Basically four principles involved in pumping water:
Atmospheric pressure.
Positive displacement
Centrifugal force
Movements of columns of water caused by the difference in specific gravity.
Pump Characteristic:
Terminology:
Capacity:
It is volume of water pumped per unite time. It is measured in liters seconds.
Static suction head:
It is vertical distance from center of pump to free level of water to be pumped.
Static discharged Head:
It is vertical distance from the center line of the pump to discharge water level.
Total Suction Head:
It is vertical distance from the center line of the pump to free level of the liquid to be pumped minus, all friction looses in suction pipe and fitting plus any pressure head existing on the suction supply.
Total Discharge head:
It is sum of static discharge head, friction and exit looses in discharge piping plus the velocity head and pressure head at the point of discharge.
Friction head:
It is the equivalent head expressed in meters of water required to over come the friction caused by flow through the pipe and pipe fittings.
Pressure head:
It is pressure expressed in meters of water in a closed vessel from which pumps takes its suction or against which the pump discharges.
Hp = P/w
Hp – Pressure head, m
P – Pressure inside vessel kg/m2
W – Specific weight of water kg/m3
Velocity Head:
It is the pressure expressed in meters of water required to create the velocity of flow.
Hv = V^2 / 2g
Hv = velocity head, m
V = velocity of water through the pipe m/sec.
g = Acceleration due to gravity, m/sec2. [g = 9.81m/sec.]
Net positive suction Head [NPSH]:
It is the total suction head minus the vapor pressure of water at the pumping temperature both expressed in meter
Maximum practical Suction lift of pump:
Maximum practical Suction lift of pump can be computed by –
Hs = Ha – Hf – es – NPSH – Fs
Where,
Hs = maximum practical suction lift or elevation of water surface, m
Ha = Atmospheric press at water surface [10.33 m at sea level]
Hf = Friction looses in strainer pipe fitting and values on suction line, m.
es = Saturated vapor pressure of water, m .
NPSH = net positive suction head.
es = Factor of safety [0.66m]
Water horse power [WHP]:
It is the theoretical horse power required for pumping.
Discharge in liters per sec. x total head in m.
WHP = ———————————————————-
75
OR,
Discharge in cubic meters per sec. x Total head in m.
= ———————————————————
273
Shaft horse power [WHP]:
It is power required at pump shaft.
WH
SHP = —————————–
Pump efficiency
[SHP is always greater than WHP]
Efficiency:
It is the ratio of power output to power input.
Water WHP
Pump efficiency = —————————
SHP
Brake Horse power:
It is actual horse power required to be supplied by the engine or electric motor for driving the pump.
With direct driven pump [ drive efficiency 100%]
BHP = SHP
With belt or other indirect drives.
WHP
BHP = —————————————————–
Pump efficiency x drive efficiency
WHP
3. H.P. Input to, electric motor = ———————————————
Pump efficiency x drive efficiency x motor efficiency
BHP x 0.746
4. Kilowatt input to electric power = ———————————-
Motor efficiency
