SUMMARY
In order to move and increase the pressure of a fluid, power is consumed by a pump, fan or compressor. The power requirement of pump depends upon many factors, including pump and motor efficiency, differential pressure, density of fluid, viscosity and fluid flow rate. In this article we are going to discuss the relationships to determine the pump power requirement.
DEFINITIONS
Ph
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:
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Hydraulic power of the pump (kW).
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Ps
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:
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Shaft power of the pump (kW).
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Pm
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:
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Required power to the Motor (kW).
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Q
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:
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Volumetric flow of fluid through the pump (m3/h).
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ρ
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Density of the fluid being pumped (kg/m3).
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g
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:
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Gravity (9.81 m/s2).
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h
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:
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Head produced by the pump (m).
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dP
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Differential pressure across the pump (kPa)
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ηp
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Pump efficiency (%).
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ηm
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Motor efficiency (%).
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HYDRAULIC POWER
Hydraulic power, also known as absorbent power, indicates the power imparted on the fluid which is to be pumped to increase the fluid's pressure and velocity. Hydraulic power can be calculated using one of the formulae below:
Units
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Formula
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P - kW
Q - m3/h
ρ - kg/m3
g - m/s2
h - m
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Ph = Qρgh/3.6 x 10^6
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P - kW
Q - m3/hr
dP - kPa
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Ph = QdP/3,600
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P - kW
Q - L/min
dP - kPa
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Ph= QdP/60,000
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P - kW
Q - L/s
dP - kPa
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Ph = QdP/1,000
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SHAFT POWER
The power supplied by the motor to the pump shaft is called "Shaft power". It is defined as the sum of the hydraulic power and power loss due to inefficiencies in the transmission of power from the shaft to the fluid. The shaft power of pump is generally calculated as the ratio of hydraulic power of the pump to the pump efficiency.
Ps = Ph/ηp
Ps = Shaft power
Ph = Hydraulic power of pump (discussed above)
ηp = Pump efficiency
MOTOR POWER
The power consumed by the pump motor in order to turn the pump shaft is called as "motor power". The motor power is the sum of shaft power and power losses while converting electrical energy into kinetic energy. Mathematically motor power is calculated as "shaft power divided by motor efficiency".
Pm = Ps/ηm
Pm = Motor power
Ps = Shaft power
ηm = Motor efficiency
Some OTHER FACTORS WHICH INCREASE REQUIRED POWER
In addition to the motor we can use some other drive features which will increase the power requirement of pump to transfer a perticular fluid.
These are :
1) Belt drives
2) Gear drives
3) VSD's (Variable speed drives)
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