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device for controlling the flow of fluids (liquids and gases). Valves vary in construction and size depending upon their function. Some are classified according to their method of operation or design, e.g., butterfly, gate, globe, lift, needle, piston, and slide valves. Valves are also named for the functions they perform, e.g., check valve (which permits flow in one direction only) and cutoff, bypass, exhaust, intake, safety (see safety valvesafety valve,
device attached to a boiler or other vessel for automatically relieving the pressure of steam before it becomes great enough to cause bursting. The common spring-loaded type is held closed by a spring designed to open the valve when the internal pressure reaches a
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), and throttle valves. Valves are operated automatically, by hand, or by special mechanism. Valves are employed in the carburetor, diesel engine, internal-combustion engine, pump, and steam engine. In Great Britain an electron tubeelectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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 may be referred to as a valve. In anatomy and physiology the term valve includes the flaps of tissue that help to control the direction of the flow of blood in the heart.



a part or device used to control the flow of a gas or fluid in machines and piping by altering the cross-sectional dimensions of the passage.

In internal combustion engines, pumps, compressors, and blowers, a valve is a part of the distribution mechanism or of the gas or fluid flow-control mechanism. As a variety of pipe fitting, a valve is used to control the flow of gas, vapor, or liquid.

Valves are made up of a housing, which is incorporated into the piping, and a gate, which moves within the housing and alters the dimensions of the bore (and, thereby, the transmissive capacity). Valves are used to develop a pressure differential (throttle valves), to prevent the reverse flow of a fluid (check valves), to release a certain amount of gas, vapor, or liquid when the pressure exceeds a set value (safety valves), to control pressure or flow rate (regulating valves), and to reduce pressure and keep it at a constant level (reducing valves). Valves are also used as stopper fittings to seal off piping, in engineering equipment, and in heat-and-power engineering facilities.

Depending on the design of the housing, valves are classed as pass, angled, three-way, and multipath. In terms of the nature of attachment to the piping, they may be flanged or coupled. The gate may be moved manually, electrically, pneumatically, or hydraulically.



A flat of tissue, as in the veins or between the chambers in the heart, which permits movement of fluid in one direction only.
A segment of a dehiscing capsule or legume.
The lidlike portion of certain anthers.
(invertebrate zoology)
One of the distinct, articulated pieces composing the shell of certain animals, such as barnacles and brachiopods.
One of two shells encasing the body of a bivalve mollusk or a diatom.
(mechanical engineering)
A device used to regulate the flow of fluids in piping systems and machinery.


A flow-control device. Valves are used to regulate the flow of fluids in piping systems and machinery. In machinery the flow phenomenon is frequently of a pulsating or intermittent character and the valve, with its associated gear, contributes a timing feature.

The valves commonly used in piping systems are gate valves (Fig. 1), usually operated closed or wide open and seldom used for throttling; globe valves, frequently fitted with a renewable disk and adaptable to throttling operations; check valves, for automatically limiting flow in a piping system to a single direction; and plug cocks, for operation in the open or closed position by turning the plug through 90° and with a shearing action to clear foreign matter from the seat. Safety and relief valves are automatic protective devices for the relief of excess pressure. See Safety valve

For hydraulic turbines and hydroelectric systems, valves and gates control water flow for (1) regulation of power output at sustained efficiency and with minimum wastage of water, and (2) safety under the inertial flow conditions of large masses of water.

To control the kinematics of the cycle, steam-engine valves range from simple D-slide and piston valves to multiported types. Many types of reversing gear have been perfected which use the same slide valve or piston valve for both forward and backward rotation of an engine, as in railroad and marine service. See Steam engine

Poppet valves are used almost exclusively in internal combustion reciprocating engines because of the demands for tightness with high operating pressures and temperatures (Fig. 2). Two-cycle engines utilize ports, alternately covered and uncovered by the main piston, for inlet or exhaust. See Cam mechanism, Internal combustion engine, Valve train

In compressors, valves are usually automatic, operating by pressure difference on the two sides of a movable, springloaded member and without any mechanical linkage to the moving parts of the compressor mechanism. Like those for compressors, pump valves are usually of the automatic type operating by pressure difference.


A device which regulates or closes off the flow of a fluid.


1. any device that shuts off, starts, regulates, or controls the flow of a fluid
2. Anatomy a flaplike structure in a hollow organ, such as the heart, that controls the one-way passage of fluid through that organ
3. Electronics an evacuated electron tube containing a cathode, anode, and, usually, one or more additional control electrodes. When a positive potential is applied to the anode, electrons emitted from the cathode are attracted to the anode, constituting a flow of current which can be controlled by a voltage applied to the grid to produce amplification, oscillation, etc.
4. Zoology any of the separable pieces that make up the shell of a mollusc
5. Music a device on some brass instruments by which the effective length of the tube may be varied to enable a chromatic scale to be produced
6. Botany
a. any of the several parts that make up a dry dehiscent fruit, esp a capsule
b. either of the two halves of a diatom cell wall
7. Archaic a leaf of a double door or of a folding door


UK term for a vacuum tube.
References in periodicals archive ?
Zanettini and colleagues noted the presence of increased mitral tenting, a geometric measure of the position of the leaflets that can be an early indicator of valvular dysfunction, in patients receiving ergot-derived dopamine agonists such as pergolide and cabergoline.
Steffee et al (2) reported 3 cases of fen-phen valvulopathy (2 MVs, 1 AV) with extensive histologic evaluation and comparison to both postinflammatory valvular disease and a normal MV obtained at autopsy.
It is the effects of serotonin that are believed to cause severe regurgitant cardiac valvular disease and pulmonary hypertension.
Valvular lesions were recorded: aortic stenosis was considered important if the aortic valve gradient was [is greater than] 50 mm mercury; mitral regurgitation was considered important when moderate or severe mitral regurgitation was associated with left atrial dilution (left atrial width [is greater than] 4 cm); mitral stenosis was considered important if the mitral valve pressure half-time was [is greater than] 100 ms (equivalent to a mitral valve area of [is less than] 2.
Another young woman with previously normal heart function was found to have newly documented valvular problems after taking fen-phen.
However, due to the rare nature of these valvular diseases in the population studied, the association between Fen-Phen treatment and valvular disease is not likely due to coincidence.
Patients with acquired, primarily left-sided, valvular heart disease may be at increased risk for development of bacterial endocarditis following certain invasive procedures.
The FDA action does not affect phentermine, which has not been associated with valvular heart disease and remains on the market.
Food and Drug Administration also distributed a letter to physicians across the country warning them of the association of fen-phen with valvular heart disease and asking doctors to alert the FDA of any cases of the disease.
A combination of 34 video clips, detailed animations (including three-dimensional anatomy of the heart), approximately 200 high-quality clinical photographs, and hundreds of full-color and black and-white drawings are used to demonstrate scanner characteristics and imaging specifics, normal cardiac anatomy, screening for coronary artery disease, assessment of aortic valvular disease, intracardiac tumors, and pulmonary thromboembolism.
He concentrates on the three diseases that account for the majority of heart problems: coronary artery disease, valvular disease, and hypertension.