Centrifugal pump energy loss and efficiency
Centrifugal pump energy loss and efficiency The power delivered by the prime mover to the pump shaft cannot be fully converted to active power, ie it cannot be used to increase the energy of the liquid. Since some of the energy is consumed during the rotation of the pump shaft, some of the energy is lost in the pump, so the pump's effective power is always less than the shaft power.
According to the energy loss form of the centrifugal pump , it can be divided into: mechanical loss, volume loss and hydraulic loss.
1. Mechanical loss and mechanical efficiency The mechanical loss includes two parts: one is the friction loss between the pump shaft and the bearing and the shaft sealing device; the other is the wheel resistance loss, also called the friction loss of the disk, that is, the original pump case filled with liquid. During internal rotation, the friction between the outer surface of the impeller and the fluid is lost.
In the mechanical loss, the proportion of wheel resistance loss is larger, while the shaft seal of the bearing has a smaller friction loss. The mechanical efficiency ηm represents the mechanical loss, and the mechanical efficiency is the ratio of the remaining power of the shaft power Pe to the shaft power after the mechanical loss, ie: 
Where Pm is the power consumed due to mechanical losses.
Theory and practice show that a reasonable reduction in the impeller outer diameter, increase impeller speed, reduce the surface roughness of the wheel cover, can increase the mechanical efficiency of the pump. If the shaft of the pump shaft is mechanically sealed, the friction loss of the shaft seal is small. If the packing is sealed with a packing, care should be taken not to overtighten the packing gland. The centrifugal pump's mechanical efficiency is generally 90%~97%.
2. Volume loss and volumetric efficiency When the centrifugal pump is in operation, the liquid pressure in the pump body is different. There are high pressure areas and low pressure areas. Due to the need of the mechanism, there are a lot of gaps in the pump body. When the pressures in the front and back of the gap are different, some of the liquids need to have the high pressure area and the low pressure area, as shown in FIG. 1 . Although this part of the liquid has gained energy but has not been effectively utilized, it circulates in the pump and is consumed to overcome the gap resistance. Some of the liquid leaks from the shaft seal after it has gained energy, so the pump's actual flow qv is less than the theoretical flow qvth.
Figure 1 Schematic diagram of liquid leakage in a centrifugal pump
The volumetric efficiency, ηv, is the volumetric loss, which is the ratio of the volumetric lost power to the undamaged power, ie: 
The volumetric efficiency ηv of a centrifugal pump is generally 90% to 95%.
For a given centrifugal pump, to increase the volumetric efficiency ηv, it is necessary to reduce the leakage amount, and it is possible to reduce the annular area of ​​the seal gap or increase the gap resistance of the seal ring. Centrifugal pumps in operation should regularly check the wear of the seal ring and replace it in time, otherwise the volumetric efficiency will be reduced.
3, hydraulic loss and hydraulic efficiency There is friction loss when the liquid flows through the flow passage parts such as the impeller, and there is impact loss when the liquid flow speed changes in size and direction. These losses all consume a part of energy, and this part of energy loss is usually called hydraulic loss.
(1) Frictional loss along the flow-through part The frictional resistance loss occurs when the liquid passes through the flow-through parts such as the liquid-absorbing chamber, the impeller, and the guide wheel. Since the friction loss along the path is proportional to the square of the flow velocity, and the flow rate is proportional to the flow rate, the friction loss along the path is proportional to the square of the flow rate. Curve I in Fig. 2 shows the friction loss along the path and the theoretical flow qvth.
Fig. 2 Relationship between hydraulic loss and flow rate of centrifugal pump
(2) Impact loss Resistance loss occurs when the size and direction of the liquid flow rate change. In the design condition, the impact loss is small and close to zero because the flow direction is consistent with the direction of the blade. When the flow rate is greater or less than the design condition, the impact loss will gradually increase due to the change of the flow direction. Curve II in FIG. 2 represents the curve of impact loss as a function of qvth.
The total hydraulic loss of the centrifugal pump is the sum of the above two items, as shown by the curve III in Figure 2.
The hydraulic efficiency is the ratio of the power lost after hydraulic loss to the power without hydraulic loss, expressed as ηh. The size of ηh is related to the structure of the centrifugal pump and is generally 70% to 90%.
In order to improve the hydraulic efficiency, the shape of the impeller flow path and the shape of the blades should be reasonably determined to make the liquid flow rate change as gentle as possible in order to prevent vortices and dead angles and reduce the surface roughness of the flow-through components.
4. The total efficiency of the centrifugal pump The total efficiency η of the centrifugal pump is equal to the ratio of the effective power P and the shaft power Pe, ie: 
The efficiency of the centrifugal pump is related to the specific speed of the centrifugal pump. It is also related to the flow and structure of the pump. When the single-stage single-suction centrifugal pump delivers normal-temperature clean water and the specific rotation speed is ns=120~210, the efficiency value can be found from FIG. 3 according to the flow rate. When ns <120 or >210, the efficiency value of the single-stage single-suction centrifugal pump is the sum of the efficiency value found in FIG. 3 and the efficiency correction value caused by the difference in the specific speed ns. When ns=20~120, the efficiency correction value is found in Figure 4(a); when ns=210~300, the efficiency correction value is found in Figure 4(b).
Figure 3 Single-stage single-suction centrifugal pump efficiency (ns=120~210)
Fig. 4 Efficiency correction value of single-stage single-suction centrifugal pump
The efficiency of the centrifugal pump is an important technical and economic index, which indicates the performance of the pump and the degree of utilization of the prime mover. Increasing the efficiency of centrifugal pumps involves the design, manufacture, installation, and operation of the pumps. It must be fully considered before reaching the goal of improving efficiency.
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