S-T-U

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

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

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

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

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

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

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

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

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

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

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

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

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

Shell – The cylindrical portion of a pressure vessel.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Suspended solids – Undissolved solids in boiler water.

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

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

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

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

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

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

Theoretical flame temperature – Same as “adiabatic temperature”.

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

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

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

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

Thermocouple – A temperature measuring instrument.

Throat – The neck portion of a passageway.

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

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

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

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

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

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

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

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

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

Trap – A receptacle for the collection of undesirable material.

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

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

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

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

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

Tube – A hollow cylinder for conveying fluids.

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

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

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

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

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

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

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

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

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