Gage cock - A valve attached to a water column or drum for checking water level.
Gage pressure - The pressure measured relative to the local atmospheric pressure. Gage pressure may be negative. A negative gage pressure is known as suction or vacuum.
Gas analysis - The determination of the constituents of a gaseous mixture.
Gasification - The process of converting solid or liquid fuel into a gaseous fuel such as the gasification of coal.
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. The size, arrangement, and angular disposition of the openings in the tip have a major effect on the size and shape of the flame.
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 two 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 atmospheric, 14.7 psi at sea level; absolute pressure minus 14.7 at sea level.
Globe valve - A stop valve using the round-disk-and-seat principle. Used where the fluid controlled is comparatively clean.
Grain - A unit of measure commonly used in water analysis for the measurement of impurities in water. (17.1 grains = 1 part per million - ppm)
Handhole - An inspection, a sight, or a cleanout opening in a boiler; often elliptical and closed by a handhole plate.
Handhole cover - A handhole enclosure.
Hardness - A measure of the amount of calcium and magnesium salts in boiler water . Usually expressed as grains per gallon or parts per million as CaCO2
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 pipe supplying one or more main pipe lines and receiving a number of supply lines tapped into it.
Heat liberation - Amount of heat released during combustion of fuels. One of the criteria for determining what burner to use in an application.
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)
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 heating 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 gas pressure switch - A switch to stop the burner if gas pressure is too high.
Higher heating value - HHV, the theoretical heat the combustion process can release if the fuel and oxidant are converted with 100% efficiency to CO2 and liquid H20.
Hot well - A tank used to receive condensate from various sources on its passage back to a boiler through the feedwater system. It usually is vented to atmosphere.
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 temperature - the temperature required to initiate combustion.
Impeller - The rotating wheel of a centrifugal pump.
Impingement - The striking of moving flame against boiler parts, causing local overheating.
Incomplete combustion - The partial oxidation of the combustible constituents of a fuel.
Induced draft fan - A fan exhausting hot gases from heat absorbing equipment.
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.
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 flow is stopped, this succession occurring in a sequence of frequent cycles.
Intermittent 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 firetube boiler having an internal furnace such as a scotch, locomotive firebox, vertical tubular, or other type having a water-cooled plate type furnace.
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.