Valve Considerations

For Feedwater Applications

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Presentation Overview

• Definitions 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

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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

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Hysteresis

• Equation: *100

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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.

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Combined Effect of Hysteresis and Dead Band

• Hysteresis may contribute to dead band.

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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

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Limits of Digital Technology

Manufacturer Calculates Resolution Capabilities of the Valve Design:

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Limits of Digital Technology

Resolution or Accuracy is Additive:

1. Add positioner accuracy to valve accuracy for system capability.

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

3. The only way to negate calculated resolution is through Electro-hydraulic or Motor Operator

Performance Comparison

Comparison Factor Premium Digital

Conventional Pneumatics

Hydraulics

Reduced maintenance Yes No No

Reliability Superior High Low

Fire Hazard No No Yes

Simple calibration and tuning

Yes No No

Lower lifecycle costs Yes Yes No

Hysteresis and deadband < 0.5% < 1 – 3% < 0.3%

Dead time on seat < 150 msec < 300 to 1000 msec

< 100 msec

Resolution and deadband < 0.25% < 0.5 – 1.5% < 0.1%

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Interpretation of Results

• “Boots” shown at both ends• Indicates loose position feedback mechanism• Common condition with Scotch Yoke type

positioner linkages. • Example:

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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

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Linkage Improvement

• Cam Style Motion Converter

• Minimizes hysteresis in the system

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Features of Modern Feedwater Valves

• Rangeability

• Equal Percentage

• Single valve for Start Up and Main

0% 20% 40% 60% 80% 100%0%

20%

40%

60%

80%

100%Valve Cv Versus Travel

% Stroke

% C

v

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Features of Modern Feedwater Valves

• Accurate Position Feedback

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Features of Modern Feedwater Valves

• Digital Positioner Technology

Standards vs. Guidelines, Steam

ISA 75 has recommendations.

Multi Stage Letdown

Multi-stage Pressure Drop

Eliminates Cavitation

Trim Analysis, Liquid Applications

• Graphical representation of Pressure Recovery Factor

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Pressure recovery. FL

Low FL (Ball valve)

Higher FL

Very high FL. (Multi-stage valve)P1

P2

Trim Analysis, Cavitation Prediction Methods

•  

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Definition of Sigma

•  

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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

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Sigma-sVibration

smv

sc

si

smr

1.0 20+

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Seat Protection Features, Min Flow / Recirc

Snap Acting Relay Function

• Pneumatic

• Digital PLUGCAGE

SEAT RING

BODY

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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)

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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