Environmental Engineering Reference
In-Depth Information
s peCifiC s peed
Specific speed ( N s ) refers to the speed of an impeller when pumping 1 gpm of liquid
at a differential head of 1 ft. Use the following equation for specific speed, where H
is at the best efficiency point:
05
.
rpm
×
Q
N
s =
(14.19)
075
.
H
where
rpm = Revolutions per minute.
Q = Flow (gpm).
H = Head (ft).
Pump specific speeds vary between pumps. No absolute rule sets the specific
speed for different kinds of centrifugal pumps; however, the following N s ranges are
quite common:
Volute, diffuser, and vertical turbine = 500-5000
Mixed flow = 5000-10,000
Propeller pumps = 9000-15,000
Note: The higher the specific speed of a pump, the higher its ef iciency.
POSITIVE-DISPLACEMENT PUMPS
The clearest differentiation between centrifugal (kinetic) pumps and positive-dis-
placement pumps can be made based on the method by which the pumping energy
is transmitted to the liquid. Centrifugal (kinetic) pumps rely on a transformation
of kinetic energy to static pressure. Positive-displacement pumps, on the other
hand, discharge a given volume for each stroke or revolution (that is, energy is
added intermittently to the fluid flow). The two most common forms of positive-
displacement pumps are reciprocating action pumps (which use pistons, plung-
ers, diaphragms, or bellows) and rotary action pumps (using vanes, screws, lobes,
or progressing cavities). No matter which form is used, all positive-displacement
pumps act to force liquid into a system regardless of the resistance that may oppose
the transfer. The discharge pressure generated by a positive-displacement pump is,
in theory, infinite.
The three basic types of positive-displacement pumps that apply to this discussion
are
Reciprocating pumps
Rotary pumps
Special-purpose pumps (peristaltic or tubing pumps)
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