Mechanical Engineering Dictionary-Steam Boilers

Important Terms and their Meaning of Mechanical Engineering-Steam Boilers:



AIR PREHEATER – A device that makes the final heat recovery from boiler flue gases and uses the same to preheat the incoming furnace air for its reaction with fuel.

ANTI-INCRUSTATOR – A substance used to prevent the formation of scale on the internal surfaces of steam boilers.

ANTIPRIMING PIPE – A pipe placed in the steam space of a boiler, so as to collect the steam while excluding entrained water.

AUTOMATIC FEED WATER REGULATOR – Device that regulates feedwater supply to the boiler according to load, and so does away with hand operation of valves on feed lines. It is controlled by temperature; its action depends upon expansion and contraction of some metal part.

AUTOMATIC INJECTOR – One that is self starting after its operation has been stopped by the interruption of its water supply.

BABCOCK and WILCOX BOILER – A water tube boiler consisting in its simplest form of a horizontal drum from which is suspended a pair of headers carrying between them an inclined bank of straight tubes.

BAG – A bulged out section of a portion of the shell, extending through the full thickness of the shell, caused by overheating and pressure.

BALANCED DRAUGHT – A system of air supply to a boiler furnace, in which one fan forces air through the grate, while a second, situated in the uptake, exhausts the flue gases. The pressure in the furnace is thus at atmospheric i.e., is balanced.

BANKING LOSS – The fuel used in maintaining a floating bank or to maintain a dead bank and then raise the steam pressure to normal.

BANKING UP – Reducing the rate of combustion in a boiler furnace by covering the fire with slack or fine coal.

BENSON BOILER – A high pressure boiler of the once through type in which water is pumped through the successive elements of the heating surface, firing being by gas, oil, or pulverized coal.

BLISTER – A separation of the metal from the shell plate, caused by impurities rolled into the shell plate when formed.

BLOWDOWN OF SAFETY VALVE – The difference between the pressure at which the safety valve pops and that at which it closes.

BLOWING OFF – Act of letting out water and steam from a boiler to carry off accumulated mud and scale.

BLOW OFF VALVE – The valve which empties the boiler for cleaning, inspection, or repair. It blows out mud, scale, or sediment when the boiler is in operation and prevents excessive concentration of soluble impurities in the boiler. Also used for rapid lowering of boiler water level if it is too high.

BOILER – A closed pressure vessel in which a fluid is heated and converted to vapour for use external to itself, by the direct application of heat resulting from the combustion of fuel (solid, liquid or gaseous) or by the use of electricity or nuclear energy.

BOILER CAPACITY –The weight of steam, usually expressed in kg/hour, which a boiler can evaporate, when steaming at full load output.

BOILER COMPOSITION – Chemicals introduced into the boiler feed water to inhibit scale formation and corrosion, or to prevent priming or foaming.

BOILER CROWN –The upper rounded plates of the boiler of shell type.

BOILER EFFICIENCY – The ratio of heat supplied by a boiler in heating and evaporating the feed water to the heat supplied to the boiler in the fuel. It may vary from 60 to 90 per cent.

BOILER PATCH – A small piece of metal used to cover and strengthen a weak spot. A soft patch is a covering over a leak or defect which is fastened with bolts, as distinguished from a hard patch which is riveted.

BOILER PLATE – Mild steel plate, generally produced by the open hearth process, used mainly for the shells and drums of steam boilers.

BOILER PRESSURE – The pressure at which steam is generated in a boiler.

BOILER SETTING – The supporting structure on which a boiler rests, usually of brick for land boilers and steel for marine boilers.

BOILER TEST – (1) A hydraulic pressure test applied to check water tightness under pressure greater than the working pressure. (2) An efficiency test carried out to determine evaporative capacity and magnitude of losses.

BOILER TRIAL – An efficiency test of a steam boiler, in which the weight of feed water and of fuel burnt are measured and various sources of losses are assessed.

BOILER TUBES – Steel tubes forming part of the heating surface in a boiler. In water tube boilers, the hot gases surround the tubes. In locomotive and some marine boilers (fire tube boilers) the gases pass through the tubes.

BREECHING – The metal duct that carries the smoke and gases of combustion from a furnace to the stack or chimney for ultimate discharge to the atmosphere.

CARRY OVER – It is entrained moisture and associated solids passing from a boiler with the steam.

CAULKING – Upsetting or burring up of the edge of the plate or strap after riveting so as to make the edges press down tightly on the plate beneath and thus form a water and steam tight joint.

CHECK VALVE – A form of non return valve used to control the flow of water as in pump operation.

CHIMNEY – A tall, hollow cylindrical column built of steel, brick or concrete used to produce the required natural draft effect.

CHIMNEY EFFECT – The upward movement of warm air or gas, compared with ambient air or gas, due to the lesser density of the warm air or gas. Chimney effect may be a cause of uneven heating in buildings two or more stories high.

CLEANING THE FIRE – Operation of removing clinkers, etc., from the burning coal at regular intervals.

CLOSED HEATER – A type of heater in which the steam and feed water are separated by a metal surface.

CLYDE BOILER – A boiler similar to a scotch boiler, but instead of a water space at the back end of the combustion chamber, a removable back which is lined with some insulating material such as asbestos or fire tile, is existing.

COCK – A device for regulating the flow of fluids through a pipe.

COLLECTOR or DRY PIPE – A pipe placed inside a boiler at a high point and having small perforations throughout its length so as to take off steam at a multiplicity of points and thus avoid turbulence caused by taking off steam at only one point.

CORROSION – Chemical action which causes destruction of the surface of a metal by oxidation, rusting. It is an electrochemical attack.

DOUBLE TUBE BOILER – Boiler having an auxiliary tube placed inside each main tube. Gases flow from one end to the other end through the auxiliary tube and from that end to the other end in the reverse direction through the annular space in the main tube.

DOWN COMER – A large vertical tube or pipe for circulating water from the water space of the steam drum to water wall headers.

DRAFT – The difference of pressure producing air flow through the boiler furnace, flue and chimney.

DUPLEX PUMP – A combination of two pumps arranged side by side and so connected that movement of each operates the steam valve of the other.

ECONOMIZER – Feed water heater placed between the boiler and chimney (exhaust) stack, so as to absorb a portion of the heat in the gases, not absorbed by the boiler.

ELECTRIC BOILER – Unit in which the resistance of water between solid metal electrodes generates heat and thereby produces steam.

EROSION – It is a mechanical action causing wear by abrasion.

EXTERNALLY FIRED BOILER – One in which the furnace is outside the boiler shell, the furnace walls being usually of fire brick. Example, the familiar horizontal return tubular boiler.

FEED WATER – The water (chemically treated water) supplied to a boiler to replace that evaporated as steam or blown off.

FEED WATER HEATER – An apparatus for raising the temperature of boiler feedwater by abstracting some of the heat from exhaust steam or from the hot gases of combustion.

FEED WATER REGULATOR – An automatic device which controls the amount of feed water admitted to the boiler so as to maintain a constant water level in the boiler drum.

FIREBOX BOILER – Boiler having the fire within a firebox, although external to the shell, is rigidly connected to it.

FIRE CRACKS – Cracks caused by radiant heat, usually around circumferential riveted seams of thick plates.

FIRE LINE – The highest point of the heating surface in most common types of boilers.

FIRE TUBE – Tube in which the products of combustion pass through and water surrounds the tube.

FLASH BOILER – A boiler consisting of a series of coils of steel tubing, water is supplied by a pump to the top coil, from where it circulates through the other coils, becoming heated in its descent and issuing from the lower coil, as highly superheated steam.

FLUID VAPOURIZER GENERATOR – A closed vessel in which a heat transfer medium, other than water, is vaporized under pressure by the application of heat.

FOAMING – It is severe priming or agitation of the water level due to dirty or impure water. Small, stable, non-coalescing bubbles are formed through the boiler water.

FOULING – A condition of the flue gas passages in a boiler or furnace that adversely affects the transfer of heat, usually in the form of soot or scale.

FURNACE – That part of the boiler designed for burning the fuel.

FUSIBLE PLUG – A safety device which acts in case of dangerously low water. It consists of an alloy of tin, lead and bismuth and a covering of brass or cast iron. The plug melts during dangerous water levels and permits steam to rush into the furnace and put out fire.

GALLOWAY TUBES – Transverse tubes placed in a flue tube and attached to the openings in the side of the flue to increase the heating surface.

GAUGE COCK – A device for determining the water level in the boiler.

GIRDER STAY – A cast steel or built up girder with its ends resting on the side or end sheets of the fire box or combustion chamber, and supporting the flat crown sheet or top sheet of the combustion chamber by means of bolts.

GOOSE NECK – A short length of pipe having one complete turn to which the steam gauge is attached.

GRATE SURFACE – The area of the grate upon which fire rests in a coal or wood fired boiler.

GROOVING – Surface cracking of boiler plates. Expansion and contraction of parts too rigidly connected cause grooving.

GROSS FEED WATER – The net feed water plus the quantity of water provided for that blown out.

GUSSET STAY – Pieces of plate iron secured to the boiler front or back near the top or bottom by means of angle irons.

HEATING SURFACE IN A BOILER – Fire side area in a boiler exposed to the products of combustion. The area consists of the surface area of tubes, fire boxes, shells, tube sheets and projected area of headers.

HORIZONTAL RETURN TUBULAR BOILER – One so arranged that the products of combustion after passing along the length of the shell, return in an opposite direction through the tubes, before passing up the stack.

HOT WATER HEATING BOILER – A hot water heating boiler is used for space water heating, with the water returned to the boiler.

HOT WATER SUPPLY BOILER – A boiler which supplies hot water to be used externally to itself for washing, cleaning etc.

INCRUSTATION – A coating over, the coating, being commonly known as scale.

INDUSTRIAL BOILER – A stationary water tube steam generator, in which some of the steam is produced in a convective tube bank.

INJECTOR – An instrument for forcing water into a boiler against the boiler pressure by means of a steam jet.

INTERMITTENT BLOW DOWN – Blow down that is taken from the bottom of the mud-drum, water wall headers, or lowest point in the circulation system, at regular intervals.

INTERNALLY FIRED BOILER – Boiler in which the furnace is within the shell, being surrounded by water.

INTERNAL TREATMENT – Treating water in the boiler while evaporation is taking place by chemically adjusting or balancing the boiler water to prevent scale formation, corrosion, steam contamination and embrittlement.

JAW STAY – A round bar having jaws forged at one end and a flat plate at the other inclined at the proper angle for riveting to the boiler shell.

LIGAMENT – The metal between tube holes in boiler practise. Section of the metal not cut away between two adjacent tube holes.

LOCOMOTIVE BOILER – A specially designed boiler, specifically meant for self propelled traction vehicles on rails.

LOG SHEET – A forced reminder to check certain components of a boiler to prevent trouble from developing later and to note if proper operation is taking place. A data sheet.

LOW WATER CUT OFF – A device that shuts down the boiler immediately if the water drops to a dangerously low level.

MAKE UP WATER – Additional water to be put into the boiler periodically to make up for the loss due to leakage or exhausting of steam without condensation.

MARINE BOILER – A low head type special design boiler meant for ocean cargo and passenger ships with an inherent fast steaming capacity.

MECHANICAL DRAFT – The draft artificially produced by mechanical devices such as fans and in some units by steam jets.

MECHANICAL STOKER – A device constructed to automatically feed fuel to a furnace. Its use results in more efficient combustion owing to constant instead of intermittent firing.

MINIATURE HIGH PRESSURE BOILER – A boiler which does not exceed the following limits: (1) 16" inside diameter of shell. (2) 5 cuft. gross volume exclusive of casing and insulation (3) 100 psi gauge. If it exceeds any of these limits, it is called a power boiler.

MISSISSIPPI COCK – A cock in which steam pressure keeps the cock closed and a push button is provided to open the cock.

NATURAL DRAFT – The draft caused by the difference in weight between the column of hot gas inside the chimney and a column of cool outside air of the same height and cross-section.

NET FEED WATER – The quantity of water necessary to supply a stated evaporation in a given interval of time.

NON-SECTIONAL BOILER – A boiler in which the tubes are divided into groups, each group communicating with a header at each end, making independent units.

ONCE THROUGH BOILER – A boiler or steam generator which receives feed water at one end of continuous tubes and discharges steam at the other end.

OPEN HEATER – An open chamber in which the exhaust steam and water to be heated are brought into intimate contact by spraying the water through the steam, both the water and condensate going to the boiler.

PACKAGED BOILER – A completely factory assembled boiler either water tube or fire tube, including boiler firing apparatus, controls and boiler safety devices.

PALM STAY – A round rod having forged on one end a plate or palm.

pH VALUE – It is a number between 0 and 14 indicating the degree of acidity or alkalinity.

PITTING – A form of corrosion resulting in a series of minute holes or pits eaten into the surface of the metal to a depth of sometimes 6 mm.

POP SAFETY VALVE – A valve so constructed that it opens very suddenly like a cork popping out of a champagne or sparkling burgundy bottle and remains open until the pressure is reduced a predetermined amount.

PORCUPINE BOILER – A boiler having a vertical drum into which are screwed a multiplicity of horizontal radial short tubes.

PORTABLE BOILER – A boiler mounted on a truck, barge, a small river boat, or any other such mobile apparatus.

POSITIVE INJECTOR – One with a hand operated overflow valve, which permits operation at high pressure by stopping the drizzle from the overflow.

POWER BOILER – A power boiler is a steam or vapour boiler operating above 15 psig and exceeding the miniature size boiler.

PRIMING – It lifts the water level and delivers steam containing spray of water. It is usually caused by forcing a boiler too hard or by a too high water level or a combination of both these causes.

RADIAL STAYS – Long threaded rods used in locomotive boilers, screwed through both the firebox crown sheet and wrapper sheet and the ends riveted.

REDUCING VALVE – An automatic throttle valve for use where low pressure steam for heating or process is taken from high pressure mains.

REGENERATIVE AIR HEATER – A type of air heater containing a rotor which comes in contact alternately with hot gases and air thereby transfers heat from hot gases to the air.

REHEATER – A superheater that heats steam which is let out from a high pressure turbine and after heating sends the steam to a low pressure turbine thereby increases the plant efficiency.

RETARDERS – Also called Spinners-These are helical strips or ribbons of metal centered in horizontal or vertical fire tubes of a fire tube boiler for increasing the wiping effect on the inner surfaces of the tubes, by the flue gases on their way to the chimney stack. The tubes cannot be cleaned by scraping or brushing until these strips are removed. Retarders also increase the boiler frictional resistance to the flue gases.

RINGLEMANN CHART – The chart used for comparing the smokes density when no instrument is available.

RIVETED JOINT EFFICIENCY – Ratio of the strength of a unit section of the joint to the same unit length of solid plate. Unit length usually taken is the pitch of the rivets (distance from centre to centre) in the row having the greatest pitch.

RIVETED STAYS – Stays in which the threaded ends are riveted instead of having a nut at each end.

SAFETY VALVE – A circular valve connecting the steam space of a boiler and loaded to such an extent that when the pressure of steam exceeds a certain point, the valve is lifted from its seat and allows the steam to escape. The valve is loaded either by weight or by a spring. The release of steam saves the boiler from explosion.

SCALE – Incrustation within a vessel caused by the mineral substances from the water. Scale is a result of the chemical effect of the heat and concentration. A hard coating, chiefly calcium sulphate. If the scale is excessive, it leads to overheating of the metal and ultimate failure.

SCOTCH BOILER – A horizontal boiler in which the combustion chamber at the end of the boiler shell is entirely surrounded by water.

SCUM SCOOP – Apparatus for blowing out water from the surface to remove fine particles of scale forming foreign matter.

SEPARATOR – The device that removes as much moisture as possible from steam after it leaves the boiler.

SINGLE TUBE BOILER – Boiler made up of plain tubes, and the gases flow from one end to the other end of the tubes.

SOCKET STAY – Also called stay bolt consisting of a rod and socket.

STATIONARY BOILER – A boiler which is installed permanently on a land installation.

STAY BOLT – Short stay bars to support flat surfaces that are only a short distance apart, such as inner and outer sheets of water legs in a locomotive boiler. Generally have screw threads cut at one end, and sometimes at both ends, to receive a nut.

STAY ROD or THROUGH STAY – A plain rod 1 ¼" to 2½" in diameter having nuts and washers for fastening the ends to the plates.

STAY TUBE – A thick tube with threads on the ends, one end being larger than the other so that the tube may be slipped through the large hole.

STEEL ANGLE STAY – Two lengths of steel angle riveted together forming a T-shape piece and riveted to the plate.

STEAM – Water in a semi gaseous condition. It is a vapour than a gas, that is a substance between the purely liquid and gaseous states. Wet steam.

STEAM BOILER – A closed vessel in which steam or other vapour is generated continuously for use external to itself by the direct application of heat resulting from the combustion of fuel (solid, liquid, or gaseous) or by the use of electricity or nuclear energy.

STEAM GAUGE – A device for indicating gauge pressure as distinguished from absolute pressure.

STEAM LOOP – An ingeneous thermal pump consisting of an arrangement of piping wherein condensate is returned to the boiler.

STEAM SEPARATOR – An apparatus for separating out moisture that may be carried in suspension by steam flowing in pipelines, and for preventing this moisture from reaching and perhaps damaging engines, pumps, or other machinery that may be driven by the steam.

STEAM SPACE – The space above the water level in a boiler where steam gets collected until it is drawn off through the steam main.

STEAM TRAP – An automatic device which allows the passage of water but prevents the passage of steam. It is used to drain pipes of condensate.

STOP VALVE – A non-return valve having a hand wheel and screw stem which acts only to close the valve.

SUBMERGED TUBE BOILER – A vertical boiler having tubes extending from the lower tube sheet to an upper submerged tube sheet.

SUPER CRITICAL BOILER – A boiler that operates above the super critical pressure of 3206. 2 psi and 705.4°F saturation temperature.

THERMAL LIQUID HEATER – A closed vessel in which a heat transfer medium other than water is heated without vaporization and the heated fluid gives up its heat and does useful work outside the closed vessel.

THROUGH TUBE BOILER – A vertical shell boiler having tubes extending from the lower tube sheet to the full length of the shell.

TRAVELLING GRATE or CHAIN GRATE – A type of overfeed stoker consisting of an endless grate composed of short sections of bars passing over sprockets at the front and rear of the furnace.

TUBE SHEET – A sheet of a water tube boiler where tubes are inserted, either in a flat sheet or a drum.

UNDERFEED STOKER – One in which the fuel is fed upward from underneath.

WASTE HEAT BOILER – A boiler which uses byproduct heat such as from a blast furnace in a steel mill, exhaust from a gas turbine, or by products from a manufacturing process. Waste heat is passed over the heat exchanger surfaces to produce steam or hot water for conventional use.

WATER ANALYSIS – Analyzing a water sample i.e., process of finding out how much of the various impurities and other chemical substances are present in the water. The results are usually expressed in parts per million (ppm).

WATER COLUMN – A boiler fixture consisting of a cylindrical piece to which are attached the water gauge and gauge cocks, thus combining the two into one unit. The top and bottom have outlets which connect it with the boiler below and above the water level.

WATER GAUGE – A device used to indicate the height of water within a boiler.

WATER GRATE – A series of pipes connected close together in parallel to a header at one end and to upflow elements at the other.

WATER LINE – The level at which water stands in the boiler.

WATER TUBE – One which is surrounded by the products of combustion, the water being inside the tube.

WATER TUBE BOILER – A boiler which contains one or more relatively small drums with a multiplicity of tubes in which water steam circulate and hot gases surround the tubes.


WINDOW PATCH – A patch used to seal a hole cut in a water tube to provide access for welding the backside of a circumferential joint, or to replace a small, sharp bag.


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