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Centrifugal Pump Calculations Energy loss in discharge valve throttling

System 1 : Pump E selection ( Exact to reaction )

We have requirement for pump with 68m³/hr flow and 47 meter height

So we had selected Pump E with 60% efficiency.

Here

ϱ = density = 1000 liter / hr

g = gravitional force = 9.81 m/s²

H = Head = 47 meter

Q = Flow of fluid = 68 m³/hr = 68 m³ / 3600 sec

η pump = 60%

η motor = 90%

Motor power = ( 68 x 47 x 1000 x 9.81 ) / ( 1000 x 3600 x 0.6 x 0.9)

= 16.1 kW

System 2 : Pump A selection ( Oversize )

We have same flow and head as previous but efficiency is 50%.

Here

ϱ = density = 1000 liter / hr

g = gravitional force = 9.81 m/s²

H = Head = 76 meter

Q = Flow of fluid = 68 m³/hr = 68 m³ / 3600 sec

η pump = 50%

η motor = 90%

Motor power = ( 68 x 76 x 1000 x 9.81 ) / ( 1000 x 3600 x 0.5 x 0.9)

= 31 kW

Extra power consumption

= Oversize pump A - Exact size pump E

= 31 - 16.1

= 14.9 kW extra power consumption.

Extra cost of pumping

= 8760 hr/yr * 14.9 kW

= 1,30,524 kWh/Annum * Cost of unit power

We can see that new pump installation cost is less than oversize pumping.