Steam locomotives are
'external combustion' machines, meaning that the power to operate the
cylinders is obtained from steam created in an exterior boiler.
The boiler is the heart of
any steam locomotive and is by far the most expensive single part. There are
two main components; the barrel and the firebox. The firebox is double
skinned so that the metal is cooled by the water contained in the boiler. The
two skins are held in place by numerous stays which retain everything in its
place. Most importantly, water must always be maintained over the crown of
the firebox. This is not easy as the locomotive may at first be climbing, so
the water may be over the crown, but when it descends, the water will
move to the front.
The water level over the
crown is measured by 2 gauge glasses. The striping behind the gauge glass,
which is protected by heavy glass, reverses behind water so the level can be
The bottom of the firebox is fixed to the thick foundation ring
which has high mechanical strength.
Near the bottom of the
firebox are laid the fire bars which are usually made in cast iron. An
ash pan is fitted to the foundation ring which also usually has doors which
control the draft through the fire bars. Some engines are able to spray water
into the ash pan to cool the red hot cinders in order to reduce the risk of
a small boiler on its side showing the
inside a large firebox
In the crown of the firebox
is fitted at least one fusible plug. This is made of a metal of a low melting
point so that if the firebox crown becomes uncovered, it will melt and allow
high pressure steam and water to flow onto the fire to extinguish it.
The boiler barrel is attached
to the firebox and many tubes are fitted into it from one end to the other.
front tube plate of small model boiler
The hot gasses pass through
the tubes which heats the water in the barrel. A smoke box is attached to the
front of the boiler which is kept as air tight as possible. The used steam
from the cylinders is directed to a jet below the chimney. This draws the
gasses through the boiler tubes. In addition, a jet of steam can be released
by a control in the cab (the blower) which will also draw the fire.
Inside a locomotive smoke box showing blast
pipes from cylinders under the chimney and the blower tubing. The distance
between the chimney and the blast pipe is important.
At the top of the boiler
barrel is the dome. Inside is a valve (regulator) which is controlled from
the cab. This collects the steam and directed it through high pressure pipes
to the cylinders. The dome is important as it allows the steam to be
collected as far from the water as possible.
Safety valves are also fitted
to the top of the barrel. These are adjusted to open when the steam pressure
reaches a preset level to prevent an explosion of the boiler.
The boiler has to be
regularly flushed to remove sludge, so at the bottom of the firebox, close to
the foundation ring, are washout plugs. These can actually have a tap or a
large hex nut head.
Boilers of greater efficiency
use superheating. The steam is passed through additional tubing where it is
dried and further heated.
Boiler pressures vary between
on average, 120 psi (pounds per square inch) and 250 psi. (some idiot tried
building an ultra high pressure engine called 'Fury'. Needless to say, it
blew up). Maintenance is
onerous. The water to be used has to be analysed and the correct boiler
treatment added to prevent scaling and corrosion. In this harsh environment
corrosion does inevitably take place. The boiler is usually tested under
pressure once a year and every ten years is removed from the locomotive to be
thoroughly examined and re-tubed.
Most commonly, boilers are
fired using steam coal, which is soft and expands when burning. Oil firing is
also used and some locomotives have operated burning wood, bagasse and even
peat. One of the less pleasant tasks is that of cleaning out the boiler
tubes. This can be done by a special wire brush attached to a long rod or by
high pressure steam wands.