15
and when pressure is falling in the suction line.
During the return run the plungers generate a depression which flow the water into the
pumping chambers; as higher is the resistance encountered by the water while running from
the reservoir to the pump, as bigger is the value of the depression created by the pump.
Consequently the cavitation risk also increases.
This resistance is due to two decisive factors.
Concentrated pressure losses: due to the presence, along the line, of elbows, curves,
fittings, taps, filter, etc. Being an obstacle to the regular water flow, they offer a re-
sistance mainly dependent on their size and shape.
Distributed pressure losses: due to the friction generated between the moving water
and the pipes sides. The value of these losses is proportional to the pipe length, rises
when the internal roughness of the pipe increases and, at the same water capacity,
increases when the internal diameter of the pipe decreases.
Other pressure losses are due to: water temperature, difference in height between the pump
and the water level in the tank
To project the plumbing should be considered that the pump inlet pressure is ever lower
than the pressure at the beginning of the suction line.
To prevent the cavitation, the minimum difference in height Hz between the pump and
the water level in the tank must respect the following condition:
Hz > (NPSHr+C)+H
1
+ H
2
– (H
atm
– H
3
) (m & °C) or (ft & °F)
Where:
NPHSr: net positive suction head required. The value of NPHSr of the pump can be ob-
tained by the following table 1:
Hz = minimum difference in height (positive or negative) between the pump and the water
level in the tank
C = 0,5m (1,65 ft);
H
1
= hydraulic losses due to pipes and fittings (see tables 2 and 3);
H
2
= hydraulic losses depending on the water temperature (see table 4)
H
atm
= atmospheric pressure at the see level = 10,33m (33,9 ft)
H
3
= hydraulic loss depending on elevation above see level (see table 5)
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