平衡鼓间隙对首级叶轮前泵腔压力及多级泵轴向力的影响

平衡鼓的轴向力平衡能力是多级离心泵发生故障并影响其寿命的关键因素。该文采用数值模拟方法,在多级泵的外特性、平衡管内压力和泄漏量的数值计算结果与试验结果基本一致的基础上,研究了平衡鼓间隙泄漏量变化对首级叶轮前泵腔的压力分布、首级叶轮及整个叶轮轴向力的影响。研究结果表明:首级叶轮前泵腔中的漩涡区是腔体内压力变化的主要原因。当平衡鼓间隙由0增大到0.5 mm时,首级叶轮的轴向力在间隙为0时最大,在间隙为0.3 mm时最小,其最小值为最大值的20.6%;整个叶轮所受轴向力随着平衡鼓间隙增大呈先减小后增大的趋势。无量纲化的平衡鼓间隙面积大于6.6×10-3时,由于平衡鼓前后压差较小,已无法有效平衡轴向力,在此范围轴承发生断轴的风险较大。该研究可为多级泵平衡鼓设计提供参考。

Influence of balance drum clearance on pressure of front cavity of first stage impeller and axial force of multistage pump

The ability of the balance drum to balance the axial force is the key factor for the failure of multistage centrifugal pump. However, during the operation of the balance drum, due to the long-term collision with the liquid or the friction with the casing, the leakage amount at the balance drum clearance is gradually increased, resulting in the balance drum being worn. Therefore, studying the leakage flow is critical to the ability of the balance drum to accurately balance the axial force of the entire impeller. In this paper, three-dimensional turbulent flow of multistage centrifugal pump was simulated by using the CFD code FLUENT. Besides, steady simulation was conducted for different operating points of the pump, the turbulence was simulated with shear stress transportation(SST) turbulence model together with automatic near wall treatment. CFD results were compared with those from the model test. And the results of the pressure and leakage in the balance pipe and the external characteristics of the multistage pump were basically consistent with the experimental results. Moreover, the maximum errors of head, efficiency and shaft power were 4.17%, 2.81% and 4.25% respectively, but the experimental flow rate of balance pipe was always greater than the simulated one. This was mainly because the influence of orifice flowmeter in the pipe had been not taken into account in the numerical simulation. The maximum error of the flow rate of the balanced pipe at the design point was 4.49%. The maximum error of pressure was 2.5%. It showed that the calculation method selected in this paper could provide a reliable guarantee for this study. The results showed that at the design flow rate, the liquid pressure in the front cavity of the first impeller increased gradually along the axial direction from the inlet section to the outlet section. When the balance drum clearance was less than 0.2 mm, the pressure distribution along the radial direction was uniform in each section. But when the clearance was more than 0.2 mm, it was asymmetric. Furthermore, with the increase of clearance, the pressure inhomogeneity became more obvious. Moreover, at 0.5Q(Q is design flow, Q=128 m3/h) and 1.5Q flow rates, there was no obvious regularity of pressure increment along the radial direction under different clearances. The pressure increment was the smallest when the balance drum clearance was 0.3 mm, and the biggest when the balance drum clearance was 0. And under the above 2 conditions, the minimum increments were 50.7% and 88.9% of the maximum, respectively. When the clearance increased from 0 to 0.5 mm, under design flow rate, the pressure increased gradually along the radial direction. Wherein, when the clearance was 0 and 0.5 mm, the pressure increment was the maximum value and the minimum value, respectively, and the minimum value was 44.6% of the maximum value. Besides, there was a large vortex region in the front cavity of the first impeller, when the clearance of balance drums was 0, 0.3 and 0.5 mm, respectively. The vortex region decreased gradually with the increase of flow rate, which indicated that the appearance of the vortex region was the main reason for the change of pressure in the cavity. In addition, with the increase of clearance, the total axial force of 11 stage impellers decreased first and then increased. And when non-dimensionalized balance drum clearance area was greater than 6.6×10-3, the balance drum could not effectively balance the axial force. Furthermore, the bearing had a greater risk of fracture in this range. This research can provide useful reference for design of balance drum and prediction of risk of bearing fracture.