The Steam Boiler

All steam engines (and other steam applications) have something in common. They all need a steam generating plant which is usually in the form of a steam boiler where water is heated in a large vessel and steam taken from the vessel to the steam engine itself. There are five main kinds of steam generator:

The Fixed Boiler. This sort of unit is semi permanently imbedded into a brick structure which is designed to both support the boiler and circulate hot gases around the boiler to heat the water. This sort of boiler would typically be found in industry supplying one or more stationary steam engines. They can still be found today as heating plant for the building where they used to service engines. The firebox is below the boiler in front. Typically they were manually stoked and burnt either wood or coal. More modern units burnt oil or natural gas. A common feature of this sort of fixed boiler is the large access hole into the boiler space where a man can enter (when the unit is cool!) to clean and repair.
The Locomotive Boiler. The most seen by the public, and hence seemingly most prevalent steam application is the mobile steam plant, whether it be a portable engine, traction engine, truck, bus or tram. All of these units have in common that they are mobile and cannot be tied to a fixed boiler. Arguably the most common type of boiler used in mobile applications is the Locomotive type. This boiler has the firebox and boiler as an integral unit with metal stays supporting the whole thing and holding all the parts together. The firebox is typically at the rear of the boiler and is fed by a fireman with coal, wood, or straw. The hot gases from the fire circulate through fire tubes within the boiler itself to emerge into the smoke box in front of the boiler and out the stack (chimney). The first picture shows the general layout of the locomotive boiler and the second picture shows some detail of tubes and stays. The tubes are the pipes running through the water space and the stays are the bolts (all thread bars) holding the various parts together.
The Return Flue Boiler Similar in initial appearance to the locomotive type of boiler, the return flue has two major differences, visually different is that the chimney is in the back of the boiler above the rear of the firebox directly in front of the operator. Internally instead of having many small tubes to carry the hot gases through the water space, the hot gases circulate under the water space where they travel to the front of the boiler and back towards the rear with several large return flue pipes. The gases then discharge out through the smoke box and stack. The return flue boiler though not as common in preservation as the more popular locomotive type is thought to have both greater efficiency and safety over the locomotive type. In the return flue it is more difficult to expose metal not covered by water on hills making failure less likely. The first picture shows the general layout and the second picture gives a better idea of the flue layout.
The Vertical Boiler The vertical boiler is common to steam trucks, buses, trams and very portable equipment such as donkey engines used for sawmilling and dockwork. The design of the vertical tube boiler lends itself well to rough handling while in steam making it ideal in situations where the work plant has to be moved frequently over non accomodating ground. They are popular for cargo trucks and people moving vehicles because they take up less room than the convential horizontal boiler styles. They are said to be the safest of all because it is almost impossible to expose bare uncovered metal to the fire. Their propensity for quick steaming in as little as 20 minutes made them popular with fire engine companies such as Shand Mason. The fire could be lit and steam raised while the boiler and pump were hauled to the fire in as little as twenty minutes. The fire is built in the bottom of the unit, with the hot gases circulating upwards though vertical tubes where they pass out through the stack. Most boilers have steam nozzles within the smokestack to help generate draft to keep the fire burning hot and bright. Vertical boilers rarely have or need this.
The Marine Boiler The Marine Boiler in large vessels is typically a cross half breed between the fixed boiler and the locomotive boiler. In small vessels it can often be a vertical boiler adapted specially.
The Water Tube Boiler John Byers and James Hansen reminded me about Water Tube Boilers. John pointed out that Westinghouse was one traction manufacturer that employed them. Most modern powerplants use them and they were popular in marine practice. Babcock and Wilcox units were/are very popular...they are still in business. On that happy day when I get my steam car project going, I hope to use a water tube along the lines of a Beckmann boiler: www.steamboating.net/boilers.html. James added The water tube design is mostly used in stationary plants, but did see usage in one traction engine I know of, the westinghouse. The biggest advantage of a water tube boiler is that it is a very safe design. It consists of a header drum, to which the water tube pipes connect. This drum and the tubes are usually surrounded by an insulating jacket or brickwork, and the fire and products of combustion are directed so as to pass through the tubes a multiple number of times, either horizontally or vertically. There is another drum, a mud drum that receives the tubes on the bottom, and as it's name suggests, the mud, or particulate sediment of carbonates in the boiler feedwater settle out into this drum and can be removed when the boiler is blown down. The vast majority of stationary plant boilers be they electrical gen stations or industrial plants use this type of boiler. Biggest advantage is safety. If a tube blows, it is extremely rare to have a boiler explosion ensue. That, and the design lend itself to be self cleaning- the tubes get very little lime deposit compared to the horizontal fire tube boiler. Also, if you replace the tubes, the majority of your heated surface gets renewed, unlike a locomotive boiler, however, that said, usually a new boiler is purchased these days, as they last so long that by the time the tubes are gone, so are the drums. and it is more cost effective and liability limiting to replace with new anyway.
The Flash Steam Boiler Not strictly speaking a boiler at all, the concept of flash steam has been around since the 1800's and is common in instant on demand hot water systems today. The idea is to heat a long single pipe with a fire and pass only enough water through the pipe to create the amount of steam required at that moment by the engine. As only a small amount of water is heated at a time, this arrangement is efficient and effective. However it takes a skilled operator and is really only suited to smaller engines. This form of steam has been used effectively on such bizarre creatures as steam motorcylces. James Hansen added Flash boilers were also used very successfully in Stanley and White steam cars, to name only two. I have heard anecdotally that Henry Ford was originally torn between using steam or gasoline in his model T. Gas engines were new, and not well refined, while steam was essentially at it's peak, and worked extremely well and efficiently. The disadvantage was the time required to steam up- 5 minutes, but still, the gas engine was relatively instant. I say relatively, because I own a T. There is nothing instant about a model T... Flash boilers have the advantage that they need little attention, and produce very dry steam, as the hottest part of the tubing effectively acts as a superheater. The Stanley brothers held the land speed record for some time with their modified steam car. The boilers were hydro tested in public demonstrations to 1000psi. Nothing unsafe about them. [:-)] Still used in steam jennies (cleaners) to my knowledge, just a copper coil with fire surrounding- poof you have steam. that's why they're called "flash" boilers, as the water tubes are at a heat sufficient to raise the feedwater to steam long before the water reaches the end of it's travel through the coil.

Some Boiler Terms

Shell

refers to the main sheets of metal from which the boiler is constructed. Typically rolled steel which is either overlapped and rivetted 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.

Firebox

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 seperates 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.

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 crownsheet is uncovered. The operator should not draw or pull the fire. You cannot safely remove the fire through the little bitty firebox door in a hurry when there is a low water condition. If the water is indeed low, and you are moving, the sloshing will keep the sheet from getting overheated. When the condition is noticed, it should be double checked with the trycocks in case the gauge glass or water column is plugged, or the glass is completely full. The moving engine should be backed into a hole, to get the back end down, so water covers the crownsheet. If it does, no worries. If not keep reading. If stationary, you need to smother the fire by covering or plugging the smokestack, and closing all sources of draft. If no oxygen gets in, no fire burns, and no further heat is made. Stopping the products of combustion from getting out does this as effectively as stopping off the draft. Doing both is insurance, but the best thing to do. Poking about in the firebox stirs the fire up, making it hotter, and exacerbating an already bad situation, perhaps from bad to blinding when your head is in the firebox door and the fusible plug melts, blowing live steam, and burning wood and ashes out the door.. The term "pulling the fire" applies to stationary plant operation when pulling the fire involves turning off a valve.
I questioned the possibility that sloshing water onto the hot crown sheet might cause an explosion and James offered some further information in a later message to clarify what he said and help my understanding - I've included this later information here as other people may have misunderstood in the same way I did, and it is vitally important that this particular aspect of steam boiler operation is done right every single time:
If you are moving, and there is no water showing in the gauge glass then the sloshing of the water will keep the crownsheet from getting red hot .....etc. If you are stationary, this is a no deal at all. Stationary engines working with a low water condition, plugged fusible plug, and hard fire should not be moved AT ALL. The fire should be immediately smothered, no engine movement at all is allowed, or what you describe can happen. BLEVE or Boiling Liquid Expanding Vapor Explosion is no school of thought or theory. Movement or non movement does to an extent dictate what you should do. If you are unsure, just make the fire stop producing heat by smothering it, go away and let it cool, then inspect for damage later. You might want to counsel others to stay away as well.... Boiler explosions are rare, but historically, in the majority of cases were caused by low water conditions. Mechanical failure from overpressure due to prv failure would probably be second, but I have no data on that.

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 convential 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 typcially 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 sandwhiched 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 disasterous 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.

Grate

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. When I said SIGNIFICANT, I meant it. James Hansen offered that the ZZ Geiser I ran would pull the fireman's hat off if he got too close when feeding the beast with the firebox door open.

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.

Diaphram Plate

The Diaphram 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.

I received some good comments on this article from John Byers and James Hansen and I'm greatful to both guys for helping make the article better and more accurate. I'm always happy to receive constructive criticism.

Last modified Sunday, 20-Jul-2003 15:28:00 BST

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