Model Engineering / Locomotives / Loco Construction / Briggs Boiler


This is how I built the steel Briggs stayless boiler for my 0-4-0 narrow gauge loco Perseverence. The boiler was built to the Australian Miniature Boiler Safety Committee (AMBSC) Code part 2 - Steel Boilers, as it was at the time. There is a revised Part 2 now that should be consulted for new constructions. NOTE: This story should not be used as permission to build one exactly the same without first seeking approval of your AMBSC Boiler Inspector.

The main aim of this photo-story is to provide a guide for anyone thinking about building a steel boiler of the Briggs type but not sure how to go about it, or what they look like. I've noticed a fair bit of predjudice against them, but they work! They are easy and relatively cheap to make compared to a copper boiler. The only drawback is having to have a certified pressure vessel welder to weld the plates together for you and having the boiler inspected every two years instead of three as for the copper variety.

See below for feedwater treatment guidelines and AMBSC contact info

Basic specifications

Shell: 6" nominal bore; 18" long; 1/4" wall thickness.
Front tube plate: 1/2 thick.
Rear tube plate: 3/8" thick.
Crown sheet: 3/8" thick.
Girder stays: 1/2" thick.
Tubes: 17; 1/2" diameter copper. No superheating.

The boiler assembly story board...

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1. The boiler profile showing the firebox cut out and the threaded BSP sockets welded in place. The two lower sockets are for blowdown valves.

2. The larger 2" BSP socket is the steam dome. Then the two 1/4" BSP sockets for the safety valves. Then the 3/8" BSP socket for the manifold over the firebox.

3. An upside-down view of the firebox cut-out and the interior welding of the sockets at the firebox end.

4. A view from the front showing the front blowdown socket. A blowdown in this area reduces the sludge build-up against the front tubeplate.

5. The girder stays attached to the crown sheet.

6. Another view of the girder stays.

7. The firebox side view of the back tubeplate welded to the crown sheet.
HINT: only drill 1/8" dia. pilot holes (through both plates clamped together) for the tubes to prevent the holes distorting during welding.

8. A waterside view of the back tubeplate welding.

9. Note the 45 degree weld preparation on the tube plate and crown sheet.

10.The crown sheet and back tubeplate assembly in trial position before welding.

11.The crown sheet and back tubeplate assembly in position after welding.

12. Looking into the boiler over the crown sheet interior weld.

13. Looking into the boiler from the firebox end.

14. Looking into the boiler from the front.

15. The front tube plate welded in place.
HINT: line up the tube holes by passing a 1/8" dia. brass rod through the pilot holes. Use winding sticks on the rods to set the parallel alignment.

16. After all the welding is complete, drill out the tube holes undersize for reaming.

17. Reaming out the tube holes with a 1/2" machine reamer.

18. The tube holes in the firebox end drilled and reamed. This method lets you drill right up to the weld and give a tube hole with no distortion.

19. Drilling out the front tubeplate prior to reaming as per the back tube plate.

After both ends are drilled and reamed, remove all sharp edges around the holes using a scraper. Don't forget to shake out any swarf inside the boiler.

20. The tubes have been fitted and expanded (see note below). The front end of the tubes are flared. The large hole near the top of the front tube plate is for the regulator and steam header fitting.
The threaded holes on both sides of the shell between the steam dome and the front tube plate are for the water inlet check valves.

21. The firebox end of the tubes are gently beaded over against the tubeplate (see note below). The center hole in the crown sheet is for the fusible plug. The four holes near the corners of the crown sheet are the connections for the water wall unions.

22. I used water walls instead of the more usual coil system. Each tube is bronze brazed together and a 16g copper web is silver soldered in between each tube.

23. The firebox is made from 1/4" steel plate and is screwed in to the boiler 1/8" thick steel strip welded to the shell exterior.
The outer shell was painted with rust resistant aluminium paint.

24. The firebox end assembled. This arrangement allows removal of the water wall tubes without taking the boiler out of the locomotive frame!
Note: The current steel code may require an inlet connection of slightly larger tube than shown.

25. The blowdown valve is shown here as well as the fire hole.

26. Now it looks like a boiler! The backhead shows the hole and studs for the regulator valve stem and the two inspection plugs.

27. The front view showing the inspection plug below the nest of tubes.

Expanding the tubes into the tubeplates

I tried using a commercial tube expander, but its bulky head prevented its use around the edges, especially the tubes near the crown sheet. I made up a tapered drift from bright mild steel about 10" long. 6" is tapered and 4" parallel for a handle. The smallest diameter of the taper is about 20 thou smaller than the tube ID. The major diameter is about 1 thou per inch diameter bigger than the hole-diameter-less-twice-the-wall-thickness-of-the-tube. I made the major diameter parallel for a length about 1-1/2 times the thickness of the front tube plate. Make sure you allow enough metal to get to the major diameter while achieving a highly polished finish. The dimension of the minor diameter is not critical. The transition point where the taper meets parallel should be radiused.

Important: Anneal both ends of each tube before you start and make sure that excessive force is NEVER used to expand the tubes into the tubeplates!

An improvement on the original design of the drift would be a length of steel rod (about 5/16" dia. up to 3/4"dia tubes, and correspondingly larger for the larger dia. tubes) screwed in to the minor dia. end of the drift. The rod would be long enough to extend past the end of the tubes before the tubes have been expanded. You then make a stepped collar with a hole to fit loosley on the drift rod and mount the collar on the opposite end of the tube you are working on. As you operate the drift, it is kept parallel to the tube by the rod passing through the guide collar at the other end.
Keeping the drift rod diameter fairly large will allow you to use it to tap the drift back out of the hole after expanding the tube. Make sure the exposed end of the rod is chamfered to reduce the mushrooming effect of prolonged tapping during the extraction process.

Warning: Make sure the tube doesn't slip along while expanding! It's hard to fix if it locks in the tubeplate after it moves out of position.

Front tubeplate flaring
Once the tubes are secure in the tubeplates you can flare the front edges. I simply tapped the back end of a lathe dead centre with a wooden mallett to gently tap the tube end to a 60 deg flare. Stop when you see that the tube end is in the correct shape and you feel just the slightest increase in resistance to the flaring process. If your tube diameter is larger than your lathe centre you will need to make a larger "centre" to suit. Don't forget to polish the contact surface to prevent damage to the tubes and make it easy to slide in during the process.

Warning: If you bash the centre too hard you can easily shear off the end of the tube flush with the tubeplate and that would be disasterous! You'll have to replace the damaged tube!

Back tubeplate beading
This part is a little harder to accomplish than the front end. However, I used a similar technique as used when securing eyelets. I made up two more tools. Start with a piece of steel about a millimeter larger diameter than the flattened end of the tube will finish at.
Tool No.1 has a stub end about 15mm long that fits neatly inside the tube as a guide. The face of the shoulder is radiused like an eyelet tool. The purpose of this tool is to GENTLY roll the end of the tube over to 90 degrees and up against the back tubeplate.
Tool No.2 is similar but without the radius on the shoulder face. Its job is to GENTLY bead the end of the tube (created by tool 1) against the tubeplate. Use a wooden mallett on the ends of both tools and keep the tapping light. The aim is to form a seal and it takes very little force on the tubes to achieve that aim.

The tubes sealed well and have never needed "touching up". The tapered drift, flaring and beading tools are very easy and cheap to make.

Taking care of a steel boiler

There are several theories about how to care for a steel boiler. I guess they all work to varying degrees, but my method of steel boiler care is a bit different to all of the other methods! You can make up your own mind.

The major difference of my method is that I leave the water in the boiler between runs. I usually blow it down "half a glass" a couple of times during the session, but never empty it until inspection time. Before you reach for the back button, I'll tell you that after ten years of operation, the boiler interior in still in excellent condition. There are no pitting or corrosion marks at all due to the protective tannin lining on all the surfaces.

The catch... When you leave the water in the boiler, it is important to make sure that it is totally full of water with no air spaces inside (see After the run for more details).

If you decide to empty your boiler after each run, you must open some plugs etc to allow air to circulate inside the shell to prevent condensation forming. You could also rig up a 25W light bulb in the firebox to keep the shell warm.

During the run

During the operation of the boiler I use a chemical produced by Maxwell Chemicals called Nu Wood Boiler Disincrustant as a dose in the water tanks. Nu Wood is a concentrate that is mixed 5ml per litre of water. I use a 10 litre plastic jerry can to dilute the concentrate and take with me to running days or visits to other clubs. Over ten years of use, I have barely used 1/2 a litre of the 5 litres of concentrate I originally purchased. The 10 litre jerry can of water with 50ml of Nu Wood can last about a year, because I only use about 1/2 a litre of diluted chemical per session.

Nu Wood has a tannin base so it looks like black tea in concentrate form, mild tea in mixed form, and very weak tea by the time it is in the boiler. A dose in the tender or water tank at the start of the session is usually enough to keep you going for the day. If the water in the boiler gets to dark, just skip a dose to weaken the mix.

After the run

I simply drop the fire and leave the loco as it is! I have an axle pump on my loco, and one day I accidentally discovered that if I leave the by-pass valve closed after a run, the vacuum created in the boiler as it cools, draws the water from the tank into the boiler to fill up void (the vacuum also helps remove oxygen from the water). If you don't have an axle pump, you'll have to manually pump the water in to fill it up. You will also need some way to allow air to escape from the highest point on your boiler as you pump in the water.

Another thing that works in my favour is the whistle mounted on top of the steam dome. After the boiler has cooled down, I simply open the whistle valve and give the loco a shake to get any trapped air to the top and then pump in a small amout of water via the hand pump until the water flows out of the whistle. The whistle valve is closed and the boiler given another pump to put a small amount of pressure on the gauge (about 20KPa) to squeeze out some more oxygen. This process is called water wedging.

The boiler is left in this condition until the next run.

When you're at the track for the next run, you simply open the blowdown valve to release some water to half-glass. You may need to open the blower valve to let some air in so the water flows out better. Then proceed with lighting up as usual.

Why I leave the water in the boiler

Sydney Water can include many other ingredients besides H2O...

Aluminium (Al), Iron (Fe), Manganese (Mn), Mineral Elements, Phosphorus (P)
Naturally occurring elements which can enter the water from the catchments.

Calcium (Ca)
A naturally occurring element which can enter the water from the catchments. It may also be added to water in the treatment process to reduce the acidity levels.

The application of chlorine to drinking water, waste water, or industrial waste to kill bacteria or to oxidise undesirable compounds.

A chemical containing iron. Used in the water filtration process to settle contaminants.

A chemical which encourages heavy contaminants to gather together and settle from water more quickly in the treatment process.

Small amounts of fluoride are added for dental health reasons in accordance with legislation.

Nitrogen (N)
A naturally occurring element which can enter the water from the catchments. Is used by plants as a nutrient.

A measure of the alkalinity or acidity of water expressed on a scale from 1 to 14: 1 is most acidic, 7 neutral and 14 most alkaline.

THMs (Trihalomethanes)
A by-product of chlorine which has been used to treat water.

Boiler Feedwater Treatment

In treating feedwater for a boiler, there are three basic items we are trying to prevent. These are: Scale, corrosion, and foaming. All of these problems can cause boiler and engine damage.

Chiefly, calcium and magnesium salts, dissolved in boiler water, deposit on tube surfaces as deposits and/or scale when water evaporates. This reduces heat transfer, increases tube metal temperature, and leads to ruptured boiler tubes; and an increase in fuel consumption.
Scale formation is prevented by removal of these hardness salts. This is done by chemicals to turn these salts into a soft sludge which can be removed by blowdown. The least expensive and most reliable method is by means of water softeners with the addition of boiler treatment chemicals.

Boilers become corroded for several reasons. Low pH water and the presence of dissolved gases such as carbon dioxide and oxygen can all cause corrosion, such as pitting of the boiler tubes and shell. To prevent this, the pH of boiler water is maintained in the alkaline range. The dissolved gases can be removed by corrosion inhibitors to protect surfaces of steam and return lines.

Foaming (called Carryover in the US) is boiler water leaving the boiler with the steam. These slugs of water in the steam line can cause mechanical damage. Several things can cause this: Alkalinity too high, poor boiler design, oil in the water, over firing, and others (apart from pumping too much water into the boiler). Foaming is controlled by the addition of anti-foam agents. It is also controlled by boiler blowdown.

My Boiler

The mineral elements are floating about in the boiler as "solids" and remain in suspension. While still in suspension, the Nu Wood chemical in the boiler helps keep the solids as a sludge to prevent them from baking on the heated surfaces like the crown sheet and girder stays as well as the boiler tubes. Blowing down the boiler a couple of times during a running session helps reduce the amount of suspended solids in the water as well as the effects of foaming.

The water wedging after the run helps reduce the oxygen content of the water for storage.

If no chemicals are used and the boiler is drained after each run, the solids are deposited on the horizontal surfaces of the boiler interior as the water level drops. The residual heat of the boiler cooks the solids on to the boiler surfaces and eventually builds up a crusty coating on the tubes etc. that will eventually reduce the heat transfer and reduce the boiler's efficiencey. All is not lost though because chemical cleaning with something like Phosphoric Acid can clear away the crust.

To me, the downside of draining the boiler after each run is having to unscrew fittings each time for air circulation which could lead to premature wear and failure of the threads/bolts etc.
The residual heat in the boiler can bake o-rings hard as a rock and reduce their sealing efficiency. The forming of scale on the interior surface means the addition of an ACID in the boiler to clean it! If you warm the shell with a light bulb in the firebox, you are using electricity you have to pay for!

So there you have it. With simple care, a steel boiler will give you great service and will last for your lifetime and beyond!!!

Where to get the Nu Wood

Nu Wood Boiler Disincrustant is available from ECOLAB at 6 Hudson St. Castle Hill, NSW, Australia. Phone (02) 9680 5444. The current availability (January 2000) is a 25 litre container for AUD$134.09 (including tax). The 25 Litre container is a bit much for individual use, so a group or club could divide the cost and decant the concentrate into smaller containers.

The mixing rate:
In a 10 Litre container, add 50ml of Nu Wood and 10 Litres of water. I also add 1/2 a teaspoon of Calgon (the brand name of a domestic water softener) to help maintain the water in the alkaline range of pH.

The mixture from the 10 litre container is poured into the tender or tanks in small doses to maintain a very weak tea colour of the boiler water. The colour should be barely noticable.

Where to get the AMBSC Code books

Australian Association of Live Steamers (AALS) Operating Codes of Practice and
Australian Miniature Boiler Safety Committee (AMBSC) Boiler Codes Part 1 (Copper boilers) and Part 2 (Steel boilers - Briggs and Wet-back)

Happy steaming!


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