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Steam: Taming the Demon

Written by Kevin H. Evans

DISCLAIMER

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

Steam Safety

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 2x4. (The steam jet will cut the stick or 2x4 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.

Figure 1

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.

Boiler Materials

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.

Boiler Types

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).

Figure 2

Figure 3

Figure 4

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 ...