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7.6
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.
7.7
7.8
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 are given on page 112-114. 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, page 111)
NPSH
= net load at intake of the place of work (page 112-114)
Hr
= load loss in metres on the whole intake duct (pipe - curves - foot valves)
pV
= vapour tension in metres of the liquid in relation to the temperature expressed in °C (see fig.
7, page 111).
Example 1: installation at sea level and fluid at t = 20°C
required N.P.S.H.:
pb :
Hr:
t:
pV:
Z1
Example 2: installation at a height of 1500 m and fluid at t = 50°C
required N.P.S.H.:
pb :
Hr:
t:
pV:
Z1
Example 3: installation at sea level and fluid at t = 90°C
required N.P.S.H.:
pb :
Hr:
t:
pV:
Z1
In the last case, in order to operate correctly the pump must be fed with a positive head of 1.99 - 2 m, that is
the free surface of the water must be 2 m higher than the axis of the pump.
N.B.: it is always good practice to leave a safety margin (0.5 m in the case of cold water) to
allow for errors or unexpected variations in the estimated data. This margin becomes
especially important with liquids at a temperature close to boiling point, because slight
temperature variations cause considerable differences in the working conditions. For
example in the third case, if instead of 90°C the water temperature reaches 95°C at any
time, the head required by the pump would no longer be 1.99 but 3.51 metres.
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.
Z1 = pb - rqd. N.P.S.H. - Hr - correct pV
3,25 m
10.33 mcw (fig. 6, page 111)
2,04 m
20°C
0.22 m (fig. 7, page 111)
10.35 - 3.25 - 2.04 - 0.22 = 4.82 approx.
3,25 m
8,6 mcw (fig. 6, page 111)
2,04 m
50°C
1,147 m (fig. 7, page 111)
8.6 - 3.25 - 2.04 - 1.147 = 2.16 approx.
3,25 m
10.33 mcw (fig. 6, page 111)
2,04 m
90°C
7.035 m (fig. 7, page 111)
10.33 - 3.25 - 2.04 - 7.035 = -1.99 approx.
ENGLISH
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