Pistol boiler
Encyclopedia
A pistol boiler is a design of steam boiler used in light steam tractors and overtype steam wagons. It is noted for the unusual shape of the firebox
, a circular design intended to be self-supporting without the use of firebox stays.
The name 'pistol boiler' derives from the smooth curve of the outer firebox flowing into the boiler barrel, and a supposed resemblance to the stock of an early 19th century pistol
.
's day; although the cost and complexity of its firebox remained a drawback, particularly for small boilers. If the top crown sheet of the inner firebox was made flat, so as to maintain a constant water depth above it, this required complex and expensive girder stays to support it. These stays were also a safety-critical part of the boiler and many past boiler explosion
s had been caused by their failure. This was especially so for boilers that were likely to be used at all carelessly, or by crews who were less skilled or well-trained.
Clearly the market for small steam traction engine
s could make use of a novel boiler design that avoided these problems. Following the lead of the London & Birmingham Railway's Bury locomotive
s, some small portable engine
s were already using cylindrical stayless fireboxes. These combined a cylindrical vertical drum with a domed top, both shapes that could support themselves well under pressure. In extreme cases for larger railway locomotives, these became the massive brass-clad 'Gothic' firebox
es that were so distinctive on early Great Western Railway
locomotives.
s and small steam tractors, developed its own version of this stayless domed firebox as the pistol boiler.
As the boiler was small, with a barrel diameter of only 2', it was practical to form the deeply curved plates for the inner and outer firebox with a hydraulic press
. The inner firebox was formed in one piece as a truncated cone with a domed top. Although a small boiler, this gave a large grate area that permitted the burning of oke][ as a fuel. The front face of this cone was flattened inwards, to form the firebox tubeplate. 54 1½inch fire-tubes were used. The boiler barrel was cylindrical, but the lower part of the rear end was cut on a diagonal, rather than straight across. The lower part of the outer firebox was also conical and wrapped over this diagonal edge. The foundation ring was circular, avoiding the problems of mud build-up in the corners. The upper corner of the outer firebox was a separate plate, approximately a quarter of a sphere. Rather than the time-consuming and costly flanging of flat plates, these curved plates could be pressed and riveted together almost immediately. The number of boiler plates was also reduced, from the usual eight to only five.Boiler barrel, upper firebox, lower firebox, inner firebox, smokebox tubeplate. A relatively high working pressure of 250 psi could be used.
The firebox door was also of novel design. As the backplate sloped so steeply, the door was top-hinged and opened inwards, rather than outwards. This also had the effect of acting as a deflector plate, directing the cold draught down onto the firebed, rather than directly across and into the tubes.
Robey used this design of boiler on their 6-ton steam wagons and 'Express' steam tractorsNine 'Express' tractors were built, two survive., also their tandem steam rollers. One of these rollers was the first artefact to be preserved by the Robey Trust. When the boiler was re-barreled in 1988, this was the last boiler to be constructed by the Robey factory before closure. On their larger engines, Robey used a conventional boiler.
The firm of Garrett
also sought a reliable stayless firebox for their smaller boilers at around the same time in the 1920s. Their solution was more conventional: a conventional outer firebox enclosed an inner firebox, where the inner crown sheet was formed into a curved valley. For a small firebox, this acted as a girder stay between the end sheets of the firebox and was sufficient to be self supporting. The portable engines used this pattern alone. For the slightly more powerful road tractor boilers, the inner firebox was still self-supporting but the outer wrapper now required crosswise sling stays to support it, where previously it would have been supported by the stays to the inner firebox.
Firebox
In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name.-Railway locomotive firebox :...
, a circular design intended to be self-supporting without the use of firebox stays.
The name 'pistol boiler' derives from the smooth curve of the outer firebox flowing into the boiler barrel, and a supposed resemblance to the stock of an early 19th century pistol
Pistol
When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...
.
Need for a self-supporting firebox
The locomotive boiler had become well established since StephensonGeorge Stephenson
George Stephenson was an English civil engineer and mechanical engineer who built the first public railway line in the world to use steam locomotives...
's day; although the cost and complexity of its firebox remained a drawback, particularly for small boilers. If the top crown sheet of the inner firebox was made flat, so as to maintain a constant water depth above it, this required complex and expensive girder stays to support it. These stays were also a safety-critical part of the boiler and many past 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...
s had been caused by their failure. This was especially so for boilers that were likely to be used at all carelessly, or by crews who were less skilled or well-trained.
Clearly the market for small steam traction engine
Traction engine
A traction engine is a self-propelled steam engine used to move heavy loads on roads, plough ground or to provide power at a chosen location. The name derives from the Latin tractus, meaning 'drawn', since the prime function of any traction engine is to draw a load behind it...
s could make use of a novel boiler design that avoided these problems. Following the lead of the London & Birmingham Railway's Bury locomotive
Bury Bar Frame locomotive
The Bury Bar Frame locomotive was an early type of steam locomotive, developed at the works of Edward Bury and Company, later named Bury, Curtis, and Kennedy....
s, some small 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...
s were already using cylindrical stayless fireboxes. These combined a cylindrical vertical drum with a domed top, both shapes that could support themselves well under pressure. In extreme cases for larger railway locomotives, these became the massive brass-clad 'Gothic' firebox
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...
es that were so distinctive on early Great Western Railway
Great Western Railway
The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...
locomotives.
Development of the pistol boiler
The firm of Robey & Co, well-known builders of both large stationary engineStationary steam engine
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars , agricultural engines used for ploughing or...
s and small steam tractors, developed its own version of this stayless domed firebox as the pistol boiler.
As the boiler was small, with a barrel diameter of only 2', it was practical to form the deeply curved plates for the inner and outer firebox with a hydraulic press
Hydraulic press
A hydraulic is a machine using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalenta mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795...
. The inner firebox was formed in one piece as a truncated cone with a domed top. Although a small boiler, this gave a large grate area that permitted the burning of oke][ as a fuel. The front face of this cone was flattened inwards, to form the firebox tubeplate. 54 1½inch fire-tubes were used. The boiler barrel was cylindrical, but the lower part of the rear end was cut on a diagonal, rather than straight across. The lower part of the outer firebox was also conical and wrapped over this diagonal edge. The foundation ring was circular, avoiding the problems of mud build-up in the corners. The upper corner of the outer firebox was a separate plate, approximately a quarter of a sphere. Rather than the time-consuming and costly flanging of flat plates, these curved plates could be pressed and riveted together almost immediately. The number of boiler plates was also reduced, from the usual eight to only five.Boiler barrel, upper firebox, lower firebox, inner firebox, smokebox tubeplate. A relatively high working pressure of 250 psi could be used.
The firebox door was also of novel design. As the backplate sloped so steeply, the door was top-hinged and opened inwards, rather than outwards. This also had the effect of acting as a deflector plate, directing the cold draught down onto the firebed, rather than directly across and into the tubes.
Robey used this design of boiler on their 6-ton steam wagons and 'Express' steam tractorsNine 'Express' tractors were built, two survive., also their tandem steam rollers. One of these rollers was the first artefact to be preserved by the Robey Trust. When the boiler was re-barreled in 1988, this was the last boiler to be constructed by the Robey factory before closure. On their larger engines, Robey used a conventional boiler.
Related designs
A similar pistol boiler was also used by Foden in their 'O-type' Speed Six and Speed Twelve steam wagons.The firm of Garrett
Garrett
-Places:United States* Garrett, Illinois* Garrett, Indiana* Garrett, Pennsylvania* Garrett, Texas* Garrett, Washington* Garrett, Wyoming* Garrett County, Maryland-Other uses:...
also sought a reliable stayless firebox for their smaller boilers at around the same time in the 1920s. Their solution was more conventional: a conventional outer firebox enclosed an inner firebox, where the inner crown sheet was formed into a curved valley. For a small firebox, this acted as a girder stay between the end sheets of the firebox and was sufficient to be self supporting. The portable engines used this pattern alone. For the slightly more powerful road tractor boilers, the inner firebox was still self-supporting but the outer wrapper now required crosswise sling stays to support it, where previously it would have been supported by the stays to the inner firebox.