93
Hs = Ha - Hf-Cs - NPSH - Fs
Where Hs = Max. practical suction lift in meters
Ha = Atmosphere pressure at water surface (10.33m at sea level)
Hf = Friction losses in pipe, foot valve etc. on suction side
Cs = Saturated vapour pressure of water in meters
NPSH= Net positive suction head of pump including losses at impeller
and Velocity head in metres
Fs = Safety factor taken as 0.6m
Thus, in the equation, the practical limit at sea level can be estimated
considering certain known factors like Hf as 5% of practical suction lift, Cs at a
temp of 28
o
C (Avg) is 0.42m and the available NPSH at 4m for good quality
pumps. Thus say,
Hs = 10.33 - 0.05 Hs - 0.42 - 4 - 0.6
Therefore, 1.05 Hs = 10.33 - 5.02
Hs = 5.31 = 5m
1.05
If a pump installed at an altitude of more than 500m above mean sea level
allowance be made at a rate of one meter pressure drop for every 1000m
increase in altitude.
Net Positive Suction Head (NPSH)
Out from a manufacturing stage a pump needs as its inherent requirement a
certain minimum suction head known as the Net Positive Suction Head. This is
a function of the rate of pumping and design of the pump and has to be supplied
by the manufacturers. The value of net positive suction head should be given by
the manufacturer for a discharge at duty point where the efficiency is
maximum. A comparatively low value of net positive suction head for the same
value of head, discharge and RPM for different make of pumps indicates that a
pump with the lowest net positive suction head is better as it can operate at
greater suction head. From the above equation it would be seen that the static
suction head (Hs) can be increased if the head loss due to friction, velocity head
(Hv) and net positive suction head (NPSH) are low.
