Power Engineering Dictionary

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PACKAGE UNITS, (REFRIGERATION) - Complete refrigerating system including compressor, condenser and evaporator located in the refrigerated space.

PACKED COLUMN - A tower filled with small objects, designed to obtain large surfaces per volume between rising vapors and a descending liquid.

PACKING - Material made usually of woven animal, plant, mineral or metal fiber and some type of lubricant, placed in rings around the shaft of a pump and used to control leakage from the stuffing box.

PACKING - The fill in a confined space in a stripping vessel, ranging from simple shaped units such as rocks or slats to complex shapes that provide large surface area per unit volume.

PACKING GLAND - The metal part that compresses and holds packing in place in a stuffing box.

PARALLEL CIRCUIT - One where all the elements are connected across the voltage source. Therefore, the voltage on each element is the same but the current through each may be different.


PARTIAL PRESSURES - Condition where two or more gases occupy a space and each one creates part of the total pressure.

PASCAL - The accepted metric unit of measurement or pressure and stress component in the measurement of viscosity. A Pascal is equal to a force of 1 Newton acting an area of 1 square meter. The symbol is Pa.

PASCAL'S LAW - Pressure imposed upon a fluid is transmitted equally in all directions.

PASSIVATING (ANODIC) INHIBITORS - A material capable of forming a protective oxide film on metal surfaces.

PASSIVATION - The process of rendering a metal surface chemically inactive, either by electrochemical polarization or by contact with a passivating agent.

PASSIVATOR - A type of corrosion inhibitor that appreciably changes the potential of a metal to a more noble (positive) value.

PASSIVATOR - A type of corrosion inhibitor that appreciably changes the potential of a metal to a more positive value.

PASSIVE-ACTIVE CELL - A corrosion cell in which the anode is a metal in that active state and the cathode is the same metal in the passive state.

PASSIVITY - A condition on metal surfaces that inhibits electrochemical action between the metal and its environment, such as with boiler water.

PATHOGENIC BACTERIA - Disease-causing bacteria.

PEAK DEMAND - The greatest amount of kilowatts needed during a demand interval.

PEAK LOAD PRICING - A pricing principle that charges more for purchases that contribute to the peak demand and, thereby, cause the expansion of productive capacity when the peak demand exceeds the peak capacity (less minimum excess capacity). In the electric power industry, this means charging more hr electricity bought on or near the seasonal peak of the utility or on or near the daily peak of the utility. The latter requires special meters; the former does not.

PEAKING LOAD - Is the electrical load drawn on the system during high power usage. Usually on very hot or cold days or during the supper hour.

PELTIER EFFECT - When direct current is passed through two adjacent metals one junction will become cooler and the other will become warmer. This principle is the basis of thermoelectric refrigeration.

PERFECT GAS - A hypothetical gas obeying the relation pv = RT.

PERMANENT GASES - Cryogenic refrigerants.

PERMEABILITY - The ability of a body to pass fluid under pressure.

PETROGRAPHIC STUDY OF BOILER SCALES The systematic and descriptive study of rocks. This method also allows the identification of boiler scales, since scale can consist of a large variety of minerals

PETROLEUM OIL - (see mineral oils).

pH - The negative logarithm of the hydrogen-ion concentration of a solution; simply a measure of the relative acidity or alkalinity of a water solution. (pH 1 very acidic; pH 14, very basic; pH 7, neutral).

pH OF SATURATION (pHs) - The pH at which a sample of water is saturated with a specific salt; for example, the pH of saturation of calcium carbonate is the pH of a saturated solution of calcium carbonate.

PHASE - Part of an AC voltage cycle. Residential electrical service is 2-phase; commercial facilities are usually 3-phase AC voltage.

PHIAL -Term sometimes used to denote the sensing element on a thermostatic expansion valve.

PHOSPHATE - An ion, compound, or salt containing phosphorus and oxygen, such as sodium phosphate (Na3P04).

PHOSPHATE TREATMENT - An internal boiler water treatment method to reduce calcium in the boiler with low hardness feedwater.

PHYSICAL STABILITY - The quality which an ion-exchange resin must possess to resist changes that might be caused by attrition, high temperatures, and other physical conditions.

PHYSICAL WATER TREATMENT - Refers to the treatment of removing dissolved gases from the boiler feedwater, using steam.

PIG IRON - High-carbon iron made by reduction of iron ore in the blast furnace.

PILOT DUTY RELAY - A relay used for switching loads such as another relay or solenoid valve coils. The pilot duty relay contacts are located in a second control circuit. Pilot duty relays are rated in volt-amperes (VA).


PIPE SCALE - Rust or mill scale found on the interior of water pipe.

PIT - Corrosion localized in a small spot.

PITOTE TUBE - An open ended tube arrangement to face against the current of a stream of fluid; used in measuring the velocity head of a flowing medium.

PITS - Petroleum Industry Training Service. (Canada)

PITTING - Localized corrosion of a metal characterized by small blisters under which holes have perforated the metal.

PLASTIGAUGE -  provides a simple but effective method for the measurement of clearance between fitted surfaces. It is particularly useful for measuring clearances in split bearings or in situations where a feeler gauge cannot be inserted. Measurement of clearance in big-end bearings can be achieved without dismantling the crankshaft.







PLASTICITY - The ability of a substance to be deformed without rupturing.

PLENUM CHAMBeR - An air compartment connected to one or more distributing ducts.

PLENUM CHAMBER - Chamber or container for moving air or other gas under a slight positive pressure.

PNEUMATIC - Operated by air pressure.

PNEUMATIC-ELECTRIC (PE) SWITCHES - Device that operates an electric switch from a change of air pressure.

pOH - An expression of the alkalinity of a solution; the negative logarithm of the hydroxyl-ion concentration.

POINT, CRITICAL - Of a substance, state point at which liquid and vapor have identical properties; critical temperature, critical pressure, and critical volume are the terms given to the temperature, pressure, and volume at the critical point. Above the critical temperature or

POLARITY - The direction of current flow in a DC circuit. By convention, current flows from plus to minus. Electron flow is actually in the opposite direction.

POLARIZE - In corrosion, to develop a barrier on the anodic or cathodic surface, disrupting the corrosion process.

POLE - An electrical connection point. In a panel, the point of connection. On a device, the terminal that connects to the power.

POLYELECTROLYTE - A water-soluble polymer that is highly charged with cationic or anionic sites. Used as an aid with coagulants.

POLYMER - A chemical compound formed by the adjoining of many molecules of the same characteristics; for example, polyvinyl chloride is a polymer of the molecule vinyl chloride.

POLYMERIC DISPERSANT - A water-soluble polymer that acts as a suspending agent; that is, it promotes suspension of particles in water so that they resist settling.

POLYPHOSPHATE - A chemical compound formed by the adjoining of phosphate ions, hence a polymer of phosphate.

POLYTROPHIC PROCESS - A non-adiabatic reversible process characterized by the equation of path, pv = constant.

POROUS VASE - A vase having a porous construction, which was used to cool its content.

POSITIVE CHARGE - The electrical potential acquired by an atom which has lost one or more electrons; a characteristic of a cation.

POTENTIAL ENERGY - The energy, that a body has by virtue of its position or state, enabling it to do work. (Water in a reservoir has potential energy by virtue of its elevation above some other point.)

POTENTIAL TRANSFORMER - A voltage transformer. The voltage supplied to a primary coil induces a voltage in a secondary coil according, to the ratio of the wire windings in each of the coils.

POTENTIOMETER - An electromechanical device having a terminal connected to each and to the resistive element, and a third terminal connected to the wiper contact. The electrical input is divided as the contact moves over the element, thus making it possible to mechanically change the resistance.

POUR POINT - The lowest temperature at which a lubricant can be observed to flow under specified conditions. For oil, the pour point is a temperature -15°C above the temperature at which the oil is solid.

POUR-POINT DEPRESSANT - An additive that lowers the pour point of a lubricant.

POWER (P) - Expressed in watts (W) or kilowatts (kW), and is equal to:

POWER ELEMENT - Sensitive element of a temperature-operated control.

POWER FACTOR (pf) - A quantity that relates the volt amperes of an AC circuit to the wattage (power = volt-amperes x power factor). Power factor also is the ratio of the circuit resistance (R) to the impedance (Z) expressed as a decimal between zero and one (pf = R/Z). When the power factor equals one, all consumed power produces useful work.

POWER FACTOR CHARGE - A utility charge for "poor" power factor. It is more expensive to provide power to a facility with a poor power factor (usually less than 0.8).

POWER FACTOR CORRECTION - Installing capacitors on the utility service’s supply line to improve the power factor of the building.

ppb - equals 0.001 ppm. (Parts per billion)

PRECIPITATE - An insoluble reaction product; in an aqueous chemical reaction, usually a crystalline compound that grows in size to become settable.

PRECIPITATING (CATHODIC) INHIBITORS - The are chemicals which form insoluble precipitates that are able to coat and protect surfaces. They are less durable then the passivating type inhibitors.

PRECIPITATION - A process whereby salts drop or come out of a water solution.

PRECISION PHOSPHATE TREATMENT - A phosphate treatment based upon maintaining 2 to 4 ppm of phosphate and 15 to 50 ppm of hydrate alkalinity in the boiler.

PREDICTING METHOD - A method for determining when load shedding should occur. A formula is used to arrive at a preset kilowatt limit. Then the actual amount of energy accumulated during the utility's demand intervals is measured. A projection is made of the actual rate of energy usage during the rest of the interval. If the predicted value exceeds the preset limit, loads will be shed.

PREIGNITION - A condition in an internal combustion engine characterized by a knocking sound and caused by the fuel-air mixture having been ignited to soon because of an abnormal condition.

PRESSURE - The normal force exerted by a homogeneous liquid or gas, per unit of area, on the wall of its container.


PRESSURE DROP - Pressure loss in fluid pressure, as from one end of a duct or pipe to the other, due to friction, dynamic losses, and changes in velocity pressure.

PRESSURE GAUGE - Instrument for measuring the pressure exerted by the contents on its container.

PRESSURE HEAD - The height to which liquid can be raised by a given pressure (sometimes referred to as pump head).

PRESSURE MOTOR CONTROL - Device which opens and closes an electrical circuit as pressures change.

PRESSURE OPERATED THERMOMETER - Temperature indicator which is controlled by bellows, a capillary or remote sensitive bulb.

PRESSURE REGULATOR, SUCTION (REFRIGERATION COMPRESSORS) - An automatic valve designed to limit the suction pressure to prevent motor overload.

PRESSURE SWITCH - Switch operated by a rise or drop in pressure.

PRESSURE VELOCITY - In moving fluid, the pressure capable of causing an equivalent velocity, if applied to move the same fluid through an orifice such that all pressure energy expended is converted into kinetic energy.

PRESSURE, ABSOLUTE - Pressure referred to that of a perfect vacuum. It is the sum of gauge pressure and atmospheric pressure.

PRESSURE, ATMOSPHERIC - It is the pressure indicated by a barometer. Standard atmosphere is the pressure equivalent

PRESSURE, CRITICAL - Vapor pressure corresponding to the substance's critical state at which the liquid and vapor have identical properties.

PRESSURE, GAGE - Gauge: Pressure above atmospheric.

PRESSURE, HYDROSTATIC - The normal force per unit area that would be exerted by a moving fluid on an infinitesimally small body immersed in it if the body were carried along with the fluid.

PRESSURE, OPERATING - Pressure at which a system is operating.

PRESSURE, PARTIAL - Portion of total gas pressure of a mixture attributable to one component.

PRESSURE, SATURATION- The saturation pressure for a pure substance for any given temperature is that pressure at which vapor and liquid, or vapor and solid, can coexist in stable equilibrium.

PRESSURE, STATIC - The normal force per unit area that would be exerted by a moving fluid on a small body immersed in it if the body were carried along with the fluid. Practically, it is the normal force per unit are at a small hole in a wall of the duct through which the fluid flows (piezometer) or on the surface of a stationary tube at a point where the disturbances, created by inserting the tube, cancel. It is supposed that the thermodynamic properties of a moving fluid depend on static pressure in exactly the same manner as those of the same fluid at rest depend upon its uniform hydrostatic pressure

PRESSURE, TOTAL - In the theory of the flow of fluids, the sum of the static pressure and the velocity pressure at the point of measurement. Also called dynamic pressure.

PRESSURE, VAPOR - The partial pressure exerted by the water vapor contained in air.

PRESSURE-HEAT DIAGRAM - Graph of refrigerant pressure, heat and temperature properties. (Mollier's diagram.)

PRESSURESTAT - A control which reacts to pressure changes in the evaporator.


PRETREATMENT - Also referred to as external treatment, consisting of treating the raw make-up water, and include removing dissolved oxygen, carbon dioxide, suspended solids, hardness, alkalinity, silica, dissolved solids, etc..

PRIMARY AIR - The initial air stream discharged by an air outlet (the air being supplied by a fan or supply duct) prior to any entrainment of the ambient air or for he purpose of combustion.

PRIMARY CONSTITUENTS - Refers to the category of dissolved solids present in water at a level of more then 5 mg/L.

PRIMARY CONTROL - A device which directly or indirectly controls the control agent in response to needs indicated by the controller. Typically a motor, valve, relay, etc.

PRIMARY ELEMENT - The portion of the controller which first uses energy derived from the controlled medium to produce a condition representing the value of the controlled variable; for example, a thermostat bimetal.

PRIMARY SYSTEM - A closed water system in which the water is circulated directly through a chiller for cooling or heat exchanger or boiler for heating.

PRIME MOVER - An engine or other device by which natural source of energy is converted into mechanical energy.

PRIMING - The phenomenon in which the level of water in a boiler is changed by bouncing rapidly. The result is, that boiler water will enter the steam flow.

PROCESS HOT WATER - Hot water needed for manufacturing processes over and above the "domestic hot water" that is for the personal use of industrial workers.

PROPANE - A paraffin hydrocarbon (C3H8) that is a gas at ordinary atmospheric conditions but easily liquefied under pressure.

PROPORTIONAL BAND - The range of values of a proportional positioning controller through which the controlled variable must pass to move the final control element through its full operating range. Commonly used equivalents are "throttling range" and "modulating range".

PSI - Symbol or initials used to indicate pressure measured in pounds per square inch.

PSIA - Symbol or initials used to indicate pressure measured in pounds per square inch absolute. Absolute pressure equals gauge pressure plus atmospheric pressure. The "A" indicates that the gauge pressure is reading in absolute.

PSIG - Symbol or initials used to indicate pressure in pounds per square inch gauge. The "G" indicates that is gauge pressure and not absolute pressure.

PSYCHROMETER - An instrument for ascertaining the humidity or hygrometric state of the atmosphere.

PSYCHROMETER OR WET BULB HYGROMETER – An instrument for measuring the relative humidity of atmospheric air.

PSYCHROMETRIC CHART – A chart that shows relationship between the temperature, pressure and moisture content of the air.

PSYCHROMETRIC MEASUREMENT - Measurement of temperature pressure and humidity using a psychometric chart.

PSYCHROMETRY - The study of air-vapor mixtures.

PULL DOWN - An expression indicating action of removing refrigerant from all or part of a refrigeration system.

PULSATION DAMPENER - Any gas or liquid charged, chambered device that minimizes periodic increases and decreases in pressure.

PULSE-ECHO TECHNIQUES - Corrosion detecting processes which, by recording the action of ultrasonic waves artificially introduced into production structures, can determine metal thickness and detect flaws.

PUMP - A device that increases the pressure on a fluid or raises it to a higher level.

PUMP DOWN - The act of using a compressor or a pump to reduce the pressure in a container or a system.

PUMP, POSITIVE DISPLACEMENT - A pump that moves a measured quantity of fluid with each stroke of a piston or each revolution of vanes or gears.

PUMP, ROTARY - A pump that moves fluid by positive displacement, using a system of rotating vanes, gears, or lobes.

PUMP-DISCHARGE PRESSURE - The point of highest pressure in a re-circulating water system which is at the discharge side of the re-circulating pump.

PURGING - Releasing compressed gas to atmosphere through some part or parts for the purpose of removing contaminants from that part or parts.

PURGING VALVES - Devices used to remove non-condensable gases from the refrigeration system.

PYROMETER - A device for measuring temperatures above the range of liquid thermometers.

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