17
tank to the pump, the higher will be the value of the vacuum created by the pump, a
condition which consequently increases the risk of cavitation.
This resistance is due to two crucial factors.
Concentrated pressure losses: due to the presence, along the line, of bends, curves,
unions, cocks, filters, etc., all obstructions to the regular flow of water which oppose a
certain resistance, mainly depending on their size and geometric shape.
Distributed pressure losses: due to the friction created between the moving water
and sides of the pipes. The value of these losses is proportional to the length of the
pipe. It increases as the roughness inside the pipe increases and, water flow rate
being equal, increases as the internal diameter of the pipe decreases.
Other pressure losses are due to: the temperature of the water and altitude of the place
where the pump is used with respect to sea level.
When a system is designed, one must therefore remember that the pressure of the water at
the pump inlet will always be lower than that at the beginning of the feeding line.
To prevent cavitation, the minimum difference in level Hz between the level of the
water and the pump must comply with the following relation:
Hz>(NPSHr+C)+H
1
+ H
2
– (H
atm
– H
3
) (m & °C) or (ft & °F)
Where:
NPSHr:
net positive suction head required at the suction port of the pump. The value to
assign to CHX pumps is 6.5 m (21.3 ft)
Hz
= minimum difference in level (positive or negative) between the pump and the water in
the tank;
C
= 0.5m (1.65 ft);
H
1
= pressure losses in the pipes and unions (see tabs.
1
and
2
);
H
2
= pressure losses depending on the temperature of the water (see tab.
3
)
H
atm
= barometric pressure at sea level = 10.33m (33.9 ft)
H
3
= pressure losses due to height above sea level (see tab.
4
)
DATA FOR THE CALCULATIONS
Table 1
Equivalent length of unions, for various dimensions, in m (ft) of steel pipe
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