Page 267 - Water Engineering Hydraulics, Distribution and Treatment

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Solution 2 (SI System):
WMP = water power in metric unit = 9.8066(Q)(H)
= 9.8066(0.0391) (41.15) = 15.7866 kW
(= 8.7b)
BMP = break power in metric unit = WMP∕E
(8.15)
= 15.7866∕0.80 = 19.733 kW
MMP = motor power in metric unit = BMP∕E
motor
(8.16)
= 19.733∕0.90 = 21.92 kW pump 8.2 Pump Characteristics (8.14)
3
where Q = water flow, m /s; and H = head or lift, m.
EXAMPLE 8.3 COMPUTATION OF TOTAL POWER INPUT
Determine the total power input if the electrical input to a motor-pump system is 220 V and 25 amp.
Solution 1 (US Customary System):
INHP = input power in US customary unit = (V)(A)∕746
= (220) (25)∕746 = 7.4hp. (8.8a)
Solution 2 (SI System):
INMP = input power in metric unit = 0.001(V)(A)

= 0.001(220) (25)∕746 = 5.5kW. (8.8b)

8.2.2 Cavitation Equation (8.17b) is a specific speed equation using the
SI or metric units:
Specific speed is an important criterion, too, of safety against
0.5
cavitation, a phenomenon accompanied by vibration, noise, S = 51.7 NQ ∕H 0.75 (SI units) (8.17b)
sv
and rapid destruction of pump impellers. Cavitation occurs
where S is the specific speed, dimensionless; N is the rotative
when enough potential energy is converted to kinetic energy 3
speed, rpm; Q is the capacity, m /s; and H is the net positive
sv
to reduce the absolute pressure at the impeller surface below
inlet or suction head, m.
the vapor pressure of water at the ambient temperature. Water
The specific speed, S, is a number that can be used
then vaporizes and forms pockets of vapor that collapse sud-
to compare the performance of specific pump impellers
denly as they are swept into regions of high pressure. Cavi-
under various conditions of rotative speed, capacity, and head
tation occurs when inlet pressures are too low or when pump
per stage.
capacity or speed of rotation is increased without a compen-
The units used to determine specific speed must be con-
sating rise in inlet pressure. Lowering a pump in relation to
sistent within the numerical system for S to become mean-
its water source, therefore, reduces cavitation.
ingful. Certain general safe limits have been established for
If we replace the head H in Eq. (8.1) by H ,the net S by experiment. The following are examples:
sv
positive inlet or suction head, the difference between the total
inlet head (the absolute head plus the velocity head in the inlet Single-suction pumps with overhung impellers
pipe) and the head corresponding to the vapor pressure of the S < 8,000–12,000
water pumped (table in Appendix 4), we obtain the suction
Single-stage pumps with shaft through eye of impeller
specific speed (S):
S ≤ 7,000–11,000
0.5
S = NQ ∕H 0.75 (US customary units) (8.17a) High-pressure, multistage pumps (single suction)
sv
S < 5,500–7,500
where S is the specific speed, dimensionless; N is the rotative
speed, rpm; Q is the capacity, gpm; and H is the net positive High-pressure, multistage pumps with special first-stage
sv
inlet or suction head, ft. impeller (single suction) S ≤ 7,500–10,000