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- a container which stores fluids under pressure as a source of hydraulic power. It may also be used as a shock absorber.


- a device which converts hydraulic power into mechanical force and motion. (examples: hydraulic cylinders and motors).

Alarm valve

- is a check valve with a method of getting pressure or slight flow from an auxiliary port. The alarm valve opens when pressure on the system side is low enough for supply pressure to open it. Then some water goes out the alarm port to a pressure switch or water motor gong. 

Air Valve

 - In general, a valve designed to control the flow of air. Specifically:

* A valve placed upon a steam-boiler to admit air, and thus prevent the formation of a vacuum by the condensation of steam within when the boiler is cooling off, and the consequent tendency to collapse.

* A valve placed at bends and summits of water-pipes, etc., for the outflow of air, as when the pipes are being filled, and for the ingress of air to prevent the formation of a vacuum when the water is drawn out.

* In an engine-cylinder, and particularly on the locomotive engine, a valve which is held shut by steam-pressure when the throttle-valve is open, but which opens by a spring to admit atmospheric air when the throttle is closed and the pistons keep on moving from the momentum of the mass of engine and train.


- (Abbreviation) level control, differential altitude pilot (2/3 ways)



- the process by which air is removed from a hydraulic system


- A secondary passage for fluid flow.

Butterfly valve

- is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk.

Back pressure

- the differential pressure between the inlet and outlet pressures.


Cam lobe motor

- a hydraulic radial piston motor in which rotational force is created by the outward movement of the pistons against the lobes of a stationary cam.


- a phenomenon which occurs when the pressure at a point in a hydraulic system is lowered below the vapour pressure of the oil in the system. This allows bubbles of oil vapour to form in the oil. If this occurs at the pump inlet, the quick pressure rise inside the pump forces it is characterized by increase in the valve head losses, typical loud noise and vibrations, quick typical erosion of the valve wall at the downstream side.


- a series of component parts connected to each other by fluid lines or passages. Usually it is part of a "system".

Closed center system

- a hydraulic system in which the control valves are closed during neutral, stopping oil flow. Flow in this system is varied, but pressure remains constant.


 - a microprocessor that controls electro-hydraulic valve functions.


- electronic valve controller designed for reliable and accurate regulation of hydraulic automatic valves, suitable for performing any control application and combination of applications.

Control valves

- valves that serve for control to open the inlet supply into the tank\reservoirs when the level is low, control to close the inlet supply into the tank\reservoirs when the level is high, thus prevent overflow. In combination with other control applications will also limit the rate of flow into the reservoir, close in a way that reduces surges in the supply line, be controlled remotely or locally by the system’s operators. 

Cooler (oil)

 - a heat exchanger which removes heat from a fluid. (see "heat exchanger").


 - a device to connect two hoses or lines, or to connect hoses to valve receptacles.


 - a device sometimes built into the end of a cylinder which restricts outlet flow and thereby slows down the piston.


 - a single complete operation of a compo- nent which begins and ends in a neutral position.


 - a device for converting fluid power into linear or circular motion. An "actuator". Basic design types include piston and vane units.

* Double-acting cylinder - a cylinder in which fluid force can be applied to the movable element in either direction.

* Piston- type cylinders - a cylinder which uses a sliding piston in a housing to produce straight movement.

* Rotary cylinders - a cylinder in which fluid force is applied to produce circular motion. Single-acting cylinder-a cylinder in which fluid force can be applied to the movable element in only one direction.

* Vane-type cylinder - a cylinder which uses a turn- ing vane in a circular housing to produce rotary movement.

Check valve (clack valve, non-return valve or one-way valve)

- is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a wide range of sizes and costs, check valves generally are very small, simple, or inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check valves are made of plastic or metal.

Cracking pressure and pressure override

- the inlet pressure at which the first indication of flow occurs (steady stream of bubbles) -the pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as pressure differential, also known as pressure override.


Deluge valve

- a normally closed valve, opened by a stimulus from other than pressure on the system side. Typically the stimulus comes from a heat detector for electrically operated systems, or loss of pressure in a pilot line. The pilot line is provided with air or water pressure, and has fusible plugs or sprinklers for detection, mounted in appropriate locations. If the pilot line loses pressure, the deluge valve opens automatically. Manual operating features are always included. Rarely, one finds deluge systems that are manually operated only. 


 - the volume of oil displaced by one complete stroke or revolution (of a pump, motor, or cylinder).


 - motion of a cylinder or motor due to internal leakage past components in the hydraulic system.



- three types of energy are available in modern hydraulics (of the normal hydrostatic type):

* Potential energy - pressure energy. The static energy of oil which is standing but is pressurized and ready to do work. Example: oil in a loaded accumulator.

* Heat energy - friction or resistance to flow. (an energy loss in terms of output.) Example: friction between moving oil and the confines of lines or passages produces heat energy.

* Kinetic energy - the energy of the moving liq- uid. Varies with the velocity (speed) of the liquid.


Filter (oil)

- a device which removes solids from a fluid.


- pressure / area - a force per area unit: bar=kg/cm2, psi=pound/inch2 the same pressure, when applied on a larger section area, generates a larger force and vice versa.

Flow meter

 - a testing device which gauges either flow rate, total flow, or both.

Flow rate

 - the volume of fluid passing a point in a given time. Flow = volume / time, can be calculated also by speed x section area.

Fluid power

 - energy transmitted and controlled through use of a pressurized fluid.


 -  (Abbreviation) differential level control by electric float


- (Abbreviation) differential level control by vertical or horizontal float-pilot.


 - a push or pull acting upon a body. In a hydraulic cylinder, it is the product of the pressure on the fluid, multiplied by the effective area of the cylinder piston. It is measured in pounds or tons.


 - the resistance to fluid flow in a hydraulic system. (an energy loss in terms of power output).

Fixed outlet PRV

- the standard pressure reducing valve that regulates a high, varying upstream pressure to a lower fixed  and stable downstream pressure, regardless of variations in demand flow.


Gal Valves

- hydraulic automatic valves, for the control and regulation of water supply systems; a direct diaphragm seal valve; the rubber diaphragm is both the actuator and the plug; only one moving part (the diaphragm). No internal seals, bearings and throttling plugs used. The Gal Valve is a patent of Dorot, being the first manufacturer worldwide to fabricate such a valve!

Gate Valve

- also known as a sluice valve, is a valve which opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is desired. The gate faces can be parallel, but are most commonly wedge-shaped. Gate valves are primarily used to permit or prevent the flow of liquids, but typical gate valves shouldn't be used for regulating flow, unless they are specifically designed for that purpose.


Heat exchanger

 - a device which transfers heat through a conducting wall from one fluid to another. (see "cooler, (oil)").


- the work produced per unit of time.


 - a flexible line.


 - the engineering science of liquid pressure and flow.


 - the engineering science of the energy of liquid pressure and flow.


 - the engineering science of the energy of liquids at rest. (all the systems covered in this manual operate on the hydro-static principle).


 - when the water flows through the pipes, the pressure drops due to vortex and friction

Hydraulic relay valve

- a type of a pilot valve that reacts to a pressure command; the pressure command is translated to an hydraulic command to the main valve.


Inert gas

- it is a non-explosive gas.


Junction rule

 - the incoming flow must be equal to the outgoing flow, the sum of the flow rates entering a junction = 0



- (Abbreviation) - an electric device which enables activation of hydraulic valves, by means of a latching (pulse) solenoid, using standard 9v batteries; the main purpose of the lcu dls is to control the maximal and minimal water levels in tanks and reservoirs, using a very sensitive pressure switch.


- a tube, pipe, or hose for conducting a fluid.

Low flow stability

- superb low flow regulation ability. Due to the diaphragm flexibility, the water-passage section area is changing according to the flow rate; minimal head-losses in the fully open valve. 



 - a fluid conductor which provides many ports.

Moter (hydraulic)

 - a device for converting fluid energy into mechanical force and motion - usually rotary motion. Basic design types include gear, vane, and piston units.


Open center system

 - a hydraulic system in which the control valves are open to continuous oil flow, even in neutral. Pressure in this system is varied, but flow remains constant.


 - a restricted passage in a hydraulic circuit. Usually a small drilled hole to limit flow or to create a pressure differential in a circuit.


 - a static and/or dynamic seal for curved or circular mating surfaces.



 - any material or device which seals by compression. Common types are u-packings’, v- packings, "cup" packings, and a-rings.


 - a line who’s outside diameter is standardized for threading/grooved/ flanged


 - a cylindrical part which moves or rotates in a cylinder and transmits or receives motion to do work.


 - the open end of a fluid passage. May be within or at the surface of a component.

Pour point

- the lowest temperature at which a fluid will flow under specific conditions.

Power beyond

 - an adapting sleeve which opens a passage from one circuit to another. Often installed in a valve port which is normally plugged.


- force of a fluid per unit area, usually expressed in pounds per square inch (psi). Pressure units: bar (kg/cm2), psi (pounds / inch2)

* Back pressure - the pressure encountered on the return side of a system.

* Breakout pressure - the minimum pressure which starts moving an actuator.

* Cracking pressure - the pressure, at which a relief valve, etc., begins to open and pass fluid.

* Differential pressure - the difference in pressure between any two points in a system or a component. (Also called a "pressure drop.")

* Full-flow pressure - the pressure at which a valve is wide open and passes its full flow.

* Operating pressure - the pressure at which a system is normally operated.

* Pilot pressure - auxiliary pressure used to actuate or control a component.

* Rated pressure - the operating pressure which is recommended for a component or a system by the manufacturer.

* Static pressure - the pressure in a fluid at rest. (a form of "potential energy.")

* Suction pressure - the absolute pressure of the fluid at the inlet side of the pump.

* Surge pressure - the pressure changes caused in a circuit from a rapidly accelerated column of oil. The "surge" includes the span of these changes, from high to low.

* System pressure - the pressure which overcomes the total resistances in a system. It includes all losses as well as useful work.

* Working pressure - the pressure which overcomes the resistance of the working device.


 - repeated small fluctuation of pressure within a circuit.


- a device which converts mechanical force into hydraulic fluid power. Basic design types are gear, vane, and piston units.

* Fixed displacement pump - a pump in which the output per cycle cannot be varied.

* Variable displacement pump - a pump in which the output per cycle can be varied.

Pump system control

 - a system that prevents water hammer, prevents returning flow through the pumps and eliminates check-valves slamming noises.

Pilot Valve

- a pilot valve is the most important device in the servo-system of a regulating valve; it serves to pre-set the required pressure\flow in the system by controlling the volume in the valve control chamber.

Proportional pressure reducer

- a valve that has a control chamber permanently connected to the downstream; this valve must be a double chamber [d] type; the balance of hydraulic forces created between the high pressure on the small seal area, and the lower downstream pressure on the larger diaphragm area, causes a fixed ratio of inlet/outlet pressure of approximately 2.7:1; no other control device is needed.


Regenerative circuit

- a circuit in which pressure fluid discharged from a component is returned to the system to reduce flow input requirements. Often used to speed up the action of a cylinder by directing discharged oil from the rod end to the piston end.


 - a hydraulic function such as a cylinder which is separate from its supply source. Usually connected to the source by flexible hoses.


 - a container for keeping a supply of working fluid in a hydraulic system.


 - a reduced cross-sectional area in a line or passage which normally causes a pres- sure drop. (examples: pinched lines or clogged passages, or an orifice designed into a system).

Reseal pressure

 - the pressure at which there is no indication of flow.



 - an electro - magnetic device which positions a hydraulic valve. 

Solenoid Valve

 - is hydro-electric valve that reacts to a low power electric signal; the electric command is translated to an hydraulic command to the main valve.


- a lack of oil in vital areas of a system. Often caused by plugged filters, etc.


 - a coarse filter.


 - (1) the length of travel of a piston in a cylinder. (2) sometimes used to denote the changing of the displacement of a variable delivery pump.


- a momentary rise of pressure in a hydraulic circuit.


 - used as a short-hand representation on drawings to represent hydraulic system components.


 - one or more series of component parts connected to each other. Often made up of two or more "circuits".


Terminal expansion

 - expansion of the fluid volume due to heat.


 - the turning effort of a hydraulic motor or rotary cylinder. Usually given in inch-pounds (in- ibs) or foot-pounds (ft-lbs).


 - a line whose size is its outside diameter.

Tanks & reservoirs

 - recipients used for used for storage, pressure control (both pressure reduction and discharge point for relief valves), energy saving by reducing pumping hours and supplying high demand by pumping at low demand\low cost hours.



 - a device which controls either 1) pressure of fluid, 2) direction of fluid flow, or 3) rate of flow.

* Bypass flow regulator valve - a valve which regulates the flow to a circuit at a constant volume, dumping excess oil.

* Check valve - a valve which permits flow in only one direction.

* Closed center valve - a valve in which inlet and outlet ports are closed in the neutral position, stopping flow from pump.

* Directional control valve - a valve which directs oil through selected passages. (Usually a spool or rotary valve design.)

* Electro-hydraulic valve - a valve that is opened and closed by a solenoid.

* Flow control valve - a valve which controls the rate of flow. (Sometimes called a "volume control valve.")

* Flow divider valve - a valve which divides the flow from one source into two or more branches. (includes "priority" and "proportional" types.)

* Needle valve - a valve with an adjustable tapered point which regulates the rate of flow.

* Open center valve - a valve in which the inlet and outlet ports are open in the neutral position, al- lowing a continuous flow of oil from pump.

* Pilot valve - a valve used to operate another valve or control.

* Pilot operated valve - a valve which is actuated by a pilot valve.

* Poppet valve - a valve design in which the seating element pops open to obtain free flow in one direction and immediately reseats when flow reverses.

* Pressure control valve - a valve whose primary function is to control pressure. (Includes relief valves, pressure reducing or sequencing valves, and unloading valves.)

* Pressure reducing valve - a pressure control valve which limits outlet pressure.

* Pressure sequence valve - a pressure control valve which directs flow in a pre-set sequence.

* Priority flow divider valve - a valve which directs oil to one circuit at a fixed rate and dumps excess flow into another circuit.

* Proportional flow divider valve - a valve which directs oil to all its circuits at all times.

* Relief valve - a valve which limits the pressure in a system, usually by releasing excess oil.

* Rotary directional valve - a valve designed in a cylindrical shape. When the valve is turned, it opens and closes drilled passages to direct oil.

* Selector valve - a valve which selects one of two or more circuits in which to direct oil, usually operated manually.

* Shuttle valve - a connecting valve which selects one of two or more circuits because of flow or pressure changes in these circuits.

* Spool directional valve - a valve designed as a spool which slides in a bore, opening and closing passages.

* Thermal relief valve - a valve which limits the pressure in a system caused by heat expansion of oil.

* Two-, three-, four-, or six-way valve - a valve having 2, 3, 4, or 6 ports for direction of oil flow.

* Unloading valve - a valve which allows a pump to operate at minimum load by dumping the pump's excess oil at a low pressure.

* Volume control valve - a valve which controls the rate of flow. Includes flow control valves, flow di- vider valves, and bypass flow regulators.



 - single phase fluid

Water hammer (pressure surge)

 - change of flow velocity, that converts kinetic energy to pressure wave / any operation in a water system, that causes change (increase or decrease) of the flow velocity generates water hammer. The sudden increase in pressure in pipe-lines due to reduction in velocity in a very short period of time is known as Water Hammer.

Wh theory

- basic equation, that defines the relation between velocity change to pressure change: ∂h= ∂v * a` / g   (Joukowsky equation) ; a`= sonic speed of pressure wave in a specific pipeline, determined by: material of the pipe (flexibility), ratio of diameter to wall thickness, restraint of the pipe, dissolved air. 

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