Page 308 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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268 Chapter 8 Pumping, Storage, and Dual Water Systems

Elevation of pressure energy grade line

v d 2 /2g
Elevation of hydraulic grade line

p /
Pressure d
gauge
p d
Outlet through
Pressure or
vacuum gauge check valve and
p s z d gate valve
t system
0
Datum
Suction energy plane
grade line z s

p /
s
Inlet through strainer
and foot valve (where
2
v /2g needed) from source
s
Figure 8.3 Head Relationships in Pumping Systems
2
2
System head II (z d z s ) (p d > p s > ) (v d >2g v s >2g). For suction lift, p s > and
2
2
z s are negative, and v s >2g is positive. For suction pressure, p s > and v s >2g are positive, and
z s is negative.
Changing the speed of a centrifugal pump will change its operating characteristics
including the water flow. If the speed of a pump (rpm) is changed, its water flow will
change as follows:
Q (Q )(N >N ) (8.2)
1
2
1
2
where
3
Q 2 water flow now, gpm (m /s)
3
Q 1 rated water flow, gpm (m /s)
N 2 pump speed now, rpm
N 1 rated pump speed, rpm.
Changing the speed of a centrifugal pump will also change its head. If the speed of a
pump (rpm) is changed, its head will change as follows:
H (H )(N >N ) 2 (8.3)
1
2
1
2
where
H 2 head now, ft (m)
H 1 rated head, ft (m)
N 2 pump speed now, rpm
N 1 rated pump speed, rpm.