PAH (polycyclic aromatic hydrocarbons) - The carcinogenic byproducts of some very sub-stoichiometric combustion processes. Usually absent in process burners.
Packaged boiler - A boiler equipped and shipped complete with fuel-burning equipment, mechanical draft equipment, automatic controls and accessories; usually shipped in one or more major sections.
Particulates - The residue left over from coal and fuel oil combustion.
Pascals - A unit of pressure. One Pascal (Pa) is equal to a force of one Newton per square meter.
Pass - A confined passageway, containing heating surface, through which fluid flows essentially one direction.
Perfect or Stoichiometric combustion - The complete oxidation of all the combustible constitutes of a fuel, utilizing all the oxygen supplied.
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.
Pilot - A small burner which is used to light the main burner.
Pilot, constant - A pilot that burns without turndown throughout the entire time the boiler is in service.
Pilot flame establishing period - The length of time fuel is permitted to be delivered to a proved pilot before the flame-sensing device is required to detect pilot flame.
Pilot, proved - A pilot flame which has been proved by flame-failure controls.
Pit - Corrosion localized in a small spot.
Plenum - An enclosure through which gas or air passes at relatively low velocities.
Postpurge - A period after the fuel valves close during which the burner motor or fan continues to run, to supply air to the combustion chamber.
Prepurge period - A period on each start-up during which air is introduced into the combustion chamber and associated flue passages in volume and manner as to completely replace the air or fuel air-mixture contained therein prior to an attempt to initiate combustion.
Pre-mixed flame - Before ignition, the fuel and air are intimately mixed. The combustion process is controlled by heat conduction and diffusion of radicals.
Pressure - As applied to boilers, the force exerted by a liquid or gas on a unit area. Three pressures may be involved: gauge pressure, the unit pressure above atmospheric pressure; absolute pressure, gauge pressure plus the atmospheric pressure; vacuum pressure; the pressure below atmospheric pressure usually expressed in inches of Hg.
Pressure, gas - The force exerted per unit area on a surface created by the collision of gas molecules with the surface.
Pressure, static - The pressure of a gas measured at a point where the gas velocity is zero.
Pressure, total - The sum of the static pressure and the velocity pressure of the gas.
Pressure, velocity or dynamic - The pressure of flowing gas attributed to the impact of gas molecules resulting from the velocity of the gas flow.
Primary air - Air introduced with fuel at the burners.
Priming - An induction of boiler water caused by the steam flow into the steam line. The water may be in the form of a spray or a solid body.
Process steam - Steam used for industrial purposes other than for producing power.
Prompt NOx - NOx formed at the initial stages of combustion that cannot be explained by either the thermal mechanism or the fuel NOx mechanism. The prompt NOx mechanism requires the CH radical as an intermediate, so the fuel must have carbon present to create prompt NOx.
Proportional control - A mode of control in which there is a continuous linear relation between value of the controller variable and position of the final control element (modulating control).
Puff - A minor combustion explosion within the boiler furnace or setting.
Pulsation - Rapid fluctuations in furnace pressure.
Purge interlock - A device so arranged that an air flow to the furnace above a minimum must exist for a definite time interval before the interlocking system will permit an automatic ignition torch to be placed in operation.