This article is not intended to provide all the information needed to design and build actual boilers. Many skills and cross checks are needed to ensure the safe design and construction of pressure vessels. This article is to promote the understanding of steam technology, and to provide a useful framework for writing stories set in the 1632 verse.
Steam can REALLY KILL YOU
Of all the substances on earth, water seems to be the most adaptable to transferring power from heat to work. When heated water boils and creates steam, the volume of water increases some 745 times (more according to some sources). This can power a simple engine capable of performing useful work with a relatively small fuel use. However, steam has its dangers. Steam will kill you if you give it the slightest chance. There are numerous things that must be done right every time. When water is enclosed and heated, it stores energy in the water. Steam will form in the “bubble” at the top of the enclosure, and pressure will increase. As the steam pressure goes up, the amount of heat required to create additional pressure increases. Also, more energy is stored in the water. As the pressure is released the “superheated” water changes to steam. If the pressure is released all at once, all the water will become steam at once. Thus 100 gallons of water becomes at least 9,950 cubic feet of steam. Most standard boilers carry from 500 to 1,000 gallons of water. That would give us 49,750 to 99,500 cubic feet of steam.
In a locomotive, this sudden expansion normally separates the boiler from the frame and has been known to throw the boiler hundreds of feet from the wreck site. Needless to say, the engineer and fireman are almost always scalded to death, and the steam cloud can drift over the train and kill every one else on board.
Steam has several hazards. First there is thermal damage. This is heat energy contained in the steam transferring to a victim. Then there is the shock wave caused by the sudden expansion of the superheated water to steam. This can cause damage similar to a chemical explosion, with shrapnel and damage to persons and structures. The cloud of steam can spread and cause further thermal damage. In addition, the steam cloud is denser than air and hugs low spots and fills spaces like rooms and compartments This cloud also excludes oxygen and will cause death from suffocation even if the cloud is cool. Lastly, a steam leak from an otherwise intact boiler can, in some circumstances, exit as a high pressure stream that is colorless and shows no vapor until it cools and spreads. A steam jet like this can cut off arms and legs or anything else exposed to it. These are best found with a broom stick or 2×4. (The steam jet will cut the stick or 2×4 in pieces or jerk it right out of your hand.)
From an NTSB report:
On June 16, 1995, the firebox crownsheet of Gettysburg Passenger Services, Inc., steam locomotive 1278 failed while the locomotive was pulling a six-car excursion train about 15 mph near Gardners, Pennsylvania. The failure resulted in an instantaneous release (explosion) of steam through the firebox door and into the locomotive cab, seriously burning the engineer and the two firemen. This accident illustrates the hazards that are always present in the operation of steam locomotives. The Safety Board is concerned that these hazards may be becoming more significant because Federal regulatory controls are outdated and because expertise in operating and maintaining steam locomotives is diminishing steadily. As a result of its investigation, the National Transportation Safety Board issued safety recommendations to the Federal Railroad Administration, the National Board of Boiler and Pressure Vessel Inspectors, and the Tourist Railway Association, Inc.
Safety appliances are those devices that protect, warn, and show the condition of a boiler in operation. If ignored and not maintained, the boiler will—not may— will eventually kill the crew and destroy the locomotive.
The first of these safety devices are the gauge cocks. These are three valves connected into the backhead of the boiler at set heights related to the crown sheet. The lowest is three inches above crown sheet height, the next an exact distance above it and the third above the second. The gauge cocks cannot be allowed to become clogged since they give the best warning as to the true level of water in the boiler. They are manually operated and must be checked before operation and at specific times during operation.
The next safety device is the water glass. This is also connected directly into the boiler, at a height so that the bottom of the glass is above the crown sheet. The 1999 Federal Regulation requires two water glasses on all steam locomotives.
Another safety appliance is the safety release valve (also known as pop valve). This is a valve set into the top of the boiler that will open if the steam pressure in the boiler exceeds the operating pressure. Usually there are three, each set at two to three pounds more than the first, and each able to vent off all the steam in the boiler.
Additionally, each boiler must have two separate ways of feeding water into it, each capable of forcing water in against operating pressure. Also of great importance is the steam gauge. This indicates the pressure in the boiler and is used to check the performance of the pop valves.
Finally, the crew must use the safety appliances. Sight glasses and gauge cocks that are clogged or not watched don’t give warning. Pop valves that are tied down or jammed won’t relieve pressure, and water injectors that don’t inject won’t raise the water level. It is worth going to the accident report and the upgraded rules. Please see note one at the end of this article for links and more information.
Design of a Boiler
Boilers are devices that allow the transfer of energy, in the form of heat to water. The water transforms to steam and is released in a controlled manner to another device where it performs work.
Another class of boiler is used to transfer heat to environmental areas, either industrial or residential heating. In this form, the water is heated to a temperature below boiling, and distributed via pipes and radiators in the spaces to be heated.
What a Boiler Needs
To successfully convert energy to work, the boiler needs a heat source, a sealed container for the water, a method to transfer the heat to the water, a way to add more water, a way to control the combustion gasses, and a way to remove the steam in a controlled manner.
Heat is usually supplied by combustion. This combustion is performed in the firebox. A firebox is an enclosed space that has inlet ports for fuel and air so as to support combustion. Usually the firebox is surrounded on the top and four sides by a water jacket. The water jacket and top account for 40% or more of the heat transfer. The back of the boiler or firebox is called the backhead, and is composed of an inner and outer wall connected by staybolts, large metal rods that provide support to the parallel walls. The sides of the firebox are called the legs and are also composed of inner and outer walls supported by staybolts. The front of the firebox is composed of an inner wall and an outer wall supported by staybolts. Of note is the inner wall (called the rear sheet) that extends upward and is pierced to support the flues that allow the combustion gasses to flow to the front of the boiler and allow more heat transfer. The top of the firebox is composed of an inner wall and an outer wall connected again by staybolts. The inner wall is called the crown sheet, and is attached to the inside walls of the backhead, the legs, and the rear sheet. The outer wall of the firebox is called the wrapper, and is connected to the outside walls of the backhead, legs, firebox front, and the drum of the boiler. The sides of the firebox that are exposed to direct flame are lined with firebrick, and a damper arch is sometimes also included. The bottom of the firebox is composed of grates (coal) or by a solid sheet covered by fire brick (oil). Air is provided by openings in the grate or bottom sheet.
The main body of the boiler is called the drum, and attaches to the firebox and the front sheet. Within the drum, between the front and rear sheets, are the flues or tubes. On top of the drum is a dome or chamber where the dry pipe collects the steam and carries it out of the boiler. Finally, the top half of the front and rear sheet may be supported by braces from the sides of the drum to the inside of the sheet. The dry pipe extends into the steam collection dome and often contains the steam supply valve. The dome is also often used as the manhole or access point into the boiler for maintenance. Connected to the front of the drum is the smoke box, that has the smoke stack and spark arrester. Exhaust steam is released up the smoke stack to promote draft.
What a Boiler Burns
Combustion requires fuel. Wood, coal, natural gas, and oil are the most common fuels used although almost anything combustible can be used. Firebox design depends greatly on fuel type. Coal and wood require grates and ash pans, with air coming from below to support the fire. Oil is normally injected from the front bottom of the firebox, and sprays from a duck foot nozzle impelled by steam. Air is supplied by vents built into a plate closing the bottom of the firebox. Oil also needs heat applied to the fuel tank because Bunker C oil at room temperature is about the same thickness and consistency of peanut butter. To start combustion with this stuff “house steam,” steam supplied by the shop or maintenance facility, is needed.
Boilers are made mostly of metal with iron and steel predominating. Copper is also used for smaller units, but cost usually prevents its use for larger installations. Early boilers, 1745 to the 1830s, were mostly iron—either cast or wrought plate.
Cast iron is still in common use for stationary boilers. Natural gas burning, cast iron sectional boilers are probably the most efficient available today, often exceeding 98% fuel to heat to water transfer. A cast iron boiler is made by casting the water containment section and placing it over the firebox.
Wrought iron plate was made by taking cast iron pigs (billets of cast iron from the foundry) and heating them in furnaces called soaking pits. Once heated to near melting temperatures, the pigs were run between rollers and formed into plates. Often many passes were required, with the plates folded in half, fluxed and welded together with the rolling mill. Once the plates are of the desired shape, size, and malleability, they are formed on rollers to make the firebox, drum, sheets, and flues. Holes are then punched preparatory to riveting
Steel is prepared much the same as wrought iron, but is preferred as it has better strength and resistance to damage. Also staybolts and dimensions can be lighter, reducing the weight of the boiler.
Copper is also formed by rollers but must have larger dimensions for the same capacity due to the weakness of the material.
Lastly, we come to the high alloy steels, titanium, vanadium, etc. These steels are like steel compared to wrought iron, only more so. They allow even lighter dimensions for a given capacity of boiler. Sadly, the metallurgy required is advanced, and it may take years to create the physical plant needed to formulate these steels.
Boilers come in many types. Mainly they can be divided into: a) Fire tube (exhaust gasses go through the tubes to the exhaust point), b) Water tube (the tubes are connected to “drums” and are filled with water with the exhaust gasses going around the water tubes to the exhaust point), and c) Sectional boilers (where prebuilt sections are assembled, contain the water, and the exhaust gasses pass between the sections). Other boiler types exist, even fireless types, but in the main they fall into these broad categories
Fire tube boilers are as described earlier, where there is a firebox connected to the body of the boiler, with the exhaust gasses flowing through the tubes to the stack. The boiler type comes in a number of variations: vertical (firebox in the base, water drum on top, exhaust above that), locomotive style (firebox at the rear, horizontal tubes through the water drum, exhaust at the front), Scots marine (firebox contained within the water drum, exhaust through tubes also in the drum). All of these can be single- or multi-pass systems.
Vertical boilers are typically stationary, that is, used in the environmental systems of structures for heat and limited steam supply. The vertical boilers are simple to make and maintain, but are not very efficient.
Locomotive type boilers are used in stationary and mobile applications, are robust, and can be very efficient. Often called horizontal boilers, they are capable of producing steam in great quantities, and are the most common type in commercial applications where robustness and large steam production are needed.
Scots marine boilers are, as the title suggests, boilers in common use aboard ships. The fuel, typically oil or gas, is burned in a large fire tube in the base of the water drum, and the ends of the boiler are covered by doors or caps that are divided so as to reverse the flow of exhaust gasses through layers of tubes, usually two sets, so that the gasses make three passes through the boiler, hence the multi-pass name. Scots marine boilers are also very common in industrial use, (I have three at my facility) very efficient, and reliable.
Water tube boilers are made from a set of drums. The drums, usually one steam drum on top and a mud drum on the bottom, have holes in the bottom (steam drum) or the top (mud drum) where the water tubes are connected. The water tubes are not straight but curved and fill the space between the drums like spider legs. They are normally only inches apart. The whole assembly is mounted over the firebox and enclosed within an insulated case. Commonly used on large ships, they are efficient producers of steam and able to produce steam quickly due to the relatively small quantities of water (in each tube by cross section) being heated. While most common on ships, they are also used in large industrial plants where rapid steam production is needed.
Side note: I once worked on a set of five water tube boilers in a dairy in northern Utah that were thirty-plus feet tall, forty feet wide and sixty feet long. The work involved crawling inside the steam drum and using a water powered descaling drill to clean the scale off of the inside of the water tubes At the end of each day I had the “privilege” of crawling into the mud drum to remove the day’s mud gleanings. This is of interest because the drums were four feet in diameter and show the size involved. They stick in my mind, though, because that was where a local maintenance guy cut our locks on the valves and proceeded to turn live steam in on us . . . I was irate when we got out, lucky to be only lightly toasted as the water in the lower half of the boiler cooled the steam somewhat.
Cast iron boilers are made of individual castings. These are bolted together. Early designs looked much like an oil drum or a water heater. These have the firebox at the bottom of the water tank. The water around the sides of the firebox are called “water legs,” which go down to the grates.
Other cast iron boilers were made in hollow flat square sections bolted together like slices of toast on edge, and placed on the frame which has the burners and fire box beneath. Steam is withdrawn through a manifold connecting to the top of each section. Sections are ganged together by ports located in the top and bottom sides of each section. Old apartment radiators can be considered this type of sectional boiler. Gaskets, usually lead or bronze (or modernly, high temp silicon rubber), seal the sections together. Combustion gasses flow between the sections in channels and spaces provided for them when the sections were cast. The entire assembly is enclosed in an insulated shell with provision for the exhaust gasses to exit by means of a smoke stack. Unfortunately, cast iron does not respond well to sudden shock and tends to break up in mobile usage. Cast iron sections also require advanced casting methods, including cores, the handling of large molds, and manipulating large pours of molten iron.
Joint and Seam Methods
Boiler parts can be held together in a number of ways. Welding and riveting are the most common, but nuts and bolts are also used, and even drilled and tapped holes in the boiler shell are common.
Wrought iron welds well, and the primary form of welding wrought iron is hammer welding. Hammer welding, also called forge welding, is where the two pieces of metal to be welded are brought to a near molten state, (hotter than the temps used for rolling) fluxed, placed on each other, and compressed until the metal intermingles.
While hammer welds are easy in wrought iron, hammer welding steel is much more difficult. Oxidization caused by heating interferes with a good connection in the two steel parts being welded. Another problem with hammer welding is the amount of heat needed. Large components need more heat applied as the heat tends to travel and try to heat the whole part. Also, large parts can be difficult to handle as the weight is more than can be handled without machinery.
Welding can also be achieved by the application of heat in a localized spot in a short time. This heat can be created by gas torch or electrical resistance.
Gas welding, normally Oxy Acetylene, is very good for small parts (less than two inches in diameter), but has the problem of needing much more time when welding larger parts. Gas welding may have issues in the Ring of Fire, as production and storage would be difficult.
Electrical resistance welding comes in at least two forms. Stick welding, where an electrode attached to a handle uses an arc from the electrode to the work piece to create local intense heat and fuse the metal parts together. The electrode also supplies additional metal to use as filler in the joint. The second form is spot welding, where the two parts to be welded are placed between two electrodes and high current is applied. This high current creates heat which fuses the metal together. This method is most suitable for sheet metal applications.
This resistance welding can be AC or DC and can be achieved under fairly low tech conditions. As an example, one time I was on a deployment in my two and a half ton truck. We were in some rough country and managed to break a bracket needed to keep the alternator in place and operational. While I had the mask, rod, and cables, the welding “box” was on another truck. Our solution was to weld the bracket on and continue our trip. This was accomplished by hooking our cables to the terminals of the truck’s battery and making the weld. This was possible because an automotive battery has fifty or more amps, and resistance welding works well at that amperage. (We had four batteries, set up for 24vdc so we had in excess of 100 amps available). In the 1632 universe, the biggest problems will be insulating the welding cables and charging the batteries.
The best way to connect the boiler parts together is riveting. Rivets provide solid, dependable connections, have well understood properties, and are relatively easy to make. Riveting is still in common use for large steel fabrication. Even the locomotive our club is restoring (built in 1944) uses large numbers of riveted connections. Riveted joints come in a number of flavors. They are lap joints, lap joints with cover plates, and butt joints. They can be single, double, or triple riveted. The Machinery’s Handbook (mine is the 1942 11th ed.) has the layout and math for setting up these joints (pp408-422). Any of the Grantville machine shops will have copies of this book, and will probably have multiple copies as new editions come out frequently (we are up to the 27th ed).
Many areas that look like they would need complicated welds are really better made with rivets. The mud ring, that joint that runs around the base of the firebox, is still made by casting a “ring” of cast iron the width of the space between the inner and outer walls, and riveting through the ring. Firebox door ports (where they throw the coal in) are also simply made using a ring of cast iron around the opening. Stay bolts are also riveted over on the ends. The riveted joint gives solid, dependable connection.
Tubes are often installed into a boiler using a rolled fit. The rolled connection is accomplished by placing the tube in the sheet in its desired location, then placing the tube roller inside the tube. The roller is then turned and compresses the tube wall against the sheet, causing the tube end to expand and lock against the sheet. In a properly executed rolling operation, you can actually see ripples in the sheet as the roller expands the tube. Of note is that it is common practice that the firebox tubes are riveted down to the sheet (beaded) and then welded, while the exhaust end of the tube is left as rolled. This reduces the tendency of the combustion in the firebox to degrade the edges of the tubes and thus reduce the tubes’ life span.
Threaded connections are also common in boiler construction. Tapered tapped holes (taper of 3/4 inch per foot) are used to put tapered threaded holes in boiler plate. These tapered holes are used to mount appliances (things needed to make the boiler/engine work) and other items (such as handrails and brackets) to the boiler. Mounting studs, tapered on the boilerside and straight threaded on the end not mounted in the boiler are a common device using this thread. Straight taps are used to mount long bolts not needing compression fit to the boiler and are similar to stay bolt taps that are used to tap the holes that the threaded staybolts put into prior to them being riveted over or welded. Machinery’s Handbook (pp1338-1339) have the common standards listed.
High vs. Low Pressure
Surprisingly, the demarcation of high pressure steam is atmospheric pressure, that is approximately fifteen pounds per square inch. Any pressure below that amount is “low” pressure and any pressure above that amount is “high” pressure.
In the early days of steam power, many machines were built to operate in the low pressure range, and do it well. Low pressure machines are, however, limited in the amount of work they can perform in comparison to their size.
High pressure machines give much greater horsepower to weight or size results and are more fuel efficient. Boilers tend to evaporate water at a given rate depending on their design. Higher pressures allow the use of smaller machinery to provide the same amount of work. Certainly higher pressures require stronger construction and thus heavier overall weights, but the tradeoff was generally that bigger was better. Another note about pressure, high altitude has little effect on boilers. Because they are regulated by pop valves, the internal pressure is what matters in a boiler; this pressure is what determines the transfer of energy to the water.
Steam Supply Control
To be useful, steam must be withdrawn from the boiler and sent to the machinery. The steam should contain as little liquid water as possible. Steam is therefore usually collected from the extreme top of the boiler in a space called the steam dome.
The steam dome extends above the boiler and contains the dry pipe. This dry pipe has an open top and goes down into the boiler and extends to wherever the steam is removed from the body of the boiler. The dome also usually has the pop valves mounted on it and may also be, through the use of a bolted on hatch, the entrance into the inside of the boiler.
The dry pipe is also connected to a valve controlling the amount of steam allowed to exit the boiler. This valve is sometimes mounted at the base of the dry pipe below the steam dome or it may be mounted in the smoke box at the exhaust end of the tubes. Mounting the valve below the dome allows the use of the control rod and its housing as a support for the top of the boiler sheet, by connecting to the dry pipe, making a continuous link from the back head to the front sheet. If mounted in the smoke box the valve may be accessed without opening the pressure vessel of the boiler. Each type of installation has its benefits and the choice is really up to the design team and the purpose of the installation.
Water is, for all intents and purposes, incompressible. This property of water presents some challenges to steam-powered machinery. For example, if you have water in your cylinder on the non-powered side, the water can blow the cylinder head right off the cylinder. Water, in comparison to dry steam, has mass and will slow down and otherwise make steam-powered machinery feel mushy in operation. And, last, the water takes the space that more energetic steam could be occupying.
The cure for water in your steam is simple, dry the steam out. Superheaters, sometimes called dryers, are tubes run from a header back through the flues (tubes) and then out to a collecting manifold that sends the steam on to the machinery. The superheater tubes also act as a sort of second pass ( like the Scots marine boiler type) and wrest more efficiency out of the fuel combusting in your firebox. At the end of the 1940s superheaters were so efficient that the machinery was really running on a pseudo-plasma of oxygen and hydrogen and actually causing new problems in cylinder wear.
Supplying Water and Fuel
Needless to say, a boiler that cannot be re-supplied with operating fluid (water) or fuel while in use is of limited utility. But, as in all else, this re-supply includes its own challenges. Injecting cold water into a hot boiler can cause problems. First, cold water on hot metal can cause shrinkage, stressing and even tearing the metal. At a minimum, it reduces the life of the boiler. Second, even if the water is put into the boiler so that it does no damage, it drops the overall temperature of the water in the boiler. This temperature drop can stop the boiler from steaming and will require extra heat to raise the water to operating temperature. Third, the water must be put into the boiler against the operating pressure of the system.
The cures for these ills are the feedwater injector, feedwater pump and the feedwater heater. Feedwater systems are always doubled. Two ways to put water into the boiler are essential for safety reasons. Injectors work by using boiler pressure to force water into the boiler much like a waterjet-style well pump. Feedwater pumps are piston arrangements that move the water against the boiler pressure by mechanical means. Heaters are heat exchangers that use waste heat from the boiler to bring the water closer to operating temperature before injection into the boiler.
Fuel can be fed into the firebox in a number of ways. Wood and coal can be thrown into the firebox through a door on the backhead. This was done primarily by hand, and is an exacting art as just dumping the fuel in the door will not provide the level of combustion needed for useful steam.
Oil is sprayed into the firebox and is normally done with steam bled off the boiler. Superpower locomotives (like our club’s) needed mechanical stokers as the amount of fuel required far surpassed the ability of even three stokers to keep up. However, superpower is probably a long way down the road for Grantville.
Safety appliances are those devices that protect, warn, and show the condition of a boiler in operation. If ignored and not maintained, the boiler will eventually kill the crew and destroy the locomotive. These devices were discussed at the beginning of this article.
Boilers are generally designed to operate in one position. Supports must maintain the boiler in the correct attitude for proper operation. Provision must be made for heat expansion and vibration. And the strength of supports must be sufficient to provide stability.
Many things must be considered in the design of a boiler. The first consideration is what the use of the boiler will be. A residential heating system has very different requirements from a locomotive boiler. The footprint of the boiler (how much room it needs) and what support and supply it requires are also of import.
Next is how much power the boiler needs to supply, and how fast it needs to recover. The machinery the boiler will supply also dictates boiler design. Mobile or stationary operation must be considered. Materials and fuel are also important.
The following list contains things that should be considered for boiler design. In the sample design area we will use this list to define what we need for a specific boiler. (Note: the list is not in order of importance):
1. Stationary or mobile 2. Fuel
3. Construction materials
4. Horse power to be produced
5. Steam or hot water, high or low pressure
6. Pressure needed
7. Fabrication method
8. Maintenance needs
9. Intended use
10. Safety systems
For the sample designs we are going to look at three categories and an extra one just for grins. The first design will be for a residential heating boiler. The boiler needs to supply heat to a large multi-residence building and be low maintenance.
2. City provided gas and electric
3. Cast iron
4. Large building so we will say 50 hp
5. Hot water, low pressure
6. 11 psi, supplied by water pump
7. Cast iron sectional
8. Very low, Water treatment, water makeup, auto fire on off control
9. Make hot water for heating
10. Low water control (2), Overheat gas shut off, pop-valves 15psi (2), water temperature burner control
This boiler is simple as it doesn’t need to move, make steam, or operate machinery. Gas is run from the supply to a regulator, then to a burner below the boiler sections. Burning gas flows between the sections to the exhaust stack and transfers heat through the walls of the sections. Boiler dimensions are determined by the amount of heat each section can transfer, with the number of sections determined by the end hp needed to heat the building. According to my sources:
1lb of gas contains about 21000 BTU/hr and 2545 BTU/hr = 1 hp.
So fifty horsepower is roughly six pounds of gas per hr. Please note that this is at 100% efficiency and I distrust the source of the thermal energy for the gas.
Cast iron boiler sections are widely different in size, but by fiat, each section is 98% efficient at heat transfer (very common for the industry) and about 2.5 by 2.5 by.3 feet in size. Each section also transfers about 2.5 hp/hr, so we will need 20 sections to do the job. Giving a dimension of 2.5 by 2.5 by 6 feet for the actual boiler, with another 1/2 foot in insulation around the sections, 8 inches for the frame holding the burners and supporting the sections, and 1.5 feet for the headers, total footprint of 3 by 8 by 5 foot (added a foot for gas regulator and controls).
Safety controls are two float valves that are attached to the sections and shut off the gas if the water level drops below the top of the section. Two pop valves that open if the pressure exceeds 15psi (note: pop valves need to have their vents piped to the ground.) And a thermostatic control that shuts off the gas if a preset water temperature is surpassed. Additionally there is a burner on/off control that keeps the water at the preset working temperature.
Maintenance on this type of system is low. Basically checking that the burners don’t foul, and verifying that the water treatment chemicals, which condition water to stop mineralization, are present in the right amounts.
Next we have an industrial plant that needs steam for a number of machines. Or a boiler that can run a medium locomotive.
1. Stationary (Mobile) 2. Coal (12,000 BTU/lb)
3. Wrought iron
4. 150 hp
5. Steam, high pressure
6. 200 psi
7. Riveted, single pass, superheated, feedwater heater
8. Water treatment, water makeup, auto fueling
9. Industrial steam supply
10. Low water control (2) stationary, Gauge cocks (mobile), popvalves, sight glass, Feedwater injector, feedwater pump, pressure gauge, and control valve.
This is a more complex boiler. It needs to provide steam on a fairly constant basis and needs replenishment of both fuel and water. First we calculate the amount of fuel to be burned per hour.
Coal gives approximately 12,000 BTU/lb, 1hp is about 2545 BTU/hr or 2.64lb of water evaporated an hr at 212° F.
One hundred fifty horsepower is equivalent to 381,750 BTU/hr so our boiler needs to consume about thirty-two pounds of coal an hr (64 lb/hr? see note 2).
Using a grate area of 3 by 4 feet, the boiler needs to consume just less than six pounds every ten minutes. The inside of the fire box is 3 feet wide, 4 feet long and 4 feet high base to top of the crown. The firebox will extend 1 foot below the drum of the boiler, and the drum will be 4 feet in diameter. The crown sheet will be 12.5 square feet and the front sheet will be approximately the same (12.5 feet square). Each side will be approximately 12 square feet and the back of the inside wall of the firebox will be about the same. This gives the firebox an inside surface of 61 square feet, allowing that the firebox gathers 40% of the heat transferred by a boiler then the remaining 60% will be made up of the flues, and will need to be about 91 square feet.
Using coal and a flue diameter of five inches (allowing for superheaters) each flue gives 15.7 square inches of surface per inch of flue. We will use a value of 15. The flue stack of five inch flues with one inch between flues allows a maximum of six flues wide in the nest. However we will use a stack: a layer of three wide, then a layer of 4 wide below, then a layer of 5, with a layer of 4 on the bottom. This will complete our flue nest. This gives sixteen flues with a combined surface of 245 square inches of surface area per inch of flue length. The flues will run to the front sheet at a slight upward angle to promote draft. We need 91 square feet, or 13,104 square inches to make up the rest of the absorption surface. This works out to about an additional four feet seven inches of flue.
Allowing for construction needs, our boiler works out to about 8 feet in length (had to add in the smokebox on front) The sides and top of the firebox need to be staybolted, and the entire assembly insulated. All of the above is to give us our dimensions so that we can work out our rough dimensions. Plugging them into the formulas we find out how thick we need to make the plates that the shell and rods that the stays are made from.
From my Machinery’s Handbook we find that the tension strength of wrought iron is 48,000 lb/in squared and the formula for the strength of materials is:
P=A*S where P=total pounds stress, A is the area of the material, and S is the working stress in lb per square inch.
Working this out gives us a thickness of.004+ to hold 200psi of pressure. However this is for a 1 by 1 inch sample and includes NO safety factors at all. To find the wall thickness for our boiler drum we need the formula of:
t=dP/2S giving (48*200)/(2*48000)
or.1 of an inch . . . It seems absurd, and is, because there is NO safety factor calculated in.
Safety factors are calculated from four factors in reference to the material under consideration. Elastic limit compared to the ultimate strength is the first of these factors ( a). Then the stress generated by the type of load incurred ( b). Next is the way the load is being applied to the device ( c). And last is the type of use the device is made for ( d). This also includes the fudge factor added in for materials quality and workmanship. The equation is:
F= a * b * c * d or by the table (pp 331) 2*1*1*2.25-3 or 4.5 to 6.
this is multiplied to the formula for thickness and so we arrive at a shell thickness of.45 to.6 of an inch. In the post-Ring of Fire environment, I would probably push the fudge factor up some and run the wrought iron shell at.75 inch. A thicker plate, while more expensive, would be easier to make and stand up to rougher handling.
Now we come to the flat sections of the firebox. Using the shear number for wrought iron (40,000), a safety factor of 10, a plate thickness of.6 inch, and a working pressure of 200psi the formula:
L=1.89*t*[(square root of) S/P]L is the length between supports, t is the plate thickness, S is the stress number for Wrought iron, and P is the working pressure.
All of this gives us an L of just over five inches between staybolts (center to center). The diameter of staybolts should be one and a half to two times the thickness of the plate making up the walls to be stayed.
Joints are those places where two plates are joined together. Early boilers had lap joints where the plates to be joined were overlapped and riveted through. By the time of my handbook, longitudinal lap joints had fallen out of favor due to numerous failures caused by fatigue of the metal. The preferred joint was the butt joint where the plates to be joined are butted together between an inner and outer plate. This joint could be double riveted, triple riveted, or even quadruple riveted, depending on the job at hand. For this boiler we will use a triple riveted butt joint with the rivets (one inch in diameter) pitched (spaced center to center) seven inches apart with each line of rivets three inches apart. This gives us six lines of rivets along the joint of the drum of the boiler. The dome, mud ring, firebox ring, firebox to firebox, firebox to drum, sheet to drum and drum to smokebox joints will all be double riveted lap joints at a pitch of three inches and the rivet lines one and one sixteenth inches apart.
From the steam collection dome we will have a dry pipe with the control valve mounted below the dome. The valve will be operated from the backhead via a “stay pipe” running from the back of the valve to the back head with a control rod in the pipe. In the smoke box (attached to the front of the drum) the dry pipe attaches to the superheater header. From the header nine superheater pipes will run back into the top twelve flues and back out the same flue (basically making a long “U” into that flue) and connect to the collecting manifold of the superheater. From this manifold the steam is piped to its point of use.
The feedwater heater will be a heat exchanger in the smoke box from which the water will be piped to the injectors and pumps.
Safety controls for stationary mode are: two low water controls that are attached to warning devices (bells, whistles), sight glasses, pressure gauge, water injector, water pump, and pop valves. In the mobile mode, gauge cocks are substituted for the low water detectors as vibration makes the detectors unreliable.
The third application will be a marine boiler. This boiler will need to power a ship, assuming a paddle wheel, driven by two 26 inch cylinders with a 3 foot stroke. The foot/lb of the cylinders is figured by area of the cylinder heads, multiplied by the psi of the boiler output to calculate the total horse power. (This is just a rough estimate and ignores the torque component of the equation.) The cylinders have an area of approximately 1062 square inches, times 150 psi, gives 159300 foot/lbs. converting this number to horse power, (divide by 550 ft/lb a second) gives 290 hp consumption. The boiler needs to provide at least this much power. In order to allow for inefficiency we will calculate the boiler at 300 hp. 300 hp is roughly equivalent to 76,500 BTU/hr. Assuming coal fire @ 12000 BTU/lb. 300hp is 763,500 BTU/hr, coal consumption works out to 63.5 lb/hr (127lb/hr? see note 1). Please note this is under ideal conditions, actual “mileage” will vary
1. mobile 2. coal (12000 BTU/lb)
3. wrought iron
4. 300 hp
5. Steam, high pressure
6. 150 psi
7. Wrought iron riveted
8. Water treatment, cleaning
9. propulsion of small steam ship 10. Gauge cocks (mobile), popvalves, sight glass, Feedwater injector, feedwater pump, pressure gauge, and control valve.
For this boiler we will use a small Scots marine boiler. This is an internal firebox three pass boiler, that uses exhaust steam to promote draft.
The burner tube has a short grate to allow ash dump and air injection to the fire. The combustion chamber will be twelve inches in diameter and three feet long. This gives a surface of just less than 1300 square inches for the firebox equivalent. Assuming this as 40% of the transfer area, we need an additional 2000 square inches of surface area. Each tube layer in a Scots marine boiler is of lesser diameter than the layer closest to the combustion chamber. This reduction of tube size in each pass allows the heat transfer to remain even in each pass of the exhaust traveling through the boiler. The tube reduction also helps regulate the resistance to the flow of the exhaust gasses. Allowing 1000 square inches for each of the remaining passes we arrive at a mid bank of three tubes three inches in diameter and a top bank of four tubes 2.25 inches in diameter. There are no stayed surfaces in this boiler. The drum is twenty-four inches in diameter. Here we hit our friend t= (d*P)/(2*S). Where d is the inside diameter, P is the pressure, and S is the stress factor (including safety factor of 10). The equation becomes:
75.4*150 / 2*4000, or 11310 divided by 8000.
Becoming 1.414 inches of wall thickness. The end sheets should be of the same thickness to ease construction. The combustion tube is set by chart to 1/2 inch of wall thickness, and the tubes to a thickness of 1/8th inch.
The end covers have a divider that creates a chamber so as to redirect the flow of exhaust gasses into each pass of tubes and out the stack. Steam is collected in a dome with a below-the-dome mounted control valve mounted in dry pipe.
Control of the valve is through a stay pipe from the valve through the back head of the boiler. Exhaust steam is injected into the combustion tube to promote draft.
Safety controls: sight glasses, pressure gauge, water injector, water pump, pop-valves, and gauge cocks.
Last, how do you power a locomotive used in a factory where any fire will cause an explosion? Gunpowder, flour, chemical, refineries, and many other products have significant hazards with an open flame present.
The solution is to have a locomotive that uses no combustion to produce its work. This locomotive has a large tank that is rated for high pressure and is charged with superheated water. The superheated water will form steam in the space within the tank not filled with water. As the steam is used, the water level lowers until there is no more usable pressure in the tank, whereupon a fresh charge is put in the tank. A stationary boiler separate from the factory is used to produce this charge.
Shear values, welding standards, and construction standards are per the Machinery’s Handbook, 11th ed. 1942. This handbook has been continuously updated since the first edition and is currently in its 27th edition. The most modern edition available to Grantville residents would be the 25th or 26th eds.
1) accident report
CFR49 part230 Form 4 and figures included
and here are a few other sites of interest.
2) Throughout the writing of this article I have felt ill at ease with the fuel consumption numbers I have been getting. The number seemed very low in terms of fuel used per hour. As luck would have it, I have a point of reference for the numbers. When I was seventeen, I sometimes had the opportunity to “guest fire” a locomotive here in the west. The locomotive in question developed about 2295 hp and required nine hours to make the trip. During the trip it would use about four and a half tons of coal up hill, and only half a ton coming back. Running the numbers:
hp * 2545 = BTU/hr (5840775) Divided by 12000 (487lb/hr @ 100% efficiency).
Compared to, total coal used (10000 lb) divided by nine hrs (1112 lb an hr @ actual efficiency) shows an efficiency of about 44%, a number not out of line for a locomotive built in 1924 and allowing for the level part of the run. My conclusion is that the fuel use numbers should be multiplied by two, to allow for actual efficiency as opposed to Ideal efficiency.