A round-topped boiler is a type of

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...

used for some designs of

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...

and

portable engine A portable engine is an engine, either a steam engine or an internal combustion engine, that sits in one place while operating (providing power to machinery), but (unlike a stationary engine) is portable and thus can be easily moved from one w ...

. It was an early form of

locomotive boiler A fire-tube boiler is a type of boiler in which hot gases pass from a fire 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 t ...

, although continuing to be used for new locomotives through to the end of steam locomotive manufacture in the 1960s.

Firebox

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 haycock 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 strip A lap is a surface (usually horizontal) created between the knee and Hip (anatomy), hips of a biped when it is in a seated or lying down position. The lap of a parent or loved one is seen as a physically and psychologically comfortable place for ...

s. 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 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 ...

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.

Staying

Portable engine firebox, transverse section (Army Service Corps Training, Mechanical Transport, 1911)

The upper

crown sheet Boilers for generating steam or hot water have been designed in countless shapes, sizes and configurations. An extensive terminology has evolved to describe their common features. This glossary provides definitions for these terms. Terms which re ...

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

girder A girder () is a support beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a stabilizing ...

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 The steam dome is a vessel fitted to the top of the boiler of a steam engine. 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. This arrangement acts as a ...

Rod and sling stays

Round-topped boiler, firebox section (Gairns, Railways for All)

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 stays 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 A boiler explosion is a catastrophic failure of a boiler. There are two types of boiler explosions. One type is a failure of the pressure parts of the steam and water sides. There can be many different causes, such as failure of the safety val ...

. 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 The Belpaire firebox is a type of firebox used on steam locomotives. It was invented by Alfred Belpaire of Belgium in 1864. Today it generally refers to the shape of the outer shell of the firebox which is approximately flat at the top and sq ...

. 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.

Examples

Locomotive designs fitted with round-topped boilers: File:NER Class C (LNER Class J21).png, NER Class C1 876 at

Beamish Museum Beamish Museum is the first regional open-air museum, in England, located at Beamish, near the town of Stanley, in County Durham, England. Beamish pioneered the concept of a living museum. By displaying duplicates or replaceable items, it wa ...

in 2001 File:65033 at Weybourne 2.jpg, Same locomotive, 876, dismantled for overhaul in 2008

References