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.