Absolute Pressure: is zero-referenced against a perfect vacuum, and is equal to gauge pressure plus atmospheric pressure, and labeled as p.s.i.a..

Babbitt bearing: Babbitt, also called Babbitt metal or bearing metal, is any of several alloys used for the bearing surface in a plain bearing.

The original Babbitt metal was invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, USA. Other formulations were later developed but Isaac Babbitt's exact formulation is not known with certainty. Babbitt metal is most commonly used as a thin surface layer in a complex, multi-metal structure, but its original use was as a cast-in-place bulk bearing material. Babbitt metal is characterized by its resistance to galling. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface. However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, which creates paths for lubricant between the hard high spots that provide the actual bearing surface. When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, which protects the bearing from wear when other lubricants are absent.

There are many Babbitt alloys in addition to Babbitt's original. Some common compositions are: 90% tin, 10% copper, or 89% tin, 7% antimony, 4% copper, or 80% lead, 15% antimony, 5% tin, or 76% copper, 24% lead, or 75% lead, 10% tin, or67% copper, 28% tin, 5% lead

Boiler charging car: A narrow gauge coal dumping rail car moving 6 tons of coal from a storage pocket hopper to the coal shoveler for a boiler stoker hopper.

Balanced Poppet Valve: was invented and patented by H. F. Frisbie, July 23,1884, then owner of the Frisbie Engine and Machine Company, Cincinnati, Ohio. A similar valve called the double-beat valve, the drop valve or equilibrium valve was invented by Jonathan Hornblower from the UK in about 1800.

The valve is arranged to allow opening against a high pressure with a minimum of force. The valve has two valve plugs that ride on a common stem, with the pressure on one plug largely balancing the pressure on the other. The force needed to open the valve is determined by the difference between the areas of the two valve openings. The valve is balanced when pressure pushes the valve in both directions, both open and closed, but slightly more in the closed position.

With steam engines the double balance poppet valve was a favorite choice because it maintained it's ability to handle a large volume steam flow yet required little force to operate.

Centrifugal "fly ball" Governor: a servomechanism that adjust the speed of the engine by regulating the amount of process steam admitted to the cylinder.

As the speed of the engine increases, the central spindle of the governor rotates at a faster rate and the kinetic energy of the balls increases. This allows the two masses on lever arms to move outwards and upwards against gravity.

If the motion goes far enough, this motion causes the lever arms to pull down on a thrust bearing, which moves a beam linkage connected to the governor hook rod attached to the knock-off cam on the Corliss valve. The knock-off cam controls the position when the valve closes early in the piston travel cycle.

Condensing Steam Engine: for a stationary steam engine a condensing engine refers to the recovery of the water from the exhaust steam using a surface condenser to produce condensate, which allows the engine to operate on absolute pressure as compared to atmospheric pressure, and increases the thermal operating efficiency.

A steam locomotive condensing apparatus differs in purpose from the closed cycle steam engine condenser, where its function is primarily either to recover water, or to avoid excessive emissions to the atmosphere (important for tunnel operations), rather than maintaining a vacuum to improve both efficiency and power. It usually takes the form of a series of pipes, valves and other ancillary equipment usually attached to an otherwise conventional steam locomotive. The apparatus takes the exhaust steam that would normally be lost up the funnel and routes it through a heat exchanger, into the normal water tanks.

Coal passer, brings coal from the coal storage building to the stokers for the boiler furnace.

Coal dump car: A two ton capacity narrow gauge coal dumping rail car.

Corliss Valve: Invented by and named after an American engineer George Henry Corliss in Providence, Rhode Island, the semi rotary Corliss valve requires very little power to operate and can be opened and closed very quickly. The valve allows steam into the cylinder without significant pressure drop which leads to the steam cooling and a consequential loss of power and thermal efficiency. Another advantage is that cold exhaust steam does not exit through the inlet valve which would cool the valve and further reduce thermal efficiency.

Condensate pump / hot well pump removes water from the hot well and returns it to the boiler pre-heater.


Deaerating: Deaeration is the mechanical process of removing scale and dissolved gasses such as oxygen, carbon dioxide and other harmful chemicals from the boiler feed water caused in part by the boiler tubes. Tap water at standard atmospheric pressure and temperature contains about 3% air, and process steam contains some of this air along with other chemicals absorbed from the metal tubing and piping

Dissolved oxygen in boiler feed water will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion.


Dash Pot: is a generic name for a mechanical motion controller or damper which resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. The shock absorber on a vehicle is a good example.

A steam engine with Corliss valve gearing often used a specially designed dual cylinder vacuum dashpot to accelerate the closure of the inlet steam valve rather than slow the motion. Rapid closure was required to accurately cut-off steam early in the piston cycle to improve thermal efficiency. A second cylinder compressed air at the end of the stroke to cushion the stopping forces.

Doctor Pump: aka "boiler feed water pump", introduced in 1840 and significantly reduced the risk of explosions on steamboats.

Economizer - used to preheat oiler feed water through a series of tubes located in the path of effluent gases from a boiler to absorb waste heat. Patented by Edward Green in 1845, and since then has been known as a Green's economizer. It consisted of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases passed. This is the reverse arrangement to that usually but not always seen in the fire tubes of a boiler; there the hot gases usually pass through tubes immersed in water, whereas in an economizer the water passes through tubes surrounded by hot gases.

Gauge Pressure: is zero-referenced against ambient air pressure and is equal to absolute pressure minus atmospheric pressure, and labeled as psig. Negative signs are often omitted. When steam engines use a condenser, pressure is often labeled as psig to eliminate confusion to absolute pressure, p.s.i.a.. The condenser will generally operate at a very low psia pressure. Since pressure gauges are referenced to ambient atmospheric pressure, a gauge on a condenser will often read around -14 psig, but the engine operates on psia.

Governor Rod: connects the fly ball governor to the knock-off cam on the Corliss valve

Heat of Saturated Liquid: This is the amount of heat required to raise the temperature of a pound of water from 32F to the boiling point at the pressure and temperature shown in the steam table. It is expressed in British thermal units (Btu).

Hook Rod: connects the eccentric bearing to the steam valve used on a steam engine.

Knock-Off Cam: a cam attached to the Corliss valve gearing used to actuate rapid closure of the valve.

Hot well: is the bottom section of a steam condenser where condensate water settles from the condensing tubes.

Jacking Engine: Also called a "Kicking Engine or a Barring Engine" is used to rotate an idle reciprocating engine or turbine to facilitate starting, inspection, or repairs. On occasion a steam engine will stop at crank-end or head-end dead center and require nudging off dead center for staring. Often a large metal bar or crowbar was used for this task thus the phrase "Jacking" or "Barring." On some engines it is possible to use a person's foot for this task thus the phrase "Kicking Engine".

Latent Heat or Heat of Vaporization: The amount of heat (expressed in Btu) required to change a pound of boiling water to a pound of steam. This same amount of heat is released when a pound of steam is condensed back into a pound of water. This heat quantity is different for every pressure/temperature combination,

Multiple expansion steam engine.

A single expansion steam engine has one cylinder and can have an efficiency between 1% and 10%, typically around 8% with enhancements.

Double expansion (usually known as compound) engines expanded the steam in two stages and increase the operating efficiency over the single expansion engine. The cylinder pairs may be duplicated or the work of the large LP cylinder can be split with one HP cylinder exhausting into one or the other, giving a 3-cylinder layout where cylinder and piston diameter are about the same making the reciprocating masses easier to balance.

Two-cylinder compounds can be arranged as:

  • Cross compounds -The cylinders are side by side.

  • Tandem compounds -The cylinders are end to end, driving a common connecting rod

  • Angle compounds - The cylinders are arranged in a vee (usually at a 90 angle) and drive a common crank.

Triple expansion extends the double expansion concept into three cylinders, the HP, the IP and the LP cylinders. When the diameter of the LP cylinder becomes excessive it was often divided into dual LP cylinders. On occasion the dual LP cylinder engine would be referred to as a quad expansion but it was not a four times expansion and remained a triple expansion engine. The coal to water efficiency of an enhanced triple expansion engine could reach 25%.


Pocket Hopper: a coal storage bin equipped with a tapered floor terminating in an adjustable coal spout.

Pre-heater: process steam or exhaust steam is used to preheat the boiler feed water arriving from the condensate pump prior to re-entering the boiler.

Re-heater: process steam passing through coils in a receiver to add additional heat to the expanded steam.

Receiver: temporary storage container for expanded steam located between stages in an multi expansion engine.


Dry Steam: In early 1900s dry steam was considered to be <5% liquid however today, 1/2% liquid or less is considered dry.

Flash Steam: When hot condensate or boiler water, under pressure, is released to a lower pressure, part of it is re-evaporated, becoming what is known as flash steam.

Process steam: the final steam product leaving the boiler plant ready for work.

Saturated steam: There is a temperature below which steam will start to condense into water droplets. This is called the saturation temperature, and it varies with the pressure of the steam. Steam that is exactly at its saturation temperature is called saturated steam. Saturated Steam is pure steam at the temperature that corresponds to the boiling temperature of water at the existing pressure.

Saturated steam has three main disadvantages in a steam engine: It contains small droplets of water which have to be periodically drained from the cylinders; being precisely at the boiling point of water for the boiler pressure in use, it inevitably condenses to some extent in the steam pipes and cylinders outside the boiler, causing a disproportionate loss of steam volume as it does so; and it places a heavy demand on the boiler because a large amount of water has to be evaporated per unit volume of steam. 

Superheated steam : is steam that is above its saturation temperature.

Wet steam: is in equilibrium with heated water at the same pressure, i.e. it has not been heated past the boiling point for that pressure. Steam that is below its saturation temperature contains droplets of moisture and is called wet steam.

Steam tables: Steam Tables

Surface condenser: is a heat exchanger that removes heat from exhaust steam and returns it to the liquid state, "condensate".

Sub cooling: is the effect of saturated liquid in the condenser removing additional heat from the steam as it falls to the hot well. Sub cooling is a desirable effect to help prevent cavitations of the condensate pump.


Steam chest: A compartment in a steam engine through which steam is delivered from the boiler to a cylinder.

Steam trap: A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically. Others, like venturi traps, are based on turbulent 2-phase flows to obstruct the steam flow.

Stuffing box: an assembly housing a gland seal that is used to prevent the leakage of fluid, such as water or steam, between sliding or turning parts of machine elements.

Super heater: is a device installed inside a boiler used to convert saturated steam into dry steam, which raises its temperature to a point where condensation is much less likely and increases its volume significantly. A super heater can be installed as straight or hairpin tubes in the upper part of the boiler between the first two steam drums. The baffles direct the gas-flow through this area first, so it may reach the highest temperature. A Foster Super heater: is a drawn steel tube with cast iron radial fins heat shrunk onto the tubing and allows better heat transfer from boiler gases to steam than tubing without fins.

Sump Pump/Bilge Pump: a pump to remove water from the pump pit caused in part by seepage in the pit floor and from leakage caused by a leaking gland seal.

Super heater is a device installed inside a boiler used to convert saturated steam into dry steam, which raises its temperature to a point where condensation is much less likely and increases its volume significantly. A super heater can be installed as straight or hairpin tubes in the upper part of the boiler between the first two steam drums. The baffles direct the gas-flow through this area first, so it may reach the highest temperature.

Steam Jacket: a second circular casing on the outside of an operating steam cylinder for a steam engine. The jacket circulates process steam to help keep the interior hot in an effort to reduce condensation inside the cylinder which will increase thermal operating efficiency.

Turbining: The removal of scale or other foreign material from the internal surface of a metallic cylinder.

Wet Air Pump: removes elastic vapors such as air and uncondensed steam from the surface condenser and will typically operate with an absolute pressure near zero.

Work Duty/Engine Duty: is the technical measure for the efficiency of the engine determined by the cost of the work done in fuel consumed.. Pumping engines are commonly rated by the work done by the consumption of a specified weight of fuel, as one hundred pounds. A duty of 100,000,000 foot-pounds, on this basis, would correspond to a consumption of 1.98 pounds of fuel per horse-power per hour.

The "horse-power," taken in British measure as 33,000 foot-pounds per minute or 1,980,000 per hour, requires the transformation of the equivalent amount of heat into work each minute or hour.

The duty of a pumping engine per 1,000 Ibs of steam, and 1,000,000 heat units; is the rate of a thousand pounds of steam, and the million heat units that result from a hundred pounds of coal, giving an evaporation in the boilers to 10 Ibs of steam for each pound of coal burned on the boiler grates. Which is 10 Ibs of water evaporated into steam per pound of coal burned on the grates, and equal to 100 Ibs of coal evaporating 1,000 Ibs. of water into steam.

Water works that use steam engines often have multiple definitions for the phrase Low Duty or High Duty. Generally speaking the phrase refers to the efficiency of the engine as high or low. However, the same phrase is used to describe the volume of water pumped per engine or per station. At GCWW Low Duty is low volume water pumping and High Duty is high volume water pumping.