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| Pascal (Pa) |
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A unit of pressure or stress equal to one newton per square metre.
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| Photocell |
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Device used to detect the presence of an object. A photocell is composed of a transmitter, which transmits an infrared signal to a receiver or reflector.
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| Photosynthesis |
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Process by which chlorophyll-containing cells in green plants convert incident light to chemical energy, capturing carbon dioxide in the form of carbohydrates.
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| Pile |
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A long, heavy timber, round or square, driven deep into the ground to provide a secure foundation for structures built on soft, wet, or submerged sites (e.g., landing stages, bridge abutments).
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| Pilot pressure |
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Auxiliary pressure used to actuate or control hydraulic components.
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| Pilot scale |
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The size of a system between the small laboratory model size (bench scale) and a full-size system.
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| Pilot valve |
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An auxiliary valve used to control the operation of another valve. The controlling stage of a 2-stage valve.
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| Piston |
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A cylindrically shaped part which fits within a cylinder and transmits or receives motion by means of a connecting rod.
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| PLC |
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Programmable logic controllers. Industrial computer with I/O capability, which programs in ladder-relay logic.
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| Plumb |
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Straight up and down, perfectly vertical.
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| Plunger |
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A cylindrically shaped part which has only one diameter and is used to transmit thrust. A ram.
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| Poppet |
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That part of certain valves which prevents flow when it closes against a seat.
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| Port |
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An internal or external terminus of a passage in a component.
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| Positive displacement |
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A characteristic of a pump or motor which has the inlet positively sealed from the outlet so that fluid cannot recirculate in the component.
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| Potentiometer |
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A control element in the servo system which measures and controls electrical potential.
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| Pound (lb) |
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A unit of weight equal to 16 ounces (0.45 kilograms).
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| Power |
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Work per unit of time. Measured in horsepower (hp) or watts (W).
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| Power boiler |
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A boiler that uses wood waste and natural gas as fuel to produce process steam.
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| Power pack |
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An integral power supply unit usually containing a pump, reservoir, relief valve and directional control.
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| Precharge pressure |
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The pressure of compressed gas in an accumulator prior to the admission of liquid.
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| Present value |
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The worth of future receipts or costs expressed in current value. To obtain present value, an interest rate is used to discount future receipts or costs.
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| Pressure |
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Force per unit area. Usually measured in pounds per square inch (PSI) or kilopascals (kPa). A kilopascal is equal to 1000 newtons per square metre or 0.0102 kg/sq cm (0.145 lb/sq in).
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| Pressure drop |
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The difference in pressure between any two points of a system or a component.
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| Pressure line |
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The line carrying the fluid from the pump outlet to the pressurized port of the actuator.
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| Pressure override |
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The difference between the cracking pressure of a valve and the pressure reached when the valve is passing full flow.
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| Pressure plate |
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A side plate in a vane pump or motor cartridge on the pressure port side.
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| Pressure reducing valve |
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A valve which limits the maximum pressure at its outlet regardless of the inlet pressure.
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| Pressure switch |
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1. An electric switch operated by fluid pressure.
2. Switches that read the available pressure in air and hydraulic lines. These switches are
often used as a safety feature, to prevent equipment from operating when there is not enough
air pressure or hydraulic fluid pressure.
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| Process heat |
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Heat used in an industrial process rather than for space heating or other housekeeping purposes.
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| Producer gas |
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Fuel gas high in carbon monoxide (CO) and hydrogen (H2), produced by burning a solid fuel with insufficient air or by passing a mixture of air and steam through a burning bed of solid fuel.
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| Proportional flow |
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In a filter, the condition where part of the flow passes through the filter element in proportion to pressure drop.
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| Proximity switch |
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Photoelectric switches that are triggered as a piece passes near them. Proximity switches are used to detect the presence or absence of material.
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| PSI |
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Pounds per square inch.
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| Pump |
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A device which converts mechanical force and motion into hydraulic fluid power.
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| Pyrolysis |
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The thermal decomposition of biomass at high temperatures in the absence of air. The end product of pyrolysis is a mixture of solids (char), liquids (oxygenated oils), and gases (methane, carbon monoxide, and carbon dioxide).
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