A BOILER SAFETY
A fire-tube
boiler is a type of boiler in which hot gases from a fire pass through one or more
tubes running through a sealed container of water. The heat of the gases is
transferred through the walls of the tubes by thermal conduction, heating the water and
ultimately creating steam.
The fire-tube
boiler developed as the third of the four major historical types of boilers:
low-pressure tank or "haystack"
boilers, flued boilers with one or two large flues,
fire-tube boilers with many small tubes, and high-pressure water-tube
boilers. Their advantage over flued boilers with a single large flue
is that the many small tubes offer far greater heating surface area for the
same overall boiler volume. The general construction is as a tank of water penetrated
by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part—being the
strongest practical shape for a pressurized
container—and this cylindrical tank may be either horizontal or
vertical.
This type of
boiler was used on virtually all steam
locomotives in the horizontal "locomotive" form. This has
a cylindrical barrel containing the fire tubes, but also has an extension at
one end to house the "firebox". This firebox has an open base to
provide a large grate area and often extends beyond the cylindrical barrel to
form a rectangular or tapered enclosure. The horizontal fire-tube boiler is
also typical of marine applications, using the Scotch boiler. Vertical
boilers have also been built of the multiple fire-tube type,
although these are comparatively rare: most vertical boilers were either flued,
or with cross water-tubes.
Operation
Schematic diagram of a "locomotive" type fire-tube
boiler
In the locomotive-type boiler, fuel
is burnt in a firebox to produce hot combustion gases.
The firebox is surrounded by a cooling jacket of water connected to the long,
cylindrical boiler shell. The hot gases are directed along a series of fire
tubes, or flues, that penetrate the boiler and heat the water
thereby generating saturated ("wet") steam. The steam rises to the
highest point of the boiler, the steam dome,
where it is collected. The dome is the site of the regulator that
controls the exit of steam from the boiler.
In the locomotive boiler, the
saturated steam is very often passed into a superheater,
back through the larger flues at the top of the boiler, to dry the steam and
heat it to superheated steam. The superheated steam is directed to the
steam engine's cylinders or very rarely to a turbine
to produce mechanical work. Exhaust gases are fed out through a chimney,
and may be used to pre-heat the feed water to increase the efficiency of the
boiler.
Draught
for firetube boilers, particularly in marine applications, is usually provided
by a tall smokestack.
In all steam locomotives since Stephenson's
Rocket, additional draught is supplied
by directing exhaust steam from the cylinders into the smokestack through a blastpipe,
to provide a partial vacuum. Modern industrial boilers use fans to provide forced
or induced draughting of the boiler.
Another major advance in the Rocket was large numbers of
small-diameter firetubes (a multi-tubular boiler) instead of a single
large flue. This greatly increased the surface area for heat transfer, allowing
steam to be produced at a much higher rate. Without this, steam
locomotives could never have developed effectively as powerful prime movers.
Types of fire-tube boiler
For more details on the related
ancestor type, see Flued boilers.
Cornish boiler
For more details on this topic, see Cornish
boiler.
The earliest form of fire-tube
boiler was Richard Trevithick's "high-pressure"
Cornish boiler. This is a long horizontal cylinder with a single large flue
containing the fire. The fire itself was on an iron grating placed across this
flue, with a shallow ashpan beneath to collect the non-combustible residue.
Although considered as low-pressure (perhaps 25 psi) today, the use of a
cylindrical boiler shell permitted a higher pressure than the earlier
"haystack" boilers of Newcomen's
day. As the furnace relied on natural draught
(air flow), a tall chimney was required at the far end of the flue to encourage a
good supply of air (oxygen) to the fire.
For efficiency, the boiler was
commonly encased beneath by a brick-built chamber. Flue gases were routed through this,
outside the iron boiler shell, after passing through the fire-tube and so to a
chimney that was now placed at the front face of the boiler.
Lancashire boiler in Germany
Lancashire boiler
For more details on this topic, see Lancashire
boiler.
The Lancashire boiler is similar to
the Cornish, but has two large flues containing the fires. It was the invention
of William Fairbairn in 1844, from a theoretical
consideration of the thermodynamics of more efficient boilers that led him to
increase the furnace
grate area relative to the volume of water.
Later developments added Galloway
tubes (after their inventor, patented in 1848),[1]
crosswise water tubes across the flue, thus increasing the heated surface area.
As these are short tubes of large diameter and the boiler continues to use a
relatively low pressure, this is still not considered to be a water-tube
boiler. The tubes are tapered, simply to make their installation through the
flue easier.[2]
Side-section of a Scotch marine boiler: the arrows show
direction of flue gas flow; the combustion chamber is on the right, the
smokebox on the left.
Scotch marine boiler
For more details on this topic, see Scotch marine boiler.
The Scotch marine boiler differs
dramatically from its predecessors in using a large number of small-diameter
tubes. This gives a far greater heating surface area for the volume and weight.
The furnace remains a single large-diameter tube with the many small tubes
arranged above it. They are connected together through a combustion chamber –
an enclosed volume contained entirely within the boiler shell – so that the
flow of flue gas through the firetubes is from back to front. An enclosed
smokebox covering the front of these tubes leads upwards to the chimney
or funnel. Typical Scotch boilers had a pair of furnaces, larger ones had
three. Above this size, such as for large steam ships,
it was more usual to install multiple boilers.[3]
Locomotive boiler
A locomotive boiler has three main
components: a double-walled firebox; a horizontal, cylindrical
"boiler barrel" containing a large number of small flue-tubes; and a smokebox
with chimney,
for the exhaust gases. The boiler barrel contains larger flue-tubes to carry
the superheater
elements, where present. Forced draught is provided in the locomotive boiler by
injecting exhausted steam back into the exhaust via a blast pipe
in the smokebox.
Locomotive-type boilers are also
used in traction engines, steam rollers,
portable
engines and some other steam road vehicles. The inherent strength of
the boiler means it is used as the basis for the vehicle: all the other
components, including the wheels, are mounted on brackets attached to the
boiler. It is rare to find superheaters designed into this type of boiler, and
they are generally much smaller (and simpler) than railway locomotive types.
The locomotive-type boiler is also a
characteristic of the overtype steam wagon, the steam-powered
fore-runner of the truck.
In this case, however, heavy girder frames make up the load-bearing chassis of
the vehicle, and the boiler is attached to this.
Taper boiler
Certain railway locomotive boilers
are tapered from a larger diameter at the firebox end to a smaller diameter at
the smokebox
end. This reduces weight and improves water circulation. Many later Great Western Railway and London, Midland and Scottish Railway
locomotives were designed or modified to take taper boilers.
Vertical Fire-Tube boiler
Main article: Vertical
boiler
A vertical fire-tube boiler (VFT),
colloquially known as the "vertical boiler", has a vertical
cylindrical shell, containing several vertical flue tubes.
Horizontal Return Tubular boiler
Horizontal Return Tubular boilers from the Staatsbad Bad Steben
GmbH
Horizontal Return Tubular boiler
(HRT) has a horizontal cylindrical shell, containing several horizontal flue
tubes, with the fire located directly below the boiler's shell, usually within
a brickwork setting
Admiralty-type direct tube boiler
Extensively used by Britain, before
and in the early days of ironclads, the only protected place was below the
waterline, sometimes under an armoured deck, so to fit below short decks, the
tubes were not led back above the furnace but continued straight from it with
keeping the combustion chamber in between the two. Hence the name, and
considerably reduced diameter, compared to the ubiquituous Scotch or return
tube boiler. It was not a great success and it's use was being abandoned after
the introduction of stronger side armouring – “the furnace crowns, being very
near the water-level, are much more liable to over-heating. Further, on account
of the length of the boiler, for an equal angle of inclination, the effect on
the water-level is much greater. Finally, the unequal expansion of the various
parts of the boiler is more pronounced, especially at the top and bottom, due
to the increased ratio between the length and the diameter of the boiler; the
local strains are also more severe on account of the comparatively feeble
circulation in long and low boilers.” All these also resulted in a shorter
life. Also, the same length of a combustion chamber was much less effective on
a direct tube than on a return tube boiler, at least without baffling.[4]
Immersion Fired Boiler
The Immersion Fired boiler is a
single pass fire-tube boiler that was developed by Sellers Engineering in the
1940s. It has only firetubes, functioning as a furnace and combustion chamber
also, with multiple burner nozzles injecting premixed air and natural gas under
pressure. It claims reduced thermal stresses, and lacks refractory brickwork completely
due to its construction.[5]
Variations
Water tubes
Fire-tube boilers sometimes have
water-tubes as well, to increase the heating surface. A Cornish boiler may have
several water-tubes across the diameter of the flue (this is common in steam
launches). A locomotive boiler with a wide firebox may have arch
tubes or thermic syphons. These increase the heating
surface and give additional support to the brick arch.
Another technique for increasing the
heating surface is to include internal rifling inside
the boiler tubes (also known as Serve tubes).
Not all shell boilers raise steam;
some are designed specifically for heating pressurised water.
Reverse flame
In homage to the Lancashire design,
modern shell boilers can come with a twin furnace design. A more recent
development has been the reverse flame design where the burner fires into a
blind furnace and the combustion gasses double back on themselves. This results
in a more compact design and less pipework.
Package boiler
The term "package" boiler
evolved in the early- to mid-20th century from the practice of delivering
boiler units to site already fitted with insulation, electrical panels, valves
and gauges. This was in contrast to earlier practice where little more than the
pressure vessel was delivered and the ancillary components were fitted on-site.
Safety considerations
Because the fire-flume boiler itself
is the pressure vessel, it requires a number of safety features to prevent
mechanical failure. Boiler explosion, which is a type of BLEVE (Boiling Liquid
Expanding Vapor Explosion), can be devastating.
- Safety valves release steam before a dangerous pressure can be built up
- Fusible plugs over the firebox melt at a temperature lower than that of the firebox plates, thereby warning the operators by the noisy escape of steam if the water level is too low to cool the firebox crown safely.
- Stays, or ties, physically link the firebox and boiler casing, preventing them from warping. Since any corrosion is hidden, the stays may have longitudinal holes, called tell-tales, drilled in them which leak before they become unsafe.
The fire-tube type boiler that was
used in the Stanley Steamer automobile had several hundred
tubes which were weaker than the outer shell of the boiler, making an explosion
virtually impossible as the tubes would fail and leak long before the boiler
exploded. In nearly 100 years since the Stanleys were first produced, no
Stanley boiler has ever exploded.[citation needed]
Maintenance
An intensive schedule of maintenance
is needed to keep a boiler in safe condition. A typical regime will involve
regular external inspections (including the inside of the firebox), washouts
(with an internal inspection), periodic detailed examination and a general
overhaul.
Daily inspection
The tube plates, the fusible plug
and the heads of the firebox stays should be checked for leaks. The correct
operation of the boiler fittings, especially the water gauges
and water feed
mechanisms, should be confirmed. Steam pressure should be raised to
the level at which the safety valves lift and compared with the
indication of the pressure gauge.
Washout
Cutaway of locomotive boiler. Note the narrow water spaces
around the firebox and the "mudhole" for access to the crown sheet:
these areas require special attention during washout
The working life of a locomotive
boiler is considerably extended if it is spared from a constant cycle of
cooling and heating. Historically, a locomotive would be kept “in steam”
continuously for a period of about eight to ten days, and then allowed to cool
sufficiently for a hot-water boiler washout. The schedule for express engines
was based on mileage.[6]
Today's preserved locomotives are not usually kept continuously in steam and
the recommended washout interval is now fifteen to thirty days, but anything up
to 180 days is possible.[7]
The process starts with a “blowdown”
while some pressure remains in the boiler, then the draining away of all the
boiler water through the “mudholes” at the base of the firebox and the removal
of all the “washout plugs”. Scale is then jetted or scraped from the interior surfaces
using a high pressure water jet and rods of soft metal, such as copper. Areas
particularly susceptible to scale buildup, such as the firebox crown and narrow
water spaces around the firebox, are given special attention. The inside of the
boiler is inspected by sighting through the plug holes, with a particular check
paid to the integrity of the firetubes, firebox crown and stays and absence of
pitting or cracking of the boiler plates. The gauge glass cocks and tubes and
fusible plug should be cleared of scale; if the core of the fusible plug shows
signs of calcination the item should be replaced.
On reassembly care should be taken
that the threaded plugs are replaced in their original holes: the tapers can
vary as a result of rethreading. The mudhole door gaskets, if of asbestos,
should be renewed but those made of lead may be reused; special instructions are in force for the
disposal of these harmful materials.[7]
At large maintenance facilities the boiler would have been both washed and
refilled with very hot water from an external supply to bring the locomotive
back to service more quickly.
Periodic examination
Typically an annual inspection, this
would require the removal and check of external fittings, such as the
injectors, safety valves and pressure gauge. High-pressure copper pipework can
suffer from work hardening in use and become dangerously
brittle: it may be necessary to treat these by annealing before refitting. A hydraulic
pressure test on the boiler and pipework may also be called for.
General overhaul
In the UK the specified maximum
interval between full overhauls is ten years. To enable a full inspection the
boiler is lifted from the locomotive frame and the lagging removed. All firetubes are removed for
checking or replacement. All fittings are removed for overhaul. Before
returning to use a qualified examiner will check the boiler’s fitness for
service and issue a safety certificate valid for ten years.
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