Environmental Engineering Reference
In-Depth Information
Specific speeds vary between pumps. Although no absolute rule sets
the specific speed for different kinds of centrifugal pumps, the following
rule of thumb for
n
s
can be used:
•
Volute, diffuser, and vertical turbine 500-5000
•
Mixed flow
5000-10,000
•
Propeller pumps
9000-15,000
3.2.9.1 Suction Specific Speed
Suction specific speed
(
n
ss
), another impeller design characteris-
tic, is an index of the suction characteristics of the impeller (i.e., the
suction capacities of the pump) (Wahren, 1997). For practical purposes,
n
ss
ranges from about 3000 to 15,000. The limit for the use of suction
specific speed impellers in water is approximately 11,000. The following
equation expresses
n
ss
:
rpm
NPSHR
×
q
0.5
n
ss
=
(3.18)
0.75
where:
rpm = revolutions per minute.
q
= flow in gpm.
NPSHR = net positive suction head required.
Ideally,
n
ss
should be approximately 7900 for single-suction pumps and
11,200 for double-suction pumps. (The value of
q
in Equation 3.18 should
be halved for double-suction pumps).
3.2.10 affinity laws—Centrifugal Pumps
Most parameters (impeller diameter, speed, and flow rate) determin-
ing the performance of a pump can vary. If the impeller diameter is held
constant and the speed varied, the following ratios are maintained with
no change of efficiency (because of inexact results, some deviations may
occur in the calculations):
q
2
/
q
1
=
d
2
/
d
1
(3.19)
h
2
/
h
1
= (
d
2
/
d
1
)
2
(3.20)
BhP
2
/
BhP
1
= (
d
2
/
d
1
)
3
(3.21)
where:
q
= flow.
d
1
= impeller diameter before change.
d
2
= impeller diameter after change.
