.

In this set-up you see the drilling of the small hole, taking up the end of the regulator rod. To get it exactly in line with the hole for the main steam pipe, a short piece of threaded rod was positioned in the vice and set to zero on the digtal read out. The body was screwed on top of it and the hole was made.

The main body with a 4 mm thick bronze disk silver soldered to it. Two holes of 5.5 mm are used for taking of the steam from the boiler when the regulator is fully opened. 

Stainless steel operating links of 3 mm thickness and 8 mm width. M4 stainless steel bolds were made to keep it in place.

It just fits in the dome. 

The main steam pipe is 12 mm round bronze, but has a 7 square hole. This was filed by hand, for taking up a piece of square steel.

This piece of steel can be used as a wrench to screw in the steam pipe in to the regulator body. This was designed this way, so that removing in the future for maintenance should be possible. Alan Beard from Bedford ME told me that he had trouble removing the steam pipe from his 7¼ Marie Estelle 0-4-0 loco after 25 years of service. This was the more often used  copper pipe arrangement (the same with my 3½" and 5" locos), which only has a small slot at the end of the pipe. A large screw driver should do the trick of removing it, but over the years it will be chalked up a bit, and more force will be needed. 

The set up in the boiler. A small piece of wire, bend in a hook, is used to catch regulator rod to position it for the square hole

The bottom part of the linkage; a piece of 1 mm stainless steel wire will secure the bold from working loose in service; which really would present a problem: How to retrieve the small bolds from the boiler.

A cloth is used to close the boiler; so no parts are accidently dropped in the boiler.

The last piece of wire in position. The V-shaped holes in the disk can be clearly seen: this way there is a gradual opening of the steam ports. A nice controlled and slow start of the locomotive should be possible.

The end stops of the regulator are on the boiler back head in the cab.