调节阀的空化现象

Valve Inlet
Pvc Pvc < Pv
Vena Contracta
Valve Outlet
cavrecov
Distance
Figure 23-10. The Potential For Cavitation As A Function Of Pressure Recovery
Strategy: Pressure Drop Strategy
Fluid Pressure vs. Vapor Pressure
Flow
P1
Fluid
P2
Pressure,
psia
Pv
Pvc
Upstream Vena Contracta
Downstream
Distance
pv_pvc
Figure 23-2. Fluid Vaporization That Occurs When Pvc<Pv.
F igure 23-9. Influences on C avitation D am age
Strategy: Low Recovery Valve Design
P1
Pressure, psia
Pv
Low Recovery Valve Pvc > Pv
P2 High Recovery Valve
Cavitation Damage
cavillus
Cavitation Damage
Figure 23-4. Cavitation Damage That Results From Imploding Vapor Cavities
Cavitation Damage
Figure 23-5. Actual Cavitation Damage of a Valve Plug
P1
Single Port Trim
Multi-Stage Trim
Pressure, psia
P2
Pv
multistg
Valve Stage 1 Inlet
Figure 23-11. Pressure Drop Staging
Stage 2 Stage 3 Distance
Valve Outlet
Flashing Damage
Figure 23-7. Actual Flashing Damage of a Valve Plug
Cavitation Damage Index for System: Ar
Ar =
P1 - P2 P1 - Pv
P1
Pressure, psia
Pv
Ar approaching 0.99: Increasing Potential for Cav Ar = 0.99: Max Potential for Cav Ar = or > 1.00: Flashing
Cavitation: Bubble Formation and Collapse
Liquid Flow
Cavitating Fluid
Flow
Vena Contracta Cavitation
Pv
P1
vccav
Figure 23-3. Cavitation
Pvc
Biblioteka Baidu
P2
Distance
Cavitation Damage
Flo w
R estriction
Vena C ontracta
V elocity P ro file P re ss u re P ro file
b rn o u li
Figure 23-1. Pressure and Velocity Profiles of a Fluid Flow ing Through a Restriction
P1-P2 P2
P1-Pv
Pvc
P1
Pvc
P2
ksa&ar
Figure 8. Cavitation Index Ar
Distance
Influences on Cavitation Damage
Influences on C avitation D am age
• P roxim ity O f Im p losion s T o B ou n d ary S u rfaces • Pressure D rop • M aterial P rop erties • C avitation in a C orrosive or E rosive F lu id • A ir A n d G as C on ten t • S in gle S p ecies F lu id s V ersu s M ixtu res • E xp osu re T im e
Pressure and Velocity Profiles
1 . A s flu id flo w s th ro u g h a res tric tio n , flu id ve lo city in c re a s es .
2 . B e rn o u li's L a w : If ve lo city in c re a s es , p re s su re w ill d e cre as e .
Flashing - P2 < Pv
Liquid Flow Flow
Fluid Vaporization (Flashing)
Vena Contracta Cavitating
Pv Flashing
P1
Pvc
P2
vcflash
Distance
Figure 23-6. Flashing Phenomena That Occurs When P2<Pv
3. Th e V en a C on tracta, d ow n stream o f th e restriction , is the po int of sm a lle s t flo w a re a , h ig h e s t v e lo c ity, a n d lo w e s t p re s su re .
Strategy: Damage Resistant Materials
Cavitation Damage
Pressure, P1
psia
Pv
Pvc
P2 Standard MaterialsOf Construction No Cavitation Damage
Valve Inlet
Vena
Valve