Pump efficiency refers to the ratio of the effective power of the pump to the shaft power. It indicates the effective degree of pump energy conversion and is an important technical and economic indicator of the pump, which is represented by η. The efficiency on the pump nameplate corresponds to the efficiency at which the flow is designed, which is the highest efficiency of the pump. The higher the efficiency of the pump, the smaller the energy loss when the pump is working.
It is impossible for the pump shaft power to be transmitted to the output liquid, and there must be some energy loss. The energy loss in the pump can be divided into three parts, namely hydraulic loss, volume loss and mechanical loss.
1. Hydraulic loss and hydraulic efficiency
Friction loss, local loss and impact loss occur when the water flows through the suction chamber, the impeller, and the pressure chamber of the water pump. The friction loss is the loss caused by the frictional resistance between the water flow and the wall of the flow passage member. Local loss is the loss of water flow in the pump due to changes in the magnitude and direction of the water flow. Impact loss is the loss of water flow at the inlet, outlet, and out of the blade when the pump is operating under non-design conditions. The greater the hydraulic loss, the smaller the lift of the pump. If the head of the hydraulic loss in the pump is not considered as the theoretical head HT, the ratio of the pump head H to the theoretical head H and the theoretical head HT is called the hydraulic efficiency ηh.
2, volume loss and volumetric efficiency
After the water flows through the impeller, a small portion of the high-pressure water leaks through the pump body gap (such as the leak-reducing ring) and the axial force balance device (such as the balance hole) to the inlet of the impeller, and another part leaks from the shaft seal device to the pump body. It consumes a part of the energy, that is, the volume loss. The larger the leakage q, the smaller the water discharge Q of the pump. The ratio of the flow rate Q through the pump outlet to the flow rate Q+q through the pump inlet is referred to as the volumetric efficiency ηV.
3. Mechanical loss and mechanical efficiency
When the impeller rotates in the liquid, the outer surface of the front and rear cover plates and the liquid cause friction loss (ie, the loss of the wheel). When the pump shaft rotates, the shaft and the shaft seal and the bearing generate friction loss, and the friction loss consumes part of the energy, that is, the machine. Loss, mechanical loss power is expressed in Pm. After the mechanical loss is subtracted from the input power of the pump, the power delivered by the impeller to the liquid is weighed, expressed in Pw.
The efficiency of a pump is the product of volumetric efficiency, hydraulic efficiency, and mechanical efficiency. To increase pump efficiency, it is necessary to reduce various losses in the pump. To improve the efficiency of the pump, in addition to improving and improving the hydraulic model, material selection, processing technology, components, etc., the user should also pay attention to the correct choice of pump type, ensure installation quality, rationally adjust operating conditions and strengthen maintenance management. The pump is often operated under high efficiency to save energy, reduce costs and improve economic efficiency.
