8 Pumps
292
(8.15)
At atmospheric pressure in the reservoir ( p r = 101.3 kPa) and water at 15 °C
( p v = 1.7 kPa), ( p r − p v )/ρg represents about a 10 m height. As a consequence, the
geometric suction height certainly cannot exceed 10 m.
In order to avoid cavitation, the available NPSH should exceed the required
NPSH. With (8.14), the NPSH required may be noted as
(8.16)
The required NPSH decreases for a given head, if specific speed decreases. With
(8.16) and Ω ss = 3, it follows that NPSH r , for a pump with a low specific speed (e.g.
Ω s = 0.4), is about 2 m (for H m ~ 30 m) to 3 m (for H m ~ 45 m). Taking into account
suction pipe losses, this implies that the suction height may amount to 6–7 m. This
result applies to pumps operating at design flow rate.
8.1.6 Increasing the Acceptable Suction Height
In case of difficulties with the suction height, there are some obvious measures,
as increasing the diameter of the suction pipe in order to diminish the head loss
(increasing NPSH a ) or choosing a pump with a lower specific speed (decreasing
NPSH r ). These measures raise costs: larger ducts and a larger pump.
It may occur that avoiding cavitation is impossible with a pump with the usual
suction specific speed ( Ω ss ≈ 3). Some special designs with a higher suction specific
speed exist. The factor σ has the greatest effect on NPSH r , as emerges from (8.13).
E. g. for σ = 0.1 and λ = 1.1, the factor in (8.13) becomes 0.10 instead of 0.25. This
gives a result better than halving NPSH r , with the corresponding
5.3.
W =
ss
Decreasing the blade load at the inlet implies, in principle, an increase of the number
of rotor blades. But this generates an adverse effect with pumps with a low specific
speed (being the most advantageous ones regarding NPSH r ), as the flow velocity
increases due to increased displacement effect. This may be solved by mounting
a helical rotor upstream of the radial rotor, as shown in Fig. 8.3. This helical rotor is termed an inducer. The helix features a very large blade surface, allowing
generation of a significant force with a small pressure difference between pressure
and suction sides. The pressure build-up within the inducer should be sufficiently
high in order to avoid cavitation within the downstream centrifugal rotor. With an
inducer, W ss may be raised to about 7. The inducer shown in Fig. 8.3 has one blade,
but types exist with several blades. There are also types with the inducer integrated
into the impeller, which then means that the inducer blades continue smoothly into
some of the blades of the post-connected centrifugal rotor.
r
r
=
−
−
−
v
r
a
s
irr ,s
p
p
gNPSH
gH q
.
W W
=
4 / 3
r o
s
ss
m
( gNPSH ) (
/
) ( gH ).
292
(8.15)
At atmospheric pressure in the reservoir ( p r = 101.3 kPa) and water at 15 °C
( p v = 1.7 kPa), ( p r − p v )/ρg represents about a 10 m height. As a consequence, the
geometric suction height certainly cannot exceed 10 m.
In order to avoid cavitation, the available NPSH should exceed the required
NPSH. With (8.14), the NPSH required may be noted as
(8.16)
The required NPSH decreases for a given head, if specific speed decreases. With
(8.16) and Ω ss = 3, it follows that NPSH r , for a pump with a low specific speed (e.g.
Ω s = 0.4), is about 2 m (for H m ~ 30 m) to 3 m (for H m ~ 45 m). Taking into account
suction pipe losses, this implies that the suction height may amount to 6–7 m. This
result applies to pumps operating at design flow rate.
8.1.6 Increasing the Acceptable Suction Height
In case of difficulties with the suction height, there are some obvious measures,
as increasing the diameter of the suction pipe in order to diminish the head loss
(increasing NPSH a ) or choosing a pump with a lower specific speed (decreasing
NPSH r ). These measures raise costs: larger ducts and a larger pump.
It may occur that avoiding cavitation is impossible with a pump with the usual
suction specific speed ( Ω ss ≈ 3). Some special designs with a higher suction specific
speed exist. The factor σ has the greatest effect on NPSH r , as emerges from (8.13).
E. g. for σ = 0.1 and λ = 1.1, the factor in (8.13) becomes 0.10 instead of 0.25. This
gives a result better than halving NPSH r , with the corresponding
5.3.
W =
ss
Decreasing the blade load at the inlet implies, in principle, an increase of the number
of rotor blades. But this generates an adverse effect with pumps with a low specific
speed (being the most advantageous ones regarding NPSH r ), as the flow velocity
increases due to increased displacement effect. This may be solved by mounting
a helical rotor upstream of the radial rotor, as shown in Fig. 8.3. This helical rotor is termed an inducer. The helix features a very large blade surface, allowing
generation of a significant force with a small pressure difference between pressure
and suction sides. The pressure build-up within the inducer should be sufficiently
high in order to avoid cavitation within the downstream centrifugal rotor. With an
inducer, W ss may be raised to about 7. The inducer shown in Fig. 8.3 has one blade,
but types exist with several blades. There are also types with the inducer integrated
into the impeller, which then means that the inducer blades continue smoothly into
some of the blades of the post-connected centrifugal rotor.
r
r
=
−
−
−
v
r
a
s
irr ,s
p
p
gNPSH
gH q
.
W W
=
4 / 3
r o
s
ss
m
( gNPSH ) (
/
) ( gH ).