DAB KDN 32-125.1 Instrucciones Para La Instalación Y El Mantenimiento página 31

7.3. Connecting the pipes
Ensure that the metal pipes do not transmit excess force to the pump apertures, so as to avoid causing
deformations or breakages. Any expansion due to the heat of the pipes must be compensated with suitable
precautions to avoid weighing down on the pump. The counterflanges of the pipes must be parallel to the flanges
of the pump.
To reduce noise to a minimum it is advisable to fit vibration-damping couplings on the intake and delivery pipes.
On completing assembly, before connecting the pump to the electricity mains,
it is advisable to check the coupling alignment again.
It is always good practice to place the pump as close as possible to the liquid to be pumped. It is advisable to
use a suction pipe with a diameter larger than that of the intake aperture of the electropump. If the head at intake is
negative, it is indispensable to fit a foot valve with suitable characteristics at intake. Irregular passages between
the diameters of the pipes and tight curves considerably increase load losses. Any passage from a pipe with a
small diameter to one with a larger diameter must be gradual. Usually the length of the passage cone must be 5 to
7 times the difference in diameter.
Check accurately to ensure that the joins in the intake pipe do not allow air infiltrations. Ensure that the gaskets
between flanges and counterflanges are well centred so as not to create resistances to the flow in the pipes. To
prevent the formation of air pockets, the intake pipe must slope slightly upwards towards the pump.
If more than one pump is installed, each pump must have its own intake pipe. The only exception is the reserve
pump (if envisaged) which, as it starts up only in the case of breakdown of the main pump, ensures the operation
of only one pump for each intake pipe.
Interception valves must be fitted upstream and downstream from the pump so as to avoid having to drain the
system when carrying out pump maintenance.
The pump must not be operated with the interception valves closed, as in these conditions there
would be an increase in the temperature of the liquid and the formation of vapour bubbles inside the
pump, leading to mechanical damage. If there is any possibility of the pump operating with the
interception valves closed, provide a by-pass circuit or a drain leading to a liquid recovery tank
(following the requirements of local legislation concerning toxic fluids).
7.4. Calculating the NPSH
To guarantee good operation and maximum performance of the electropump, it is necessary to know the level of
the N.P.S.H. (Net Positive Suction Head) of the pump concerned, so as to determine the suction level Z1. The
curves for the N.P.S.H. of the various pumps may be found in the technical catalogue.
This calculation is important because it ensures that the pump can operate correctly without cavitation phenomena
which occur when, at the impeller intake, the absolute pressure falls to values that allow the formation of vapour
bubbles in the fluid, so that the pump works irregularly with a fall in head. The pump must not cavitate because,
as well as producing considerable noise similar to metallic hammering, it would cause irreparable damage to the
impeller. To determine the suction level Z1, the following formula must be applied:
where:
Z1 = difference in level in metres between the axis of the pump and the free surface of the liquid to be pumped
pb = barometric pressure in mcw of the place of installation (fig. 6 on page 164)
NPSH = net load at intake of the place of work (see characteristic curves in the catalogue)
= load loss in metres on the whole intake duct (pipe - curves - foot valves)
Hr
= vapour tension in metres of the liquid in relation to the temperature expressed in °C
pV
(see fig. 7 on page 164)
Example 1: installation at sea level and fluid at t = 20°C
required N.P.S.H. : 3.25 m
pb : 10.33 mcw
Hr: 2.04 m
t: 20°C
pV: 0.22 m
Z1 10.33 - 3.25 - 2.04 - 0.22 = 4.82 approx.
Example 2: installation at a height of 1500 m and fluid at t = 50°C
required N.P.S.H. : 3.25 m
pb : 8.6 mcw
Hr: 2.04 m
t: 50°C
pV: 1.147 m
Z1 8.6 - 3.25 - 2.04 - 1.147 = 2.16 approx.
Example 3: installation at sea level and fluid at t = 90°C
required N.P.S.H. : 3.25 m
pb : 10.33 mcw
Hr: 2.04 m
t: 90°C
pV: 7.035 m
Z1 10.33 - 3.25 - 2.04 - 7.035 = -1.99 approx.
ENGLISH
Z1 = pb - rqd. N.P.S.H. - Hr - correct pV
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