29_Cavitation_&_Flashing.ppt

7/22/2019 29_Cavitation_&_Flashing.ppt

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Cavitation Flashing

and

Flashing


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VaporizationEffects in

Fluid Flow

Vaporization

Effects inFluid Flow

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FusionLine

FusionLine

Critical

PointLiquid

Phase

Vapor

Phase

Phase

Solid

Triple

Point

Pr

essure

Triple Point Diagram

Temperature


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Water 32 0.0886

oF psia

Triple Point

CO2 - 69.88 75.09


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FusionLine

FusionLine

Critical

PointLiquid

Phase

Vapor

Phase

Phase

Solid

Triple

Point

Pr

essure

Triple Point Diagram

Temperature

Water (Solid, Liquid, Vapor)

CO2 (Solid, Vapor, NO Liquid)


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Cavitation?

What is


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Cavitation?What is


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1 2 3

Varying Area Flow Channel


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Flow with Pressure RecoveryFlow with Pressure Recovery

pp

1 2 3


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V12 P1 V2

2 P2

+ + gz 1 = + + gz 2

2 1 2 2

1V1A1 = 2V2A2

Basic Equations:

Bernoullis Energy Equation

X X

Continuity


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1 2 3

11Flow Channel with sharp edge orifice


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1211

p

1 2 3

Flow Channel with out Pressure Recovery


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ValvePressure

Drop

CavitationCavitation

PP 22(Outlet

Pressure)(Outlet

Pressure)

Single Seated Valve Pressure Recovery Diagram

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PPvcvc (Pressure at Vena Contracta)

PV

PP 11

(InletPressure)

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P Line( Pressure)Drop


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Bubblecollapseformation

andmechanism

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1 diameter from wall

Bubble Implosion

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1 diameter from wall

Bubble Implosion

Within a relatively

short time the total

damage can become

very signi f icant.


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CavControl trim


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CavControl trim


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316

Stainless

Steel

with

TungstenCarbide

Insert


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316

Stainless

Steel

with

TungstenCarbide

Insert

With higher magnification


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Pitting and erosion

Noise and vibration

Corrosion

Combination

Resultsof Cavitation


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V12 P1 V2

2 P2

+ + gz 1 = + + gz 22 1 2 2

Basic Equations:

Bernoullis Energy Equation

V12 P1 V22 P2+ = +

2 1 2 2

X X


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V1

A1

= V2

A2

V1 P1 V2 P2+ = +

2 1 2 2

[ ]( )A 2

2 1/2 2 PV1 = 1 +

A1

GfW

therefore

Capacity

CV TermP/Gf


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PQ = V2 A2 = CVGf

CV is defined for

units/area used

where:

Reduces to

ISA Sizing Equation


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Slope = CV

Ideal Fluid

P

q


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qq

Actual Liquid

PP

q A


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qq

P

Actual Liquid

CBTheoretical choking pointActual choking point


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ChokedPoint B

Pc

= FL

2 (P

1- F

FP

V)

ChokedChoked


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Pressu re drop produces no ise

and vibrat ion fromvapo r free

f low turbu lence.

No Cav itat ion :

Simp ly Flow Noise


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In term it ten t Noise and cavi ty

format ion.

Constant(o r Crit ic al)

Steady cons tant Noise and cons tant

cavi ty format ion .

Cavitat ion:

Cavitat ion

Incipient


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Increases in vib rat ion and

Noise intensi ty to the level of

"max imum v ibrat ion"

Ful l

"max imum v ibrat ion""max imum v ibrat ion""max imum v ibrat ion""max imum v ibrat ion"

Cavitat ion:

Significant Damage"oftenoccu rs somewhere in th is regime.


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`

Erosion damage from high velocity

can occur giving a damageappearance.

Super

A decrease in noise and vibration.

Choking often occurs in this

area.

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Cavitation:

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P1 - Pv1 =P1 - P2

P1 - based index:

Sigma1


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P2 - based index:

P2 - Pv2 =P1 - P2

Sigma2


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Testing forCavitation in

"LowPressureRecovery"

Control Valves

Testing forCavitation in

"LowPressureRecovery"

Control Valves


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Ways of

of Cavitation

Ways of

of Cavitation

MeasuringIntensity Soft Aluminum Plate

Accelerometer

MeterMeter

1 m. down

1 m. out

and

Sound


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TestingTesting1. Noise / Acceleration2. Damage3. Visual


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44/90

46 = (P1- PV) / P

Acceler

ation


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47 = (P1- PV) / P

Acceler

ation

i

c

mv


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Testing1. Noise / Acceleration2. Damage3. Visual

Testing1. Noise / Acceleration2. Damage3. Visual


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Cavitation


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Velocity Erosion


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Corrosion


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Testing1. Noise/Acceleration2. Damage3. Visual

Testing1. Noise/Acceleration2. Damage3. Visual


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55 Transparent pipe Strobe Light

Visual Cavitation


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Visual Cavitation


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Control of Cavitation DamageControl of Cavitation Damage1. Remove all nuclei


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Control of Cavitation DamageControl of Cavitation Damage

2. Inject air or gas1. Remove all nuclei


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3. Increase the FL value

FL = .92


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0 20 40 60 80 1000 20 40 60 80 100

% of Rated Capacity% of Rated Capacity

FL Flow-to-CloseFL Flow-to-Close

F L Flow-to-OpenFL Flow-to-Open

0 20 40 60 80 1000 20 40 60 80 100

% of Rated Capacity% of Rated Capacity

0 30 60 900 30 60 90

ValdiskValdisk

Fi Flow-to-CloseF i Flow-to-Close

F i Flow-to-OpenF i Flow-to-Open

Valve Recovery FactorsValve Recovery Factors

FL Fi FL

Globe Valve FL Values Globe Valve Fi Values Rotary Valve FL Values

1.00

0.90

0.80

0.70

0.60

0.50

0.40

1.00

0.90

0.80

0.70

0.60

0.50

0.40

1.00

0.90

0.80

0.70

0.60

0.50

0.40

Degrees Open

ShearStream

FL = .92?

FL .92Flow Under

FL = .70

Flow Over


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4. Hardened materials (brute force)


Estimated force = approx. 250,000# per in2

Control of Cavitation Damage

No materials available that have such strength


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62Hardened Trim

Flowserve prefers: 316 ss / #6 Stellite

Note: For2 valves and smaller, the entire plug

head and seat ring are solid Stellite.


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5. Remove metal boundary


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FLOW

CavControl


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Counterbored holes from outside in.

CavControl


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Flow

CavControl


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A. Single Orifice

B. Multiple Orifice

This looses less energy


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5. Remove metal boundary




6. Multi-stage trim design

Control of Cavitation Damage


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PP

PP

PP

11

22

vcPP

Liquid

The Problem

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Liquid and Vapor

PV


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PV

The SolutionPP

PP

11

22

Liquid

Liquid and Vapor

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PV

The SolutionPP

PP

11

22

Liquid

(Multi-Stage trim)

Liquid and Vapor

Since, Pressure does not go below PV ,

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71/90

74 doesNOToccur!Cavitation

PP

PP

11

22PV

Liquid

The Solution

(Multi-Stage trim)

Since, Pressure does not go below PV ,

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outside to

increase

Channelwidths

Inside.


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NO CAVITATION


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Visual Comparison

CHANNELSTREAMCAVITATION CONTROL

CAVITATES DOES NOT CAVITATE

SINGLE STAGE MULTI-STAGE


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Tiger-Tooth

Flow

Tiger-Tooth Trim also eliminates cavitation.77

.


77/90

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FlashingFlashing


78/90

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PP

FlashingFlashing

PV

Pressure)(Outlet

P2

(ValvePressureDrop)

PP

1

(Pressure at Vena Contracta)

Pressure Profile Single Seated Valve

VC

Liqu id

Liqu id / Gas combinat ion


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c. Erosion

ProblemsProblemsa. High Velocitiesa.High Velocities

b. Choking


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h1 = hf(1-x) + hg(x)

Since the Enthalpy of a control valveis considered constant between P1 & P2 :

x = % of liquid mass

flashed to vapor

Constant Enthalpy, h


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T

S

Vapor

Liquid

1

2

P2

P1

Liquid Gas

Typical T S Diagram


82/90

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P = 15 psia

Example

= 200 psia

Water @ 250o

F

P1

2


83/90


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hf1 = 218.59 B/lb m

hf2 = 181.19 B/lb m

hfg2 = 967.70 B/lb m

Enthalpies from

Steam Tables


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= 3.86% flash(218.59 - 181.19)

967.70

Enthalpy o f

Sat. Liq uid at

Inlet Temp

Enthalpy o f

Evaporat ion at

Outlet Press

Enthalpy o f

Sat. Liq uid at

Outlet Press

Percent Flash


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Sp. Vol. = (1 - x)vf+ xvg

vfm = 1.04 ft3/ lb m

vfm = .017(96.1%) + 26.29(3.09%)Liquid Vapor

61+ times as much

volume as the

original liquid.


87/90


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Flashing DamageFlashing Damage

1. Harden the trim2. Control velocities

3. Select Proper materials

To minimize


89/90

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`

Vaporization

RequiresCareful

Engineering


90/90

Return to Performance!

Documents

29_Cavitation_&_Flashing.ppt