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Valve Cavitation

This document discusses cavitation in valves. Cavitation occurs when the pressure downstream of a valve drops below the vapor pressure of the liquid, causing bubbles to form. If the pressure recovers above vapor pressure downstream, the bubbles will collapse, causing cavitation. If pressure remains below vapor pressure, the bubbles will not collapse and can cause damage. A cavitation index called Sigma is used to quantify cavitation risk, with values below 1 indicating flashing and values above 2 indicating no cavitation. The document provides categories of Sigma values and recommendations for cavitation control based on the index value.

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Driss Eddenia
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137 views8 pages

Valve Cavitation

This document discusses cavitation in valves. Cavitation occurs when the pressure downstream of a valve drops below the vapor pressure of the liquid, causing bubbles to form. If the pressure recovers above vapor pressure downstream, the bubbles will collapse, causing cavitation. If pressure remains below vapor pressure, the bubbles will not collapse and can cause damage. A cavitation index called Sigma is used to quantify cavitation risk, with values below 1 indicating flashing and values above 2 indicating no cavitation. The document provides categories of Sigma values and recommendations for cavitation control based on the index value.

Uploaded by

Driss Eddenia
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Cavitation in Valves

Saleh
Daryabari
Introduction
Pressure drop occurs through the valves
because as velocity increases to its maximum
through the flow restriction, pressure
decreases to its minimum. As velocity
normalizes over distance downstream,
pressure recovers as well but not to the level
it was at upstream of the valve. The point at
which pressure is at its minimum is Vena
Contracta. In liquids at this point the local
pressure may drop below vapor pressure
forming bubbles.
Velocity Profile Pressure Profile

2
Cavitation and Flashing
Cavitation: If the liquid pressure Flashing: If liquid pressure
downstream of the Vena Contracta downstream does not recover and
recovers to a point higher than vapor remains below the vapor pressure,
pressure, bubbles will collapse and bubbles do not collapse and cause
cause cavitation. further damage in downstream
piping, especially at elbows.

3
Cavitation and Flashing Profile

4
Cavitation Index
A cavitation index, called Sigma (σ), has been developed by ISA-RP75 to
quantify cavitation in control valves. Sigma represents the ratio of the
potential for resisting cavity formation to the potential for causing cavity
formation. This cavitation index is defined as follows:

σ = (P1-Pv)/(P1-P2)

• Where:
• P1 = Upstream pressure
• P2 = Downstream pressure
• Pv = Vapor pressure of the liquid at flowing temperature

5
Cavitation Index Categories
Through laboratory and field-testing results, acceptable operating Sigmas for
eliminating cavitation (and its associated choking, noise, and damage) have been
established.
Following is general categories for a typical globe valve’s operating conditions:

• σ > 2.0 No cavitation is occurring.


• 1.7 < σ < 2.0 No cavitation control required. Hardened trim provides protection.
• 1.5 < σ < 1.7 Some cavitation control required. Mutual impingement trim may work.
• 1 < σ < 1.5 Potential for severe cavitation. Use staged pressure drop trim.
• σ < 1 Flashing is occurring.

6
Cavitation Control
Use staged pressure drop Upgrade the material to resist
trim. Reduce the pressure energy from energy of the
from inlet to outlet gradually, imploding bubbles
thus avoiding a large
pressure drop at the Vena
Contracta.

7
Thank you
Saleh Daryabari
Refinery Senior Process Engineer
daryabari@gmail.com

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