Boiler Glossary

on . Posted in Boiler


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  • Absolute Pressure  -  A pressure at absolute zero can only exist in a total vacuum and any pressure above this is called absolute 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.
  • Air-fuel Ratio  -  The ratio of air weight to fuel weight consumed in a furnace.
  • Allowable Stress  -  The stress values to be used in the design are temperature-dependent. 
  • Allowable Working Pressure  -  The maximum pressure for which the boiler was designed and constructed.
  • Ambient Temperature  -  When outdoors the ambient temperature is the current surrounding environment air temperature.  This temperature has nothing to do with high or low forcasts.
  • Atmospheric Pressure  -  The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth.


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  • Baffle  -  A plate or wall for deflecting gases or liquids.
  • Blastpipe  -  Part of the exhaust system that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.
  • 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.
  • 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.
  • Boiler Crown  -  The upper round plates of a boiler of the shell type boiler.
  • Boiler Horsepower  -  The evaporation of 34-1/2 lbs of water per hour from a temperature of 212\(^\circ\)F into dry saturated steam at the same temperature.
  • Boiler Plate  -  A mild steel plate, generally produced by the open hearth process, used mainly for the shells and drums of steel boilers.
  • Boiler Pressure  -  The pressure at which steam is generated in a boiler.
  • Boiler Rating  -  The heat capacity of a boiler expressed in boiler horsepower, Btu/hr, or pounds of steam/hr.
  • Boiler Shell  -  The outer cylindrical portion of a pressure vessel.
  • Boiler Shutdown  -  A sequence of operation completed when taking a boiler off line.
  • 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.
  • Boiling Point  -  Temperature at which water changes into steam.
  • Bottom Blowdown  -  Periodic draining of part of the water in the boiler to remove the heavy sludge that settle to the bottom of a vessel.
  • Breeching  -  Ducting from boiler flue gas outlet to stack.
  • Bridgewall  -  A wall in the furnace over which the products of combustion pass.
  • Brinell Hardness Number  -  Is a value assigned to the hardness of metals and alloys.
  • Brittle  -  A metal is brittle when it permits little or no plastic deformation prior to fracture.
  • Bulge  -  A local distortion or swelling outward caused by internal pressure on a tube wall or boiler shell due to overheating.
  • 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 Capacity  -  Amount of heat release a burner can deliver 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.


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  • Cavitation  -  The creation and collapse of bubbles in a liquid.
  • Check Valve  -  These valves are designed to allow the process fluid to flow in only one direction to prevent backflow.
  • Cladding  -  The layer of insulation and outer wrapping around a boiler shell.
  • Combustion  -  A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen, and heat.
  • Continuous Blowdown  -  Small stream of water that constantly drains from a boiler to control the quantities of impurities in a boiler on a continuous basis.
  • Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.
  • Corrosion Allowance  -  A designer shall consider whether to add corrosion allowance when designing thickness of steam boiler parts.
  • Cracking Open  -  Slowly opening a valve, generally to allow equalization.
  • Cut-in Pressure  -  Automatic pressure control setting at which the boiler turns on.
  • Cut-out Pressure  -  Automatic pressure control setting at which the boiler turns off.


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  • Damper  -  A device for introducing a variable resistance of regulating the volumetric flow of gas or air.
  • Design Pressure  -  The pressure used for calculating the minimum thickness requirement for steam boiler parts.
  • Design Temperature  -  The maximum mean wall temperature of pressure parts (tube, pipe, header and drum) that is to say the sum of the outside and inside pressure parts surface temperatures divided by two.  A steam boiler is designed for design metal temperature which influence allowable stress values.  The design metal temperature of an individual component may be much higher than the design steam temperature of the steam boiler.
  • Dissolved Solids  -  Impurities that have passed into solution.
  • Downcomer  -  A tube or pipe in a boiler or waterwall circulating system through which fluid flows downward between headers.
  • Downed Tube  -  Either a fire tube or water tube that is entirely below the water level of the operating boiler.  As corrosion and scaling is most active in the region of the water level, this reduces wear and maintenance requirements.
  • Dry Steam  -  Does not contain water held in suspension.  It is similar to superheated steam.


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  • 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.
  • Enthalpy  -  Measures the sum of internal energy changes in heat under constant pressure of the system.
  • 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  -  Air more than the theoretical amount needed for combustion.
  • Exhaust Gas Damper  -  Used to control the pressure in an exhaust gas boiler.
  • Expansion Joint  -  Heat and cold causes piping systems to expand and contract. 


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  • Feedwater  -  The water supplied to a boiler to replace that evaporated as steam or blown off.
  • Feedwater 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.
  • Feedwater Line  -  The pipeline through which the water is fed to the boiler from the feedwater tank with the help of a pump.
  • Fire Point  -  Temperature at which fuel oil burns continuously when exposed to an open flame.
  • Firing Rate  -  Amount of fuel the burner is capable of burning in a given unit of time.
  • Flame Failure  -  When the burner pilot or main flame goes out on its own.
  • 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.
  • Flash Point  -  Temperature at which fuel oil, when heated produces a vapor that flashes when exposed to an open flame.
  • Flash Steam  -  When hot condensate is released from a high pressure to a lower pressure steam.
  • Foaming  -  Formation of steam bubbles on the surface of boiler water due to high surface tension of the water.
  • Friction Loss  -  How much loss of flow through a pipe is due to the viscosity, the measure of the internal friction/resistance to the flow of a liquid near the surface of the pipe.
  • 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  -  That part of the boiler designed for burning the fuel.


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  • Gage Cock  -  A valve attached to a water column or drum for checking water level.
  • Gasification  -  The process of converting solid or liquid fuel into a gaseous fuel such as the gasification of coal.
  • Gas Analysis  -  The determination of the constituents of a gaseous mixture.
  • Gas Leak Detector  -  A device to locate gas leaks in the boiler room.
  • 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.
  • Gauge Glass  -  Part of the water level gauge, which normally consists of a vertical glass tube connected top and bottom to the boiler backplate.  The water level must be visible within the glass at all times.
  • Gauge Pressure  -  Pressure above atmospheric pressure.  Assumes atmospheric pressure being zero.


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  • Handhole  -  An inspection, a sight, or a cleanout opening in a boiler; often elliptical and closed by a handhole plate.
  • Hard Water  -  Water which contains calcium or magnesium in an amount which requires an excessive amount of soap to form a lather.
  • Header  -  Manifold that feeds several branch pipes or takes in steam or water from several smaller pipes.
  • Head Loss  -  A pressure loss due to the resistance of the fluid and obstructions along the way in a pipe.
  • Heat  -  A form of energy that causes physical change in what is being heated.
  • Heat Exchanger  -  A device used to transer heat from one medium to another at different temperatures.  The heat transfer can be air or a liquid such as water or oil.
  • Heat Transfer by Conduction  -  It is the flow of energy between two objects, or within one object, where there is a temperature differential.
  • Heat Transfer by Convection  -  Convection is the energy transfer of heat by air, water, or any other fluid to and object.
  • 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.
  • Hotwell  -   A tank for the condensate returns and from which the feed water pump takes its suction.
  • Hot Water Boiler  -  Boiler that is completely full of water that produces only hot water, not steam.
  • Hydrostatic Head  -  This is the load caused by the weight of steam boiler part and its content.


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  • 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.
  • 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.


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  • Joint Efficiency  -  The ratio of the strength of a section of riveted or welded joint to the strength of a related section of solid plate.



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  • Laminar Flow  -  Generally happens when dealing with low Reynolds numbers in pipes. This could be due to low velocities, large diameters or high viscosities.
  • Latent Heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.
  • 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.
  • Live Steam  -  Steam available directly from a boiler under full pressure.
  • Local loads  -  Can result from steam boiler mountings, lugs, external piping, and the like. 
  • Lowest Possible Water Level  -  The lowest water level at which the boiler can be safely operated without damaging or overheating any part of the boiler.
  • Low Gas Pressure Switch  -  A control to stop the burner if gas pressure is too low.
  • Low Water Fuel Cutoff  -  A safety device which cuts off the fuel supply to the burner if the boiler water level drops below a safe level.


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  • Makeup Water  -  Raw water that has been filtered and softened and then introduced into the hotwell to compensate for the water loss in the system.
  • Manifold  -  A pipe or header for collecting a fluid from, or the distributing of a fluid to a number of pipes or tubes.
  • Maximum Allowable Working Pressure  -  The maximum gage pressure or the pressure above the atmospheric pressure that is permitted in the steam boiler.  It’s based on the lowest design pressure of any steam boiler part.
  • Motive Pressure  -  Line velocity pressure in steem piping.


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  • Natural Circulation  -  The circulation of water in a boiler caused by the differences in density.
  • Non-return Valve  -  Combustion shutoff and check valve that allows steam to pass out of the boiler, but a back flow of steam from a drop in pressure causes the valve to close.


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  • Oil Burner  -  A burner that atomizes fuel oil and blows it into the combustion chamber in the form of a fine mist or vapor.
  • 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.
  • Operating Pressure  -  The actual pressure that be operated in steam pressure parts.
  • Operating Temperature  -  The actual temperature that be operated in steam boiler parts.


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  • Particulates  -  The residue left over from coal and fuel oil combustion.
  • Pilot  -  A small burner which is used to light the main burner.
  • Plenum  -  An enclosure through which gas or air passes at relatively low velocities.
  • Popping Pressure  -  Predetermined pressure at which a safety relief valve opens and remains open until the pressure drops.
  • Post Purge  -  The passing of air through the boiler fireside after normal burner shutdown.
  • Pour Point  -  The lowest temperature at which a fuel oil flows as a liquid.
  • Pre-purge  -  The passing of air throught the boiler fireside prior to pilot and main burner flame lightoff.
  • Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.
  • Pressure Differential  -  The pressure difference between two points of a system.
  • Pressure Drop  -  See pressure loss.
  • Pressure Indicating Transmitter  -  An instrument for measuring, controlling and indicating positive, negative or differential pressures of different fluids.
  • Pressure Loss  -  The difference in pressure between two points, usually caused by friction resistance in the pipe, but moisture can also affect it.
  • Pressure Safety Valve  -  Also known as a pressure relief valve and is used to protect vessels and tanks from overpressure.
  • Primary Air  -  Air supplied to the burner that regulates the rate of combustion.
  • Process Steam  -  Steam used for industrial purposes other than for producing power.
  • Proving Pilot  -  Sighting the pilot through a flame scanner to verify that the pilot is lit.
  • Puff  -  A minor combustion explosion within the boiler furnace or setting.
  • Pulsation  -  Rapid fluctuations in furnace pressure.
  • Purge Period  -  Before ignition and after burner shutdown when explosive combustibles are removed.



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  • Radiant Section  -  The part of a process heater into which the burners fire.
  • Rated Capacity  -  The manufacturer’s stated capacity rating for mechanical equipment.
  • Raw Water  -  Water supplied to the plant before any treatment.
  • Reheater  -  A device using highly superheated steam or high temperature flue gases as a medium serving to restore superheat to partly expanded steam.
  • Return Trap  -  A trap designed to discharge its condensate against boiler pressure and feed to the boiler without additional mechanical equipment.
  • Riser Tube  -  A tube through which steam and water pass from an upper waterwall header to a drum.
  • Rupture Disk  -  A safety device which acts like a safety valve to protect against excessive pressure buildup in a system.  The disk shatters when its maximum pressure is reached and must be replaced each time it activates.


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  • Safety Relief Valve  -  Safety valves provide relief from pressure.  This valve opens quickly to avoid overpressurization when a set pressure is reached, preventing a potential safety incident.
  • Scale  -  Dissolved minerals from hard water that precipitate out in the steam space around the water level.  Where this scale falls to the bottom of the boiler and mixes with other contaminants, it is called mud.
  • Scum Valve  -  A blowdown valve mounted at the water level of a boiler, used to blow down lighter oily or foamy deposits within a boiler that float on the water-level.
  • Sediment  -  Particles of foreign matter present in the boiler water.
  • Seismic Loads  -  General effects of seismic loads fall into two categories: bending due to moments imposed and dynamic effects due to vibration.
  • Smoke Density  -  Varies from clear to dark.  Determined by the amount of light that passes through the smoke as it leaves the boiler.
  • Spalling  -  Hairline cracks in boiler refractory due to changes in fireside temperatures.
  • Steam  -  The invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.
  • Steam Gauge  -  A device for indicating gauge pressure.
  • Steam Quality  -  The porportion of saturated steam (vapor) in a saturated condensate (liquid) / steam (vapor) mixture.
  • Steam Purity  -  The degree of contamination.
  • 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.
  • Steam Space  -  The space above the water line in a steam boiler.
  • Steam Trap  -  Mechanical device used to remove condensate from steam piping.
  • Stop-check Valve  -  Essentially, two valves built into one.  It can act as a globe valve to isolate or control the flow rate.  It also acts as a check valve by preventing reverse flow.
  • Stop Valve  -  Stop valves are used to shut off or partially shut off the flow of liquids.
  • Strainer  -  This is a closed vessels that collect solid particles to be separated while passing a fluid through a removable screen.
  • Structural Stability  -  This allowance is provided for pipes and tubes to consider threading and mechanical loads.
  • Superheated Steam  -  Steam at any given pressure which is heated to a temperature higher than the temperature of saturated steam.
  • Surface Blowoff  -  Removal of water, foam, etc. from the surface at the water level in a boiler.
  • 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.
  • Syphon -  A protective device used to prevent steam from entering the internal works of a steam gage.  Often a syphon only consists of a single coil of high pressure pipe with threaded ends.


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  • Temperature  -  Normally described as the amount of heat or cold, but it is neither heat or cold.
  • Tertiary Air  -  Air for combustion supplied to the furnace to supplement the primary and secondary air.
  • Thermal Conductivity  -  The ability to transfer heat within a material without any motion of the material.
  • Thermal Efficiency  -  The fraction of heat that is converted to work or desired output divided by required input.  Thermal efficiency is the heat exchanger's ability to transfer heat from the heating process to water or steam, leaving out radiation and convection loss.
  • Thermal Load  -  This type of load occurs when steam boilers are used in cyclic service, with frequent start-ups and shutdowns that causes high thermal stress.
  • Thermal Shock  -  Caused by temperature difference when cold water is directly introduced on hot surfaces of a steam boiler parts.
  • 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.
  • Total Pressure  -  The sum of the static and velocity pressures.
  • Treated Water  -  Water which has been chemically treated to make suitable for boiler feed.
  • 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.
  • 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.



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  • Vacuum Breaker  -  Vent on top of vessel that allows air to be pulled into the tank to prevent formation of a vacuum.
  • Vacuum Gauge  -  Pressure gauge used to measure pressures below atmospheric pressure.
  • 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.
  • Vapor  -  The gas state of a liquid or solid.
  • Vertical Firing  -  An arrangement of a burner such that air and fuel are discharged into the furnace, in practically a vertical direction.
  • Viscosity  -  The measure of the internal friction/resistance to the flow of a liquid.
  • Volume of Air  -  The number of cubic feet of air per minute expressed at fan outlet conditions.
  • Vortex  - The swirling motion of a liquid in a vessel at the entrance to a discharge nozzle.


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  • Waste Heat  -  Sensible heat in non-combustible gases discharged to the environment.
  • Windbox  -  The box surrounding the burner damper on a boiler.  The fan blows combustion air into the box and the damper regulates and directs the air into the burners.
  • Water Hammer  -  Occurs when a valve is suddenly opened or closed.
  • Water Line  -  The level at which water stands in the boiler.
  • Water Softener  -  Used to remove dissolved minerals from water.  Hard water can leave mineral scale after evaporation.
  • Wet Steam  -  Contains both water and steam held in suspension just below the satutation temperature.
  • Wind Loads  -  General effects of wind loads fall into two categories: bending due to moments imposed and dynamic effects due to vibration.
  • Working Pressure  -  The normal pressure that a system operates at.



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  • Yield Point  -  The point where an elastic material is permanent change in length with no extra load force.
  • Yield Strength  -  The minimum stress that leads to permanent deformation of the material.


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Tags: Glossary Boiler