PowerPoint Presentation

Valve ConsiderationsFor Feedwater Applications

Presentation OverviewDefinitions of operating characteristics pertaining to Feedwater control valves

Features of the modern Feedwater Control Valve

Velocity head considerations in Feedpump Recirc applications and Condensate Recirc Applications

Strictly personal and confidential2About 27 slides, 45 minutes. The goal of the presentation is to give simple take aways to guide valve and positioner selection for feedwater systems. Specifically:Limits of digital positioner technology.Optimum positioner mountingInterpretation of vendor sizing data2HysteresisDefined by ISA 51.1Hysteresis is the ability to position to the same point for a given instrument signal, regardless of the travel direction.Hysteresis is the result of non-elasticity in mechanical componentsStrictly personal and confidential3On a valve diagnostic trace, the gap between the opening and closing line is a measure of hysteresis. 3HysteresisStrictly personal and confidential4

Notice the hysteresis is measured on the y axis.4Dead BandTime it takes for positioner to vary the output signal to move the valve.Dead band is quantified with signal reversals.Dead band is the result of friction and mechanical connections.Strictly personal and confidential5

Notice the characteristics of dead band are measured on the X axis. 5Combined Effect of Hysteresis and Dead BandHysteresis may contribute to dead band.Strictly personal and confidential6

This may also be combined to a general valve characteristic called Resolution. This is the ability of the valve to hit within a desired setpoint of valve opening as measured by percentage. 6Features that Influence Hysteresis and DBFriction

Stem PackingBalance Seal

Compromise may be required to minimize hysteresis and dead band.

Actuator Size Seat Leakage RequirementStrictly personal and confidential7

Valve manufacturers prefer teflon when possible to reduce friction and the resultant hysteresis. 7Limits of Digital TechnologyManufacturer Calculates Resolution Capabilities of the Valve Design:Strictly personal and confidential8

The resolution calculation shown is only for a pneumatic actuator. The air spring characteristic is why the resolution is worse at mid stroke. Hydraulic operators do not have this issue because of incompressible fluid. 8Limits of Digital TechnologyResolution or Accuracy is Additive:

Add positioner accuracy to valve accuracy for system capability.

Digital systems (QT, FT, DVC, SVI) do not lower valve resolution.

The only way to negate calculated resolution is through Electro-hydraulic or Motor OperatorStrictly personal and confidential9When writing specifications, the end user should have reasonable expectations of valve performance. International specifications are beginning to use 1% Accuracy as a benchmark, which is not possible. 9Performance ComparisonComparison FactorPremium DigitalConventional PneumaticsHydraulicsReduced maintenanceYesNoNoReliabilitySuperiorHighLowFire HazardNoNoYesSimple calibration and tuningYesNoNoLower lifecycle costsYesYesNoHysteresis and deadband< 0.5%< 1 3%< 0.3% Dead time on seat< 150 msec< 300 to 1000 msec< 100 msecResolution and deadband< 0.25%< 0.5 1.5%< 0.1%Numbers given are for teflon packing and balance seal. For nuclear applications with graphite packing and balance seals, the numbers are additive to the previous slide showing resolution. Interpretation of ResultsBoots shown at both endsIndicates loose position feedback mechanismCommon condition with Scotch Yoke type positioner linkages. Example:Strictly personal and confidential11

Causes of Excessive Dead BandDigital Positioner SetupPositioner not configured for Scotch Yoke linkagePositioner not configured for spring return pistonTuning parameters depend upon system setupSloppy linkageStrictly personal and confidential12

Linkage ImprovementCam Style Motion ConverterMinimizes hysteresis in the system

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Features of Modern Feedwater ValvesRangeability

Equal Percentage

Single valve for Start Up and MainStrictly personal and confidential14Features of Modern Feedwater ValvesAccurate Position FeedbackStrictly personal and confidential15

Features of Modern Feedwater ValvesDigital Positioner TechnologyStrictly personal and confidential16

As digital control systems become the norm, digital positioners will become a necessity for the associated valves in the feedwater systerm. 16Standards vs. Guidelines, SteamISA 75 has recommendations.

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Multi Stage LetdownMulti-stage Pressure DropEliminates CavitationTrim Analysis, Liquid ApplicationsGraphical representation of Pressure Recovery FactorPage 19

Trim Analysis, Cavitation Prediction MethodsPage 20Definition of Sigma Page 21How is Sigma Determined?Manufacturer Determines Sigma by TestApplication ExperienceCavitation Damage Testing of Aluminum TrimsVaries according toStyleSizeCapacityApplication PressureEstablished by:10% Increments of liftTwo valve sizes of like geometryTwo upstream pressures to determine scaling factorsPage 22Sigma-sVibrationsmvscsismr1.020+Seat Protection Features, Min Flow / RecircSnap Acting Relay Function

Pneumatic

DigitalStrictly personal and confidential23

PLUGCAGESEAT RINGBODYMaterial Considerations for High Vel. AppsAngle ValveCarbon Steel Valve Body (A216-WCB,WCC)Globe ValveChromoly (A216-WC6 Casting, F11 Forging)High Chrome for Fossil (A216-C5, F5 Forging)

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SummaryValve design will limit digital positioner performance. Critical valve positioners require modern linkagesCam plate type linkageComputer compensation of four bar linkageElimination of Scotch Yoke type positioner linkageCavitation index warnings should be evaluated on a case by case basis

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Pressure recovery. FL
Low FL (Ball valve)
Higher FL
Very high FL. (Multi-stage valve)
P1
P2

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