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Absolute pressure - The pressure above zero pressure, equal; to gauge pressure plus atmospheric pressure.

Acid cleaning  - The process of cleaning the interior surfaces of steam-generating units by filling the unit with a dilute acid accompanied by an inhibitor to prevent corrosion and by subsequently draining, washing, and neutralizing the acid by a further wash of alkaline water.

Acidity - Represents the amount of free carbon dioxide, mineral acids, and salts (especially sulfates or iron and aluminum) which hydrolyze to give hydrogen ions in water; is reported as milliequivalents per liter of acid, or ppm acidity as calcium carbonate, or pH, the measure of hydrogen ion concentration.

Agglomeration - Groups of fine dust particles clinging together to form a larger particle.

Air-atomizing oil burner - A burner for firing oil in which the oil is atomized by compressed air which is forced into and through one or more streams of oil, breaking the oil into a fine spray.

Air-fuel ratio - The ratio weight, or volume, of air to fuel.

Air heater or air preheater - Heat-transfer apparatus through which air is passed and heated by a medium of higher temperature, such as the products of combustion or steam.

Air purge - The removal of undesired matter by replacement with air.

Air vent - A valved opening in the top of the highest drum of a boiler or pressure vessel for venting air.

Alkalinity - The amount of carbonates, bicarbonates, hydroxides, and silicates or phosphates in the water; reported as grains per gallon, or parts per million as calcium carbonate.

Allowable working pressure - The maximum pressure for which the boiler was designed and constructed; the maximum gauge pressure on a complete boiler; and the basis for the setting on the pressure relieving devices protecting the boiler.

Amplitude - In ultrasonic testing, the vertical pulse height of a signal, usually base to peak, when indicated by an A-scan presentation.

A - scan - In ultrasonic testing, a method of data presentation on a CRT with the horizontal baseline indicating distance or time and the vertical deflections from the baseline indicating amplitude of the ultrasonic reflection.

Atomization - The process whereby a volume of liquid is converted into a multiplicity of small drops. The principal goal is to produce a high surface area to mass ratio so that the liquid will vaporize quickly and thus be susceptible to combustion.

Atomizer - part of an oil gun which breaks up the fuel oil flow into tiny particles by both mechanical means the use of an atomizing medium. The oil and atomizing medium mix together in the atomizer and then flow to the oil tip to be discharged into the furnace.

Attemperator - Apparatus for reducing and controlling the temperature of a superheater vapor or of a fluid.

Audible sound - Vibrations in a gas, liquid, or solid with components falling in the frequency range of 16Hz to 20Hz.

Automatic lighter or igniter - A means for starting ignition of fuel without manual intervention. Usually applied to liquid, gaseous, or pulverized fuel.

Available draft - The draft which may be utilized to cause the flow of air for combustion or the flow of products for combustion.



Backing ring - A strip of thin plate used on the inner surfaces of the abutting ends of pipe, tubes, or plates which are butt-welded. Its purpose is to prevent irregularities at the base weld and to permit penetration at its root.

Baffle - A plate or wall for deflecting gases or liquids.

Bag - A deep bulge in the bottom of the shell or furnace of a boiler.

Bag filter - A device containing one or more cloth bags for recovering particles from dust laden gas or air which is blown through it.

Balanced draft - The maintenance of a fixed value of draft in a furnace at all combustion rates by control of incoming air and outgoing products of combustion.

Barometric pressure - Atmospheric pressure as determined by a barometer usually expressed in inches of mercury.

Barrel - The cylindrical portion of a firetube boiler shell that surrounds the tubes.

Base load - The term applied to that portion of a station or boiler load that is practically constant for long periods.

Beta Ratio - For a single orifice the beta ratio is the ratio of the orifice bore diameter to that of the upstream pipe diameter. However, since in burner designs typically there is more than one orifice at a riser pipe exit, the beta ratio is equal to the square root of the ratio between total area of the fuel ports to that of the upstream pipe area.

Bias - The output plus (or minus) some arbitrary value.

Black light - In magnetic particle inspection, light in the near ultraviolet range of wavelengths, just shorter than visible light.

Blast furnace gas - Lean combustible by-product gas resulting from burning coke with a deficiency of air in a blast furnace.

Blowback - The number of pounds per square inch of pressure drop in a boiler from the point where the safety valve pops to the point where the safety valve reseats.

Blowback ring - An adjustable ring in a safety valve, used to control the amount of blowback.

Blowdown - The drain connection including the pipe and the valve at the lowest practical part of a boiler, or at the normal water level in the case of a surface blowdown. The amount of water blown down.

Blowdown valve - A valve generally used to continuously regulate concentration of solids in the boiler, not the drain valve. 

Blower - A fan used to force air under pressure.

Boiler - A closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum by the application of heat from combustible fuels, electricity, or nuclear energy. The term dose not include such facilities of an integral part of a continuous processing unit but does include fired units of heating or vaporizing liquids other than water where these units are separate from processing systems and are complete within themselves.

Boiler header (box) - A pressure part of the boiler consisting of a flat tube sheet into which the ends of the water tubes are rolled. In a parallel plane is a tube cap or handhole sheet. The two sheets are spaced about 4 to 8 in. or more apart. The top and bottom and both ends are flanged together and riveted or may be closed by a narrow flanged strip of plate riveted to each sheet. Circulating nipples connect the top of the header and drum, or the header may be flanged and riveted directly to the drum. Welding would be used today instead of rivets.

Boiler, high-pressure, steam or vapor - A boiler in which steam or vapor is generated at a pressure exceeding 15 psig.

Boiler, hot-water-heating - A boiler in which no steam is generated and from which hot water is circulated for heating purposes and then returned to the boiler.

Boiler, hot-water-supply - A boiler functioning as a water heater.

Boiler, low-pressure-steam or vapor - A boiler in which steam or vapor is generated at a pressure not exceeding 15 psig.

Boiler horsepower - The evaporation of 34 ½ lbs. of water per hour from a temperature of 212oF into dry saturated steam at the same temperature. Equivalent to 33,475 Btu.

Boiler water - A term construed to mean a representative sample of the circulating boiler water, after generated steam has been separated and before the incoming feed water or added chemical becomes mixed with it so that its composition is affected. (ASTM - D860)

Boiling - The conversion of a liquid into vapor with the formation of bubbles.

Boiling out - The boiling of a highly alkaline water in boiler pressure parts for the removal of oils, greases, etc. prior to normal operation or after major repairs.

Bourdon tube - A hollow, metallic tube, bent semicircular, which forms the actuating medium of a pressure gauge.

Breeching - A duct for transport of the products of combustion between parts of a steam-generating unit to the stack.

Bridgewall - A wall in the furnace over which the products of combustion pass.

Brinell test
- A hardness test performed by pressing a steel ball of standard hardness into a surface by a standard pressure.

Btu (British Thermal Unit)  - A standard measure of energy in the British unit system. 1 Btu is the amount of heat required to raise the temperature of a liquid by 1 degree. 

Brittle - A metal is brittle when it permits little or no plastic deformation prior to fracture.

Buckstay - A structural member placed against a furnace or boiler wall to limit the motion of the wall against furnace pressure.

Bulge - A local distortion or swelling outward caused by internal pressure on a tube wall or boiler shell due to overheating. Also applied to similar distortion of a cylindrical furnace due to external pressure when overheated, provided the distortion is of a degree that can be driven back.

Bunker C oil - Residual fuel oil (no. 6 fuel oil) of high viscosity commonly used in marine and stationary steam power plants.

Burner - A device which combines fuel and air in proper proportions for combustion and which enables the fuel-air mixture to burn stably to give a specified flame size and shape.

Burner assembly - A burner that is factory-built as a single assembly or as two or more assemblies which include all parts necessary for its normal function when installed as intended.

Burner, atmospheric - A gas burner in which all air for combustion is supplied by natural draft, the inspiriting force being created by gas velocity.

Burner, natural-draft type - A burner which depends primarily on the natural draft created in the flue to induce the air required for combustion into the burner.

Burner, power - A burner in which all air for combustion is supplied by a power-driven fan that overcomes the resistance through the burner to deliver the quantity of air required for combustion.

Burner block - Also called "burner tile", "muffler block," or "quarl". The specially formed refractory pieces which mount around the burner opening inside the furnace. The burner block forms the burner's airflow opening and helps stabilize the flame.

Burner capacity - Amount of heat release a burner can deliver (i.e., amount of fuel which can be completely burned through a burner) at a given set of operating conditions.

Burner windbox - A plenum chamber around a burner in which an air pressure is maintained to ensure proper distribution and discharge of secondary air.

Bypass temperature control - Control of vapor or air temperature by diverting part of or all the heating medium from passing over the heat-absorbing surfaces, usually by means of a bypass damper.



Calorie - The amount of energy required to raise the temperature of 1 gram of water by 1 degree C. The kilocalorie (kcal) is a typical unit of measure in the process industry, 1 kcal = 1000 calories.

Carryover - The moisture and entrained solids forming the film of steam bubbles; a result of foaming in the boiler. Carryover is caused by a poor water condition within the boiler.

Casing - A covering of sheets of metal or other material such as fire resistant composition board used to enclose all or a portion of a steam-generating unit.

Caustic cracking - Also called caustic embrittlement cracking, usually occurring in carbon steels or iron-chromium nickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 400 to 480 deg. F.

Check valve - A valve designed to prevent reversal of flow. Flow in one direction only is permitted.

Circulating tube - A boiler tube used to connect the water spaces of two drums or the pressure parts of a boiler.

Closed feedwater heater - An indirect -contact feedwater heater; that is, one in which the steam and water are separated by tubes or coils.

Closing-in-line - The sealing by plastic refractory between a boiler shell or head and the firebrick wall; used to prevent hot gases from contacting the boiler above the lowest waterline.

Colloid - A finely divided organic substance which tends to inhibit the formation of dense scale and results in the deposition of sludge, or causes it to remain in suspension, so that it may be blown from the boiler.

Combined feeder cutoff - A device that regulates makeup water to a boiler in combination with a low water fuel cut off.

Combustible - The heat-producing constituents of a fuel.

Combustible loss - The loss representing the unliberated thermal energy occasioned by failure to oxide completely some of the combustible matter in the fuel.

Combustion - The rapid reaction of fuel and oxidant (usually oxygen in air) to produce light, heat and noise. Major products of combustion for hydrocarbon fuels (e.g., natural gas, refinery gas, fuel oils) are carbon dioxide and water vapor. Trace products include carbon monoxide and nitrogen oxides, which are pollutants. 

Combustion chamber - See furnace

Combustion efficiency - The fraction of carbon in the fuel that is converted into CO2 in the flue gas, customarily expressed as a percent.

Combustion rate - The quantity of fuel fired per unit of time, as pounds of coal; per hour or cubic feet of gas per hour.

Combustion (flame) safeguard - A system for sensing the pressure or absence of flame and indicating, alarming or initiating control action.

Condensate - Condensed water resulting from the removal of latent heat from steam.

Conduction - The transfer of heat by molecular collision. This process is more efficient in metals and other thermal conductors and poorer in fluids and insulators such as refractory.

- The amount of heat (Btu) transmitted in 1 hour through 1 sq. ft. of a homogenous material 1 in. thick for a difference in temperature of 1o F. between two surfaces of the material.

Continuous blowdown - The uninterrupted removal of concentrated boiler water from a boiler to control total solids concentration in the remaining water.

Control - A device designed to regulate the fuel, air, water, steam, or electrical supply to the controlled equipment. It may be automatic, semi-automatic or manual.

Control, limit - An automatic safety control responsive to changes in liquid level, pressure, or temperature; normally set beyond the operating range for limiting the operation of controlled equipment.

Control, operating - A control, other than s safety control or interlock, to start or regulate input according to demand and to stop or regulate input on satisfaction of demand. Operating controls may also actuate auxiliary equipment.

Control, primary safety - A control responsive directly to flame properties, sensing the presence of flame and, in the event of ignition failure or unintentional flame extinguishments, causing safety shutdown.

Control, safety - Automatic controls and interlocks (including relays, switches, and other auxiliary equipment used in conjunction to form a safety control system) which are intended to prevent unsafe operation of the controlled equipment.

Convection - The transfer of heat or mass by large-scale fluid movements. When the process occurs, due to density and temperature differences, it is termed natural convection. When the process occurs due to external devices (such as fans), it is termed forced convection.

Convection section - The part of a furnace between the radiant section and the stack. The area is filled with tubes or pipes which carry process steam and which absorb heat via convection heat transfer from the hot gases passing through the area on their way out of the stack. The convection section forms an obstacle to the combustion gas flow and can greatly affect furnace draft in the radiant section of the furnace.

Corrosion - The wasting away of metals as a result of chemical action. In a boiler, usually caused by the presence of O2, CO2, or an acid.

Corrosion fatigue - Cracks produced by the combined action of repeated or fluctuating stress and a corrosive environment, which produces the cracking at lower stress levels or fewer cycles of stress than would be the case if no corrosive environment were present.

Course - A circumferential section of a boiler shell or drum. With usual diameters, the number of courses will equal the number of plates forming the shell or drum.

Creep - The time-dependent stretching or strain, heavily influenced by temperature, of a material under stress.

Crimping tool - A tool used to reduce the diameter of the end of a boiler tube preparatory to its removal from a boiler.

Critical pressure and critical temperature - That point at which the difference between the liquid and vapor states for water completely disappears.

Cross-box - A boxlike structure to the longitudinal drum of a sectional header boiler forconnecting circulating tubes.

Crown sheet - The plate forming the roof of an internally fired furnace or a combustion chamber.

C-scan - In ultrasonic testing a means of data presentation to show a plan view of the material, and of any discontinuities therein.


Dba - "A" weighted average of the sound pressure levels over the entire frequency band. Intended to be a more accurate representation of how a human hears sound.

Damper - A device for introducing a variable resistance of regulating the volumetric flow of gas or air.

Decibel - Unit of sound pressure or power. Abbreviation is "dB". 1 Watt of sound power is equal to 120 dB.

Deaerator - A type of feedwater heater operating with water and steam in direct contact. It is designed to heat the water and drive off oxygen.

Deflector - A device used to change the direction of a stream of air or of a mixture of pulverized fuel and air.

Design pressure - The pressure used in the design of a boiler for the purpose of determining the minimum permissible thickness or physical characteristics of the different parts of a boiler.

Differential - (of a control) the difference between cut in and cut out points.

Diffusion (raw gas) flame - Combustion state controlled by mixing phenomena. Fuel and air diffuse into one another until a flammable mixture ratio is achieved.

Downcomer - A tube or pipe in a boiler or waterwall circulating system through which fluid flows downward between headers.

Draft - The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of the steam-generating unit.

Draft control, barometric - A device that controls draft by means of a balanced damper which bleeds air into the breeching on changes of pressure to maintain steady draft.

Draft differential - The difference in static pressure between two points in a system.

Drip leg - The container placed at a low point in a system of piping to collect condensate and from which it may be removed.

Drum - A cylindrical shell closed at both ends, designed to withstand internal pressure.

Dry back - The baffle provided in a firetube boiler joining the furnace to the second pass to direct the products of combustion, that is so constructed to be separate from the pressure vessel and constructed of heat resistant material. (Generally refractory and insulating material)

Dry steam - Steam containing no moisture. Commercially dry steam containing not more than 0.5 percent moisture.

Duct - A passage for air or gas flow.

Economizer - A series of tubes located in the path of flue gases. Feedwater is pumped through these tubes on its way to the boiler in order to absorb waste heat from the flue gas.

Efficiency - Of boiler operation: Output in heat units divided by input in heat units. The number of Btus contained in all steam evaporated is useful output. The number of Btus contained in all fuel supplied to the boiler is input.

Ejector - A device which utilizes the kinetic energy in a jet of water or other fluid to remove a fluid or fluent material from tanks or hoppers.

Electric boiler - A boiler in which electric heating means serve as the source of energy.

Embrittlement - An intercrystalline corrosion of boilerplate occurring in highly stressed zones. Cracking may result.

Emissivity - The efficiency with which a material radiates thermal energy, expressed as a fraction between 0 and 1.

Enthalpy - A thermal property of a fluid which is a function of state and is defined as the sum of stored mechanical potential energy and internal energy. It is generally expressed in Btu per pound of fluid (joules per kilogram).

Entrainment - The conveying of particles of water or solids from the boiler water by the steam.

Evaporation rate - The number of pounds of water evaporated in a unit of time.

Evaporator - A pressure vessel used to evaporate raw water by means of a steam coil. The steam is condensed by means of cooling water coils, and this distilled water is used as make-up for boiler feed.

Excess air - The amount of air needed by a burner which is in excess of the amount required for perfect or stoichiometric combustion. Some amount of excess air, depending on the available fuel/air mixing energy, is required to assure through mixing of the fuel and air for complete combustion.

Explosion door - A door in a furnace or boiler setting designed to be opened by a predetermined gas pressure.


Fan - A machine consisting of a rotor and housing for moving air or gases at relatively low-pressure differentials.

Fan performance - A measure of fan operation in terms of volume, total pressures, static pressures, speed, power input, and mechanical and static efficiency, at a stated air density.

Fan performance curves - The graphical presentation of total pressure, static pressure, power input, and mechanical and static efficiency as ordinates and the range of volumes as absciassas, all at constant speed and air density.

Feedwater regulator - A device for admitting feedwater to a boiler automatically on demand. Practically a constant water level should result.

Firebox - The equivalent of a furnace. A term usually used for furnaces of locomotive boilers and similar types of boilers.

Firetube - A tube in a boiler having water on the outside and carrying the products of combustion on the inside.

Firing rate control - A pressure temperature or flow controller which controls the firing rate of a burner according to the deviation from pressure or temperature set point. The system may be arranged to operate the burner on-off, high-low or in proportion to load demand.

Flame - A luminous body of burning gas or vapor.

Flame detector - A device which indicates if fuel, such as liquid, gaseous, or pulverized, is burning or if ignition has been lost. The indication may be transmitted to a signal or to a control system.

Flame propagation rate - Speed of travel of ignition through a combustible mixture. (See flame speed)

Flame speed - The rate at which a flame can propagate in a combustion mixture. If the flame is lower than the speed of the reacting flow, the flame may lift off the burner. If the flame speed is higher than the speed of the reacting flow the flame may flash back into the burner.

Flammability limits - The upper and lower bounds of the fuel/air mixture which will support combustion. The upper flammability limit indicates the maximum fuel concentration in air that will support combustion. The lower flammability limit indicates the minimum fuel concentration in the air that will support combustion. Outside of these bounds the mixture does not burn.

Flareback - A burst of flame from a furnace in a direction opposed to the normal flow, usually caused by the ignition of an accumulation of combustible gases.

Flashback - A phenomenon occurring only in pre-mix gas burners when the flame speed overcomes the gas-air mixture flow velocity exiting the gas tip. The flame rushes back to the gas orifice and can make an explosive sound when flashback occurs. Flashback is most common when hydrogen is present in fuel gas.

Flashing - The process whereby a drop in pressure or increase in temperature cause vaporization.

Flue - A passage for products of combustion.

Flue gas - The gaseous products of combustion in the flue to the stack.

Foaming - Formation of steam bubbles on the surface of boiler water due to high surface tension of the water. See carryover.

Forced circulation - The circulation of water in a boiler by mechanical means external to the boiler.

Forced-draft fan - A fan supplying air under pressure to the fuel burning equipment.

Fouling - The accumulation of refuse in gas passages or on heat absorbing surfaces which results in undesirable restrictions to the flow of gas or heat.

Fuel NOx - NOx that is formed from nitrogen that is organically bound to the fuel molecule. Fuel NOx is most often a problem with liquid fuel or coal burning. Once the nitrogen has been cracked from the fuel molecule, the mechanism follows basically the same path as the prompt NOx mechanism.

Furnace - An enclosed space provided for the combustion of fuel.

Furnace arch - Uppermost part of a radiant furnace (also called the "bridgewall", a term which came from the original furnace designs and has remained in use). The last area in an upflow furnace before the convection section.

Furnace Draft - The negative air pressure generated by buoyancy of hot gases inside a furnace. The temperature difference between gases within the furnace and in the atmosphere along with furnace and stack height basically determine the amount of draft generated by the furnace. Draft is generally measured in negative inches of water column. ("-w.c."; 27.7 inches w.c. = 1 psig)

Furnace explosion - A violent combustion of dust or gas accumulations in a furnace or combustion chamber of a boiler.

Furnace release rate - The heat available per square foot of heat-absorbing surface in the furnace. That surface is the projected area of tubes and extended metallic surfaces on the furnace side including walls, floors, roof, partition walls, and platens and the area of the plane of the furnace exit which is defined as the entrance to the convection tube bank.

Furnace volume - The cubical contents of the furnace or combustion chamber.


Gage cock - A valve attached to a water column or drum for checking water level.

Gage pressure - The pressure measured relative to the local atmospheric pressure. Gage pressure may be negative. A negative gage pressure is known as suction or vacuum.

Gas analysis - The determination of the constituents of a gaseous mixture.

Gasification - The process of converting solid or liquid fuel into a gaseous fuel such as the gasification of coal.

Gas recirculation - The reintroduction of part of the combustion gas at a point upstream of the removal point, in the lower furnace for the purpose of controlling steam temperature.

Gas tip - The part of a burner which discharges the gas fuel via one or more openings into the furnace. The size, arrangement, and angular disposition of the openings in the tip have a major effect on the size and shape of the flame.

Gate valve - A stop valve using the wedge-and-double-seat principle. It may be used to control fluids containing some solids, for when wide open, it operates on a straight-through flow. There is little likelihood of its becoming obstructed.

Gauge glass - A glass-enclosed visible indicator of the water level in a boiler. Many gauge glasses are tubular, but modern high-pressure practice and railroad locomotives use two thick, flat strips of glass bolted between flanged plates, with the water and steam between the glass strips.

Gauge pressure - The pressure above that of atmospheric, 14.7 psi at sea level; absolute pressure minus 14.7 at sea level.

Globe valve - A stop valve using the round-disk-and-seat principle. Used where the fluid controlled is comparatively clean.

Grain - A unit of measure commonly used in water analysis for the measurement of impurities in water. (17.1 grains = 1 part per million - ppm)

Handhole - An inspection, a sight, or a cleanout opening in a boiler; often elliptical and closed by a handhole plate.

Handhole cover - A handhole enclosure.

Hardness - A measure of the amount of calcium and magnesium salts in boiler water . Usually expressed as grains per gallon or parts per million as CaCO2

Hard water - Water which contains calcium or magnesium in an amount which requires an excessive amount of soap to form a lather.

Header - A distribution pipe supplying a number of smaller lines tapped off of it. A main receiving pipe supplying one or more main pipe lines and receiving a number of supply lines tapped into it. 

Heat liberation - Amount of heat released during combustion of fuels. One of the criteria for determining what burner to use in an application.

Heating surface - That surface which is exposed to the heating medium for absorption and transfer of heat to the heat medium per American Boiler Manufacturers Association (ABMA)

Heat release - The total quantity of thermal energy above a fixed datum introduced into a furnace by the fuel, considered to be the product of the hourly fuel rate and its high heating value, expressed in Btu per hour per cubic foot of furnace volume or square foot of heating surface.

High fire - The input rate of a burner at or near maximum.

High gas pressure switch - A switch to stop the burner if gas pressure is too high.

Higher heating value - HHV, the theoretical heat the combustion process can release if the fuel and oxidant are converted with 100% efficiency to CO2 and liquid H20.

Hot well - A tank used to receive condensate from various sources on its passage back to a boiler through the feedwater system. It usually is vented to atmosphere.

Hydrostatic test - A pressure test by water at room temperature applied to a boiler to determine its safety, as a check on repairs or to trace suspected leakage.

Igniter - A burner smaller than the main burner, which is ignited by a spark or other independent and stable ignition source and which provides proven ignition energy required to immediately light off the main burner.

Ignition temperature - the temperature required to initiate combustion.

Impeller - The rotating wheel of a centrifugal pump.

Impingement - The striking of moving flame against boiler parts, causing local overheating.

Incomplete combustion - The partial oxidation of the combustible constituents of a fuel.

Induced draft fan - A fan exhausting hot gases from heat absorbing equipment.

Input rating - The fuel burning capacity of a burner at sea level in Btu per hour as specified by the manufacturer.

Insulation - A material of low thermal conductivity used to reduce heat loss.

Interlock - A device to prove the physical state of a required condition and to furnish that proof to the primary safety control circuit.

Intermittent firing - A method of firing by which fuel and air are introduced and burned in a furnace for a short period after which flow is stopped, this succession occurring in a sequence of frequent cycles.

Intermittent ignition - An igniter which burns during light off and while the main burner is firing and which is shut off with the main burner.

Internally fired boiler - A firetube boiler having an internal furnace such as a scotch, locomotive firebox, vertical tubular, or other type having a water-cooled plate type furnace.

Ion - A charge atom or radical which may be positive or negative.

Ion exchange - A reversible process by which ions are interchanged between solids and a liquid. These ions exist throughout the solution and act almost independently.


Lagging - A covering, usually of insulating material, on pipe or ducts.

Laminar flow - Very smooth flow in which all the molecules are traveling in generally the same direction. For internal flows, it occurs at Reynolds numbers less than 2000.

Lift-off - This condition occurs when the fuel or fuel/air mixture velocity is too high, thus allowing the fuel to exit the stabilizing zone before it has achieved its ignition temperature.

Lining - The material used on the furnace side of a furnace wall. It is usually high-grade refractory tile or brick or plastic refractory material.

Live steam - Steam which has not performed any of the work for which it was generated.

Low draft switch - A control to prevent the burner operation if the draft is too low. Used primarily with mechanical draft.

Low fire start - The firing of a burner with controls in a low-fire position to provide safe operating condition during light off.

Low gas pressure switch - A control to stop the burner if gas pressure is too low.

Lower heating value - LHV, The theoretical heat the combustion process can release if the fuel and oxidant are converted to 100% efficiency to CO2 and H2O vapor.

Low-oil-temperature switch - A cold-oil switch; a control to prevent burner operation if the temperature of the oil is too low. 

Low-water cutoff - A device to stop the burner on unsafe water conditions in the boiler.

Lug - As applied to boiler suspension, a steel eyepiece fitted and riveted or welded to the curvature of a boiler shell or drum and connected by a steel U-bolt or sling rod to overhead steel structure; used to support the weight of a boiler.

Makeup water - The amount of raw water necessary to compensate for the amount of condensate that is not returned in the feedwater supply to the boiler.

Manhole - An access opening to the interior of a boiler, elliptical and 11 in. by 15 in. or larger or circular 15-in. diameter or larger.

Manifold - A pipe or header for collecting a fluid from, or the distributing of a fluid to a number of pipes or tubes.

Manual reset device - A component of a control which requires resetting by hand to restart the burner after safe operating conditions have been restored.

Mechanical draft - The negative pressure created by mechanical means.

Micron - One millionth of a meter, or 0.000039 in. or 1/25400 in The diameter of dust particles is often expressed in microns.

Mixer - The part of a pre-mix burner (also gas-air mixer) which uses the kinetic energy of the high velocity fuel gas stream to draw in part or all of the air required by the burner for combustion.

Mud or lower drum - A pressure chamber of a drum or header type located at the lower extremity of a water-tube boiler convection bank which is normally provided with a blowoff valve for periodic blowing off of sediment collecting in the bottom of the drum.

Multifuel burner - A burner by means of which more than one fuel can be burned either separately or simultaneously, such as pulverized fuel, oil and gas.

Natural circulation - The circulation of water in a boiler caused by differences in density; also referred to as thermal or thermally induced circulation.

Natural gas - Gaseous fuel occurring in nature.

Net fan requirements - The calculated operating conditions for a fan excluding tolerances.

Noise - An undesirable sound.

Normal cubic meter - (Nm3) The quantity of a gas that is present in 1 m3 at the thermodynamic conditions of 1 atm and 0o C. For an ideal gas there are 22.41 Nm3 in 1 kmol.

Nozzle - A short flanged or welded neck connection on a drum or shell for the outlet or inlet of fluids; also a protecting spout for the outlet or inlet of fluids; also a projecting spout through which fluid flows.

NOx - Any combination of nitrogen and oxygen in a compound form. The most common in terms of environmental considerations is NO, which constitutes 90% of combustion NOx emissions, and NO2. All NO is eventually converted to NO2 in the atmosphere. Hence, most regulations are written to assume that the NOx which is emitted is in the form of NO2. Nox emissions are influenced by many factors, including furnace temperature, flame temperature, burner design, combustion air temperature, nitrogen content of liquid fuels, ammonia content of gas fuels, and other factors.

Oil block - Usually a monolithic block located at the center of a burner assembly. The oil block acts to stabilize the oil flame.

Oil burner - A burner that atomizes fuel oil and blows it into the combustion chamber in the form of a fine mist or vapor. Steam or mechanical motion plus air may be used as the operating medium.

Oil gun - The assembly of parts in a burner which provides atomized fuel oil mixture to the furnace for burning.

Oil tip - Part of the oil gun which discharges the atomized fuel oil mixture into the furnace through multiple openings. The hole pattern in the tip has a great effect on flame size and shape.

Operating control - A control to start and stop the burner; it must be set below the high limit control.

Orifice - (1) The opening from the whirling chamber of a mechanical atomizer or the mixing chamber of a steam atomizer thru which the liquid fuel is discharged. (2) A calibrated opening in a plate, inserted in a gas stream for measuring the velocity of flow.

Orifice discharge coefficient (Cd) - The ratio of the actual flow through an orifice to that of the theoretical or isentropic flow through an orifice. Basically this parameter is a measure of the orifice efficiency. Valves are dimensionless and range from 0.61 for a thin-plate orifice to 0.85 for thick plate square-edged orifices, and up top 0.90 - 0.95for tapered orifices.

Orsat - An instrument for determining the chemical analysis of flue gas.

Oxidation - Chemical combination with oxygen.

Oxygen attack - Corrosion or pitting in a boiler caused by oxygen.


PAH (polycyclic aromatic hydrocarbons) - The carcinogenic byproducts of some very sub-stoichiometric combustion processes. Usually absent in process burners.

Packaged boiler - A boiler equipped and shipped complete with fuel-burning equipment, mechanical draft equipment, automatic controls and accessories; usually shipped in one or more major sections.

Particulates - The residue left over from coal and fuel oil combustion.

Pascals - A unit of pressure. One Pascal (Pa) is equal to a force of one Newton per square meter.

Pass - A confined passageway, containing heating surface, through which fluid flows essentially one direction.

Perfect or Stoichiometric combustion - The complete oxidation of all the combustible constitutes of a fuel, utilizing all the oxygen supplied.

pH - The hydrogen ion concentration of a water to denote acidity or alkalinity. A pH of 7 is neutral. A pH above 7 denotes alkalinity while one below 7 denotes acidity. 

Pilot - A small burner which is used to light the main burner.

Pilot, constant - A pilot that burns without turndown throughout the entire time the boiler is in service.

Pilot flame establishing period - The length of time fuel is permitted to be delivered to a proved pilot before the flame-sensing device is required to detect pilot flame.

Pilot, proved - A pilot flame which has been proved by flame-failure controls.

Pit - Corrosion localized in a small spot.

Plenum - An enclosure through which gas or air passes at relatively low velocities.

Postpurge - A period after the fuel valves close during which the burner motor or fan continues to run, to supply air to the combustion chamber.

Prepurge period - A period on each start-up during which air is introduced into the combustion chamber and associated flue passages in volume and manner as to completely replace the air or fuel air-mixture contained therein prior to an attempt to initiate combustion.

Pre-mixed flame - Before ignition, the fuel and air are intimately mixed. The combustion process is controlled by heat conduction and diffusion of radicals.

Pressure - As applied to boilers, the force exerted by a liquid or gas on a unit area. Three pressures may be involved: gauge pressure, the unit pressure above atmospheric pressure; absolute pressure, gauge pressure plus the atmospheric pressure; vacuum pressure; the pressure below atmospheric pressure usually expressed in inches of Hg.

Pressure, gas - The force exerted per unit area on a surface created by the collision of gas molecules with the surface.

Pressure, static - The pressure of a gas measured at a point where the gas velocity is zero.

Pressure, total - The sum of the static pressure and the velocity pressure of the gas.

Pressure, velocity or dynamic - The pressure of flowing gas attributed to the impact of gas molecules resulting from the velocity of the gas flow. 

Primary air - Air introduced with fuel at the burners. 

Priming - An induction of boiler water caused by the steam flow into the steam line. The water may be in the form of a spray or a solid body.

Process steam - Steam used for industrial purposes other than for producing power.

Prompt NOx - NOx formed at the initial stages of combustion that cannot be explained by either the thermal mechanism or the fuel NOx mechanism. The prompt NOx mechanism requires the CH radical as an intermediate, so the fuel must have carbon present to create prompt NOx. 

Proportional control - A mode of control in which there is a continuous linear relation between value of the controller variable and position of the final control element (modulating control).

Puff - A minor combustion explosion within the boiler furnace or setting.

Pulsation - Rapid fluctuations in furnace pressure.

Purge interlock - A device so arranged that an air flow to the furnace above a minimum must exist for a definite time interval before the interlocking system will permit an automatic ignition torch to be placed in operation.


Radiant - As applied to heat, having the property that permits heat to be transmitted by rays similar to those of light. To absorb radiant heat, an object must be in the "light" of the fire.

Radiant section - The part of a process heater into which the burners fire. Tubes mounted in this area of the furnace receive heat principally via direct radiation from both burner flames and furnace refractory. Physical volume arrangement of the radiant section has a great effect on burner choice and required flame patterns.

Radiation - All warm bodies emit light (electromagnetic radiation - mostly infrared). When this radiation is absorbed or emitted by a body, heat is transferred and termed "heat transfer by radiation". Such heat transfer requires a line of sight (view factor) and is proportional to the fourth power of the absolute temperature difference between bodies and the emissivity of the bodies.

Rated capacity - The manufacturer's stated capacity rating for mechanical equipment, for instance, the maximum continuous capacity in pounds of steam per hour for which a boiler is designed.

Ratio of specific heats (k) - Also known as isentropic coefficient. Is equal to the quotient of the heat capacity at constant pressure and the heat capacity at constant volume. (Cp/Cv). This parameter is tabulated for many pure components at standard conditions, but is technically dependent on the gas composition and temperature. The values are dimensionless and range from 1.0 to 1.6.

Raw water - Untreated feedwater.

Recycle - The process of sequencing a normal burner start-up following shutdown.

Refractory - A heat-insulating material, such as firebrick or plastic fire clay, used for purposes as lining combustion chambers.

Reheater - A device using highly superheated steam or high-temperature flue gases as a medium serving to restore superheat to partly expanded steam; used often between high - and low-pressure turbines.

Relay - A device that is operative by a variation in the conditions of one electric circuit to start the operation of other devices in the same or another electric circuit (such as pressure or temperature relay).

Return trap - A trap designed to discharge its condensate against boiler pressure and feed to the boiler without additional mechanical equipment.

Ringlemann chart - A series of four rectangular grids of black lines of varying widths printed on white background, used in criterion of blackness for determining smoke density from chimneys.

Riser tube - A tube through which steam and water pass from an upper waterwall header to a drum.

Regulator, gas pressure - A spring loaded, dead weighted or pressure balanced device which will maintain the gas pressure to the burner supply line.


Safety shut down - The action of shutting off all fuel and ignition energy to the burner by means of safety control or controls such that restart cannot be accomplished without operator action.

Safety valve - A valve that automatically opens when pressure attains the valve setting which is adjustable; used to prevent excessive pressure from building up in a boiler.

Safety valve drain - A hole of at least 3/8 in diameter required through the body below the valve-seat level in safety valves larger than 2-in diameter; used to prevent condensate from collecting at this point.

Safety valve escape - A pipe conducting steam discharged from a safety valve to a safe location.

Safety-valve lifting lever - A lever by which safety valve may be lifted from its seat. 

Safety-valve muffler - A silencer designed so that it will not cause appreciable restriction to steam flow.

Safety-valve nozzle - A flanged nozzle by which a safety valve is connected to a boiler shell or drum.

Scale - A deposit of medium to extreme hardness occurring on water heating surfaces of a boiler because of an undesirable condition of boiler water.

Scrubber - An apparatus for the removal of solids from gases by entrainment in water.

Seal weld - A weld used primarily to obtain tightness and prevent leakage.

Secondary combustion - Combustion which occurs as a result of ignition at a point beyond the furnace.

Secondary treatment - Treatment of boiler feedwater or internal treatment of boiler water after primary treatment.

Separator - A tank-type pressure vessel installed in a steam pipe to collect condensate to be trapped off and thus providing comparatively dry steam to connect machinery.

Shell - The cylindrical portion of a pressure vessel.

Silica - A scale-forming element found in some boiler feedwaters.

Sinous header - A header of a sectional header-type boiler in which the sides are curved back and forth to suit the stagger of the boiler tubes connected to the header faces.

Siphon - A pigtail-shaped pipe or a drop leg in the pipe leading to a steam pressure gauge, serving to trap water in the gauge and prevent overheating from direct contact with steam.

Slug - A solid body of boiler water passed into the steam flow by priming or picked up from a pocket of condensate in the steam line.

Softening - The act of reducing scale-forming calcium and magnesium impurities from water.

Soot blower - A tube from which jets of steam or compressed air are blown for cleaning the fireside of tubes or other parts of the boiler.

Sonic flow - When the flow velocity is equal to the speed of sound. The point at which the flow turns sonic is called critical pressure. This transition occurs at about 12.2 psig for natural gas at 60o F.

Spalling - The breaking off of the surface refractory material as a result of internal stresses.

Specific gravity - The ratio of the weight of a unit volume of a material to the weight of the same unit volume of water.

Specific heat - The quantity of heat, expressed in Btu (joule) required to raise the temperature of 1 lb. (kilogram) of a substance 1oF (oC)

Spontaneous combustion - Ignition of combustible material following slow oxidation without the application of high temperature from an external source.

Spray nozzle - A nozzle from which a liquid fuel is discharged in the form of a spray.

SSU - (seconds, Saybolt Universal) units of kinematic viscosity.

Stack - A vertical conduit, which due to the difference in density between internal and external gases creates a draft at its base.

Stack draft - The magnitude of the draft measured at inlet to the stack.

Stack effluent - Gas and solid products discharged from the stack.

Stack loss - The fraction of total heat which exits with the flue gas through the stack. The quantity is customarily expressed as a percent of the total heat input. The stack loss is directly proportional to the stack exit temperature; the higher the temperature, the greater the stack loss.

Staged air - NOx reduction technique predominantly used for fuel oil firing. The fuel is injected into a fuel-rich primary zone. This stoichiometry helps to control the fuel Nox mechanism. When firing gas, staged air produces higher NOx emissions than staged fuel.

Staged fuel - NOx reduction technique whereby a small portion of the fuel is injected in a lean primary combustion zone. The flue products from this region flow to the secondary combustion zone where the remainder of the fuel is burned out. The lengthening of the flame creates cooler flame temperatures, thus lowering thermal NOx.

Static pressure - The measure of potential energy of a fluid.

Staybolt - A bolt threaded through or welded at each end, into two spaced sheets of a firebox or box header to support flat surfaces against internal pressure.

Steam - The vapor phase of water substantially unmixed with other gases.

Steam atomizing oil burner - A burner for firing oil which is atomized by steam. It may be of the inside or outside mixing type.

Steam binding - A restriction in circulation due to a steam pocket or rapid steam formation.

Steam Gage - A gage for indicating the pressure of steam.

Steam generating unit - A unit to which water, fuel, and air are supplied and in which steam is generated. It consists of a boiler furnace, and fuel burning equipment, and may include as component parts water walls, superheater, reheater, economizer, air heater, or any combination thereof.

Steam purity - The degree of contamination. Contamination usually expressed in ppm.

Steam quality - The percent by weight of a vapor in a steam and water mixture.

Steam scrubber -A series of screens, wires, or plates through which steam is passed to remove entrained moisture.

Steam separator - A device for removing entrained water from steam.

Strainer - A device, such as a filter, to retain solid particles allowing the liquid to pass.

Sulphate - carbonate ratio - The proportion of sulphates to carbonates, or alkalinity expressed as carbonates, in boiler water. The proper maintenance of this ratio has been advocated as a means of inhibiting caustic embrittlement.

Superheat - To raise the temperature of steam above its saturation temperature. The temperature in excess of its saturation temperature.

Superheated steam - Steam at higher temperature than its saturation temperature.

Surface blowoff - Removal of water, foam, etc. from the surface at the water level in a boiler. The equipment for such removal.

Surge - The sudden displacement or movement of water in a closed vessel or drum.

Suspended solids - Undissolved solids in boiler water.

Swell -The sudden increase in the volume of steam in the water steam mixture below the water level.

Swinging load - A load that changes at relatively short intervals.


Tempering air - Air at a lower temperature added to a stream of pre-heated air to modify its temperature.

Tertiary air - Air for combustion supplied to the furnace to supplement the primary and secondary air.

Theoretical air - The quantity of air required for perfect combustion.

Theoretical draft - The draft which would be available at the base of a stack if there were no friction or acceleration losses from the stack.

Theoretical flame temperature - Same as "adiabatic temperature".

Therm - A unit of heat applied especially to gas. One therm = 100,000 Btu.

Thermal conductivity - The ability of a material to conduct heat, expressed as thermal power conducted per unit temperature and thickness. Metals and other thermal "conductors" have a large thermal conductivity. Refractories and other thermal "insulators" have a low thermal conductivity.

Thermal NOx - NOx formed via the Zeldovich mechanism. The rate-limiting step in this mechanism is the formation of the O radical. This occurs only at high temperatures (above about 2400o F.). Hence the term thermal NOx, since it is NOX produced in the highest temperature regions of the flame.

Thermo acoustical efficiency - Equal to the sound power level/heat release. A value used to characterize the amount of combustion noise emitted from a flame. Defined as the ratio of the acoustical power emitted from the flame to the total heat release of the flame. Approximately equal to 1 X 10-6 for premixed and turbulent flames and equal to 1 X 10-9 for diffusion and laminar flames.

Thermocouple - A temperature measuring instrument.

Throat - The neck portion of a passageway.

Through stay - A brace used in fire-tube boilers between the heads or tube sheets.

Tie plate - A plate, through which a bolt or tie rod is passed to hold brick in place.

Tie rod - A tension member between buckstays or tie plates.

Tile - A preformed, burned refractory, usually applied to shapes other than standard brick.

Time delay - A deliberate delay of a predetermined time in the action of a safety device or control.

Total air - The total quantity of air supplied to the fuel and products of combustion. Percent total air is the ratio of total air to theoretical air expressed as per cent.

Total pressure - The sum of the static and velocity pressures.

Total solids concentration - The weight of dissolved and suspended impurities in a unit weight of boiler water, usually expressed as ppm.

Tramp air - Any air that enters (infiltrates) the furnace through leaks. This air may be measured by the O2 analyzer and often contributes to the burning of the fuel.

Trap - A receptacle for the collection of undesirable material.

Treated water - Water which has been chemically treated to make suitable for boiler feed.

Trail-for-ignition - That period of time during which the programming flame failure controls permit the burner fuel valves to be open before the flame sensing device is required to detect the flame.

Trail for main flame ignition - A timed interval when with the ignition means proved, the main valve is permitted to remain open. If the main burner is not ignited during this period, the main valve and ignition means are cut off. A safety switch lockout follows.

Trail for pilot ignition - A timed interval when the pilot valve is held open and an attempt made to ignite and prove it. If the presence of the pilot is proved at the termination of the interval, the main valve is energized; if not the pilot and ignition are cut off followed by a safety lockout.

Try cock - One of three valves mounted on a boiler or water column within the visible range of the gauge glass and used to check the water level.

Tube - A hollow cylinder for conveying fluids.

Tube cleaner - A device for cleaning tubes by brushing, hammering, or by rotating cutters.

Tube plug - A solid plug driven into the end of a tube.

Turbulent burner - A burner in which fuel and air are mixed and discharged into the furnace in such a manner as to produce turbulent flow from the burner.

Turbulent flow - Characteristically random flow patterns that form eddies from large to small scales. For internal flows, it occurs at Reynolds numbers greater than 4000. Turbulence is integral to the mixing process between the fuel and air for combustion.

UHC - Any unburned hydrocarbon that is emitted in a combustion process. Also termed VOC (volatile organic compound).

Unaccounted for loss - That portion of a boiler heat balance, which represents the difference between 100 per cent and the sum of the heat absorbed by the unit and all the classified losses expressed as per cent.

Unburned combustible - The combustible portion of the fuel, which is not completely oxidized.

Unfired pressure vessel - A vessel designed to withstand internal pressure, neither subjected to heat from products of combustion nor an integral part of a fired pressure vessel system.

Use factor - The ratio of hours in operation to the total hours in that period.

Vane - A fixed or adjustable plate inserted in a gas or air stream used to change the direction of flow.

Vane control - A set of movable vanes in the inlet of a fan to provide regulation of airflow.

Vane guide -
A set of stationary vanes to govern direction, velocity and distribution of air or gas flow.

Valve, manual gas shutoff - A manually operated valve in a gas line for the purpose of completely turning on or shutting off the gas supply.

Valve, manual oil shutoff - A manually operated valve in the oil line for the purpose of completely turning on or shutting off the oil supply to the burner.

Valve, manual reset safety shutoff - A manually opened, electronically latched, electrically operated safety shut-off valve designed to automatically shut off fuel when de-energized. 

Valve, motor driven reset safety shutoff - An electrically operated safety shut-off valve designed to automatically shut off fuel flow upon being de-energized. The valve is opened and reset automatically by integral motor device only.

Valve, fuel control - An automatically or manually operated device consisting essentially of a regulating valve and an operating mechanism. It is used to regulate fuel flow and is usually in addition to the safety shut-off valve. Such valve may be of the automatic or manually opened type.

Vapor - The gaseous product of evaporation.

Vapor generator - A container of liquid, other than water, which is vaporized by the absorption of heat.

Vaporization - The change from liquid or solid phase to the vapor phase.

Velocity pressure - The measure of kinetic energy of a fluid.

Velocity thermocouple - (suction pyrometer) a device for measuring furnace gas temperature. It is comprised of a thermocouple, which has been recessed into an insulating shroud, and a suction device such as an eductor, which aspirates large volumes of furnace gas through the shroud and past the thermocouple. The high velocity of a gas ensures good convective heat transfer to the thermocouple and surrounding furnace. The velocity thermocouple represents the most accurate means to measure flue gas temperature. Bare thermocouples are unacceptable for this purpose, being in error often by more than 100o F due to radiation losses.

Vent - An opening in a vessel or other enclosed space for the removal of gas or vapor.

Vertical firing - An arrangement of a burner such that air and fuel are discharged into the furnace, in practically a vertical direction.

Viscosity - Measure of the internal friction of a fluid or its resistance to flow.

Volatile matter - Those products given off by a material as gas or vapor, determined by definite prescribed methods.

Volume of air - The number of cubic feet of air per minute expressed at fan outlet conditions.

Vortex - (1) The swirling motion of a liquid in a vessel at the entrance to a discharge nozzle. (2) The point in a cyclonic gas path where the two spirals change general direction by 180o.


Waste fuel - Any by-product fuel that is waste from a manufacturing process.

Waste heat - Sensible heat in non-combustible gases.

Water column - A vertical tubular member connected at its top and bottom to the steam and water space respectively to a boiler, to which the water gage, gage cocks, high and low water alarms and fuel cutoff may be connected.

Water gage - The gage glass and its fittings for attachment.

Water hammer - A sudden increase in pressure of water due to an instantaneous conversion or momentum to pressure.

Water level - The elevation of the surface of the water in a boiler.

Water tube - A tube in a boiler having the water and steam on the inside and heat applied to the outside.

Water vapor - A synonym for steam, usually used to denote steam of absolute low pressure.

Weep - A term usually applied to a minute leak in a boiler joint which forms droplets (or tears) of water very slowly.

Wet back - Baffle provided in a firetube boiler joining the furnace to the second pass to direct the products of combustion that is completely water-cooled.

Wet steam - Steam containing moisture.

Windbox - A chamber below the grate or surrounding a burner, thru which air under pressure is supplied for combustion of the fuel.

Windbox pressure - The static pressure in the windbox of a burner or stoker.

Zero governor - A regulating device that is normally adjusted to deliver gas at atmospheric pressure within its flow rating.

All Steam Supply Inc.
13903 Bluewater Circle, Orlando, FL 32828-8320
Phone: 407-282-5731 • Toll Free: 877-21-STEAM • Fax: 407-282-5732
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