Page 307 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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8.2 Pump Characteristics 267
Today most water and wastewater pumping is done by either centrifugal pumps or
propeller pumps. These are usually driven by electric motors, less often by steam turbines,
internal combustion engines, or hydraulic turbines. How the water is directed through the
impeller determines the type of pump. There is (1) radial flow in open- or closed-impeller
pumps, with volute or turbine casings, and single or double suction through the eye of the
impeller; (2) axial flow in propeller pumps; and (3) diagonal flow in mixed-flow, open-im-
peller pumps. Propeller pumps are not centrifugal pumps. Both centrifugal pumps and pro-
peller pumps can be referred to as rotodynamic pumps.
Open-impeller pumps are less efficient than closed-impeller pumps, but they can pass
relatively large debris without being clogged. Accordingly, they are useful in pumping
wastewaters and sludges. Single-stage pumps have but one impeller, and multistage pumps
have two or more, each feeding into the next higher stage. Multistage turbine well pumps
may have their motors submerged, or they may be driven by a shaft from the prime mover
situated on the floor of the pumping station.

8.2 PUMP CHARACTERISTICS

A centrifugal pump is defined by its characteristic curve, which relates the pump head
(head added to the system) to the flow rate. Pumping units are chosen in accordance with
system heads and pump characteristics. As shown in Fig. 8.3, the system head is the sum
of the static and dynamic heads against the pump. As such, it varies with required flows
and with changes in storage and suction levels. When a distribution system lies between
pump and distribution reservoir, the system head also responds to fluctuations in demand.
Pump characteristics depend on pump size, speed, and design. For a given speed N in rev-
olutions/min, they are determined by the relationships between the rate of discharge Q,
3
usually in gpm (or L/m or m /s) and the head H in ft (or m), the efficiency E in %, and the
power input P in horsepower (or kilowatt). For purposes of comparison, pumps of given
geometrical design are characterized also by their specific speed N , the hypothetical speed
s
of a homologous (geometrically similar) pump with an impeller diameter D such that it
will discharge 1 gpm (3.78 L/m) against a 1-ft (0.30-m) head. Because discharge varies as
2 1 2
the product of area and velocity, and velocity varies as H 1 2 , Q varies as D H . But veloc-
ity varies also as DN>60. Hence, H 1 2 varies as DN, or N varies as H 3 4 >Q 1 2 , and the
specific speed becomes
N NQ 1 2 >H 3 4 (8.1)
s
where
N s specific speed, rpm
N speed, rpm
3
Q capacity, gpm (m /s)
H head, ft (m).
To obtain the specific speed based on U.S. customary units of head and capacity, multiply
the specific speed based on metric units of head and capacity by 52.
Generally speaking, pump efficiencies increase with pump size and capacity. Below
specific speeds of 1,000 units, efficiencies drop off rapidly. Radial-flow pumps perform
well between specific speeds of 1,000 and 3,500 units; mixed-flow pumps in the range of
3,500 to 7,500 units; and axial-flow pumps after that up to 12,000 units. As shown in Eq. 8.1,
for a given N, high-capacity, low-head pumps have the highest specific speeds. For double-
suction pumps, the specific speed is computed for half the capacity. For multistage pumps,
the head is distributed between the stages. In accordance with Eq. 8.1, this keeps the spe-
cific speed high and with it, the efficiency.