低比转速复合叶轮离心泵停机过程水力特性

离心泵的瞬态水力特性对于系统的安全可靠运行至关重要,因此掌握其在过渡过程中的水力性能对于优化水力设计、提升可靠性具有重要价值。为探索低比转速带分流叶片的复合叶轮离心泵在突然断电停机过程中的水力特性,在8个不同稳态流量比的情况下,测量了一台比转速为45的复合叶轮离心泵的转速、进出口压力、扬程、流量、扭矩和轴功率等性能参数随时间的动态变化过程。作为对比参考,还同时测量相同叶片形状和尺寸的普通闭式叶轮离心泵停机过程的水力性能。结果表明:随着停机前稳定流量的增大,叶轮停止转动所需的时间越来越短;转速曲线变得更为陡峭,转速下降曲线基本上为四次多项式函数形式。流量在停机初期较为稳定,大大延迟于转速下降历程;随着停机前稳定流量的增大,流量较为平稳的持续时间呈现出轻微缩短的趋势,而流动完全停止所需的时间却越来越长,与转速曲线变化特性完全相反。扬程和出口静压力与转速的变化规律类似。进口静压变化十分剧烈,但在6.0 s左右趋于稳定。轴扭矩与轴功率的变化趋势基本上一致的,均与转速的变化规律类似。性能参数特征时间随流量比的增加而线性减小。同一流量比条件下,性能参数特征时间由长到短的顺序为流量、扬程、转速、扭矩和轴功率。与普通叶轮离心泵相比,在相同流量比条件下,复合叶轮离心泵性能参数的特征时间有延长的趋势,特别是流量、扬程和转速。

Hydraulic performance of low specific-speed centrifugal pump with compound impeller during stopping period

Abstract: Usually, centrifugal pumps run under steady working conditions, i.e. the pump rotational speed or its working point is basically steady or changes slowly in a long period of time during operation. Hereby the existing investigations into the performance of centrifugal pumps are mainly limited to the steady operating conditions. However, with the expansion of centrifugal pump applications and the increasing complexity of fluid flow systems, centrifugal pumps need to be operated in transient operating conditions, such as startup, stopping and rapid change of rotational speed etc. These transient operational conditions can result in a few remarked negative effects on electric grids and pipelines related. For example, the abrupt startup of a large-scale pump in pumping stations can cause a considerable impact on a local electric grid and induce a pressure surge in the pipeline. Nonetheless, the transient hydraulic characteristics of a centrifugal pump are very important to the safe and reliable operation of a pumping system. Furthermore, a full understanding of transient hydraulic performance of a centrifugal pump can be helpful for the optimization of pump hydraulic design and the enhancement of its reliability. As such, studying the pump hydraulic performance during transient operating periods has become a more necessary and urgent technical issue, and has drawn significant attention in the past 30 years. On one hand, centrifugal pumps with closed-type compound impeller of long and short vanes in a low specific-speed and high head are extensively used to transport a variety of liquid and are indispensable liquid conveying equipment in aerospace, petrochemical, automobile industries and other important sectors. On the other hand, however, the existing investigations are mainly associated with centrifugal pumps with closed-type ordinary impeller. In this paper, the transient performance of the centrifugal pump with a specific-speed of 45 and compound impeller was measured to reveal the transient characteristics of a low specific-speed centrifugal pump with closed-type compound impeller during stopping period due to power outage. The time-history profiles of performance parameters were obtained, such as rotational speed, static pressures at the pump inlet and outlet, pump head, flow rate, torque and shaft power at eight flow rate ratios where a stopping period starts to occur. As a reference, the transient performance of the centrifugal pump with ordinary impeller in the same specific-speed and dimensions and blade shape was tested as well. The experimental results showed that the time for the impeller stopping rotation was shorter and shorter, and the rotating speed curves become steeper with increasing flow rate ratio. Interestingly, the rotational speed time-history curves can be fitted quite well with a 4th-order polynomial function. In the initial period, the instant flow rate curve was kept constant basically. With increasing flow rate ratio, the period was extended slightly, but the time at the instant flow rate zero became longer. This behavior was completely different from that of the instant rotational speed curves. The instant pump head and outlet pressure exhibited a similar pattern as the rotational speed curves. Even though the inlet static pressure varied considerably during the stopping period, it gradually became relatively stable at about 6 s. Once again, the instant torque and shaft-power curves shared nearly identical profiles with the rotational speed. The length of characteristic time of performance parameters got longer with increasing flow rate ratio. At a flow rate ratio, the length of characteristic time of performance parameters was in the following order from the longest to the shortest: flow rate, head, rotational speed, torque and shaft-power; further, the length of characteristic time of the centrifugal pump with compound impeller was extended in comparison with the pump with ordinary impeller, especially for flow rate, head and rotational speed.