Page 265 - Water Engineering Hydraulics, Distribution and Treatment

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8.2 Pump Characteristics
1/2
where
varies as DN,or N varies
also varies as DN∕60. Hence, H
3∕4
1∕2
as H
∕Q
, and the specific speed becomes
3
Q = water flow, gpm (m /s)
1∕2
3∕4
2
(8.1)
N = NQ
∕H
s
3
Q = rated water flow, gpm (m /s)
1
where
N = pump speed now, rpm
2
N = rated pump speed, rpm
1
N = specific speed, rpm
s
N = speed, rpm
Changing the speed of a centrifugal pump will also
3
Q = capacity, gpm (m /s)
change its head. If the speed of a pump (rpm) is changed, its
head will change as follows:
H = head, ft (m)
H = (H )(N ∕N ) 2 (8.3)
1
2
1
2
To obtain the specific speed based on US customary
units of head and capacity, multiply the specific speed based where
on metric units of head and capacity by 52.
Generally speaking, pump efficiencies increase with H = head now, ft (m)
2
pump size and capacity. Below specific speeds of 1000 units,
H = rated head, ft (m)
1
efficiencies drop off rapidly. Radial-flow pumps perform well
between specific speeds of 1,000 and 3,500 units; mixed- N = pump speed now, rpm
2
flow pumps in the range of 3,500–7,500 units; and axial-flow N = rated pump speed, rpm
1
pumps after that up to 12,000 units. As shown in Eq. (8.1), for
agiven N, high-capacity, low-head pumps have the highest On changing the speed of a centrifugal pump its power
specific speeds. For double-suction pumps, the specific speed requirement will change as follows:
is computed for half the capacity. For multistage pumps, the 3
head is distributed between the stages. In accordance with P = (P )(N ∕N ) (8.4)
1
2
2
1
Eq. (8.1), this keeps the specific speed high and with it, the
where
efficiency.
Changing the speed of a centrifugal pump will change
its operating characteristics including the water flow. If the P = power requirement now, hp (kW)
2
speed of a pump (rpm) is changed, its water flow will change P = rated power requirement, hp (kW)
1
as follows: N = pump speed now, rpm
2
Q = (Q )(N ∕N ) (8.2) N = rated pump speed, rpm
1
2
1
2
1
EXAMPLE 8.1 DETERMINATION OF PUMP’S NEW POWER REQUIREMENT
Changing the speed of a centrifugal pump will change its operating characteristics, including the power requirement.
Determine the new power requirement, if
P = rated head = 20 hp = 14.914 kW
1
N = pump speed now = 1,200 rpm
2
N = rated pump speed = 1,425 rpm
1
Solution 1 (US Customary System):
P = (P )(N ∕N ) 3
1
2
2
1
3
= (20 hp)(1,200∕1,425) = 12 hp.
Solution 2 (SI System):
P = (P )(N ∕N ) 3
2
1
2
1
3
= (14.914 kW)(1,200∕1,425) = 8.948 kW.