Glossary of boiler terminology

Glossary of boiler terminology

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

Terms which relate solely to boilers used for space heating or generating hot water are identified by (HVAC).

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See also


Definitions Points of Interest
  • Ashpan: A container beneath the furnace, catching ash and clinker that falls through the firebars. This may be made of brickwork for a stationary boiler, or steel sheet for a locomotive. Ashpans are often the location of the damper. They may also be shaped into hoppers, for easy cleaning during disposal.

  • Blow-down cock: a valve mounted low-down on the boiler, often around the foundation ring, which is used to periodically vent water from the boiler. This water contains the most concentrated precursors for sludge build-up, so by venting it whilst still dissolved, the build-up is reduced. When early marine boilers were fed with salt water, they would be blown-down several times an hour.
  • Blower: the blower provides a forced draught on the fire, encouraging combustion. It consists of a hollow ring mounted either in the base of the chimney or on top of the blastpipe. Holes are drilled in the top of the blower ring, and when steam is fed into the ring, the steam jets out of the holes and up the chimney, stimulating draught.[1]
  • Boilermaker: a craftsman skilled in the techniques required for construction and repair of boilers. Historically known as a boilersmith.
  • Boiler suit: heavy-duty one-piece protective clothing, worn when inspecting the inside of a firebox for steam leaks, for which task it is necessary to crawl through the firehole door.
  • Boiler ticket: the safety certificate issued for a steam (locomotive) boiler on passing a formal inspection after a major rebuild, and generally covering a period of ten years. Additional annual safety inspections must also be undertaken, which may result in the locomotive being withdrawn from service if the boiler requires work. When the ticket "expires" the locomotive cannot be used until the boiler has been overhauled or replaced, and a new ticket obtained.[citation needed]
  • Brick arch: A horizontal baffle of firebrick within the furnace, usually of a locomotive boiler. This forces combustion gases from the front of the furnace to flow further, back over the rest of the furnace, encouraging efficient combustion. The invention of the brick arch, along with the blastpipe and forced draught, was a major factor in allowing early locomotives to begin to burn coal, rather than coke.
  • Bridge clamp:
Steam locomotive blastpipe
Section through manhole and bridge clamp
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Definitions Points of Interest
  • Carryover: the damaging condition where water droplets are carried out of the boiler along with the dry steam. These can cause scouring in turbines or hydraulic lock in cylinders. The risk is accentuated by dirty feedwater.
See also priming.
  • Check valve: or clack valve, from the noise it makes. A non-return valve where the feedwater enters the boiler drum. They are usually mounted halfway along the boiler drum, or else as a top feed, but away from the firebox, so as to avoid stressing it with the shock of cold water.
  • Cladding: The layer of insulation and outer wrapping around a boiler shell, particularly that of a steam locomotive. In early practice this was usually wooden strips held by brass bands. Later and modern practice is to use asbestos insulation matting (or other, less hazardous, fibres) covered with rolled steel sheets. The outer shape of the cladding is often a simplification of the underlying boiler shell.
Also termed "clothing" in LMS practice.
  • Crinolines: The framework of hoops used to support cladding over a boiler. Named from the similar hoops under a crinoline skirt.
  • Crown sheet: The upper sheet of the inner firebox on a locomotive boiler. It is the hottest part of the firebox, and sometimes at risk of boiler explosion, should the water level drop and the crown sheet be exposed and thus allowed to overheat. Supported from above by complex stays.
  • Damper: An adjustable flap controlling the air admitted beneath the fire-bed. Usually part of the ashpan.
  • Dome: a raised location on the top of the main boiler drum, providing a high point from which to collect dry steam, reducing the risk of priming.
  • Downcomer: large external pipes in many water-tube boilers, carrying unheated cold water from the steam drum down to the water drum as part of the circulation path.
  • Drowned tube: Either a fire-tube or water-tube that is entirely below the water-level of the operating boiler. As corrosion and scaling is most active in the region of the water-level, this reduces wear and maintenance requirements.
  • Exhaust injector: a feedwater injector that economizes on steam consumption by using waste steam, such as engine exhaust.
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Definitions Points of Interest
  • Firebar: Replaceable cast-iron bars that form the base of the furnace and support the fire. These wear out frequently, so as designed for easy replacement.
    See Rocking grate
  • Firebox
  • Firedoor
  • Fire dropping: Emptying out the remains of the fire after a day's work. A time-consuming and filthy task; labour-saving ways to improve this became important in the final days of steam locomotives.
  • Fire-tube:
  • Flue: A large fire tube, either used as the main heating surface in a flued boiler, or used as enlarged firetubes in a locomotive-style boiler where these contain the superheater elements.
  • Foundation ring: The base of the firebox, where the inner and outer shells are joined.
  • Fusible plug: A safety device that indicates if the water level becomes dangerously low. It melts when overheated, releasing a jet of steam into the firebox and alerting the crew.
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Definitions Points of Interest
  • Galloway tubes: tapered thermic syphon water-tubes inserted in the furnace of a Lancashire boiler.
  • Gauge glass: part of the water level gauge, which normally consists of a vertical glass tube connected top and bottom to the boiler backplate. The water level must be visible within the glass at all times.[2]
  • Grooving: erosion of a boiler's plates from the internal water space, particularly where there is a step inside the shell. This was a problem for early boilers made from lapped plates rather than butted plates, and gave rise to many boiler explosions. In later years it was a problem for the non-circular water drums of Yarrow boilers.
  • Handhole: A small manhole, too small for access but useful for inspection and washing out the boiler.
See: mudhole
  • Injector: a feedwater pump without moving parts that uses steam pressure and the Bernoulli effect to force feedwater into the boiler, even against its pressure.
  • Klinger gauge glass: A form of gauge glass where the water level is visible through a flat glass window in a strong metal frame, rather than a cylindrical tube. These were popular with some operators, and increasingly so for high pressure boilers.
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Definitions Points of Interest
  • Manhole: an oval access door into the boiler shell, used for maintenance and cleaning. Manholes are sealed with a removable door from the inside. As they are oval, this door may be turned and lifted out through the hole. Doors are clamped in place from the outside with one or two bridge clamps spanning the hole and tightened down with a nut on a stud. As the cutting of a manhole weakens the boiler shell, the surrounding area is strengthened with a patch.

  • Mud: a sludge of boiler scale particles, precipitates and general impurities that builds up in the lower parts of a boiler. Mud reduces water circulation and so a local buildup may lead to localized overheating and possibly explosion.
  • Mud drum: a water drum, particularly one mounted low on the boiler whose function is primarily to trap mud from circulation.
  • Mudhole: A small manhole, too small for access but useful for washing out the boiler, either as an inlet for a hose or as a drain for removed mud.
See: washout plug

  • Priming
  • Regulator
  • Rocking grate: An advanced form of firebar, where sections of the grate may be rocked or tipped to either break up clinker within the fire, or to drop the fire after a day's work.
Mudhole and bridge clamp on a vertical boiler
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Definitions Points of Interest
  • Safety valve: an automatic valve used to release excess pressure within the boiler.
  • Scale dissolved minerals from hard water that precipitate out in the steam space around the water-level. Where this scale falls to the bottom of the boiler and mixes with other contaminants, it is termed mud.
  • Scum valve: A blow-down valve mounted at the water-level of a boiler, used to blow-down lighter oily or foamy deposits within a boiler that float on the water-level.
  • Sludge, another term for mud.
  • Smokebox: an enclosed space at the extremity of a fire-tube boiler, where the exhaust gases from the tubes are combined together and pass to the flue or chimney.
  • Steam drum: a cylindrical vessel mounted at a high point of a water-tube boiler, where dry steam may separate above the water level, so that it may be drawn off without risk of priming.
This is similar to the function of a dome in a fire-tube boiler.
  • Steam & water drum: a steam drum that contains a turbulent mixture of steam and water, with a substantial part of this being water. The terms are used somewhat interchangeably.
  • Steam drier, a form of mild superheater that adds additional heat to wet- or saturated steam, thus ensuring that all water in the steam has been evaporated, thus avoiding problems with water droplets in the cylinders or turbine. Unlike the superheater, the steam drier does not attempt to raise the temperature of the steam significantly beyond the boiling point.
  • Suction valve: an automatic non-return valve, which opens when the boiler is at less than atmospheric pressure. This avoids any risk of vacuum collapse, when a hot boiler is allowed to cool down out of service.
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Definitions Points of Interest
  • Three-drum boiler: A generic term for water-tube boilers of the Yarrow pattern.
In Royal Navy practice, a reference to the specific Admiralty example of this.[3][4]
  • Top-feed: in locomotive boilers, a feed water check valve placed on the top of the boiler drum. This encourages rapid mixing of the cold feedwater with the hot steam, reducing the risk of thermal shock to the heated parts of the boiler.
  • Water level:
  • Water-wall: a furnace or other wall within a boiler enclosure that is composed of numerous closely set water-tubes. These tubes may be either bare, or covered by a mineral cement.
  • Washout plug: A small mudhole used for washing out the boiler. Plugs, as compared to mudholes, are usually screwed into a taper thread, rather than held by clamps.

  • Water drum:
See: mud drum
  • Water-tube boiler: a boiler whose primary heating surface is composed of many small tubes, filled with water. Tubes of 3 inch diameter and above are termed "large-tube" boilers. Later water-tube designs used smaller "small-tubes" of 2 inches or less.
Dismantled steam locomotive, showing the smokebox tubeplate
Inside a water drum, showing the tube ends
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  1. ^ Semmens, P.W.B.; Goldfinch, A.J. (2003) [2000]. How Steam Locomotives Really Work. Oxford: Oxford University Press. p. 79. ISBN 0 19 860782 2. 
  2. ^ Handbook for Railway Steam Locomotive Enginemen. Hersham: Ian Allan. April 2006 [1957]. pp. 49–52. ISBN 0 7110 0628 8. 0604/A1. 
  3. ^ BR 77 Machinery Handbook. later replacement for the Stokers Manual. Admiralty, via HMSO. 1941. pp. 12–13. 
  4. ^ Naval Marine Engineering Practice. later replacement for the Stokers Manual. vol 1. Royal Navy, via HMSO. 1971 [1959]. p. 4. ISBN 11-770223-4. 
  5. ^ Kennedy, Rankin (1912). The Book of Modern Engines and Power Generators. VI. London: Caxton. 
  6. ^ Milton, J. H. (1961) [1953]. Marine Steam Boilers (2nd ed.). Newnes. 
  7. ^ Borthwick, Alastair (1965). Yarrows: the first hundred years.. Yarrows. 
  8. ^ Stokers Manual ((1912 edition) ed.). Admiralty, via HMSO, via Eyre & Spottiswoode. 1901. 
  9. ^ Cisin, Harry G.. Modern Marine Engineering. New York: Van Nostrand. pp. 84. 
  10. ^ Brassey, Thomas Allnutt. The Naval Annual. Brassey. pp. 118–119. ISBN 1-4212-4178-1. 

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