Valves

VALVES

INTRODUCTIONValves are the most important components in any piping segment. Estimates real that of a substantial portion, approximately 8 10% of the total capital expenditure of the process or power industry is used for procurement of Valves.Valves are classified based on function, end connection and material of constructionFUNCTIONS OF VALVESIsolationRegulationPreventing Reversal of FlowSpecial Purpose

TYPES OF VALVESBased on Functions

IsolationRegulationNon - ReturnSpecial PurposeGate ValveGlobe ValveCheck ValveMulti port ValveBall ValveNeedle ValveFlush Bottom ValvePlug ValveButterfly ValveFloat ValvePiston ValveDiaphragm ValveFoot ValveDiaphragm ValvePiston ValveLine Blind ValveButterfly ValvePinch ValveKnife Gate Valve

Based on End ConnectionsEnd Connection means the arrangement of attachment of the valves to the equipment of piping system

Screwed EndsSocket Welded EndsFlanged EndsButt Weld EndsWafer Type Ends

Based on Material of ConstructionThe most commonly available materials are

Cast IronDuctile IronCarbon SteelStainless SteelAlloy SteelHDPE, Poly Propylene, Etc.

GATE VALVESThey function as block valves75% of all valves in process industries are gate valvesGate valves are not suitable to throttle flow, because it will pass maximum flow when it is only partially open.The end flanges could be integrally cast into the body. (Refer ANSI B16.5 for flanged connection)There are two types of Port Designs. Full Port Design Net area of the bore through the seat be nearly as equal to the pipe size.Regular Port Design Port Diameter is normally one size less than the size of the pipe.

GATE VALVESFlow InFlow Out

BALL VALVESThey function as block valves and flow regulating (special design) valves.Quarter turn positive shut- off valves.Suited for conditions where quick on / off / bubble tight shut off is required.Soft seats are usually used only for low temperatureMetal seated ball valves are used for high temperature services.Soft seats are not normally used for throttling because of erosion or distortion / displacement caused by fluid flow. Contd..

The Pressure-temperature ratings are established based on the seating material.Offers minimum resistance to flow.Two designs are available.

Regular Port Design Port Diameter will be smaller than the pipe inside diameter. Normally the port diameter is one size less than the pipe diameter.Full Port Design In this case the Port Diameter of the Valve will be same as that of the inside diameter of the Pipe.

BALL VALVES

CROSS SECTIONAL VIEW OF BALL VALVES

PLUG VALVESPlug Valves are quarter turn positive shutoff valve.Well suited for quick on/off and /or Bubble tight shutoff.Not Normally used for throttling, because soft seats are subject to erosion.Lesser resultant total pressure drop across the valve.In certain designs, a low friction Poly Tetra Fluoro Ethylene (PTFE) is impregnated on the surface structure of the valve plug.

Contd..

Lubricated Metal Seated Plug Valves- the lubrication of the seating surface is by means of lubricant, which is fed into the operating surface.Regular Port Design - Teflon sleeved plug valves can have flanged, butt welded, screwed or socket weld ends. The Pressure classification is same as that specified for gate valves.Contd..TYPES OF PLUG VALVES

Regular Pattern- they have plug ports generally rectangular and have area substantially equal to full bore of the pipe.Short Pattern- Face to face dimensions corresponding to Gate valves.Venturi Pattern- They have reduced port area. They produce a venturi effect to restore a large percentage of velocity head loss through the valve and produce a resultant total pressure drop of relatively low order.PLUG VALVE DESIGN PATTERN

Cross Section of a Plug Valve

BUTTERFLY VALVESButterfly Valves are quarter turn positive shutoff valve.Major Parts-- Body.- Disc- Shaft- Body Seat or Seal- Shaft Seal- Shaft Bearing- HandleIn certain design soft seats are provided instead of linear.Contd..

Seats are made of PTFE with certain reinforcement.Valves upto 12 NB are operated with LeverHigher Diameter valves are provided with gear unit and hand wheel.These valves are wear resistance.Elastomers have higher life expectance than conventional metallic seated valves when used in high density mineral slurry services.Generally used for line sizes > 8.BUTTERFLY VALVES

Cross Sectional Top View

DIAPHRAGM VALVEDiaphragm valves are used for low pressure corrosive services as shut off valve.These can also be used as control valves.Here the diaphragm moves up and down to operate the valve.The major parts of diaphragm valves are,1.Body2.Diaphragm3.Bonnet4.Stem5.Stem bushing6.Compressor7.Hand wheel8.Bonnet bolting.

GLOBE VALVEGlobe valves are mainly used to throttle the fluid flow.Smaller sizes are hand controlled.Application of larger size valves are limited to bypass control valve stations.Provide relatively tight shut off.Valve patterns involve change in flow directions.The discs of globe valves shall be flat-faced type,plug type,ball type,needle type,port type.Commonly used for pipe sizes up to 8High pressure drop across valve.

Cross Section of Globe ValveFlow InFlow Out

Check valves prevent flow reversal.Typical check valve applications are in pump and compressor discharge piping.Valves with discs provide lower resistance to flow of working fluid than those that contain balls or pistons.The later are often used where there is an angular change in flow stream.Ball and lift check valves are used for sizes 2 and smaller.Swing check and Plate check valves are used for higher pipe sizes.CHECK VALVE

SWING CHECK VALVESThe disc swings freely in the form of an arc.Valve is kept open by the flow and disc seating is accomplished by gravity and/or flow reversal.When used in vertical lines the flow should be upwards only.

SWING CHECK VALVESFlow InFlow Out

Basic types are piston lift check valve,Ball lift check valve etc., Used for sizes to 2.Reverse flow forces piston or ball against seat.Used to control vertical flow.LIFT CHECK VALVESWAFER CHECK VALVESWafer body is a value body without flanges. It is also called flangeless body. Such valves are held between any two flanges and held in place by compressive force between the flange.

There are certain valves, which perform duties other than Isolation, control and check. These are classified as Special Purpose Valves. A few of them and their uses are

Multi-Port Valve Many ports, after flow line.Flush Bottom Valves After Flow line.Float Valve Control Fluid Level in Reservoir.Foot Valves NRV with strainers.Line Blind Valves Replace Spectacle Blinds.Knife Gate Valves Slurry Services(MSS-SP)SPECIAL PURPOSE VALVES

General Procedure for selecting Valve Components

Identify the necessary design information. This includes Design Pressure and Temperature, Valve Function, Material, Etc.Identify potentially appropriate valve types( i.e Ball, Butterfly, etc.)and components based on application and function( i.e block, throttle or reverse flow prevention).Determine Valve application requirements( i.e. design or service limitations).Finalize valve selection. Check which factors need consideration if two or more valves are suitable.Provide a full technical description. This is done by specifying the valve type, material, flange rating, etc. VALVES SELECTION

Valve bodies are produced in almost as many different materials as there are available in pipe. Moreover, within a valve, a number of different materials are used.One point that should be taken care of is that while forgings are acceptable in place of castings, the reverse may not always be true.A careful study of the material would reveal that trim material is usually superior to the body / bonnet material. Trims mean the moving parts of a valve that come in contact with the fluid. VALVES MATERIALSContd..

Among the principal factors which influence the performance of trim materials are Tensile properties, Chemical Stability and Corrosion Resistance at the operating temperature.Hardness and ToughnessCo-efficient of Expansion which corresponds closely to that of the valve body, andA difference of sufficient magnitude in the properties of a seat and disk facings to prevent seizing of their surfaces when slides over the other.VALVES MATERIALS

NAME OF PARTCARBON STEELLTCS3 NiSSBody & Bonnet (upto 1 )A 105A350 LF2A350 LF2A182 Gr F304 / F318 / F318 LBody & Bonnet (2 & above)A216 Gr WCBA352 Gr LCBA352 Gr LC3A351 Gr CF8 / CF8MBonnet Gasket Class 150, 300-600, 900 & aboveCorrugated soft Iron SPW SS304 + CAF soft Iron Ring JointCorrugated 304 SPW SS304 + CAF Soft Iron Ring JointSPW SS304 + CAF 3 % Cr Ring Joint SPW SS304 + CAF 304/316/316L Ring JointGland PackingGraphited asbestos with Inconel wireAsbestos Impregnated PTFEAsbestos Impregnated PTFEGraphited asbestos with Inconel wireBonnet BoltA193 GrB7A320 GrL7A320 GrL7A193 GrB7Bonnet NutA194 Gr2HA194 Gr4A194 Gr4A194 Gr2HStem (no casting)13% Cr18 / 8 SS18 / 8 SSSS304 / 316 / 316LOther Trim Material13% Cr (Stellited for 600# & above)StellitedStellitedSS304 / 316 / 316L

Pressure Temperature RatingsIt is the maximum allowable sustained non shock pressure at the corresponding tabulated temperature. These are listed in ANSI B16.34 and ANSI B16.5. ClassThe valve is specified by the pressure rating of the body of the valves. The American Standard specifies the following classes 150#, 300 #, 600 #, 900 #, 1500 #, 2500 #, 800 # and 4500 #.TERMS USED FOR VALVE SPECIFICATIONContd..

TrimTrim is composed of Stem, Seat Surfaces, Back Seat Bushing and other small internal parts that normally contact the surface fluid. The table below indicates trim of common types of valves. API 650 specifies the materials that can be used with typical specification.

TRIM PARTS ON COMMON VALVESTERMS USED FOR VALVE SPECIFICATION

GATEGLOBESWING CHECKLIFT CHECKStem StemSeat RingDisc GuideSeat RingSeat RingDisc HolderSeat RingWedge RingDisc NutSide PlugBushingBushingHolder Pin, Disc Nut Pin

Pressure DropPressure drop is the loss of pressure through resistance across the valve while flowing, and is expressed in terms of equivalent length in pipe diameters.

Wire DrawingThis term is used to indicate the premature erosion of the valve seat caused by excessive velocity between seat and seat disc. An erosion pattern is left as if wire had been drawn between the seat and seat disc. Excessive velocity can occur when valve is not closed tightly, which leads to erosion. Disc of harder material are to be closed carefully to prevent wire drawingTERMS USED FOR VALVE SPECIFICATION

VALVES TYPES FOR SPECIFIC SERVICES

ServiceMainSecondaryGasesButterfly ValvesCheck ValvesDiaphragm ValvesLubricated Plug valvesScrew down stop valvesPressure Control ValvesPressure Reducing ValvesPressure Relief ValvesSafety ValvesRelief ValvesLiquids, clear up to sludges and sewageButterfly ValvesScrew down stop valvesGate ValvesLubricated Plug ValvesDiaphragm ValvesPinch Valves-----Slurries and liquids heavily contaminated with solidsButterfly ValvesPinch ValvesGate ValvesScrew down stop valves Lubricated Plug Valves-----SteamButterfly ValvesGate ValvesScrew down stop valvesTurbine ValvesCheck ValvesPressure Control ValvesPresuperheated ValvesSafety and Relief Valves

S Suitable ChoiceLS Limited SuitabilityM May be suitable in modified form

VALVE TYPE SUITABILITYS Suitable ChoiceLS Limited SuitabilityM May be suitable in modified form

DESIGN AND TESTING STANDARDS

TYPE OF VALVEDESIGN STANDARDSTESTING STANDARDSGATE < 2 2API 602API 600API 598API - 598GLOBE < 2 2BS 5352BS 1873BS 6755 (I)BS 6755 (I)CHECK < 2 2BS 5352BS 1868BS 6755 (I)BS 6755 (I)BALLBS 5351 / API 6DBS 6755 / API 607PLUGBS 5353BS 6755 (I)BUTTERFLYBS 5155 / MNF STDBS 6755 (I)