DAB KV 10/2, Instruction For Installation And Maintenance

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ENGLISH
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7.5 It is always good practice to place the pump as close as possible to the liquid to be pumped.
The internal diameter of the pipes must never be smaller than that of the apertures of the pump. If
the head at intake is negative, it is indispensable to fit a foot valve with suitable characteristics at
intake.
Fig. D. For suction depths of over four metres or with long horizontal stretches it is
advisable to use an intake pipe with a diameter larger than that of the intake aperture of the pump.
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.
Fig. D.
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.
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 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.
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 97-98-99. This calculation is important because it ensures that the pump can operate orrectly
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:
Z1 = pb - rqd. N.P.S.H. - Hr - correct pV
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 (fig. 3, page 96)
NPSH = net load at intake of the place of work (fig. 5-6-7. page 97-98-99)
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. 4, page 96) .
Example 1: installation at sea level and fluid at t = 20°C
3,25 m
pb : 10.33 mcw (fig. 3, page 96)
Hr: 2,04 m
t: 20°C
pV: 0.22
m (fig. 4, page 96)
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 (fig. 3, page 96)
Hr: 2,04 m
t: 50°C
pV: 1,147
m (fig. 4, page 96)
Z1 8,6 - 3,25 - 2,04 - 1,147 = 2,16 approx.