Brochure

NPSH [m]
Q[m
3
/h]
4040
2. Performance curves
2.10 NPSH, Net Positive Suction Head
NPSH is a term describing conditions related to cavitation, which is
undesired and harmful.
Cavitation is the creation of vapour bubbles in areas where the pressure
locally drops to the fluid vapour pressure. The extent of cavitation depends
on how low the pressure is in the pump. Cavitation generally lowers the
head and causes noise and vibration.
Cavitation first occurs at the point in the pump where the pressure is
lowest, which is most often at the blade edge at the impeller inlet, see
figure 2.10.
The NPSH value is absolute and always positive. NPSH is stated in meter [m]
like the head, see figure 2.11. Hence, it is not necessary to take the density of
dierent fluids into account because NPSH is stated in meters [m].
Distinction is made between two dierent NPSH values: NPSH
R
and NPSH
A
.
NPSH
A
stands for NPSH Available and is an expression of how close the fluid
in the suction pipe is to vapourisation. NPSH
A
is defined as:
[ ]
Pa
ppp
dyn
(2.1)
(2.2)
(2.5)
(2.6)
(2.7)
stattot
+ =
[ ]
PaV
2
1
2
1
2
1
p
2
dyn
⋅ ⋅ = ρ
[ ]
Papppp
geodynstattot
∆ + ∆ + ∆
p∆
p∆
∆ =
[ ]
Papp
stat, instat, outstat
− =
[ ]
PaVV
2
in
2
outdyn
⋅⋅−⋅ ⋅ = ρ ρ
(2.8)
2
1
[
]
Pa
D
1
D
1
4
Q
p
4
in
4
out
2
dyn








− ⋅








⋅ ⋅ =
π
ρΔ
(2.9)
[ ]
Pagzp
geo
⋅ ⋅ ∆ = ∆ ρ
(2.10)
(2.3)
(2.4)
(2.11)
(2.13)
(2.14)
(2.12)






= ⋅ + +
2
22
s
m
Constantzg
2
V
p
ρ
[ ]
Pappp
barrelabs
+ =
[ ]
m
g
p
H
tot
⋅
=
ρ
Δ
[ ]
WQpQgHP
tothyd
⋅ ∆ = ⋅⋅ ⋅ = ρ
[ ]
⋅ 100 %
[ ]
⋅ 100 %
[ ]
⋅ 100 %
=
2
hyd
hyd
P
P
η
=
1
hyd
tot
P
P
η
[ ]
WP
2
P
1
P
hyd
> >
(2.15)
(2.16)
(2.17)
(2.17a)
(2.18)
(2.19)
⋅⋅=
hydmotorcontroltot
ηηηη
( )
[ ]
m
g
pp
NPSH
vapourabs,tot,in
A
⋅
−
=
ρ
[ ]
mNPSH = NPSH
3%
NPSH
RA
0.5 +>
NPSH
A
>
[ ]
mNPSH = NPSH
3%
or
R
S
A
.
[ ]
m
g
p
H
g
p
NPSH
p
vapour
suction pipe
,loss
geo
bar
A
⋅
∆
− −
+
⋅ ⋅
=
ρ
ρ
9.81m
23
A
Pa 7375 3500 Pa
m 3
sm992.2kg
101300 Pa
NPSH −−−
⋅ 9.81m
23
sm992.2kg ⋅ 9.81m
23
sm992.2kg ⋅
=
9.81m
23
A
47400 Pa
1 m 3 m
sm973 kg
-27900 Pa + 101000 Pa
+ 500 Pa
NPSH − −+
⋅ 9.81m
23
sm973 kg ⋅
=
6.3mNPSH
A
=
4.7mNPSH
A
=
[ ]
m
g
p
HH
g
pp
NPSH
vapour
loss, pipegeo
barstat,in
A
⋅
−−+
⋅
+ +
=
ρ
ρ
[
( )
0.5
.
ρ
.
V 1
2
where
p
vapour
= The vapour pressure of the fluid at the present temperature [Pa].
The vapour pressure is found in the table ”Physical properties of
water” in the back of the book.
p
abs,tot,in
= The absolute pressure at the inlet flange [Pa].
Figure 2.10: Cavitation.
Figure 2.11: NPSH curve.