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8.2 Pump Characteristics 271

EXAMPLE 8.3 COMPUTATION OF TOTAL POWER INPUT
Determine the total power input if the electrical input to a motor-pump system is 220 volts and
25 amps.

Solution 1 (U.S. Customary System):
INHP input power in U.S. customary unit (V)(A)/746 (8.8)
(220)(25) 746 7.4 hp

Solution 2 (SI System):
INMP input power in metric unit 0.001 (V)(A) (8.8a)
0.001 (220)(25) 746 5.5 kW

8.2.2 Cavitation
Specific speed is an important criterion, too, of safety against cavitation, a phenomenon
accompanied by vibration, noise, and rapid destruction of pump impellers. Cavitation oc-
curs when enough potential energy is converted to kinetic energy to reduce the absolute
pressure at the impeller surface below the vapor pressure of water at the ambient tempera-
ture. Water then vaporizes and forms pockets of vapor that collapse suddenly as they are
swept into regions of high pressure. Cavitation occurs when inlet pressures are too low or
when pump capacity or speed of rotation is increased without a compensating rise in inlet
pressure. Lowering a pump in relation to its water source, therefore, reduces cavitation.
If we replace the head H in Eq. 8.1 by H , the net positive inlet or suction head,
sv
namely, the difference between the total inlet head (the absolute head plus the velocity
head in the inlet pipe), and the head corresponding to the vapor pressure of the water
pumped (table in Appendix 4), we obtain the suction specific speed (S):
0.75
0.5
S NQ /H sv (U.S. Customary Units) (8.11)
where S is the specific speed, dimensionless; N is the rotative speed, rpm; Q is the capac-
ity, gpm; H is the net positive inlet or suction head, ft.
sv
Equation 8.11a is a specific speed equation using the SI or metric units
0.5
S 51.7 NQ /H 0.75 (SI Units) (8.11a)
sv
where S is the specific speed, dimensionless; N is the rotative speed, rpm; Q is the capac-
3
ity, m /s; H is the net positive inlet or suction head, m.
sv
The specific speed, S, is a number that can be used to compare the performance of spe-
cific pump impellers under various conditions of rotative speed, capacity, and head per stage.
The units used to determine specific speed must be consistent within the numerical
system for S to become meaningful. Certain general safe limits have been established for
S by experiment. The following are examples:
Single-suction pumps with overhung impellers S 8,000–12,000
Single-stage pumps with shaft through eye of impeller S 7,000–11,000
High-pressure, multistage pumps (single suction) S 5,500–7,500
High-pressure, multistage pumps with special first-stage impeller S 7,500–10,000
(single suction)
2
H p v 2g p , where p is the absolute pressure, v , the velocity of
s
sv
s
s
w
s
the water in the inlet pipe, and p , the vapor pressure of the water pumped, with being
w