| Term | Definition |
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| Packaged Boiler | A boiler supplied with all of its components - burner, controls and auxiliary equipment, designed as a single engineered package, and ready for on-site installation.
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| Packaged Steam Generator | See Packaged Boiler.
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| Particle Size | A measure of dust size, expressed in microns or per cent passing through a standard mesh screen. PASS - A confined passageway, containing heating surface, through which a fluid flows in essentially one direction.
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| Perfect Combustion | The complete oxidation of all the combustible constituents of a fuel, utilizing all the oxygen supplied.
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| Petroleum | Naturally occurring mineral oil consisting predominately of hydrocarbons.
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| pH | The hydrogen ion concentration of a water to denote Acidity or Alkalinity. A pH of 7 is neutral. A pH above 7 denotes alkalinity while one below 7 denotes acidity. This pH number is the negative exponent of 10 representing hydrogen ion concentration in grams per liter. For instance a pH of 7 represent 10-7 grams per liter.
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| Pilot | (See also "Ignitor.") A flame which is utilized to ignite the fuel at the main burner or burners.
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| Pitot Tube | An instrument which will register total pressure and static pressure in a gas stream, used to determine its velocity.
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| Pitting | A concentrated attack by oxygen or other corrosive chemicals in a boiler, producing a localized depression in the metal surface.
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| Port | An opening through which fluid passes.
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| Post Purge | A method of scavenging the furnace and boiler passes to remove all combustible gases after flame failure controls have sensed pilot and main burner shutdown and safety shut-off valves are closed.
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| ppm | Abbreviation for parts per million. Used in chemical determinations as one part per million parts by weight.
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| Precipitate | To separate materials from a solution by the formation of insoluble matter by chemical reaction. The material which is removed.
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| Precipitation | The removal of solid or liquid particles from a fluid.
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| Preheated Air | Air at a temperature exceeding that of the ambient air. PRESSURE - Force per unit of area.
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| Pressure Drop | The difference in pressure between two points in a system, caused by resistance to flow.
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| Pressure Vessel | A closed vessel or container designed to confine a fluid at a pressure above atmospheric.
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| Primary Air | Air introduced with the fuel at the burner.
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| Priming | The discharge of steam containing excessive quantities of water in suspension from a boiler, due to violent ebullition.
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| Process Steam | Steam used for industrial purposes other than for producing power.
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| Products of Combustion | The gases, vapors, and solids resulting form the combustion of fuel.
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| Pulsation | Rapid fluctuations in pressure.
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| Purge | To introduce air into the furnace and the boiler flue passages in such volume and manner as to completely replace the air or gas-air mixture contained therein.
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