After deciding which series of pumps to choose, the specific type can be determined on the type spectrum or series characteristic curve by taking the main parameters of the head after enlarging the 5%-10% allowance, according to the z* large flow rate (1.1 times of the normal flow rate is usually taken as the z* large flow rate when there is no z* large flow rate).
The operation is as follows: using the pump characteristic curve, find the required flow value in the abscissa, find the required lift value in the longitudinal coordinate, and lead the vertical or horizontal line upward and right respectively from the two values. The intersection point of the two lines just falls on the characteristic curve, then the pump is the pump to be selected, but this ideal situation is generally rare.
Usually there are two situations: the first one is that the intersection point is above the characteristic curve, which indicates that the flow rate meets the requirements, but the head is not enough. At this time, if the head is similar, or the difference is about 5%, the pump with larger head can still be selected. The second kind: under the characteristic curve, in the fan trapezoidal range of the pump characteristic curve, the type of impeller is initially determined. Then, according to the difference of lift, the diameter of the impeller is decided whether to cut. If the difference of lift is very small, the impeller is not cut. If the difference of lift is very large, the diameter of the impeller is cut according to the required Q, H, ns and cutting formula. If the intersection point does not fall in the fan trapezoidal, the impeller diameter is cut according to the required Q, H, ns and cutting formula. Within the scope, the water ring vacuum pump with smaller lift should be selected.