The volume loss of centrifugal pump includes leakage loss of sealing ring, leakage loss of balancing mechanism and leakage loss of interstage.
1. Leakage loss of sealing ring
At the inlet of the impeller, there is a sealing ring, when the pump is working, because there is a pressure difference on both sides of the sealing ring, one side is approximately the impeller outlet pressure, and the other side is the impeller inlet pressure, so there will always be a part of the liquid leakage from the impeller outlet to the impeller inlet. This part of the liquid gets energy in the impeller, but the liquid is not sent out, which reduces the amount of water supplied to the pump. All the energy of the leaked liquid is used to overcome the resistance of the sealing ring.
Obviously, the larger the diameter of the sealing ring, the greater the difference in pressure between the two sides, the greater the leakage. In order to reduce the amount of leakage and improve the efficiency of the pump, the gap between the sealing rings should be reduced if possible. Generally, the total clearance is approximately 0002, if dw = 200 mm, the total clearance is 040 mm. When assembling, the sealing ring should not be too eccentric, otherwise, the leakage will also increase. In addition, the leakage can be reduced by the method of increasing the resistance of the sealing ring, and the main measure of increasing the resistance is to make the sealing ring into a labyrinth, zigzag and the like, and this also increases the sealing length of the sealing ring, and increases the resistance along the way.
The leakage of the sealing ring will cause the disturbance of the impeller inlet in some cases, so it is necessary to design the form of the sealing ring reasonably.
2. Leakage loss of balance mechanism
In many centrifugal pumps, there are mechanisms for balancing axial thrust: such as balance holes, balance tubes, balance plates, etc. Due to the pressure difference between the two sides of the balance mechanism, some liquid leaks from the high-pressure area to the low-pressure area. The leakage of the balance hole will reduce the efficiency of the pump by about 5%. In the balance disc mechanism, the leakage accounts for 3% of the working flow, but some high-pressure pumps are larger than this value;In order to reduce leakage losses, the diameter d' of the balance disc can be reduced without affecting the balance force.
3. Inter-stage leakage loss
In the vortex shell type multistage pump, the pressure on both sides of the stage spacer plate is unequal, so there is also a leakage loss, according to the different arrangements of the mechanism, the pressure difference on both sides of the stage spacer plate may be first, second or **, the more the difference in the number of stages, the more serious the leakage between the stages of the partition, so the stepped type interstage seal is widely used here.
In addition, in segmented multistage pumps, there is also interstage leakage. However, this is different from the interstage leakage mentioned above, because this part of the leakage liquid does not pass through the impeller, so it is not a volume loss. Here, the pressure difference between the front and back of the stage spacer is caused by the pressurization of the diffuse part of the guide vane and the suction action of the impeller backlash (equivalent to a centrifugal impeller). Under the action of pressure difference, the leakage liquid enters the side gap of the forestage impeller along the gap of the stage spacer, and flows back to the interstage gap through the guide vane, and the anti-guide vane (suction guide vane) flows back to the interstage gap, and the above process is repeated. Although the interstage leakage of a segmented multistage pump is not a volume loss, it flows back and forth so that it consumes the power of the pump. In addition, when this part of the liquid passes through the guide vane, the effective cross-section of the guide vane throat decreases (i.e., the leaking liquid takes up a part of the cross-section), so the flow velocity here increases, causing additional hydraulic losses. According to the book "Centrifugal Pump Design Basics", a multistage pump, when the flow rate is 20 liters per second, the interstage gap is from 075 mm reduced to 025 mm, the interstage leakage q was reduced by 07 liters of seconds, due to the reduction of Q, the flow through the guide vane is reduced, the flow rate of the guide vane throat is reduced, the hydraulic loss in the guide vane is reduced, and at the same time, due to the reduction of Q, the relative velocity of the liquid in the gap between the impeller and the guide vane is reduced, thus also reducing the friction loss of the impeller disc, so the efficiency of the pump is increased by about 5%.