UNDERSTANDING VALVESBy:Salman Tariq

What is a Valve?A valve is a mechanical device that controls the flow of fluidWhat functions do valves perform?Control fluid flowStop and start fluid flowControl the amount of fluid flow (throttling) Control the direction of fluid flowPrevent backflowProtect piping and equipment from over pressure

Valve Classification based on functionFlow ControllingValvesGate ValveGlobe ValveNeedle ValveBall ValvePlug ValveButterfly ValveDiaphragm Valve

BackflowPreventingValvesCheck ValvePressure ControllingValvesRelief ValveSafety Valve

Gate ValveA linear motion valve used to start or stop fluid flow

AdvantagesGood choice for ON-OFF serviceFull flow-low pressure dropBidirectional

Gate ValveA linear motion valve used to start or stop fluid flow.

DisadvantagesNot for throttling; use fully opened or fully closedMetal-to-metal seating means not best choice for frequent operation. Bubble-tight seating should not be expected with metal-to-metal design Are not quick opening or closing valvesRequire large space for installation, operation, and maintenanceDifficult to repairDifficult to automate

Globe ValveA linear motion valve used to stop, start, and regulate fluid flowAdvantagesGood shutoff capabilityModerate to good throttling capabilityGood for frequent operationShorter stroke (compared to a gate valve)Available in zed, wye, and angle patterns, each offering unique capabilitiesEasy to machine or resurface the seats

Globe ValveA linear motion valve used to stop, start, and regulate fluid flowDisadvantagesHigher pressure drop (compared to a gate valve)Requires greater force or a larger actuator to seat the valve (with pressure under the seat)More costly than alternate valves

Globe Valve ConfigurationsGlobe Valve Body DesignsZ-Body Design Y-Body Design Angle Valve Design

Needle ValveUsed to make relatively fine adjustments in the amount of fluid flowAdvantagesVery accurate throttling is possibleTypical ApplicationsUsed for instrument, gauge, and meter line serviceUsed in some components of automatic combustion control systems where very precise flow regulation is necessary

Ball ValveA rotational motion valve that uses a ball-shaped element to stop or start fluid flowAdvantagesProvides bubble-tight serviceQuick to open and close, not torque-dependent for seatingOffers high cycle lifeEspecially useful for low flow situationsEasier to automate than multi-turn valves

Ball ValveA rotational motion valve that uses a ball-shaped element to stop or start fluid flowDisadvantagesThey are not suitable for sustained throttling applicationsIn slurry or other applications, the suspended particles can settle and become trapped in body cavities causing wear, leakage, or valve failureTemperature range limited by seat material

Plug ValveA rotational motion valve used for the same full-flow service as gate valves, where quick shutoff is requiredAdvantagesSimple design with few partsQuick to open or closeCan be serviced in placeOffers minimal resistance to flowProvides reliable leak tight service

Plug ValveA rotational motion valve used for the same full-flow service as gate valves, where quick shutoff is requiredDisadvantagesRequires greater force to actuate, due to high frictionTypically, plug valves may cost more than ball valves

Plug Valve ConfigurationsMultiport Plug ValvesA single multiport valve may be installed in lieu of three or four gate valves or other types of shutoff valve

Butterfly ValveA rotary motion valve that is used to stop, regulate, and start fluid flowAdvantagesRequire considerably less space, compared to gate, globe, or other valvesLight in weightQuick 90 open/closeAvailable in large sizes, ranging from NPS 1-12 to over NPS 200Have low-pressure dropEasier to automate than multi-turn valves

Butterfly ValveA rotary motion valve that is used to stop, regulate, and start fluid flowDisadvantagesThrottling service is limited to low differential pressureDo not normally create a tight seal

Diaphragm ValveA linear motion valve that is used to start, regulate, and stop fluid flow

Diaphragm ValveAdvantagesOffer good chemical resistance due to variety of linings availableStem leakage is eliminatedProvides bubble-tight serviceDoes not have pockets to trap solids, slurries, and other impuritiesParticularly suitable for hazardous chemicals and radioactive fluidsDo not permit contamination of flow medium, thus they are used extensively in food processing, pharmaceutical, brewing

Diaphragm ValveDisadvantagesThe weir may prevent full drainage of pipingWorking temperatures and pressures are limited by the diaphragm material. Generally the pressures are limited to 200 psi (1380 kPa) and temperatures up to 400F (204C)The diaphragm may experience erosion when used extensively in severe throttling service containing impuritiesAvailable in limited sizes, usually NPS 12 to 12

Check ValveA valve designed to automatically prevent the reversal of flow in a pipeline systemAdvantagesSelf-actuated and require no external means to actuate the valve either to open or closeFast actingDisadvantagesDifficult to determine whether the valve is open or closedValve disc can stick in open position

Check Valve ConfigurationsCheck Valve DesignsSwing Check Lift Check Stop Check

Relief and Safety ValvesPrevent equipment damage by relieving accidental over-pressurization of fluid systems Relief Valve Safety Valve

Relief and Safety ValvesDifferences Relief ValveOpens only as necessary to relieve the over-pressure conditionTypically used for incompressible fluids such as water or oil

Safety ValveRapidly pops fully open as soon as the pressure setting is reached and will stay fully open until the pressure drops below a reset pressureCan often be distinguished by the presence of an external leverTypically used for compressible fluids such as steam or other gases

Valve ActuatorsA device that utilizes a source of power to operate a valveActuator ClassificationManual (Hand Wheel, Lever, Gears, Chains etc)Automatic (Pneumatic, Motorized, Hydraulic, Solenoid etc)Manual ActuatorsHand Wheel Lever Gears

Valve ActuatorsA mechanism that causes a valve to perform its function.Automatic ActuatorsPneumatic Motorized Solenoid

SummaryGate valves are generally used in systems where low flow resistance for a fully open valve is desired and there is no need to throttle the flow.Globe valves are used in systems where good throttling characteristics and low seat leakage are desired and a relatively high head loss in an open valve is acceptable.Ball valves allow quick, quarter turn on-off operation and have poor throttling characteristics.Plug valves are often used to direct flow between several different ports through use of a single valve.Diaphragm valves are used in systems where it is desirable for the entire operating mechanism to be completely isolated from the fluid.

SummaryButterfly valves provide significant advantages over other valve designs in weight, space, and cost for large valve applications.Check valves automatically open to allow flow in one direction and seat to prevent flow in the reverse direction.A stop check valve is a combination of a lift check valve and a globe valve and incorporates the characteristics of both.Safety/relief valves are used to provide automatic over- pressurization protection for a system.

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