8 Pumps
316
impeller pumps, in the form as used with slurry pumps (Fig. 8.25, right) may even
be used up to 1000 · 10
−6
m
2
/s.
8.8 Exercises
8.8.1 Looking up Pump Characteristics
With the following web site, pump manufacturers may be identified: http://www.
pumps-directory.com. Search for a particular type of pump. With this information,
one may consult the web site of a manufacturer. Some manufacturers provide information on the characteristics of some of their products. One may learn on practical
aspects of pumps with the information provided by manufacturers. One manufacturer offers free access to a pump handbook with material that complements very
well the present chapter on pumps and the previous chapter on similitude. Search
for Grundfos pump handbook. The result is: http://net.grundfos.com/doc/webnet/
mining/_downloads/pump-handbook.pdf.
8.8.2 Verification of an NPSH-Value
On the web sites of some pump manufacturers, characteristic curves may be found,
including the variation of the required NPSH as a function of the flow rate. Make
the choice of a pump and verify the required NPSH at the design flow rate against
the value obtained from W =
ss
3 according to Eq. (8.16). In the Grundfos pump
handbook, an example of a set of pump characteristics is discussed at the very beginning of the book in Sect. 1.1.2. We may read the data for the flow rate at optimum
efficiency. These are: flow rate
/ ,
=
3
Q 70m h head
,
=
H 42m power
,
P 10kW
=
efficiency
%,
.
h =
=
r
80 NPSH
3m First, we may verify the efficiency. The rotational speed is not mentioned, but we may guess the value by considering the possible range of specific speed of single-stage centrifugal pumps, since the head is a
typical value of such a pump. Verify that n 2900 rpm
=
leads to
.
W =
s 0.463 We
take this value, since the rotational speed of
1450
n
rpm
=
leads to an unlikely
low value. Calculate the required NPSH with Eq. (8.16) for W =
ss
3. The result is
.
=
r
NPSH
3.478m This means that about 0.5 m safety margin has to be provided
with respect to the value obtained by the manufacturer from measurement of 3 %
head drop. This value is the typical safety margin advised by manufacturers. Since
W =
ss 3 is the optimum value for incidence-free flow, normally a safety margin of
0.5 m cannot be sufficient for avoiding cavitation erosion. It looks wise to provide
an NPSH value of 4 m for the considered case, but a safety factor of 2, as recommend by Europump [2], seems exaggerated.
316
impeller pumps, in the form as used with slurry pumps (Fig. 8.25, right) may even
be used up to 1000 · 10
−6
m
2
/s.
8.8 Exercises
8.8.1 Looking up Pump Characteristics
With the following web site, pump manufacturers may be identified: http://www.
pumps-directory.com. Search for a particular type of pump. With this information,
one may consult the web site of a manufacturer. Some manufacturers provide information on the characteristics of some of their products. One may learn on practical
aspects of pumps with the information provided by manufacturers. One manufacturer offers free access to a pump handbook with material that complements very
well the present chapter on pumps and the previous chapter on similitude. Search
for Grundfos pump handbook. The result is: http://net.grundfos.com/doc/webnet/
mining/_downloads/pump-handbook.pdf.
8.8.2 Verification of an NPSH-Value
On the web sites of some pump manufacturers, characteristic curves may be found,
including the variation of the required NPSH as a function of the flow rate. Make
the choice of a pump and verify the required NPSH at the design flow rate against
the value obtained from W =
ss
3 according to Eq. (8.16). In the Grundfos pump
handbook, an example of a set of pump characteristics is discussed at the very beginning of the book in Sect. 1.1.2. We may read the data for the flow rate at optimum
efficiency. These are: flow rate
/ ,
=
3
Q 70m h head
,
=
H 42m power
,
P 10kW
=
efficiency
%,
.
h =
=
r
80 NPSH
3m First, we may verify the efficiency. The rotational speed is not mentioned, but we may guess the value by considering the possible range of specific speed of single-stage centrifugal pumps, since the head is a
typical value of such a pump. Verify that n 2900 rpm
=
leads to
.
W =
s 0.463 We
take this value, since the rotational speed of
1450
n
rpm
=
leads to an unlikely
low value. Calculate the required NPSH with Eq. (8.16) for W =
ss
3. The result is
.
=
r
NPSH
3.478m This means that about 0.5 m safety margin has to be provided
with respect to the value obtained by the manufacturer from measurement of 3 %
head drop. This value is the typical safety margin advised by manufacturers. Since
W =
ss 3 is the optimum value for incidence-free flow, normally a safety margin of
0.5 m cannot be sufficient for avoiding cavitation erosion. It looks wise to provide
an NPSH value of 4 m for the considered case, but a safety factor of 2, as recommend by Europump [2], seems exaggerated.