Abstract：In order to adjust working conditions of variable speed operations with variable frequency drives applied in low head pumping stations in the South to North Water Diversion Project, the relationship between heads and discharge rates of pump and pumping system under different rotational speeds were analyzed with affinity law. It is found out that the relationship between heads and discharge rates of pump corresponds to the conventional affinity law under different speeds, but that of pump system does not. The flow fields in passages, especially those in discharge passages affected by the outflow of pump, don′t follow the kinetic similarity rules when the pumping system is operated under different rotational speeds. Errors produced when the conventional affinity law is applied to predict the low head pumping system performances. The flow fields distributions of a typical tubular pumping system under three different rotational speeds were analyzed by CFD technology, which shows that the velocity distributions are different or even in the reversed direction when the variation of rational speed reaches a certain degree. The relationship at different speeds does not correspond to the affinity laws and the conventional ones cannot be adopted.
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