DAB KC, Instruction For Installation And Maintenance

Page: 19 / 64

ENGLISH
17
5.3.2 Delivery pipe
– Connect the delivery pipe to the delivery mouth
of the pump (Fig. 2).
– The delivery pipe must be at least of the same
diameter as the pump delivery mouth so as to
minimise load losses and decrease the flow rate
and the noise production
.
Fig. 2
5.3.3 Connecting the by-pass
–
The pump must not be run with the interception
valves closed, since in these conditions there
would be an increase in temperature of the liquid
and the formation of vapour bubbles inside the
pump with consequent mechanical damage. If
possible, provide a by-pass circuit or a discharge
leading to a liquid recovery tank..
5.4 Minimum pressure at intake
The minimum pressure at intake “H” in metres of head,
required during operation to avoid cavitation of the
pump, may be calculated using the following formula:
H p
b
x 10,2 – NPSH – H
f
– H
v
– H
s
p
b
Barometric pressure in bar.
(The barometric pressure may be set at 1 bar).
In closed systems p
b
indicates the preload
pressure of the system in bar.
NPSH NPSH (Net Positive Suction Head) expressed
in metres of head (to be read in the NPSH
curves on pages 50-51 at the maximum flow
rate supplied by the pump).
H
f
Load loss in the suction manifolds expressed in
metres of head.
H
v
Vapour pressure in metres of head, see Fig. E
on page 52.
t
m
= temperature of the liquid.
H
s
Safety margin = min. 0.5 metres of head.
If the resulting value “H” is positive, the pump can work
with a max. suction height of “H” metres of head.
If the calculated value “H” is negative, a minimum
suction pressure of “H” metres is required to avoid
cavitation.
For example:
– P
b
= 1bar
– Type of pump: KC – KCV 200
–
Flow rate: 24m³/h.
–
NPSH (from Fig.B on page 50): 1.7 metres of head.
– H
f
= 3.0 metres of head
– Liquid temperature: 20°C
–
H
V
(from Fig. E on page 52): 0.24 metres of head.
–
H = p
b
x 10.2 – NPSH – H
f
– H
V
– H
S
(metres of head).
– H = 1x10.2-1.7-3.0-0.24-0.5 = 4.8 metres of head .
This means that the pump can work with a lift on intake
equal to a maximum of 4.8 metres of head.
Pressure calculated in bar: 4.8 x 0.0981 = 0.47 bar
Pressure calculated in kPa: 4.8 x 9.81 = 47.1 kPa
5.5 Maximum pressure at intake
The actual suction pressure added to the pressure of the
pump running with the delivery valve closed must
always be lower than the max. working pressure.
Delivery mouth
Suction mouth