Environmental Engineering Reference
In-Depth Information
h
1
= head before change.
h
2
= head after change.
BhP
1
= brake horsepower before change.
BhP
2
= brake horsepower after change.
The relationships for speed (
n
) changes are as follows:
q
2
/
q
1
=
n
2
/
n
1
(3.22)
h
2
/
h
1
= (
n
2
/
n
1
)
2
(3.23)
BhP
2
/
BhP
1
= (
n
2
/
n
1
)
3
(3.24)
where:
n
1
= initial rpm.
n
2
= changed rpm.
Example 3.9
Problem:
Change an 8-in.-diameter impeller for a 9-in.-diameter impel-
ler, and find the new flow (
q
), head (
h
), and brake horsepower (
BhP
)
when the 8-in.-diameter data are:
q
1
= 340 rpm
h
1
= 110 ft
BhP
1
= 10
Solution:
Data for the 9-in. impeller diameter would be as follows:
q
2
= 340 × 9/8 = 383 gpm
h
2
= 110 × (9/8)
2
= 139 ft
BhP
2
= 10 × (9/8)
3
= 14
3.2.11 net Positive suction head
In Volume I of this handbook, we referred to the
net positive suction
head required
(NPSHR); also important in pumping technology is
net
positive
suction head
(NPSH) (Lindeburg, 1986; Wahren, 1997). NPSH is
different from both suction head and suction pressure. This important
point tends to be confusing for those first introduced to the term and
to pumping technology in general. When an impeller in a centrifugal
pump spins, the motion creates a partial vacuum in the impeller eye.
The NPSHA is the height of the column of liquid that will fill this partial
vacuum without allowing the vapor pressure of the liquid to drop below
its flash point; that is, this is the NPSH required (NPSHR) for the pump
to function properly.
