Page 418 - Civil Engineering Formulas

344 CHAPTER TWELVE

the vapor pressure of the water, expressed in meters (feet). Cavitation occurs
when the NPSH is less than the NPSH . The NPSH is found by adding the
R
A
A
term P atm P vapor to the right-hand side of Eq. (12.148) or to the energy
(Bernoulli’s) equation applied to the suction side of the pump. Thus,
2
V 3 P atm P vapor
NPSH A h s h fs
h ms (12.160)
2g
2
NPSH A P s V s z s P atm P vapor (12.161)
2g
where NPSH available net positive suction head, m (ft)
A
2
2
P atm atmospheric pressure, N/m (lb /ft )
f
2
2
P vapor absolute vapor pressure of water, N/m (lb /ft )
f
3
2
specific weight of water, N/m (lb /ft )
f
2
In the computation of NPSH , including the velocity head, V 3 2g at the suction
A
nozzle is somewhat illogical because it is not a pressure available to prevent
vaporization of the liquid. However, in practice this term cancels out because it
is also included in the NPSH required by the pump.
HYDRAULIC TURBINES*
Generation of electricity by hydraulic turbines is gaining popularity because of
the emphasis on global warming and “clean” power. Hydro power is considered
as one of the cleanest, nonpolluting sources of electricity available today,
alongside wind turbines and solar panels. Using the nomenclature below:
D diameter of runner, in (mm)
d jet diameter of impulse turbine, in (mm)
e overall efficiency of turbine e h e m
e hydraulic efficiency (including draft tube)
h
e mechanical efficiency
m
g acceleration of gravity
H net effective head, ft (m)
h head change due to load change, ft (m)
n rpm
n rpm at 1 ft head n H
1
n specific speed n P H 5/4 n 1 P 1
s
P horsepower (kW)
P horsepower at 1 ft head P H 3/2
1
*Marks, Mechanical Engineer’s Handbook, McGraw–Hill.