The Boiler
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The boiler consists of a steel shell which includes
the boiler barrel, the outer firebox wrapper plate, the back plate,
throat plate, smokebox tube plate, also the inner firebox and smoke
tubes.
The inner firebox is supported from the outer
firebox by the foundation ring at the bottom, by the crown stays at
the top and the palm stays between the firebox tube plate and the bottom
of the boiler barrel. In addition, the inner and outer firebox plates,
back plate and throat plate are stayed together by about 1,000 steel
or copper stays at approximately 4" pitch and longitudinal stays
between the boiler back plate and smokebox tube plate. when the boiler
is fitter with a superheater, a number of large flue tubes (approximately
5" in diameter) are fitted, in which the superheater elements are
placed.
THE SMOKEBOX
The smokebox is an extension at the front of
the boiler barrel, which together with the blast pipe and chimney form
the means of inducing air required for combustion to the firebox.
Apart from the chimney orifice, the smokebox
is airtight. Other fittings in the smokebox are: main steam pipes, blower
ring, brake ejector exhaust pipe, washout plugs and where fitted, superheater
elements.
When preparing or disposing a locomotive, the
crew should always check the smokebox to see if any of the fittings
are leaking or are displaced from their correct position.
WATER GAUGES
Water gauges are mounted on the boiler back plate
and are positioned so that when the water is in sight at the bottom
of the glass, the firebox crown is covered. (See Figure 2 - Water Gauge)
When working under normal conditions the water
level should be kept in sight in the top half of the glass and before
descending or ascending steep gradients, a higher level should be carried.
Running with too high a water level can cause
priming.
PRIMING
The term "priming" describes a situation
where water is carried with the steam into the cylinders, resulting
at best in an adverse effect on lubrication and at worst severe damage
to pistons, piston rods or cylinder covers, due to the incompressibility
of water.
Priming is associated with an excess of indissoluble
solids or salts in the feed water, a sudden demand for steam or too
high level of water in the boiler. These causes can largely be avoided
by regular boiler wash-outs and sensible engine management by the Driver
and Fireman.
BLAST PIPE
Exhaust steam after leaving the cylinders passes
through the exhaust passages to the blast pipe cap, where it is slightly
throttled so as to form a jet. The cap and the chimney are fixed on
the same centre line and are proportioned in relation to one.another
so that the escaping jet of exhaust steam, when passing through the
chimney, carries with it waste gases from the smokebox. Due to its air
tightness, a partial vacuum is produced within the smokebox, thereby
pulling air through the grate, the fire and the tubes. For this reason,
the smokebox door must be airtight, the blast pipe centred with the
chimney and the blower ring seated properly.
BRICK ARCH
Erected above the grate and immediately below
the first row of tubes, the arch slopes upwards towards the firehole
door. Its purpose is to direct unburnt gases towards the firehole door
to mix with air entering there to ensure complete combustion.
The brick arch works in conjunction with a baffle
plate fitted in the firehole door; this directs air into the path of
the flame and gases to complete combustion and burn off excessive smoke.
Further, the baffle plate directs the relatively cold air entering the
firebox away from the tube plate.
SAFETY VALVES
Safety valves are fitted to prevent the boiler
pressure from exceeding the registered working pressure for which the
boiler is designed.
Older locomotives are usually fitted with simpler
types of safety valve
(Ramsbottom or Salter), but all work on the same
principle of a valve held
on its seating by a spring which can be adjusted
to the required pressure.
More modern locomotives are fitted with the Ross
"pop" pattern.
FUSIBLE PLUGS
Fusible plugs are screwed into the firebox crown.
They are made of brass and have a lead core which
will melt at a comparatively low temperature. If the water level in
the boiler drops too low and uncovers the plug, the lead core melts
and allows steam to escape into the firebox, which acts as a warning
to the loco crew.
Should this occur, both injectors should be put
on immediately and steps taken to remove or deaden the fire.
WASHOUT PLUGS AND MUDHOLE DOORS
Washout plugs and mudhole doors are fitted to
boilers to facilitate cleaning and inspection of the water spaces.
They should be checked for signs of leakage before
a locomotive is lit up. Under no circumstances must washout plugs be
removed by Firelighters to fill boilers with water.
BLOWER
The blower consists of a perforated ring fitted
around the top of the blastpipe cap. Its function is to create a smokebox
vacuum for the following purposes.
a. To increase draught on the fire when the locomotive
is stationary, in order to raise steam pressure.
b. To clear smoke.
c. To counteract blow back of the fire.
DOME
The dome is situated midway along the top of
the boiler barrel and houses the regulator valve and is also the take
off point of steam for the auxiliary controls e.g. injectors etc.
HOPPER ASHPANS AND ROCKING GRATES
Hopper ashpan doors should always be opened prior
to dropping the fire during disposal, to prevent hot fire from damaging
the ashpan. When using this method of disposing of the fire, it must
be undertaken over an authorised disposal point and all clinker on the
firebars broken up using the fire irons before being dropped.
