Boiler Terminology


refers to the main sheets of metal from which the boiler is constructed. Typically rolled steel which is either overlapped and riveted at the join or in more modern units seam welded to make a cylinder.

Boiler Heads

The end plates of the boiler.

Tube Sheets

The steel plates at each end of the boiler which hold the fire tubes or flue pipes.

Fire Tubes

The fire tubes carry fire, heated gases, through the water space to heat the water. In the case of a return flue boiler there is one or more large tubes known as flues which carry the spent gases (smoke) back to the stack above the smoke box.


The space at one end of the boiler, or under the boiler, where the fire is made. Typically surrounded by a double wall with water filling the space between the walls. All flat surfaces have to be held and braced by the stay bolts lest the box warp under the intense heat. James Hansen added that the flat surfaces are held by stay bolts- not bars. Stays are actually threaded their entire length, the holes they go into are threaded with a very long tap so the thread remains in pitch, and the stay is screwed into both surfaces, then riveted and beaded on both sides. Stay bolts keep parallel surfaces parallel to one another.

Water Leg

This is the space to the sides of and sometimes below the firebox containing water. Typically the sediment and mud gathers within this space. See Mud Ring.

Crown Sheet

The Crown Sheet is the steel plate immediately above the fire, it separates the fire from the water and is typically not double walled. This is the hardest working part of the fire box as it is heated the most. The sheet should never melt as a skilled operator will always keep it covered in water. Should it become uncovered at any time it is likely that the boiler will fail unless the operator cools the unit in time.

Fusible Plug

James Hansen added that the crown sheet Is fitted with a fusible plug. This plug is in any ASME boiler and most others even if not built to code, as it is really stupid to not have one. It consists of a brass threaded plug with a tapered hole filled with tin. When the plug gets hot, the tin melts and the steam and water puts the fire out. This is the most basic safety device on any boiler. Since top of the plug is actually into the water space by 1" (2.54cm) it should melt out long before the crown sheet is uncovered.

Smoke Box

The smoke box is at the cooler end of the fire/flue tubes away from the fire. The smoke accumulates here before being vented out the stack in the conventional boiler or passing through a return flue to the stack in a return flue boiler. The space is provided to both keep the non fire end of the boiler warm and to collect cinders and sparks before they vent out through the stack.

Steam Dome

Steam Engines need hot dry steam to operate effectively without damage. Taking steam from near the water picks up very wet steam and may even suck up water which (being uncompressible) would destroy a steam engine. The Steam Dome is a space as high as possible above the boiler but directly open to the boiler sometimes with a baffle in place to stop splashing. The steam from this area is as dry as it can be without external help - see Super Heater.

Mud Ring or Mud Drum

A typically cylindrical shaped space at the bottom of the water space. Sediment, mud, and other impurities gather there. There is a special valve designed to vent the accumulated rubbish before it can become baked onto the boiler plate. See blow down. Mud drum refers to a water tube boiler, the lowermost drum, mud ring, to an upright. In a locomotive style boiler the blow down is at the lowest point of the boiler and sometimes there is another valve in the water space around the firebox to be used when the boiler has cooled off.

Man Holes

Every large boiler has at least one man sized hole where the boiler space can be opened and a man enter to clean and repair the unit. This hole must not be opened when the boiler is under pressure, and typically cannot be as the boiler pressure keeps it closed. Usually the man hole is a plate inside the boiler which is held on by clamps on the outside. The pressure inside pushes on the plate helping it to seal.

Hand Holes

Almost all boilers of any size have at least one hand hole to allow the boiler to be inspected and cleaned with a pressure or force hose. There may also be hand holes dedicated to inspection in areas of the boiler in which problems often occur. Such as the mud ring. In a more modern boiler there are hand or man holes to allow inspection of the entire interior surface.

Boiler Jacket

is an insulating layer around the entire boiler. Typically an insulating material such as hair, plaster, mud, wood or more recently fibreglass wool is sandwiched between the boiler plate and a thin metal sheet. The jacket keeps the boiler warm reducing lost heat.

Steam Jacket

A Steam Jacket is a space around the engine cylinder which is filled with live steam to stop the engine cooling when paused and to reduce lost heat. If the engine cools, the steam in it condenses which can be disastrous under compression.

Ash Pit

The ash pit is the space where ashes accumulate under the fire. The ashes fall through the fire grate into a (often) removable box.

Dead Plates

Dead plates replace some or all of the fire grate. Typically only found in special purpose boilers such as those burning straw, oil, or natural gas and other non solid fuels. The dead plates stop cold air from being sucked into the flues cooling the water.


The Grate is a series of bars or narrow plates in the bottom of the fire box. The fire/fuel sits on the grate and burns. The ashes fall through the grate into the ash pit.

Forced Draft

When first starting a fire, or when the engine is not working very hard, forced draft may be required to keep the fire burning and the flues free of still gases. The draft is usually induced by blowing steam up the smoke stack sucking air through the fire and through the flues. Forced draft is also induced by venting the exhaust steam into the stack via nozzles in the same manner as you describe, causing SIGNIFICANT induced draft. This might be done on say a steam engine running a sawmill. When working hard continuously a hotter fire is needed to keep the steam up. Most traction engines had their exhaust directed into the smokestack to cause draft.

Heating Surface

This is the entire surface area of the boiler which is exposed directly to hot gases and the radiant heat of the fire.

Steam Space

The space within the boiler which contains no water and accumulates steam whenever the boiler is in operation. The size of the steam space has to be well matched for the engine being powered from the boiler. Not enough and the engine will be starved, too much and the heat is simply wasted.

Diaphragm Plate

The Diaphragm plate or more correctly bezel is a plate with many small holes in the bottom of the steam dome typical to the locomotive style boiler. The plate attempts to stop water rising into the steam dome during hilly travel or splashing from rough roads.

Safety Valve (pressure relief valve)

The Safety Valve is a device designed to safely release excess pressure within a boiler by opening at a preset pressure limit. If boiler pressure reaches this limit, the valve opens and steam vents until the pressure has been reduced. A skilled operator will rarely produce so much pressure that the valve opens. One of the keys to being a good operator is to only produce as much steam as required for any given job.

The safety valve can actually make a low water situation worse by using up water from the boiler and making it even hotter and making even more steam until it gets to the point where the valve is unable to vent the steam as fast as it is made meaning pressure increases again, possibly to dangerous levels if left unchecked. Of course, at times it is impossible to stop the valve lifting, such as when an operator is running an engine under load on a sawmill and the belt slips off or the mill operator stops a cut quickly the steam that was being used in the engine suddenly isn't and may just escape through the safety instead if the pressure was close to the limit. James Hansen offered some extra information on safety valves...

Pressure should be brought up to the point of release, and observed for results daily. What is the use of having a safety device whose operation is unknown. It could be stuck. Doing so once a day under a controlled condition will test it under controlled conditions, and ensure it does not stick on a less than attentive operator (no-one that we know right?) If it does not pop, and giving the hand lever a bump does not cause it to release, the boiler pressure should be reduced, and the engine taken out of service until the cause can be determined- either a faulty valve or maybe a faulty pressure gauge indicating too high. This should be entered into the engine log by the operator as being done.

Super Heater

Some boilers have a Super Heater fitted. This is a simple device where the live steam from the boiler is passed through the smoke box to further heat and dry the steam. After the super heater the steam then passes directly to the engine