Superintendents & Resident Managers Technical Association - Glossary of Terms For Boilers & Heating

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 mill equivalents 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 of the 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.

1. Regenerative air preheater. An air heater in which heat is first stored up in the structure itself by the passage of the products of combustion, and which then gives up the heat so stored to the subsequent passage of air.

2. Recuperative air heater. An air heater in which the heat from products of combustion passes through a partition, which separates the products from the air.

(a ) Tubular air heater. An air heater containing a group of tubular elements through the walls of which heat is transferred from a flowing heating medium to an airstream.

(b ) Plate air heater. An air heater containing passages formed by spaced plates through which heat is transferred from a flowing heating medium to an airstream.

AIR PURGE The removal of undesired matter by replacement with air

AIR-SWEPT PULVERIZERS A pulverizer through which air flows and from which pulverized fuel is removed by the stream of 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. Also known as Maximum Allowable Working Pressure

ANTHRACITE ASTM coal classification by rank: Dry fixed carbon 92 percent or more and less than 98 percent; and dry volatile matter 8 percent or less and more than 2 percent on a mineral-matter-free basis.

APPROVED The world approved as used in a Code means acceptable to the authority having jurisdiction.

ASH The incombustible inorganic matter in the fuel.

ASH SLUICE A trench or channel used for transporting refuse from ash pits to a disposal point by means of water.

ATOMIZING MEDIA A supplementary medium, such as steam or air, which assists in breaking the fuel oil into a fine spray.

ATTEMPERATOR Apparatus for reducing and controlling the temperature of a superheater vapor or of a fluid. See also desuperheater.

1. Shell-and tube type. An attemperator consisting of a pressure vessel containing tubular elements through the walls of which heat is transferred.

2. Spray type. An attemperator in which a lower-temperature fluid is injected at relatively high velocity in an atomized state into the superheater vapor to reduce its temperature by direct contact with the atomized fluid.

3. Submerged type. An attemperator consisting of tubular elements located in the boiler circulation blow the waterline.

AUTHORIZED INSPECTION AGENCY The inspection agency approved by the appropriate legal authority of a state or municipality of the United States or a province of Canada, which has adopted a section of the ASME Code.

AUTOMATIC LIGHTER OR IGNITER A means for starting ignition of fuel without manual intervention. Usually applied to liquid, gaseous, or pulverized fuel. See igniter.

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

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

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 the 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 rate by control of incoming air and outgoing products of combustion.

BANKING Burning solid fuels on a grate at rates sufficient to maintain ignition only.

BARREL The cylindrical portion of a fire-tube-boiler shell that surrounds the tubes.

BITUMINOUS COAL ASTM coal classification by rank on a mineral matter free basis and with bed moisture only.

1. Low volatile. Dry fixed carbon 78 percent or more and less than 89 percent; dry volatile matter 22 percent or less and more than 14 percent.

2. Medium volatile. Dry fixed carbon 69 percent or more and less than 78 percent. Dry volatile matter 22 percent or less and more than 31 percent.

3. High volatile (A). Dry fixed carbon less than 69 percent; dry volatile matter more than 31 percent. Btu value equal to or greater than 14,000 moist, mineral-matter-free basis.

4. High volatile (B). Btu value 13,000 or more and less than 14,000 moist, mineral-matter-free basis.

5. High volatile (C). Btu value 11,000 or more and less than 13,000 moist, mineral-free basis commonly agglomerating, or 8300 to 11,500 Btu agglomerating.

BLACK LIQUOR Liquid by-product fuel extracted from wood in the alkaline pulp-manufacturing process and containing the chemical used to accomplish the extraction.

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 that is blown down.

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 does 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 ASSEMBLER Means a corporation, company, partnership, or individual who assembles a boiler which has been delivered knocked down in multiple pieces by bolting, threading, welding, or other methods of fastening to produce a finished pressure vessel. A boiler assembler may also be a boiler installer.

BOILER, AUTOMATICALLY FIRED A boiler which cycles automatically in response to a control system

BOILER HEADER (BOX) A pressure part of a boiler consisting of a flat tube sheet into which the ends of the water tubs are rolled. In a parallel plane is a tube cap or handhole sheet. The tow 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.

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.

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 the transport of the products of combustion between parts of a steam-generating unit or the stack.

BRIDGEWALL A wall in a 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

BRITISH THERMAL UNIT The mean British thermal unit (Btu) is ¹/₁₈₀ of the heat required to raise the temperature of 1 lb of water from 32 to 212ºF at a constant atmospheric pressure. It is about equal to the quantity of heat required to raise 1 lb of water 1ºF [251.9957 cal or 1054.35 joules (J)].

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 for the introduction of fuel and air properly mixed in correct proportions to the combustion zone.

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

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

BURNER, AUTOMATICALLY LIGHTED A burner in which fuel to the main burner is normally turned on and ignited automatically.

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 driver fan that overcomes the resistance through the burner to deliver the quantity of air required for combustion.

BURNER WIND BOX 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.

CAKING Property of certain coals to become plastic when heated and form large masses of coke.

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

CALORIE The mean calorie is ¹/₁₀₀ of the heat required to raise the temperature of 1 g of water from 0 to 100ºC at a constant atmospheric pressure. It is about equal to the quantity of heat required to raise 1 g of water 1ºC (4.184 J).

CARRYOVER The moisture and entrained solids forming the film of stream bubbles; a result of foaming in a boiler. Carryover is caused by a faulty boiler-water condition. See also foaming. Carry over usually results in the boiler sending water {wet Steam} up with the steam into the pipes.

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.

CAULK To make the contacting surfaces of a seam tight against leakage by a upsetting or forcing (by distortion) the edge or abutment of the plate into the surface of the adjoining plate. Also, to close any pinhole or fissure in a metal plate, by virtue of the ductility of boiler plate, bye distorting its surface to close a slight opening. A blunt tool is used in caulking.

CHECKER WORK An arrangement of alternately spaced brick in a furnace with openings through which air or gas flows.

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

CINDER Particles of partially burned fuel from which volatile gases have been driven off, which are carried from the furnace by the products of combustion.

CIRCULATING TUBE A boiler tube used to connect the water spaces of tow 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 form contacting the boiler above the lowest safe 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 form the boiler.

COMBINED FEEDER CUTOFF A device that regulates makeup water to a boiler in combination with a low-water fuel cutoff.

COMBUSTION Chemical combination of the combustible that part which will burn) in a fuel with oxygen in the air supplied for the process. Temperatures may range form 1850 to over 3000ºF.

COMBUSTION (FLAME) SAFEGARD A system for sensing the presence or absence of flame and indicating, or initiating control action.

CONDENSATE Condensed water resulting from the removal of latent heat from steam.

CONDUCTION The transmission of heat through and by means of matter unaccompanied by any obvious motion of the matter.

CONDUCTIVITY The amount of heat (Btu) transmitted in 1 hr through 1 ft²of a homogeneous material 1 in, thick for a difference in temperature of 1ºF between the two surfaces of the material.

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

CONTROL, LIMIT An automatic safety control responsive to changes in liquid lev3el, pressure, or temperature, normally set beyond the operating range for limiting the operation of the controlled equipment.

CONTROL MUNUFACTURER A corporation or company which manufactures operating and safety controls for use ion boiler and furnace units.

CONTROL, OPERATING, A control, other than a safety control or interlock, to star or regulate input according to demand and the stop 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 event of ignition failure or unintentional flame extinguishment, 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.

CONSTANT IGNITION Usually a gas pilot that remains lighted at full volume whether the main burner is in operation or not.

CONVECTION The transmission of heat by the circulation of a liquid or a gas such as air. Convection may be natural or forced.

CORNER FIRING A method of firing liquid, gaseous, or pulverized fuel in which the burners are located in the corners of the furnace. Se also tangential firing.

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

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

CRIMPING TOOL A tool used to reduce the diameter of the end of a boiler tube preparatory to tits 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 for connecting circulating tubes.

CROWFOOT The end of a brace in a boiler, split in two dir3ections for riveting to the plate.

CROWN SHEET The plate forming the roof of an internally fired furnace or a combustion chamber.

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

1. Butterfly type. A single-blade damper pivoted about its center.

2. Curtain type. A damper consisting of one or more blades, each pivoted about one edge.

3. Flap type. A damper consisting of one or more blades, each pivoted about one edge.

4. Louvre type. A damper consisting of several blades, each pivoted about its center and linked together for simultaneous operation.

5. Slide type. A damper consisting of a single blade which moves substantially normal to the flow.

DEAERATING HEATER A type of feedwater heater operating with water and steam in direct contact. It s designed to heat the water and to drive off oxygen.

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

DESUPERHEATER Apparatus for reducing and controlling the temperature of a superheated vapor. See also attemperator.

1. Shell-and-tube type. A desuperheater consisting of a pressure vessel containing tubular elements through the walls of which heat is transferred.

2. Spray type. A desuperheater in which a lower-temperature fluid is injected at relatively high velocity in an atomized state in to the superheater vapor to reduce its temperature by direct contact with the atomized fluid.

3. Submerged type. A desuperheater consisting of tubular elements located in the boiler circulation below the waterline.

DIAGONAL STAY A brace used in fire-tube boilers between a flathead or tube sheet and the shell.

DISTILLATE OIL Light fraction of oil which has been separated form crude oil by fractional distillation. See fuel oil.

DOLLY A riveting tool.

DOWNCOMER A tube or pipe in a boiler or waterwall circulation system through which fluid flows downward between headers.

DOWTHERM An organic chemical with an exceedingly high boiling point, sometimes used in special types of boilers for high-temperature service. It is composed of diphenyl and diphenyloxide.

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 a steady draft.

DRAFT DIFFERNTIAL The difference in static pressure between two points in a system.

DRIFT PIN A tapered steel bar used to drive into and align rivet holes or bolt holes in plates or pipe flanges.

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.

DRYBOTTOM FURNACE A pulverized-fuel furnace in which the ash particles are deposited on the furnace bottom in a dry, nonadherent condition.

DRY PIPE A perforated or slotted pipe or box inside the drum and connected to the steam outlet.

DRY STEAM Steam containing no moisture. Commercially dry steam containing not more than 0.5 percent moisture.

DUCTILITY A plastic property of metal to withstand deformation without failure.

DUMP GRATE STOKER One equipped with movable ash trays, or grates, by means of which the ash can be discharged at any desirable interval.

DUTCHMAN A wedge or tapered plug used in butt-and-double-strap longitudinal seams of some boilers to fill the space between the abutting edges of the plate form the end of the inside butt strap to the edge of the adjoining course.

DUTCH OVEN An extended furnace, external to the main setting of a boiler, used to increase the volume of an existing furnace.

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 Btu’s contained in all steam evaporated is the useful output. The number of Btu’s contained in all fuel supplied to the boiler is the input. Of a riveted seam: Ratio of the strength of a unit length of a riveted seam to the same unit length of the seamless plate.

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.

ELASTIC LIMIT The maximum tensile load to which a metal may be subjected with out becoming permanently deformed upon cessation of the load.

ELECTRIC BOILER A boiler converting electric energy to heat energy.

ELECTRIC FURNACE A furnace used for the refinement of high-grade steel.

ELECTROSTATIC PRECIPITATOR A device for collecting dust, mist, or fume form a gas stream, by placing an electric charge on the particle and removing that particle onto a colleting electrode.

EMBRITTLEMENT An intercrystalline corrosion of boiler plate occurring in highly stressed zones. Cracking may result.

ENTHALPHY 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.

EQUALIZING TUBE A boiler tube used to connect the steam spaces of two steam drums, or pressure parts of a boiler.

EROSION The wearing away of refractory or of metal parts by the action of slag or fly ash.

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 makeup boiler feed.

EVAPORATOR CONDENSER That section of an evaporator installation which condenses the vapor.

EXCESS AIR Air supplied for combustion in excess of that theoretically required for complete oxidation.

EXPANDED JOINT The pressure tight joint formed by enlarging a tube end in a tube seat.

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

EXPLOSION FIRESIDE Combustion which proceeds so rapidly that a high pressure is generated suddenly.

EXTENDED SURFACE Heating surface in the form of fins, rings, or studs, added to heat-absorbing elements.

EXTERNAL CORROSION A chemical deterioration of the metal on the fireside of boiler heating surfaces.

EXTRACTION FEEDWATER HEATER A closed feedwater heater supplied with steam extracted or bled from a stage of a steam turbine. See also feedwater heater.

FACTOR OF SAFETY The ratio between that stress which will cause failure and the working stress. This ration often applies to pressures instead of stresses

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 ordinate and the range of volumes as abscissas, all at constant speed and air density.

FATIGUE LIMIT A measure of the ability of a material to withstand repeated stress reversals without fracture or damage to the crystalline structure. A piece of soft iron wire may be broken easily by hand when it is bent back and forth a few times. Its fatigue limit is low. Conversely, a piece of spring steel may be flexed many thousands of times without showing any indication of distress. In this case, the fatigue limit is high. This property is of special value in steam-boiler construction.

FEED THROUGH A trough or pan form which feedwater overflows in the drum.

FEEDWATER HEATER A device used to heat feedwater with stream. See also extraction feedwater heater.

FEEDWATER REGULATOR A device for admitting feedwater to a boiler automatically on demand. Practically a constant water level should result.

FERRULE A short, metallic ring rolled into a tube hole to decrease in diameter. Also a short, metallic ring rolled inside of a rolled tube end. Also, a short, metallic ring for making up handhole joints.

FIN Usually a strip of steel welded longitudinally or circumferentially to a tube.

FIN TUBE A tube with one or more fins.

FIRE CRACK A crack starting on the heated side of a tube, shell, or header resulting from excessive temperature stresses.

FIRE-TUBE BOILER A boiler that has water on the outside of the tubes with the heat and product of combustion flowing inside the tube. Heat through convection is transmitted through the inside of the tube to the water that is on the outside of the tube. It is the reverse of a water tube boiler

FIRING RATE CONTROL A pressure or temperature flow controller which controls the firing rate of a burner according to the deviation form pressure or temperature set point. The System may be arranged to operate the burner on-off, high-low, or in proportion to load demand.

FIXED CARBON The carbonaceous residue less the ash remaining in the test container after the volatile matter has been driven off in making the proximate analysis of a solid fuel.

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.

FLANGE A circular metal plate threaded or otherwise fastened to an end of a pipe for connection with a companion flange on an adjoining pipe. Also that part of a boiler head (dished or flat) which is fabricated to a shape suitable for riveted or welded attachment to a drum or shell.

FLANGE To fabricate the flange in a head or similar plate.

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.

FLARED TUBE END The projecting end of a rolled tube which is expanded or rolled to a conical shape.

FLUE GAS The gaseous products of combustion in the flue to the stack.

FLY ASH Suspended ash particles carried in the flue gas.

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

FORCED-DRAFT FAN A fan supplying air under pressure to the fuel burning equipment.

FORGE-WELD The welding together of metal by raising the temperature to the plastic point and by applying pressure or blows.

FREE-BLOW A pipe open and free to blow to atmosphere.

FUEL OIL A petroleum product, requiring comparatively minor refinement, used as a combustible for steam boilers. The following terms are used to describe its properties:

1. Flash point. The flash point of a fuel oil is an indication of the maximum temperature at which it can be stored and handled without serious fire hazard.

2. Pour point. The pour point is an indication of the lowest temperature at which a fuel oil can be stored and still be capable of lowing under very low forces.

3. Viscosity. The viscosity of an oil is a measure of its resistance to flow. In fuel oil it is highly significant since it indicates both the relative ease with which the oil will flow or may be pumped and the ease of atomization. See also viscosity.

FUEL-OIL HEATER A tank and coil-type heater using steam as a heating medium to reduce heavy low-priced fuel oil to the proper viscosity for good atomization and combustion. Also an electric-coil heater making use of an electric-resistance coil because the heating medium is used sometimes where steam is not available for starting up a “cold” boiler plant.

FURNACE EXPLOSION A violent combustion of dust or gas accumulations in a furnace or combustion chamber of a boiler.

FURNACE RELEASE RATE Furnace release rate is 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, floor, 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.

FUSIBLE PLUG A brass bushing having a tapered core composed of 99 percent pure tin and a melting temperature of 400 to 500ºF and installed at the lowest safe water level of a boiler. The large end of the tapered core is exposed to a boiler pressure, the other end is exposed to products of combustion. The core of fusible plug is designed to melt if the boiler water level approaches a dangerously low level. When the core melts, escaping steam will sound warning.

FUSION-WELD To weld the edges or surfaces of metal by raising the temperature to the fusion point and adding a “filler” metal of the same-characteristics as the metal being welded at the same temperature.

GAG A clamp designed to prevent a safety valve from lifting. Used in applying a hydrostatic test at higher pressure than the safety-valve setting.

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.

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 tow 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 the atmosphere, 14.7 psi at sea level, absolute pressure minus 14.7 at sea level

GENERATING TUBE A boiler tube used for evaporation.

GIRTH SEAM A roundabout, or circumferential, seam connecting two courses of a boiler shell or drum.

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

GRAINS PER CUBIC FOOT The term for expressing dust loading in weight per unit of gas volume (7000 grains equals 1 lb).

GRATE The surface on which fuel is supported and burned and through which air is passed for combustion.

GRINDABILITY Grindability is the characteristic of coal representing its ease of pulverizing and is one of the factors used in determining the capacity of a pulverizer. The index is relative, with the large valves, such as 100, representing coals easy to pulverize such as Pocahontas and smaller values such as 40 representing coals difficult to pulverize.

GROOVED TUBE SEAT A tube seat having one or more shallow grooves into which the tube may be forced by the expander.

GROUND A conducting connection, whether intentional or accidental, between an electric circuit or equipment and either the earth or a conducting body which serves in place of the earth.

GROUNDED Connected to earth or the some conducting body which serves in place of the earth.

GROUNDED CONDUCTOR A system or circuit conductor which is intentionally grounded

GROUNDING CONDUCTOR, EQUIPMENT The conductor used to connect non-current-carrying metal parts of equipment, raceways, and other enclosures to the system-grounded conductor at the service and or the grounding electrode conductor.

GUARDED Covered, shielded, fenced, enclosed or otherwise protected by means of suitable covers, casings, barriers, rails, screens, mats or platforms to remove the likelihood of contact by persons or objects.

GUN (1) A pneumatic riveter. (2) A gun-type ail burner, of the kind having a long-shaped flame. (3) An injector, in railroad terminology.

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

HAND LANCE A manually manipulated length of pip carrying air, steam or water for blowing ash and slag accumulations from heat-absorbing surfaces.

HARDNESS A measure of the amount of calcium and magnesium silts in boiler water. Usually expressed as grains per gallon or parts per million as Ca CO₂.

HARD PATCH A riveted patch made pressure tight by caulking.

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 pip supplying one or more main pipe lines and receiving a number of supply lines tapped into it. Typical is the boiler header and superheater header.

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) rules as follows:

1. Boiler and Waterwall Heating Surface. This surface consists of all the apparatus in contact on one side with the water or wet steam being heated and on the other side with gas or refractory being cooled in which the fluid being heated forms part of the circulating system, this surface is measured on the side receiving heat.

Waterwall heating surface in the furnace, including walls, floor, roof, partition walls and platens, consisting of bare or covered tubes, is measured as the sum of the projected areas of the tubes and the extended metallic surface on the furnace side.

Continuation of furnace tubes beyond the furnace gas outlet is included as boiler heating surface, and this surface is measured on that portion on the circumferential and the extended metallic surface receiving heat.

All other boiler surfaces, including furnace screen tubes, are measured on that portion of the circumferential and the extended metallic surface receiving heat. The surface is not included in more than one category.

2. Superheater and Reheater Surface. This heating surface consists of all the heat transfer apparatus in contact on one side with steam being heated and on the other side with gas or refractory being cooled; this surface is measured on the side receiving heat.

The radiant superheating or radiant reheating surface in the furnace, including walls, floor, roof, partition walls and platens, is measured as the sum of the projected areas of the tubes’ extended metallic surface on the furnace side.

Continuation of superheater tubes beyond the furnace gas outlet is included as convection superheater surface, and this surface is measured on that portion of the circumferential and the extended metallic surface receiving heat.

All other superheater and reheater surface, including screen tubes, is measured on the basis of the circumferential and the extended metallic surface receiving heat.

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 heat 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-HEAT VALUE OR HIGHER HEATING VALUE The total heat obtained form the combustion of a specified amount of fuel which is at 60 F before the quantity of heat released is measured.

HOPPER BOTTOM FURNACE A furnace bottom with one or more inclined sides forming a hopper for the collection of ash and for the easy removal of same.

HOT-SHORT Brittle when hot.

HOT WELL A tank used to receive condensate from various sources on its passage back to a boiler through the feed water system. It usually is vented to atmosphere.

HUDDLING CHAMBER A space provided under the valve disks of many safety valves, permitting the steam pressure in the boiler to act on an increased area when the valve disk lifts, to permit the valve to pop open rather than to rise gradually.

HYDROGEN DAMAGE Temporary reduction in ductility of steel without significant reduction in tensile strength as a result of absorption of hydrogen by the steel.

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 A system in which the fuel to a main burner or gas or oil pilot is ignited directly either by an automatically energized spark or glow coil or by a gas or oil pilot.

IGNITION TEMPERATURE Lowest temperature of a fuel at which combustion becomes self-sustaining.

IMPELLER The rotating wheel of a centrifugal pump.

IMPINGEMENT The striking of moving flame against boiler parts, causing local over heating.

INCOMPLETE COMBUSTION The partial oxidation of the combustible constituents of a fuel.

INHIBITOR A substance which selectively retards a chemical action. An example in boiler work is the use of an inhibitor, when using acid to remove scale, to prevent the acid from attacking the boiler metal.

INJECTOR A device for feeding water into a boiler, making use of the high-velocity-momentum principle to feed water back against boiler pressure by use of steam at the same pressure.

INPUT RATING The fuel-burning capacity of a burner at sea level in Btu per hour (watts) as specified by the manufacturer.

INTEGRAL ECONOMIZER A segregated portion of a watertube boiler in which the feedwater is preheated before its admixture with the circulating boiler water.

INTERBANK SUPERHEATER A superheater located in a space between the tube banks of a bent-tube boiler.

INTERDECK SUPERHEATER A superheater located in a space between tube banks of a straight-tube boiler.

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 the flow is stopped, this succession occurring in a sequence of frequent cycles.

INTERMETTENT 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 fire-tube boiler having an internal furnace such as a scotch, locomotive firebox, vertical tubular or other type having a water-cooled plate-type furnace.

INTERTUBE ECONOMIZER An economizer, the elements of which are located between tubes of a boiler convection bank.

INTERTUBE SUPERHEATER A superheater, the elements of which are located between tubes of a boiler convection bank.

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 Blocks of asbestos or magnesia insulation wrapped on the outside of a boiler shell or steam piping.

LAMINATION As applied to boiler plate, a slag stratum or inclusion rolled into a piece of steel plate during rolling-mill operation.

LAZY BAR A bar fitting across the latches of firing doors of hand-fired boilers, used as a balance and rest for long, heavy firing tools.

LIGAMENT A series of holes in one or more rows.

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.

LISTED Equipment or materials included in a list published by a nationally recognized testing laboratory that maintains periodic inspections of production of listed equipment or materials. Listing indicates compliance with nationally recognized standards.

LIVE STEAM Steam which has not performed any of the work for which it was generated.

LONGITUDINAL SEAM A riveted or welded seam along the longitudinal axis of a boiler shell or drum.

LOW-HEAT VALUE The high heating value minus the latent heat of vaporization of the water formed by burning the hydrogen in the fuel.

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 condition 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 the boiler.

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

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. The feed water can be either automatic or manual.

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

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-ATOMIZING OIL BURNER A burner which uses the pressure of the oil for atomizing.

MECHANICAL STOKER A device consisting of a mechanically operated fuel-feeding mechanism and a grate; is used for the purpose of feeding solid fuel into a furnace, distributing it over the grate, admitting air to the fuel for the purpose of combustion, and providing a means for removal or discharge of refuse.

MICROMETER One millionth of a meter, or 0.00039 in. (1/25400 in.); formerly called a micron. The diameter of dust particles is often expressed in micrometers.

MILL SCALE An iron oxide scale formed on the surface of a steel plate by cooling and exposing the plate to air just after it has been rolled at high temperatures.

MILL TEST REPORT An affidavit from a steel mill testifying as to the physical and chemical properties of the steel referred to by the report.

MINIATURE BOILER A boiler, the dimensions and working pressure of which do not exceed the lower extremity of a watertube-boiler convection bank which is normally provided with a blowoff valve for periodic blowing off of sediment collecting in the bottom of the drum.

MUD OR LOWER DRUM A pressure chamber of a drum or header type located at the lower extremity of a watertube-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 on fuel can be burned either separately or simultaneously, such as pulverized fuel, oil or gas.

MULTIPLE-RETORT STOKER An underfeed stoker consisting of two or more retorts, parallel and adjacent to one another, but separated by a line of tuyeres’ and arranged so that the refuse is discharged at the ends of the retorts.

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

NFPA National Fire Protection Association.

NIPPLE A short length of pipe or tubing.

NONRETURN TRAP A trap designed to discharge its condensate at atmospheric pressure or at considerably lower pressure than at its inlet.

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

OGEE FLANGE A flange in the form of a reverse curve, used to connect the edges of two concentric shells.

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.

ONCE-THROUGH BOILER A steam-generating unit usually operated above the critical pressure in which there is no recirculation of the working fluid in any part of the unit. In the case of a supercritical steam generator, there is a constant increase in temperature and enthalpy from inlet to outlet.

OPERATING CONTROL A control to start and stop the burner, it must be in addition to the high-limit control.

ORSAT An instrument for determining the chemical analysis of flue gas.

OXYGEN ATTACK corrosion or pitting in a boiler caused by oxygen.

PACKAGED STEAM GENERATOR 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.

PALM The end of a brace in a boiler, forged flat or riveted to the shell plate, used to stay flat surfaces.

PATCH A piece of boiler plate used to replace a defective section cut out of a boiler.

PENDANT-TUBE SUPERHEATER An arrangement of heat-absorbing elements which are substantially vertical and suspended from above.

PERFECT OR STOICHOMETRIC COMBUSTION The complete oxidation of all the combustible constituents 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. This pH number is the negative exponent of 10 representing hydrogen ion concentration in grams per liter. For instance, a pH of 7 represents 10^-7 g/L.

PILOT A small burner which is used to light off 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.

PITCH The unit spacing of a series of holes, tube holes, or other holes in a plate.

PLATEN A plane surface receiving heat from both sides and constructed with a width of one tube and a depth of two or more tubes bare or with extended surface.

PLATEN SUPERHEATER A superheater made up of close back-spaced tubes forming plane elements located so as to absorb heat primarily by radiation.

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

PORCUPINE BOILER A boiler consisting of a vertical shell from which project a number of dead end tubes.

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.

POWER-ACTUATED RELIEF VALVE A safety or relief valve, actuated by a separate power source, usually electrical or pneumatic; set to operate slightly below the spring-loaded pressure-actuated valve. This valve is for the express purpose of saving wear and tear on the Code valve and may be installed with an isolating valve to permit maintenance and repair without the necessity of shutting down the boiler. The relieving capacity of this valve is not to be included in the relieving capacity in calculating total Code required.

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 ignition.

PRESSURE Absolute pressure, the pressure above a perfect vacuum, gauge pressure plus 14.7, at sea level.

PRIMARY AIR Air introduced with the fuel at the burners. In direct-fired systems this may be the same as pulverizer air bypassed around the pulverizer or bled in at the exhauster suction.

PRIMING An introduction 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.

PROJECTED GRATE AREA The horizontal projected area of the stoker grate.

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).

PULVERIZED FUEL Solid fuel reduced to a fine size, such as 70 percent through a 200-mesh screen.

PULVERIZER A machine which reduces a solid fuel to a fineness suitable for burning in suspension. Types used are:

1. High speed (over 800 r/min)

(a) Impact pulverizer. A machine in which the major portion of the reduction in particle size of the fuel to be pulverized is affected bye fracture of larger sizes by sudden shock, impingement, or collision of the fuel with rotating members and causing.

(b) Attribution pulverizer. A machine in which the major portion of the reduction on particle size is by abrasion, either by pulverizer parts on coal or by coal on coal.

2. Medium speed (between 70 and 300 r/min)

(a) Roller pulverizer. A machine having grinding elements consisting of conical or cylindrical rolls and a bowl, bull-ring mating rings or table, any of which may be the

rotating member, the fuel to be pulverized being reduced in size by crushing and attrition between the rolls and the rings.

(b) Ball pulverizer. A machine in which the grinding elements consist of one or more circular rows of metal balls arranged in suitable raceways, wherein the fuel to be pulverized is reduced in size by crushing and attrition between the balls and raceways.

3. Low speed (under 70 r/min)

(a) Ball or tube pulverizer. A machine having a rotating cylindrical or conical casing charged with metal ball or slugs and the fuel to be pulverized, with reduction in particle size being affected by crushing and attrition resulting from continuous relative movement of the charge on rotation of the casing.

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 SUPERHEATER A superheater exposed to the direct radiant heat (light) of the fire.

RAM A form of plunger used in connection with underfeed stokers to introduce fuel into retorts; a form of pusher.

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.

RAW WATER Untreated feedwater.

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

RED LIQUOR An acid-water mixture of organic material (wood residue) and spent inorganic pulping chemicals, generally associated with a sulfite pulping process in the paper mill industry.

REFRACTORY A heat-insulating material, such as firebrick or plastic fire clay, used for such 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).

RETARDER A straight or helical strip inserted in a fire tube primarily to increase the turbulence.

RETRACTABLE BLOWER A soot blower in which the blowing elements can be mechanically extended into the boiler and retracted or pulled back.

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 a white background, used as 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.

ROLLED JOINT A point made by expanding a tube into a hole by a roller expander.

ROTARY OIL BURNER A burner in which atomization is accomplished by feeding oil to the inside of a rapidly rotating cup.

SAFE-END To replace a deteriorated end of a fire tube by cutting off the end and welding on a short length of new tube.

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 vale to a safe location.

SAFETY-VALVE LIFTING LEVER A lever by which a 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 in the 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. See also delayed combustion.

SECONDARY TREATMENT Treatment of boil feedwater or internal treatment of boiler water after primary treatment.

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

SHORE SCLEROSCOPE A device to test the hardness of a material, performed by dropping a diamond-pointed hammer form a standard height.

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

SINGLE-RETORT STOKER An underfeed stoker using only one retort in the assembly of a complete stoker. A single furnace may contain one or more single-retort stokers.

SINUOUS 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 its overheating from direct contact with steam.

SLAG A residue deposited by ash particles that have attained their softening temperature (1900 to 2700ºF) depending on their composition. Slag may be plastic and viscous when hot. It hardens and is rather porous and brittle when cool.

SLAG-TAP FURNACE A pulverized-fuel-fired furnace in which the ash particles are deposited and retained on the floor in molten condition, and from which molten ash is removed by tapping either continuously or intermittently.

SLICER A slicing bar, a long steel bar used for breaking up a fuel bed in coked or caked condition.

SLUG A solid body of boiler water passed into the steam flow by printing or picked up form a pocket of condensate in the steam line.

SMOKE Small gas-borne particles of carbon or soot, less than 1 µm in size, resulting form incomplete combustion of carbonaceous materials and of sufficient number to be observable.

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

SOFT PATCH A patch applied with tap bolts, with a gasket under the patch plate to prevent leakage.

SOOT BLOWER A tube form which jets of steam or compressed air are blown for cleaning the fireside of tubes or other parts of a boiler.

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(C).

SPONTANEOUS COMBUSTION Ignition of combustible material following slow oxidation without the application of high temperature from an external source.

SPRAY NOZZLE A nozzle form which a liquid fuel is discharged in the form of a spray.

SPUD A flange nut wrench, open at one end and pointed at the other, as a drift pin. The pointed end is used for aligning boltholes of pipe flanges.

STACK A steel “chimney”.

STAY BOLT A stay threaded and riveted over at each end, used to connect two flat or curved pressure parts of a boiler.

STEAM Water vapor produced by evaporation. Dry saturated steam contains no moisture and is at a specific temperature for every pressure, it is colorless. The white appearance of escaping steam is due to condensation at the lowered temperature, it is the water vapor that shows white.

STEAM-AND-WATER DRUM A pressure chamber located at the upper extremity of a boiler circulatory system in which the steam generated in the boiler is separated from the water and from which steam is discharged at a position above a water level maintained there.

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 a rapid steam formation.

STEAM-GENERATING UNIT A unit to which water, fuel, and air or waste heat, are supplied and in which steam is generated. It can consist of a boiler furnace and fuel-burning equipment and may include as component parts waterwalls, superheater, reheater, economizer, air heater or any combinations.

STEAM QUALITY The percentage by weight of vapor in a steam-and-water mixture.

STRESS The internal resistance of a material to an external force changing, or tending to change, the shape or position of the material. Also total stress or unit stress.

STRESS-RELIEVE To dissipate pent-up stresses caused by welding, by means of heat treatment.

STRONGBACK A heavy steel bar bolted to tube sheets of fire-tube boilers during construction, while braces are being installed, to prevent the tube sheet form buckling before installation of the tubes.

STUD A projecting pin serving as a support or means of attachment.

STUD TUBE A tube having short studs welded to it.

SUBPUNCH To drive a pilot hole through a plate preparatory to drilling a larger hole.

SUPERHEATED STEAM Steam heated to a temperature higher than that corresponding to the temperature equivalent to the pressure.

SUPERHEATER A series of tubes exposed to high-temperature gases or to radiant heat. Steam from the boiler passes through these tubes to attain a higher temperature than would be possible otherwise. This superheated steam ensures dryness. See also radiant superheat.

SUPERHEATER HEADER A large-diameter (about 4- to 8-ft) thick-walled shell or drum into which a row of superheater tubes is rolled.

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 solid in boiler water.

SWITCH, AIR-FLOW-PROVING A device installed in an airstream which senses air flow or loss thereof and electrically transmits the resulting impulses to the flame-failure circuit.

SWITCH, HIGH-PRESSURE A device to monitor liquid, steam, or gas pressure and arranged to open and/or close contacts when the pressure value is exceeded.

SWITCH, LOW-PRESSURE A device to monitor liquid, steam or gas pressure and arranged to open and/or close contacts when pressure drops below the set value.

SWITCH, OIL-TEMPERATURE-LIMT A device to monitor the temperature of oil between preset limits and arranged to open and/or close contacts should improper oil temperature be detected.

TACK To hold edges of plate in correct position for riveting by a few scattered bolts, known as “tack bolts”, placed through rivet holes or by small, scattered spot welds known as “tack welds” or “stitch welds”.

TANGENTIAL FIRING A method of firing by which a number of fuel nozzles are located in the furnace walls so that the centerlines of the nozzles are tangential to a horizontal circle. Corner firing is usually included in this type.

TANGENT TUBE WALL or TUBE-TO-TUBE WALL A waterwall in which the tubes are substantially tangent to one another with practically no space between the tubes.

TAP HOLE An opening for the removal of slag from a slag tap furnace.

TELLTALE HOLE A hole drilled into the ends of a stay bolt. The hole extends at least ¹/₂ in. inside the point of diameter reduction. The purpose is to show leakage, through the telltale hole, if the stay bolt breaks or cracks.

TENSILE STRENGTH (ULTIMATE) That stress which causes breaking in tension.

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.

THERM A unit of heat applied especially to gas. One therm equals 100,000 Btu.

THERMAL SLEEVE A spaced internal sleeve lining of a connection for introducing a fluid of one temperature into a vessel containing fluid at a substantially different temperature, used to avoid abnormal stresses.

THERMOSTATIC TRAP A nonreturn trap using a thermostatic expansion and contraction principle as its actuating medium.

THROUGH-STAY A brace used in fire-tube boilers between the heads or tube sheets.

TIE BAR A structural member designed to maintain the spacing of furnace waterwall tubes.

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.

TITRATION A chemical process used in analyzing feedwater.

TITRATION POINT The point at which a solution changes color when an indicating chemical is introduced drop by drop.

TOTAL STRESS The total resistance of a material to an external force on its entire cross-sectional area in a plane perpendicular to the direction of the force. See also stress.

TRAP A device designed to rove condensate from steam automatically, with negligible loss of steam. See nonreturn trap, return trap, and thermostatic trap.

TRAVELING-GRATE STOKER A stoker similar to a chain-grate stoker with the exception that the grate is separate from but is supported on and driven by chains. Only enough chain strands are used as may be required to support and drive the grate.

TRIAL 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.

TRIAL 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.

TRIAL FOR PILOT IGNITION A timed interval when the pilot valve is held open and an attempt is 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 CAP An elliptical or a circular handhole plate used opposite the end of a watertube in a header of a watertube boiler; used for inspection, cleaning, or tube removal.

TUBE RATTLER A vibrating tool designed to be passed through fire tubes to crack scale loose from the tube as a result of vibration.

TUBE SHEET A flat head of a boiler or that part of a boiler drum into which boiler tubes are rolled.

TUBE TURBINE A rotating tool used with water or compressed air pressure, designed to be passed through watertubes to move scale.

TUBULAR-TYPE COLLECTOR A dust collector utilizing a number of essentially straight-walled cyclone tubes in parallel.

TURBIDITY The optical obstruction to the passing of a ray of light through a body of water, caused by finely divided suspended matter; used to check feedwater.

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.

TUYERES Forms of grates, located adjacent to a retort, through which air is introduced.

UL Underwriters Laboratories, Inc.

UNIT STRESS A value expressed in pounds per square inch and found by dividing the total stress or force by the cross-sectional area stressed. See stress, total stress.

UPSET To enlarge or increase the cross-sectional area of any part of a metal by forging it back to a shorter length.

VALVE See check valve, gate valve, globe valve, safety valve.

VALVE, MANUAL-RESET SAFETY SHUTOFF A manually opened, electrically latched, electrically operated safety shutoff valve designed to automatically shutoff fuel when de-energized.

VALVE, SAFETY SHUTOFF A valve automatically closed by the safety control system or by an emergency device to completely shut off fuel supply to the burner.

VANE A fixed or adjustable plate inserted in a gas or airstream 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.

VAPOR GENERATOR A container of liquid, other than water, which is vaporized by the absorption of heat.

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

VENT VALVE (GAS BURNER) A normally open, power closed valve piped between the two safety shutoff valves, vented to a safe location.

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

VORTEX ELIMINATOR Baffles, screens or plates at the entrance to a large down-comer designed to prevent the formation of a free vortex.

WASHOUT PLUG An inspection, sight and cleanout opening, circular, threaded and fitted with a threaded pipe plug and not to be used for any pip connection.

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

WATER COLUMN A vertical, hollow chamber located between a boiler and the gauge glass for the purpose of steadying the water level in the glass through the reservoir capacity of the column. Also, the column may eliminate the obstruction of small-diameter gauge-glass connections by serving as a sediment chamber.

WATER HAMMER A sudden increase in pressure of water due to an instantaneous conversion of momentum to pressure.

WATER LEG That space which is full of boiler water between two parallel plates. It usually forms one or more sides of internally fired furnaces.

WATER SCREEN A screen formed by one or more rows of water tubes spaced above the bottom of a pulverized-fuel furnace.

WATER-TUBE A boiler tube through which the fluid under pressure flows. The products of combustion surround the tube.

WATER-TUBE BOILER A boiler in which the water or other fluid flows through the tubes and the products of combustion surround the tubes. This kind of boiler is mainly used for high pressure steam but also can be used to produce low pressure steam.

WATER-WALL A row of watertubes lining a furnace or combustion chamber, exposed to the radiant heat of the fire; used to protect refractory and to increase capacity of the boiler.

WELD To join two edges or surfaces of metal by the application of heat. Also forged-weld, fusion-weld.

WELDED WALL A furnace closure wall made up of closely spaced waterwall tubes welded together or to an intermediate fin to form a continuous airtight structure.

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

WIRE DRAWING A cutting of surfaces caused by the abrasive action of high-velocity flow under restricted outlet.

WRAPPER SHEET The outside plate enclosing the firebox in a firebox or locomotive boiler. Also the thinner sheet in the shell of a two-thickness boiler drum.

YIELD POINT The point at which a metal, under a mounting tensile load, exceeds its elastic limit. At the yield point the metal becomes permanently deformed and will not return to its original shape or position upon cessation of the load.