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Cavitation and NPSH Analysis

The document discusses cavitation and provides equations for calculating net positive suction head (NPSH). It defines terms like vapor pressure head, atmospheric pressure head, suction head, and loss coefficient. Graphs illustrate pump operation and losses. The document also discusses the Law of Thoma related to cavitation in hydraulic similar flows.

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Fabio Folpini
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12 views7 pages

Cavitation and NPSH Analysis

The document discusses cavitation and provides equations for calculating net positive suction head (NPSH). It defines terms like vapor pressure head, atmospheric pressure head, suction head, and loss coefficient. Graphs illustrate pump operation and losses. The document also discusses the Law of Thoma related to cavitation in hydraulic similar flows.

Uploaded by

Fabio Folpini
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Cavitation

2
c
H A = h v + h s + (1 − ς s ) + ∆h
s

2g
hv vapour pressure head
HA atmospheric pressure head
hs suction head
cs mean velocity
ςs loss coefficient
∆h resulting pressure head
4

z4

c22 c32 c32


h2 + z 2 + = h3 + z3 + +ς3 ⋅
2⋅ g 2⋅ g 2⋅ g Losses
⇓
c22 c32 c32 c22
h2 + z 2 + = h4 + z 4 + +ς3 ⋅ = h4 + z 4 + ς s ⋅
2⋅ g 2⋅ g 2⋅ g 2⋅ g
⇓
c22
h2 = h4 + (z 4 − z 2 ) + (ς s − 1) ⋅
2⋅ g

Atmospheric pressure: HA = h4
4

z4

2
h2 = h4 + ( z 4 − z 2 ) + (ς s − 1) ⋅
c 2
2⋅ g

Let us introduce the vapor pressure, hv :

h2 = hv + ∆h

2
hv + ∆h = h4 + (z4 − z2 ) + (ς s −1) ⋅
c 2
2⋅ g
NPSH
Net Pressure Suction
Head
4

z4

c22
hv + ∆h = h4 + ( z 4 − z 2 ) + (ς s − 1) ⋅
2⋅ g
⇓
c22
∆h = h4 − hv + ( z 4 − z 2 ) + (ς s − 1) ⋅
2⋅ g
⇓
c22
NPSH = H A − hv + ( z 4 − z 2 ) + (ς s − 1) ⋅
2⋅ g
Suction Head
4

hs
z4

c22
NPSH = H A − hv + ( z 4 − z 2 ) + (ς s − 1) ⋅
2⋅ g
hs
Law of Thoma

NPSH
σ=
H
Provided that similar hydraulic cavitating flow
remain unchanged relative to the flow canals, the
relations of hydraulic similar flow, are valid also
for flow including cavitation.

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