Round-topped boiler
Encyclopedia
A round-topped boiler is a type of boiler used for some designs of steam locomotive
and portable engine
. It was an early form of locomotive boiler, although continuing to be used for new locomotives through to the end of steam locomotive manufacture in the 1960s.
They use the early form of firebox, where the outer wrapper of the firebox is a semi-circular continuation of the cylindrical boiler barrel. They are relatively simple in shape and manufacture, but their design and service is complicated by the difference in shape between the outer and the flat-topped inner wrapper of the firebox. This requires complex staying to support it.
The first boilers of this form had raised fireboxes that were considerably larger, particularly higher, than the boiler barrel. In the extreme case, this gave rise to the Gothic boiler
, where the firebox was raised into a vaulted dome. In most cases though, the firebox was merely larger by the width of the internal iron framing section. Around 1850, at the instigation of Crampton
, the firebox adopted the flush-topped firebox casing, where it was of the same diameter as the boiler barrel and joined to it by riveted lap strips.
The early boiler fireboxes had flat parallel sides (see figure 1). As locomotives grew in power their boilers expanded in diameter, but the width of the firebox grate was still constrained by the need to fit between the locomotive frame
s, which were themselves constrained by the gauge between the wheels. This led to the development of the "waisted" form (see figure 2), where a narrow firebox flared upwards and outwards to meet the boiler barrel. Staying of these firebox side walls could still use simple short rod stays, although these were progressively tilted in the middle of the firebox, so as to remain perpendicular to the sheets.
Because the shape of the firebox follows the shape of the boiler barrel, they are used for saddle tank
locomotives. The Hunslet Austerity
, the last production steam locomotives to be constructed in the UK, used this form.
s spanning the length or width, on the outside of the inner firebox (i.e. in the water space). The crown sheet is bolted to these. The force on the ends of the girder stays is supported by the ends or sides of the inner firebox. The upper part of the boiler barrel is unconnected to the firebox. This has the advantage that a large hole may be cut in the boiler directly above the firebox, to mount the steam dome
.
Thermal expansion was a problem for these long rod stays, both in their expansion tending to elongate them and reduce their staying effect . Also as the inner firebox expanded in length, this would bend the stays forwards. Where a long firebox was stayed, the forward rod stays were replaced with sling stays, a rod stay with a hinge at the upper end, allowing it to move forwards with the firebox.
These stays were also placed in the most active part of the boiler, where corrosion was at its worst. Careful inspection and periodic replacement of them was needed, to avoid the risk of a boiler explosion
.
The main difficulty of rod stays was that it required a large number of rod stays, each of which was placed at a different angle to its neighbour. The complexity of this rod staying was what drove Alfred Belpaire
to develop his Belpaire firebox
. This used a more complicated squared-off outer firebox, which was more difficult to manufacture, but could now use simple short rod stays throughout, as for the firebox sides.
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
and portable engine
Portable engine
A portable engine is a small steam engine, mounted on wheels or skids, which is used for driving machinery using a belt from its flywheel. It is not self-propelled and is towed to the work site by horses or bullocks, or even a traction engine. Portable engines were used mainly for driving...
. It was an early form of locomotive boiler, although continuing to be used for new locomotives through to the end of steam locomotive manufacture in the 1960s.
They use the early form of firebox, where the outer wrapper of the firebox is a semi-circular continuation of the cylindrical boiler barrel. They are relatively simple in shape and manufacture, but their design and service is complicated by the difference in shape between the outer and the flat-topped inner wrapper of the firebox. This requires complex staying to support it.
The first boilers of this form had raised fireboxes that were considerably larger, particularly higher, than the boiler barrel. In the extreme case, this gave rise to the Gothic boiler
Gothic boiler
A gothic boiler is an early form of steam locomotive boiler with a prominently raised firebox of "Gothic arch", "haystack", or "coppernob" shape. The term haystack is most commonly used, but is avoided here as it is confusingly used for three quite different forms of boiler. This particularly large...
, where the firebox was raised into a vaulted dome. In most cases though, the firebox was merely larger by the width of the internal iron framing section. Around 1850, at the instigation of Crampton
Thomas Russell Crampton
Thomas Russell Crampton, MICE, MIMechE was an English engineer born at Broadstairs, Kent, and trained on Brunel's Great Western Railway....
, the firebox adopted the flush-topped firebox casing, where it was of the same diameter as the boiler barrel and joined to it by riveted lap strips.
The early boiler fireboxes had flat parallel sides (see figure 1). As locomotives grew in power their boilers expanded in diameter, but the width of the firebox grate was still constrained by the need to fit between the locomotive frame
Locomotive frame
A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure of some kind...
s, which were themselves constrained by the gauge between the wheels. This led to the development of the "waisted" form (see figure 2), where a narrow firebox flared upwards and outwards to meet the boiler barrel. Staying of these firebox side walls could still use simple short rod stays, although these were progressively tilted in the middle of the firebox, so as to remain perpendicular to the sheets.
Because the shape of the firebox follows the shape of the boiler barrel, they are used for saddle tank
Saddle tank
Saddle tank may refer to:* Saddle tank , a water tank carried above the boiler of a steam locomotive; also the type name of a locomotive so fitted.* Saddle tank , an early type of ballast tank...
locomotives. The Hunslet Austerity
Hunslet Austerity 0-6-0ST
The Hunslet Engine Company Austerity 0-6-0ST is a steam locomotive designed for shunting. The class became the standard British shunting locomotive during the Second World War, and production continued until 1964 at various locomotive manufacturers....
, the last production steam locomotives to be constructed in the UK, used this form.
Staying
The upper crown sheet of the inner firebox is, as with most locomotive boilers, approximately flat and horizontal, so as to maintain a constant depth of water over this hottest part of the firebox. This flat surface, with steam pressure on its upper side, requires stays to support it. As it is not a constant distance from the boiler barrel, unlike the firebox sides, this staying is difficult to arrange.Girder stays
Early examples soon used girder stays to support the inner firebox crown. These are iron girderGirder
A girder is a support beam used in construction. Girders often have an I-beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams...
s spanning the length or width, on the outside of the inner firebox (i.e. in the water space). The crown sheet is bolted to these. The force on the ends of the girder stays is supported by the ends or sides of the inner firebox. The upper part of the boiler barrel is unconnected to the firebox. This has the advantage that a large hole may be cut in the boiler directly above the firebox, to mount the steam dome
Steam dome
A Steam dome is a vessel fitted to the top of the boiler of a steam locomotive. It contains the opening to the main steam pipe and its purpose is to allow this opening to be kept well above the water level in the boiler...
.
Rod and sling stays
Larger boilers could not support their firebox crowns entirely on the inner firebox and required a means to support it from the boiler barrel. As this is also forced outwards by the steam pressure, the total force could be shared between many small stays. Each stay carried enough force on each end to balance itself, avoiding the large bending forces at the ends of the girder staysThermal expansion was a problem for these long rod stays, both in their expansion tending to elongate them and reduce their staying effect . Also as the inner firebox expanded in length, this would bend the stays forwards. Where a long firebox was stayed, the forward rod stays were replaced with sling stays, a rod stay with a hinge at the upper end, allowing it to move forwards with the firebox.
These stays were also placed in the most active part of the boiler, where corrosion was at its worst. Careful inspection and periodic replacement of them was needed, to avoid the risk of a boiler explosion
Boiler explosion
A boiler explosion is a catastrophic failure of a boiler. As seen today, boiler explosions are of two kinds. One kind is over-pressure in the pressure parts of the steam and water sides. The second kind is explosion in the furnace. Boiler explosions of pressure parts are particularly associated...
.
The main difficulty of rod stays was that it required a large number of rod stays, each of which was placed at a different angle to its neighbour. The complexity of this rod staying was what drove Alfred Belpaire
Alfred Belpaire
Alfred Jules Belpaire was a Belgian locomotive engineer who invented the square-topped Belpaire firebox in 1860....
to develop his Belpaire firebox
Belpaire firebox
The Belpaire firebox is a type of firebox used on steam locomotives. It was invented by Alfred Belpaire of Belgium. It has a greater surface area at the top of the firebox, improving heat transfer and steam production...
. This used a more complicated squared-off outer firebox, which was more difficult to manufacture, but could now use simple short rod stays throughout, as for the firebox sides.