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A fire-tube boiler is a type of boiler in which hot gases from the fire pass through one or more tubes within the boiler. A boiler or steam generator is a device used to create Steam by applying Heat energy to Water. It is one of the two major types of boilers, the other being the water-tube boiler. A water-tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire A fire tube boiler can be either horizontal or vertical. A fire-tube boiler is sometimes called a "smoke-tube boiler" or "shell boiler" or sometimes just "fire pipe".
This type of boiler was used on virtually all steam locomotives in the horizontal "locomotive" form. A steam locomotive is a Locomotive powered by Steam. The term usually refers to its use on Railways but can also refer to a "road locomotive" It is also typical of early marine applications and small vessels, such as the small riverboat used in the movie The African Queen. A riverboat is Ship designed for Inland navigation. These vessels are usually less sturdy than ships built for the open seas with limited navigational and The African Queen is a Drama film directed by John Huston and produced by Sam Spiegel (billed as "S Small fire-tube boilers used for various auxiliary services were often called "donkey boilers". Today, they find extensive use in the stationary engineering field, typically for low pressure steam use such as heating a building.
Types of fire-tube boiler
Cornish boiler
Cornish boiler has a single large flue containing the fire.
Lancashire boiler
Lancashire boiler has two large flues containing the fires
Scotch marine boiler
This is similar to a Lancashire boiler but the flues lead into combustion chambers where the hot gases are reversed to flow through a large number of horizontal fire tubes located within the boiler shell above the flues [1].
Locomotive boiler
Locomotive boiler has a double-walled firebox and a large number of small flue-tubes. Larger flue-tubes 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. The blastpipe is part of a Steam Locomotive that discharges exhaust steam from the cylinders into the Smokebox beneath the Chimney
Vertical Firetube boiler
Vertical Firetube boiler (VFT) has a vertical cylindrical shell, containing several vertical flue tubes
Horizontal Return Tubular boiler
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
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 steamboat or steamship, sometimes called a steamer, is a ship in which the primary method of propulsion is steam power, typically driving a Propeller A locomotive boiler with a wide firebox may have arch tubes or thermic syphons. These both increase the heating surface and give additional support to the brick arch. 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 (aka twin flue) 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 generally results in a more compact design and can do away with a whole pass of tubes.
Package boiler
Another type is the "package" boiler. This comes from the practice which evolved in the early to mid 20th century when the units were delivered to site with insulation, electrical panels, valves and gauges already fitted. This was in contrast to earlier practice where little more than the pressure part was delivered and the ancillary components were fitted on site.
Taper boiler
Locomotive boilers are often tapered from a large diameter at the firebox end to a smaller diameter at the smokebox end. A smokebox is one of the major basic parts of a steam Locomotive. This reduces weight and improves water circulation.
Operation
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 tube. The hot gases are directed along a series of fire tubes, or flues, that penetrate the boiler and heat the water thereby generating saturated 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. See Superheating for the physics process A superheater is a device in a Steam engine that heats the steam generated by the The superheated steam is directed to the steam engine's cylinders or very rarely to a turbine to produce mechanical work. A steam engine is a Heat engine that performs Mechanical work using Steam as its Working fluid. A steam turbine is a mechanical device that extracts Thermal energy from pressurized Steam, and converts it into useful 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. A boiler is a closed vessel in which Water or other Fluid is heated A flue gas stack is a type of Chimney, a vertical pipe channel or similar structure through which Combustion product gases called Flue gases are exhausted In all steam locomotives, since Stephenson's Rocket, additional draught was supplied by directing exhaust steam from the cylinders into the smokestack through a blastpipe, to provide a partial vacuum. George Stephenson ( 9 June 1781 &ndash 12 August 1848) was an English Civil engineer and mechanical engineer Replica Rocket and coachjpg|thumb|right|A replica coach and Rocket at the Rocket 150 event]] Stephenson's Rocket was an early Steam locomotive of 0-2-2 This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. 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 instead of a single large flue (a multi-tubular boiler). Replica Rocket and coachjpg|thumb|right|A replica coach and Rocket at the Rocket 150 event]] Stephenson's Rocket was an early Steam locomotive of 0-2-2 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. A locomotive is a railway Vehicle that provides the motive power for a Train. In Locomotives, a prime mover is the primary source of power to run the locomotive
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. Boiler explosions are catastrophic failures of Boilers As seen today boiler explosions are of two kinds BLEVE, pronounced /ˈblɛvi/ ("blevvy" is an Acronym for " boiling liquid expanding vapor explosion "
- 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. A safety valve is a Valve mechanism for the automatic release of a gas from a Boiler, Pressure vessel, or other System when the pressure A fusible plug is a threaded metal plug usually made out of Bronze, Brass, or Gunmetal.
- Stays, or ties, physically link the firebox and boiler casing, preventing them from warping.
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. The Stanley (nicknamed Stanley Steamer) was a steam-powered Automobile produced by the Stanley Motor Carriage Company In nearly 100 years since the Stanleys were first produced, no Stanley boiler has ever exploded.
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. A fusible plug is a threaded metal plug usually made out of Bronze, Brass, or Gunmetal. The correct operation of the boiler fittings, especially the water gauges and water feed mechanisms, should be confirmed. A sight glass or water gauge is a transparent tube through which the operator of a tank or Boiler can observe the level of liquid contained within An injector, ejector, steam ejector or steam injector is a pump-like device that uses the Venturi effect of a converging-diverging Steam pressure should be raised to the level at which the safety valves lift and compared with the indication of the pressure gauge. A safety valve is a Valve mechanism for the automatic release of a gas from a Boiler, Pressure vessel, or other System when the pressure
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. [2] 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. [3]
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. Components subject to fouling The following lists examples of components that may be subject of fouling and the direct effects of fouling heat exchanger surfaces 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. Asbestos is a group of Minerals with long thin fibrous Crystals The word "asbestos" (῾ἀσβεστος is derived from a Greek adjective Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly [3] 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. Work hardening, strain hardening, or cold work is the strengthening of a material by macroscopically speaking plastic deformation (which has the Annealing, in Metallurgy and Materials science, is a Heat treatment wherein a material is altered causing changes in its properties such as strength 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. The term thermal insulation can refer to materials used to reduce the rate of Heat transfer, or the methods and processes used to reduce heat transfer 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. [3]
External links
- A locomotive boiler
- A "hot" boiler washout in pictures
- Picture gallery showing boiler internal features and defects.
- BS EN 12953 is the relevant contemporary standard, which supersedes BS2790.
References
- ^ General Ship Layout
- ^ Bell, A M (1957): Locomotives, seventh edition, Virtue and Company, London.
- ^ a b c Unknown authors (2005):The management of steam locomotive boilers. Health and Safety Executive, Sudbury, Suffolk, England. The Health and Safety Executive (HSE is a United Kingdom Non-departmental public body.
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