Flow of metering pump -- there are three types of lift characteristic curve: flat, steep, and hump. The water pump used in the sewage treatment system should have a flat characteristic, and the range between zero flow and maximum flow should not be greater than 10% to 15%. The measurement pump with steep descending characteristic is too large for the maximum flow and the minimum flow, so it is not suitable to choose. Hump characteristics of the pump is adopted, for the two pumps in parallel runtime may cause the periodic change of load and head, and when the change of frequency is equal to the natural frequencies of the system to produce dangerous "oscillation phenomenon", and this phenomenon will cause certain influence to the normal operation of the system.
The best working point of circulating water pump is the intersection point of the characteristic curve of the pump and the characteristic curve of the system pipe network. However, for a variety of reasons, the actual flow of the system is always greater than the design calculation flow, and the result is that the design of the pump working point is offset to the right along the characteristic curve of the pump.
The bias in the pump working point to the right, produced by circulating water pump head is reduced, it is extremely unfavorable for the normal operation of the system, especially the most unfavorable loop system will lead to the flow of the loop further reduce, affect the normal use function.
Causes of the set-point moves to the right there are mainly two aspects: first is the safety factor of the hydraulic calculation in the design of using too much and not the actual pressure drop calculation method, followed by the design of the system is not a serious hydraulic balance calculation, and after construction and strict system debugging. Therefore, in order to make the system according to the operation of design, besides should be related to calculated carefully, also should choose when choosing water pump will pump working point on the best working point on the left side of the position, to prevent pump real working point beyond a certain range in economic operation condition, affecting normal operation of the system.