8.1 Cavitation
289
(see next section). For the study of incipient cavitation, the cavitation number s i
must be introduced. This number significantly changes with incidence. The NPSH
value corresponding to start of erosion is denoted with NPSH e . In practice, the effect on performance is mostly determined with a criterion of 3 % head drop due
to cavitation. We indicate the corresponding value with NPSH 3 . The s -values are
s ≈
e
0.3 and
.
s ≈
3
0.15 NPSH 1 (1 % decrease) is sometimes used as well.
Figure 8.2 sketches the variation of NPSH i , NPSH e and NPSH 3 [2, 3]. The deviating variation of NPSH i at very low flow rate is due to circulating flow at the pump
inlet. Typically, manufacturers only publish curves of NPSH 3 in their catalogues,
as this quantity can easily be determined experimentally. The value of NPSH i may
only be determined reliably the optical way. This is feasible when the pump inlet
is observable through transparent parts, especially manufactured for this purpose.
Measuring of NPSH e cannot be performed easily. It requires long duration operation of a pump and damage observation. A pump user should know that the NPSH e
value may significantly exceed the NPSH 3 value. Individual manufacturers do not
supply precise information concerning this aspect. It is commonly advised to keep a
safety margin of 0.5 m on the value of NPSH 3 . This margin is mostly not sufficient
to prevent erosion. Complete prevention of erosion may require a safety factor of
about 2 on the NPSH 3 value, according to the European association of manufactures
[2], since s e is about twice as high as 3
s (see exercise 8.8.2).
8.1.4 Optimisation of the Inlet of a Centrifugal Pump Rotor
With operation in the design point, coefficients λ and σ in expression (8.9) for
NPSH r have a fixed value. NPSH r may easily get minimised [9]. First, we remark
that the NPSH value cannot be improved by introducing pre-swirl, since λ is larger
than σ. Formula (8.9) takes the form
−
=
+
2
r
2gNPSH
a x
b x,
Fig. 8.2 Variation of NPSH i ,
NPSH e and NPSH 3
289
(see next section). For the study of incipient cavitation, the cavitation number s i
must be introduced. This number significantly changes with incidence. The NPSH
value corresponding to start of erosion is denoted with NPSH e . In practice, the effect on performance is mostly determined with a criterion of 3 % head drop due
to cavitation. We indicate the corresponding value with NPSH 3 . The s -values are
s ≈
e
0.3 and
.
s ≈
3
0.15 NPSH 1 (1 % decrease) is sometimes used as well.
Figure 8.2 sketches the variation of NPSH i , NPSH e and NPSH 3 [2, 3]. The deviating variation of NPSH i at very low flow rate is due to circulating flow at the pump
inlet. Typically, manufacturers only publish curves of NPSH 3 in their catalogues,
as this quantity can easily be determined experimentally. The value of NPSH i may
only be determined reliably the optical way. This is feasible when the pump inlet
is observable through transparent parts, especially manufactured for this purpose.
Measuring of NPSH e cannot be performed easily. It requires long duration operation of a pump and damage observation. A pump user should know that the NPSH e
value may significantly exceed the NPSH 3 value. Individual manufacturers do not
supply precise information concerning this aspect. It is commonly advised to keep a
safety margin of 0.5 m on the value of NPSH 3 . This margin is mostly not sufficient
to prevent erosion. Complete prevention of erosion may require a safety factor of
about 2 on the NPSH 3 value, according to the European association of manufactures
[2], since s e is about twice as high as 3
s (see exercise 8.8.2).
8.1.4 Optimisation of the Inlet of a Centrifugal Pump Rotor
With operation in the design point, coefficients λ and σ in expression (8.9) for
NPSH r have a fixed value. NPSH r may easily get minimised [9]. First, we remark
that the NPSH value cannot be improved by introducing pre-swirl, since λ is larger
than σ. Formula (8.9) takes the form
−
=
+
2
r
2gNPSH
a x
b x,
Fig. 8.2 Variation of NPSH i ,
NPSH e and NPSH 3