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Valve Considerations

Valve Considerations. For Feedwater Applications. Presentation Overview. Definitions of operating characteristics pertaining to F eedwater control valves Features of the modern Feedwater Control Valve

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Valve Considerations

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  1. Valve Considerations For Feedwater Applications

  2. Presentation Overview • Definitions of operating characteristics pertaining to Feedwatercontrol valves • Features of the modern Feedwater Control Valve • Velocity head considerations in FeedpumpRecirc applications and Condensate Recirc Applications Strictly personal and confidential

  3. Hysteresis • Defined by ISA 51.1 • Hysteresis 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 components Strictly personal and confidential

  4. Hysteresis • Equation: *100 Strictly personal and confidential

  5. Dead Band • Time 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 confidential

  6. Combined Effect of Hysteresis and Dead Band • Hysteresis may contribute to dead band. Strictly personal and confidential

  7. Features that Influence Hysteresis and DB Friction • Stem Packing • Balance Seal Compromise may be required to minimize hysteresis and dead band. • Actuator Size • Seat Leakage Requirement Strictly personal and confidential

  8. Limits of Digital Technology Manufacturer Calculates Resolution Capabilities of the Valve Design: Strictly personal and confidential

  9. Limits of Digital Technology Resolution 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 Operator Strictly personal and confidential

  10. Performance Comparison

  11. Interpretation of Results • “Boots” shown at both ends • Indicates loose position feedback mechanism • Common condition with Scotch Yoke type positioner linkages. • Example: Strictly personal and confidential

  12. Causes of Excessive Dead Band • Digital Positioner Setup • Positioner not configured for Scotch Yoke linkage • Positioner not configured for spring return piston • Tuning parameters depend upon system setup • Sloppy linkage Strictly personal and confidential

  13. Linkage Improvement • Cam Style Motion Converter • Minimizes hysteresis in the system Strictly personal and confidential

  14. Features of Modern Feedwater Valves • Rangeability • Equal Percentage • Single valve for Start Up and Main Strictly personal and confidential

  15. Features of Modern Feedwater Valves • Accurate Position Feedback Strictly personal and confidential

  16. Features of Modern Feedwater Valves • Digital Positioner Technology Strictly personal and confidential

  17. Standards vs. Guidelines, Steam • ISA 75 has recommendations.

  18. Multi Stage Letdown Multi-stage Pressure Drop Eliminates Cavitation

  19. Trim Analysis, Liquid Applications • Graphical representation of Pressure Recovery Factor

  20. Trim Analysis, Cavitation Prediction Methods

  21. Definition of Sigma

  22. smv sc smr Vibration si 1.0 20+ Sigma-s How is Sigma Determined? • Manufacturer Determines Sigma by Test • Application Experience • Cavitation Damage Testing of Aluminum Trims • Varies according to • Style • Size • Capacity • Application Pressure • Established by: • 10% Increments of lift • Two valve sizes of like geometry • Two upstream pressures to determine scaling factors

  23. Seat Protection Features, Min Flow / Recirc Snap Acting Relay Function • Pneumatic • Digital CAGE PLUG SEAT RING BODY Strictly personal and confidential

  24. Material Considerations for High Vel. Apps Angle Valve • Carbon Steel Valve Body (A216-WCB,WCC) Globe Valve • Chromoly (A216-WC6 Casting, F11 Forging) • High Chrome for Fossil (A216-C5, F5 Forging) Strictly personal and confidential

  25. Summary • Valve design will limit digital positioner performance. • Critical valve positioners require modern linkages • Cam plate type linkage • Computer compensation of four bar linkage • Elimination of Scotch Yoke type positioner linkage • Cavitation index warnings should be evaluated on a case by case basis Strictly personal and confidential

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