大型低扬程泵装置nD值的选取

从水泵选型、能量性能、汽蚀性能等3个方面,讨论了减小nD值对大型低扬程泵装置水力性能的影响;提出了减小nD值的低扬程泵装置水泵选型设计思路;借助于叶片泵相似律,推导了减小nD值与增径降速的一致关系,在设计流量一定的条件下,若叶轮直径增大5%,则水泵转速和nD值将分别下降13.6%和9.3%;从叶轮直径对流道水力损失的影响上,分析了减小nD值对提高泵装置流道效率的作用;根据nD值与水泵扬程的关系,低扬程泵装置选型时,宜适当减小nD值,以便在较低扬程下选用到更优秀的轴流泵水力模型;根据叶片泵汽蚀相似律,分析了减小nD值对低扬程泵装置汽蚀性能的影响;同时,还讨论了泵装置汽蚀性能的考核指标,以及增径降速对流道控制尺寸及设备投资的影响等问题.结果表明：对于平均扬程为4 m、单泵设计流量为33.5 m3/s的泵站,若将叶轮直径由2.9 m增大至3.1 m,则流道效率可提高2.9%;在设计流量一定的条件下,若将nD值由435降为387.5,由水力模型TJ04-ZL-06换算的原型泵高效区扬程可由5 m左右降为4 m左右,水泵必需汽蚀余量可降低20.6%;对于年运行时数较长的大型低扬程泵站,宜采用较小的nD值.

Selection of nD value for low head pump system of large pumping station

The influence of reducing nD value on hydraulic performance of a large pump system with low head was discussed from the point of view of pump type selection,energy performance and cavitation performance,and the design idea of reducing nD value for the pump system was put forward.The consistent relationship between reducing nD value and increasing impeller diameter and reducing pump speed was derived according to the affinity law of pumps.If the pump diameter increases by 5% under the condition of a certain design discharge,the pump speed and nD value will decrease by 13.6% and 9.3%,respectively.The effect of reducing nD value on increasing pump system efficiency was analyzed from the point of view of influence of impeller diameter on the conduit hydraulic loss.Based on the relation between nD value and pump head,it was put forward to reduce nD value for selecting the more excellent axial pump hydraulic model under the condition of low head.According to the cavitation affinity law of pumps,the influence of reducing nD value on the cavitation performance of the pump system was analyzed.Meanwhile examination index for the cavitation performance of the pump system and the influence of increasing impeller diameter and reducing pump speed on the conduit control sizes and equipment investment were also discussed.The research results indicate that if the pump diameter increases from 2.9 m to 3.1 m for a pump system with the average head of 4 m and single pump design discharge of 33.5 m3/s the conduit efficiency will increase by 2.9%.If the nD value decreases from 435 to 387.5 under the condition of a certain design discharge,the pump head at the high efficiency zone of the hydraulic model TJ04-ZL-06 will lower from 5 m to 4 m and the pump NPSHR will reduce by 20.6%.It suggests that a lower nD value should be adopted for a large pumping station with low head and long running operation time.