调节阀的空化现象
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Pressure and Velocity Profiles
1. As fluid flows through a restriction, fluid velocity increases. 2. Bernouli's Law: If velocity increases, pressure will decrease. 3. The Vena Contracta, downstream of the restriction, is the point of smallest flow area, highest velocity, and lowest pressure. Restriction Flow Vena Contracta
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2
Cavitation: Bubble Formation and Collapse
Liquid Flow Flow Cavitating Fluid
Vena Contracta Cavitation
Pv
P1
cavrecov
Pvc > Pv P2
Pvc < Pv Valve Outlet
Vena Contracta Distance
Figure 23-10. The Potential For Cavitation As A Function Of Pressure Recovery
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Velocity Profile
Pressure Profile
brnouli
Figure 23-1. Pressure and Velocity Profiles of a Fluid Flowing Through a Restriction
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P1 Pressure, psia Pv Pvc P1
ksa&ar
P1-P2 P2
P1-Pv
Pvc Distance
P2
Figure 8. Cavitation Index Ar
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8
Influences on Cavitation Damage
vccav
பைடு நூலகம்
Pvc Distance
P2
Figure 23-3.
Cavitation
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3
Cavitation Damage
Cavitation Damage
Cavitation Damage
cavillus
Figure 23-4.
Influences on Cavitation Damage
Proximity Of Implosions To Boundary Surfaces Pressure Drop Material Properties Cavitation in a Corrosive or Erosive Fluid Air And Gas Content Single Species Fluids Versus Mixtures Exposure Time
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5
Flashing - P2 < Pv
Liquid Flow Flow Fluid Vaporization (Flashing)
Vena Contracta Cavitating
Pv Flashing P1
vcflash
Pvc Distance
P2
Figure 23-6. Flashing Phenomena That Occurs When P2<Pv
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6
Flashing Damage
Figure 23-7. Actual Flashing Damage of a Valve Plug
Figure 23-9. Influences on Cavitation Damage
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9
Strategy: Low Recovery Valve Design
Low Recovery Valve P1 Pressure, psia Pv High Recovery Valve Pvc Valve Inlet
Cavitation Damage That Results From Imploding Vapor Cavities
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4
Cavitation Damage
Figure 23-5. Actual Cavitation Damage of a Valve Plug
1
Fluid Pressure vs. Vapor Pressure
Flow
P1
Fluid Pressure, psia Pv Pvc Upstream
pv_pvc
P2
Vena Contracta Distance
Downstream
Figure 23-2. Fluid Vaporization That Occurs When Pvc<Pv.
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7
Cavitation Damage Index for System: Ar
P1 - P2 Ar = P1 - Pv
Ar approaching 0.99: Increasing Potential for Cav Ar = 0.99: Max Potential for Cav Ar = or > 1.00: Flashing
1. As fluid flows through a restriction, fluid velocity increases. 2. Bernouli's Law: If velocity increases, pressure will decrease. 3. The Vena Contracta, downstream of the restriction, is the point of smallest flow area, highest velocity, and lowest pressure. Restriction Flow Vena Contracta
©Educational Services - Duplication Prohibited
2
Cavitation: Bubble Formation and Collapse
Liquid Flow Flow Cavitating Fluid
Vena Contracta Cavitation
Pv
P1
cavrecov
Pvc > Pv P2
Pvc < Pv Valve Outlet
Vena Contracta Distance
Figure 23-10. The Potential For Cavitation As A Function Of Pressure Recovery
©Educational Services - Duplication Prohibited
Velocity Profile
Pressure Profile
brnouli
Figure 23-1. Pressure and Velocity Profiles of a Fluid Flowing Through a Restriction
©Educational Services - Duplication Prohibited
P1 Pressure, psia Pv Pvc P1
ksa&ar
P1-P2 P2
P1-Pv
Pvc Distance
P2
Figure 8. Cavitation Index Ar
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8
Influences on Cavitation Damage
vccav
பைடு நூலகம்
Pvc Distance
P2
Figure 23-3.
Cavitation
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3
Cavitation Damage
Cavitation Damage
Cavitation Damage
cavillus
Figure 23-4.
Influences on Cavitation Damage
Proximity Of Implosions To Boundary Surfaces Pressure Drop Material Properties Cavitation in a Corrosive or Erosive Fluid Air And Gas Content Single Species Fluids Versus Mixtures Exposure Time
©Educational Services - Duplication Prohibited
5
Flashing - P2 < Pv
Liquid Flow Flow Fluid Vaporization (Flashing)
Vena Contracta Cavitating
Pv Flashing P1
vcflash
Pvc Distance
P2
Figure 23-6. Flashing Phenomena That Occurs When P2<Pv
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6
Flashing Damage
Figure 23-7. Actual Flashing Damage of a Valve Plug
Figure 23-9. Influences on Cavitation Damage
©Educational Services - Duplication Prohibited
9
Strategy: Low Recovery Valve Design
Low Recovery Valve P1 Pressure, psia Pv High Recovery Valve Pvc Valve Inlet
Cavitation Damage That Results From Imploding Vapor Cavities
©Educational Services - Duplication Prohibited
4
Cavitation Damage
Figure 23-5. Actual Cavitation Damage of a Valve Plug
1
Fluid Pressure vs. Vapor Pressure
Flow
P1
Fluid Pressure, psia Pv Pvc Upstream
pv_pvc
P2
Vena Contracta Distance
Downstream
Figure 23-2. Fluid Vaporization That Occurs When Pvc<Pv.
©Educational Services - Duplication Prohibited
7
Cavitation Damage Index for System: Ar
P1 - P2 Ar = P1 - Pv
Ar approaching 0.99: Increasing Potential for Cav Ar = 0.99: Max Potential for Cav Ar = or > 1.00: Flashing