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15 1 3 Calculation example 1 for suction head (metric)
Assumptions
Atmospheric pressure:
Suction pipe diameter:
Suction pipe length:
Fittings:
Valves:
Max. flow rate:
Calculation factor:
Safety margin:
Calculation
Step 1: Reading the NPSH graph
The NPSH curves for the corresponding pump type can be found in COMPASS, cf. "Figure
31: Reading NPSH graph, example 1" below. For a max. flow rate of 20 m
value is 2.0 m.
Step 2: Determination of the friction losses
H
= H
/ 100 . (l
+ l
+ l
)
f
f,pipe
pipe
elbow
valve
The friction losses of the fitting and the valve can be read off from "Table 12: Friction loss
for valves and fittings – equivalent length of straight pipe" on page 31, as equivalent
length of straight pipe. For a pipe diameter of 3"the following applies:
ƒ l
= 2.4 m
elbow
ƒ l
= 9.8 m
valve
H
can be read from "Table 10: Friction loss in Schedule 40 steel pipe with Standard
f,pipe
Inside Diameter (SIDR)" on page 30. Since the max. flow rate is Q = 20 m
be interpolated with formula A.3:
From the table, the following value can be read:
Q
/h H
Q
= 18.2 m
= 1.57 m
3
0
f0
H
(Q) = 1.86 m
f,pipe
H
= 1.86 m / 100 . (150 + 2.4 + 9.8) m
f
H
= 3.02 m head
f
BERNT LORENTZ GmbH & Co. KG Kroegerskoppel 7, 24558 Henstedt-Ulzburg, Germany, Tel. +49 (0) 4193 7548 - 0, Fax - 29, www.lorentz.de
All specifi cations and information are given with good intent, errors are possible and products may be subject to change without notice. Pictures may
differ from actual products depending on local market requirements and regulations.
p
= 1.0 bar
0
3"
15 m
1x normal 90° elbow pipe
1x check valve (spring)
Q = 20 m³/h
a = 10.2 m (constant for all applications)
H
= 0.60 m (minimum margin for all applications)
s
3
/h the NPSH
/h, H
3
/h H
= 22.7 m
= 2.39 m
3
1
f1
Step 3: Determination of the vapor pressure
The value for Hv can be read from "Table 11: Vapour pressure" on page 30. At a temper-
ature of 40 °C the allowed suction head is reduced by 0.75 m.
ƒ H
: 0.75 m head
v
Step 4: Determination of the suction head
H = p
. a – NPSH – H
0
H = 1.0 ∙ 10.2 m – 2.0 m – 3.02 m – 0.75 m – 0.6 m
H = 3.83 m head
Result
The result means that the pump can operate at a maximum suction lift of
H = 3.83 m head at 1 bar atmospheric pressure.
must
f,pipe
A.3
Figure 31: Reading NPSH graph, example 1
NPSH [m]
3
3
2
2
1
0
0
2
4
32
– H
– H
f
v
s
6
8
10
12
14
16
Sun. Water.
2
1
18
20
22
24
20
[m³/h]
Life.
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