The axial force of the segmented multistage centrifugal pump is the
superposition of the axial forces of the impellers at all levels. The value of
the axial force is very large, and it can not be completely borne by the
bearings. Therefore, effective balancing measures must be taken.
① The
symmetrical arrangement of impellers makes every two impellers of centrifugal
pump symmetrically installed on the same pump shaft in the opposite direction,
so that the axial force generated by each two impellers counteracts each other.
This scheme is complicated and expensive. When the number of stages is large,
the axial force can not be completely balanced due to different leakage
conditions and different impeller diameters, and the auxiliary balancing device
is often needed.
② Because the impeller of segmented multi-stage centrifugal
pump is installed on the shaft along one direction, the total axial force of the
balance plate device is very large. The balance plate is often installed behind
the last stage impeller to balance the axial force. There is an axial clearance
B between the balance disc 5 and the balance ring 4, and a radial clearance B0
between the balance disc 5 and the balance sleeve 3. The balance chamber behind
the balance disc 5 is connected with the suction of the pump by pipes, so that
the pressure before the radial clearance is the pressure P2 on the back of the
last stage impeller, The pressure behind the balance plate is close to the
pressure PL at the suction port. After the pump starts, the high-pressure liquid
from the last stage impeller of the multistage pump flows through the radial
clearance B0, and the pressure drops to P ｀. Due to the pressure P ｀ > PL,
the pressure P ｀ - PL acts on the balance disc 5. This force is the balance
force, and the direction is opposite to the axial force acting on the
impeller.
When the centrifugal pump is working, when the axial force on the
impeller is greater than the balance force on the balance disk 5, the rotor of
the pump will move towards the suction direction, which reduces the axial
clearance B0 of the balance disk 5, increases the fluid resistance of the
liquid, and thus reduces the leakage. With the decrease of the leakage, the
pressure drop of the liquid flowing through the radial gap B0 decreases, thus
increasing the pressure P ｀ in front of the balance disc 5, that is, increasing
the balance force on the balance disc 5. As the balance disc 5 moves to the
left, the balance force increases gradually. When the balance disc 5 moves to a
certain position, the balance force is equal to the axial force and reaches
balance.
Similarly, when the axial force is less than the balance force, the
rotor will move to the right, and after a certain distance, the axial force and
balance force will reach a new balance. Due to the inertia, the moving rotor
will not stop at the new balance position immediately, but continue to move,
causing the balance to be destroyed, resulting in the condition that the rotor
moves in the opposite direction.
When the water pump is working, the rotor
will never stop at a certain position, but move axially at a certain balance
position. When the working point of the pump changes, the rotor will
automatically move to another balance position for axial movement. Due to the
characteristics of balancing the axial force automatically, the balancing disc
has been widely used.
Balancing device of single stage pump:
① The
pressure on both sides of the impeller can be basically balanced by opening a
balance hole on the impeller. But due to the liquid through the balance hole has
a certain resistance, so there is still a small part of the axial force can not
be completely balanced, and the efficiency of the pump will be reduced. The main
advantage of this method is that the structure is simple, and it is often used
in small centrifugal pumps.
② A balance pipe is installed on the pump body to
connect the liquid at the back of the impeller with the liquid at the pump inlet
through the balance pipe to balance the axial force. This method is superior to
the opening of balance hole, it does not interfere with the liquid flow at the
pump inlet, and the efficiency is relatively high.
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