3.1.2 Centrifugal pump losses and efficiency
Centrifugal pump losses and efficiency
Centrifugal pump losses and efficiency are the sum of mechanical and hydraulic losses in the pump. The shaft power P supplied is defined as the product of rotary moments and angular velocity at the pump’s shaft coupling. A part of the shaft power is used in overcoming friction resistance in bearings and seals. Figure 3.7 gives a schematic diagram of the power flow through a pump.
This loss of power does not generally affect the pump medium but is transformed into heat which is given off to the environment. The associated efficiency is called the mechanical efficiency and is designated by ηm.
The internal power (Pi) is thus the shaft power which remains from that supplied after the mechanical losses have been overcome. The internal power is applied to the pumped liquid and causes it to change state. The internal power is associated with the previously mentioned technical work according to the following relationship.
where ṁ denotes the mass flow through the pump
A part of the internal power supplied is converted inside the pump to an undesired increase in the internal energy of the pumped medium. This loss of power which appears as an increase in temperature of the liquid, is traditionally divided into three parts. The first pan comprises so-called impeller friction losses, i.e. such losses which occur through the effect of friction between the outer sides of the impeller and the surrounding medium. The associated efficiency is called impeller friction efficiency and denoted
The second part consists of flow losses in the pump casing and blade ducts. The size of these losses (blade losses) expresses how well the pump’s hydraulic flow walls are designed. The associated efficiency is called hydraulic efficiency or blade efficiency and designated
The third part is caused by inner leakage which cannot be ignored. The medium which has already passed the impeller leaks back and causes further blade work. These losses
are usually characterised by the internal volumetric efficiency In the event that external leakage occurs an external volumetric efficiency is also formed in the same manner.
There now remains of the power flow, the useful power.
where the volume flow through the pump, Q = ṁ/ρ is introduced in the last line
The following equation holds for the overall efficiency of the pump.
where
ρ = density (kg/m³)
Q = volume flow (m³/s)
g = acceleration due to gravity (m/s²)
H = delivery head (m)
P = shaft power (W)
Or expressed with the help of partial efficiencies
