Basic Fluid Dynamics - Control Valves

WHEN ACCURACY MATTERS

Introduction Introduction Basic Fluid DynamicsBasic Fluid Dynamics


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


Valve Capacity or Rated CvValve Capacity or Rated Cv


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


Required Flow Capacity = CRequired Flow Capacity = Cvv

P

QCv


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.


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


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


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


• 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?


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


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


Why Worry About Cavitation?(Bubble implosion)


PvPv

Static Static PressurePressure CavitationCavitation

P1P1

P2P2

Cavitation


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


Localized ErosionSounds like rocks Will attack any

materialWorse with high

pressure

Cavitation


Why Worry About Cavitation Why Worry About Cavitation

Highmaintenance

Poorcontrol

Systemshutdown

Lostproduction

Noise

Pipevibration

Trim & bodywear

Downstreampipe erosion


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)


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)


Control Valve GeometriesControl Valve Geometries

• Where is the Vena Contracta?

• How do we use FL to assess the potential for cavitation?


Technology SelectionTechnology Selection

• Single Stage Pressure Letdown

• Double Stage Pressure Letdown

• Multi-Stage Pressure Letdown


Energy ManagementEnergy Management• Single Step

Letdown

DP

DP

= P

1 –

P2


Energy ManagementEnergy Management• Single Step

Letdown

• Multiple Step Letdown

DP

DP


Single Stage Cavitation


Multi Stage No Cavitation


Hydrodynamic Flow RegimesHydrodynamic Flow Regimes

Static

Pressure

P1

Pressure Letdown Path

Subcritical

Cavitating

Flashing

Vapor Pressure


Single Stage Cavitation Containment


Turn

Recombine

Split

Two Stage Cavitation Containment


Premier High Pressure Liquid Letdown

Separates Flow

Re-combines Flow

Constricts Flow

Expands Flow

Turns Flow


Lincoln Log Flow Path


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


Multi Stack Plate


V-LOG OperationV-LOG Operation

• Multiple Turns and Flow Restrictions• Gradual Pressure Reduction• Maintains Low Fluid Velocities

90O TURNS


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


Flashing in Control Flashing in Control ValvesValves


What is Flashing?What is Flashing?

P1

Pv

P2

Vapor Bubble Formation

Outlet Pressure

Inlet Pressure

Vena Contracta Flow

Pressure


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


• Smooth cuts• Material Loss• Body Damage• Trim Damage• Piping Damage

Effects of Flashing


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


Erosion


• Erosion• Seat leakage at high

pressure• Cavitation

secondary

Wire Drawing


• Chemical induces• All wetted parts are

affected• Material

Corrosion


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


Sizing Equation - Liquid

P

QCv


Sizing Equation – Gas/VaporSizing Equation – Gas/Vapor


Questions? Questions?

Basic Fluid Dynamics - Control Valves

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