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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).


- A mechanical or electrical device to sound an alarm on operation of the valve

Source: UL 260 Standard

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)


- A valve connected in the open position to the intermediate chamber of a dry pipe valve so as to drain water from the chamber and vent the chamber to atmosphere when  the valve is in the set position, and limits water flow from the chamber after the valve is tripped.

Source: UL 260 Standard



- 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".


- That portion of a valve mechanism on which air pressure or water pressure, or both, act and which opens to allow water to flow through the valve when operated.

Source: UL 260 Standard

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

- An automatic water-supply control valve that is intended to be operated by:
a) Mechanical, electrical, hydraulic, pneumatic, or a thermal fire detection system installed in
the same area as sprinklers;
b) Manual means; or
c) A combination of these methods.
A deluge valve is intended to admit water into:
a) A piping system having open sprinklers (deluge systems); or
b) A piping system with closed sprinklers (preaction systems).

Source: UL 260 Standard

Deluge System

- A sprinkler system employing open sprinklers attached to a piping system connected to a water supply through an automatic water control valve that is opened by the operation of a detection system installed in the same areas as the sprinklers. When this valve opens, water flows into the piping system and discharges from all sprinklers attached thereto.

Deluge sprinkler systems have open sprinklers or open spray nozzles and are used where it is desirable to discharge water through all of the system's sprinklers or nozzles simultaneously. Prior to discharge there is no water in the sprinkler piping. The water supply is held back by an automatic water control valve which is operated manually or automatically by the actuation of a fire detection system. The fire detection system is required to be one of the following types: wet pilot sprinkler line, dry pilot sprinkler line, hydraulic rate-of-rise, pneumatic rate-of-rise or electric.

An Electrical detection system may consist of an electrical Releasing Fire Alarm Control Panel, using a heat detector, smoke detector or radiant energy detector as an initiating device or signal line circuit device.

Source: FM standard 1011-1012-1013


- The ratio of system water supply pressure to system air pressure, expressed as gauge pressures measured at the trip point.

Source: UL 260 Standard


- A dry pipe valve having an air clapper of large diameter relative to the diameter of the water clapper with the two separated by an intermediate chamber maintained at atmospheric pressure.

Source: UL 260 Standard


 - 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.


- An automatic sprinkler water-supply control valve constructed so that air pressure in a system of piping will retain water pressure and flow until the air pressure in the system is released by actuation of an automatic sprinkler, manual release, or other similar devices. (revised July 6, 2007)

Source: UL 260 Standard



- 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.


- That portion of a differential or low-differential dry pipe valve which separates the air and/or water clapper seating surfaces and that is at atmospheric pressure when the valve is in the set position.

Source: UL 260 Standard


Junction rule

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



- A latch mechanism provided in a differential dry pipe valve designed to prevent the clapper from returning to its set position after operation.

Source: UL 260 Standard


- A valve in which a mechanism acts to produce a force which is multiplied through a series of levers, links, or latches to maintain the water clapper in the closed position. Such a valve is sometimes referred to as a mechanical type.

Source: UL 260 Standard


- (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.


- The system air pressure and service water pressure at which water begins to emit from the intermediate chamber drain during the tripping sequence of a valve.

Source: UL 260 Standard


- 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 dry pipe valve having a single clapper member whose effective area exposed to system air pressure is only slightly larger than the effective area exposed to supply water pressure. Such a valve has a differential of 1.5:1 or lower.

Source: UL 260 Standard



 - 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.


- An automatic sprinkler connected to an air or water line, which in turn is connected to a deluge, pre-action, or other water-supply control valve; when actuated, the sprinkler will cause the valve to operate.

Source: UL 260 Standard


 - 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.

Preaction System, Single-Interlock

- A sprinkler system employing automatic sprinklers attached to a piping system containing supervisory pressure with a supplemental detection system installed in the same areas as the sprinklers. Actuation of the detection system signals a fire alarm control system which opens the automatic water control valve which permits water to flow into the sprinkler piping system and to be discharged from any sprinklers that are open.

Single-Interlock Preaction systems are used where it is important to prevent the accidental discharge of water.  These systems may also be used where an alarm is desired in advance of sprinkler operation or where it is desired to minimize the water delivery delay inherent in a standard dry-pipe system.

Source: FM standard 1011-1012-1013

Preaction System, Double-Interlock (Also referred to as a Refrigerated Area System)

- A sprinkler system employing automatic sprinklers in the system piping which contains supervisory pressure. Installed in the area of the system sprinklers are detectors/releasing devices, which are either electric heat (or smoke) detectors or pneumatic release detectors such as a fixed temperature sprinkler head in a dry pilot line. This arrangement requires two independent detector/releasing activations in order to trip the automatic water control valve and flow water into the sprinkler piping. This system provides an additional safeguard against accidental water discharge than that of Single-Interlock Preaction Systems.

The system is also referred to as a Refrigerated Area System because they are predominately installed in refrigerated areas where the accidental charging of the distribution lines with water could have detrimental effects on the installation.

In contrast, the Single-Interlock Preaction System would fill the system piping with water upon one detector activation only. This would result in frozen sprinkler heads and piping without an activated sprinkler. The sprinkler system would have to be dismantled, resulting in a prolonged time without fire protection.

However, these double-interlock systems are also employed in sensitive non-freezer applications where the accidental water discharge would cause damage and production downtime of expensive equipment, such as found in semi-conductor manufacturing.

Source: FM standard 1011-1012-1013


- A port provided in the valve body to allow water to enter above the clapper when the valve is in the set position.

Source: UL 260 Standard


 - 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.



- The position of a valve clapper and other operating parts with system pressure and system air pressure applied when the valve is ready for operation. (Revised July 6, 2007)

Source: UL 260 Standard


 - 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".


- The static water pressure at the main outlet of a valve when the valve  is in the set condition. (Added July 6, 2007)

Source: UL 260 Standard


- The static air pressure in the system piping when the valve is in the set condition. (Added July 6, 2007)

Source: UL 260 Standard


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).


- That system air pressure and service water pressure at which the valve clapper begins to move away from the set position to allow water to flow into the system.

Source: UL 260 Standard


- TA device for operating a dry pipe or deluge valve, initiated by a fire detection system or by manual means.

Source: UL 260 Standard


 - 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.


- Any supplementary means which prevents normal operation.

Source: UL 260 Standard



 - 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.


- A condition occurring in a dry, deluge, or preaction fire suppression system where water accumulates downstream of the clapper of a system control valve, preventing the control valve from operating.

Source: UL 260 Standard


- A mechanical alarm device (operated by energy or water flow) attached to a dry pipe, pre-action, or deluge valve and supplied with water from the intermediate chamber or discharge side of the valve, to sound an alarm on operation of the valve.

Source: UL 260 Standard


- A hydraulic detection and actuation piping system equipped with heat responsive devices, commonly automatic sprinklers which when subjected to heat from a fire, operate to release water pressure from the piping system causing the automatic operation of a deluge valve.

Source: UL 260 Standard

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.