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Fig. 4
positive slope towards
the pump for the
suction pipe
non-return valve
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. Do not run the pump with closed
valve on the plant.
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.
To reduce noise to a minimum it is advisable to fit
vibration-damping couplings on the intake and
delivery pipes and between the motor feet and the
foundation.
If more than one pump is installed, each pump
must have its own intake pipe. The only exception
is the reserve pump (if envisaged).
9.2 Minimum inlet pressure ( Z1 )
(negative suction head pump)
To have good performances of the pump avoiding
cavitation phenomenon it is necessary to calculate the
suction lift Z1.
Fig. 5
Z1
secure the pump to the
basement by a solid and strong
installation
Hr
pb
NPSH
pV
ENGLISH
To determine the suction level Z1, the following formula
must be applied:
Z1= pb – rqd. N.P.S.H – Hr – correct pV– Hs
where:
Z1 =
difference in level in metres between the
intake mouth of the pump and the free
surface of the liquid to be pumped.
pb =
barometric pressure in mcw of the place of
installation.( chart 1, pag. 132 )
NPSH =
net load at intake of the place of work.
Hr =
load loss in metres on the whole intake
duct.
pV =
vapour tension in metres of the liquid in
relation to the temperature expressed in
°C. ( chart 2, pag. 133 )
Hs =
safety margin = minimum 0.5 metres head.
If the calculated "Z1" is positive, the pump can operate
at a suction lift of maximum "Z1" metres head.
If the calculated "Z1" is negative, than the pump has to
be fitted with a positive lift of at least "Z1" mt.
Ex. : installation at sea level
and fluid at 20°C
required N.P.S.H:
pb :
Hr :
t :
pV :
Z1 :
10,33 – 3,25 – 2,04 – 0,22 – 0,5 = 4,32 approx.
This means that the pump can operate at a suction lift of
maximum 4,32 metres head.
9.3 Minimum suction pressure
(positive suction head pump)
It is important to maintain the sum of the inflow pressure
and that developed by the pump, the latter with feeder
closed, always lower than the maximum pressure rating
(PN) permitted by the pump.
P1max + P2max ≤ PN (fig.6A)
P1max + P2max + P3max ≤ PNHP (fig.6B)
9.4 Minimum nominal capacity
The function of the pump at a lower level than the
minimum permitted nominal capacity may cause
excessive and detrimental overheating of the pump.
For liquid temperatures higher than 40°C, the minimum
capacity should be increased in relation to the
temperature of the liquid (see fig. 6A)
The pump must never operate with the
delivery valve closed.
20
3,25 m
10,33 mcw ( chart 1, pag. 132 )
2,04 m.
20°C
0,22 m. ( chart 2, pag. 133 )
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