pag. 8
7 INSTALLATION
7.1 SUCTION AND INFLOW CONDITIONS
(NPSH = Net Positive Suction Head)
NPSH of system
(available NPSH)
In order to ensure that pump operation is free from cavitation, it is essential to observe the maximum
permitted suction lift
ha geo max
or the minimum allowable head
hc geo min.
NPSH of pump
(required NPSH)
The centrifugal pumps can operate correctly only if vapour has not formed inside. For this reason
the static head at the reference point for the NPSH is the centre of the impeller, that is the point of
intersection of the pump shaft axis with the vertical plane that passes through the external points
of the blade inlet corners.
NPSHnec. is the value required by the pump, expressed in metres, obtained from the performance
curve. In practice 0.5 m should be added to this value as a safety margin.
7.2 PIPING
In order to prevent the creation of harmful stresses, the suction and discharge pipes must be con-
nected to the pump ports without the use of force. These pipes must also be supported independently
avoiding causing stresses on the pump. The internal diameter must be the same size as the pump
connections. It must in any case not be smaller to avoid head loss and/or poor performances. Always
use elbows with large radius. If the pipe diameter changes along the line, use reduction cones,
choosing the ones that are most suitable for avoiding the formation of air pockets (Pict. 1).
Pict. 1
NO
YES
The suction pipe must be as short as possible and rise as it moves towards the pump if it is sucking
from a tank, if on the other hand the pump is below the level of the liquid, the pipe should descend
slightly. If the pump is used for transporting hot liquids, fit expansion joints to compensate any
expansion of the piping. The maximum velocity of the liquid in the suction pipe must not be greater
than 3 m/s. Velocities between 1 and 2 m/s are recommended.
The suction pipe must be designed in such a way as to prevent air from entering the pump. For
this reason, when sucking from a tank located at a lower level, the pipe must reach below the free
surface of the liquid. Avoid creating obstacles which could increase suction losses disrupting smooth
fluid flow. Make sure that there are no restrictions, sharp turns or tight elbows on the discharge line,
since these increase disturbance near the pump.
7.3 ELECTRICAL CONNECTION
Make the electrical connection only after the hydraulic connection has been completed; set up the
motor control system in conformity with the technical standards and regulations in force (EN 60204-
1): in particular a manual electric power switch must be installed with adequate current switching
capacity; devices for overcurrent and overload protection (e.g. fuses, automatic switches, etc.) must
also be fitted, plus, if necessary, a device to prevent accidental restarting.
Check that the main frequency and voltage and the available power are suitable for the motor
installed. All the material used for the electrical connection (cables, cable clamps, switches and
shielding) must have a suitable level of protection for the environment in which it is installed. Be sure
to use cables of sufficient cross-section for the current shown on the motor plate so as to prevent
them from overheating. Before doing anything else, make the motor's earth connection, using the
terminal on the motor and a cable of sufficient cross-section. The cables may be connected to the
terminal board using either a delta or star arrangement. Follow the data given on the motor plate
for the main voltage, as shown in the diagram in pict.2; ensure that the terminals are clean and tight
and not under stress.
When starting, the motor's current absorption increases briefly to 5-6 times the nominal value. If the
mains supply is unable to sustain this increase in absorption, use a star-delta starter or other kind
of device (e.g. an autotransformer).