According to the principles and basic conditions of pump selection, the specific operations are as follows:
1. Based on the layout, terrain, water level, and operating conditions of the device, horizontal, vertical, and other types of pumps (pipeline, submersible, submerged, non clogging, self priming, gear, etc.) are selected.
2. Based on the nature of the liquid medium, determine whether to use a clean water pump, hot water pump, oil pump, chemical pump, corrosion-resistant pump, or impurity pump, or use a non clogging pump. Pumps installed in explosive areas should use corresponding explosion-proof motors based on the level of the explosive area.
3. Determine whether to choose a single suction pump or a double suction pump based on the flow rate; According to the height of the lift, choose single stage pump or multi-stage pump, high speed pump or low speed pump (air conditioning pump), multi-stage pump efficiency is lower than single stage pump. If both single stage pump and multi-stage pump can be used, first choose single stage pump.
4. Determine the specific model of the pump
After determining which series of pumps to choose, the specific model can be determined on the type spectrum or series characteristic curve based on the maximum flow rate (when there is no maximum flow rate, 1.1 times the normal flow rate is usually taken as the maximum flow rate), and the head after enlarging the margin by 5% -10%. The operation is as follows:
Using the pump characteristic curve, find the required flow value on the horizontal axis and the required head value on the vertical axis. Draw a vertical or horizontal line from the two values up and right respectively. If the intersection of the two lines happens to fall on the characteristic curve, the pump is the one to be selected. However, this ideal situation is usually rare, and the following two situations usually occur:
The first one: the intersection point is above the characteristic curve, which indicates that the flow meets the requirements, but the head is not enough. At this time, if the head difference is not much, or about 5%, it can still be selected. If the head difference is much, choose a pump with a larger head Or try to reduce pipeline resistance loss.
The second one: The intersection point is below the characteristic curve and within the fan-shaped trapezoidal range of the pump characteristic curve. This model is initially determined, and then the impeller diameter is cut based on the difference in head. If the difference in head is small, the impeller diameter is not cut. If the difference in head is large, the impeller diameter is cut according to the required Q, H, ns and cutting formula. If the intersection point does not fall within the fan-shaped trapezoidal range, the pump with smaller head should be selected. When selecting pumps, it is sometimes necessary to consider production process requirements and choose Q-H characteristic curves with different shapes.
5. After the pump model is determined, for pumps with physical and chemical media that are similar to water, it is necessary to go to the relevant product catalog or samples and make corrections based on the performance table or curve of the model to see if the normal working point falls in the priority working area of the pump? Is the effective NPSH greater than (NPSH). Can NPSH also be used to correct the geometric installation height in reverse?
6. For liquid pumps with a viscosity greater than 20mm2/s (or a density greater than 1000kg/m3), it is necessary to convert the characteristic curve of the water experimental pump into the performance curve of that viscosity (or at that density), especially for careful calculation or comparison of suction performance and input power.