29
Sun. Water.
Life.
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.
v150106
P
0
H
H
f
H
v
15 Appendix
15 1 Inlet pressure
The maximum suction head is limited by the local
air pressure, the water temperature of the medium,
the pipe losses and the NPSH value of the pump
WARNING – Running the pump with
a suction lift which is outside of the
pumps specification will lead to cav-
itation and damage the pump
Cavitation is not covered by
warranty!
15 1 1 Minimum inlet pressure and NPSH
If the suction pressure in the pump is lower than the
vapor pressure of the fluid, cavitation will occur. Cavitation
creates noise and will damage the pump. Damage due to
cavitation is not subject to warranty. To avoid cavitation,
the pressure of the fluid must be maintained above its
vapor pressure at all points as it passes through the pump.
The maximum possible suction head (H) can be calculated
with the following formula A.1:
H = p
0
. a – NPSH – H
f
– H
v
– H
s
A.1
with:
p
0
= atmospheric pressure, typically 1 bar, “15.1
Inlet pressure” on page 29; in a closed system,
p
0
means system pressure [bar]
a
= calculation factor = 10.2 m/33.5 ft head
NPSH
= net positive suction head
(It can be read out from the point of possible
Figure 30: Schematic view of an open pump system
15 1 2 Inlet/suction pipeline
The target is to reduce the inlet pressure losses to
a minimum
In order to achieve maximum suction lift and reduce the
risk of cavitation, the inlet suction pipe diameter must be
sized with great care. Use smooth pipes and as few bends
in pipes as possible.
Size the pipe diameters large enough Consult
pressure loss tables to size the pipeline correctly
(see “Table 11: Vapour pressure” on page 30,
and “Table 10: Friction loss in Schedule 40 steel
pipe with Standard Inside Diameter (SIDR)” on
page 30)
Avoid bends! When they cannot be avoided,
calculate their impact properly (see “Table 12:
Friction loss for valves and fittings – equivalent
length of straight pipe” on page 31)
Table 9: Atmospheric pressure
Height
Pressure
[m]
[ft]
[bar]
0
0
1.013
250
820
0.984
500
1,640
0.955
750
2,460
0.926
1,000
3,280
0.899
1,250
4,100
0.872
1,500
4,920
0.846
Height
Pressure
[m]
[ft]
[bar]
1,750
5,740
0.820
2,000
6,560
0.795
2,250
7,380
0.771
2,500
8,200
0.747
2,750
9,020
0.723
3,000
9,840
0.701
max. flow rate shown on the NPSH curve; this
curve can be found in COMPASS and in the
COMPASS sizing report.)
H
f
= Pipeline friction loss at the inlet pipe
(Must be determined individually with formula
A.2. Depends on pipe length and diameter,
the amount of valves and fittings, …, which is
combined in
l
pipe
)
H
f
= H
f,pipe
/ 100 . l
pipe
A.2
H
v
= vapour pressure of the water
H
s
= safety margin = min. 0.6 m/2 ft delivery head
If the calculated result “H” is positive, it represents the
max. allowed suction head for the calucated application.
In case the calculated result H is negative, it represents the
necessary positive feed height above the pump inlet.