Brochure

H
Q
•
•
•
•
•
•
•
•
•
•
•
•
Referencekurve
3% kurve
Målt løftehøjde
NPSH
3%
NPSH
A
109109
6.3.7 Barometric pressure
In practise the inlet pressure is measured as a relative pressure in relation to
the surroundings. It is therefore necesarry to know the barometric pressure
at the place and time where the test is made.
6.3.8 Calculation of NPSH
A
and determination of NPSH
3%
NPSH
A
can be calculated by means of the following formula:
(6.1)
(6.2)
(6.3)
(6.4)
(6.5)
12
HHH − =
( ) ( )
12
''
loss, friction,2 loss, friction,1
HHHHH − − +=
Dynamic pressureStatic pressure
Geodetic pressure
g
UU
g
pp
zzH
⋅
−
+
⋅
−
+ − =
2
2
1
2
212
12
ρ






−
⋅
+








+
⋅
+
−






+
⋅
+








+
⋅
+ =
2
1
1
1
1
2
2
2
2
2
2
'
'
'
'
2
'
'
'
'
loss, friction,1 M
M
loss, friction, 2 M
M
H
g
U
z
g
p
z
H
g
U
z
g
p
zH
ρ
ρ
g
p
Hz
g
Vpp
NPSH
vapour
loss, friction, geo
barstat,in
A
⋅
− − +
⋅
⋅ ⋅ + +
=
ρρ
ρ
2
1
5.0
p
stat,in
= The measured relative inlet pressure
p
bar
= Barometric pressure
V
1
= Inlet velocity
z
geo
= The pressure sensor’s height above the pump
H
loss,friction
= The pipe loss between pressure measurement and pump
p
vapour
= Vapour pressure (table look-up)
ρ = Density (table look-up)
The NPSH
3%
value can be found by looking at how the head develops during
the test, see figure 6.14. An NPSH
3%
value is determined by the NPSH
A
value
which is calculated from the closest data point above the 3% curve.
6.4 Measurement of force
Measurement of axial and radial forces on the impeller is the only reliable
way to get information about the forces’ sizes. This is because these forces
are very dicult to calculate precisely since this requires a full three-dimen-
sional numerical simulation of the flow.
Figure 6.14: Determination of NPSH
3%
.
Reference curve
3% curve
Measured head
NPSH
3%
NPSH
A