Brochure

4141
NPSH
R
stands for NPSH Required and is an expression of the lowest NPSH
value required for acceptable operating conditions. The absolute pressure
p
abs,tot,in
can be calculated from a given value of NPSH
R
and the fluid vapour
pressure by inserting NPSH
R
in the formula (2.16) instead of NPSH
A
.
To determine if a pump can safely be installed in the system, NPSH
A
and
NPSH
R
should be found for the largest flow and temperature within the
operating range.
A minimum safety margin of 0.5 m is recommended. Depending on the
application, a higher safety level may be required. For example, noise
sensitive applications or in high energy pumps like boiler feed pumps,
European Association of Pump Manufacturers indicate a safety factor S
A
of
1.2-2.0 times the NPSH
3%
.
[ ]
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
The risk of cavitation in systems can be reduced or prevented by:
• Loweringthepumpcomparedtothewaterlevel-opensystems.
• Increasingthesystempressure-closedsystems.
• Shorteningthesuctionlinetoreducethefrictionloss.
• Increasingthesuctionline’scross-sectionareatoreducetheuid
velocity and thereby reduce friction.
• Avoidingpressuredropscomingfrombendsandotherobstaclesin
the suction line.
• Loweringuidtemperaturetoreducevapourpressure.
The two following examples show how NPSH is calculated.