CIVIL ENGINEERING FORMULAS

344 CHAPTER TWELVE

the vapor pressure of the water, expressed in meters (feet). Cavitation occurs
when the NPSHAis less than the NPSHR. The NPSHAis found by adding the
term 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,

(12.160)

(12.161)

where NPSHAavailable net positive suction head, m (ft)
Patmatmospheric pressure, N/m^2 (lbf/ft^2 )
Pvaporabsolute vapor pressure of water, N/m^2 (lbf/ft^2 )
specific weight of water, N/m^3 (lbf/ft^2 )

In the computation of NPSHA, including the velocity head, at the suction
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:

Ddiameter of runner, in (mm)

djet diameter of impulse turbine, in (mm)

eoverall efficiency of turbine

ehhydraulic efficiency (including draft tube)

emmechanical efficiency

gacceleration of gravity

Hnet effective head, ft (m)

hhead change due to load change, ft (m)

nrpm

n 1 rpm at 1 ft head

nsspecific speed

Phorsepower (kW)

P 1 horsepower at 1 ft headPH3/2

nPH5/4n 1 P 1

nH

ehem

V^23  2 g

NPSHA

Ps





V^2 s

2 g

zs

Patm





Pvapor



NPSHAhshfs

hms

V^23

2 g



Patm





Pvapor



PatmPvapor

*Marks,Mechanical Engineer’s Handbook, McGraw–Hill.