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Building A Model Boiler

A Layman’s Guide To Steam Fittings

Steam fittings are generally made of brass, usually being cast and then machined to shape, although they can be fabricated by an individual if he chooses.

The problem with using brass has been mentioned before regarding the transposition of the zinc within the brass being moved through electrolysis into the water, and for this reason these fittings may need replacing after a few years use.

By not using brass fittings on the outlet side of the boiler (keeping the brass away from the flow of the steam) there tends to be less damage done to the fittings and that way a longer period of usefulness is achieved, although the same problem occurs on the feed side, but at a slower rate.

Normally the steam control valve is the main fitting used on the outlet side between the boiler and the engine and being unable to shut off the steam flow is a sure sign that the valve needs replacing.


As a general rule the fittings needed on a model boiler are:

  •  One–way valves to allow extra water to be pumped in but at the same time keep any pressure within your boiler (often called clack or check valves)
  •  A visual guide for the water level within the boiler (your sight gauge)
  • A pressure gauge to show what the current available working pressure is
  • A safety valve to control the maximum pressure within your boiler
  • A cylinder lubrication device (a direct oil pump, or more often on small boilers a displacement lubricator)
  • Valves to allow your boiler to be drained (known as blow-down valves or drain-cocks)
  • A steam control valve (for the steam take-off and control)

There are others like injector valves, water lifters, blower and bypass valves that need not concern us with the smaller models we are dealing with at the moment, along with the much needed whistle, as these tend to be used on the larger coal-fired models capable of pulling three or four people around.


Taking the clack valves or check valves first, they are made with a stainless steel ball sealing a hole, usually with the aid of spring pressure, althoughCheck valve it can simply be gravity and boiler pressure, so that the force of the boiler acts with the aid of the spring or gravity to keep the boiler contents inside the boiler.

At the other side of the ball, the inlet side, water is pumped past the ball and into the boiler, and as the pump reduces the pressure on the supply side to draw in more water, the ball snaps tight into its seating to seal the passageway from the pump into the boiler barrel.


The Sight Gauge is normally two hollow fittings sat at different heights on the boiler barrel, with a clear tube sat between them, that way allowing the water height within the boiler barrel to be seen. In its simplest form it would be a glass or perspex type of disc mounted in the end of the boiler, but here there are heat and pressure considerations that need to be addressed for the material in use.

There are various types of sight gauge, but the clear tube one is the most common and is readily available in different sizes and is supplied with rubberised seals for the glass – a simple (but can be very fiddly) bolt together job.

Again the material used is not glass, but some sort of polycarbonate produced specifically for the pressure intended.


Pressure gauges again come in different styles and the loop of piping below them serves the purpose of keeping any water away from the gauge's internal workings. Their function is to simply show you, the operator, what the steam pressure is so that you can control it.


Pressure relief valves are of utmost importance with a boiler and it is usually easy to adjust the pressure relief setting to match your particular model. The thread sizes vary according to the scale of model you are producing and may seem a little confusing, but more important than this is the need for the valve to release the steam quickly enough to stop any boiler failure.

Sometimes it may be necessary to have two valves on your boiler, with one set at your working pressure and the second a little higher to act a secondary safety valve, as they do stick shut sometimes.


The cylinder lubrication device (if you use a displacement lubricator) is needed in the steam line feeding from the boiler and sittingDisplacement lubricator very close to your cylinder block, or directly entering into the cylinder block itself. This closeness is necessary to make sure any oil is not drawn back into the boiler barrel once the pressure is reduced at cool-down.

If you have a mechanical pump, this usually has minute amounts of neat steam oil being forced into the cylinder block by mechanical means, usually as the engine rotates.

What you do not want to do is add oil to the water feed going into your boiler.

If this was the case, as the water heats up inside your boiler, the oil – having floated to the surface of the water, gradually forms a sludge sitting on top of the water which does not go through into your cylinder, but also clogs up the outlet pipes, pressure gauge, relief valve, whistle, whatever other fittings you may have, along with making it impossible to see the water level through your sight glass or water gauge.

Eventually you would have a boiler filled with sludge and it would need to be cleaned with all sorts of chemicals to remove the thick gunge, but this would be after your piston had give up the ghost and seized rock solid in its cylinder.


Blow-down valves and drain cocks are needed for two purposes.

Cylinder drain cocksCylinder drain cocks make warming up the cylinder and cylinder block easier by allowing you to vent steam and any condensed water through them before any live steam running commences.

Blow-down valves (the red one to the right) on the other hand are generally attached to the boiler itself to allow boiler drainage by usingSteam Boiler blowdown valve any pressure within the boiler to force out any liquid remaining in the boiler. For this reason they tend to be mounted low on the side or base of the boiler and the top nut is turned fractionally to allow the ball to lift off its seat to allow steam to pass to the floor.

They are used after a steam run when the machinery is to be parked up for a period of time with an empty boiler and can be left open to allow any pressure within the boiler to become equal to outside atmospheric pressure so that any distortion of the boiler itself is not experienced.

Both types can be simple tap-like affairs or can have stainless balls acting as valves on the inside, being nudged off their seats by a small actuating rod of some sort.


Steam control valves (basic regulators) again are simply a tap to control the rate of flow of steam through to your driven motor or fittings.


Blower valves are a direct feed from your boiler into a fine jet in the smoke stack (chimney) which forces the smoke and air in your smoke-stack away from your boiler. This causes a slight vacuum in the fire-box and flue tubes and forces air through your coal fire to replace it, meaning more heat is produced as more steam is used. Obviously this item is used solely for coal fired boilers, although trials have gone on with gas firing.


Injectors and water lifters work in a similar way in that a fine jet of steam is introduced into a chamber and a vacuum is created causing water to replace the steam. Because of the shapes of chamber involved this can be used to raise water from the roadside into your water tank on the full sized road steam engine, or to force a lesser amount of water directly from the water tank and into your boiler. This basically ‘injects’ water into your boiler.

In this case both would need a steam control valve in the steam feed line to regulate the steam flow, and hence the water flow, when needed.


A by-pass valve is simply a valve sitting between the water tank and a mechanical water pump, but sitting on the return side of the pump into the tank. Normally with the valve shut, the water feeds from the tank into the pump and then goes directly through a one way valve and into the boiler itself, but with this valve open, the water returns to the tank after passing the pump, that way no further water is being added to your boiler. Obviously whether the valve operates when shut or open depends on the water circuits of your engine.

By regulating this bypass valve a steady supply of water can be added to your boiler to keep the water level just right with the surplus provided by the pump being returned to the tank.


And now a word on the bushes, the parts sitting between your boiler and these brass fittings.

Normally these are made of bronze for the particular reason that bronze, although similar to brass in its chemical make-up, is not affected by electrolysis so much as brass and will likely outlast your boiler’s lifetime anyway.

For this reason all bushes should be made of bronze (or another less durable option is gun metal) and should be silver soldered/brazed into position, with the brass fittings attached to these bushes. By putting that bridge between the boiler itself and the brass fittings, a longer life of your fittings is achieved without too much extra effort.


I hope that has explained a little to you.


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