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Valves and Sealing Glossary


- A -

Abrasion :  The wearing away of a materiel surface by friction. Particles become detached by a combined cutting, shearing and tearing action. Furnace carbon blacks are the best ingredients found for increasing the resistance of rubber compounds to abrasion.

Accumulation : The pressure increase above the Maximum Allowable Working Pressure of a vessel during discharge through a pressure relief valves or rupture disc. It is either expressed in pressure units or percentage.

Acme thread : a trapezoidal shaped (flat-topped) thread that is used for power transmission rather than for fastening.

Actual Discharge Area : The measured minimum net area that determines the flow through a valve.

Actuator : Device used to open/close or control a valve. The three main types include electrical, hydraulic and pneumatic actuators. Movement may be quarter-turn, multi-turn or linear. Often used for remote control, in hazardous areas or for sequencing of valve operations.

Aging (of rubber) : A progressive change in the chemical and physical properties or rubber, especially vulcanized rubber, usually marked by deterioration. Aging may be retarded by the use or antioxidants.

Air valve : Valve that is used to control the flow of air. Flows are normally small, so small valves or solenoid valves are used.

Alloy steel : A steel consisting primarily of iron with some percentage of one or more other elements such as chromium, nickel, titanium, manganese, or vanadium deliberately added to enhance certain properties.

Ambient temperature : The prevailing temperature of the environment immediately surrounding an object - generally considered to be -25°C to +35°C (-20° F to +100° F).

Angle valve : A variation of the globe valve, in which the end connections are at (right) angles to each other, rather than being inline.

Annealing : A process involving high-temperature heating and cooling of the as-rolled cold rolled steel substrate to make it softer and more formable.

ANSI – American National Standards Institute : The principal standards writing organization in the U.S. who sets standards for a wide variety of items, including the design, fabrication, and testing of pressure piping, systems, and components for various pipeline services.

Anti-extrusion ring : A ring which is installed on the low pressure side of an O-ring or similar sealing element to prevent extrusion of the sealing material into the gap between metal pieces.

Antioxidant : Usually organic and nitrogenous. A substance which inhibits, or retards, oxidation and certain other kinds of aging. Some antioxidants cause staining or discoloration of the rubber compound on exposure to light and are used only in black or dark-colored goods. Others (phenolic), described as non-staining, are used in white or light-colored goods.

API – American Petroleum Institute : The principal U.S. oil company trade association.  It has some standards and specification writing functions, such as well head components and pipeline valves.

ASME – American Society of Mechanical Engineers : This professional society publishes many technical books, papers, codes and standards.  Of principal interest is the ASME Boiler and Pressure Vessel Code, which is referenced for many aspects of valve making, although not for the valves themselves.

Asperities : Minute imperfections on the seal face of the mating ring which are the result of normal surface finishing processes.

ASTM – American Society for Testing & Materials : A professional society governing detailed physical and chemical analysis of all basic metals and alloys used in construction.  The valves of most manufacturers have components whose materials correspond to ASTM standards.

Austenitic (stainless steel) : The common stainless steel, where the primary micro structure is austenite and the composition primarily iron but also includes both chromium and nickel. The steels are designated as 300 series such as 304, 316, CF8M, etc. and are generally non-magnetic.

Automatic U-joints : Also called u-cups, u-cups or u cups. A "U" shaped sealing ring made from a strong pliable plastic or rubber.

Axial movement : Movement along the axis or parallel to the center line of a shaft or stem.

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- B -

Back pressure : The pressure exerted on the downstream side of a valve seat as a result of the pressure in the discharge system e.g. flare system, blowdown system etc. It is the sum of the superimposed and built up back pressure.

Backrinding : Defect in which the rubber adjacent to the mold parting line shrinks below the level of the molded product, often leaving the parting line ragged and torn.
Backringing : Distortion at the mold parting line usually in the form of wrinkles, folds, tears or indention's. In severe cases may cause over-all dimensional changes.

Back seat : A shoulder on the stem of a gate or globe valve which seals against a mating surface inside the bonnet to prevent leakage of media through the bonnet stuffing box when the valve is fully opened and/or to permit replacement, under pressure, of stem seals or packing

Baffle Rings : A ring used to slow the flow of fluids along a shaft.

Balanced Pressure Relief Valve : A spring loaded pressure relief valve that incorporates a means for minimising or “balancing” the effect of back pressure on the valve's performance characteristics.

Balanced seal : A mechanical seal arrangement whereby the hydraulic pressure in the seal chamber has its effect on the seal faces substantially reduced through seal design.

Ball valve : A quarter-turn valve with a ball-type closing element which is rotated through 90º to open and close the valve, held between two seats. Characteristics include quick opening and tight shut-off. Ball valves are widely used as on/off valves in the chemical process and other industries. Special designs (with V notches or fingers) are available for throttling applications. Larger valves with heavier balls (eg, on pipelines) may use trunnion-mounted design to help support the ball and prevent damage to soft internals. Designs are typically, one, two or three piece.

Ball check (valve) : A non-return valve with a small ball that seals against a seat preventing flow in one direction and allowing flow in the other direction.

Belleville spring : A spring resembling a dished washer or other form, used in some ball valves to push the seats against the ball or to keep the stem packing under sufficient pressure, to compensate for variations because of temperature or wear.

Bellows : Flexible sealing device fixed on the valve disc on one side and around the stem at the other side which prevents line media leaking between the stem and the body. Often used for dangerous fluids or cryogenic applications.

Bellows pitch : the distance between convolutions.

Bevel gear box : Device facilitating operation of a gate or globe valve by means of a set of bevel gears having the axis of the pinion gear at right angles to that of the larger ring gear. The reduction ratio of this gear set determines the multiplication of torque eventually achieved.

Bill of Material : Total list of all components/materials required to manufacture the product.

Binder : is a substance (usually organic) that is used to bond the different components of a gasket material into a matrix.

Blowdown : The difference between the set pressure and the closing pressure of a pressure relief valve, expressed either as a percentage of the set pressure, or in pressure units.

Blowdown valve : Valve used ocasionally to blow off the solids & deposits collected at the bottom of a boiler vessel. When the valve is opened, it uses the pressure in the boiler to push out these solids. Often a spring return feature is added to have a quick and automatic shut-off after blowing down.

Body : The principle pressure containing part of a valve in which the closure element and seats are located. It can be split in different parts and bolted, screwed or welded together.

Bolted bonnet : A bonnet which is connected to a valve body with bolts or studs and nuts and so can be taken apart and repaired in the field.

Bonding Agents : Substances or mixtures of substances that are used for attaching rubber to metal, fabrics or other substrates. Generally the rubber compound is vulcanized by heat in the process. Cyclized rubber or rubber isomers, halogenated rubber, rubber hydrochloride, reaction product of natural rubber and acrylonitrile, polymers containing diisocyanates, are all used.

Bonnet : The top part of a valve, attached to the body, which contains the packing gland, guides the stem, and adapts to extensions or operators.

Bore (or port) : The inside diameter of the smallest opening through a valve, e. g., inside diameter of a seat ring, diameter of hole through ball in a ball valve.

Brittleness : Tendency of material to crack/break when subjected to deformation.

Bubble-tight shut-off : A phrase used in describing the sealing ability of a valve. During air pressure testing of a new valve in the closed position, leakage past the seats is collected and bubbled through water. To qualify as “bubble tight,” no bubbles should be observed in a prescribed time span.

Buffer fluid : A fluid which is introduced between two seal elements, quite often at a pressure which is higher than the pressure of the fluids on either side of the seal assembly. Also called a barrier fluid.

Built Up Back Pressure : The increase in pressure in the discharge header that develops as a result of flow after the relief device opens.

Bushing : A square or rectangular cross section device used to restrict flow between (A) two different fluids, (B) liquid to gas, (C) a hot fluid from a cold fluid, (D) a clean fluid from a dirty fluid.

Butterfly valve :A short face-to-face quarter-turn valve which has a circular disc in the center of the flow stream as its closing element, which rotates 90 degrees as the butterfly valve opens and closes. The standard design has the valve stem running through the disk, giving a symmetrical appearance. Later designs off-set the stem, so that the disk ‘cams’ into the valve seat. Advantages include less wear and tear on the disk and seats, and tighter shut-off capabilities. Many design types are available including inexpensive Teflon or resilient seats for use in water (treatment) plants, etc. More expensive metal seats can be used where high temperatures or aggressive chemicals are encountered. So-called “High Performance” butterfly valves offer zero leakage designs and have been applied in both the chemicals and hydrocarbon processing sectors.

Butt Joint : Joining two ends of material whereby the junction is perpendicular to the ID of an O-ring.

Butt weld end (BWE) : The end connection of a valve suitably prepared for butt welding to a connecting pipe.

Bypass valve : A small bore valve fitted in parallel to a larger main valve. Bypass valves are used to reduce the differential pressure across the main valve before this latter valve is opened (as otherwise this larger, more expensive valve, may suffer damage to internal components).

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- C -

Capacity : rate of flow through a valve under predefined conditions.

Carbon steel (CS) : Iron containing carbon in the form of carbides, about 0.1 to 0.3 percent carbon with no other alloying elements other than the sulfur, phosphorus, and other elements present in almost all steels.

Cartridge seal : A completely self-contained assembly including primary and secondary seals, gland, and sleeve, usually needing no installation measurement. The result is a convenient unit which can be inserted in a properly sized recess in a pump or similar device.

Cascaded seals : Seals which are staged or arranged in series and which have a regulated pressure between adjacent pairs.

Casting : A product or the act of producing a product made by pouring molten metal into a mold and allowing it to solidify, thus taking the shape of the mold. Used for mass produced valves and usually cheaper than forged valves.

Cast iron : The common term for cast gray iron or iron containing flake carbon in the range of 2,5% to 4%. Cast iron is brittle, exhibiting very little ductility before fracturing and is hardly weldable.

Cavitation : The rapid formation and collapse of vapor pockets in a flowing liquid in localized regions of very low pressure—often a cause of erosive damage to pumps, throttling type valves, and to the piping itself.  Can be the cause of excessive noise.

Charpy impact test : A destructive mechanical test conducted on a precisely machined coupon of steel to be tested. The coupon is clamped in a special machine and subjected to lateral hammer blow. The test provides a relative measure of the toughness of the steel or its resistance to shock or impact loads and is usually required for material used in low temperature applications.

Checking : The short, shallow cracks on the surface of a rubber product, resulting from damaging action by environmental conditions.

Check valve :A one-directional valve that is designed to allow the fluid to flow in a given direction but closes to prevent backflow. Types include swing check, tilting disc check and wafer check. Check valves (also called non-return valves) are usually self-acting.

Chevron packing : A type of packing used in packing boxes consisting of a nest of “V” cross-section rings.

Clapper : The hinged closure element of a swing check valve.

Class : a classification system to differentiate valves according to their pressure-rating.

Clogging : The condition in a mechanical seal where foreign materials and suspended solids interfere with the free movement of any movable device.

Closure element : The moving part of a valve, positioned in the flow stream, which controls the flow through the valve, e. g., wedge, plug, clapper, ball.

Closing Pressure : The value of decreasing inlet static pressure at which the valve disk re-establishes contact with the seat or at which lift becomes zero.

Cock (valve) : a small plug valve, often used with level indicators.

Coil spring : A type of spring which is formed from wire wrapped in a helix. The spring so formed encloses a volume in the shape of a right cylinder. It is used as a single or in multiples transmitting a uniform closing force on a seal face or in single acting pneumatic actuators to provide the fail-safe action when the air pressure is lost.

Cold Differential Test Pressure : The pressure at which the pressure relief valve is adjusted to open on the bench. It includes corrections for back pressure and/or temperature.

Cold Flow : Continued deformation under stress.
Compression set : The residual deformation of a material after removal of the compressive stress.

Conduit gate valves : are a type of gate valve that have a rectangular disk as the closing element. One half of the disk is solid, to close the valve, the other has a circular port, which can be used to open the valve.

Control valve : A valve which regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters, temperature gauges, etc. Control valves are normally fitted with actuators and positioners. Pneumatically-actuated globe valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball and butterfly valves may also be used.

Corrugated metallic gasket : is a metal gasket, usually incorporating a filler material in the well of the corrugations, in which the seal is formed between the peaks of the corrugations and the mating flanges.

Crazing : A surface effect on rubber articles characterized by many minute cracks.

Creep : The deformation, in either cured or uncured rubber under stress, which occurs with lapse of time after the immediate deformation.

Creep Relaxation : In a flange gasket, loss of stress accompanied by constantly decreasing compressed thickness. This type of relaxation is encountered in bolted flange joints.

Cross linked : The establishment of a chemical bond between the molecular chains of a given polymer, thereby enhancing physical properties.

Cryogenic valves : These are valves suited for use at temperatures below - 100 degrees Celsius, often and typically -196°C. (cryogenic service). Correct selection of used valve materials is critical.

Cv-value : Flow coefficient expressed as the number of gallons of water that would flow through an opening, such as a valve port, in 1 minute under a differential pressure of 1 psi. Equivalent in SI-units is called Kv-value.

CWP : Cold working pressure - the maximum allowable pressure under non-shock conditions at ambient temperature ( -20° F to +100° F).

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- D -

Dezincification : A form of pitting corrosion which attacks certain zinc bearing copper-based alloys, often called "yellow brasses", when in contact with sea water or fresh water that is high in oxygen and carbon dioxide. (ASTM B61 and B62 are "red brasses" and not susceptible to dezincification.)

Diaphragm valve : A bi-directional valve which is operated by applying an external force to a flexible element, or diaphragm (typically an elastomer). Diaphragm valves may be used for slurries (where other valve designs might clog) or in hygienic applications. The two main types are the weir type and the straight type.

Differential pressure : The difference in pressure across a valve in a pressurized line.  The difference in pressure between any two points in a pressurized system under flowing conditions.

Dilution rate : The rate at which a liquid flush migrates into the product fluid.

Diverter valve : A valve which can change the direction of the flow of a medium to two or more different directions. This will save the user having to connect several single valves and piping together to achieve the same thing

DN : an alpha-numeric designation to indicate the nominal size of pipeline components. (DN stands for Diamètre Nominal – which is french for nominal diameter)

Double acting : see pneumatic actuator

Double Acting Seals : Seal rings which seal in two directions, on the push and the pull stroke of a hydraulic or pneumatic cylinder.

Double block and bleed : A valve configuration in which positive shut-off is achieved at both the inlet and outlet sides. A small port is fitted to discharge fluid in the intermediate space (“bleed”). This port can help a user check if the valve is leaking. Fitting a gas detector to the port provides assurance of the integrity of the upstream seal. This configuration is often required to isolate high pressure sections of a system to facilitate safe maintenance, etc.

Drain plug : A fitting at the bottom of a valve, the removal of which permits draining and flushing the body cavity.

Dry face seal : One which effectively seals without the need for a liquid film between the faces.

Ductility : The ability to permit change of shape without fracture. In flat rolled steel, ductility is usually measured by hardness or mechanical properties in a tensile test.
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- E -

Eccentric : Not having the same center. Use for indicating the position of the disc/axis of a butterfly valve.

Elastic Modulus : The value of the load (in pounds per square inch of original cross-section) required to give an intermediate elongation, is usually called the modulus at that elongation. The expression used is "modulus at 300 percent elongation." Tensile-stress observations of this sort arc exceedingly useful in characterizing a particular compound, since by indicating the position and shape of the stress-curve. They show the relative toughness of the vulcanization.

Elastomer : A natural or synthetic elastic material, often used for o-ring seals. Typical materials are viton, buna-N, EPDM (ethylene propylene dimonomer), etc.

Electric actuators : Actuator which uses an electric motor to operate the valve stem.

Encapsulated O-rings : A rubber o-ring with a thin jacket of PTFE or Teflon surrounding the softer core material, which allows it to be used in chemical applications.

End connection : The type of connection supplied on the ends of a valve which allows it to be connected to piping - may be weld end, flanged end, threaded or socket weld.

Envelope gasket : type of gasket in which the actual gasket is enclosed in an outer cover (typically PTFE) to protect the gasket material from chemical attack by the sealed fluid.

Equal Percentage Flow Characteristic : An inherent flow characteristic of a control valve which produces equal percentage of changes in the existing flow for equal increments of rated travel.

ESDV – Emergency Shut Down Valve : A critical valve or a system of valves which, when activated, initiate a shut-down of the plant, process, or platform they are tied to.

Extended bonnet : Used when the media is at high or low temperatures, to avoid damage to the stem sealing elements.

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- F -

Face-to-face : The overall dimension from the inlet face of a valve to the outlet face of a valve (one end to another) allowing valves of the same size and pressure class to be mutually interchangeable, regardless of manufacturer.

Face seal : A device which prevents leakage of fluids along rotating shafts. Sealing is accomplished by a stationary primary seal ring bearing against the face of a mating ring mounted on a shaft. Axial spring force and fluid pressure maintains the contact between seal ring and mating ring. Also called mechanical seal.

Facing : The finish of the gasket contact surface of a flange.

Fail-closed : a condition in which the valve closes or remains closed when the actuator power (air pressure or electricity) would fall away.

Fail-open : a condition in which the valve opens or remains open when the actuator power (air pressure or electricity) would fall away.

Filler : Any compounding material, usually in powder form, added to rubber in a substantial volume to improve quality or lower cost. The most important reinforcing filler is carbon black. The most important inert filler, diluent or extender is whiting.

Fitting : Any component, other than valves, used with pipe as part of the pressure system and normally referring to items covered by a national standard.

Fire-safe : A valve design that is capable of passing a fire test with specified limits on leakage to the atmosphere and downstream after being closed subsequent to fire exposure. Must be referenced to a particular specification.  See API Spec. 6FA and BS Spec 6755.

Flash : Imperfections on the elastomeric portion of a seal, formed by extrusion of the elastomer at the parting lines in the mold cavity.

Flat Face (FF) : A flange surface in which the gasket sealing area is the entire surface from the ID to the outside edge. Usually used for class 125 cast iron valves.

Floating ball : A ball valve design in which the ball is not rigidly held on its rotational axis (see trunnion) and so is free to float between the seat rings. Line pressure is used to get tight sealing against the downstream seat.

Float valve : A valve which automatically opens or closes as the level of a liquid changes. The valve is operated mechanically by a float which rests on the top of the liquid.

Flow characteristic : is the relationship that exists between the flow rate through the valve and the percent of rated travel/opening that is varied from 0 to 100%. The inherent flow characteristic is the flow characteristic of a valve when the pressure drop across the valve is held constant.

Flow coefficient : The number of gallons of water per minute that will flow through a valve with a pressure drop of 1 psi.  Also referred to as the Cv of the valve.

Flush : A small amount of fluid which is introduced into the seal chamber in close proximity to the sealing faces and usually used for cooling or other protection of the seal faces.

Forging : A metalworking process that involves hammering or squeezing, with or without a die, at hot working temperatures to form a specific shape. Generally gives better material characteristics than casting.

Free length : The uncompressed axial length of a face seal assembly, spring or bellows.

Fretting : A combination of corrosion and wear which occurs when a secondary seal continually whipes the protective oxide coating from a shaft or sleeve.

Fugitive emissions : are emissions of gases or vapors from pressurized equipment due to leaks and various other unintended or irregular releases of gases, mostly from industrial activities. As well as the economic cost of lost commodities, fugitive emissions contribute to air pollution and climate change.
Emissions of volatile organic compounds such as benzene from oil refineries and chemical plants pose a long term health risk to workers and local communities. In situations where large amounts of flammable liquids and gases are contained under pressure, leaks also increase the risk of fire and explosion. Leaks from pressurized process equipment generally occur through valves, pipe connections, mechanical seals, or related equipment. Because of the large number of potential leak sources at large facilities and the difficulties in detecting and repairing some leaks, fugitive emissions can be a significant proportion of total emissions. Even though the quantities of leaked gases may be small, leaks of gases that have serious health or environmental impacts can cause a significant problem.
To minimize and control leaks at process facilities operators carry out regular leak detection and repair (LDAR) activities. Routine inspections of process equipment with gas detectors can be used to identify leaks and estimate the leak rate in order to decide on appropriate corrective action. Proper routine maintenance of equipment reduces the likelihood of leaks.

Full bore (full port) : Describes a valve (typically a ball valve) in which the bore (port) is nominally the same as the bore of the connecting pipe.

Full penetration weld : Describes the type of weld wherein the weld metal extends through the complete thickness of the parts being joined.

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- G -

Galling : The tearing of metal when two elements rub against each other.  Usually caused by lack of lubrication or extreme contact pressure.

Gasket : A component whose purpose is to seal a joint between two components, softer than the surfaces of the joint being sealed and usually squeezed by means of bolting to effect the seal.

Gate : The closure element of a gate valve (sometimes called wedge or disc).

Gate valve : A multi-turn valve which has a gate-like disk and two seats to close the valve. The gate moves linearly, perpendicular to the direction of flow. This type of valve is normally used in the fully opened or fully closed position; it is not suited to throttling applications. Wedge gate valves have a wedge-shaped gate which ‘wedges’ between floating seats to close the valve tightly.Gate valves provide robust sealing, and are used extensively in the petrochemicals industries.

Gearbox : Box enclosure with a gear set inside and a handwheel, used to ensure easier operation of larger valves like butterfly valves and ball valves.

Gland follower or gland flange : The component used to hold down or retain the gland in the stuffing box.

Gland or Gland bushing : That part of a valve which retains or compresses the stem packing in a stuffing box (where used) or retains a stem O-ring, lip seal, or stem O-ring bushing.  Sometimes manually adjustable.  See “Packing,” “Stuffing Box.”

Globe valve : A multi-turn valve with a closing element (typically a flat disc or conical plug) that moves perpendicularly to the valve body seat and generally seals in a plane parallel to the direction of flow. This type of valves is suited both to throttling and general flow control.

Graphite: Flexible carbon material used to make gaskets and packing. The gaskets may be flat graphite sheet or have metal inserts for added strength. The packing is a combination of lattice braided rings used as anti-extrusion or wiper rings and die-formed rings which are compressed to effect the seal.

Grease fitting : A device which permits injection of grease into a bearing surface. (see also plug valves)

Grommets : A rubber ring used to fit into a hole in sheet metal allow wires, shafts or rod to exit the housing without touching the metal.
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- H -

Hard facing : A surface preparation in which an alloy is deposited on a metal surface usually by weld overlay to increase resistance to abrasion and or corrosion.

Hand wheel : A wheel-shaped valve operating device intended to be grasped with one or both hands which allows turning the valve stem or operator shaft to which it is attached.

Hard face : A seal face or valve closing element surface made of or coated with a material of high hardness. Common hard face materials are : Tungsten Carbide, Stellite, Ceramic, Silicon Carbide.

Heat analysis : A chemical analysis conducted by a foundry immediately prior to pouring which measures the exact chemical composition of a particular batch of molten metal.

Heat treatment : Describes any process or procedure by which the internal structure of steel is altered by heating to produce desired physical and mechanical characteristics.

High Performance Butterfly valve : see butterfly valve.

Hot tap: A connection made to a pipeline while the line is under pressure or in service. A special procedure is required to make an opening in the pipe without leaking any of the line contents.

Hot tears : A defect occurring in castings caused where partially solidified or weak, newly solidified sections are subjected to a pull resulting from the contraction of thinner parts that have solidified earlier. A hot tear is an intergranular failure.

Huey test : A corrosion resistance test for stainless steels, most useful for predicting resistance to intergranular corrosion.

Hydraulic actuator : A device fitted to the valve stem than uses hydraulic energy to open and close the valve. Depending on the configuration, the hydraulic fluid may both open and close the valve, or just open the valve. In that latter case, a spring will typically be fitted inside the actuator to return it (and the valve) to the closed position.

Hydrostatic test : A pressure test in which a valve is completely filled with water and pressure tested to detect leaks - may be a shell test or a seat closure test.

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- I -

IBBM : Iron body, bronze mounted - common term for valves with cast iron body and bonnet and bronze trim (seating surfaces, stem, bushings).

IBC gasket : Inner Bolt Circle gasket – gasket that lies wholly within the ring of boltholes of a flange.

ID : The measurement of the inside diameter of a circular part.

Internal pressure relief : A self relieving feature in non-independent seating valves that automatically relieves excessive internal body pressure caused by sudden changes in line pressures.  By means of the piston effect principal the excessive body pressure will move the seat away from its seating surface and relieve it to the lower pressure side.

ISRS : Inside screw, rising stem - common term for any valve design in which the stem threads are exposed to the fluid below the packing and the stem rises up through the packing when the valve is opened.

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- J -

Jacketed valve : This valve is design incorporates a so-called jacket around the valve body. Steam is introduced into the jacket to keep the fluids being controlled at the required temperature.

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- K -

Kammprofile gasket : This type of gaskets have a solid corrugated (many equally spaced bumps) core with a flexible covering layer, typically graphite (or PTFE). This arrangement allows for very high compression and an extremely tight seal along the ridges of the gasket. Since generally the graphite will fail instead of the metal core, Kammprofile can be repaired when the seal is not needed such as during a shutdown of some kind. Kammprofile has a high initial cost for most applications but this can be justified both by long term savings and increased reliability

Kv-value : see Cv-value

Knife gate valves : have much thinner gates than gate valves with a knife-like edge, “cutting through the media transported in the pipe, making them suited to use with floating solids, eg, as in the pulp & paper industries.

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- L -

Labyrinth Seals : A non-contacting, rotary seal with a series of internal grooves to divert the flow and lubricating fluids in the direct of its source used on a shaft.

Laminar flow : sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers.It is the opposite of turbulent flow. In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough."

Laminate : Product made by bonding together two or more layers of like, or unlike materials.

Lantern ring : often used in a stem packing set with globe valve or piston valve, it is a metal spacer.

Lapping : A finishing operation using small free-floating abrasives in a fluid on a flat surface or diamond charged plates. Used to achieve extremely smooth and flat surface on a seating surface, for instance with safety valves.

Leak Test Pressure : The inlet static pressure at which a seat leak test is performed. API 527 (Seat Tightness of Pressure Relief Valves) specifies 90% of set pressure.

Lever : A handle type operating device for quarter-turn valves.

Lift : The actual travel of the disk away from the seat when a valve is opened or relieving.

Lift check : This non-return valve design incorporates a piston to damp the disk during operation.

Linear valve : See multi-turn

Line blind valve : A pipeline shut-off device, whereby a flat disk is forced & clamped between two flanges. Line blinds are less expensive than valves, but require much more time to operate.

Liner : is a covering (often PTFE or rubber) used mostly with butterfly valves, ball valves and diaphragm valves.

Limit switch : An electrical device providing a signal to a remote observation station indicating when the valve is in the fully open or fully closed position.  Usually a component of a valve operator.

Lip seal : A circular seal ring of “U” shaped cross section encompassing an elastomeric O-ring which provides resiliency and ensures a seal at the inner and out lips of the “U.”

Liquid penetrant inspection : A nondestructive method of detecting the presence of surface cracks and imperfections through use of a special red dye. Abbreviated as LPI or PT.

Locking device : Any valve attachment (padlock or other system) whose purpose is to prevent the operation of the valve by unauthorized persons.

Lug-type : type of body of butterfly valves that has “lugs” with threaded holes around its body for fastening to the pipe flange.

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- M -

Magnetic particle inspection : A non-destructive method of detecting the presence of surface cracks and imperfections through the use of fine iron particles in an electrical field. Abbreviated as MPI or MT.

Magnetic seal : A seal which uses magnetic material, instead of springs or a bellows, to provide the closing force to keep the seal faces together.

Manifold (Header) : A common pipe or chamber having several lateral outlets.

Material Test Reports Certificates : provided by the steel manufacturer indicating the chemical analysis and mechanical properties of a specific batch of steel traced by sequentially assigned heat numbers or codes. Most used in Europe is the the 3.1 Material Certificate (formerly 3.1B – DIN 50049)
(see also Mill certificate)

Maximum Operating Pressure : The maximum pressure expected during operation.

Maximum Allowable Working Pressure : The maximum pressure allowed in a vessel at its designated temperature. This is the basis for the set pressure of the relief devices that protect the vessel.

Mechanical seal : In a valve, a shut off that is accomplished by a mechanical means rather than with fluid or line pressure.  The wedging action of a gate against the seats or the seat springs pushing the seat against the ball or gate are examples of mechanical seals in a valve.

Metal fatigue : A condition usually caused by repeated bending and flexing of a metal part, and resulting in fracture of the metal.

Mill certificate : Certificates provided by the steel mill, indicating the chemical analysis and physical properties of a specific batch of steel.  “Mill Certs” are usually required only for pressure containing parts and the customer’s need for such “Certs” must be made known at the time an order is first placed, otherwise traceability of a vale part, back to the mill, is not possible

Mold : A hollow cavity, frequently in packed sand, for giving a desired shape to a material in a molten or plastic shape.

MSS – Manufacturers Standardization Society of the Valve and Fitting Industry : An American-based technical association of valve, fitting and actuator manufacturers that writes standards and practices for the valve and fittings industry.  Recommendations of this society are advisory only.

Multi-ported : Multi-ported valves include additional inlet/outlet ports, to allow fluids to be directed. The ball and plug valve types are ideally suited to multi-port designs.

Multi-turn : Category of valves (such as gate, globe, needle) which require multiple turns of the stem to move the valve from the fully open to the fully closed position. Also known as linear valves. See also quarter-turn valves.

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NACE – National Association of Corrosion Engineers : This technical association publishes papers, articles and standards on all aspects of corrosion, and has written the definitive standards for valve materials for sour gas service.

Needle valve : This multi-turn valve derives its name from the needle-shaped closing element and a seat having a small orifice. The design resembles that of a small globe valve. Typically available in smaller sizes, they are often used on secondary systems for on/off applications, sampling, flow metering, etc.

NPS : Nominal Pipe Size - dimensionless number used to indicate sizes of pressure pipe and valves - used interchangeably with valve size in inches.

NPT : National Pipe Taper - standard tapered thread for pressure pipe and components. Requirements defined in ASEM B1.20.1.

NRS Non-Rising Stem - A gate valve having its stem threaded into the gate. As the stem turns the gate moves but the stem does not rise. Stem threads are exposed to the line fluid.

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O.D. – Outside Diameter : The measurement of the outermost diameter of a circular part like a pipe.

Opening Pressure : The value of an increasing inlet static pressure at which there is a measurable lift of the disc or at which discharge of the fluid becomes continuous.

O-ring : An elastomeric or synthetic seal ring of circular cross section.

OS&Y : Outside Stem & Yoke - A valve design in which the stem threads are above the packing gland or outside the valve body and there is a yoke to support the top or outer end of the stem. The stem sealing elements are between the valve body and the stem threads. The fluid does not come in contact with the stem threads. 

Overcure : A state of excessive vulcanization resulting from overstepping the optimum cure, i.e., vulcanizing longer than necessary to attain full development of physical strength. Manifested by softness or brittleness, and impaired age resisting quality of the vulcanization.

Overpressure : The pressure increase over the set pressure of the relieving device, expressed in pressure units or as a percentage. It is the same as accumulation when the relieving device is set at the maximum allowable working pressure of the vessel.

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Packing gland : See “Gland”.

Packing : The deformable sealing material inserted into a valve stem stuffing box, which, when compressed by a gland, provides a tight seal about the stem.  (See “Gland”; “Stuffing Box)”

Parachute Packings : Also called V-Packing, Vee packing, Chevron Packing, Parachute packing or v-set packing. A complete vee packing set contains multiple "V" shaped sealing rings stacked and nested together with a male adapter on one end and a female adapter on the other end.

Parbacks : A back-up ring with a concave shape on one side, used as an anti-extrusion ring for an o-ring.

Pattern : A duplicate made of wood or metal of a part to be cast. Used to form the mold into which the molten metal is poured.

Penstock valve : A type of simple gate valve, used to contain fluids in open channels. Often found in waste water treatment plants.

Permeability : a measure of the ease with which a fluid can pass through a gasket material.

Pickling : Removing surface oxides from metals by a chemical reaction.

Pilot valve : Small valve requiring little power which is used to operate a larger valve. See also solenoid valves.

Pilot Operated Pressure Relief Valve : A pressure relief valve in which the main valve is combined with and controlled by an auxiliary pressure relief valve.

Pinhole : Numerous small gas holes at the surface or just below the surface of castings, generally occurring in the thicker parts of the casting as a reduction in the solubility of gases in the metal as the metal cools.

Pinion shaft : The external input shaft of certain gear operators which drive the internal reduction gearing.

Pinch valve : A valve in which a flexible hose is pinched between one or two moving external elements to stop the flow. This valve is often used in slurry and mining applications, as its operation is not affected by solid matter in the medium. It is also used with certain gases, as the absence of possible leak paths to the atmosphere ensures good emission control.

Piston T-Seals : A "T" shaped rubber seal, with back-up rings of a harder material on each side, used as a piston seal.

Pitting : Surface voids usually caused by mechanical erosion, chemical corrosion, or cavitation.

Plastics : A broad classification covering a variety of non-metallic, synthetic or organic materials capable of being molded or formed into desired shapes. Typical materials include nylons and tetrafluoroethylenes such as Teflon (PTFE).

Plug valve : This multi-turn valve derives its name from the rotating plug which forms the closing element. The plug may be cylindrical or truncated. In the open position, the fluid flows through a hole in the plug. Lubricated plug valves rely on a sealing compound injected between the plug and the valve body, whilst sleeved plug valves are fitted with a ‘soft’ insert between the plug and the body.

PMI : Positive Material Identification - a method for cross checking the identity of a piece of material, often using a portable spectrometer, usually with x-rays (TN 9266, nuclear analyzer) or a welding arc (Arc Met 900, optical spectrometer).

Pneumatic actuator : A device fitted to the valve stem than uses pneumatic energy (compressed air) to open/close or regulate the valve. Depending on the configuration, the compressed air may both open and close the valve (“double acting”), or just open the valve (“single acting”). In that latter case, a spring will typically be fitted inside the actuator to return the valve to the closed position.

Pneumatic test : A test in which a valve is tested with air - usually a seat closure test.

Poisson's Ratio : The ratio of lateral concentration per unit of diameter to longitudinal extension per unit of length in a bar of material longitudinally stressed. For a body which does not change its volume on deformation, it is 0.5. For metals, the ratio is usually considerably less than 0.5. In the case of vulcanized rubber, pure gum, having practically no volume change on extension, shows a ratio of approximately 0.5 for small deformations; compounded rubber may increase in volume on extension, consequently the ratio drops below 0.5. For rubber the ratio is constant only for small extensions.

Popping Pressure : The pressure at which the valve disk rapidly moves from a simmer position to a full open position.

Porosity : A defect found in castings or welds consisting of gas bubbles or voids in the solidified metal.

Positioner : Device that ensures the closing or throttling element of a valve moves to or maintains the correct position.

Position indicator : Any external device which visually indicates the open and closed position of valve.

Preload : clamping force which a bolt excerts on a joint when tightened.

Pressure class : A pressure rating expressed as a dimensionless number. The class rating charts give actual pounds per square inch maximum allowable pressure at a given temperature.

Pressure gradient : The pressure drop which occurs across seal faces ranging from the seal chamber pressure to atmospheric pressure. Often illustrated by means of a wedge.

Pressure reducing valve : A self-operating valve used to reduce any the pressure in a system, eg steam. Also known as a PRV. The valve opens if the internal pressure exceeds that holding the closing element onto the seat.Two main types are direct operated and pilot operated PRVs.

Pressure Relief Device : A device actuated by inlet static pressure and designed to open during an emergency or abnormal condition to prevent a rise of internal fluid pressure in excess of a specified value. A Pressure Relief Device can also be designed to prevent an excessive internal vacuum.      

Pressure-Temperature Ratings : The maximum allowable working pressures at specified temperatures. For steel valves, the ratings are defined by "classes" and found in ASME B16.34. For iron and bronze valves, the ratings are defined in the applicable MSS specifications.

Product Analysis : The chemical analysis of a material done on a finished component to show compliance with the material specifications. Usually has tolerances defined for each element to allow for differences in the completed product compared to the molten metal.

PSI : Pounds per square inch - the force per unit area exerted against a resisting body.

Pusher type seal : A mechanical seal in which a secondary seal is pushed along the shaft or sleeve to compensate for face wear.

Pusher Rings : A ring that fits against another sealing device to push it in order to activate it or energize it in the absence of pressure or in low pressure applications.

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Quarter-turn : Category of valves (such as ball, plug, butterfly) which require just a 90 degree turn of the stem to move from the fully open to the fully closed position. (See also multi-turn.) Note: some larger valves may, for simpler manual operation, be fitted with gearboxes, giving them the appearance of a multi-turn valve.

Quench : A neutral fluid which is introduced on the atmospheric side of the seal for the purpose of diluting fluid which may have leaked by the faces of the seal.

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Ra : Abbreviation for "arithmetic average roughness height" - the measure of the roughness of a surface expressed in microinches. The higher the number, the rougher the surface. Used to designated the desired surface finish for end flange raised faces.

Rack-and-pinion actuator : type of actuator (opposite to scotch-and-yoke type) that uses a gear pinion and rack to convert the linear movement of the actuator piston into rotary motion of the quarter-turn valve to be operated.

Radial Shaft Seals : Also called grease seals, rotary seals or shaft seals. Made of rubber to seal grease in housing with a rotating shaft.

Radiographic inspection : A nondestructive inspection method using x-rays to locate internal flaws in castings, fabricated parts and welds. Abbreviated as RT.

Raised faced (RF) : The raised area of a flange face which is the gasket sealing surface between mating flanges. Defined in ASME B16.5. Class 150 and 300 valves have 0.06" RF and Classes 600 and up have a 0.25" RF.

Recovery : increase (in %) of thickness over the compressed thickness of the gasket , once the compressive load has been removed.

Reduced bore/port : A valve seat bore/port opening that is smaller than the line size or the valve end connection size. (see also full bore)

Regulating valve : This valve type is used to regulate flows to provide a certain wanted output (pressure, flow rate,...).

Relief Valve : A spring loaded pressure relief valve actuated by the static pressure upstream of the valve. The valve normally opens in proportion to the pressure increase over the opening pressure. Used primarily with incompressible fluids.

Residual stress : stress remaining in a gasket after service for a given time.

Resilient seat : A valve seat containing a soft seal such as an o-ring or plastic to assure tight shut-off.

Rim pull : The force required at the edge of the handwheel to generate the required torque at the center of the handwheel.

Ring type joint (RTJ) : A flange connection using a specially shaped soft metal ring as a gasket. Generally used on high pressure valves. May be the body and bonnet connection and/or the end flange connection.

RMS : Root Mean Square - The measure of surface roughness, obtained as the square root of the sum of the squares of micro-inch deviation from true flat.

ROTT : Room temperature Operational Tightness Test, as defined by the PVRC

RS : Rising stem - A valve stem with threads arranged so that as the stem turns, the threads engage a stationary threaded area and lift the stem along with the closure element attached to it.

Rupture Disk Device : An emergency over-pressure relief device, employing a relatively thin metal diaphragm, designed to burst at a specified pressure.  Cannot be reset – must be replaced after rupture event.

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Safety Integrity Level (SIL) : is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function (SIF).
Four SILs are defined in the IEC standard 61508, with SIL4 being the most dependable and SIL1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. The requirements for a given SIL are not consistent among all of the functional safety standards.

Sampling valve : A valve which is fitted to a reactor or pipeline to allow small sample of a fluid to be withdrawn for further testing. In simple cases a standard gate or needle valve, for example, may be used. The disadvantage is, that inappropriate use may result in spillage. As an alternative, valves are available which ‘trap’ a small quantity of fluid in a chamber, and only this small amount of fluid is released when the valve is operated.

Safety Valve : A spring loaded pressure relief valve actuated by the static pressure upstream of the valve and charachterised by rapid opening or pop action. Normally used with compressible fluids.

Schedule : A system for indicating the wall thickness of pipe. The higher the schedule number, the thicker the wall for a certain pipe size.

Scorching : A term frequently used to denote premature vulcanization of a rubber compound, occurring on a mill or calendar, or in an extruder. Same as burning or "setting up".

Scotch & yoke operator (Used on quarter turn valves) : A quarter turn operator using a scotch yoke mechanism rather than gears.  The “Scotch Yoke” has a torque output at the beginning and ending of its stroke that is generally twice the magnitude of the torque output in the center of its stroke.

Seal weld : A weld that does not contribute anything to the mechanical integrity of an assembly, but is made purely to seal or prevent leakage from, for instance, a threaded joint.

Seat : The part of a valve against which the closure element effects shut-off.

Self-relieving : The process by which excessive internal body cavity pressure is automatically relieved either into the upstream or downstream line - generally found in ball valves.

Set Pressure : The inlet gauge pressure at which the pressure relief valve is set to open under service conditions.

Shaft : see stem

Shims : Flat, thin metal gasket-like parts used as spacers to position machinery or align equipment.

Shrinkage : Internal defect in castings that are internal voids, irregular in shape, caused by volume contraction during solidification. Can be caused by not maintaining a fluid channel to the riser during solidification.

Shut-off valve : A valve designed only for on/off service. Not a throttling valve. Sometimes referred to as a “block valve.”

SIL : see Safety Integrity Level

Single acting : see pneumatic actuators.

Single Acting Seals : Rings which are designed to seal only in one axial direction.

Solenoid valve : Solenoid valves, typically of the needle globe type, are operated by an electrical solenoid. They are often deployed as pilot valves, i.e., fitted to actuators which in turn control larger valves.

Socket weld end (SWE) : The end connection of a valve suitably prepared for socket welding to a connecting pipe.

Sour gas : Natural gas containing significant amounts of hydrogen sulfide (H2S). Requires special material treatments to avoid failures from sulfide corrosion cracking (SCC).

Specification : A document that defines the requirements that a finished product must conform to - may include chemical and mechanical properties, tolerances, marking, shipping, etc.

Spiral-wound gasket : type of gasket that utilizes a mix of metallic material and "filler material" generally the gasket has a chosen metal, normally a carbon rich or stainless steel, wound (hence the name) outwards in a circle (although other shapes are possible this is the primary) with the filler material, generally a flexible graphite, starting at the opposite side of the circle and winding in the same direction. This leads to a growing circle of alternating layers of filler and metal. These gaskets have proven to be reliable in most applications and although more expensive than solid material they do not require as high of bolt forces to be effective. This is possible mainly because the graphite makes the primary seal with the flange and the metal only acts to keep the gasket structurally sound.

Spring Loaded Pressure Relief Valve : A pressure relieve device designed to automatically re-close and prevent the further flow of liquid. Reclosure supplied by spring force.

Spring return : See Pneumatic actuator.

Spur gear : The simplest of gears - in a gear set, the pinion and ring gear are aligned on parallel shafts. Can be added to another gear operator to further increase the mechanical advantage afforded by the gear.

Square operating nut : A nut, usually 2" x 2", which is attached to a valve stem or the pinion shaft of a gear operator allowing use of wrenches to quickly operate the valve.

Stainless steel (SS) : Any of a number of types of steel alloy with chrome, nickel, titanium or other elements that does not oxidize in free air.

Stamped Capacity : The rated relieving capacity that appears on the device nameplate. It is based on the set pressure or burst pressure plus the allowable overpressure for compressible fluids and the differential pressure for incompressible fluids.

Stem : The rod or shaft transmitting motion from an operator (handwheel or gear operator) to the closure element of the valve.

Stem nut (yoke nut) : The threaded nut that surrounds a reciprocating valve stem and causes the stem to move when the nut is rotated.

Strain : change in dimensions or shape of a body (gasket) due to applied force or stress.

Stress : effect of load per unit area excerted on a body.

Stress corrosion cracking (SCC) : a common form of stress cracking in which an electrolyte encourages the growth of a crack in a bolt unders stress.

Stud : A bolt, threaded on both ends, often used in bolting together bodies and bonnets or bodies and closures.

Stuffing box : The annular chamber provided around a valve stem in a sealing system into which deformable packing is placed. Sometimes called packing chamber.

Subsea valve : A valve which is designed for use in sea water. For example, installed in a pipeline on the sea bed.

Superimposed Back Pressure : The static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system e.g. flare system, and therefore may be constant or variable.

Surge : To rise suddenly to an excessive or abnormal value; a transient sudden rise of pressure in a pipeline.  Pipeline surges can be positive or negative and are caused most frequently by the sudden closure of a block valve or emergency shutdown of a pump.  Surge pressures in excess of the rated capacity of a pipeline can cause ruptures of the piping system.

Swing check (valve) : A check valve in which the closure element is a hinged clapper which swings or rotates about a supporting shaft.

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Tack : The ability to adhere to itself; a sticky or adhesive quality. (gaskets)

Tank (bottom) valve : A valve arranged for fitting at the bottom of a tank or process vessel.

Tensile strength : The highest tensile stress that a material can withstand before failure or rupture occurs - the force being applied in a direction tending to elongate the material.

Tensile test : A destructive test performed on a specially machined specimen taken from material in its delivered condition to determine mechanical properties, such as tensile strength, yield strength, and percent elongation.

Throttling : The intentional restriction of flow by partially closing or opening a valve to regulate the flow.

Thrust : The net force applied to a part in a particular direction - e.g., on the end of a valve stem.

Top entry : The design of a particular valve or regulator where the unit can be serviced or repaired by leaving its body in the line, and its internals can be accessed by removing a top portion of the unit.

Torque : The rotational force imposed on or through a shaft to operate a valve, usually expressed in Newton-meters (Nm) or “pound-feet” and referred to the stem nut, handwheel or operator pinion shaft.

Trim : Commonly refers to the valve’s working parts and to their materials. Usually includes seat ring sealing surfaces, closure element sealing surfaces, stems, and back seats. Trim numbers which specify the materials are defined in API 600 and API 602.

Trim : The process Involving removal of mold flash.  (sealing)

Triple eccentric (Butterfly Valves) : A particular design of a butterfly valve where the stem is located behind the disc, below the centerline of the disc, and its cone axis is offset from the centerline of the disc.  This particular design is capable of a very tight shutoff at temperatures well above 100ºF.

Trunnion : The part of a ball valve which holds the ball on a fixed vertical axis and about which the ball turns. The seat are pressed against the ball using springs.

Turlulent flow : see laminar flow.

Turns to operate : The number of complete revolutions of a handwheel or the pinion shaft of a gear operator required to stroke a valve from fully open to fully closed or vice versa.

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Ultrasonic inspection : An inspection procedure using high frequency sound waves to detect wall thickness or flaws throughout the thickness of metal parts. Abbreviated as UT.

Unidirectional seal : A seal which prevents the passage of fluid from one direction only.

Union bonnet : A type of valve construction in which the bonnet is held on by a union nut with threads on the body.

U-Packings - A "U" shape cup sealing ring designed to seal in one direction along a shaft or rod in a hydraulic or pneumatic cylinder.
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Valve : A device used to shut-off or control the flow of fluid contained in a pipe line or vessel.

Vapour pressure : The pressure at a given temperature below which a liquid changes to gas. (see also cavitation)

Vena Contracta : is the point in a valve where the pressure and the cross-sectional area of the flow stream is at its minimum, whereas the fluid velocity is at its highest level. Normally occurs just downstream of the actual physical restriction in a valve.

V-Packing : Also known as Vee Packing, Vee Sets, Chevron Packing, and Parachute Packing. A multiple ring set of packings whose center rings or sealing rings are V-shaped to form sealing lips. The V-rings stack on top of each other and have a male and female adaptor on each end to make the set flat. This packing type is adjustable.

Vulcanisation : cross-linking reaction of elastomer with sulphur, creating a matrix of greater stability. Irreversable process.

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Wafer design : The construction of wafer design valves allows them to be ‘sandwiched’ between flanged sections of pipeline. The benefit is lower bolting requirements. Typically used with certain butterfly and check valves.

Wall thickness : The thickness of the wall of the pressure vessel or valve. For ASME steel valves, minimum thickness requirements are defined in ASME B16.34, API 600, and API 602.

Washer : is a thin plate (typically disk-shaped) with a hole in the middle that is normally used to distribute the load of a threaded fastener. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, and locking device.

Water hammer : The physical effect, often accompanied by a typical, loud banging, produced by pressure waves generated within the piping by rapid change of velocity in a liquid system.

Weepage : A minute amount of liquid leaked by a seal. It has rather arbitrary limits, but is commonly considered to be a leakage rate of less than one drop of liquid per minute.

Wellhead valve : Wellhead valves are used to isolate the flow of oil or gas at the takeoff from an oil or gas well. The design is usually a plug or gate valve.

Wiper Rings : Also, called a wiper ring - A ring which rides tight against a rod, with a sharp lip to scrape or wipe off excess oil, dirt or dust in a hydraulic or pneumatic cylinder.

WOG : Water-Oil-Gas - a rating designation generally used for small valves chiefly in low ratings. Indicates maximum working pressure at ambient + 32° F to +100º F. Also called Nonshock Rating.

Working pressure : The pressure at which a valve is designed to operate.

Worm gears : A gear set in which the input shaft is offset from and perpendicular to the output shaft, and driving gear is very small and perpendicular to the driven gear. Worm gear operators are used on ball valves.

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Yield Point :The load or stress at which a marked increase in the deformation of the sheet occurs without increasing the applied load. Yield point is one of the characteristics of low-carbon steels after they have been annealed. The yield point is usually calculated using a tensile-test specimen, and it is the load that is commensurate with the point beyond the elastic limit at which the specimen lengthens considerably without an additional increase in load.

Yield strength : The limiting stress beyond which a certain material will sustain permanent deformation.

Yoke : That part of a gate valve which acts as a bracket between the bonnet and the operator or actuator to support the top end of the stem and stem bearing.

Young's modulus : The ratio of normal stress to corresponding stress or compressive stresses below the proportional limit of the material.

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ZigBee : Zigbee is an established set of specifications for wireless personal area networking (WPAN), i.e., digital radio connections between computers and related devices. This kind of network eliminates use of physical data buses like USB and Ethernet cables. The devices could include valve sensors and controls located within a few meters of each other. ZigBee is one of the global standards of communication protocol formulated by the relevant task force under the IEEE 802.15 working group. The fourth in the series, WPAN Low Rate/ZigBee is the newest and provides specifications for devices that have low data rates, consume very low power and are thus characterized by long battery life.

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