Glossary of steam locomotive terms

Back pressure
Back pressure is caused by the resistance of the exhaust steam exiting the cylinders. Overcoming this resistance represents negative work done by the piston and it decreases cylinder efficiency. On a large locomotive this can easily mean the loss of several hundred horsepower. To minimize it, it’s important that the exhaust valve openings be made as wide as possible. The final choke point for the exhausting steam needs to be at the exhaust nozzle in the smokebox and not at the valves. Also see Valve events.

Blowdown valve
Boiler water contains dissolved and suspended solids. When the water evaporates these impurities remain behind and they collect as sludge and scale in the lowest part of the boiler — the two side water legs of the firebox. Valves, operable remotely from the cab, are located at the low point of each water leg. By opening them, this residue can be blown out of the boiler. Also see Carryover.

Blower
When a locomotive is standing or drifting, there are times when it is necessary to induce a draft to either lift some smoke or enhance combustion. This is done by operating the blower. It consists of a steam jet that is incorporated into the exhaust nozzle in the smokebox. It is aimed upward toward the base of the stack and when steam exits the jet, it induces a draft through the boiler and up the stack. This has the effect of stimulating the fire and raising the smoke above the engine. See also Induced drafting.

Carryover
Any contaminant — solid, liquid, or vapor — that leaves the boiler along with the steam and goes into the dry pipe is called carryover. Boiler water containing dissolved or suspended solids is the most common reason. Carryover can travel into the superheater tubes. It will often accumulate at the first bend of the tube right at the rear tube sheet. At some point, excessive accumulation will block off the steam flow and cause a failure of the superheater unit. Carryover can be minimized by chemical treatment of the boiler water, and by not carrying the water too high in the boiler. Also see Blowdown valve.

Booster
A booster is an auxiliary steam engine mounted on a locomotive’s trailing truck, or on one of the tender trucks, which can be used during starting and at low speeds to boost tractive effort. At speeds up to about 20 mph, a boiler produces more steam than can be used by the cylinders, and some of it can be used by a booster. As the speed increases, this changes and the booster is cut out. A booster typically adds about 10,000 lbs. of additional tractive effort.

Brick arch
A brick arch is located in the firebox. It rests on arch tubes that extend from the throat to the back inside sheet. The arch, made of firebrick, rests on the arch but does not extend all the way to the rear sheet. It lengthens the path of the fire gases by forcing them backward under the arch, around the end of it, and then forward into the firebox/combustion chamber furnace volume. By lengthening the path of gas travel, the fuel and air are better mixed and have more time to burn. It was very common in coal-burners, but, except in Canada, little used in oil-burners. See also Firebox.

Combustion chamber
A combustion chamber increases the furnace volume of the firebox. It extends from the front of the firebox, where the rear tube sheet would be located, into the barrel of the boiler. Its front becomes the new location of the rear tube sheet. The increased furnace volume provides a greater opportunity for the complete combustion of fuel. Once the hot gases enter the tubes, all burning ceases. The combustion chamber also adds additional firebox heating surface. Also see Firebox and Tube sheet.

Counterbalance
This is the weight added to a driving wheel to balance revolving or reciprocating weights. Three situations can require it:

1. Rotational balance. Weight is added to each driving wheel to counter the weights of the crank pin, side rods, eccentric crank, and the rotating weight of the main rod. The counterbalance added exactly balances each wheel.
    
2. Reciprocating balance. The reciprocating action of the pistons, crossheads, and the main rods will create a jigging and nosing motion. It is countered by adding some more weight to each wheel at the same locations as that added for rotational balance. This is called “overbalance” and it upsets the rotational balance. The reciprocating weights are only partially balanced. The Association of American Railroads recommended that just 31½ percent of the reciprocating weight be balanced. Since the drivers are coupled by the side rods, the same amount of weight does not have to be applied to each wheel; it can be distributed, if for example there is not enough room for it on the main driver. Too much overbalance could create a rotational unbalance that might cause high-speed wheel slips (wheels lifting of rail) and rail-pounding (dynamic augment).
    
3. Dynamic balance. This imbalance is created by the fact that effective forces of the rotating weights and the counterweight are applied at different points on the axle. For example, on the main driver, the pin and rods can be located 8 or 9 inches away from the wheel. These weights, and the wheel’s counterweight, are rotating in parallel but separate planes. If one viewed the axle as if they were in space, unattached to the locomotive, you would see it spinning end over end. When attached to the locomotive, it also causes nosing. The term dynamic balance comes from the fact the unbalance becomes significant only with speed. It is balanced with weight placed on the opposite driving wheel. This is called cross-balancing. Not all locomotives had this and it wasn’t done on all driving wheels. In later years passenger engines received cross-balancing as did some large freight engines with big heavy rods on the main driver.

Crown sheet
The crown sheet is essentially the top cover of the firebox. It is attached to the other interior sheets of the firebox and is supported by radial staybolts hung from the boiler roof sheet over the firebox. Being directly over the fire, it is in a critical location, as this is the hottest point in the boiler. Water is converted to steam here with tremendous vigor. It is absolutely imperative that the crown sheet remain covered with water at all times. Not doing so will permit the rapid failure of the metal, which in all probability will result in a boiler explosion. See Firebox.

Exhaust steam injector
This injector has two separate steam nozzles. One uses exhaust steam to begin the motion of the feed water to the boiler. The second nozzle uses live steam to provide the remaining necessary velocity to the water to overcome the boiler pressure. Like a feedwater heater appliance, the exhaust steam injector also pre-heats the feedwater using exhaust steam, and it has the advantage of recycling heat that would have otherwise been lost going up the stack. All exhaust steam injectors are non-lifting. Also see Injector and Feedwater heater.

Feedwater heater
This is an appliance that taps into the steam exhausted from the cylinders and then uses it to pre-heat the water being fed into the boiler. There were two basic designs. One was a closed system. It used a heat exchanger to transfer the steam’s heat. In the process the exhaust steam condensed and it was run through an oil separator and then returned to the tender. The second design employed an open system in which the exhaust steam heated the feedwater by being directly combined with it. In both systems, a live-steam piston pump was used to force the heated feedwater water into the boiler. Feedwater heaters improved locomotive efficiency by 8 to 10 percent through recovering heat energy that otherwise would have gone up the stack. Also see Exhaust steam injector.

Firebox
The firebox is the furnace of a locomotive boiler. It contains a grate at its base if coal is burned, or a burner and firepan if oil is the fuel. The interior side, top, and back sheets of the firebox are surrounded by boiler water. The front opens to the rear tube sheet. Air is drawn through the bottom of the firebox. For a coal-burner this is through the grate; for an oil-burner it is via a damper in the fire pan. It mixes with the fuel to support combustion. Much of this occurs in the furnace volume of the firebox where combustion of the fuel in gaseous form takes place. Heat absorption in the firebox is primarily through radiation directly from the fire itself. This is extremely efficient and steam generation at this end of the boiler is very high. See also Combustion chamber, Brick arch, Crown sheet, and Tube sheet.

Front-end multiple-valve throttle
A front-end throttle, as opposed to a dome throttle, is located in the smokebox between the superheater header and the steam pipes leading to the cylinders. Because of its position, it allows the superheater units to always be filled with superheated steam. The short distance between the throttle and the cylinders also provides a quick response to throttle actions. As the throttle is opened, operating cams open a series of poppet valves – each a bit larger than the last. This results in the ability to finely control the admission of steam to the cylinders.

Induced drafting
Exhaust steam from the cylinders is released through a nozzle at the base of the smokebox. This nozzle is set directly beneath the smoke stack extension into the smokebox. As the exhaust steam expands after its release, it entrains the smoke and other products of combustion and then pushes it all up the stack into the atmosphere. This action induces a partial vacuum in the smokebox, and as a consequence, air is drawn into the firebox. This air is needed to support combustion of the fuel. Importantly, this action is self-regulating. When the engine is working hard, and more air is needed for combustion, the heavier exhausts will increase the smokebox partial vacuum and automatically increase the volume of air drawn into the firebox. Good drafting is essential for a free-steaming locomotive. Also see Smokebox.

Injector
This is a device that forces water into a steam boiler. A jet of live steam is fed through a nozzle so that it strikes the feedwater. In doing so it imparts its velocity to the water. This moving flow of water now has enough force to overcome the boiler pressure and enter the boiler. There are two basic types of injectors. The lifting type is self-priming and may be mounted above the water level of the tender. The non-lifting type is not self-priming and must be located lower than the tender floor, but it has a greater capacity than the lifting injector. Injectors also pre-heat the feedwater so that cold water is not put into the boiler. See also Exhaust steam injector.

Limited cut-off
A full-stoke engine is capable of admitting steam to the cylinders for as long as 85 percent of its stoke. The steam in the cylinders would be acting as ram and would not be used expansively. Operation in this mode would be typical when starting a heavy train and at low speeds. A limited cut-off engine cannot admit steam for nearly as long. Cut-off after 60 percent of the stroke was typical. This allowed some use of expansive steam – even when starting. Obviously, less power can be developed with this arrangement, and to compensate for it, limited cut-off engines tended to have slightly larger cylinders than comparable full-stroke engines. See also Valve events.

Rated tractive effort (or tractive force)
The rated tractive effort for a locomotive is a figure of merit that is calculated from a locomotive’s specifications. It is used to estimate the force the locomotive can exert at the rim of its driving wheels while starting and operating at low speeds. It is a widely published specification. For example, the AAR formula for a two-cylinder, full-stroke simple (as opposed to compound) steam locomotive is: TE = .85 x BP x C² x S/D where: TE = tractive effort (pounds), BP = boiler pressure (PSI), C = cylinder diameter (inches), S = cylinder stroke (inches), and D = driver diameter (inches).

Smokebox
The smokebox is located at the very front of the boiler. Its front has an airtight door; its rear is the front tube sheet. It is the place where smoke and other products of combustion are entrained with exhaust steam from the cylinders and the resulting mixture is ejected upward through the stack. Also see Induced drafting.

Steam circuit
The steam circuit is name given to the path taken by steam once it has been drawn from the top of the boiler by a collector and routed to the cylinders. On a modern locomotive with a front-end throttle, the various piping and appliances would include the following, in order: dry pipe, superheater header, superheater tubes, superheater header, throttle, piping to steam chests, and finally the piston valves. Good design practice called for minimizing obstructions, sharp bends, or anything that might restrict the steam flow. The object was to deliver steam to the cylinders with as little pressure loss as possible.

Superheater
The superheater adds additional heat energy to steam that has been generated in the boiler. Steam is drawn off from the top of the boiler and routed to a superheater unit, where it is no longer in contact with the boiler water, and then the additional heat is added. This can increase the steam temperature by as much as 300ºF. The Schmidt superheater was the most common type used in North America. Its superheater units consisted of a series of bundled tubes that came from a header in the smokebox and then ran back into the boiler flues. Steam from the header entered these tubes and, after receiving the additional heat, it was returned back to the header and routed to the cylinders. See also Tubes and flues and Steam circuit.

Tubes and flues
The tubes and flues are pipes that stretch between the front and rear boiler tube sheets. Flues are larger in diameter than tubes because they contain superheater units. A typical tube diameter is 2¼ inches, whereas flues can run from 3½ to 5½ inches depending on the size of the superheater unit. Tubes and flues provide the boiler with the majority of its heating surface. They absorb heat by convection from the mass of hot gases passing through them. Also see Tube sheet.

Tube sheet
There are front and rear tube sheets for every boiler. They are welded in place to form the ends of the boiler’s cylindrical pressure vessel. Holes are cut into the sheets for the tubes and flues that stretch between them. These are then welded to the tube sheets. The tubes and flues provide the structural integrity needed for these sheets to withstand the boiler’s pressure. Also see Tubes and flues.

Thermic syphons
Thermic syphons are triangular water legs hung from the firebox crown sheet. The flat sides of the triangle are about 3” apart. The top edge of the triangle is flanged and open to water the space above the crown. The front edge is closed over. The rear edge opens to a 6” neck that continues forward beyond the triangle as a pipe that connects to a low point in the boiler such as the throat or base of the combustion chamber. Water at this low point is the coolest in the boiler and it is drawn upward through the broad sides of the syphons and on to the crown. Being directly above the fire and in the middle of the fire, the crown and syphons are at hottest part of the boiler. This greatly enhances heat transfer by having the greatest differences in temperatures between the heat source (fire) and the material (water) absorbing the heat. Testing has shown syphons can increase boiler efficiency by as much as 8 percent. Syphons also add valuable firebox heating surface.

Valve events
There are four valve events:

1. Admission – steam is admitted to the cylinder and it pushes against the piston.

2. Cut-off – steam admission is cut off and the steam in the cylinder is allowed to expand against the piston.

3. Release – the exhaust port is opened and steam begins to leave the cylinder. The piston reverses direction and then pushes the exhaust steam out the exhaust port. The resistance it encounters doing this is called back pressure.

4. Closure – the exhaust port is closed. Some steam remains in the cylinder and it is compressed as the piston pushes against it. The pressure of the compressed steam reaches that of the boiler pressure. This acts to cushion the piston as it slows and begins to reverse its direction of movement. The energy lost in compressing the trapped steam is regained at cut-off when the steam in the cylinder is allowed to expand. See also Back pressure.