Download - Basic Fluid Dynamics - Control Valves
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WHEN ACCURACY MATTERS
Introduction Introduction Basic Fluid DynamicsBasic Fluid Dynamics
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WHEN ACCURACY MATTERS
Liquid Applications(incompressible media)
Considerations: -Specific Gravity
-Viscosity
-Vapor Pressure
-Critical Pressure
-Temperature
-P1 - Upstream Pressure
-P2 - Down Stream Pressure or Delta P (P1 - P2)
-Min. Flow Rate / Max Flow Rate
-Clean or dirty media
-Corrosive potential
-Erosion Potential
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WHEN ACCURACY MATTERS
Valve Capacity or Rated CvValve Capacity or Rated Cv
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Required Flow Capacity = CRequired Flow Capacity = Cvv
One COne Cvv is equal to a flow rate of one is equal to a flow rate of one
gallon per minute of water atgallon per minute of water at60 degrees F at one pound 60 degrees F at one pound
per square inch pressure differentialper square inch pressure differential
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WHEN ACCURACY MATTERS
Required Flow Capacity = CRequired Flow Capacity = Cvv
P
QCv
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WHEN ACCURACY MATTERS
Bernoulli’s LawBernoulli’s Law
Bernoulli's principle states that for an Incompressible flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure.
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WHEN ACCURACY MATTERS
Conservation of Energy & MassConservation of Energy & Mass
Mass
1 2
Energy
Bernoulli’s Equation for
Incompressible Flow
½V2 + P = Constant
V P
1A1V1 = 2A2V2
A V
Higher velocity through a smaller area
Pressure decreases as velocity increases
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WHEN ACCURACY MATTERS
Control Valve Fluid MechanicsControl Valve Fluid Mechanics
Letdown Path
Control valves throttle by convertingstatic pressure to kinetic energy.
Pressure is reduced by creatingresistance along the fluid’s flow path.
DynamicPressure
P1
P2
Pvc
FL
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WHEN ACCURACY MATTERS
Conservation of Mass/EnergyConservation of Mass/Energy
Vena Contracta Pressure
Pressure P1
P2
A1 A2
Vena Contracta Area
V1V2
Velocity at Vena Contracta
Velocity
Area
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WHEN ACCURACY MATTERS
• Cavitation is defined as the phenomenon of formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure.
• Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and Incipient cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave.
• Incipient cavitation is the point at where cavitation begins, but has not reached a destructive state
What is Cavitation?
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WHEN ACCURACY MATTERS
Cavitation Phenomena
P1
V1
V2
P2
Pvapor
Vvc
CavitationFlashpoint Pvc
Pressure
Velocity
Local Fluid Pressure Drops Below Fluid Vapor Pressure
Formation of Vapor Bubbles
Downstream Pressure Recovery Above Vapor Pressure
Bubbles Collapse
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WHEN ACCURACY MATTERS
What is Cavitation?What is Cavitation?
1. Local Pressure Drops Below Fluid Vapor Pressure, Pv
2. Vapor “Bubble” Forms
3. Pressure Recovers Above Pv
4. Collapse of Vapor Back to Liquid
5. Addition of Turbulence & Noise
P<Pv
P>Pv
Flow
Vena Contracta
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WHEN ACCURACY MATTERS
Why Worry About Cavitation?(Bubble implosion)
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WHEN ACCURACY MATTERS
PvPv
Static Static PressurePressure CavitationCavitation
P1P1
P2P2
Cavitation
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WHEN ACCURACY MATTERS
Valve Cavitation DamageValve Cavitation Damage
• Pitting and erosion of exposed surfaces– Accelerates valve wear
– Increases maintenance and process down-time
• Larger Scale Cavitation: potential catastrophic failure of valve body & pressure vessel walls
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WHEN ACCURACY MATTERS
Localized ErosionSounds like rocks Will attack any
materialWorse with high
pressure
Cavitation
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WHEN ACCURACY MATTERS
Why Worry About Cavitation Why Worry About Cavitation
Highmaintenance
Poorcontrol
Systemshutdown
Lostproduction
Noise
Pipevibration
Trim & bodywear
Downstreampipe erosion
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WHEN ACCURACY MATTERS
Cavitation Prediction MethodsCavitation Prediction Methods
1. Method based on the Valve Pressure Recovery
2. Method based on the Valve Cavitation Index ISA-RP75.23-1995: “Considerations for Evaluating
Control Valve Cavitation”
P1-P2
P1-PVC
FL =
(Sigma) = (Sigma) = (P1-PV)
(P1-P2)
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WHEN ACCURACY MATTERS
Cavitation Management Cavitation Management StrategiesStrategies
• Prevention– Control Pressure Recovery by Reducing Velocity
• Increase Flow Resistance via Staging
• Containment– Control “Bubble” Collapse Location & Size
• Removed From Surfaces• In Low-Impact Regions (e.g. Cage Hole)• High FL Not Necessary• Reduce orifice size ( Small hole diameter-Drilled cages)
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WHEN ACCURACY MATTERS
Control Valve GeometriesControl Valve Geometries
• Where is the Vena Contracta?
• How do we use FL to assess the potential for cavitation?
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WHEN ACCURACY MATTERS
Technology SelectionTechnology Selection
• Single Stage Pressure Letdown
• Double Stage Pressure Letdown
• Multi-Stage Pressure Letdown
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WHEN ACCURACY MATTERS
Energy ManagementEnergy Management• Single Step
Letdown
DP
DP
= P
1 –
P2
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WHEN ACCURACY MATTERS
Energy ManagementEnergy Management• Single Step
Letdown
• Multiple Step Letdown
DP
DP
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WHEN ACCURACY MATTERS
Single Stage Cavitation
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WHEN ACCURACY MATTERS
Multi Stage No Cavitation
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WHEN ACCURACY MATTERS
Hydrodynamic Flow RegimesHydrodynamic Flow Regimes
Static
Pressure
P1
Pressure Letdown Path
Subcritical
Cavitating
Flashing
Vapor Pressure
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WHEN ACCURACY MATTERS
Single Stage Cavitation Containment
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WHEN ACCURACY MATTERS
Turn
Recombine
Split
Two Stage Cavitation Containment
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WHEN ACCURACY MATTERS
Premier High Pressure Liquid Letdown
Separates Flow
Re-combines Flow
Constricts Flow
Expands Flow
Turns Flow
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WHEN ACCURACY MATTERS
Lincoln Log Flow Path
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WHEN ACCURACY MATTERS
VARIABLE NUMBER OF STAGESDrilled Hole Stacked Plate Design
Pressure Drop Control Technology (Cavitation Prevention)
Pressure Drop Always Distributed Across Various Stages
Stages Disengage with Increasing
Plug Lift
High Staging at Low Lift
Low Staging at High Lift
Wide Cv Ranges are Possible
Axial Flow Multi-Stage Variable Resistance Trim (VRT®)
PLUG
P1
P2
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WHEN ACCURACY MATTERS
Multi Stack Plate
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WHEN ACCURACY MATTERS
V-LOG OperationV-LOG Operation
• Multiple Turns and Flow Restrictions• Gradual Pressure Reduction• Maintains Low Fluid Velocities
90O TURNS
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WHEN ACCURACY MATTERS
77000 Series Multistage Adiabatic Trim
• Product Highlights – Two-Phase Flow– Heavy Guiding– Model 84/85 Piston Actuator
• Compound Leverage 4:01 & 6:01• Spring Return
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WHEN ACCURACY MATTERS
Flashing in Control Flashing in Control ValvesValves
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WHEN ACCURACY MATTERS
What is Flashing?What is Flashing?
P1
Pv
P2
Vapor Bubble Formation
Outlet Pressure
Inlet Pressure
Vena Contracta Flow
Pressure
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WHEN ACCURACY MATTERS
What is Flashing?What is Flashing?
P1
Pv
P2Outlet Pressure
Inlet Pressure
Vena Contracta Flow
Pressure
Vapor Pressure
High Pressure Recovery = Low FL
Low Pressure Recovery = High FL
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WHEN ACCURACY MATTERS
• Smooth cuts• Material Loss• Body Damage• Trim Damage• Piping Damage
Effects of Flashing
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WHEN ACCURACY MATTERS
Treatment of Flashing SurfaceTreatment of Flashing Surface
• Flashing cannot be prevented by valve design• Armoring of valve trim and body
– Upgrade body to Chrome-Moly or Stainless Steel– Hardening of trim parts or hardfacing
• Back pressure devices to increase P2 pressure and postpone flashing
• Expanded outlet valve to slow impact of water droplets • Control the valve and line velocities via selection of valve and
line size• Multistep valves to dissipate initial energy release within the
valve trim
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WHEN ACCURACY MATTERS
Erosion
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WHEN ACCURACY MATTERS
• Erosion• Seat leakage at high
pressure• Cavitation
secondary
Wire Drawing
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WHEN ACCURACY MATTERS
• Chemical induces• All wetted parts are
affected• Material
Corrosion
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WHEN ACCURACY MATTERS
Gas and Steam Applications(compressible media)
Considerations:
-Specific Gravity / Density / Molecular Weight
-Compresability
-Temperature
-P1 - Upstream Pressure
-P2 - Down Stream Pressure or Delta P (P1 - P2)
-Min. Flow Rate / Max Flow Rate
-Clean or dirty media
-Corrosive potential
-Erosion Potential
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WHEN ACCURACY MATTERS
Sizing Equation - Liquid
P
QCv
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WHEN ACCURACY MATTERS
Sizing Equation – Gas/VaporSizing Equation – Gas/Vapor
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WHEN ACCURACY MATTERS
Questions? Questions?